Purine loss after repeated sprint bouts in humans

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1 Purine loss after repeated sprint bouts in humans C. G. STATHIS, 1 S. ZHAO, 1 M. F. CAREY, 1 AND R. J. SNOW 2 1 Exercise Metabolism Unit, Centre for Rehabilitation, Exercise and Sport Science, Victoria University of Technology, Footscray 3011; and 2 School of Human Movement, Deakin University, Burwood 3125, Australia Stathis, C. G., S. Zhao, M. F. Carey, and R. J. Snow. Purine loss after repeated sprint bouts in humans. J. Appl. Physiol. 87(6): , The influence of the number of sprint bouts on purine loss was examined in nine men (age yr, weight kg, peak O 2 consumption l/min) who performed either one (B1), four (B4), or eight (B8) 10-s sprints on a cycle ergometer, 1 wk apart, in a randomized order. Forearm venous plasma inosine, hypoxanthine (Hx), and uric acid concentrations were measured at rest and during 120 min of recovery. Urinary inosine, Hx, and uric acid excretion were also measured before and 24 h after exercise. During the first 120 min of recovery, plasma inosine and Hx concentrations, and urinary Hx excretion rate, were progressively higher (P 0.05) with an increasing number of sprint bouts. Plasma uric acid concentration was higher (P 0.05) in B8 compared with B1 and B4 after 45, 60, and 120 min of recovery. Total urinary excretion of purines (inosine Hx uric acid) was higher (P 0.05) at 2hofrecovery after B8 ( µmol) compared with the other trials (B1: ; B4: µmol). These results indicate that the loss of purine from the body was enhanced by increasing the number of intermittent 10-s sprint bouts. intermittent exercise; inosine; hypoxanthine; uric acid; metabolism THE METABOLIC DEMAND of intense sprint exercise requires a high skeletal muscle ATP turnover. This high turnover usually results in the failure of ATP resynthesis to match ATP hydrolysis rates, thereby causing a reduction in skeletal muscle ATP content (4, 22, 27). During short-duration, high-intensity exercise, the fall in muscle ATP content is matched by an equimolar increase in IMP and ammonia (15). A small proportion of the accumulated IMP can undergo further degradation to produce inosine, which can be subsequently converted to hypoxanthine (Hx) (30). Both inosine and Hx are either resynthesized to IMP via the purine salvage pathway (7, 19) or leave the muscle and accumulate in the plasma (3, 10, 12). Importantly, once Hx has diffused from the muscle, it appears to be permanently lost as a precursor to muscle ATP synthesis because Hx uptake into the recovering muscle after intense exercise does not occur (14). Hx and inosine may be removed from the plasma via renal excretion (21, 28) or by the liver (12). The liver converts Hx to uric acid (12, 31), which is released into the circulation and excreted by the kidney (21, 28), or it may be taken up by the recovering muscle where it is possibly used as an The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. oxygen free radical scavenger (13). The latter removal pathway only occurs when plasma uric acid levels are high (14). The elevated plasma accumulation and urinary excretion of inosine, Hx, and uric acid after intense exercise represent a loss of ATP from the active and recovering muscle. The high-intensity, intermittent exercise associated with sprint training provides an opportunity for increased flux through the purine nucleotide catabolic pathways, which may lead to a chronic reduction of resting muscle ATP content. The effect of sprint training on resting skeletal muscle ATP content is controversial. Several studies have reported no change (5, 22, 29), whereas others have demonstrated a reduced resting skeletal muscle ATP content after sprint training (11, 27). The reasons for the discrepant results are not obvious but may be related to variations in training intensity, exercise bout duration, recovery interval between bouts, and variation in the recovery time between the last training session and the posttraining biospy. Several studies have indicated that a greater production of purine bases occurs when the intensity of exercise is high compared with less intense exercise (17, 25). Additionally, an increase in sprint duration from 2- to 6-s bouts during intermittent exercise was also associated with an elevated plasma Hx concentration (1). Furthermore, a reduction in recovery duration between bouts (2) has also been shown to increase plasma purine base concentration. The number of sprint bouts completed per training session may also explain, at least in part, why some studies have observed an attenuated resting muscle ATP content after sprint training. Unfortunately, there are no studies that have examined the influence of the number of sprint bouts on muscle purine loss. Several studies (8, 20) have reported that most of the fall in muscle ATP content occurs during the initial bouts of intermittent, high-intensity exercise with little, if any, change occurring in the latter bouts. Because a recovery duration of 3 min is too short to allow significant recovery of ATP (9, 26, 27), then an increased number of exercise bouts per training session may elevate muscle purine loss. This may occur because the enzymes associated with the degradation pathway of IMP have a longer period in which they are exposed to an elevation of their substrate concentrations. Therefore, the aim of the present study was to investigate the influence of single and multiple intermittent sprint exercise protocols on the accumulation and excretion of purine bases in the plasma and urine, respectively. We hypothesized that increasing the number of sprint bouts will result in an elevation of plasma purine concentration and urinary purine excretion after sprint exercise, thereby /99 $5.00 Copyright 1999 the American Physiological Society 2037

2 2038 PURINE LOSS AND SPRINT EXERCISE indicating an increased purine loss from the active muscle. METHODS Subjects. Nine active, nonspecifically trained men (age yr, weight kg, peak oxygen consumption l/min), volunteered for the study, which was approved by the Victoria University of Technology Human Experimentation Ethics Committee. All subjects were fully informed of the experimental procedures and signed an informed consent statement. Exercise protocols. Peak oxygen consumption of each subject was determined 1 wk before he began the experimental trials. The exercise protocol involved cycling on a cycle ergometer (Lode, Gronigen, The Netherlands) for 3 min at three submaximal work rates; subsequently, the work rate was increased every 1 min thereafter until volitional exhaustion. Expired air was directed, by a Hans Rudolph valve, through a ventilometer (Pneumoscan S30) into a mixing chamber and analyzed for oxygen and carbon dioxide content by gas analyzers (Applied Electrochemistry S-3A O 2 and CD-3A CO 2, respectively). These analyzers were calibrated before each test by using commercially prepared gas mixtures. Oxygen consumption was calculated by a microprocessor by using standard equations. Subjects were asked to perform three separate experimental exercise trials, consisting of either one (B1), four (B4), or eight (B8) 10-s all-out sprint bouts on an air-braked cycle ergometer (series A, Repco, Melbourne, Australia) modified to enable computerized determination of peak and mean power. The power output of the air-braked cycle ergometer is approximately proportional to the cube of the wheel velocity, which was measured by using a tachometer (Hall-effect device and a cog at the wheel hub). The subjects were instructed to remain seated and pedal as fast as possible for the complete 10-s exercise periods. Multiple bouts were separated by 50 s of passive rest on the cycle ergometer, and all trials were assigned in random order. Subjects were familiarized with the intermittent sprint task by performing the B8 protocol, without blood or urine collection procedures, at least 1 wk before testing. Each subject was instructed to refrain from strenuous exercise and from caffeine and alcohol consumption 24 h before each experimental trial. In addition, subjects recorded their diet for the 24 h before the first experimental trial and were asked to consume similar foods before the remaining trials. The experimental trials were conducted in the morning after an overnight fast and were separated by 1 wk. Blood and urine sampling, treatment, and analysis. Blood was sampled from an antecubital vein, via an indwelling catheter, at rest, and where appropriate after the first, fourth, and eighth bout of the trials. Blood samples (10 ml) were also obtained after 5, 10, 15, 20, 30, 45, 60, and 120 min of passive recovery after the final exercise bout. The samples were placed into lithium-heparin tubes and spun in a centrifuge. Subsequently, 500 µl of plasma were added to 1 ml of ice-cold 3 M perchloric acid and spun, and the supernatant was stored at 80 C before analysis for lactate. The remaining plasma was stored in liquid nitrogen for analysis of inosine, Hx, and uric acid. The plasma stored for these metabolites was deproteinized with 1.5 M perchloric acid and subsequently neutralized with 2.1 M KHCO 3 immediately before analysis. Plasma lactate was determined in duplicate, by using an enzymatic spectrophotometric technique (18). Plasma Hx, inosine, and uric acid were determined on neutralized perchloric acid extracts, by using a modification of the reverse-phase HPLC technique described by Wynants and Van Belle (32). Urine was collected for at least 60 min ( min) before the first exercise bout. In addition, urine was collected for the first 2-h period and the subsequent 6- and 16-h periods after the completion of the last sprint bout. Urine volume was determined and then the samples were deproteinized, neutralized, and analyzed for Hx and inosine by using the same sample treatment and HPLC procedures as described for plasma. Urinary uric acid concentration was determined by an enzymatic colorometric method by using a Beckman Synchron CX system. Statistical analysis. All metabolite data were analyzed by using ANOVA with repeated measures (BMDP statistical software). Simple main-effects analyses and Newman-Keuls post hoc tests were used to locate differences when ANOVA revealed a significant interaction. Sprint performance data were analyzed by using ANOVA or paired t-tests where appropriate. The level of probability to reject the null hypothesis was set at P All values are reported as means SE. RESULTS Exercise performance. Mean power was not different (P 0.05) between the first bout of all trials and for the fourth bout of the B4 and B8 trials (Table 1). Similarly, peak power was not different between the first bout of all trials; however, it was 4% lower (P 0.05) in the fourth sprint bout of B8 compared with that of B4 (Table 1). Plasma metabolites. With the exception of lactate, all of the measured plasma metabolites were similar (P 0.05) at rest between the trials (Figs. 1 and 2). Plasma inosine increased (P 0.05) in concentration during recovery in all trials (Fig. 1A). The plasma inosine levels peaked at 15 min in B1 and 30 min in the other trials. The plasma inosine concentration returned to basal levels after 30, 45, and 60 min in the B1, B4, and B8 trials, respectively. The inosine concentration was higher (P 0.05) during most of the recovery period in B8 and B4 compared with B1. Furthermore, the inosine concentration in B8 was greater than in B4 after 20, 30, and 45 min of recovery. The plasma Hx concentration Table 1. Peak and mean power during the first, fourth, and eighth bouts of 10-s sprint cycling in the three experimental protocols First Bout Fourth Bout Eighth Bout B1 B4 B8 B4 B8 B8 Peak power, W 1, , , * Mean power, W Values are means SE; n 9 men. B1, B4, and B8: exercise trials consisting of 1, 4, and 8 10-s sprint cycling bouts, respectively. *Different from B4 in fourth bout (P 0.05).

3 PURINE LOSS AND SPRINT EXERCISE 2039 trials for at least 120 min of recovery. The plasma uric acid concentration was higher (P 0.05) in B8 compared with B1 at 45, 60, and 120 min of recovery, but no differences were observed between B4 and the other trials at these times. Plasma lactate concentration increased (P 0.05) above resting values during the early stages of recovery in all trials (Fig. 2). At some sampling times during the first 45 min of recovery, plasma lactate concentrations were progressively higher when a greater number of exercise bouts were performed. Urine metabolites. Statistical analysis of the urinary uric acid (Fig. 3C) and inosine excretion rates (Fig. 3A) revealed no significant interaction (time trial) or main effect for trial. The uric acid data, however, displayed a significant main effect for time. This demonstrated that the uric acid excretion rates were elevated during the 2 h immediately after exercise compared with basal rates and those measured in recovery after the initial 2-h period. The basal rate of urinary Hx excretion was similar (P 0.05) between trials (Fig. 3B). Increases (P 0.05) in this rate were observed in the 2 h after exercise in all trials. The rate of Hx excretion during this time period was greater (P 0.05) in B8 compared with B4 and B1. Similarly, this rate was higher (P 0.05) in B4 compared with B1. There were no differences (P 0.05) between trials during the later stages of recovery, and these rates were similar to basal rates. The total amounts (µmol) of inosine, Hx, and uric acid excreted above basal levels into the urine during the 2-h recovery period are reported in Table 2. The amount of Hx excreted after B8 was 1.4- and 2.8-fold greater (P 0.05) than after B4 and B1, respectively. The amounts of inosine and uric acid excreted were not different between trials. The total urinary purine (inosine Hx uric acid) excretion was greater in B8 compared with B1 and B4. The latter two trials were not different from each other. Fig. 1. Forearm venous plasma inosine (A), hypoxanthine (B), and uric acid (C) concentrations at rest (R) and during 120 min of recovery after 1 (B1), 4 (B4), and 8 (B8) 10-s sprint cycling bouts. Values are means SE; n 9 men. *Different from B1, P # Different from B4, P (Fig. 1B) peaked between 15 and 20 min after the cessation of exercise and had not returned (P 0.05) to basal levels after 60 min of recovery in all trials. In B1, the plasma Hx concentration was similar to preexercise levels after 120 min of recovery; however, the Hx levels remained elevated in the other two trials. At most sampling times during recovery the plasma Hx concentration was higher (P 0.05) after B8, compared with B4 and B1. Furthermore, the plasma Hx concentration was also greater in B4 compared with B1. Plasma uric acid concentration increased (P 0.05) above resting levels after 20 min of recovery in all exercise trials (Fig. 1C) and remained above preexercise levels in these Fig. 2. Forearm venous plasma lactate concentration at rest (R) and during 120 min of recovery after B1, B4, and B8 trials. Values are means SE; n 9 men. *Different from B1, P # Different from B4, P 0.05.

4 2040 PURINE LOSS AND SPRINT EXERCISE major source of plasma inosine, Hx, and uric acid, during and in recovery from intense exercise, is the degradation of the ATP stores within contracting skeletal muscle. The present data, coupled with the findings of previous research, suggest that skeletal muscle purine loss is enhanced when the number of 10-s sprint cycling bouts performed in a single session is increased from one to eight bouts. Although speculative, a difference in the number of sprint bouts performed per exercise session may explain, at least in part, why some studies have reported an attenuated resting muscle ATP content after sprint training (11, 27), whereas others have not (5, 22, 29). The increase in plasma inosine, Hx, and uric acid concentration with more sprint bouts observed in the present study must be due to a greater difference between the rates of entry of these metabolites into, and removal from, the circulation. The present study is unable to determine the relative importance of the inosine, Hx, and uric acid plasma entry and removal rates among the three trials. However, it is highly likely that an increase in sprint bout number results in an enhanced rate of entry of these metabolites into the plasma. The increased entry rate of inosine and Hx into the plasma is likely to occur for the following reasons. First, it has been demonstrated that one 10-s sprint bout is sufficient to acutely deplete the contracting muscle ATP pool by 21% (4). Second, a more marked reduction (i.e., 41%) in muscle ATP content has been reported immediately after ten 6-s sprints (8). Third, the fall in ATP content is highly likely to be associated with a concomitant increase in intramuscular IMP concentration (15, 27). Finally, the elevated muscle IMP content probably occurs throughout the entire sprint protocol in the present study because the reamination of IMP and the return of ATP stores to resting levels require several minutes (9, 26, 27). Taken together, these data strongly indicate that the muscle IMP content is likely to be elevated for a longer period of time in B8 compared with B4 and in B4 compared with B1. In the present study, the accumulation of IMP in the initial bouts is likely to increase the activity of 5 -nucleotidase (6) and consequently the production rate of inosine and Hx. Because the Michaelis constant of 5 -nucleotidase for IMP is low (i.e., 100 µm; Ref. 6), Fig. 3. Urinary inosine (A), hypoxanthine (B), and uric acid (C) excretion rate at rest (basal) and during the first 2 h and the subsequent 6 and 16 h of recovery after B1, B4, and B8 trials. Values are means SE; n 9 men, except for inosine where n 8 men. *Different from B1, P # Different from B4, P Main effect for time, P DISCUSSION The present study demonstrated that the increases in plasma inosine, Hx, and uric acid concentrations after sprint exercise were greater when more sprint bouts were performed. Furthermore, urinary Hx excretion and whole body purine loss after 2 h of recovery were also elevated when more sprint bouts were undertaken. Previous studies (3, 12, 24) indicate that the Table 2. Urinary purine loss after 2hofrecovery from one, four, or eight 10-s sprint cycling bouts B1 B4 B8 Hx lost to urine after 2 h of recovery * * Uric acid lost to urine after2hofrecovery Inosine lost to urine after2hofrecovery Total urinary purine loss after2hof recovery * Values are means SE given in µmol; n 9 men except urinary inosine loss where n 8. Hx, hypoxanthine. *Different from B1, P Different from B4, P 0.05.

5 PURINE LOSS AND SPRINT EXERCISE 2041 the enzyme is likely to be saturated with substrate in intensely contracting muscle and during the early stages of recovery. For example, a 10% decrease in muscle ATP content equates to an increase in IMP concentration of 0.8 mmol/l of intracellular water. Consequently, the flux generating step for inosine and Hx formation is at maximal capacity; hence, the production of these metabolites is mainly influenced by the exposure time of 5 -nucleotidase to its substrate. Therefore, each additional sprint performed in the present study allows further time for IMP degradation to proceed. The above evidence suggests that the elevated plasma inosine and Hx concentrations during and after sprint exercise were due to an increased rate of entry into the plasma. Plasma removal rates of these metabolites are also a factor; however, they cannot explain the rise in plasma concentration in early recovery because removal rates are likely to be increasing during this period. This contention is based on the following observations. First, the elevation in plasma Hx concentration observed in the present study (Fig. 1B) provides indirect evidence that the rate of inosine oxidation to Hx has increased. Some of this conversion may occur in the circulation because purine nucleoside phosphorylase activity has been found in the plasma (10) and endothelial cells (23). Second, our data demonstrated that the rate of urinary Hx excretion was elevated above basal levels during the initial 2-h period of recovery in all trials (Fig. 3B). Furthermore, the rate of Hx excretion during this period was greater when more bouts were performed. Third, the elevated plasma uric acid concentration during recovery from B8 compared with B1 observed in the present study (Fig. 1C) provides indirect evidence that the rate of Hx removal and uric acid production by the liver (12) and capillary endothelial cells (16) was also increased during recovery in the B8 trial. A proportion of the uric acid can also be taken up by the recovering muscle (12, 14) where it may be converted to allantoin (13). Unfortunately, our experimental design does not allow us to account for this uric acid uptake. Finally, the elevated urinary purine excretion (Table 2) observed during the 2 h after the B8 trial also suggests that the rate of purine removal from the plasma was enhanced during recovery from sprinting. The total urinary purine (inosine Hx uric acid) excretion, a marker of whole body purine loss, was greater (P 0.05) in B8 compared with the other two trials during the 2-h recovery period (Table 2). On the basis of the elevated plasma Hx and uric acid concentrations at2hofrecovery in the B8 and B4 trials, it may be expected that further urinary excretion of these metabolites would occur after the 2-h collection period. This expectation, however, appears to be unfounded because the urinary excretion rate of Hx and uric acid in the subsequent 6- and 16-h recovery period (Fig. 3, B and C) was not different between trials. It is unclear why this was the case. It is possible that we were unable to detect an increased urinary Hx excretion during the latter 22 h of recovery in B8 and B4 trials because the accumulated plasma Hx after the 2-h recovery period was converted to uric acid and excreted in this form. It is also possible that we failed to detect an increased urinary uric acid excretion during the latter stages of recovery in the intermittent sprint trials because the urinary uric acid excretion rate was quite variable. In addition, it cannot be ruled out that some of the uric acid that accumulated in the plasma was not excreted by the kidney. For example, several studies have demonstrated that uric acid is taken up by recovering skeletal muscle (12, 14). It should be noted, however, that this process only occurs during the early stages of recovery (e.g., first 20 min) and is therefore unlikely to explain our results. The increased urinary excretion of purines in the first 2 h of recovery in B8 compared with the other trials is best explained by an elevated purine loss from the musculature recruited during the sprint exercise. Because we did not measure changes in muscle purine content or purine fluxes across the active and recovering muscle, this conclusion is clearly speculative. Nevertheless, previous research (3, 12) has demonstrated that purine loss does occur from contracting and recovering skeletal muscle. Furthermore, there is currently no reason to believe that other tissues would be contributing to the elevated urinary excretion of purines after repeated sprint exercise observed in this study. If, as we contend, the increased excretion of purines reflects the magnitude of purine loss from skeletal muscle, then our data indicate that the performance of more sprints (e.g., 8 sprints) results in a greater muscle purine loss than when fewer sprints are undertaken. Our data therefore support the possibility that changes in resting muscle ATP content after sprint training may be influenced by the number of sprint bouts performed in the training sessions. In conclusion, this study demonstrated that there was an elevated plasma inosine and Hx concentration during recovery with an increasing number of sprint bouts. Furthermore, plasma uric acid concentration was greater in B8 compared with B1 after 45, 60, and 120 min of recovery. The present study also demonstrated an increased urinary purine excretion during the first 2 h of recovery in B8 compared with the other trials. These data demonstrate that whole body purine loss was enhanced as a function of increasing sprint bout number. Although speculative, this increased urinary purine loss is best explained by an enhanced loss of purines from contracting and recovering skeletal muscle. If this proves to be correct, then the extent of muscle adenine nucleotide loss is dependent on not only exercise intensity, exercise bout duration, and recovery interval but also on the number of sprint bouts performed in an exercise session. We thank Dr. Glenn McConell for help in the preparation of the manuscript and Ian Fairweather for excellent technical support. We also acknowledge Peter Maddy at the Western Hospital (Footscray, Australia) for assistance with some of the urinary biochemical analysis. Address for reprint requests and other correspondence: R. J. Snow, School of Human Movement, Deakin Univ., 221 Burwood Highway, Burwood 3125, Australia ( rsnow@deakin.edu.au). Received 25 January 1999; accepted in final form 11 August 1999.

6 2042 PURINE LOSS AND SPRINT EXERCISE REFERENCES 1. Balsom, P. D., J. Y. Seger, B. Sjodin, and B. Ekblom. Physiological responses to maximal intensity intermittent exercise. Eur. J. Appl. Physiol. 65: , Balsom, P. D., J. Y. Seger, B. Sjodin, and B. Ekblom. Maximal-intensity intermittent exercise: effect of recovery duration. Int. J. Sports Med. 13: , Bangsbo, J., B. Sjodin, and Y. Hellsten-Westing. Exchange of hypoxanthine in muscle during intense exercise in man. Acta Physiol. Scand. 146: , Bogdanis, G. C., M. E. Nevill, H. K. A. Lakomy, and L. H. Boobis. Power output and muscle metabolism during and following recovery from 10 and 20 s of maximal sprint exercise in humans. Acta Physiol. Scand. 163: , Boobis, L. H., C. Williams, and S. A. Wootton. Influence of sprint training on muscle metabolism during brief maximal exercise in man (Abstract). J. Physiol. (Lond.) 342: 36P, Camici, M., C. Fini, and P. L. Ipata. Isolation and kinetic properties of 5 -nucleotidase from guinea-pig skeletal muscle. Biochim. Biophys. Acta 840: 6 12, Edwards, N. L., D. Recker, and I. H. Fox. Overproduction of uric acid in hypoxanthine-guanine phosphoribosyltransferase deficiency: contribution by impaired purine salvage. J. Clin. Invest. 63: , Gaitanos, G. C., C. Williams, L. H. Boobis, and S. Brooks. Human muscle metabolism during intermittent maximal exercise. J. Appl. Physiol. 75: , Graham, T. E., J. Bangsbo, P. D. Gollnick, C. Juel, and B. Saltin. Ammonia metabolism during intense dynamic exercise and recovery in humans. Am. J. Physiol. 259 (Endocrinol. Metab. 22): E170 E176, Harkness, R. A., R. J. Simmonds, and S. B. Coade. Purine transport and metabolism in man: the effect of exercise on concentrations of purine bases, nucleosides and nucleotides in plasma, urine, leucocytes and erthrocytes. Clin. Sci. (Colch.) 64: , Hellsten-Westing, Y., P. D. Balsom, B. Norman, and B. Sjodin. Decreased resting levels of adenine nucleotides in human skeletal muscle after high-intensity training. J. Appl. Physiol. 74: , Hellsten-Westing, Y., L. Kaijser, B. Ekblom, and B. Sjodin. Exchange of purines in human liver and skeletal muscle with short-term exhaustive exercise. Am. J. Physiol. 266 (Regulatory Integrative Comp. Physiol. 35): R81 R86, Hellsten, Y. Adenine nucleotide metabolism. A role in free radical generation and protection? In: Human Muscular Function During Dynamic Exercise, edited by P. Marconnet, B. Saltin, P. Komi, and J. Poortmans. Basel, Karger, 1996, vol. 41, p (Med. Sport Sci. Ser.) 14. Hellsten, Y., B. Sjodin, E. A. Richter, and J. Bangsbo. Urate uptake and lowered ATP levels in human muscle after highintensity intermittent exercise. Am. J. Physiol. 274 (Endocrinol. Metab. 37): E600 E606, Jansson, E., G. A. Dudley, B. Norman, and P. A. Tesch. ATP and IMP in single human muscle fibres after high intensity exercise. Clin. Physiol. 7: , Jarasch, E.-D., C. Grund, G. Bruder, H. W. Heid, T. W. Keenan, and W. W. Franke. Localisation of xanthine oxidase in mammary-gland epithelium and capillary epithelium. Cell 25: 67 82, Ketai, L. H., R. H. Simpson, J. W. Kreit, and C. M. Grum. Plasma hypoxanthine and exercise. Am. Rev. Respir. Dis. 136: , Lowry, O. H., and J. V. Passoneau. A Flexible System of Enzymatic Analysis. New York: Academic, Manfredi, J. P., and E. W. Holmes. Control of the purine nucleotide cycle in extracts of rat skeletal muscle: effects of energy state and concentrations of cycle intermediates. Arch. Biochem. Biophys. 233: , McCartney, N., L. L. Spriet, G. J. F. Heigenhauser, J. M. Kowalchuk, J. R. Sutton, and N. L. Jones. Muscle power and metabolism in maximal intermittent exercise. J. Appl. Physiol. 60: , Nasrallah, S., and U. Al-Khalidi. Nature of purines excreted in urine during muscular exercise. J. Appl. Physiol. 19: , Nevill, M. E., L. H. Boobis, S. Brooks, and C. Williams. Effect of training on muscle metabolism during treadmill sprinting. J. Appl. Physiol. 67: , Rubio, R., and R. M. Berne. Localization of purine and pyrimidine nucleoside phosphorylases in heart, kidney, and liver. Am. J. Physiol. 239 (Heart Circ. Physiol. 8): H721 H730, Sahlin, K., K. Ekberg, and S. Cizinsky. Changes in plasma hypoxanthine and free radical markers during exercise in man. Acta Physiol. Scand. 142: , Sjodin, B., and Y. Hellsten Westing. Changes in plasma concentration of hypoxanthine and uric acid in man with shortduration running at various intensities. Int. J. Sports Med. 11: , Snow, R. J., M. J. Mckenna, S. E. Selig, J. Kemp, C. G. Stathis, and S. Zhao. Effect of creatine supplementation on sprint exercise performance and muscle metabolism. J. Appl. Physiol. 84: , Stathis, C. G., M. A. Febbraio, M. F. Carey, and R. J. Snow. Influence of sprint training on human skeletal muscle purine nucleotide metabolism. J. Appl. Physiol. 76: , Sutton, J. R., C. J. Toews, G. R. Ward, and I. H. Fox. Purine metabolism during strenous exercise in man. Metabolism 29: , Thorstensson, A., B. Sjodin, and J. Karlsson. Enzyme activities and muscle strength after sprint training in man. Acta Physiol. Scand. 94: , Tullson, P. C., J. Bangsbo, Y. Hellsten, and E. A. Richter. IMP metabolism in human skeletal muscle after exhaustive exercise. J. Appl. Physiol. 78: , Vincent, M.-F, G. Van den Berghe, and H.-G. Hers. Metabolism of hypoxanthine in isolated rat hepatocytes. Biochem. J. 222: , Wynants, J., and H. Van Belle. Single-run high performance liquid chromatography of nucleotides, nucleosides, and major purine bases and its application to different tissue extracts. Anal. Biochem. 144: , 1985.