This is one of the other articles that includes a discussion of the mechanisms by which fructose acutely increases plasma uridine and also urinary uridine excretion [Yamamoto et al., 1997: (http://www.ncbi.nlm.nih.gov/pubmed/9160822)], but Yamamoto et al. (1997) didn't show the decreases in plasma uridine, to levels below the baseline concentrations, that occur after the increases (see a recent posting). Yamamoto et al. (1997) also didn't address the mechanism by which the fructose-induced inorganic phosphate (Pi) sequestration leads to purine degradation, but a key mechanism is that the decrease in intracellular Pi disinhibits adenosine monophosphate (AMP) deaminase. AMP deaminase is normally inhibited by Pi. Yamamoto et al. (1997) cited a lot of interesting research, however. They suggested that the ethanol-induced (and, by less direct mechanisms, fructose-induced) increases in hypoxanthine and xanthine might have resulted from the elevations in the cytosolic NADH/NAD+ ratio that results from the metabolism of ethanol to acetaldehyde, given that NADH inhibits xanthine dehydrogenase activity. Fructose could also produce that effect, albeit to a lesser extent than ethanol. In addition to the ATP depletion that ultimately can occur through the disinhibition of AMP deaminase, resulting from fructose-induced Pi sequestration, Yamamoto et al. (1997) referred to the direct consumption of ATP in the fructokinase reaction that forms fructose-1-phosphate and thereby sequesters Pi [see also Phillips and Davies, 1985: (http://jp.physoc.org/content/520/3/909.full)(http://www.ncbi.nlm.nih.gov/pubmed/2992452)]. It's worth noting that fructose also depletes guanosine triphosphate (and guanosine nucleotides in general, as shown in multiple articles), partly because fructokinase activity is apparently GTP-dependent (Phillips and Davies, 1985). Fantastic. It depletes all the major nucleotide pools. Cytidine depletion would also be expected to occur (I'll bet there's some research showing that, too), given that cytidine is formed from uridine. But the point I was going to make is that changes in intracellular Pi could regulate xanthine dehydrogenase activity by buffering the intracellular pH, given that increases in the intracellular pH tend to activate phosphofructokinase and glycolytic activity overall. That increase in glycolysis would then increase the NADH/NAD+ ratio and reduce xanthine dehydrogenase activity, and that could conceivably allow for more salvage of hypoxanthine (and even xanthine, which can be salvaged to a minimal extent by a two-enzyme pathway). Yamamoto et al. (1997) cited research showing that lactate can decrease the rate of urinary uric acid excretion but apparently doesn't reduce the excretion of hypoxanthine or xanthine [the oxypurines that Yamamoto et al. (1997) are referring to]. Does Pi repletion increase or decrease ischemia-induced glycolytic activity? Pi repletion generally does increase the activities of glycolytic enzymes, in many of the articles I've seen, but it could also reduce the kinds of wild fluctuations in the intracellular pH that can occur during ischemia. The Pi-induced increases in glycolytic activity by allosteric mechanisms could increase the cytosolic NADH/NAD+ ratio [Zhou et al., 2005: (http://jp.physoc.org/content/569/3/925.full.pdf+html)(http://www.ncbi.nlm.nih.gov/pubmed/16223766?dopt=Abstract)] and inhibit xanthine dehydrogenase activity (meaning that, from a simplistic standpoint, that effect could decrease uric acid formation and enhance purine salvage, conceivably), and, in the absence of a high intake of a phosphate salt displaying an abnormal ratio of monobasic to dibasic orthophosphate (orthophosphate refers to [HPO4(2-) + H2PO4(-) + the less-than-1-% contribution of PO4(3-)]), Pi repletion can produce an alkalinizing effect that could also activate glycolysis and further reduce xanthine dehydrogenase activity. But it could also exert more of a neutral effect. Those are just speculative thoughts.
For that matter, I wonder if the alkalotic effects of excesses of Pi might abolish or decrease the mitochondrial proton gradient under some circumstances, by mimicking the effects of uncouplers. Pi could conceivably stimulate respiration by that mechanism [that commonly occurs as a compensatory response (http://scholar.google.com/scholar?hl=en&q=stimulate+uncoupler+mitochondrial+respiration)], and that could explain those articles I cited, in a past posting, showing that Pi can increase the postprandial metabolic rate in humans, etc. That could conceivably account for some of its supposed psychiatric or psychoactive effects, and the "pseudodepression" and other effects of Pi depletion could be due to the poor "regulation" of the mitochondrial membrane potential. There are all sorts of articles showing that the stimulation of respiration is associated with phosphate influx into mitochondria, and phosphate influx interacts with ADP-stimulated respiration, etc. The point is that the effects of different concentrations of intracellular or intramitochondrial Pi on respiration could conceivably be either "bad" or "good," depending on the way you look at the effects.
It would be interesting to see some in vivo research on the effects of Pi depletion or repletion on the exercise-induced loss of purine nucleotides, for example, because it could be a complex set of effects. It's interesting that Hellsten et al. (1999) [Hellsten et al., 1999: (http://jp.physoc.org/content/520/3/909.full)(http://www.ncbi.nlm.nih.gov/pubmed/10545153?dopt=Abstract)] argued that the initial effect of exercise had been to increase Pi availability, thereby inhibiting AMP deaminase activity, but that the decreases in intracellular pH that had subsequently occurred had activated AMP deaminase activity. It's interesting that an increase in the inhibition of AMP deaminase by Pi would tend to lead to a relative increase in adenosine availability, and some of that adenosine would presumably serve to increase blood flow to the exercising muscles. I wonder if that increase could lead to a greater loss of adenosine, however, or if the Pi-mediated inhibition of AMP deaminase activity (as in the endothelial cells in which much of the adenosine deaminase-mediated deamination of interstitial-fluid adenosine occurs) would mean that more adenosine could be released and then also salvaged. The intracellular and extracellular adenosine concentrations are not usually very different, and there's a slight, inwardly-directed, transmembrane adenosine gradient. Usually, one thinks of adenosine release as being a unidirectional process that's "coupled" to an increase in the degradation, by adenosine deaminase in endothelial cells, of the adenosine to inosine and hypoxanthine. But, presumably, that's not always going to be the case. It's interesting that uncouplers are used to increase extracellular adenosine concentrations [see the reference to "respiratory uncouplers" on the first page of Rubio et al., 1972: (http://www.ncbi.nlm.nih.gov/pubmed/5022662)], and my overall point is that excessive concentrations of intracellular Pi, to the extent that they are achievable, could conceivably have some adverse effects that would go beyond the well-known increases in the risk of calcification, etc.
Showing posts with label Pyrimidine Nucleotides. Show all posts
Showing posts with label Pyrimidine Nucleotides. Show all posts
Saturday, September 12, 2009
Thursday, September 10, 2009
Depletion of Intracellular Uridine in Response to Intracellular Phosphate Depletion: Potential Relevance to mtDNA & Nuclear DNA Turnover and Repair
In this article [Makras et al., 2008: (http://www.ncbi.nlm.nih.gov/pubmed/18252791)], Makras et al. (2008) described a person who had X-linked hypophosphatemic rickets (XLHR), a genetic disorder that impairs the reabsorption of phosphate, from the tubular fluid, in the proximal tubules, and in whom roughly seven years of phosphate supplementation was ultimately required to completely ameliorate his myopathy (muscle weakness, etc.). The authors noted the mysterious quality of the myopathy and their finding that the severity of the myopathy had generally been independent of the person's serum phosphate levels. The authors also noted that the myopathy had been worsened during periods of vitamin D intoxication. I'm not sure if they're talking about calcitriol or vitamin D, but it probably doesn't matter, to some extent. Hypercalcemia could conceivably result from supplementation with either vitamin D (at the high doses used in patients with XLHR) or calcitriol and could cause excessive calcium influx into myocytes, thereby impairing mitochondrial ATP formation, or cause hypercoagulability, etc.
Although the authors wrote that vitamin D usually causes rapidly-emerging improvements in muscle weakness in people who do not have inherited mutations that affect phosphate homeostasis, as in XLHR, it's conceivable to me that the myopathy could have resulted from mitochondrial dysfunction, perhaps as a result of acquired mitochondrial DNA (mtDNA) mutations, as a consequence of the phosphate depletion. The fact that the degree of muscle weakness was independent of the serum phosphate is not surprising, and the intracellular phosphate levels are known to frequently, if not generally, be independent of steady-state serum phosphate levels in normal humans given low dosages of supplemental phosphate. Given that intracellular phosphate depletion is known to deplete ATP and purine nucleotide pools and that the depletion of the pools of purine deoxyribonucleotides can impair mtDNA replication (see past postings), it's conceivable that intracellular phosphate depletion could impair DNA repair and lead to a gradual accumulation of mtDNA or even nuclear DNA mutations. It's worthwhile to note that the maintenance of an adequate pool of each of the major intracellular purine nucleotides is a prerequisite for the maintenance of pyrimidine salvage. I think some of that has been shown in the context of fructose-induced hepatic ATP depletion. I think researchers have shown that fructose can deplete uridine from the liver and transiently elevate plasma uridine, as one might expect in response to fructose loading. Here are some references on that [see page 33 of the chapter of the book by Davies et al., 1998, who found that the plasma uridine levels increased soon after fructose administration in humans and then decreased a lot by 4 hours after a meal; sounds fantastic: (http://scholar.google.com/scholar?q=fructose+uridine+plasma+OR+serum&hl=en)].
Thus, intracellular phosphate depletion could conceivably contribute to the development of mutations in nuclear DNA and to the development of some of those more severe myopathies or intractable disease states, such as chronic fatigue syndrome, by leading to a depletion of both purines and pyrimidines. That's just my opinion, however. It's noteworthy that DNA repair consumes a lot of ATP, and some authors have suggested, as I noted in my old folic acid paper (see past posting), that the depletion of intracellular total folates might cause apoptotic cell death in neurons by "DNA-repair-associated" ATP depletion. They meant that there would be a futile cycle of DNA damage, in response to folate depletion and increases in the dUMP/dTMP ratio, and DNA repair and that the DNA repair would ultimately consume so much ATP as to lead to apoptotic cell death, such as in response to ischemic episodes or strokes that can cause a lot of DNA damage. Davies et al. (1998) argued that fructose-induced phosphate depletion in the liver had caused both the purine depletion, as evidenced by the elevations in serum uric acid, and the uridine export from the liver. I'm not suggesting that more is always going to be better, and those articles on the overlapping mechanisms governing the efflux of uric acid and inorganic phosphate, as discussed in recent postings, suggest that the metabolic cost or competitive inhibitory effects of excesses of intracellular inorganic phosphate could become significant, past a certain point, and derange the transport of organic anions other than uric acid, etc. Although the research suggests that a lot of phosphate would be required to create that type of state, it's worthwhile to discuss these things with one's doctor.
My view is that the data on the dosages of phosphate used in people with XLHR (and in other genetic disorders that reduce phosphate reabsorption) is relevant to normal humans, with regard to the risk of nephrocalcinosis, but I can think of a number of possible objections to that view. The first would be that, in people with XLHR, the rate of phosphate (Pi) reabsorption would be lower than it would in normal people and that that would decrease the risk, in comparison to normal people, of intracellular calcium phosphate precipitation. I should mention that, in that long review on nephrocalcinosis that I recently discussed, the author noted that calcification can occur either extracellularly (and "luminally" or intraluminally), on the luminal membranes of the proximal or distal tubule cells or in the interstices of the tight junctions, or intracellularly, in the cells of the renal tubules. Thus, one could argue that normal people would have the same risk of intraluminal calcification as people who have XLHR would but that normal people would have a higher risk of intracellular calcification, in response to a given dosage of supplemental phosphate, as people who have XLHR would. In normal people, however, the proximal tubules are able to vary the percent reabsorption to between something like 80 and 99 percent, and that means there would be a lot of potential for the proximal tubules to increase the urinary excretion of phosphate in response to some dosage of supplemental Pi. I just think the risks are basically similar for normal people as they are for people who have XLHR.
The reason I'm focusing on the reabsorption is that XLHR doesn't affect the glomerular filtration of serum phosphate, except to the extent that the cells of people with XLHR might be more "hungry" for phosphate and might clear the serum phosphate, from a dosage of phosphate, more rapidly than a normal person's cells might (thereby producing an indirect decrease in the amounts of phosphate filtered per unit time). In other words, the acute elevations in serum phosphate could conceivably be larger in normal people than in people who have XLHR. But that presupposes that a person has no capacity to tell if some change in his or her phosphate to calcium intake ratio, for example, is producing any benefit. If there's no obvious benefit, presumably there wouldn't be an impetus to continue taking any reasonable amount of phosphate, with the approval of one's doctor. It would also, obviously, be important to spread the dosage out across the day as much as possible and to consider limiting any dosage of supplemental vitamin D to 2000-4000 IU or less, given that hypercalciuria is thought to be a major factor that can increase the risk of nephrocalcinosis. Some authors have suggested splitting the total daily dosage of phosphate, in people who have XLHR, into 8 dosages, spread out across the day, instead of the usual practice of splitting the dosage into 4-5 increments. Another objection I can think of would be that the PHEX protein or the Na(+)/Pi cotransporter might be expressed in myocytes or myogenic satellite cells or some other extrarenal cell type. That could mean that mtDNA replication or some other Pi-sensitive metabolic process would be specifically affected in the muscles and would not be likely to show up in normal people. But I don't see how a pure and severe case of Fanconi's syndrome couldn't produce the same kinds of long-term problems in postmitotic cell types as something like XLHR can. It probably wouldn't take 7 years to treat the problem in a normal person, but I just think that there's a need to think of this type of thing with the long view in mind. It's necessary for someone to do long-term safety research using supplemental phosphate in normal people and to use reasonable amounts of dietary calcium, etc. Or someone could do that type of research in people who have chronic fatigue syndrome. I don't know what the best approach would be. One could argue that reasonable and low dosages of phosphate would improve both purine and pyrimidine salvage and could help limit something like the age-associated reductions in mtDNA copy number in different cell types. These are just my off-the-cuff thoughts, but I think the notion that 7 days of "phosphate loading" is enough to make anyone "A-okay," in view of the mechanisms by which both the purine and pyrimidine ribonucleotide pools could become depleted intracellularly, for example, doesn't make a whole lot of sense to me. If the intracellular phosphate depletion is brief, then it makes sense to me that a brief period of time would be required to correct that depletion. But one is not even going to be able to tell if the intracellular phosphate levels are being maintained in some cases, given the frequently-observed independence of the intracellular and extracellular phosphate concentrations. So someone would have to do muscle biopsies or use 31P-MRS intermittently or measure red blood cell 2,3-diphosphoglycerate levels as a surrogate for the measurement of the intracellular Pi levels in myocytes, etc.
Although the authors wrote that vitamin D usually causes rapidly-emerging improvements in muscle weakness in people who do not have inherited mutations that affect phosphate homeostasis, as in XLHR, it's conceivable to me that the myopathy could have resulted from mitochondrial dysfunction, perhaps as a result of acquired mitochondrial DNA (mtDNA) mutations, as a consequence of the phosphate depletion. The fact that the degree of muscle weakness was independent of the serum phosphate is not surprising, and the intracellular phosphate levels are known to frequently, if not generally, be independent of steady-state serum phosphate levels in normal humans given low dosages of supplemental phosphate. Given that intracellular phosphate depletion is known to deplete ATP and purine nucleotide pools and that the depletion of the pools of purine deoxyribonucleotides can impair mtDNA replication (see past postings), it's conceivable that intracellular phosphate depletion could impair DNA repair and lead to a gradual accumulation of mtDNA or even nuclear DNA mutations. It's worthwhile to note that the maintenance of an adequate pool of each of the major intracellular purine nucleotides is a prerequisite for the maintenance of pyrimidine salvage. I think some of that has been shown in the context of fructose-induced hepatic ATP depletion. I think researchers have shown that fructose can deplete uridine from the liver and transiently elevate plasma uridine, as one might expect in response to fructose loading. Here are some references on that [see page 33 of the chapter of the book by Davies et al., 1998, who found that the plasma uridine levels increased soon after fructose administration in humans and then decreased a lot by 4 hours after a meal; sounds fantastic: (http://scholar.google.com/scholar?q=fructose+uridine+plasma+OR+serum&hl=en)].
Thus, intracellular phosphate depletion could conceivably contribute to the development of mutations in nuclear DNA and to the development of some of those more severe myopathies or intractable disease states, such as chronic fatigue syndrome, by leading to a depletion of both purines and pyrimidines. That's just my opinion, however. It's noteworthy that DNA repair consumes a lot of ATP, and some authors have suggested, as I noted in my old folic acid paper (see past posting), that the depletion of intracellular total folates might cause apoptotic cell death in neurons by "DNA-repair-associated" ATP depletion. They meant that there would be a futile cycle of DNA damage, in response to folate depletion and increases in the dUMP/dTMP ratio, and DNA repair and that the DNA repair would ultimately consume so much ATP as to lead to apoptotic cell death, such as in response to ischemic episodes or strokes that can cause a lot of DNA damage. Davies et al. (1998) argued that fructose-induced phosphate depletion in the liver had caused both the purine depletion, as evidenced by the elevations in serum uric acid, and the uridine export from the liver. I'm not suggesting that more is always going to be better, and those articles on the overlapping mechanisms governing the efflux of uric acid and inorganic phosphate, as discussed in recent postings, suggest that the metabolic cost or competitive inhibitory effects of excesses of intracellular inorganic phosphate could become significant, past a certain point, and derange the transport of organic anions other than uric acid, etc. Although the research suggests that a lot of phosphate would be required to create that type of state, it's worthwhile to discuss these things with one's doctor.
My view is that the data on the dosages of phosphate used in people with XLHR (and in other genetic disorders that reduce phosphate reabsorption) is relevant to normal humans, with regard to the risk of nephrocalcinosis, but I can think of a number of possible objections to that view. The first would be that, in people with XLHR, the rate of phosphate (Pi) reabsorption would be lower than it would in normal people and that that would decrease the risk, in comparison to normal people, of intracellular calcium phosphate precipitation. I should mention that, in that long review on nephrocalcinosis that I recently discussed, the author noted that calcification can occur either extracellularly (and "luminally" or intraluminally), on the luminal membranes of the proximal or distal tubule cells or in the interstices of the tight junctions, or intracellularly, in the cells of the renal tubules. Thus, one could argue that normal people would have the same risk of intraluminal calcification as people who have XLHR would but that normal people would have a higher risk of intracellular calcification, in response to a given dosage of supplemental phosphate, as people who have XLHR would. In normal people, however, the proximal tubules are able to vary the percent reabsorption to between something like 80 and 99 percent, and that means there would be a lot of potential for the proximal tubules to increase the urinary excretion of phosphate in response to some dosage of supplemental Pi. I just think the risks are basically similar for normal people as they are for people who have XLHR.
The reason I'm focusing on the reabsorption is that XLHR doesn't affect the glomerular filtration of serum phosphate, except to the extent that the cells of people with XLHR might be more "hungry" for phosphate and might clear the serum phosphate, from a dosage of phosphate, more rapidly than a normal person's cells might (thereby producing an indirect decrease in the amounts of phosphate filtered per unit time). In other words, the acute elevations in serum phosphate could conceivably be larger in normal people than in people who have XLHR. But that presupposes that a person has no capacity to tell if some change in his or her phosphate to calcium intake ratio, for example, is producing any benefit. If there's no obvious benefit, presumably there wouldn't be an impetus to continue taking any reasonable amount of phosphate, with the approval of one's doctor. It would also, obviously, be important to spread the dosage out across the day as much as possible and to consider limiting any dosage of supplemental vitamin D to 2000-4000 IU or less, given that hypercalciuria is thought to be a major factor that can increase the risk of nephrocalcinosis. Some authors have suggested splitting the total daily dosage of phosphate, in people who have XLHR, into 8 dosages, spread out across the day, instead of the usual practice of splitting the dosage into 4-5 increments. Another objection I can think of would be that the PHEX protein or the Na(+)/Pi cotransporter might be expressed in myocytes or myogenic satellite cells or some other extrarenal cell type. That could mean that mtDNA replication or some other Pi-sensitive metabolic process would be specifically affected in the muscles and would not be likely to show up in normal people. But I don't see how a pure and severe case of Fanconi's syndrome couldn't produce the same kinds of long-term problems in postmitotic cell types as something like XLHR can. It probably wouldn't take 7 years to treat the problem in a normal person, but I just think that there's a need to think of this type of thing with the long view in mind. It's necessary for someone to do long-term safety research using supplemental phosphate in normal people and to use reasonable amounts of dietary calcium, etc. Or someone could do that type of research in people who have chronic fatigue syndrome. I don't know what the best approach would be. One could argue that reasonable and low dosages of phosphate would improve both purine and pyrimidine salvage and could help limit something like the age-associated reductions in mtDNA copy number in different cell types. These are just my off-the-cuff thoughts, but I think the notion that 7 days of "phosphate loading" is enough to make anyone "A-okay," in view of the mechanisms by which both the purine and pyrimidine ribonucleotide pools could become depleted intracellularly, for example, doesn't make a whole lot of sense to me. If the intracellular phosphate depletion is brief, then it makes sense to me that a brief period of time would be required to correct that depletion. But one is not even going to be able to tell if the intracellular phosphate levels are being maintained in some cases, given the frequently-observed independence of the intracellular and extracellular phosphate concentrations. So someone would have to do muscle biopsies or use 31P-MRS intermittently or measure red blood cell 2,3-diphosphoglycerate levels as a surrogate for the measurement of the intracellular Pi levels in myocytes, etc.
Monday, June 29, 2009
Endogenous Formation of Dinucleotide Polyphosphates: Crude Discussion of Interactions with the Mevalonate Pathway, Nucleotide Metabolism, and aa-tRNAs
This is an interesting article [Jankowski et al., 2009: (http://www.brjpharmacol.org/view/0/earlyView.html)], and the authors discuss the fact that dinucleotide polyphosphates (DNPP) are made endogenously and stored in platelets, adrenal chromaffin cells, and also neurons in the brain. The general formula for a DNPP is N-p(x)-N, where N = adenosine, guanosine, uridine, etc., and x = 2 to 7 phosphates linking two 5'-carbons of the ribose moieties of the nucleotides. Some common examples are diadenosine tetraphosphate (Ap4A) and Ap5A, Up4A (U = uridine), etc. That article doesn't discuss that much about their biosynthesis, but it turns out they're formed by aminoacyl-tRNA synthetases, just as the isoprenoid-based nucleotide polyphosphates are (see recent posting). They can also be formed by Ap4A phosphorylases, luciferase enzymes, and guanylyltransferases [Schluter et al., 1998: (http://www.pubmedcentral.nih.gov/picrender.fcgi?doi=10.1172/JCI119882&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/9449703)]. The different DNPP's have agonist and antagonist effects, at low concentrations, on purinergic receptors, and the article by Jankowski et al. (2009) catalogues a lot of those effects on platelet aggregation, etc. They're apparently degraded fairly slowly by various hydrolase enzymes.
So I guess to understand the interactions of cholesterol and nucleotide metabolism, one should read about the regulation of aminoacyl-tRNA synthetases. That's bizarre, but it seems interesting. It looks like the DNPP's have both "good" and "bad" effects, as one might expect, and it sounds like the concentrations can be fairly high, in the millimolar range, in vesicles in neurons or in platelet secretory granules. They basically are thought to act locally, the way purines and other nucleotides act on purinergic receptors. The plasma concentrations of different diadenosine oligophosphates/polyphosphates are 0.18 to 0.89 uM, but Jankowski et al. (2009) think that they may act locally on platelets and influence platelet aggregation under some circumstances. It sounds like one effect might be on phosphate homeostasis, as the authors of that article on isopentenyl-ATP derivatives were implying and suggesting. It seems like there may be a tendency to focus a lot on the effects of DNPP's on purinergic receptors, but it sounds like the intracellular effects might be more important. I haven't read much of anything on this topic yet, and there's probably some research about that type of thing. Maybe they regulate cholesterol metabolism or interfere with nucleotide transport. Some genetic mutations that affect nucleotide transport can cause mtDNA depletion, and then there are the more short-term effects (of DNPP's), such as on respiration, that could potentially occur via the inhibition of the adenine nucleotide translocase transporter. I wonder if there's anything on mRNA stability or something like that or on transcription. Presumably there are all sorts of potential pathological effects, but I actually don't know anything about the mechanisms governing the intracellular transport of DNPP's. I also don't know what factors regulate their formation by the aminoacyl-tRNA synthetases or by other enzymes. Schluter et al. (1998) say that the half lives of various DNPP's range from 49 to 69 minutes, and those are much longer than the half-lives of purines. The half-life of plasma adenosine is about 0.5 to 1.5 seconds.
It's interesting that Schluter et al. (1998) say that the aminoacyl-tRNA synthetases transfer the AMP of an aminoacyl-AMP to a nucleotide diphosphate or triphosphate and form the DNPP and also release an amino acid. I wonder if there's some kind of specificity to the aminoacyl-tRNAs that form specific DNPP's. If there were (I have no idea if there is), then that could explain some of these strange effects of different amino acids. That could explain some of the puzzling effects of glutamine, such as its antiapoptotic effects. For example, glutaminyl-tRNA synthetase [Ko et al., 2001: (http://www.jbc.org/cgi/content/full/276/8/6030)(http://www.ncbi.nlm.nih.gov/pubmed/11096076?dopt=Abstract)] produces antiapoptotic effects, in a glutamine-dependent manner, by obscure mechanisms. Ko et al. (2001) discuss some of the mechanisms by which glutaminyl-tRNA synthetase may contribute to the supposed antiapoptotic effects of glutamine. Maybe there's some intermediary influence of glutaminyl-tRNA-derived DNPP's in the antiapoptotic effects of glutamine, under some circumstances. I mean, maybe the DNPP formation is facilitated in some way by protein-protein interactions of glutaminyl-tRNA synthetase and some other protein [even the apoptosis signal-regulating kinase 1, discussed by Ko et al. (2001), that interacts with glutaminyl-tRNA synthetase, etc.]. Those are crude ideas, but it's interesting. The article by Ko et al. (2001) discusses the effects of glutamine on various mitogen-activated protein kinase pathways.
So I guess to understand the interactions of cholesterol and nucleotide metabolism, one should read about the regulation of aminoacyl-tRNA synthetases. That's bizarre, but it seems interesting. It looks like the DNPP's have both "good" and "bad" effects, as one might expect, and it sounds like the concentrations can be fairly high, in the millimolar range, in vesicles in neurons or in platelet secretory granules. They basically are thought to act locally, the way purines and other nucleotides act on purinergic receptors. The plasma concentrations of different diadenosine oligophosphates/polyphosphates are 0.18 to 0.89 uM, but Jankowski et al. (2009) think that they may act locally on platelets and influence platelet aggregation under some circumstances. It sounds like one effect might be on phosphate homeostasis, as the authors of that article on isopentenyl-ATP derivatives were implying and suggesting. It seems like there may be a tendency to focus a lot on the effects of DNPP's on purinergic receptors, but it sounds like the intracellular effects might be more important. I haven't read much of anything on this topic yet, and there's probably some research about that type of thing. Maybe they regulate cholesterol metabolism or interfere with nucleotide transport. Some genetic mutations that affect nucleotide transport can cause mtDNA depletion, and then there are the more short-term effects (of DNPP's), such as on respiration, that could potentially occur via the inhibition of the adenine nucleotide translocase transporter. I wonder if there's anything on mRNA stability or something like that or on transcription. Presumably there are all sorts of potential pathological effects, but I actually don't know anything about the mechanisms governing the intracellular transport of DNPP's. I also don't know what factors regulate their formation by the aminoacyl-tRNA synthetases or by other enzymes. Schluter et al. (1998) say that the half lives of various DNPP's range from 49 to 69 minutes, and those are much longer than the half-lives of purines. The half-life of plasma adenosine is about 0.5 to 1.5 seconds.
It's interesting that Schluter et al. (1998) say that the aminoacyl-tRNA synthetases transfer the AMP of an aminoacyl-AMP to a nucleotide diphosphate or triphosphate and form the DNPP and also release an amino acid. I wonder if there's some kind of specificity to the aminoacyl-tRNAs that form specific DNPP's. If there were (I have no idea if there is), then that could explain some of these strange effects of different amino acids. That could explain some of the puzzling effects of glutamine, such as its antiapoptotic effects. For example, glutaminyl-tRNA synthetase [Ko et al., 2001: (http://www.jbc.org/cgi/content/full/276/8/6030)(http://www.ncbi.nlm.nih.gov/pubmed/11096076?dopt=Abstract)] produces antiapoptotic effects, in a glutamine-dependent manner, by obscure mechanisms. Ko et al. (2001) discuss some of the mechanisms by which glutaminyl-tRNA synthetase may contribute to the supposed antiapoptotic effects of glutamine. Maybe there's some intermediary influence of glutaminyl-tRNA-derived DNPP's in the antiapoptotic effects of glutamine, under some circumstances. I mean, maybe the DNPP formation is facilitated in some way by protein-protein interactions of glutaminyl-tRNA synthetase and some other protein [even the apoptosis signal-regulating kinase 1, discussed by Ko et al. (2001), that interacts with glutaminyl-tRNA synthetase, etc.]. Those are crude ideas, but it's interesting. The article by Ko et al. (2001) discusses the effects of glutamine on various mitogen-activated protein kinase pathways.
Saturday, June 27, 2009
Adenosine vs. Ribose vs. AICAriboside for the Restoration of Adenosine Nucleotides in the Heart Following Ischemia
In this article [Mauser et al., 1985: (http://circres.ahajournals.org/cgi/reprint/56/2/220.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/3918804)], Mauser et al. (1985) show that the intraarterial infusion of adenosine produced a 90-fold increase in the rate of adenosine nucleotide resynthesis (mostly by purine salvage pathways), following cardiac ischemia in dogs, and infusions of equimolar dosages of ribose or AICAriboside produced only between 5-fold and 9-fold increases in the rate of adenosine nucleotide formation, by either the salvage or de novo pathways. Only adenosine significantly restored ATP levels, following ischemia. All of the compounds were infused intraarterially, into the left coronary arteries of the dogs.
This article shows, in my opinion, that adenosine is far superior to ribose as an approach to restoring adenosine nucleotide levels following their depletion, and the research casts doubt on the idea, as suggested by the authors of some articles, that the entries of the carbons of ribose, derived from exogenous purine nucleotides, into the nonoxidative pentose cycle and into glycolytic pathways make a substantial contribution to the purine-mediated protection of cultured cells, such as astrocytes, against death due to glucose deprivation or other conditions. In most of those articles, the only evidence that ribose mediates the protective effects is that purine nucleoside phosphorylase (PNP) inhibitors sometimes block the protective effects of exogenous purines. But, as discussed by other authors, that doesn't mean that the use of ribose as a glycolytic substrate mediates the protective effects of purines or that ribose can substitute for the preformed nucleotides. It may just mean that ribose-1-phosphate has to be derived from nucleoside phosphorolysis to maintain purine salvage and that, paradoxically, more purine nucleobases are lost when ribose is "locked" in nucleosides and nucleotides than are lost when some turnover of nucleoside-derived ribose is allowed to occur, via the PNP-mediated formation of ribose-1-phosphate and purine bases. Also, the inhibition of nucleoside phosphorolysis prevents the formation of uric acid from the exogenous purines, and some authors have suggested that uric acid-induced peroxynitrite scavenging may partially mediate the protective effects of purines on cultured cells. There's a lot of research showing that uric acid can maintain mitochondrial functioning, in cells in the liver or in other cells, by preventing the inactivation of respiratory-chain enzymes by peroxynitrite, etc. Additionally, adenosine is normally kept at a very low concentration intracellularly, and massive amounts of adenosine have to be supplied, normally, to produce adenosine-mediated toxic effects and S-adenosylhomocysteine accumulation, etc., in cells [usually 1 mM (1000 uM) or higher of extracellular adenosine is required, a concentration that is supraphysiological] [Adair, 2005: (http://ajpregu.physiology.org/cgi/content/full/289/2/R283)(http://www.ncbi.nlm.nih.gov/pubmed/16014444)]. If adenosine were to accumulate as a result of PNP inhibition, that accumulation could produce toxic effects. But adenosine is normally metabolized extremely rapidly.
The rate of de novo purine biosynthesis is extremely slow, and this is one reason, as Mauser et al. (1985) discussed, that AICAr did not produce very significant restorative effects on adenosine nucleotide levels. Ribose did not even appear to contribute much to purine salvage, in my opinion, in comparison to the effects of adenosine. Even if the effects of ribose depended on its metabolism into glycolytic intermediates, the presence of the preformed purine nucleotides, such as can be derived from exogenous adenosine, appear to be crucial and to be a limiting factor in the rate of ATP resynthesis and nucleotide replenishment following ischemia. There are other articles that provide similar data.
In the case of pyrimidines, the research showing that uridine phosphorylase (UP) inhibition can abolish the cytoprotective effects of exogenous uridine, as in astrocytes, tends to not take into account the role that UP is thought to play in the salvage of uracil in rodents and cultured astrocytes, etc. The traditional view is that pyrimidine bases are not salvaged and that pyrimidine salvage occurs only at the level of the whole nucleoside, meaning that the main salvage pathway for uridine, for example, is its phosphorylation to UMP by uridine kinase. Some of these articles discuss the fact that, in the brains of rodents, UP appears to play an "anabolic," rather than catabolic, role and to be required for pyrimidine salvage [Mascia et al., 1999, etc.: (http://scholar.google.com/scholar?q=%22uridine+phosphorylase%22+salvage&hl=en&lr=)]. Also, inhibition UP may indirectly inhibit purine salvage, given that UP inhibition would prevent uridine from serving as a source of ribose-1-phosphate. This could decrease the formation of PRPP from uridine-derived ribose-1-phosphate, thereby increasing the loss of purines and compromising both ATP formation and the ATP-dependent salvage of uridine, etc. The depletion of purines has been shown to lead to secondary depletion of pyrimidines, even in the context of the fructose-induced depletion of ATP from the liver. Fructose has been shown to transiently elevate plasma uridine levels, and that's very much a pathological effect, in my opinion. Some people seem to think that the fructose-induced elevation of plasma uric acid levels is "good" or desirable, given that uric acid scavenges peroxynitrite. But the elevations in uric acid levels, following fructose ingestion or infusion, are mainly the result of pronounced ATP depletion in the liver, in my opinion (and as shown by countless articles). That's not desirable. In any case, the articles in these areas are interesting.
This article shows, in my opinion, that adenosine is far superior to ribose as an approach to restoring adenosine nucleotide levels following their depletion, and the research casts doubt on the idea, as suggested by the authors of some articles, that the entries of the carbons of ribose, derived from exogenous purine nucleotides, into the nonoxidative pentose cycle and into glycolytic pathways make a substantial contribution to the purine-mediated protection of cultured cells, such as astrocytes, against death due to glucose deprivation or other conditions. In most of those articles, the only evidence that ribose mediates the protective effects is that purine nucleoside phosphorylase (PNP) inhibitors sometimes block the protective effects of exogenous purines. But, as discussed by other authors, that doesn't mean that the use of ribose as a glycolytic substrate mediates the protective effects of purines or that ribose can substitute for the preformed nucleotides. It may just mean that ribose-1-phosphate has to be derived from nucleoside phosphorolysis to maintain purine salvage and that, paradoxically, more purine nucleobases are lost when ribose is "locked" in nucleosides and nucleotides than are lost when some turnover of nucleoside-derived ribose is allowed to occur, via the PNP-mediated formation of ribose-1-phosphate and purine bases. Also, the inhibition of nucleoside phosphorolysis prevents the formation of uric acid from the exogenous purines, and some authors have suggested that uric acid-induced peroxynitrite scavenging may partially mediate the protective effects of purines on cultured cells. There's a lot of research showing that uric acid can maintain mitochondrial functioning, in cells in the liver or in other cells, by preventing the inactivation of respiratory-chain enzymes by peroxynitrite, etc. Additionally, adenosine is normally kept at a very low concentration intracellularly, and massive amounts of adenosine have to be supplied, normally, to produce adenosine-mediated toxic effects and S-adenosylhomocysteine accumulation, etc., in cells [usually 1 mM (1000 uM) or higher of extracellular adenosine is required, a concentration that is supraphysiological] [Adair, 2005: (http://ajpregu.physiology.org/cgi/content/full/289/2/R283)(http://www.ncbi.nlm.nih.gov/pubmed/16014444)]. If adenosine were to accumulate as a result of PNP inhibition, that accumulation could produce toxic effects. But adenosine is normally metabolized extremely rapidly.
The rate of de novo purine biosynthesis is extremely slow, and this is one reason, as Mauser et al. (1985) discussed, that AICAr did not produce very significant restorative effects on adenosine nucleotide levels. Ribose did not even appear to contribute much to purine salvage, in my opinion, in comparison to the effects of adenosine. Even if the effects of ribose depended on its metabolism into glycolytic intermediates, the presence of the preformed purine nucleotides, such as can be derived from exogenous adenosine, appear to be crucial and to be a limiting factor in the rate of ATP resynthesis and nucleotide replenishment following ischemia. There are other articles that provide similar data.
In the case of pyrimidines, the research showing that uridine phosphorylase (UP) inhibition can abolish the cytoprotective effects of exogenous uridine, as in astrocytes, tends to not take into account the role that UP is thought to play in the salvage of uracil in rodents and cultured astrocytes, etc. The traditional view is that pyrimidine bases are not salvaged and that pyrimidine salvage occurs only at the level of the whole nucleoside, meaning that the main salvage pathway for uridine, for example, is its phosphorylation to UMP by uridine kinase. Some of these articles discuss the fact that, in the brains of rodents, UP appears to play an "anabolic," rather than catabolic, role and to be required for pyrimidine salvage [Mascia et al., 1999, etc.: (http://scholar.google.com/scholar?q=%22uridine+phosphorylase%22+salvage&hl=en&lr=)]. Also, inhibition UP may indirectly inhibit purine salvage, given that UP inhibition would prevent uridine from serving as a source of ribose-1-phosphate. This could decrease the formation of PRPP from uridine-derived ribose-1-phosphate, thereby increasing the loss of purines and compromising both ATP formation and the ATP-dependent salvage of uridine, etc. The depletion of purines has been shown to lead to secondary depletion of pyrimidines, even in the context of the fructose-induced depletion of ATP from the liver. Fructose has been shown to transiently elevate plasma uridine levels, and that's very much a pathological effect, in my opinion. Some people seem to think that the fructose-induced elevation of plasma uric acid levels is "good" or desirable, given that uric acid scavenges peroxynitrite. But the elevations in uric acid levels, following fructose ingestion or infusion, are mainly the result of pronounced ATP depletion in the liver, in my opinion (and as shown by countless articles). That's not desirable. In any case, the articles in these areas are interesting.
Sunday, June 14, 2009
Folate and UVB Papers in Pdf Format
Here are my old papers, with the diagrams included, in pdf format. I now think that reduced folates, such as L-leucovorin (L-folinic acid) and L-methylfolate, are far superior to folic acid, for many reasons, but the concepts discussed in the folic acid paper are still "valid." Here's the paper on folate metabolism in relation to purine and pyrimidine metabolism (http://www.mediafire.com/?zmfojwc4am2), and here's the paper on the effects of cutaneous UVB/UVA exposure on sensory neurons and projection neurons/wide-dynamic-range neurons in the dorsal horn and caudal trigeminal nucleus, etc. (http://www.mediafire.com/?410jmzmrjjm).
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, 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.
Thursday, April 16, 2009
The Mesolimbic Reward Pathways in Depression
The authors of this article [Nestler and Carlezon, 2006: (http://www3.utsouthwestern.edu/contecenter/refs/Nestler2.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/16566899)] discuss the potentially-important roles that dysregulations of the mesolimbic and mesocortical, dopaminergic pathways may play in the etiology of major depression. Carlezon is a researcher at Harvard Medical School and has contributed to a lot of really interesting research, such as the research showing antidepressant effects of uridine or cytidine in animal models of depression [Carlezon et al., 2002; Carlezon et al., 2005, both cited and discussed here: (http://hardcorephysiologyfun.blogspot.com/2009/02/potential-psychiatric-pitfalls-in.html)]. Nestler and Carlezon (2006) note that the tendency has been to focus on changes in the hippocampus or prefrontal cortex in depression, etc. It's stunning to me that someone had to write an article like this. Of course one would expect, in my opinion, the mesolimbic reward pathways to be dysregulated in people with depression. It's also noteworthy that some hippocampal neurons project to the prefrontal cortex and are thought to be important in the regulation of dopaminergically-mediated working memory performance. For example, Seamans et al. (1998) [Seamans et al., 1998: (http://www.jneurosci.org/cgi/content/full/18/4/1613)(http://www.ncbi.nlm.nih.gov/pubmed/9454866?dopt=Abstract)] discussed the way in which dopaminergic neurons, originating in either the ventral tegmental area (VTA) or nucleus accumbens or other sites in the striatum [Carr and Sesack, 2000, cited here: (http://hardcorephysiologyfun.blogspot.com/2009/03/adenosine-and-guanosine-in-animal.html)], project to and form synapses with GABAergic interneurons in the prefrontal cortex and are thought, via the activation of D1 dopamine receptors on those GABAergic interneurons, to exert a largely inhibitory influence on the burst firing patterns of glutamatergic pyramidal neurons that originate in the prefrontal cortex and provide monosynaptic inputs back to the dopaminergic neurons in the VTA, in the midbrain. The "regulatory" effects of glutamatergic inputs, originating in the prefrontal cortex, to the VTA are important for maintaining the normal burst firing patterns of VTA neurons, and the burst firing patterns of dopaminergic neurons in the VTA are crucially important for processes such as reward-based learning, cognitive functioning, and motivation, etc. Dopaminergic neurons in the VTA can also produce excitatory or inhibitory effects on glutamatergic neurons in the prefrontal cortex by modifying the excitatory or inhibitory effects of hippocampal neurons that provide inputs to the same classes of GABAergic interneurons that regulate the layer V glutamatergic neurons that project back to the VTA. In any case, those are some ways the reward pathways interact with the hippocampus and prefrontal cortex, and many of those interactions would, in my opinion, be relevant to an understanding of dysregulations of the reward pathways in people with major depression.
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.
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