In this article, Levine et al. (1992) [Levine et al., 1992: (http://radiology.rsnajnls.org/cgi/reprint/185/2/537.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/1410369)] used magnetic resonance spectroscopy (MRS) scans to look at the "phosphocreatine index," which is [PCr]/([PCr] + [Pi]), such that Pi = inorganic phosphate, as a way of quantifying the energy states of cells in different parts of the brain. Levine et al. (1992) also calculated the pH in the brain, by comparing measurements of Pi and PCr, and found a number of changes in the correlations between different pairs of indices in people who had recently had strokes. The authors noted that acidosis generally decreases the PCr/Pi ratio (Levine et al., 1992), and they found that, during the acidotic conditions of ischemia, in the brains of people who had recently (within days or weeks, depending on the patient) had strokes, the pH correlated directly with the phosphocreatine index and with the PCr content (as indicated by the magnitude of the MRS signal) and inversely with Pi. I'm not quite sure how to interpret a change in the intracellular inorganic phosphate levels, but I guess that makes sense that the pH would decrease as the Pi increases. The intracellular (and extracellular, to some extent) Pi levels are thought to generally increase in parallel with the hypoxia or ischemia-associated increases in intracellular AMP levels [Gorman et al., 1997: (http://ajpheart.physiology.org/cgi/reprint/272/2/H913.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/9124455)], and Gorman et al. (1997) also found that a "steady-state," or prehypoxic, depletion of the intracellular phosphocreatine contents, in the hearts of guinea pigs, caused the hypoxia-induced release of Pi and adenosine to decrease. Gorman et al. (1997) also noted that Pi stimulates glycolytic activity, and there's a considerable amount of research showing that the depletion of intracellular levels of either Pi or organic phosphates causes fairly severe deficits in glycolytic and oxidative ATP synthesis. Brautbar et al. (1983) [Brautbar et al., 1983: (http://www.ncbi.nlm.nih.gov/pubmed/6620852)] found that, in response to dietary phosphate depletion/restriction in rats, the total intracellular protein contents of creatine kinase (CK, "creatine phosphokinase"), among the rats, correlated directly with the intracellular Pi concentrations, and both the intracellular Pi levels and creatine kinase levels were decreased in response to dietary Pi depletion. The authors also cited research, on the last page, discussing the capacity of Pi to activate glutaminase and phosphofructokinase activities, and the activities of those enzymes are important in relation to energy metabolism, especially during ischemia or hypoxia. The decrease in the CK protein content was drastic (more than 50 percent) (Brautbar et al., 1983) and is really interesting, in part because the authors only found, in response to dietary Pi restriction, nonsignificant reductions in the intracellular adenosine nucleotide levels and inconsistent changes in the PCr levels (Brautbar et al., 1983). The authors apparently equated the CK protein content with CK activity, and I'm not sure that that would always be a valid assumption (the CK protein content might not correlate linearly with the CK activity, because of phosphorylation of CK by AMPK or by effects of pH on CK activity, etc.). But in humans with renal failure, phosphate supplementation increased ATP levels in the skeletal muscle, compared to controls, but did not change serum phosphate/phosphorus levels. I'd bet that Pi depletion produces changes in PCr or in the PCr index in humans as well as in animals.
Lyoo et al. (2003) [Lyoo et al., 2003: (http://www.ncbi.nlm.nih.gov/pubmed/12850248)] found that the levels of Pi in the brains of humans increased by more than 9 percent in response to creatine (Cr) supplementation, but the authors evidently didn't find any differences in pH between the control and Cr-supplemented groups. Lyoo et al. (2003) also found decreases in the beta-nucleotide triphosphate (beta-NTP) levels (a reflection of ATP levels) and marginal increases in PCr levels, and that trend toward an increase in the PCr/ATP ratio is generally consistent with the increases in PCr/NTP ratio that have been found, in the brains of humans, in response to triacetyluridine or SAM-e administration (http://hardcorephysiologyfun.blogspot.com/2009/06/deoxyribonucleotides-in-mtdna-depletion.html). Lyoo et al. (2003) and others have explained that change in terms of a mass action effect (it's not really ATP depletion and is thought to be evidence of an increase, not a decrease, in intracellular high-energy phosphate levels) that would operate in the overall equilibrium of the intramitochondrial CK enzymatic reaction (the more important CK equilibrium, as I recall) (it's noteworthy that the intramitochondrial ADP pool can be somewhat independent of the cytosolic ADP pool, however):
Cr + ATP <---> PCr + ADP + H(+)
I think that ATP disodium is much more potent than SAM-e and would require lower dosages (expressed both in terms of mg of adenosine that can be derived from ATP disodium, in comparison to mg adenosine that can be derived from SAM-e, and in terms of the dosages of ATPNa2 and SAM-e) to produce effects. Low-dose creatine (1-3 grams/day) has potential, in my opinion, to work in concert with ATPNa2 and uridine or triacetyluridine or cytidine (pyrimidines), etc. Those are just my opinions. Pyrimidines have repeatedly been shown to increase the retention of intracellular purine nucleotides during hypoxia or other metabolic insults. I was going to discuss the research showing that CK activity, induced by cellular creatine depletion, generally appears [O'Gorman et al., 1996: (http://www.ncbi.nlm.nih.gov/pubmed/8816948)] or has been shown to [Dzeja et al., 1996: (http://www.ncbi.nlm.nih.gov/pubmed/8662747)] vary inversely with adenosine kinase activity in different cell types. That effect could be partially due to changes in inorganic phosphate availability, but the relationships between the intracellular Pi levels and adenosine kinase (AK) and creatine kinase activities are likely to be complex. Some research has shown that Pi produces allosteric activation of AK (http://hardcorephysiologyfun.blogspot.com/2009/07/these-are-some-articles-showing-that.html), but Gorman et al. (1997) found that Pi inhibited the activity of AK that had been taken from the hearts of guinea pigs. I don't have time to go into the potential mechanisms, but one has to take into account the adenylate charge and the activating effects that AMP (or an increase in the AMP/ATP ratio) and Pi can have on glycolytic enzymes, etc. It's interesting that Gorman et al. (1997) discussed an index that I've almost never seen researchers use. They referred to the cytosolic phosphorylation potential as being an index, or ratio, that correlates (positively) with the energy state of the cell [similar to the adenylate charge, or {[ATP] + 0.5[ADP])/([ATP] + [ADP] + [AMP]), with [ ]'s referring to the intracellular molarities of the nucleotides}. The cytosolic phosphorylation potential is (log [ATP]/[ADP][Pi]). That's sort of similar to the phosphocreatine index, but it seems that one might expect the phosphocreatine index to vary almost inversely with the cytosolic phosphorylation potential.
Showing posts with label Creatine and Phosphocreatine. Show all posts
Showing posts with label Creatine and Phosphocreatine. Show all posts
Wednesday, August 19, 2009
Wednesday, July 1, 2009
Mild Uncoupling as a Factor That May Mediate Antidepressant Effects of Tricyclics and Other Compounds
This article [Berson et al., 1996: (http://www.ncbi.nlm.nih.gov/pubmed/8964414)] discusses the capacity of tacrine to produce uncoupling in mitochondria and to increase respiration, and I was noticing the crude structural similarity between tacrine and some of the tricylic antidepressants (http://scholar.google.com/scholar?q=tacrine+imipramine+tricyclic&hl=en&safe=off&num=100&um=1&ie=UTF-8&oi=scholart). Tacrine is sort of an extreme example, in my opinion, but some tricyclics also can produce some slight uncoupling effects in mitochondria (i.e. of redox reactions from oxidative phosphorylation) [see, for example, references in Wong et al., 2004: (http://149.142.238.229/pgxlab/docs/Wong-SJW-2004-MP.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/14743185)], and hypericin (which is a planar molecule that, in the sense that a planar ring system is one structural feature of some uncouplers, is structurally "similar," in a very general sense, to tricyclics), for example, can either facilitate or inhibit uncoupling in mitochondria (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&q=hypericin+mitochondrial+respiration). Some of the articles on uncoupling that I've cited in past postings provide better discussions of planar, "heterocyclic" ring systems being a structural feature of uncouplers, but here are some references (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&q=planar+uncoupling+mitochondria+orbital). I'm having trouble getting full texts of these types of articles, but I think the tricyclics can either assume planar or stacked-planar conformations that may explain their uncoupling effect [Maxwell et al., 1970: (http://jpet.aspetjournals.org/cgi/content/abstract/173/1/158)]. I forget what the conformational flexibility of that type of heterocyclic ring system, with the seven-carbon ring, is like. (In any case, it's not even really relevant to the discussion.)
Uncoupling is not always bad and is thought to be protective under many sets of circumstances. For example, uncoupling can protect against the generation of reactive oxygen species. I wonder if mild uncoupling contributes to the antidepressant effects of some tricyclics or to the St. John's Wort (SJW) extract. People may well have discussed this possibility in some articles, and I haven't really looked into it. The main problem some people have with SJW extract, which has considerable evidence to support its effects or usefulness or whatever, as an adjunctive antidepressant approach, is the photosensitivity. And I should mention that extended, planar ring systems, in compounds, are thought to mediate the photosensitizing effects of many compounds. There's actually a surprising amount of evidence suggesting that rutin, which I think is not photosensitizing and is sold separately, may contribute to the supposed antidepressant effects of SJW extracts (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&q=rutin+antidepressant). That research on rutin actually looks kind of questionable to me, and I'm inclined to doubt it does actually contribute to the effects of SJW. But that's just my opinion, and I put these types of things on the blog (as I come across them). I have no idea about that.
But Wong et al. (2004) note that, in animals, tricyclics have sometimes been shown to initially (within the first 7 days) increase glucose uptake or oxidation in the brain initially, consistent with a stimulation of respiration/oxygen uptake, resulting from a mild uncoupling effect. Subsequently, after 28 days, the rate of glucose oxidation was decreased, in response or as an adaptation to the presence or higher concentration of the tricyclic (reference 49, discussed on page 11). I wonder if that might partly explain the delay in the onset of the effects of tricyclics, etc.
These are crude thoughts, but the initial effect of an uncoupler on ATP production tends to be sort of neutral, from what I can tell. For example, the research on the apparent and mild uncoupling effects of beta-hydroxybutyrate or acetoacetate or sodium butyrate infusions in humans or animals, discussed in a fairly recent posting, shows that there can be a slight, initial decrease in the rate of ATP production in response to the presence of an uncoupler (in response to the proton cycling by the protonated form of the short-chain fatty acid, although it's not clear that ketones or short-chain fatty acids produce true uncoupling--they appear to produce something similar to it or may produce it indirectly). Then, there is a stimulation of respiration, in response to that initial effect, and an increase in oxygen uptake (producing the thermal effect, or thermogenic effect that is observed in response to i.v. beta-hydroxybutyrate infusion, etc.) that tends to offset some of the pH changes (at least in response to ketone infusion) and other initial effects of the mild uncoupling. That's thought to account for the increase in glucose utilization. Maybe there's some sort of adaptation that contributes to antidepressant effects. Creatine, for example, stimulates respiration, although it's not really an uncoupler, as far as I know. It's sort of the opposite. But maybe it's not so much about uncoupling vs. the inhibition of uncoupling. Maybe there needs to be some sort of shock to the status quo, in terms of state III vs. state IV respiration, such that drugs or compounds can produce opposite effects initially but lead to the same net, adaptive increase or decrease in respiration (the animal research seems to suggest that tricyclics can produce some adaptive decrease in respiration, almost in a way that's reminiscent of the effects of an increase in ketone oxidation by neurons or astrocytes). Tianeptine, for example, actually increases serotonin reuptake and has been used as an antidepressant (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&q=tianeptine+%22serotonin+reuptake%22), and the serotonin reuptake inhibitors are obviously one class of antidepressants. That's not much of an argument and is not evidence of much of anything, for many reasons, but the point is that one can, in some instances, arrive at the same net effect (among different individuals or even in the same individual) by administering either two treatments or compounds that have more or less opposite effects, etc.
In any case, an increase in noradrenergic transmission, such as would occur in response to tricyclics, also tends to stimulate oxidative metabolism in astrocytes [this is a really crude search: (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&q=noradrenergic+glycogen+astrocyte+respiration+OR+oxidation)]. So the effects of uncoupling could work in concert with the supposed astrocyte-glycogenolytic (and subsequent rebound increase in glycogen formation, as a phenomenon that displays a crude similarity to glycogen supercompensation) effect of an increase noradrenergic transmission, and the uncoupling effect and noradrenergic mechanisms would not be mutually exclusive. There's actually some evidence to suggest that there is a kind of glycogen supercompensation in the brain, in astrocytes, in response to glycogen depletion (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&q=%22glycogen+supercompensation%22+brain). In any case, creatine can increase glycogen storage/supercompensation in skeletal muscles (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&q=creatine+glycogen) and has been shown to produce antidepressant effects at low dosages (i.e. 3 grams/day or less) in three small studies (discussed in previous postings). In this search, one can see additional studies in which creatine's effects on "mood" were evaluated in a sort of informal or subjective way (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&q=creatine+monohydrate+antidepressant+OR+mood). There are other lines of evidence, but some of those articles are interesting.
Uncoupling is not always bad and is thought to be protective under many sets of circumstances. For example, uncoupling can protect against the generation of reactive oxygen species. I wonder if mild uncoupling contributes to the antidepressant effects of some tricyclics or to the St. John's Wort (SJW) extract. People may well have discussed this possibility in some articles, and I haven't really looked into it. The main problem some people have with SJW extract, which has considerable evidence to support its effects or usefulness or whatever, as an adjunctive antidepressant approach, is the photosensitivity. And I should mention that extended, planar ring systems, in compounds, are thought to mediate the photosensitizing effects of many compounds. There's actually a surprising amount of evidence suggesting that rutin, which I think is not photosensitizing and is sold separately, may contribute to the supposed antidepressant effects of SJW extracts (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&q=rutin+antidepressant). That research on rutin actually looks kind of questionable to me, and I'm inclined to doubt it does actually contribute to the effects of SJW. But that's just my opinion, and I put these types of things on the blog (as I come across them). I have no idea about that.
But Wong et al. (2004) note that, in animals, tricyclics have sometimes been shown to initially (within the first 7 days) increase glucose uptake or oxidation in the brain initially, consistent with a stimulation of respiration/oxygen uptake, resulting from a mild uncoupling effect. Subsequently, after 28 days, the rate of glucose oxidation was decreased, in response or as an adaptation to the presence or higher concentration of the tricyclic (reference 49, discussed on page 11). I wonder if that might partly explain the delay in the onset of the effects of tricyclics, etc.
These are crude thoughts, but the initial effect of an uncoupler on ATP production tends to be sort of neutral, from what I can tell. For example, the research on the apparent and mild uncoupling effects of beta-hydroxybutyrate or acetoacetate or sodium butyrate infusions in humans or animals, discussed in a fairly recent posting, shows that there can be a slight, initial decrease in the rate of ATP production in response to the presence of an uncoupler (in response to the proton cycling by the protonated form of the short-chain fatty acid, although it's not clear that ketones or short-chain fatty acids produce true uncoupling--they appear to produce something similar to it or may produce it indirectly). Then, there is a stimulation of respiration, in response to that initial effect, and an increase in oxygen uptake (producing the thermal effect, or thermogenic effect that is observed in response to i.v. beta-hydroxybutyrate infusion, etc.) that tends to offset some of the pH changes (at least in response to ketone infusion) and other initial effects of the mild uncoupling. That's thought to account for the increase in glucose utilization. Maybe there's some sort of adaptation that contributes to antidepressant effects. Creatine, for example, stimulates respiration, although it's not really an uncoupler, as far as I know. It's sort of the opposite. But maybe it's not so much about uncoupling vs. the inhibition of uncoupling. Maybe there needs to be some sort of shock to the status quo, in terms of state III vs. state IV respiration, such that drugs or compounds can produce opposite effects initially but lead to the same net, adaptive increase or decrease in respiration (the animal research seems to suggest that tricyclics can produce some adaptive decrease in respiration, almost in a way that's reminiscent of the effects of an increase in ketone oxidation by neurons or astrocytes). Tianeptine, for example, actually increases serotonin reuptake and has been used as an antidepressant (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&q=tianeptine+%22serotonin+reuptake%22), and the serotonin reuptake inhibitors are obviously one class of antidepressants. That's not much of an argument and is not evidence of much of anything, for many reasons, but the point is that one can, in some instances, arrive at the same net effect (among different individuals or even in the same individual) by administering either two treatments or compounds that have more or less opposite effects, etc.
In any case, an increase in noradrenergic transmission, such as would occur in response to tricyclics, also tends to stimulate oxidative metabolism in astrocytes [this is a really crude search: (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&q=noradrenergic+glycogen+astrocyte+respiration+OR+oxidation)]. So the effects of uncoupling could work in concert with the supposed astrocyte-glycogenolytic (and subsequent rebound increase in glycogen formation, as a phenomenon that displays a crude similarity to glycogen supercompensation) effect of an increase noradrenergic transmission, and the uncoupling effect and noradrenergic mechanisms would not be mutually exclusive. There's actually some evidence to suggest that there is a kind of glycogen supercompensation in the brain, in astrocytes, in response to glycogen depletion (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&q=%22glycogen+supercompensation%22+brain). In any case, creatine can increase glycogen storage/supercompensation in skeletal muscles (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&q=creatine+glycogen) and has been shown to produce antidepressant effects at low dosages (i.e. 3 grams/day or less) in three small studies (discussed in previous postings). In this search, one can see additional studies in which creatine's effects on "mood" were evaluated in a sort of informal or subjective way (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&q=creatine+monohydrate+antidepressant+OR+mood). There are other lines of evidence, but some of those articles are interesting.
Tuesday, June 30, 2009
Adenine Nucleotide Translocase and Mitochondrial Creatine Kinase Activities in the "Maintenance" of mtDNA Integrity; Relevance to Cytotoxic Therapies
This article [Palmieri et al., 2005: (http://hmg.oxfordjournals.org/cgi/content/full/14/20/3079)(http://www.ncbi.nlm.nih.gov/pubmed/16155110?dopt=Abstract)] is really interesting, and the authors described a patient who had no functional activity of the ANT1 isoform of the adenosine/adenine nucleotide translocase protein, which transports ADP into mitochondria and ATP out of the mitochondrial matrix, and had mitochondrial DNA (mtDNA) deletions in the skeletal muscles (and probably also the heart). The ANT1 isoform is the predominant isoform expressed in cardiac and skeletal muscles, and the authors describe the way the person had exercise intolerance and very gradual deterioration in fitness but did not demonstrate decompensated heart failure (Palmieri et al., 2005). The ANT1 isoform is also expressed in the brain, the authors say, but the authors say that the expression of the other isoforms is thought to be significant enough to compensate for the loss of functional ANT1 in different cell types in the brain (Palmieri et al., 2005).
That research may help explain the capacity of creatine supplementation to treat people who have mitochondrial disorders, and the mechanisms by which creatine can sometimes be beneficial in those conditions are not well-understood (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&q=creatine+monohydrate+mitochondrial). Not all of those studies have shown benefits, however. But creatine has sometimes been shown to decrease or abolish paracrystalline inclusions and other types of structural abnormalities in skeletal muscle mitochondria (or in the myocytes, more broadly). Even though creatine is clearly not a magic bullet, in any way, in the treatment of mitochondrial disorders, in my opinion, I do think there could be some potential, at low dosages, for creatine to prevent the accumulation of mtDNA deletions by its capacity to increase ADP recycling, via the functional coupling of mitochondrial creatine kinase activity (which is in the intermembrane space and intercristae space, outside the mitochondrial matrix) [Dolder et al., 2003: (http://www.jbc.org/cgi/content/full/M208705200)(http://www.ncbi.nlm.nih.gov/pubmed/12621025)] (it's actually thought to form complexes with ANT proteins) to adenine nucleotide translocase activity [Barbour et al., 1984: (http://www.jbc.org/cgi/reprint/259/13/8246)(http://www.ncbi.nlm.nih.gov/pubmed/6330105?dopt=Abstract); (http://scholar.google.com/scholar?q=%22nucleotide+translocase%22+%22creatine+kinase%22&hl=en&lr=)]. Essentially, ANT exports ATP from the mitochondrial matrix, and the activity of mitochondrial creatine kinase ensures, in the presence of an adequate pool of creatine, that ADP is recycled (it's the kinetics of the coupling reactions and spatial proximity, I think, that allow this to occur and to limit the loss of ADP, by diffusion, from the intermembrane space to the cytosol) back into the mitochondrial matrix. That's an oversimplification, but it conveys the concept. The intramitochondrial ADP pool has repeatedly been shown to be maintained, in a creatine-sensitive manner, somewhat or even largely independently of the cytosolic ADP pool, and the effect of creatine can be especially significant during ischemia, etc. This occurs despite the fact that there is no meaningful physical barrier (nothing like the inner mitochondrial membrane) to the diffusion of ADP from the intermembrane space to the cytosol proper. I guess the diffusion can be restricted to some extent, by the outer mitochondrial membrane, but I think the effect of the membrane is not all that significant.
It's important to note, though, that I think the therapeutic dosage range of creatine is fairly small (maybe 1-3 grams/day), but that's just my opinion. I think it has the potential, at higher dosages (or, conceivably, at any dosage in someone with liver disease, for example) to interfere with the transport of other guanidino compounds, including urea cycle intermediates, but that's just my opinion. I've discussed those issues in past postings.
There's actually more research showing that some bisphosphonates can induce apoptosis by forming either xenobiotic-and nucleotide-containing polyphosphates or can inhibit mevalonate-pathway enzymes and increase the formation of endogenous dinucleotide triphosphates, such as inosine triphosphoadenosine (IpppA), and those dinucleotides or dinucleotide-mimetics can inhibit adenine nucleotide translocase activity and thereby contribute to the pro-apoptotic effects of some bisphosphonates in cultured cells, etc. [Monkkonen et al., 2006: (http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1616989)(http://www.ncbi.nlm.nih.gov/pubmed/16402039)]. This is not always "bad," providing one can effectively target the cells one wants to target. In any case, I have to say that that mechanism doesn't sound very good to me, for many reasons, but that's just my opinion. And a person would obviously want to weigh the risks vs. the benefits in all of these cases and discuss all of those issues with one's doctor before doing anything. There are some case studies included in this search result list that are disturbing to me, and I really shouldn't get into some of these obscure topics sometimes (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&q=bisphosphonate+mitochondrial+translocase). I end up just finding more and more things I'd almost rather not know about, but I'm probably too idealistic.
That research may help explain the capacity of creatine supplementation to treat people who have mitochondrial disorders, and the mechanisms by which creatine can sometimes be beneficial in those conditions are not well-understood (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&q=creatine+monohydrate+mitochondrial). Not all of those studies have shown benefits, however. But creatine has sometimes been shown to decrease or abolish paracrystalline inclusions and other types of structural abnormalities in skeletal muscle mitochondria (or in the myocytes, more broadly). Even though creatine is clearly not a magic bullet, in any way, in the treatment of mitochondrial disorders, in my opinion, I do think there could be some potential, at low dosages, for creatine to prevent the accumulation of mtDNA deletions by its capacity to increase ADP recycling, via the functional coupling of mitochondrial creatine kinase activity (which is in the intermembrane space and intercristae space, outside the mitochondrial matrix) [Dolder et al., 2003: (http://www.jbc.org/cgi/content/full/M208705200)(http://www.ncbi.nlm.nih.gov/pubmed/12621025)] (it's actually thought to form complexes with ANT proteins) to adenine nucleotide translocase activity [Barbour et al., 1984: (http://www.jbc.org/cgi/reprint/259/13/8246)(http://www.ncbi.nlm.nih.gov/pubmed/6330105?dopt=Abstract); (http://scholar.google.com/scholar?q=%22nucleotide+translocase%22+%22creatine+kinase%22&hl=en&lr=)]. Essentially, ANT exports ATP from the mitochondrial matrix, and the activity of mitochondrial creatine kinase ensures, in the presence of an adequate pool of creatine, that ADP is recycled (it's the kinetics of the coupling reactions and spatial proximity, I think, that allow this to occur and to limit the loss of ADP, by diffusion, from the intermembrane space to the cytosol) back into the mitochondrial matrix. That's an oversimplification, but it conveys the concept. The intramitochondrial ADP pool has repeatedly been shown to be maintained, in a creatine-sensitive manner, somewhat or even largely independently of the cytosolic ADP pool, and the effect of creatine can be especially significant during ischemia, etc. This occurs despite the fact that there is no meaningful physical barrier (nothing like the inner mitochondrial membrane) to the diffusion of ADP from the intermembrane space to the cytosol proper. I guess the diffusion can be restricted to some extent, by the outer mitochondrial membrane, but I think the effect of the membrane is not all that significant.
It's important to note, though, that I think the therapeutic dosage range of creatine is fairly small (maybe 1-3 grams/day), but that's just my opinion. I think it has the potential, at higher dosages (or, conceivably, at any dosage in someone with liver disease, for example) to interfere with the transport of other guanidino compounds, including urea cycle intermediates, but that's just my opinion. I've discussed those issues in past postings.
There's actually more research showing that some bisphosphonates can induce apoptosis by forming either xenobiotic-and nucleotide-containing polyphosphates or can inhibit mevalonate-pathway enzymes and increase the formation of endogenous dinucleotide triphosphates, such as inosine triphosphoadenosine (IpppA), and those dinucleotides or dinucleotide-mimetics can inhibit adenine nucleotide translocase activity and thereby contribute to the pro-apoptotic effects of some bisphosphonates in cultured cells, etc. [Monkkonen et al., 2006: (http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1616989)(http://www.ncbi.nlm.nih.gov/pubmed/16402039)]. This is not always "bad," providing one can effectively target the cells one wants to target. In any case, I have to say that that mechanism doesn't sound very good to me, for many reasons, but that's just my opinion. And a person would obviously want to weigh the risks vs. the benefits in all of these cases and discuss all of those issues with one's doctor before doing anything. There are some case studies included in this search result list that are disturbing to me, and I really shouldn't get into some of these obscure topics sometimes (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&q=bisphosphonate+mitochondrial+translocase). I end up just finding more and more things I'd almost rather not know about, but I'm probably too idealistic.
Tuesday, May 26, 2009
Uridine-Induced Maintenance of Glycogen and Total Adenosine Nucleotide Concentrations During Hypoxia: Apparent Increases In Glucose Uptake, etc.
This article is great [Rosenfeldt et al., 1998: (http://www.ncbi.nlm.nih.gov/pubmed/9794090)], and it's about uridine and not orotic acid. Orotic acid ("orotate") is a precursor of uridine but is generally toxic to the liver, in my opinion (http://scholar.google.com/scholar?q=%22fatty+liver%22+orotate+OR+orotic&hl=en&lr=), and those effects are, paradoxically, the opposite of those of uridine. Uridine has been used to treat fatty liver disease and decreases orotate formation by causing the uridine-nucleotide-mediated inhibition carbamoyl phosphate synthetase II, etc. Rosenfeldt et al. (1998) found that exogenous uridine maintained the glycogen content in the heart, increased the lactate output from the heart, and prevented much of the loss of adenine nucleotides from the heart during hypoxia. There's a typo that shows up in a couple of places, but the authors knew what they were talking about. The article is fantastic. But the concentration of uridine is listed as having been 17 mM, and the authors mean 17 uM (micromolar). The authors refer to the 17 uM concentration in the discussion section, but the mM concentration showed up in the results section. That's the Greek letter "mu," which can be an "m" in fonts other than symbol font, etc.
But the authors' analysis of the metabolic effects of uridine is really terrific. They measured the lactate output and the amount of glycogen formed and estimated that uridine had increased the rate of glucose uptake by about 50 percent but had not increased the minimal level of oxygen uptake that had occurred during the experiment. They discuss similar research and discuss the fact that the uridine-induced stimulation of glycolysis and glucose uptake (the glycogen concentration was almost double the concentration in the hearts subjected to hypoxia in the absence of uridine) is likely to have produced the uridine-induced increase in purine salvage during hypoxia. The total amount of purine nucleotides that was lost during hypoxia was almost cut in half by uridine. 17 uM is not much higher than the normal physiological plasma concentrations of uridine in humans. The major circulating pyrimidine in humans is uridine, but the major circulating pyrimidine in rats is cytidine. That's the reason the baseline plasma uridine concentration is so low in the rats. There's a lot of other research showing that nucleotides can increase glucose uptake and increase lactate output, but the effects are more complex than that.
These effects of uridine on glycogen formation and glucose uptake and even lactate output help to explain its supposed antidepressant and neuroprotective effects in humans and animals. Researchers have used uridine or its prodrugs to treat depression in a number of trials, and it's been used to treat mitochondrial disorders (encephalopathy and cardiomyopathy due to mitochondrial dysfunction, etc.). But the effects of uridine and other nucleotides are generally quite different from something like AICAR, even though other nucleotides, such as adenosine, have sometimes been shown to enhance AMPK activation (phosphorylation) and activity (by their effects of maintaining the total adenine nucleotide pool or increasing the intracellular AMP concentration more than the intracellular concentrations of other adenine nucleotides), much as AICAR activates AMPK [AMPK activation occurs when specific residues on it are phosphorylated, and the activity of AMPK is its phosphorylation of its target proteins, such as phosphofructokinase, etc.] ([Jaswal et al., 2007: (http://0-ajpheart.physiology.org.library.pcc.edu/cgi/reprint/292/4/H1978)(http://www.ncbi.nlm.nih.gov/pubmed/17172269)]. The concentration of adenosine used in that experiment was 500 uM, however, and that's a supraphysiological concentration. It's by no means a toxic concentration, because adenosine exerts various trophic effects on endothelial cells up to 1000 uM. But the extracellular adenosine concentrations don't usually exceed about 100 uM. The effects of nucleotides on AMPK activation and and activity are likely to depend on the concentrations used, and it's also important to consider the effects of nucleotides on the phosphocreatine to creatine (PCr/Cr) ratio. Many other articles show that uridine, alone or in combination with exogenous purine nucleotides, increases the PCr/Cr ratio. That effect would tend to produce allosteric inhibition of AMPK activity, etc. The main issue I have with AMPK activators is not that AMPK activation per se is "bad." In fact, the inhibition of AMPK activity or activation by specific, drug inhibitors produces toxic effects during ischemia. It's fairly clear that AMPK activation plays a role in maintaining glycolytic activity during hypoxia or ischemia. But my problem is with this assumption that "more" AMPK activation and activity is always going to be "better," and it's evident, in my opinion, that this is not always (or even usually) going to be the case, especially in the long term. This is a great article that discusses some of these issues with research on AMPK in the context of ischemia and elevated contractile activity in the heart [Dyck and Lopaschuk, 2006: (http://jp.physoc.org/content/574/1/95.full.pdf+html)(http://www.ncbi.nlm.nih.gov/pubmed/16690706?dopt=Abstract)].
But the authors' analysis of the metabolic effects of uridine is really terrific. They measured the lactate output and the amount of glycogen formed and estimated that uridine had increased the rate of glucose uptake by about 50 percent but had not increased the minimal level of oxygen uptake that had occurred during the experiment. They discuss similar research and discuss the fact that the uridine-induced stimulation of glycolysis and glucose uptake (the glycogen concentration was almost double the concentration in the hearts subjected to hypoxia in the absence of uridine) is likely to have produced the uridine-induced increase in purine salvage during hypoxia. The total amount of purine nucleotides that was lost during hypoxia was almost cut in half by uridine. 17 uM is not much higher than the normal physiological plasma concentrations of uridine in humans. The major circulating pyrimidine in humans is uridine, but the major circulating pyrimidine in rats is cytidine. That's the reason the baseline plasma uridine concentration is so low in the rats. There's a lot of other research showing that nucleotides can increase glucose uptake and increase lactate output, but the effects are more complex than that.
These effects of uridine on glycogen formation and glucose uptake and even lactate output help to explain its supposed antidepressant and neuroprotective effects in humans and animals. Researchers have used uridine or its prodrugs to treat depression in a number of trials, and it's been used to treat mitochondrial disorders (encephalopathy and cardiomyopathy due to mitochondrial dysfunction, etc.). But the effects of uridine and other nucleotides are generally quite different from something like AICAR, even though other nucleotides, such as adenosine, have sometimes been shown to enhance AMPK activation (phosphorylation) and activity (by their effects of maintaining the total adenine nucleotide pool or increasing the intracellular AMP concentration more than the intracellular concentrations of other adenine nucleotides), much as AICAR activates AMPK [AMPK activation occurs when specific residues on it are phosphorylated, and the activity of AMPK is its phosphorylation of its target proteins, such as phosphofructokinase, etc.] ([Jaswal et al., 2007: (http://0-ajpheart.physiology.org.library.pcc.edu/cgi/reprint/292/4/H1978)(http://www.ncbi.nlm.nih.gov/pubmed/17172269)]. The concentration of adenosine used in that experiment was 500 uM, however, and that's a supraphysiological concentration. It's by no means a toxic concentration, because adenosine exerts various trophic effects on endothelial cells up to 1000 uM. But the extracellular adenosine concentrations don't usually exceed about 100 uM. The effects of nucleotides on AMPK activation and and activity are likely to depend on the concentrations used, and it's also important to consider the effects of nucleotides on the phosphocreatine to creatine (PCr/Cr) ratio. Many other articles show that uridine, alone or in combination with exogenous purine nucleotides, increases the PCr/Cr ratio. That effect would tend to produce allosteric inhibition of AMPK activity, etc. The main issue I have with AMPK activators is not that AMPK activation per se is "bad." In fact, the inhibition of AMPK activity or activation by specific, drug inhibitors produces toxic effects during ischemia. It's fairly clear that AMPK activation plays a role in maintaining glycolytic activity during hypoxia or ischemia. But my problem is with this assumption that "more" AMPK activation and activity is always going to be "better," and it's evident, in my opinion, that this is not always (or even usually) going to be the case, especially in the long term. This is a great article that discusses some of these issues with research on AMPK in the context of ischemia and elevated contractile activity in the heart [Dyck and Lopaschuk, 2006: (http://jp.physoc.org/content/574/1/95.full.pdf+html)(http://www.ncbi.nlm.nih.gov/pubmed/16690706?dopt=Abstract)].
Monday, May 18, 2009
Allosteric Inhibition of AMPK by Phosphocreatine; Inhibition of Creatine Kinase Activity by AMPK
This article is interesting [Ponticos et al., 1998: (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1170516&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/9501090)], and the authors show that the activity of the purified, muscle-specific isoform of creatine kinase is inhibited significantly upon its phosphorylation by AMPK. The other main findings are that creatine kinase binds to (i.e. heterodimerizes or forms heterooligomers, given that the stoichiometry of the interaction is unknown) AMPK, that phosphocreatine inhibits AMPK activity, that creatine reverses that inhibition, and that the inhibition by phosphocreatine becomes more pronounced as the pH increases, within physiological pH ranges. Assuming that these effects are not going to be attenuated in situ (in a living organism, "in place"), these are really important findings and provide more evidence (fairly devastating evidence) that pharmacological AMPK activation, such as by AICAR, is really not a good idea, in my opinion. The inhibition of creatine kinase activity, by 60 percent, upon its phosphorylation by AMPK, is not likely to be a desirable effect, under many conditions, in my opinion. The effects of AICAR on the liver and on metabolism in general can be very bad. That's just my opinion, and I've discussed this in past postings. AMPK activation increases glycolytic activity, and that's invariably going to tend to intensify the acidosis, especially under the conditions of diminished respiratory chain activity that are likely to exist in the cells of people who are supposedly going to benefit from AICAR. So how is AMPK activation really going to cause a "cycling" back up toward a high phosphocreatine/creatine ratio (as discussed in the article)? It might, in the context of something like recovery from high-intensity exercise, in which the profligate supply of lactate, free fatty acids, ketones, and other energy substrates will tend to restore the ATP/AMP ratio and preclude the development of the kind of mitochondrial pathology that tends to result, in my opinion, from chronic, pharmacologically-mediated activation of AMPK. But the ends don't always justify the means, particularly when it comes to the regulation of glucose transport by AMPK. Increasing glucose transport at any cost, such as by pharmacologically-mediated AMPK activation, is a legitimate goal, but it's not possible, in my opinion, to address and ameliorate profound pathologies in energy metabolism by only pharmacological means. It tends to be necessary, in my opinion, to make some attempt to restore the actual metabolic activity, instead of only using pharmacological stopgap measures. It should tell people something that insulin generally decreases AMPK activity [Hue et al., 2003: (http://www.biochemsoctrans.org/bst/031/0213/bst0310213.htm); countless other articles]. Here's an article that shows that a decrease in AMPK activation during experimental ischemia (in a model of ischemic stroke) in mice lessened the damage due to ischemia. The article also shows that AICAR worsened the damage [McCullough et al., 2005: (http://www.jbc.org/cgi/content/full/280/21/20493)(http://www.ncbi.nlm.nih.gov/pubmed/15772080?dopt=Abstract)]. That's only under acute conditions, too. All of the responses that are supposed to neatly occur in the longer term, following AMPK activation, also just don't seem to show up, in my opinion, as reliably as one might expect them to show up.
Monday, April 20, 2009
Increase in Nucleotide Absorption and Retention During the Fasted State: Bioavailability Issues and Potential Problems With Enteric-Coated Tablets
These articles [Gross et al., 1988: (http://www.ncbi.nlm.nih.gov/pubmed/3390463); Gross and Savaiano, 1991: (http://www.ncbi.nlm.nih.gov/pubmed/2009279)] are really good and show that the retention, by the intestinal tissues themselves, of intrajejunally-administered nucleosides or nucleic acids can be roughly twice as great in the fasted state as in the "fed" state. Those articles are potentially confusing, because, normally, the retention of nucleosides or nucleotides in the intestines (i.e. the salvage of nucleosides and incorporation into nucleotide pools in the epithelial or smooth muscle cells, etc.) would be less-than desirable, from the standpoint of bioavailability. But what those authors are saying is that xanthine oxidase activity is lower in the fasted state. Those articles tell me that administering purines, in particular, during the fasted state, as discussed below and in past postings [see Carver and Walker, 1995, cited and discussed here: (http://hardcorephysiologyfun.blogspot.com/2009/03/adenosine-and-guanosine-in-animal.html); see here, also: (http://hardcorephysiologyfun.blogspot.com/2009/01/details-on-nucleotides-bioavailability.html)], is likely to produce both greater bioavailability, as discussed in those past postings, and greater salvage, by the target cells that the nucleotides or their metabolites enter, of those purines. That's just my opinion. I didn't know the effect had been shown to be that large. Ho et al. (1979) [Ho et al., 1979: (http://jn.nutrition.org/cgi/reprint/109/8/1377.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/458492)] found even larger increases in the absorption of nucleosides or nucleic acids in the fasted state, although those large, relative increases in tissue content, in the fasted state vs. the fed state, appear to be partially or largely due to increases in the absorption of the nucleotides or nucleic acids (purine bases).
It's important to note that the bioavailability is also likely, in my opinion, to be enhanced in the fasted state. The bioavailability is partly a function of how rapidly a physiological compound, such as a nucleotide, enters solution in the intraluminal fluid. I forget where the reference is, but the intraluminal fluid volume in the stomach or along a segment of the small intestine can be remarkably small and can be something like 15-20 mL. The general concept is that many physiological compounds (nucleotides in particular) can be transported into and metabolized by any cell they come in contact with. If a person takes something like guanosine or adenosine, as a free nucleoside, the low solubilities will, in my opinion, significantly limit the bioavailability of those nucleosides by slowing the rate of dissolution in the GI tract. The undissolved nucleosides will slowly enter solution, as the fraction that is dissolved is transported into cells or has diffused away, by passive diffusion. Savaiano et al. (1980) [Savaiano et al., 1980: (http://jn.nutrition.org/cgi/reprint/110/9/1793.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/7411237)] found evidence suggestive of extreme differences in the bioavailabilities of nucleic acids and nucleosides, such that the intravenous (i.v.) administration produced levels of tissue retention that were 3-59 times the levels produced by oral administration. Usually, the ratio of the i.v. to oral bioavailability of a drug, expressed as the ratio of the areas under the serum concentration vs. time curves [AUC(i.v.)/AUC(oral)], is maybe between 2 and 5 or 7 or something like that. Those differences found by Savaiano et al. (1980) are not especially relevant for human dosing, however, because the solubilities of both nucleic acids and nucleosides are drastically lower than the solubilities of the disodium salts of guanosine monophosphate (GMP and adenosine monophosphate (AMP) or triphosphate (ATP), for example [or the disodium salt of inosine monophosphate (IMP)]. Those solubility differences could essentially mean that most of the nucleic acids or nucleosides would be degraded to uric acid, in humans, or to uric acid and then allantoin, in animals, before they could even enter the portal circulation, etc, in my opinion. Other salts of AMP or GMP or ATP also display deficient solubilities, and those solubility data are freely available on countless sites on the internet. Many of the researchers who have used oral guanosine or GMP as anticonvulsants, in animal experiments, have discussed those solubility issues. The authors of many of those older articles were evidently not aware of those issues, however, in my opinion, and they're very important issues.
Another major problem with oral purine dosing is the use of enteric coatings, and I've discussed this in detail previously (http://hardcorephysiologyfun.blogspot.com/2009/01/details-on-nucleotides-bioavailability.html). Most enteric coatings would be expected to severely and unnecessarily reduce the bioavailabilities of orally-administered nucleotides or nucleosides, in my opinion. Many enteric coated tablets could potentially not dissolve in the GI tract, in my opinion, because the pH in many people would not be expected to be high enough to allow the coatings to dissolve, as discussed by Fallingborg et al. (1999), cited below. Additionally, the use of tablets could be expected to produce the same, drastic slowing of entry into solution that a low level of solubility would be expected to produce, in my opinion. Persky et al. (2003) [Persky et al., 2003: (http://www.pharmacy.unc.edu/pkpd/AMP%20Articles/Persky%20et%20al%20Clin%20Pharmacok%202003.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/12793840)] discussed the fact that the rate of dissolution of physiological substrates, such as creatine, can be an important pharmacokinetic variable to consider, and these types of pharmacokinetic considerations are potentially more important for maximizing the bioavailabilities of nucleotides or other physiological compounds, in my opinion, than those considerations are for maximizing the bioavailabilities of drugs. Many drugs cannot be extensively or even partially metabolized by every cell in the body. With physiological substrates, time (i.e. pharmacokinetics) is of the essence, so to speak, because there is both the rate of uptake, by endothelial cells or cells in the liver, and the rates of degradation by every cell the substrates are available to. Even in the case of creatine, Deldicque et al. (2008) [Deldicque et al., 2008: (http://www.ncbi.nlm.nih.gov/pubmed/17851680)], discussed here (http://hardcorephysiologyfun.blogspot.com/2009/03/adenosine-and-guanosine-in-animal.html), found that the Cmax of plasma creatine, a reflection of an "improvement" in the kinetics of absorption or entry into the systemic circulation, etc., was higher in response to the administration of creatine monohydrate in a water solution (i.e. pre-dissolved) than in response to its administration in foods, which slow the rate of entry of creatine monohydrate into solution. A lower Cmax could be expected, in my opinion, to decrease the fraction of nucleotides, for example, that would gain entry into the brain and be salvaged, as opposed to being degraded into uric acid, by cells in the brain.
Some of the confusion surrounding these issues may be the result of some lingering misconceptions that many people, even researchers, evidently are holding onto. The fasted state in a human means any time 12 or more hours after the previous meal, although I've seen the 10-hour time point used as a marker for the beginning of the "fasted" state. So this means the only true fasted state is likely to be the time period in the morning, before breakfast. Why is this the case? Fallingborg (1999) [Fallingborg et al., 1999: (http://www.ncbi.nlm.nih.gov/pubmed/10421978)] discussed large numbers of studies on the time course with which food moves out of the stomach and into the duodenum and jejunum and so on, and much of the data that Fallingborg (1999) discussed had been collected from experiments using different types of devices that transmit data on the pH and other variables in the gastrointestinal (GI) tract. Some of those are pill-sized devices that have tiny video cameras in them, but I'm not sure those were in use in 1999. After a person eats the first meal of the day, that food may, depending on the sizes and frequencies of the subsequent meals (meaning any food that is eaten), remain in the stomach for between ~2.6 and 14.5 hours (Fallingborg et al., 1999). The gastric residence time (GRT) of tiny, mechanized capsules, with pH sensors in them (the pH is a measure of acidity, such that pH values below or above 7 are "acidic" or "basic"/"alkaline," respectively) is between 1.1 and 1.9 hours in the fasted state, but the GRT for the same capsule can be *up to 14.5 hours* in a person who takes the capsule at breakfast and eats every couple of hours during the rest of the day. Fallingborg (1999) discusses the fact that a single, small meal, eaten in the morning, has been shown to only increase the GRT of the capsule to ~2.6 hours. Fallingborg (1999) discusses the fact that, in the fasted state, the interdigestive migrating myoelectric complex (IMMC), which is phase III of a series of cyclic, contractile events in the smooth muscle that lines the stomach, allows solid food to exit the stomach about every 2 hours. When a person eats a single meal or, in particular, meals every 2-3 hours, the cyclic or "phasic" aspects of these contractions are abolished or "frozen", and food may not exit the stomach for many hours (up to 14 or 14.5). This is very important for understanding the major problems that exist, in my opinion, with enteric coatings for many preparations of (i.e. products containing) SAM-e or ATP disodium, etc., as discussed previously (http://hardcorephysiologyfun.blogspot.com/2009/01/details-on-nucleotides-bioavailability.html).
This residence time applies to solid substances that cannot enter solution and diffuse into the small intestine. If a person eats a water-soluble nutrient or sugar or amino acid or nucleotide, those substances can exit the stomach in aqueous (water) solution very rapidly. But even the slowing effect that is produced by the food in the stomach can limit the bioavailability of a water-soluble substance, such as creatine, in the "fed" state. So the stomach may not be completely empty until ~4 am or even later, if a person eats his or her last meal at 6 pm or something. The paper by Fallingborg (1999) is superb, and the author cites 183 papers and goes into exhaustive detail on all of these considerations.
A major point that Fallingborg (1999) makes is that statements about taking enteric-coated (acid-resistant) tablets "between meals" or "on an empty stomach, between meals" make no sense, because the stomach does not empty between meals. When a person is told to take some of these enteric-coated SAM-e or ATP disodium tablets (or other enteric-coated tablets) "between meals, on an empty stomach," the tablet may not exit the stomach and have any hope of releasing its contents until up to 14.5 hours after the person has taken it and eaten many subsequent meals. More importantly, Fallingborg (1999) discusses research showing that the mean pH in the duodenum of humans is ~6.22. The duodenal pH has been shown to range from 5.66 to 6.4 in other articles (Fallingborg, 1999). In the jejunum, the upper part of the true small intestine proper, is about 4.92 in the fasted state (a median value) and 6.08 after a meal. The pH is thought to only increase to above 7, to 7.4-7.6 (Fallingborg, 1999), in the distal ileum, which is an almost shocking fact that helps to explain the many problems, such as intestinal strictures and so on, with enteric-coated tablets that have been reported in the literature. The pH in the proximal small intestine, therefore, ranges from ~6.08 to ~6.49 (Fallingborg, 1999), when one looks at the data from multiple articles. But many of the enteric coatings do not dissolve until the pH is some amount greater than 7, and yet the jejunum is the site at which enteric-coated tablets are supposed to dissolve. Enteric coatings are polymeric substances, generally, whose solubility is pH-dependent. That means they can't dissolve in fluids that display pH values below some critical range of numbers, and the lower limit of the range may be 7.5 or 8 or some other value and may depend on the particular formulation used by the manufacturer.
Those data on the pH of the intraluminal fluid mean that the dissolution of enteric coatings could be very problematic, in my opinion. One explanation for the misconceptions about the pH in the intraluminal fluid might be that the pH of bile is ~8.03 (Fallingborg, 1999), and maybe people have thought that the pH of bile will be equivalent to the pH of the intraluminal fluid. It's just not the case. There can be a tendency to rely on 30- and 40-year-old data or research in some of these areas, and that tendency can become problematic, in my opinion. I should mention that, in many disease states, such as in people with liver disease, the jejunal pH can be substantially lower than those median or mean values, measured in apparently healthy people and can decrease progressively throughout the day. The pH-sensitivities of something like an enteric coating should obviously, in my opinion, be engineered so as to allow dissolution at the lower range of intraluminal pH values for anyone. This would not be difficult to do, but it's not something that many manufacturers or other people seem to be aware of the need for (if enteric coatings are still going to be used). Here are some articles reporting gastric or intestinal injuries (i.e. obstruction of the pyloric sphincter or intestinal obstructions/strictures) from poorly-formulated enteric-coated tablets (this poor formulation extends to more or less all enteric-coated tablets, in my opinion, when one looks at the data on the pH-dependences of the polymers used in the coatings) [Harris, 1973: (http://www.ncbi.nlm.nih.gov/pubmed/4764749); Sogge et al., 1977: (http://www.ncbi.nlm.nih.gov/pubmed/22308); Davies, 1999: (http://www.ualberta.ca/~csps/JPPS2(1)/N.Davies/NSAID.htm)(http://www.ncbi.nlm.nih.gov/pubmed/10951657); Sherry, 1979: (http://www.ncbi.nlm.nih.gov/pubmed/287936); (http://scholar.google.com/scholar?num=50&hl=en&lr=&safe=off&q=%22enteric+coated%22+stricture+OR+obstruction)]. Obviously, non-enteric-coated aspirin could cause damage to the stomach or small intestine for other reasons, and one should always talk to one's doctor before making any change in any medication. The benefits of enteric-coated preparations may outweigh any potential problems with the preparations, for many people in many specific disease states. But my point is to show the many problems that exist with the approach, in a functional sense, and with many of the individual preparations, in my opinion.
When researchers refer to a "pyloric obstruction" from an enteric-coated aspirin tablet, the researchers mean that the tablet become "stuck" in the valve-like muscle that opens, periodically, to allow food to pass from the stomach into the duodenum. In some cases, minor or not-so-minor surgical procedures are required to remove these obstructions from the undissolved tablets.
It's important to note that the bioavailability is also likely, in my opinion, to be enhanced in the fasted state. The bioavailability is partly a function of how rapidly a physiological compound, such as a nucleotide, enters solution in the intraluminal fluid. I forget where the reference is, but the intraluminal fluid volume in the stomach or along a segment of the small intestine can be remarkably small and can be something like 15-20 mL. The general concept is that many physiological compounds (nucleotides in particular) can be transported into and metabolized by any cell they come in contact with. If a person takes something like guanosine or adenosine, as a free nucleoside, the low solubilities will, in my opinion, significantly limit the bioavailability of those nucleosides by slowing the rate of dissolution in the GI tract. The undissolved nucleosides will slowly enter solution, as the fraction that is dissolved is transported into cells or has diffused away, by passive diffusion. Savaiano et al. (1980) [Savaiano et al., 1980: (http://jn.nutrition.org/cgi/reprint/110/9/1793.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/7411237)] found evidence suggestive of extreme differences in the bioavailabilities of nucleic acids and nucleosides, such that the intravenous (i.v.) administration produced levels of tissue retention that were 3-59 times the levels produced by oral administration. Usually, the ratio of the i.v. to oral bioavailability of a drug, expressed as the ratio of the areas under the serum concentration vs. time curves [AUC(i.v.)/AUC(oral)], is maybe between 2 and 5 or 7 or something like that. Those differences found by Savaiano et al. (1980) are not especially relevant for human dosing, however, because the solubilities of both nucleic acids and nucleosides are drastically lower than the solubilities of the disodium salts of guanosine monophosphate (GMP and adenosine monophosphate (AMP) or triphosphate (ATP), for example [or the disodium salt of inosine monophosphate (IMP)]. Those solubility differences could essentially mean that most of the nucleic acids or nucleosides would be degraded to uric acid, in humans, or to uric acid and then allantoin, in animals, before they could even enter the portal circulation, etc, in my opinion. Other salts of AMP or GMP or ATP also display deficient solubilities, and those solubility data are freely available on countless sites on the internet. Many of the researchers who have used oral guanosine or GMP as anticonvulsants, in animal experiments, have discussed those solubility issues. The authors of many of those older articles were evidently not aware of those issues, however, in my opinion, and they're very important issues.
Another major problem with oral purine dosing is the use of enteric coatings, and I've discussed this in detail previously (http://hardcorephysiologyfun.blogspot.com/2009/01/details-on-nucleotides-bioavailability.html). Most enteric coatings would be expected to severely and unnecessarily reduce the bioavailabilities of orally-administered nucleotides or nucleosides, in my opinion. Many enteric coated tablets could potentially not dissolve in the GI tract, in my opinion, because the pH in many people would not be expected to be high enough to allow the coatings to dissolve, as discussed by Fallingborg et al. (1999), cited below. Additionally, the use of tablets could be expected to produce the same, drastic slowing of entry into solution that a low level of solubility would be expected to produce, in my opinion. Persky et al. (2003) [Persky et al., 2003: (http://www.pharmacy.unc.edu/pkpd/AMP%20Articles/Persky%20et%20al%20Clin%20Pharmacok%202003.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/12793840)] discussed the fact that the rate of dissolution of physiological substrates, such as creatine, can be an important pharmacokinetic variable to consider, and these types of pharmacokinetic considerations are potentially more important for maximizing the bioavailabilities of nucleotides or other physiological compounds, in my opinion, than those considerations are for maximizing the bioavailabilities of drugs. Many drugs cannot be extensively or even partially metabolized by every cell in the body. With physiological substrates, time (i.e. pharmacokinetics) is of the essence, so to speak, because there is both the rate of uptake, by endothelial cells or cells in the liver, and the rates of degradation by every cell the substrates are available to. Even in the case of creatine, Deldicque et al. (2008) [Deldicque et al., 2008: (http://www.ncbi.nlm.nih.gov/pubmed/17851680)], discussed here (http://hardcorephysiologyfun.blogspot.com/2009/03/adenosine-and-guanosine-in-animal.html), found that the Cmax of plasma creatine, a reflection of an "improvement" in the kinetics of absorption or entry into the systemic circulation, etc., was higher in response to the administration of creatine monohydrate in a water solution (i.e. pre-dissolved) than in response to its administration in foods, which slow the rate of entry of creatine monohydrate into solution. A lower Cmax could be expected, in my opinion, to decrease the fraction of nucleotides, for example, that would gain entry into the brain and be salvaged, as opposed to being degraded into uric acid, by cells in the brain.
Some of the confusion surrounding these issues may be the result of some lingering misconceptions that many people, even researchers, evidently are holding onto. The fasted state in a human means any time 12 or more hours after the previous meal, although I've seen the 10-hour time point used as a marker for the beginning of the "fasted" state. So this means the only true fasted state is likely to be the time period in the morning, before breakfast. Why is this the case? Fallingborg (1999) [Fallingborg et al., 1999: (http://www.ncbi.nlm.nih.gov/pubmed/10421978)] discussed large numbers of studies on the time course with which food moves out of the stomach and into the duodenum and jejunum and so on, and much of the data that Fallingborg (1999) discussed had been collected from experiments using different types of devices that transmit data on the pH and other variables in the gastrointestinal (GI) tract. Some of those are pill-sized devices that have tiny video cameras in them, but I'm not sure those were in use in 1999. After a person eats the first meal of the day, that food may, depending on the sizes and frequencies of the subsequent meals (meaning any food that is eaten), remain in the stomach for between ~2.6 and 14.5 hours (Fallingborg et al., 1999). The gastric residence time (GRT) of tiny, mechanized capsules, with pH sensors in them (the pH is a measure of acidity, such that pH values below or above 7 are "acidic" or "basic"/"alkaline," respectively) is between 1.1 and 1.9 hours in the fasted state, but the GRT for the same capsule can be *up to 14.5 hours* in a person who takes the capsule at breakfast and eats every couple of hours during the rest of the day. Fallingborg (1999) discusses the fact that a single, small meal, eaten in the morning, has been shown to only increase the GRT of the capsule to ~2.6 hours. Fallingborg (1999) discusses the fact that, in the fasted state, the interdigestive migrating myoelectric complex (IMMC), which is phase III of a series of cyclic, contractile events in the smooth muscle that lines the stomach, allows solid food to exit the stomach about every 2 hours. When a person eats a single meal or, in particular, meals every 2-3 hours, the cyclic or "phasic" aspects of these contractions are abolished or "frozen", and food may not exit the stomach for many hours (up to 14 or 14.5). This is very important for understanding the major problems that exist, in my opinion, with enteric coatings for many preparations of (i.e. products containing) SAM-e or ATP disodium, etc., as discussed previously (http://hardcorephysiologyfun.blogspot.com/2009/01/details-on-nucleotides-bioavailability.html).
This residence time applies to solid substances that cannot enter solution and diffuse into the small intestine. If a person eats a water-soluble nutrient or sugar or amino acid or nucleotide, those substances can exit the stomach in aqueous (water) solution very rapidly. But even the slowing effect that is produced by the food in the stomach can limit the bioavailability of a water-soluble substance, such as creatine, in the "fed" state. So the stomach may not be completely empty until ~4 am or even later, if a person eats his or her last meal at 6 pm or something. The paper by Fallingborg (1999) is superb, and the author cites 183 papers and goes into exhaustive detail on all of these considerations.
A major point that Fallingborg (1999) makes is that statements about taking enteric-coated (acid-resistant) tablets "between meals" or "on an empty stomach, between meals" make no sense, because the stomach does not empty between meals. When a person is told to take some of these enteric-coated SAM-e or ATP disodium tablets (or other enteric-coated tablets) "between meals, on an empty stomach," the tablet may not exit the stomach and have any hope of releasing its contents until up to 14.5 hours after the person has taken it and eaten many subsequent meals. More importantly, Fallingborg (1999) discusses research showing that the mean pH in the duodenum of humans is ~6.22. The duodenal pH has been shown to range from 5.66 to 6.4 in other articles (Fallingborg, 1999). In the jejunum, the upper part of the true small intestine proper, is about 4.92 in the fasted state (a median value) and 6.08 after a meal. The pH is thought to only increase to above 7, to 7.4-7.6 (Fallingborg, 1999), in the distal ileum, which is an almost shocking fact that helps to explain the many problems, such as intestinal strictures and so on, with enteric-coated tablets that have been reported in the literature. The pH in the proximal small intestine, therefore, ranges from ~6.08 to ~6.49 (Fallingborg, 1999), when one looks at the data from multiple articles. But many of the enteric coatings do not dissolve until the pH is some amount greater than 7, and yet the jejunum is the site at which enteric-coated tablets are supposed to dissolve. Enteric coatings are polymeric substances, generally, whose solubility is pH-dependent. That means they can't dissolve in fluids that display pH values below some critical range of numbers, and the lower limit of the range may be 7.5 or 8 or some other value and may depend on the particular formulation used by the manufacturer.
Those data on the pH of the intraluminal fluid mean that the dissolution of enteric coatings could be very problematic, in my opinion. One explanation for the misconceptions about the pH in the intraluminal fluid might be that the pH of bile is ~8.03 (Fallingborg, 1999), and maybe people have thought that the pH of bile will be equivalent to the pH of the intraluminal fluid. It's just not the case. There can be a tendency to rely on 30- and 40-year-old data or research in some of these areas, and that tendency can become problematic, in my opinion. I should mention that, in many disease states, such as in people with liver disease, the jejunal pH can be substantially lower than those median or mean values, measured in apparently healthy people and can decrease progressively throughout the day. The pH-sensitivities of something like an enteric coating should obviously, in my opinion, be engineered so as to allow dissolution at the lower range of intraluminal pH values for anyone. This would not be difficult to do, but it's not something that many manufacturers or other people seem to be aware of the need for (if enteric coatings are still going to be used). Here are some articles reporting gastric or intestinal injuries (i.e. obstruction of the pyloric sphincter or intestinal obstructions/strictures) from poorly-formulated enteric-coated tablets (this poor formulation extends to more or less all enteric-coated tablets, in my opinion, when one looks at the data on the pH-dependences of the polymers used in the coatings) [Harris, 1973: (http://www.ncbi.nlm.nih.gov/pubmed/4764749); Sogge et al., 1977: (http://www.ncbi.nlm.nih.gov/pubmed/22308); Davies, 1999: (http://www.ualberta.ca/~csps/JPPS2(1)/N.Davies/NSAID.htm)(http://www.ncbi.nlm.nih.gov/pubmed/10951657); Sherry, 1979: (http://www.ncbi.nlm.nih.gov/pubmed/287936); (http://scholar.google.com/scholar?num=50&hl=en&lr=&safe=off&q=%22enteric+coated%22+stricture+OR+obstruction)]. Obviously, non-enteric-coated aspirin could cause damage to the stomach or small intestine for other reasons, and one should always talk to one's doctor before making any change in any medication. The benefits of enteric-coated preparations may outweigh any potential problems with the preparations, for many people in many specific disease states. But my point is to show the many problems that exist with the approach, in a functional sense, and with many of the individual preparations, in my opinion.
When researchers refer to a "pyloric obstruction" from an enteric-coated aspirin tablet, the researchers mean that the tablet become "stuck" in the valve-like muscle that opens, periodically, to allow food to pass from the stomach into the duodenum. In some cases, minor or not-so-minor surgical procedures are required to remove these obstructions from the undissolved tablets.
Wednesday, April 15, 2009
Elevation of Plasma Hypoxanthine During and After Glycogen-Depleting Exercise: Relevance to Energy Metabolism and Nucleotide Turnover in the Brain
This is a really interesting article [Sahlin et al., 1999: (http://www.ncbi.nlm.nih.gov/pubmed/10502075)], and the authors found that plasma hypoxanthine increased by a mean of 7.78-fold during exercise in humans. There was some variation in the magnitudes of the increases in plasma hypoxanthine among individuals. The increases in plasma uric acid (urate) and xanthine, which is derived from the metabolism of hypoxanthine by xanthine oxidoreductase (xanthine oxidase is technically a modified form of xanthine reductase that's been modified by proteolytic cleavage, etc., but people typically refer to the enzyme activity as being "xanthine oxidase" activity), were larger in terms of the amounts of those purines formed, but the percent increases were lower (mean increases of 5 percent for urate and 223%, or a 2.23-fold increase, for xanthine). The effect of that increase in plasma hypoxanthine on the brain should not be underestimated. In a past posting, I discussed some of the research showing neuroprotective effects of remarkably low doses of hypoxanthine [Mink and Johnston, 2007, cited and discussed here: (http://hardcorephysiologyfun.blogspot.com/2009/03/protection-against-postischemic-damage.html)]. There's still the fact that the exercise-induced increases in neuronal activity in the brain would be expected to substantially, albeit transiently, increase the rates of purine export from neurons. I can't immediately find any articles showing elevated cerebrospinal fluid hypoxanthine and xanthine and urate levels, following exercise, but it's very likely that those elevations would occur, in my opinion, especially following high-intensity exercise. So that supposed depleting effect of exercise on neuronal and astrocyte purine nucleotide levels would be expected to lessen the impact of an increase in the plasma hypoxanthine level. I would think that, as a person's muscle mass increased over time, the muscles' capacity to export hypoxanthine would be increased and would produce more significant effects on the brain. But the person would, in my opinion, keep having to push the limits and produce glycogen depletion, as discussed below. Other researchers have discussed the effects of muscle-derived purines on the brain. There's one article from 1978 or 1979 that discusses the potential effects of exercise-induced elevations in extracellular ATP, released from endothelial cells, on the brain, but the authors focused too much on ATP per se. Extracellular ATP is rapidly degraded to ADP and then adenosine and hypoxanthine, etc., and so one would expect to see much more of an effect of exercise on adenosine or hypoxanthine than on ATP. I mean that the purines that might be expected to enter the brain in significant amounts, during exercise, and to also exert meaningful effects on nucleotide pools in neurons or astrocytes would be hypoxanthine or, conceivably, adenosine and not ATP, in my opinion. Xanthine is not salvaged efficiently (but can, in fact, be salvaged in small amounts to xanthosine and then guanosine), but hypoxanthine and adenosine are salvaged relatively efficiently by cells in the brain.
Sahlin et al. (1999) discuss the fact that researchers have generally found glycogen depletion from the skeletal muscles to be a prerequisite for the most pronounced, exercise-induced increases in plasma purines, including hypoxanthine. True glycogen depletion from a muscle group generally requires exhaustive exercise, and the research has generally shown, in my opinion, that resistance exercise produces more-pronounced degrees of glycogen depletion and purine depletion from the muscles [the general idea is that ATP depletion causes a loss of the capacity to salvage adenosine (and also guanosine), and this causes inosine monophosphate to accumulate and be converted into hypoxanthine and xanthine in the muscle cells] than exercise at low intensity does. Hellsten et al. (1998) [Hellsten et al., 1998: (http://ajpendo.physiology.org/cgi/content/full/274/4/E600)(http://www.ncbi.nlm.nih.gov/pubmed/9575819)] discuss research showing that a high-intensity exercise program causes a 20 percent decrease in the total adenine nucleotide contents of skeletal muscles (when people are not exercising, meaning post-exercise and all the time). That's a remarkable fact and suggests to me that some benefit might be derived from low-dose adenosine or guanosine supplementation, particularly early in an exercise program. But that's my opinion. The effects of hypoxanthine on the brain should not be underestimated, and a single exercise session that produces an 8-fold elevation in plasma hypoxanthine levels, during the hour or few hours following exercise, could have a significant effect on the pools of adenine nucleotides, in particular, in the brain. Mink and Johnston (2007), cited above, discuss the fact that hypoxanthine appeared to be salvaged to a large extent, even during ischemia. The brain has an extremely low capacity to make purines de novo and depends almost entirely on purines exported from either the endothelial cells lining the cerebral blood vessels or from the blood.
Of course, creatine and glutamine have been shown to augment the salvage of purine or pyrimidine nucleotides in various articles [cited and discussed here or in other postings: (http://hardcorephysiologyfun.blogspot.com/2009/02/interactions-of-glutamine-and-arginine.html)], but any supposed improvement in purine or pyrimidine salvage, in the brain, that might occur in response to the administration of those types of supplements might be offset by the increase in, for example, purine nucleotide export that could accompany a creatine- or glutamine-induced increase in exercise intensity. I don't think that's the way it would work, though, as long as the doses of creatine or glutamine are kept low, but that's just my opinion. For example, the combination of glutamine and inosine, which is hypoxanthine riboside (hypoxanthine attached to ribose to make a nucleotide), [Hodges and Snyder, 2004, cited here: (http://hardcorephysiologyfun.blogspot.com/2009/02/contribution-of-glutamine-to-pool-of.html)]. That's just my opinion. In my opinion, low doses of glutamine, creatine and adenosine and guanosine might produce some sort of buffering effect on brain energy metabolism and purine nucleotide pools. Creatine increases or "stimulates" oxidative metabolism, in part by maintaining the intramitochondrial ADP pool, and this would be expected to increase the extent to which glutamine, upon its metabolism into glutamate, can be metabolized into alpha-ketoglutarate and undergo oxidative metabolism in neurons or astrocytes in the brain. People discuss creatine as if it participates only in anaerobic metabolism, but this is just not the case. Creatine can prolong "aerobic" exercise and is known to increase oxidative glucose utilization in cells, etc.
Similarly, cytidine was recently shown to decrease the glutamine+glutamate pool in parts of the brains of humans [Yoon et al., 2009: (http://www.ncbi.nlm.nih.gov/pubmed/19194376)], and that type of effect could be explained in terms of an increase in the flux of substrates through the nonoxidative pentose cycle, via the ribose-1-phosphate derived from cytidine catabolism in the brain, and glycolytic pathways. Uridine is converted into cytidine, and ribose derived from purines might be expected to produce a similar effect, to some extent. Hodges and Snyder (2004), in fact, discuss research showing that either glucose or glutamine can serve as an energy substrate for cultured cells but that inosine or some other "pentose source," meaning ribose derived from inosine, had to be present, in combination with glutamine, to maintain ATP levels in cultured cells that lacked a source of glucose. Exogenous nucleotides have been shown to elevate lactate levels in many articles, and those effects could be partly attributable to the ribose-induced increases in the activities of glycolytic enzymes. The effect of cytidine could also have been the result of some effect on receptors that bind pyrimidine triphosphates, etc., but that type of mechanism, in my opinion, is less plausible. Nonetheless, I don't think ribose, by itself would substitute for uridine, past a certain point.
My overall point is that some of these energy substrates, such as creatine and glutamine and ribose, can increase the flux of intermediates through one pathway and produce beneficial effects that might, in my opinion, be expected to be limited by the "depleting" effect that might result from that shift in the metabolism of one or another intermediates. At high doses, however, these types of supplements start to produce really complicated effects. The glutamine-induced decreases in plasma free fatty acids, for example, could, in my opinion, begin to become counterproductive, with respect to the brain, at higher dosages. Obviously, one would want to discuss any of these supplements with one's doctor before taking any of them or beginning any kind of exercise program.
Sahlin et al. (1999) discuss the fact that researchers have generally found glycogen depletion from the skeletal muscles to be a prerequisite for the most pronounced, exercise-induced increases in plasma purines, including hypoxanthine. True glycogen depletion from a muscle group generally requires exhaustive exercise, and the research has generally shown, in my opinion, that resistance exercise produces more-pronounced degrees of glycogen depletion and purine depletion from the muscles [the general idea is that ATP depletion causes a loss of the capacity to salvage adenosine (and also guanosine), and this causes inosine monophosphate to accumulate and be converted into hypoxanthine and xanthine in the muscle cells] than exercise at low intensity does. Hellsten et al. (1998) [Hellsten et al., 1998: (http://ajpendo.physiology.org/cgi/content/full/274/4/E600)(http://www.ncbi.nlm.nih.gov/pubmed/9575819)] discuss research showing that a high-intensity exercise program causes a 20 percent decrease in the total adenine nucleotide contents of skeletal muscles (when people are not exercising, meaning post-exercise and all the time). That's a remarkable fact and suggests to me that some benefit might be derived from low-dose adenosine or guanosine supplementation, particularly early in an exercise program. But that's my opinion. The effects of hypoxanthine on the brain should not be underestimated, and a single exercise session that produces an 8-fold elevation in plasma hypoxanthine levels, during the hour or few hours following exercise, could have a significant effect on the pools of adenine nucleotides, in particular, in the brain. Mink and Johnston (2007), cited above, discuss the fact that hypoxanthine appeared to be salvaged to a large extent, even during ischemia. The brain has an extremely low capacity to make purines de novo and depends almost entirely on purines exported from either the endothelial cells lining the cerebral blood vessels or from the blood.
Of course, creatine and glutamine have been shown to augment the salvage of purine or pyrimidine nucleotides in various articles [cited and discussed here or in other postings: (http://hardcorephysiologyfun.blogspot.com/2009/02/interactions-of-glutamine-and-arginine.html)], but any supposed improvement in purine or pyrimidine salvage, in the brain, that might occur in response to the administration of those types of supplements might be offset by the increase in, for example, purine nucleotide export that could accompany a creatine- or glutamine-induced increase in exercise intensity. I don't think that's the way it would work, though, as long as the doses of creatine or glutamine are kept low, but that's just my opinion. For example, the combination of glutamine and inosine, which is hypoxanthine riboside (hypoxanthine attached to ribose to make a nucleotide), [Hodges and Snyder, 2004, cited here: (http://hardcorephysiologyfun.blogspot.com/2009/02/contribution-of-glutamine-to-pool-of.html)]. That's just my opinion. In my opinion, low doses of glutamine, creatine and adenosine and guanosine might produce some sort of buffering effect on brain energy metabolism and purine nucleotide pools. Creatine increases or "stimulates" oxidative metabolism, in part by maintaining the intramitochondrial ADP pool, and this would be expected to increase the extent to which glutamine, upon its metabolism into glutamate, can be metabolized into alpha-ketoglutarate and undergo oxidative metabolism in neurons or astrocytes in the brain. People discuss creatine as if it participates only in anaerobic metabolism, but this is just not the case. Creatine can prolong "aerobic" exercise and is known to increase oxidative glucose utilization in cells, etc.
Similarly, cytidine was recently shown to decrease the glutamine+glutamate pool in parts of the brains of humans [Yoon et al., 2009: (http://www.ncbi.nlm.nih.gov/pubmed/19194376)], and that type of effect could be explained in terms of an increase in the flux of substrates through the nonoxidative pentose cycle, via the ribose-1-phosphate derived from cytidine catabolism in the brain, and glycolytic pathways. Uridine is converted into cytidine, and ribose derived from purines might be expected to produce a similar effect, to some extent. Hodges and Snyder (2004), in fact, discuss research showing that either glucose or glutamine can serve as an energy substrate for cultured cells but that inosine or some other "pentose source," meaning ribose derived from inosine, had to be present, in combination with glutamine, to maintain ATP levels in cultured cells that lacked a source of glucose. Exogenous nucleotides have been shown to elevate lactate levels in many articles, and those effects could be partly attributable to the ribose-induced increases in the activities of glycolytic enzymes. The effect of cytidine could also have been the result of some effect on receptors that bind pyrimidine triphosphates, etc., but that type of mechanism, in my opinion, is less plausible. Nonetheless, I don't think ribose, by itself would substitute for uridine, past a certain point.
My overall point is that some of these energy substrates, such as creatine and glutamine and ribose, can increase the flux of intermediates through one pathway and produce beneficial effects that might, in my opinion, be expected to be limited by the "depleting" effect that might result from that shift in the metabolism of one or another intermediates. At high doses, however, these types of supplements start to produce really complicated effects. The glutamine-induced decreases in plasma free fatty acids, for example, could, in my opinion, begin to become counterproductive, with respect to the brain, at higher dosages. Obviously, one would want to discuss any of these supplements with one's doctor before taking any of them or beginning any kind of exercise program.
Friday, April 10, 2009
Report of Liver Dysfunction or Damage Associated With Creatine/Whey Protein Ingestion: Potential Relation to the Urea Cycle
This article [Whitt et al., 2008: (http://www.ncbi.nlm.nih.gov/pubmed/18452122)] discusses a case of reversible liver dysfunction/damage associated with creatine intake. I can't get the full text of the article at the moment, but there's also a study showing liver damage in animals on high-dose creatine [Tarnopolsky et al., 2003: (http://ajpregu.physiology.org/cgi/content/full/285/4/R762)(http://www.ncbi.nlm.nih.gov/pubmed/12959920?dopt=Abstract)]. I do think it might be possible for creatine to produce liver dysfunction at excessive dosages, in the long term, but that's just my opinion. The articles on the "alternate urea cycle" that occurs in people with chronic renal failure suggest that creatine or other guanidino compounds that have sometimes been shown to accumulate, at relatively low levels in humans taking 5 grams/d of creatine [Derave et al., 2004: (http://jap.physiology.org/cgi/content/full/97/3/852)(http://www.ncbi.nlm.nih.gov/pubmed/15107411?dopt=Abstract)], could inhibit urea cycle enzymes or interfere with the transport of urea cycle intermediates, etc., and produce liver dysfunction, in my opinion. This doesn't mean that creatine is "toxic" or "bad" but just means that there tends to be a therapeutic dosage range for a physiological substrate, such as creatine, and then a dosage range across which derangements in the transport or utilization of substrates with similar structures may begin to occur. That's just my opinion, but I discussed research, in a past posting, showing that high levels of glutamine, for example, could reduce endothelial nitric oxide production from eNOS, and the effect was thought to be due to the inhibition, by glutamine, of citrulline uptake or reutilization [Kawaguchi et al., 2005, cited and discussed here: (http://hardcorephysiologyfun.blogspot.com/2009/03/inhibition-of-nitric-oxide-dependent.html)]. Citrulline is a urea cycle intermediate, and so it's conceivable, in my opinion, that abnormally-high levels of glutamine or a guanidino compound derived from the transamination of arginine could, in the liver, interfere with the metabolism of citrulline in the urea cycle, etc. Glutamine, citrulline, and creatine are structurally similar, to some extent, and there's a lot of research showing competition for transport among those compounds or competitive inhibitory effects on enzymes, etc. I don't have time to get into the research on guanidinosuccinate, which has been shown to decrease in response to creatine supplementation, or argininic acid and the other guanidino compound that can accumulate during creatine supplementation (Derave et al., 2004, cited above). This could be another reason to consider using the lower end of the dosage range of creatine monohydrate used in clinical trials (~3-10 grams/d), as discussed previously, and to discuss this with one's doctor.
Sunday, April 5, 2009
Research on the Use of Creatine Monohydrate as an Adjunctive to Prescription Antidepressants
These are some more articles discussing the use of creatine in combination with prescription antidepressants or medications to treat Parkinson's disease [Roitman et al., 2007: (http://www.ncbi.nlm.nih.gov/pubmed/17988366); Amital et al., 2006: (http://ajp.psychiatryonline.org/cgi/reprint/163/10/1840-b.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/17012702); Bender et al., 2006: (http://www.ncbi.nlm.nih.gov/pubmed/17030762)]. Amital et al. (2006) researched the effects of creatine at 3 grams per day, for one week, and then 5 grams per day, for four weeks (and then also for eight weeks after the end of the trial), in a person who was being treated with prescription medications for post-traumatic stress disorder and depression with fibromyalgia. The authors found that, during the course of the trial, the person's scores on the Hamilton Depression Rating Scale had decreased, indicating that the creatine had evidently produced an antidepressant effect in the person, and that the person's symptoms of fibromyalgia had improved. Roitman et al. (2007) found that the full antidepressant effect in the patients had required four weeks to emerge, but the greatest reductions in the Hamilton Depression Rating Scale had occurred within the first two weeks in most of the people. Roitman et al. (2007) also found that some people experienced a greater improvement in mood from 3 grams/day of creatine than from 5 grams/day.
Bender et al. (2006) used 4 grams a day of creatine for two years (after an initial phase of 20 grams per day for 6 days and then 2 grams per day for six months), in a randomized, placebo-controlled trial and found that the people who had taken creatine had not had to increase the dosages of their dopaminergic medications for Parkinson's disease to the extent that the people taking the placebo had had to increase their dosages, and that effect was statistically-significant. The other main result of the trial was the finding that creatine had produced a statistically-significant reduction in symptoms of depression, as measured by the "score" on Item 3 of Part I of the Unified Parkinson's Disease Rating Scale (UPDRS). Incidentally, I don't think the "loading dose" concept of creatine makes much sense, especially given the potential for high-dose creatine to produce plasma and extracellular fluid volume expansion in some people [Powers et al., 2003: (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=155510&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/12937471)]. To the extent that that effect would occur more with high-dose than with low-dose creatine, one would expect the body to be less able to adapt or compensate to that effect in the context of a "loading dose." But that's just my opinion. Obviously, one would want to discuss this with one's doctor before using creatine or any supplement, particularly given that Roitman et al. (2007) found that, in one of the ten patients, creatine produced transient improvement in the person's mood and then worsened it. Roitman (2007) also found that creatine produced mania or hypomania in the people in the trial who had been diagnosed with bipolar disorder.
In my opinion, creatine would be more effective in this context in combination with adenosine monophosphate/triphosphate or guanosine monophosphate or both, for complex reasons. I think it's a really important point, but it's just my opinion. I could explain my reasoning in more detail, but I'd have to cite a lot more articles and can't do that right now. I discussed some of the rationale for the combination in a past posting (http://hardcorephysiologyfun.blogspot.com/2009/03/creatine-cr-phosphocreatine-pcr-and.html), and I discussed some of the research and details on orally-administered guanosine monophosphate and adenosine monophosphate or adenosine triphosphate in this posting (http://hardcorephysiologyfun.blogspot.com/2009/03/adenosine-and-guanosine-in-animal.html). Other approaches that might potentiate the effects of creatine, in my opinion, would be the use of methylcobalamin at ~5 mg/day or something (with the intent of disinhibiting creatine kinase enzymes and tricarboxylic acid cycle enzymes by reducing methylmalonic acid levels), the use of L-methylfolate or levoleucovorin in combination with methylcobalamin (with the intent of reducing the accumulation of sarcosine, which may be produced from high-dose creatine, given that folate depletion can compromise the metabolism of sarcosine and dimethylglycine and betaine, as I've discussed in the context of research cited in past postings), and the use of adenosine and guanosine to elevate serum uric acid to high-normal levels, under a doctor's supervision (with various "purposes" in mind, including the disinhibition of creatine kinase, which is inhibited by peroxynitrite, through the scavenging of peroxynitrite by uric acid) [discussed and cited here: (http://hardcorephysiologyfun.blogspot.com/2009/03/creatine-cr-phosphocreatine-pcr-and.html)]. Poortmans et al. (2005) [Poortmans et al., 2005: (http://www.ncbi.nlm.nih.gov/pubmed/16260971)] discussed the fact that the elevation of urinary methylamine and formaldehyde, in response to high-dose creatine administration, could have resulted from the metabolism of some of the creatine to sarcosine, evidently by the creatinase enzymatic activity in microorganisms in the G.I. tract, if memory serves (humans don't express a creatinase enzyme, as far as is known). Sarcosine can be metabolized to methylamine by sarcosine reductase or to formaldehyde and glycine, by sarcosine oxidase, and methylamine can be metabolized to formaldehyde and ammonia by semicarbazide-sensitive amine oxidase (SSAO) (Poortmans et al., 2005). Formaldehyde can then be converted into formate, and the activities of the folate cycle enzymes, which are dependent upon adequate pools of reduced folates, such as L-methylfolate or levoleucovorin, and vitamin B12 (i.e. methylcobalamin), are important for the normal metabolism of formate.
Bender et al. (2006) used 4 grams a day of creatine for two years (after an initial phase of 20 grams per day for 6 days and then 2 grams per day for six months), in a randomized, placebo-controlled trial and found that the people who had taken creatine had not had to increase the dosages of their dopaminergic medications for Parkinson's disease to the extent that the people taking the placebo had had to increase their dosages, and that effect was statistically-significant. The other main result of the trial was the finding that creatine had produced a statistically-significant reduction in symptoms of depression, as measured by the "score" on Item 3 of Part I of the Unified Parkinson's Disease Rating Scale (UPDRS). Incidentally, I don't think the "loading dose" concept of creatine makes much sense, especially given the potential for high-dose creatine to produce plasma and extracellular fluid volume expansion in some people [Powers et al., 2003: (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=155510&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/12937471)]. To the extent that that effect would occur more with high-dose than with low-dose creatine, one would expect the body to be less able to adapt or compensate to that effect in the context of a "loading dose." But that's just my opinion. Obviously, one would want to discuss this with one's doctor before using creatine or any supplement, particularly given that Roitman et al. (2007) found that, in one of the ten patients, creatine produced transient improvement in the person's mood and then worsened it. Roitman (2007) also found that creatine produced mania or hypomania in the people in the trial who had been diagnosed with bipolar disorder.
In my opinion, creatine would be more effective in this context in combination with adenosine monophosphate/triphosphate or guanosine monophosphate or both, for complex reasons. I think it's a really important point, but it's just my opinion. I could explain my reasoning in more detail, but I'd have to cite a lot more articles and can't do that right now. I discussed some of the rationale for the combination in a past posting (http://hardcorephysiologyfun.blogspot.com/2009/03/creatine-cr-phosphocreatine-pcr-and.html), and I discussed some of the research and details on orally-administered guanosine monophosphate and adenosine monophosphate or adenosine triphosphate in this posting (http://hardcorephysiologyfun.blogspot.com/2009/03/adenosine-and-guanosine-in-animal.html). Other approaches that might potentiate the effects of creatine, in my opinion, would be the use of methylcobalamin at ~5 mg/day or something (with the intent of disinhibiting creatine kinase enzymes and tricarboxylic acid cycle enzymes by reducing methylmalonic acid levels), the use of L-methylfolate or levoleucovorin in combination with methylcobalamin (with the intent of reducing the accumulation of sarcosine, which may be produced from high-dose creatine, given that folate depletion can compromise the metabolism of sarcosine and dimethylglycine and betaine, as I've discussed in the context of research cited in past postings), and the use of adenosine and guanosine to elevate serum uric acid to high-normal levels, under a doctor's supervision (with various "purposes" in mind, including the disinhibition of creatine kinase, which is inhibited by peroxynitrite, through the scavenging of peroxynitrite by uric acid) [discussed and cited here: (http://hardcorephysiologyfun.blogspot.com/2009/03/creatine-cr-phosphocreatine-pcr-and.html)]. Poortmans et al. (2005) [Poortmans et al., 2005: (http://www.ncbi.nlm.nih.gov/pubmed/16260971)] discussed the fact that the elevation of urinary methylamine and formaldehyde, in response to high-dose creatine administration, could have resulted from the metabolism of some of the creatine to sarcosine, evidently by the creatinase enzymatic activity in microorganisms in the G.I. tract, if memory serves (humans don't express a creatinase enzyme, as far as is known). Sarcosine can be metabolized to methylamine by sarcosine reductase or to formaldehyde and glycine, by sarcosine oxidase, and methylamine can be metabolized to formaldehyde and ammonia by semicarbazide-sensitive amine oxidase (SSAO) (Poortmans et al., 2005). Formaldehyde can then be converted into formate, and the activities of the folate cycle enzymes, which are dependent upon adequate pools of reduced folates, such as L-methylfolate or levoleucovorin, and vitamin B12 (i.e. methylcobalamin), are important for the normal metabolism of formate.
Sunday, March 29, 2009
Adenosine and Guanosine in Animal Models of Depression
Several articles show that adenosine [Kaster et al., 2004: (http://www.ncbi.nlm.nih.gov/pubmed/14729225); Kaster et al., 2005a: (http://www.ncbi.nlm.nih.gov/pubmed/16140163); Kaster et al., 2005b: (http://www.ncbi.nlm.nih.gov/pubmed/16202183); Kaster et al., 2007a: (http://www.ncbi.nlm.nih.gov/pubmed/17868670); Kaster et al., 2007b: (http://www.ncbi.nlm.nih.gov/pubmed/17296254)] or guanosine [Eckeli et al., 2000: (http://www.ncbi.nlm.nih.gov/pubmed/10884029)] can produce antidepressant ("antidepressant-like") effects in animal models of depression. It's important to note that guanosine and adenosine have not been proven to be effective or safe in the treatment of depression or any other disease, and one should always talk to his or her doctor before taking any of these supplements. It's also important to take the results from these animal studies with a grain of salt, but the results of a lot of these experiments are actually generally-consistent with the results of other research in humans. My sense is that guanosine would display more of a mild anticonvulsant effect than adenosine would, mainly because of the large amount of research showing anticonvulsant effects of oral or intraperitoneal guanosine in animals [(http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&cites=13842524336757775794); (http://hardcorephysiologyfun.blogspot.com/2009/01/anticonvulsant-effects-of-oral.html)]. The apparent effectiveness of SAM-e in the treatment of depression (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&q=antidepressant+%22S-adenosylmethionine%22+OR+%22S-adenosyl-L-methionine%22), in spite of the relatively poor biovailability that I see SAM-e as having, is, in my opinion, an important line of evidence suggesting that adenosine could, in fact, display antidepressant effects and serve some kind(s) of adjunctive role(s) in that regard. SAM-e is rapidly metabolized into adenosine, and, as discussed below, some of the effects of SAM-e on neurons and other cell types can be blocked by adenosine receptor antagonists or mimicked by adenosine itself. Although adenosine is generally viewed as exerting primarily an inhibitory influence on catecholaminergic and glutamatergic transmission, S-adenosylmethionine (SAM-e), which is generally known to increase the pool of adenosine and its nucleotides in the cells it reaches [Smolenski, 2000: (http://www.actabp.pl/pdf/4_2000/1171-1178s.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/11996106)], can nonetheless induce mania or hypomania in some people (an effect that is not consistent with an anticonvulsant effect) (http://scholar.google.com/scholar?q=S-adenosylmethionine+mania+OR+hypomania&hl=en&lr=). Thus, SAM-e can exert excitatory effects under some conditions, such as in the presence of medications that affect adenosine release or adenosine receptor sensitivities in the brain (many drugs used in psychopharmacology). Consistent with this, Saletu et al. (2002) [Saletu et al., 2002: (http://www.ncbi.nlm.nih.gov/pubmed/12486491)] found that the changes in the electroencephalograms of people who had been receiving parenteral SAM-e for 7 days indicated that SAM-e had been producing a mixture of both excitatory and inhibitory effects on overall brain activity. This article is a bit confusing at times but discusses some of the complexities of "adenosinergic" signalling in the context of dopaminergic transmission and psychiatric conditions [Cunha et al., 2008: (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=2423946&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/18537674)]. What they're saying is that adenosine itself can exert excitatory effects on striatal dopamine release by activating (apparently-predominantly-presynaptic) "extrastriatal" A2A receptors on glutamatergic neurons, even in the face of the inhibition of dopamine release by the adenosine-induced, tonic activation of A1 receptors on neurons in the striatum (on dopaminergic terminals or interneurons, etc.). Adenosine could, for example, modulate the release of glutamate from glutamatergic, pyramidal neurons that project from the dorsolateral prefrontal cortex and provide excitatory inputs to dopaminergic neurons in the ventral tegmental area (VTA), thereby providing an excitatory influence on dopamine release in the striatum (given that many dopaminergic neurons in the VTA project to the ventral striatum, etc.) [Carr and Sesack, 2000: (http://www.jneurosci.org/cgi/reprint/20/10/3864)(http://www.ncbi.nlm.nih.gov/pubmed/10804226?dopt=Abstract)].
That's a fairly well-established neuronal pathway that's important for cognitive functioning and mood, but the research on the effects of adenosine (and guanosine) in the brain is very vast and extremely complicated. That's one reason I tend to focus more on in vivo studies in animals and to not focus too heavily on articles that discuss, for example, the intracellular signalling cascades that guanosine and adenosine activate, via the binding of GTP to G-proteins or the binding of adenosine to adenosine receptors or intracellular binding sites, etc. But given the importance of dopaminergic transmission to cognitive functioning and mood disorders, it is noteworthy that, as discussed by Carr and Sesack (2000), glutamatergic inputs to striatal dopaminergic neurons are crucially important for the burst firing patterns of those dopaminergic neurons to be maintained normally. Too much or too little dopamine release in the prefrontal cortex, from dopaminergic neurons that project to the prefrontal cortex from different sites in the basal ganglia (including the striatum), is detrimental to working memory and therefore to cognitive functioning, and the firing patterns of glutamatergic neurons in the prefrontal cortex and other "extrastriatal" sites, as would be regulated by, for example, the degree of activation of A2A receptors outside the striatum by adenosine, are a major factor that regulates dopamine release in the prefrontal cortex. In some articles, a discussion of "a receptor in the striatum" can mean that the receptor is on an axon terminal, in the striatum, of a dopaminergic neuron whose cell body is not in the striatum but is in, for example, the midbrain, in the VTA. This type of discussion can just become absurdly complicated and can just get crazy, especially given the concentration-dependence of the effects of adenosine, etc. Cunha et al. (2008) discussed the fact that A2A receptors seem to be activated more by adenosine that is derived from extracellular ATP hydrolysis than adenosine that is derived from other sources (such as by the export from the cytosol, via equilibrative adenosine transporters, etc.). That's just one example of an obscure "mechanistic" explanation for the complexity of the adenosinergic or purinergic modulation of neuronal activity. In any case, those excitatory effects that adenosine, such as can be derived from SAM-e or oral adenosine monophosphate or triphosphate, can sometimes have on some glutamatergic neurons are one mechanism that could account for the acute increases in dopamine and noradrenaline release that have been shown to occur in animals in response to SAM-e administration [cited in Benelli et al., 1999: (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1566059&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/10401554)]. In a related vein, Eckeli et al. (2000), cited above, found that the apparent antidepressant effects of guanosine monophosphate in animals was partially dependent on serotonin biosynthesis or release. The antidepressant-like effect of guanosine monophosphate was mimicked by an NMDA receptor antagonist or fluoxetine, a serotonin reuptake inhibitor and prescription antidepressant. In that case, the general idea is essentially that a stressor is producing excessive glutamatergic activation and that the glutamatergic activity is indirectly disturbing the feedback inhibition of the firing rates, which increase in response to acute stress, of noradrenergic neurons in the locus ceruleus. Guanosine or NMDA receptor antagonism may attenuate this excessive glutamatergic activity, as implied by the results of Eckeli et al. (2000). There are more than a dozen articles showing that oral or intraperitoneal guanosine can exert anxiolytic (anti-anxiety) or anticonvulsant or neuroprotective effects that are at least partially a result of the attenuation of glutamate release or augmentation of synaptic glutamate reuptake by guanosine (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&cites=13842524336757775794). This seems confusing, but under conditions of chronic stress or neuronal injury, for example, decreasing glutamatergic transmission (such as by NMDA receptor antagonism) can actually increase dopamine release or, more precisely, increase the responsiveness of (postsynaptic) neurons receiving dopaminergic inputs to, for example, D1 dopamine receptor activation in response to dopamine release [see, for example, Peeters et al., 2002: (http://www.ncbi.nlm.nih.gov/pubmed/12213297); Deep et al., 1999: (http://www.ncbi.nlm.nih.gov/pubmed/10529725); Arai et al., 2003: (http://www.ncbi.nlm.nih.gov/pubmed/12711097); Konradi et al., 1996: (http://www.jneurosci.org/cgi/reprint/16/13/4231)(http://www.ncbi.nlm.nih.gov/pubmed/8753884?dopt=Abstract); Tokuyama et al., 2001: (http://www.ncbi.nlm.nih.gov/pubmed/11408088); Boyce-Rustay et al., 2006: (http://www.ncbi.nlm.nih.gov/pubmed/16482087)]. In the absence of stress (and even in the "presence of stress," up to a point), however, decreasing glutamatergic transmission (let's say to the same "degree" as in the presence of stress and at the same dosages of the same drug, for example) will tend, with some degree of predictability, to decrease dopamine release (many more references on this). In the former case, mild NMDA receptor antagonism, such as by amantadine (which also exerts numerous other effects that confound the discussion, but so be it), produces an excitatory effect on normal, burst-firing-associated dopaminergic transmission, but NMDA receptor antagonism in the latter case--antagonism that is stronger or that occurs in the absence of some factor that is producing an "abberant" or excessive increase in the firing rates of the neurons in question--will tend to inhibit dopaminergic (or noradrenergic) transmission. Here's an article that highlights some of the complexity with which adenosine can modulate the release of dopamine in response to NMDA receptor (a glutamate receptor) activation in the striatum [Quarta et al., 2004: (http://www.ncbi.nlm.nih.gov/pubmed/15525341)].
Again, my opinion is that many of the effects of SAM-e on the brain are mediated by adenosine, which SAM-e is metabolized into, and by the nucleotides derived from adenosine and its metabolites (including hypoxanthine and inosine and also some guanosine formed from inosine monophosphate, derived from adenosine nucleotides, etc.). I've discussed some of the evidence for this in past postings, but, for example, Renshaw et al. (2001) suggested that SAM-e may exert antidepressant effects, in part, via its metabolism into adenosine [cited here: (http://hardcorephysiologyfun.blogspot.com/2009/01/details-on-nucleotides-bioavailability.html)]. Additionally, the effects of exogenous adenosine or S-adenosylmethionine or inosine monophosphate, in combination with guanosine and other nucleotides, can produce increases in the phosphocreatine (PCr) to creatine (Cr) ratio (PCr/Cr ratio) that are reminiscent of the effect of exogenous SAM-e on the PCr/NTP ratio [see Silveri et al. (2003) and other articles cited and discussed here: (http://hardcorephysiologyfun.blogspot.com/2009/03/creatine-cr-phosphocreatine-pcr-and.html)]. In reference to the article by Silveri et al. (2003), it is noteworthy that NTP refers to the pool of beta-nucleotide triphosphates, and this "quantity" is thought to primarily reflect the pool of ATP that is measured, in a given part of the brain, by magnetic resonance spectroscopy (MRS). Also, some of the antiinflammatory effects of SAM-e can be antagonized by adenosine receptors and mimicked by the administration of adenosine, as a substitute for SAM-e [Song et al., 2005: (http://www.ncbi.nlm.nih.gov/pubmed/15843034)]. Song et al. (2005) also found that SAM-e elevated the pool of intracellular adenosine in cultured monocytes. Travagli et al. (1994) [Travagli et al., 1994: (http://www.ncbi.nlm.nih.gov/pubmed/7698179)] found that SAM-e reduced the firing rates of neurons in the vagal motor nucleus, and these effects were not only abolished by adenosine receptor antagonists but were shown to be "reversed" in the presence of those antagonists. SAM-e increased the firing rates of those neurons in the presence of the adenosine receptor antagonists, and Travagli et al. (1994) noted that increases in the availability of adenosine, produced from SAM-e via the hydrolysis of S-adenosylhomocysteine (SAH), had probably caused the changes in the neuronal firing rates. The attenuation of the SAM-e-induced increases in the firing rates of the neurons by SAH, in the presence of the adenosine receptor antagonists, might be explained by the inhibition of methyltransferases by SAH. Inhibiting methyltransferase enzymes in the presence of an excess of SAM-e would prevent much of the conversion of the excess SAM-e into SAH and then into adenosine. Alternatively, the excess SAH may itself have been converted into adenosine and thereby produced effects on adenosine receptors or purinergic receptors that were, as a result of the higher concentration of adenosine, in opposition to the effects that had been produced by the exogenous-SAM-e-derived adenosine (i.e. by that lower concentration). In either case, the results could conceivably be explained in terms of the effects of nucleotides or nucleosides derived from adenosine (i.e. inosine, hypoxanthine, xanthine, uric acid, or guanosine, etc.) or the effects of different concentrations of extracellular adenosine, derived from exogenous SAM-e or SAH, on different adenosine receptor subtypes or on other intracellular or extracellular adenosine binding sites on other proteins. There are lots of other articles that show similarly-comparable effects of adenosine and either SAM-e or 5'-methylthioadenosine (MTA), which is converted into adenosine [these are two that I found without any effort, but there are many others: Song et al., 2004: (http://www.ncbi.nlm.nih.gov/pubmed/15566950); Song et al., 2003: (http://www.ncbi.nlm.nih.gov/pubmed/12736147)]. I view the adenosine-receptor dependence of the anti-inflammatory effects of SAM-e, along with the elevation of intracellular adenosine by SAM-e (Song et al., 2005, cited above; Smolenski, 2000, cited above), as evidence that the adenosine nucleotide pool is being elevated and equilibrating with the extracellular pool, thereby causing increases in the activation of extracellular adenosine receptors. There are other interpretations, but that's just my opinion.
As far as the dosage considerations go, a number of articles have shown that adenosine and guanosine can exert effects in animal models at remarkably low dosages. Guanosine has been shown to exert neuroprotective and anticonvulsant effects at 7.5 mg/kg bw per day, given orally or intraperitoneally, in rats. That works out to something like ~111 mg per day, scaled to a dosage for a 70-kg human, and that's an extremely low dosage. I would think there might be some kind of issue with that dosage or the scaling factor (4.71) that I used, but essentially the same dosage (8 mg/kg bw/day, given intraperitoneally), was shown to promote restoration of functioning in animals following experimental spinal cord injuries [Jiang et al., 2008: (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=2072916&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/18404454)]. If one were to not scale the dosage and use 8 mg/kg bw per day in a 70-kg human, the dosage would be 560 mg/d. Even under those circumstances, that's a very low dosage, as purines go. That suggests that guanosine is fairly potent, particularly in comparison to inosine. The dosages of inosine used to promote recovery after spinal cord injuries, in animal models, are much higher. For example, Liu et al. (2006) [Liu et al., 2006: (http://www.ncbi.nlm.nih.gov/pubmed/16317421)] used inosine to prevent the ongoing degneration that occurs after a spinal cord injury in adult rats, and the dosage used was 75 mg/kg bw, given intraperitoneally, every eight hours. That's 225 mg/kg bw per day in rats. The fact that Jiang et al. (2008) found therapeutic effects of guanosine at 8 mg/kg bw/day is remarkable and shows, along with the many articles showing anticonvulsant and neuroprotective effects of oral or i.p. guanosine or guanosine monophosphate at 7.5 mg/kg bw/day, that, in my opinion, guanosine would be expected to produce meaningful effects on the brain at doses that would be compatible with human physiology. I say that because the dosage of inosine of 225 mg/kg bw/day (or the 100 mg/kg bw/day used in many other animal experiments showing anti-inflammatory effects of inosine, for example) scales to 3344 mg/day of inosine for a 70-kg human. If one looks at the trials using inosine to elevate uric acid in people with multiple sclerosis, one sees that that dose of inosine could produce hyperuricemia in many humans. Using that dose in the short term, such as after a spinal cord injury, would not be expected to produce hyperuricemia, and researchers would get larger effects on the brain by administering inosine intravenously or intraperitoneally than they would by administering it orally. But guanosine appears to exert many of its effects on the brain at remarkably low dosages.
One can, in my opinion, get a sense of the possible "dosage ranges" for guanosine monophosphate and adenosine monophosphate by looking at the dosages of S-adenosylmethionine and inosine that have been used in trials in humans. Guanosine monophosphate is available, evidently only in combination with other nucleotides, from a limited number of manufacturers [for example: (http://www.google.com/products?q=bluebonnet+nucleotide+complex+&hl=en)], but I'm not sure how much guanosine monophosphate is in that product. Again, I don't have any financial interest in any of these products or in anything I've discussed on this blog. The combinations of nucleotides that are used in research (http://scholar.google.com/scholar?q=dietary+nucleotide&hl=en&lr=) typically contain guanosine monophosphate and the other nucleoside monophosphates (a nucleotide, for the purposes of this discussion, is a nucleoside that's been phosphorylated in the 5'-position) in a either roughly 1:1:1:1 mass ratio or molar ratio. If the nucleotides and nucleosides were in a 1:1:1:1 mass ratio, each of those capsules would contain ~50 mg guanosine monophosphate and 10 mg guanosine and 15 mg guanine (and those same amounts for adenosine monophosphate, adenosine, and adenine). But I don't know if that's correct. One could call the manufacturer, and I'm sure they'd probably tell you what the composition is. But, from the standpoint of uric acid production, the elevation in uric acid is going to be different for every person. That's one reason it's necessary to have one's uric acid monitored by one's doctor, if one's going to take the full range of dosages for oral adenosine or guanosine monophosphates. One way to determine a dosage, with one's doctor, in the context of antidepressant medication, would be to look at the "percent adenosine" in S-adenosylmethionine and the dosage range of inosine used to treat multiple sclerosis. Less uric acid is made from adenosine than from inosine, in general, because adenosine is more efficiently salvaged. The ratio is something like 1.3/2.5, meaning the ratio of the AUC or Cmax (I forget which) for the serum uric acid response to oral adenosine to the uric acid response to oral inosine is about 0.52 or 0.55 or something. SAM-e is ~67.1 percent adenosine (the ratio of the molar mass of adenosine to the molar mass of SAM-e is 267.241/398.44), and the maximum dosage of SAM-e that I've seen used to treat depression is 3600 mg/day [Di Rocco et al., 2000: (http://www.ncbi.nlm.nih.gov/pubmed/11104210)]. If one takes 67.1 percent of that dosage, one gets 2415 mg/day of adenosine (or adenosine + guanosine). That works out to about 16 of those nucleotide capsules per day, assuming about half of the 300 mg is adenosine + guanosine (or purine bases, which are nucleic acids that can would probably be converted into uric acid). I'm not suggesting anyone would want to take any of these sorts of full dosage ranges without talking to one's doctor, because elevations in uric acid are not the only potential side effect. It's conceivable that adenosine or guanosine could produce vasodilation or reduce blood pressure and interact with sedatives or blood pressure medications or any number of medications and produce serious side effects. It's actually the case that adenosine monophosphate and guanosine monophosphate are 70-some percent adenosine and guanosine (the rest is phosphate), and that would affect the "calculation," crude as it is (that would mean the dose would be a little bit higher to be equivalent to the 2415 mg of adenosine that can be derived from 3,600 mg of SAM-e). Adenosine triphosphate disodium is more widely available than guanosine [(http://hardcorephysiologyfun.blogspot.com/2009/02/brief-note-on-purines.html); (http://hardcorephysiologyfun.blogspot.com/2009/01/details-on-nucleotides-bioavailability.html)].
One way to minimize the production of uric acid from adenosine monophosphate and guanosine monophosphate would be to dissolve them in water and take them on an empty stomach (which means before breakfast, not between meals), but I'm not sure if that would meaningfully increase the bioavailabilities of those nucleotides. There's increasingly been a recognition that the bioavailability of physiological substrates can depend on seemingly-trivial factors, such as the rate of dissolution, and this has been discussed in the context of the bioavailability of creatine. When researchers [Deldicque et al., 2008: (http://www.ncbi.nlm.nih.gov/pubmed/17851680)] gave 2 grams of creatine monohydrate dissolved in aqueous solution (water) or in a protein bar or in some kind of complex with "beta-glucan" (I have no idea what the rationale for testing that is), the Cmax for creatine in water was 299 uM (up from a baseline of 10 uM), and that was higher than the Cmax for creatine from the protein bar (237 uM) or the beta-glucan source (174 uM). I've seen multiple articles in which the researchers have dissolved creatine monohydrate in water, ostensibly with the aim of improving the availability of creatine for entry into the brain. I don't know if that type of factor would really matter all that much, and it might be cumbersome to do. But the same concept has been discussed in the context of orally-administered nucleotides. The activity of xanthine oxidase ("xanthine oxidoreductase") is thought to be lower in the fasting state, and this means that less of a dose of adenosine or guanosine may be converted, following the conversion into hypoxanthine or xanthine, into uric acid, in the fasting state, in the short term [see references 75-77, cited on p. 63 of Carver and Walker, 1995: (http://cat.inist.fr/?aModele=afficheN&cpsidt=3477199)]. Note that that paper by Carver and Walker (1995) is not especially up-to-date, as far as information on nucleotide bioavailability is concerned.
I'll put the discussion of mechanisms in a separate posting. This is too long to put in one posting.
That's a fairly well-established neuronal pathway that's important for cognitive functioning and mood, but the research on the effects of adenosine (and guanosine) in the brain is very vast and extremely complicated. That's one reason I tend to focus more on in vivo studies in animals and to not focus too heavily on articles that discuss, for example, the intracellular signalling cascades that guanosine and adenosine activate, via the binding of GTP to G-proteins or the binding of adenosine to adenosine receptors or intracellular binding sites, etc. But given the importance of dopaminergic transmission to cognitive functioning and mood disorders, it is noteworthy that, as discussed by Carr and Sesack (2000), glutamatergic inputs to striatal dopaminergic neurons are crucially important for the burst firing patterns of those dopaminergic neurons to be maintained normally. Too much or too little dopamine release in the prefrontal cortex, from dopaminergic neurons that project to the prefrontal cortex from different sites in the basal ganglia (including the striatum), is detrimental to working memory and therefore to cognitive functioning, and the firing patterns of glutamatergic neurons in the prefrontal cortex and other "extrastriatal" sites, as would be regulated by, for example, the degree of activation of A2A receptors outside the striatum by adenosine, are a major factor that regulates dopamine release in the prefrontal cortex. In some articles, a discussion of "a receptor in the striatum" can mean that the receptor is on an axon terminal, in the striatum, of a dopaminergic neuron whose cell body is not in the striatum but is in, for example, the midbrain, in the VTA. This type of discussion can just become absurdly complicated and can just get crazy, especially given the concentration-dependence of the effects of adenosine, etc. Cunha et al. (2008) discussed the fact that A2A receptors seem to be activated more by adenosine that is derived from extracellular ATP hydrolysis than adenosine that is derived from other sources (such as by the export from the cytosol, via equilibrative adenosine transporters, etc.). That's just one example of an obscure "mechanistic" explanation for the complexity of the adenosinergic or purinergic modulation of neuronal activity. In any case, those excitatory effects that adenosine, such as can be derived from SAM-e or oral adenosine monophosphate or triphosphate, can sometimes have on some glutamatergic neurons are one mechanism that could account for the acute increases in dopamine and noradrenaline release that have been shown to occur in animals in response to SAM-e administration [cited in Benelli et al., 1999: (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1566059&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/10401554)]. In a related vein, Eckeli et al. (2000), cited above, found that the apparent antidepressant effects of guanosine monophosphate in animals was partially dependent on serotonin biosynthesis or release. The antidepressant-like effect of guanosine monophosphate was mimicked by an NMDA receptor antagonist or fluoxetine, a serotonin reuptake inhibitor and prescription antidepressant. In that case, the general idea is essentially that a stressor is producing excessive glutamatergic activation and that the glutamatergic activity is indirectly disturbing the feedback inhibition of the firing rates, which increase in response to acute stress, of noradrenergic neurons in the locus ceruleus. Guanosine or NMDA receptor antagonism may attenuate this excessive glutamatergic activity, as implied by the results of Eckeli et al. (2000). There are more than a dozen articles showing that oral or intraperitoneal guanosine can exert anxiolytic (anti-anxiety) or anticonvulsant or neuroprotective effects that are at least partially a result of the attenuation of glutamate release or augmentation of synaptic glutamate reuptake by guanosine (http://scholar.google.com/scholar?num=100&hl=en&lr=&safe=off&cites=13842524336757775794). This seems confusing, but under conditions of chronic stress or neuronal injury, for example, decreasing glutamatergic transmission (such as by NMDA receptor antagonism) can actually increase dopamine release or, more precisely, increase the responsiveness of (postsynaptic) neurons receiving dopaminergic inputs to, for example, D1 dopamine receptor activation in response to dopamine release [see, for example, Peeters et al., 2002: (http://www.ncbi.nlm.nih.gov/pubmed/12213297); Deep et al., 1999: (http://www.ncbi.nlm.nih.gov/pubmed/10529725); Arai et al., 2003: (http://www.ncbi.nlm.nih.gov/pubmed/12711097); Konradi et al., 1996: (http://www.jneurosci.org/cgi/reprint/16/13/4231)(http://www.ncbi.nlm.nih.gov/pubmed/8753884?dopt=Abstract); Tokuyama et al., 2001: (http://www.ncbi.nlm.nih.gov/pubmed/11408088); Boyce-Rustay et al., 2006: (http://www.ncbi.nlm.nih.gov/pubmed/16482087)]. In the absence of stress (and even in the "presence of stress," up to a point), however, decreasing glutamatergic transmission (let's say to the same "degree" as in the presence of stress and at the same dosages of the same drug, for example) will tend, with some degree of predictability, to decrease dopamine release (many more references on this). In the former case, mild NMDA receptor antagonism, such as by amantadine (which also exerts numerous other effects that confound the discussion, but so be it), produces an excitatory effect on normal, burst-firing-associated dopaminergic transmission, but NMDA receptor antagonism in the latter case--antagonism that is stronger or that occurs in the absence of some factor that is producing an "abberant" or excessive increase in the firing rates of the neurons in question--will tend to inhibit dopaminergic (or noradrenergic) transmission. Here's an article that highlights some of the complexity with which adenosine can modulate the release of dopamine in response to NMDA receptor (a glutamate receptor) activation in the striatum [Quarta et al., 2004: (http://www.ncbi.nlm.nih.gov/pubmed/15525341)].
Again, my opinion is that many of the effects of SAM-e on the brain are mediated by adenosine, which SAM-e is metabolized into, and by the nucleotides derived from adenosine and its metabolites (including hypoxanthine and inosine and also some guanosine formed from inosine monophosphate, derived from adenosine nucleotides, etc.). I've discussed some of the evidence for this in past postings, but, for example, Renshaw et al. (2001) suggested that SAM-e may exert antidepressant effects, in part, via its metabolism into adenosine [cited here: (http://hardcorephysiologyfun.blogspot.com/2009/01/details-on-nucleotides-bioavailability.html)]. Additionally, the effects of exogenous adenosine or S-adenosylmethionine or inosine monophosphate, in combination with guanosine and other nucleotides, can produce increases in the phosphocreatine (PCr) to creatine (Cr) ratio (PCr/Cr ratio) that are reminiscent of the effect of exogenous SAM-e on the PCr/NTP ratio [see Silveri et al. (2003) and other articles cited and discussed here: (http://hardcorephysiologyfun.blogspot.com/2009/03/creatine-cr-phosphocreatine-pcr-and.html)]. In reference to the article by Silveri et al. (2003), it is noteworthy that NTP refers to the pool of beta-nucleotide triphosphates, and this "quantity" is thought to primarily reflect the pool of ATP that is measured, in a given part of the brain, by magnetic resonance spectroscopy (MRS). Also, some of the antiinflammatory effects of SAM-e can be antagonized by adenosine receptors and mimicked by the administration of adenosine, as a substitute for SAM-e [Song et al., 2005: (http://www.ncbi.nlm.nih.gov/pubmed/15843034)]. Song et al. (2005) also found that SAM-e elevated the pool of intracellular adenosine in cultured monocytes. Travagli et al. (1994) [Travagli et al., 1994: (http://www.ncbi.nlm.nih.gov/pubmed/7698179)] found that SAM-e reduced the firing rates of neurons in the vagal motor nucleus, and these effects were not only abolished by adenosine receptor antagonists but were shown to be "reversed" in the presence of those antagonists. SAM-e increased the firing rates of those neurons in the presence of the adenosine receptor antagonists, and Travagli et al. (1994) noted that increases in the availability of adenosine, produced from SAM-e via the hydrolysis of S-adenosylhomocysteine (SAH), had probably caused the changes in the neuronal firing rates. The attenuation of the SAM-e-induced increases in the firing rates of the neurons by SAH, in the presence of the adenosine receptor antagonists, might be explained by the inhibition of methyltransferases by SAH. Inhibiting methyltransferase enzymes in the presence of an excess of SAM-e would prevent much of the conversion of the excess SAM-e into SAH and then into adenosine. Alternatively, the excess SAH may itself have been converted into adenosine and thereby produced effects on adenosine receptors or purinergic receptors that were, as a result of the higher concentration of adenosine, in opposition to the effects that had been produced by the exogenous-SAM-e-derived adenosine (i.e. by that lower concentration). In either case, the results could conceivably be explained in terms of the effects of nucleotides or nucleosides derived from adenosine (i.e. inosine, hypoxanthine, xanthine, uric acid, or guanosine, etc.) or the effects of different concentrations of extracellular adenosine, derived from exogenous SAM-e or SAH, on different adenosine receptor subtypes or on other intracellular or extracellular adenosine binding sites on other proteins. There are lots of other articles that show similarly-comparable effects of adenosine and either SAM-e or 5'-methylthioadenosine (MTA), which is converted into adenosine [these are two that I found without any effort, but there are many others: Song et al., 2004: (http://www.ncbi.nlm.nih.gov/pubmed/15566950); Song et al., 2003: (http://www.ncbi.nlm.nih.gov/pubmed/12736147)]. I view the adenosine-receptor dependence of the anti-inflammatory effects of SAM-e, along with the elevation of intracellular adenosine by SAM-e (Song et al., 2005, cited above; Smolenski, 2000, cited above), as evidence that the adenosine nucleotide pool is being elevated and equilibrating with the extracellular pool, thereby causing increases in the activation of extracellular adenosine receptors. There are other interpretations, but that's just my opinion.
As far as the dosage considerations go, a number of articles have shown that adenosine and guanosine can exert effects in animal models at remarkably low dosages. Guanosine has been shown to exert neuroprotective and anticonvulsant effects at 7.5 mg/kg bw per day, given orally or intraperitoneally, in rats. That works out to something like ~111 mg per day, scaled to a dosage for a 70-kg human, and that's an extremely low dosage. I would think there might be some kind of issue with that dosage or the scaling factor (4.71) that I used, but essentially the same dosage (8 mg/kg bw/day, given intraperitoneally), was shown to promote restoration of functioning in animals following experimental spinal cord injuries [Jiang et al., 2008: (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=2072916&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/18404454)]. If one were to not scale the dosage and use 8 mg/kg bw per day in a 70-kg human, the dosage would be 560 mg/d. Even under those circumstances, that's a very low dosage, as purines go. That suggests that guanosine is fairly potent, particularly in comparison to inosine. The dosages of inosine used to promote recovery after spinal cord injuries, in animal models, are much higher. For example, Liu et al. (2006) [Liu et al., 2006: (http://www.ncbi.nlm.nih.gov/pubmed/16317421)] used inosine to prevent the ongoing degneration that occurs after a spinal cord injury in adult rats, and the dosage used was 75 mg/kg bw, given intraperitoneally, every eight hours. That's 225 mg/kg bw per day in rats. The fact that Jiang et al. (2008) found therapeutic effects of guanosine at 8 mg/kg bw/day is remarkable and shows, along with the many articles showing anticonvulsant and neuroprotective effects of oral or i.p. guanosine or guanosine monophosphate at 7.5 mg/kg bw/day, that, in my opinion, guanosine would be expected to produce meaningful effects on the brain at doses that would be compatible with human physiology. I say that because the dosage of inosine of 225 mg/kg bw/day (or the 100 mg/kg bw/day used in many other animal experiments showing anti-inflammatory effects of inosine, for example) scales to 3344 mg/day of inosine for a 70-kg human. If one looks at the trials using inosine to elevate uric acid in people with multiple sclerosis, one sees that that dose of inosine could produce hyperuricemia in many humans. Using that dose in the short term, such as after a spinal cord injury, would not be expected to produce hyperuricemia, and researchers would get larger effects on the brain by administering inosine intravenously or intraperitoneally than they would by administering it orally. But guanosine appears to exert many of its effects on the brain at remarkably low dosages.
One can, in my opinion, get a sense of the possible "dosage ranges" for guanosine monophosphate and adenosine monophosphate by looking at the dosages of S-adenosylmethionine and inosine that have been used in trials in humans. Guanosine monophosphate is available, evidently only in combination with other nucleotides, from a limited number of manufacturers [for example: (http://www.google.com/products?q=bluebonnet+nucleotide+complex+&hl=en)], but I'm not sure how much guanosine monophosphate is in that product. Again, I don't have any financial interest in any of these products or in anything I've discussed on this blog. The combinations of nucleotides that are used in research (http://scholar.google.com/scholar?q=dietary+nucleotide&hl=en&lr=) typically contain guanosine monophosphate and the other nucleoside monophosphates (a nucleotide, for the purposes of this discussion, is a nucleoside that's been phosphorylated in the 5'-position) in a either roughly 1:1:1:1 mass ratio or molar ratio. If the nucleotides and nucleosides were in a 1:1:1:1 mass ratio, each of those capsules would contain ~50 mg guanosine monophosphate and 10 mg guanosine and 15 mg guanine (and those same amounts for adenosine monophosphate, adenosine, and adenine). But I don't know if that's correct. One could call the manufacturer, and I'm sure they'd probably tell you what the composition is. But, from the standpoint of uric acid production, the elevation in uric acid is going to be different for every person. That's one reason it's necessary to have one's uric acid monitored by one's doctor, if one's going to take the full range of dosages for oral adenosine or guanosine monophosphates. One way to determine a dosage, with one's doctor, in the context of antidepressant medication, would be to look at the "percent adenosine" in S-adenosylmethionine and the dosage range of inosine used to treat multiple sclerosis. Less uric acid is made from adenosine than from inosine, in general, because adenosine is more efficiently salvaged. The ratio is something like 1.3/2.5, meaning the ratio of the AUC or Cmax (I forget which) for the serum uric acid response to oral adenosine to the uric acid response to oral inosine is about 0.52 or 0.55 or something. SAM-e is ~67.1 percent adenosine (the ratio of the molar mass of adenosine to the molar mass of SAM-e is 267.241/398.44), and the maximum dosage of SAM-e that I've seen used to treat depression is 3600 mg/day [Di Rocco et al., 2000: (http://www.ncbi.nlm.nih.gov/pubmed/11104210)]. If one takes 67.1 percent of that dosage, one gets 2415 mg/day of adenosine (or adenosine + guanosine). That works out to about 16 of those nucleotide capsules per day, assuming about half of the 300 mg is adenosine + guanosine (or purine bases, which are nucleic acids that can would probably be converted into uric acid). I'm not suggesting anyone would want to take any of these sorts of full dosage ranges without talking to one's doctor, because elevations in uric acid are not the only potential side effect. It's conceivable that adenosine or guanosine could produce vasodilation or reduce blood pressure and interact with sedatives or blood pressure medications or any number of medications and produce serious side effects. It's actually the case that adenosine monophosphate and guanosine monophosphate are 70-some percent adenosine and guanosine (the rest is phosphate), and that would affect the "calculation," crude as it is (that would mean the dose would be a little bit higher to be equivalent to the 2415 mg of adenosine that can be derived from 3,600 mg of SAM-e). Adenosine triphosphate disodium is more widely available than guanosine [(http://hardcorephysiologyfun.blogspot.com/2009/02/brief-note-on-purines.html); (http://hardcorephysiologyfun.blogspot.com/2009/01/details-on-nucleotides-bioavailability.html)].
One way to minimize the production of uric acid from adenosine monophosphate and guanosine monophosphate would be to dissolve them in water and take them on an empty stomach (which means before breakfast, not between meals), but I'm not sure if that would meaningfully increase the bioavailabilities of those nucleotides. There's increasingly been a recognition that the bioavailability of physiological substrates can depend on seemingly-trivial factors, such as the rate of dissolution, and this has been discussed in the context of the bioavailability of creatine. When researchers [Deldicque et al., 2008: (http://www.ncbi.nlm.nih.gov/pubmed/17851680)] gave 2 grams of creatine monohydrate dissolved in aqueous solution (water) or in a protein bar or in some kind of complex with "beta-glucan" (I have no idea what the rationale for testing that is), the Cmax for creatine in water was 299 uM (up from a baseline of 10 uM), and that was higher than the Cmax for creatine from the protein bar (237 uM) or the beta-glucan source (174 uM). I've seen multiple articles in which the researchers have dissolved creatine monohydrate in water, ostensibly with the aim of improving the availability of creatine for entry into the brain. I don't know if that type of factor would really matter all that much, and it might be cumbersome to do. But the same concept has been discussed in the context of orally-administered nucleotides. The activity of xanthine oxidase ("xanthine oxidoreductase") is thought to be lower in the fasting state, and this means that less of a dose of adenosine or guanosine may be converted, following the conversion into hypoxanthine or xanthine, into uric acid, in the fasting state, in the short term [see references 75-77, cited on p. 63 of Carver and Walker, 1995: (http://cat.inist.fr/?aModele=afficheN&cpsidt=3477199)]. Note that that paper by Carver and Walker (1995) is not especially up-to-date, as far as information on nucleotide bioavailability is concerned.
I'll put the discussion of mechanisms in a separate posting. This is too long to put in one posting.
Saturday, March 28, 2009
Creatine (Cr), Phosphocreatine (PCr), and Exogenous Purines in ADP Repletion and Recycling: Significance of the PCr/Cr and PCr/NTP Ratios
This article [Ceddia and Sweeney, 2004: (http://jp.physoc.org/cgi/reprint/555/2/409)(http://www.ncbi.nlm.nih.gov/pubmed/14724211?dopt=Abstract)] helps to shed light on the type of mechanism that could account for the cases of rhabdomyolysis in response to excessive creatine supplementation. The same mechanism, namely an increase in the phosphorylation, meaning activation, of adenosine monophosphate-activated protein kinase (AMPK) in response to exogenous creatine, could produce a U-shaped, or biphasic, dose-response to creatine supplementation in the context of neurodegenerative or psychiatric disorders. Ceddia and Sweeney (2004) found that creatine produced a decrease in the phosphocreatine to creatine (PCr/Cr) ratio but simultaneously increased the rate of glucose oxidation and decreased the rate of lactate formation that was occurring in the absence of insulin. The increase in the PCr/Cr ratio was suggested to be the "cause" of the AMPK activation, and the authors noted that the creatine pool increased more than the phosphocreatine pool (the intracellular Cr concentration was roughly twice that of the intracellular PCr concentration). There are some potential strategies that could minimize this type of effect [which is not entirely undesirable and is not harmful in and of itself (AMPK activation), given that resistance exercise produces AMPK activation in skeletal muscle cells, etc.], in my opinion, but my main point is that one would want to discuss these safety issues with one's doctor before taking creatine. Even under a doctor's supervision, it would be worthwhile to increase the dose slowly and consider limiting the dosage. Rhabdomyolysis can be life-threatening and can cause major kidney damage or kidney failure and death. I'm not commenting on any specific cases or saying that creatine routinely produces rhabdomyolysis, because I don't think it does. Most of the reports of associations discuss dosages of 20 grams/d, and that's a really high dose [(http://scholar.google.com/scholar?num=100&hl=en&lr=&q=creatine+supplement+rhabdomyolysis+OR+renal+OR+kidney); (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=creatine+monohydrate+rhabdomyolysis+OR+renal+OR+kidney)].
The general idea is that this type of effect (AMPK activation) would have the potential, in my opinion, to be problematic in the context of an existing deficit in mitochondrial energy metabolism. AMPK activation doesn't always lead to neat, predictable increases in mitochondrial mass to compensate for the cellular energy deficit that produced the AMPK activation in the first place. If one is doing resistance exercise and has adapted to the training, a combination of factors tend to produce fairly reliable "trophic" effects on mitochondrial functioning in the muscle cells. But in a neurodegenerative disease or a neurological or neurodegenerative condition that is associated with inflammation or oxidative stress, creatine could simply accelerate the turnover of adenine nucleotides or inappropriately increase the rate of glucose oxidation in some cells (perhaps at the expense of other cells, etc.). If the creatine kinase enzymes are being inhibited in neurons or astrocytes in the brain, such as in response to increases in peroxynitrite formation or in nitrosative or oxidative stress in general, the magnitude of the AMPK activation could be increased. Simultaneously, the cellular response to AMPK activation could be deranged and, in my opinion, lead to problematic effects, such as maladaptive mitochondrial proliferation [discussed here: (http://hardcorephysiologyfun.blogspot.com/2009/01/folate-cobalamin-iron-accumulation-and.html); Sebastiani et al., 2007: (http://content.onlinejacc.org/cgi/content/full/50/14/1362) (http://www.ncbi.nlm.nih.gov/pubmed/17903636)].
When one looks at the research on rhabdomyolysis, one sees that rhabdomyolysis tends to be associated with strong oxidative stress in combination with some existing deficit in oxidative metabolism. For example, reperfusion-induced rhabdomyolysis, such as can occur after some surgeries restoring blood flow to the legs, could be explained as being a result of major oxidative damage, an effect that would itself impair mitochondrial functioning, and the mitochondrial impairment that TNF-alpha and other pro-inflammatory cytokines are known to produce, via the activation of various mitogen-activated protein kinase signalling cascades. The same type of combination could account for exertional rhabdomyolysis in a person who exercises during a viral illness, etc. The point is that, although creatine can obviously produce beneficial effects on mitochondrial functioning in the long term, it is worthwhile to not assume that higher doses will produce "more" or better effects. Roitman et al. (2007) [cited and discussed here: (http://hardcorephysiologyfun.blogspot.com/2009/03/arginine-agmatine-and-nitric-oxide-in.html)] found that creatine at a dose of 5 grams/day was less effective as a strategy for augmenting conventional antidepressants, in some of the people in the trial, than creatine at a dose of 3 grams/d was. There's also research showing a U-shaped dose response to neuroprotection by creatine, in an animal model of Huntington's disease (one of the articles using 3-nitropropionic acid-induced neurotoxicity, I think).
The authors discuss research showing that the activation of AMPK can be increased in response to a decrease in the PCr/Cr ratio and not just in response to a decrease in the ATP/AMP ratio. The AMPK enzyme is a heterotrimer (three subunits), and there are multiple isoforms of each subunit. AMPK is regarded as a "fuel-sensing" or "glucose-sensing" enzyme that's generally activated by a decrease in ATP or, more precisely, a decrease in the ATP/AMP ratio, but other factors can activate AMPK independently of a decrease in the ATP/AMP ratio.
I don't have time to cite all the articles, but there's a whole series of articles showing that exogenous nucleotides [especially adenosine monophosphate (or intravenous inosine monophosphate, which would be expected, in my opinion, to produce effects that would be roughly more comparable to the effects of oral adenosine triphosphate or monophosphate than to the effects of oral inosine) and guanosine monophosphate, usually given in combination with some uridine or cytidine] can increase the PCr/Cr ratio or preserve the PCr/Cr ratio (and the overall PCr + Cr pool) during ischemia [(some of them are here: (http://scholar.google.com/scholar?num=100&hl=en&lr=&cites=15081505722792949318)]. Also, there's obviously a lot of research showing creatine supplementation can help to preserve the adenylate pool (adenine nucleotides, meaning AMP, ADP, & ATP) and also the adenylate charge, which is ([ATP] + 0.5 [ADP])/([ATP] + [ADP] + [AMP]) and essentially means the more high-energy adenine nucleotide pool (ATP and ADP), in various cell types [Ronca-Testoni et al., 1985: (http://www.ncbi.nlm.nih.gov/pubmed/4087306)], in vivo, during ischemia or during other conditions. I tend to think that exogenous adenosine monophosphate/triphosphate and guanosine monophosphate could produce some synergistic effects with low-dose (1-5 grams/d) creatine, under a doctor's supervision, in the context of cognitive functioning or depression or other psychiatric conditions, but that's just my opinion. Past a certain point, however, it is conceivable that high doses of creatine could derange purine metabolism, such as by increasing the export of purines from cells. Nomura et al. (2003) [Nomura et al., 2003: (http://www.nature.com/bjp/journal/v139/n4/pdf/0705316a.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/12812994)], for example, found that exogenous creatine augmented the export of ATP from endothelial cells. That effect was shown to produce anti-inflammatory effects in cultured cells (Nomura et al., 2003) but could become undesirable in other contexts, in my opinion, such as in the context of some existing deficit in energy metabolism that is impairing purine salvage, etc.
Satoh et al. (1993) [Satoh et al., 1993: (http://www.ncbi.nlm.nih.gov/pubmed/8173706)] discussed a really interesting mechanism to explain the increases in phosphocreatine that can occur in cells in response to exogenous purines (and pyrimidines, usually given in combination with purines in these articles). The authors' idea is basically that the ADP pool is increased and that the exogenous adenosine doesn't have to increase the ATP levels to produce an increase in the PCr/Cr ratio (Satoh et al., 1993). I think there's something to that mechanistic explanation, given that the most fundamental effect of creatine is essentially to cause an increase in the transport of ADP in and out of the mitochondria, mediated by the combined activities of mitochondrial and cytosolic creatine kinase (CK) enzymes, and thereby increase "ADP recycling" [Meyer et al., 2006: (http://www.jbc.org/cgi/reprint/281/49/37361)(http://www.ncbi.nlm.nih.gov/pubmed/17028195?dopt=Abstract)]. Ceddia and Sweeney (2004) (cited above) discussed the fact that creatine is likely to have increased the rate of glucose oxidation, in their experiments, by essentially increasing ADP transport into the mitochondria. That's an oversimplification, but that could explain the increase in the PCr/NTP levels, measured by MRS, found in the brains of humans given supplemental S-adenosylmethionine (SAM-e) [Silveri et al., 2003: (http://www.ncbi.nlm.nih.gov/pubmed/14550683)]. NTP refers to the pool of beta-nucleotide triphosphates and is mostly thought to reflect the size of the ATP pool. In my opinion, that effect on phosphocreatine by SAM-e was not primarily produced by an increase in guanidinoacetate N-methyltransferase activity but was the result of an increase in the adenylate and purine nucleotide pools in neurons and astroyctes, etc. Other authors have discussed the possibility that the effects of SAM-e on depression are mediated by increases in adenosine (or its "metabolites," etc.), and I've cited that type of thing in past postings. I think that's the case, and, in my opinion, the use of adenosine monophosphate and guanosine monophosphate would be superior to the use of most of the presently-available formulations of SAM-e for any purpose that SAM-e has been used for. Another possibility is that adenosine could act as a mild anticonvulsant and limit the PCr depletion that can result from excessive, excitatory glutamatergic transmission in the brain.
One strategy for maintaining creatine kinase activity would, in my opinion, be the maintenance of the serum uric acid (urate) in the high-normal range. Urate scavenges peroxynitrite and can protect against the cellular energy deficits in myocardial tissue, following ischemia, and the effect may be due to the protection by urate against the nitrosylative inactivation of CK by peroxynitrite (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=peroxynitrite+%22creatine+kinase%22) or nitric oxide, as discussed by Xie et al. (1998) [Xie et al., 1998: (http://circres.ahajournals.org/cgi/reprint/82/8/891)(http://www.ncbi.nlm.nih.gov/pubmed/9576108?dopt=Abstract)]. Relatively low doses of adenosine or guanosine would accomplish that elevation of urate. That's a reason one would want to measure one's uric acid levels, under the supervision of a doctor. Methylcobalamin can lower methylmalonic acid (MMA) levels, and MMA and its associated "metabolites" (methylcitric acid, etc.) can produce inhibition of creatine kinase activity (http://scholar.google.com/scholar?q=methylmalonic+%22creatine+kinase%22&hl=en&lr=). Magnesium is a major factor that influences both the activity and Keq of creatine kinase enzymes, in a manner that tends to increase the PCr/Cr ratio at equilibrium (that ratio is a reflection of the Keq values for the reversible creatine kinase enzymatic reactions) (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=magnesium+%22creatine+kinase%22).
The general idea is that this type of effect (AMPK activation) would have the potential, in my opinion, to be problematic in the context of an existing deficit in mitochondrial energy metabolism. AMPK activation doesn't always lead to neat, predictable increases in mitochondrial mass to compensate for the cellular energy deficit that produced the AMPK activation in the first place. If one is doing resistance exercise and has adapted to the training, a combination of factors tend to produce fairly reliable "trophic" effects on mitochondrial functioning in the muscle cells. But in a neurodegenerative disease or a neurological or neurodegenerative condition that is associated with inflammation or oxidative stress, creatine could simply accelerate the turnover of adenine nucleotides or inappropriately increase the rate of glucose oxidation in some cells (perhaps at the expense of other cells, etc.). If the creatine kinase enzymes are being inhibited in neurons or astrocytes in the brain, such as in response to increases in peroxynitrite formation or in nitrosative or oxidative stress in general, the magnitude of the AMPK activation could be increased. Simultaneously, the cellular response to AMPK activation could be deranged and, in my opinion, lead to problematic effects, such as maladaptive mitochondrial proliferation [discussed here: (http://hardcorephysiologyfun.blogspot.com/2009/01/folate-cobalamin-iron-accumulation-and.html); Sebastiani et al., 2007: (http://content.onlinejacc.org/cgi/content/full/50/14/1362) (http://www.ncbi.nlm.nih.gov/pubmed/17903636)].
When one looks at the research on rhabdomyolysis, one sees that rhabdomyolysis tends to be associated with strong oxidative stress in combination with some existing deficit in oxidative metabolism. For example, reperfusion-induced rhabdomyolysis, such as can occur after some surgeries restoring blood flow to the legs, could be explained as being a result of major oxidative damage, an effect that would itself impair mitochondrial functioning, and the mitochondrial impairment that TNF-alpha and other pro-inflammatory cytokines are known to produce, via the activation of various mitogen-activated protein kinase signalling cascades. The same type of combination could account for exertional rhabdomyolysis in a person who exercises during a viral illness, etc. The point is that, although creatine can obviously produce beneficial effects on mitochondrial functioning in the long term, it is worthwhile to not assume that higher doses will produce "more" or better effects. Roitman et al. (2007) [cited and discussed here: (http://hardcorephysiologyfun.blogspot.com/2009/03/arginine-agmatine-and-nitric-oxide-in.html)] found that creatine at a dose of 5 grams/day was less effective as a strategy for augmenting conventional antidepressants, in some of the people in the trial, than creatine at a dose of 3 grams/d was. There's also research showing a U-shaped dose response to neuroprotection by creatine, in an animal model of Huntington's disease (one of the articles using 3-nitropropionic acid-induced neurotoxicity, I think).
The authors discuss research showing that the activation of AMPK can be increased in response to a decrease in the PCr/Cr ratio and not just in response to a decrease in the ATP/AMP ratio. The AMPK enzyme is a heterotrimer (three subunits), and there are multiple isoforms of each subunit. AMPK is regarded as a "fuel-sensing" or "glucose-sensing" enzyme that's generally activated by a decrease in ATP or, more precisely, a decrease in the ATP/AMP ratio, but other factors can activate AMPK independently of a decrease in the ATP/AMP ratio.
I don't have time to cite all the articles, but there's a whole series of articles showing that exogenous nucleotides [especially adenosine monophosphate (or intravenous inosine monophosphate, which would be expected, in my opinion, to produce effects that would be roughly more comparable to the effects of oral adenosine triphosphate or monophosphate than to the effects of oral inosine) and guanosine monophosphate, usually given in combination with some uridine or cytidine] can increase the PCr/Cr ratio or preserve the PCr/Cr ratio (and the overall PCr + Cr pool) during ischemia [(some of them are here: (http://scholar.google.com/scholar?num=100&hl=en&lr=&cites=15081505722792949318)]. Also, there's obviously a lot of research showing creatine supplementation can help to preserve the adenylate pool (adenine nucleotides, meaning AMP, ADP, & ATP) and also the adenylate charge, which is ([ATP] + 0.5 [ADP])/([ATP] + [ADP] + [AMP]) and essentially means the more high-energy adenine nucleotide pool (ATP and ADP), in various cell types [Ronca-Testoni et al., 1985: (http://www.ncbi.nlm.nih.gov/pubmed/4087306)], in vivo, during ischemia or during other conditions. I tend to think that exogenous adenosine monophosphate/triphosphate and guanosine monophosphate could produce some synergistic effects with low-dose (1-5 grams/d) creatine, under a doctor's supervision, in the context of cognitive functioning or depression or other psychiatric conditions, but that's just my opinion. Past a certain point, however, it is conceivable that high doses of creatine could derange purine metabolism, such as by increasing the export of purines from cells. Nomura et al. (2003) [Nomura et al., 2003: (http://www.nature.com/bjp/journal/v139/n4/pdf/0705316a.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/12812994)], for example, found that exogenous creatine augmented the export of ATP from endothelial cells. That effect was shown to produce anti-inflammatory effects in cultured cells (Nomura et al., 2003) but could become undesirable in other contexts, in my opinion, such as in the context of some existing deficit in energy metabolism that is impairing purine salvage, etc.
Satoh et al. (1993) [Satoh et al., 1993: (http://www.ncbi.nlm.nih.gov/pubmed/8173706)] discussed a really interesting mechanism to explain the increases in phosphocreatine that can occur in cells in response to exogenous purines (and pyrimidines, usually given in combination with purines in these articles). The authors' idea is basically that the ADP pool is increased and that the exogenous adenosine doesn't have to increase the ATP levels to produce an increase in the PCr/Cr ratio (Satoh et al., 1993). I think there's something to that mechanistic explanation, given that the most fundamental effect of creatine is essentially to cause an increase in the transport of ADP in and out of the mitochondria, mediated by the combined activities of mitochondrial and cytosolic creatine kinase (CK) enzymes, and thereby increase "ADP recycling" [Meyer et al., 2006: (http://www.jbc.org/cgi/reprint/281/49/37361)(http://www.ncbi.nlm.nih.gov/pubmed/17028195?dopt=Abstract)]. Ceddia and Sweeney (2004) (cited above) discussed the fact that creatine is likely to have increased the rate of glucose oxidation, in their experiments, by essentially increasing ADP transport into the mitochondria. That's an oversimplification, but that could explain the increase in the PCr/NTP levels, measured by MRS, found in the brains of humans given supplemental S-adenosylmethionine (SAM-e) [Silveri et al., 2003: (http://www.ncbi.nlm.nih.gov/pubmed/14550683)]. NTP refers to the pool of beta-nucleotide triphosphates and is mostly thought to reflect the size of the ATP pool. In my opinion, that effect on phosphocreatine by SAM-e was not primarily produced by an increase in guanidinoacetate N-methyltransferase activity but was the result of an increase in the adenylate and purine nucleotide pools in neurons and astroyctes, etc. Other authors have discussed the possibility that the effects of SAM-e on depression are mediated by increases in adenosine (or its "metabolites," etc.), and I've cited that type of thing in past postings. I think that's the case, and, in my opinion, the use of adenosine monophosphate and guanosine monophosphate would be superior to the use of most of the presently-available formulations of SAM-e for any purpose that SAM-e has been used for. Another possibility is that adenosine could act as a mild anticonvulsant and limit the PCr depletion that can result from excessive, excitatory glutamatergic transmission in the brain.
One strategy for maintaining creatine kinase activity would, in my opinion, be the maintenance of the serum uric acid (urate) in the high-normal range. Urate scavenges peroxynitrite and can protect against the cellular energy deficits in myocardial tissue, following ischemia, and the effect may be due to the protection by urate against the nitrosylative inactivation of CK by peroxynitrite (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=peroxynitrite+%22creatine+kinase%22) or nitric oxide, as discussed by Xie et al. (1998) [Xie et al., 1998: (http://circres.ahajournals.org/cgi/reprint/82/8/891)(http://www.ncbi.nlm.nih.gov/pubmed/9576108?dopt=Abstract)]. Relatively low doses of adenosine or guanosine would accomplish that elevation of urate. That's a reason one would want to measure one's uric acid levels, under the supervision of a doctor. Methylcobalamin can lower methylmalonic acid (MMA) levels, and MMA and its associated "metabolites" (methylcitric acid, etc.) can produce inhibition of creatine kinase activity (http://scholar.google.com/scholar?q=methylmalonic+%22creatine+kinase%22&hl=en&lr=). Magnesium is a major factor that influences both the activity and Keq of creatine kinase enzymes, in a manner that tends to increase the PCr/Cr ratio at equilibrium (that ratio is a reflection of the Keq values for the reversible creatine kinase enzymatic reactions) (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=magnesium+%22creatine+kinase%22).
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