Physiology and Occupational Physiology Springer-Verlag 1993

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1 Eur J Appl Physiol (1993) 66:49-54 European Applied Journal of Physiology and Occupational Physiology Springer-Verlag 1993 Influence of sodium bicarbonate ingestion on plasma ammonia accumulation during incremental exercise in man C. P. Lambert*, P. L. Greenhaff**, D. Ball, and R. J. Maughan Department of Environmental and Occupational Medicine, University Medical School, Foresterhill, Aberdeen, AB9 2ZD, Scotland Accepted August 20, 1992 Summary. This investigation evaluated the influence of metabolic alkalosis on plasma ammonia (NH3) accumulation during incremental exercise. On two occasions separated by at least 6 days, six healthy men cycled at 70, 80, and 90% of maximum oxygen consumption (I202max) for 5 min; each exercise period was followed by 5 min of seated recovery. Exercise was then performed at 100% 1202max until exhaustion. Beginning 3 h prior to exercise, subjects ingested 3.6 mmol.kg body mass- 1 NaHCO3 (test, T) or 3.0 mmol. kg body mass - CaCO3 (placebo, P) (both equivalent to 0.3 g.kg -1) over a 2-h period. Trials were performed after an overnight fast and the order of treatments was randomized. Arterialized venous blood samples for the determination of acid-base status, blood lactate and plasma NH3 concentrations were obtained at rest before treatment, 15 s prior to each exercise bout (Pre 70%, Pre 80%, Pre 90%0, and Pre 100%0), and at 0, 5 (5'Post), and 10 (10'Post) min after exhaustion. Additional samples for blood lactate and plasma NH3 determination were obtained immediately after each exercise bout (Post 70%, Post 80%, Post 90%) and at 15min after exercise (15'Post). Time to exhaustion at 100% of ~ro2max was not significantly different between treatments [mean (SE): 173 (42) s and 184 (44) s for T and P respectively]. A significant treatment effect was observed for plasma ph with values being significantly higher on T than on P Pre 70% [7.461 (0.007) vs (0.008)], Pre 90o7o [7.410 (0.010) vs (0.016)], and 10'Post [7.317 (0.032) vs (0.036)]. The change in plasma ph was significantly greater following the 90% bout (Pre 100%- Pre 90%) for T [-0.09 (0.02)] than for P [-0.06 (0.01)]. Blood base excess and plasma bicarbonate concentrations were significantly higher for T than P before * Present address: Department of Exercise Science, School of Public Health, University of South Carolina, Columbia, SC 29208, USA ** Present address: Department of Physiology and Pharmacology, Queen's Medical Centre, University of Nottingham, NG7 2UH, England Correspondence to: R. J. Maughan each exercise bout but not at the point of exhaustion. During recovery, base excess was higher for T than P at 5'Post and 10'Post while the bicarbonate concentration was higher for T than P at 10'Post. A significant treatment effect was observed for the blood lactate concentration with T on the average being higher than P [7.0 (1.0) and 6.3 (1.1) mmol.1-1 for T and P averaged across the 12 sampling times]. Plasma NH3 accumulation was not different between treatments at any point in time. In addition, no differences were observed between treatments in blood alanine accumulation. The resuits suggest that under the conditions of the present investigation metabolic alkalosis does not influence plasma NH3 accumulation or endurance capacity during intense incremental exercise. Key words: High-intensity exercise - Metabolic alkalosis - Ammonia - Adenine nucleotide metabolism Introduction It is generally agreed that intramuscular acidosis is one of the probable causes of fatigue during high-intensity exercise of short duration (Costill et al. 1984; Hultman and Sahlin 1980; Sahlin 1978). The decline in force-generating capacity with acidosis has often been attributed to reductions in the rate of adenosine diphosphate (ADP) rephosphorylation resulting from inhibition of glycolysis (Sutton et al. 1981) and glycogenolysis (Spriet et al. 1989), and/or displacement of the creatine kinase equilibrium (Harris et al. 1977). A reduction in the ability to rephosphorylate ADP will result in increases in the concentrations of ADP, adenosine monophosphate (AMP), inosine monophosphate (IMP) ammonia (NH3), and the development of fatigue. The conversion of AMP to IMP and NH3 is catalysed by AMP deaminase. This enzyme is activated by transient increases in AMP and ADP and in vitro has a ph optimum between 6.2 and 6.5 (Setlow and Lowenstein 1967). Thus, the reduction in adenine nucleotides and

2 50 the accumulation of NH3 that occur during intense contraction have previously been attributed to an imbalance between ADP formation and rephosphorylation as well as activation of AMP deaminase. Extracellular alkalosis has been shown to facilitate the removal of lactate and H + from muscle (Hirche et al. 1975; Mainwood and Worsley-Brown 1975), and sodium bicarbonate (NaHCO3) ingestion in man can, under certain circumstances, improve high-intensity exercise capacity (Costill et al. 1984; Goldfinch et al. 1988; Sutton et al. 1981). One possible mechanism for this improved exercise capacity with alkalosis is maintenance of an optimal rate of ADP rephosphorylation. If exerciseinduced acidosis has a significant inhibitory effect on the ADP rephosphorylation mechanisms and/or a stimulatory effect on AMP deaminase activity, increasing buffering capacity through NaHCO3 administration may reduce adenine nucleotide loss during intense exercise and thus decrease NH3 production. With this in mind, Greenhaff et al. (1991) recently evaluated the effect of NaHCO3 administration on plasma NH3 accumulation during intense exercise in the thoroughbred horse. Plasma NH3 accumulation was significantly lower after this treatment when compared with the control condition. To our knowledge the influence of NaHCO3 administration on plasma NH3 accumulation during intense exercise has not been evaluated in man. Consequently, the purpose of the present investigation was to determine the influence of NaHCO3 administration on plasma NH3 accumulation and endurance capacity during intense exercise in man. Methods Subjects. Six healthy males [mean (SE): age 30 (2) years, height 1.78 (0.04) m and mass 77.1 (4.7) kg] participated in this investigation after giving written informed consent. All subjects participated in some form of regular physical exercise at the time of the investigation and had relatively high values for maximum oxygen consumption [~ro2max: 56 (3) ml. kg 1.min-1]. This investigation was approved by the local ethics committee before it initiation. Preliminary testing. The subjects' 1202mat x was determined using a discontinuous protocol on an electrically braked cycle ergometer. Initially, subjects cycled for 5 min at 100 or 150 W followed by 2 min of recovery. Thereafter, the power output was increased by 50 W on each subsequent bout. The exercise bouts decreased to 3 min while the recovery time increased to 5 min as the subject approached fatigue. Expired gases were collected and analysed for % CO2, % 02 and ventilatory volume using a semi-automated on- line system (Gould, Salford, UK). 1202max was considered the point at which PO: levelled off, i.e., <2 ml-kg.min -1 increase with increasing power output. 1202m, was verified in a second test approximately 2 days later by having the subject cycle at a power output 25 W below and above that corresponding to 1202max attained in the previous testing session. Six or 7 days prior to the first experimental trial, each subject participated in a familiarization trial which was identical to the experimental trials except that there was no treatment administration and no blood sampling. Experimental testing. On two occasions separated by at least 6 days, subjects cycled at workloads equivalent to 70, 80 and 90% 12Ozm~ for 5 min with 5 min of seated recovery after each bout. Exercise was then carried out to exhaustion at 100% The subjects attempted to maintain a pedal frequency of 70 rev' rain-1 during exercise. Exhaustion was defined as the point at which the subjects could no longer maintain 50 rev. rain-1. The subjects recorded their dietary intake for 48 h prior to the first experimental trial and duplicated this diet prior to the second trial. In addition, subjects abstained from alcohol consumption and strenuous exercise for 48 h prior to each trial. Over a 2-h period beginning 3 h prior to exercise subjects ingested 3.6 mmol.kg body mass -1 NaHCO3 (test, T) or 3.0 mmol.kg -1 CaCO3 (placebo, P) (0.3 g. kg-1 body mass for both T and P). Treatments were administered in double-blind fashion after an overnight fast and the order of treatment was randomized. Arterialized venous blood samples for the determination of blood acid-base parameters (1.5 ml) and metabolite concentrations (2.5 ml) were obtained via a 21-gauge butterfly cannula from a vein on the dorsal surface of the hand at rest prior to treatment administration, 15 s prior to each exercise bout (Pre 70 70, Pre 80%, Pre 90%, Pre 100%) and at 0, 5 (5'Post) and 10 (10'Post) rain after exhaustion. Additional blood samples (2.5 ml) for metabolite analyses were obtained immediately after the exercise bouts at 70 (Post 70%), 80 (Post 80%), and 90% (Post 90%) of 1202m~ as well as 15min after exhaustion (15'Post). Samples were obtained while the subject was seated on the ergometer. The cannula was kept patent by flushing with a small volume of isotonic saline. A schematic representation of the experimental protocol is provided in Fig. 1. Blood was arterialized by having the subject immerse his hand in hot (42 C) water for 10 min prior to the initial sampling as described by Forster et al. (1972) and by keeping his hand immersed in water at 42 during the recovery period between exercise bouts. Samples for the determination of acid-base status were collected anaerobically into heparinized syringes, capped, placed in iced water, and analysed within 2 h for plasma ph and blood PCO2 with a BMS3 system (Radiometer, Copenhagen, Denmark). Plasma bicarbonate and blood base excess were calculated using the method of Siggard-Andersen (1963). Blood for metabolite determinations was placed in tubes containing K3EDTA. Duplicate aliquots (100 ~tl) from the K3EDTA tube were immediately deproteinized in 1 ml of 0.3 N perchloric acid, centrifuged, and analysed for lactate and alanine using the methods of Maughan (1982). A 1.5 ml aliquot from the K3EDTA tube was immediately centrifuged and the plasma frozen in liquid N 2 and stored at - 55 C. Plasma NH3 was determined spectrophotometrically using a commercially available kit (Sigma procedure no. 170-UV) within 5 days of blood sam- Treatment~3h ~ i i E 5 10 % (/O 2 max 15 Time (mln) Fig. 1. Schematic representation of the experimental protocol. Metabolites (,L) measured were plasma ammonia (NH3), blood alanine and blood lactate; haemoglobin and haematocrit were also measured at these times. Blood acid-base parameters (O) include plasma ph and blood PCO2; plasma bicarbonate concentration and blood base excess were calculated. R, Rest before treatment; E, point of exhaustion; maximum oxygen consumption

3 51 pling. Haemoglobin (cyanamethaemoglobin method) and haematocrit (microcentrifuge method) were determined on the remaining blood and the results were used to calculate plasma volume changes according to the method of Dill and Costill (1974). Statistical analysis was by two-factor analysis of variance with repeated measures on both factors (treatment x time). When a significant main effect or interaction was present a Newman-Keuls post hoc test was used to locate the differences. The probability level of 0.05 was selected for statistical significance All values in text and tables are presented as mean (SE). Deviations between subjects on the same treatment in the figures are represented by standard error bars. Results As expected, the ingestion of NaHCO3 resulted in a significant elevation in both plasma bicarbonate (Fig. 2a) and blood base excess (Fig. 2b) prior to the 70% exercise bout relative to the pre-ingestion values and to the values attained on treatment P. Bicarbonate and base excess remained significantly higher on T relative to P at all times prior to the point of exhaustion. During recovery, bicarbonate (10'Post) and base excess (5'Post and 10'Post) were significantly higher on T than on P. Plasma ph (Fig. 3) declined significantly in both groups following the 70% bout and remained significantly lower than the pre-exercise value throughout the testing session In addition, plasma ph values were higher on treatment T than on P at Pre 7007o [7.461 (0.007) vs (0.008)], Pre 90O7o [7.410 (0.010) vs (0.016)] and 10'Post [7.317 (0.032) vs (0.036)]. Calculation of the change in blood ph (Table 1) revealed a significantly greater decline following the 90% bout (Pre 100%-Pre 90%) on T than P. The present exercise protocol resulted in a significant elevation in plasma NH3 concentration (Fig. 4) with the peak values occurring immediately after exercise at 100% [~ro2max. The plasma NH3 concentration did not differ between treatments at any time. Blood alanine concentration (Fig. 5) increased significantly after the exercise bout at 80% of ~ZO2max relative to the pre-exercise value (Pre 70%) and remained elevated at all subsequent sampling points. No differences were observed between treatments at any point in time. The blood lactate concentration (Fig. 6) increased with each increase in exercise intensity Peak values for blood lactate occurred 5 min after the point of exhaus- 30- (a) 7.5' ' E g ~ 10 --~ 7.3' o. 7.2' \ nlo~ 90 1 O ~' "~" N % ~f02 max 0 5'. O' E g -5" (b) f~ E 5 10 Time (mln) 7,1 I~ Jl 0 1' E 5 10Time(mini Fig. 3. Plasma ph before treatment administration (R), 15 s before each exercise bout (Pre 70 70, Pre 80%, Pre 90%, Pre 100%), at the point of exhaustion (E), and 5 (5'Post) and 10 (10'Post) min after exhaustion following NaHCO3 (-- --) and placebo (--e--) administration 1o- Table 1. The change in blood ph after exercise in the test and placebo treatments % ~/O2 max II R E 5 10Time (rain) Fig. 2. Plasma bicarbonate (HCO3) concentration (a) and blood base excess (b) before treatment administration (R), 15 s before each exercise bout (Pre 70%, Pre 80%, Pre 90%, Pre 100%), at the point of exhaustion (E), and 5 (5'Post) and 10 (10'Post) rain after exhaustion following NaHCO3 (--O--) and placebo (--e--) administration. Deviations between subjects on the same treatment are represented by standard error bars Test Placebo PRe 80%-Pre 70% (0.01) (0.01) Pre 90%-Pre 80% (0.01) (0.01) Pre 100%-Pre 90% (0.02) (0.01)* Post 100%-Pre 100% (0.04) (0.03) 5'Post-Post 100% (0.02) (0.02) 10'Post-5'Post 0.07 (0.04) 0.03 (0.00) * Significant difference (p < 0.05) between treatments All values are relative to the sample taken at rest before exercise at 70O7o of VO2m~ Values are mean (SE)

4 Table 2. Percentage change in plasma volume during exercise and recovery for the test and placebo treatments 140 ~ l Test Placebo ~-~ 120 ~ill 100'..~ 80' N ~ 0.,, ~. N N.~. %VO2max R I= Time(min) Fig. 4. Plasma ammonia (NH3) concentration before treatment administration (R), 15 s before (Pre 70%, Pre 80%, Pre 90070, Pre 100%), and immediately after (Post 70070, Post 80 70, Post 90o70, E) each exercise bout and 5 (5'Post) and 10 (10'Post) min after the point of exhaustion; --C)-- NaHCO3, --e-- placebo 2. E o 450' 400' 350' 300" 250 ' 200 ' Post 70% (0.5) (2.4) Pre 80 7o (1.1) (2.6) Post 80% (1.6) (1.5) Pre 90 7o (1.8) (1.6) Post 90 7o (1.7) (1.6) Pre (2.4) (2.5) Exhaustion (1.1) (2.0) 5'Post (2.3) (1.9) 10'Post (3.1) (2.1) 15'Post (2.7) (2.0) All values are relative to the sample taken at rest before exercise at 70 7o of ~ro2max Values are mean (SE) tion [11.9 (1.4) and 11.1 (1.8) mmol.1-1 for T and P respectively]. In addition, a significant treatment effect was observed with values on treatment T being higher than on treatment P, but there was no treatment-time interaction. Plasma volume fell slightly during treatment administration (pre-ingestion to Pre 70 7o); the change was similar for both treatments [-2.2 (2.5) and -3.3 (1.1 70)] for T and P, respectively. Plasma volume was reduced (Table 2) following the initial exercise bout on both treatments but was not different between treatments at any time. The mean peak change in plasma volume for the two groups was and occurred immediately after the bout. Time to exhaustion at of I702ma. was not different between treatments [173 (42) s for T and 184 (44) s for P]. Time to exhaustion during the familiarization trial was 180 (46) s, which was similar to the values obtained during the experimental trials. This suggests that there was not a negative effect of the experimental treatments on endurance capacity. v 0 R E Time (min) Discussion Fig. 5. Blood alanine concentration. Blood sampling points as in Fig " 10.0" 8.0" 6.0" "~ 1.0" 2.0" ~... % I~ 6 5 1"0 1"5 2() 2" I= 5 I() 15 Time (min) Fig. 6. Blood lactate concentration. Blood sampling points as in Fig. 4 The major finding of this investigation was that metabolic alkalosis did not influence plasma NH3 accumulation during intense exercise in man. Intramuscular acidbase status was not assessed in this investigation. However, if the greater blood lactate concentration and change in plasma ph on T than P (Pre 100o70-Pre 90O7o) reflect a difference in intramuscular acid-base status, it is possible that the lactic acidosis that occurs during intense submaximal and maximal exercise (100o70 I202max) may not be a significant factor influencing plasma NH3 accumulation in man. In contrast to our findings using human subjects, Greenhaff et al. (1991) reported that NaHCO3 administration reduced plasma NH3 accumulation during 2 rain of intense exercise in the thoroughbred horse. There are many possible explanations for these divergent results. First, the medial gluteal (a prime mover) of the thoroughbred horse is composed of at least 80o70 type II fi-

5 53 bres (Snow 1983), whereas the type II fibre content of the vastus lateralis of the human is lower and much more variable (Gollnick et al. 1972). Sahlin et al. (1989) have reported that IMP formation and NH3 production during exercise to exhaustion at 100% of l)'ozmax were twice as high in type II as in type I fibres. The greater IMP and NH3 formation in type II than type I fibres is probably the result of higher AMP deaminase activity (Bockman and McKenzie 1983; Schultz and Lowenstein 1976; Winder et al. 1974), a higher ATP turnover rate (Katz et al. 1986), a lower oxidative capacity (Essen et al. 1975), and/or a higher glycolytic capacity (Essen et al. 1975; Harris et al. 1976). Thus, any effect of alterations in acid-base status on ADP rephosphorylation or removal (glycogenolysis, glycolysis, the reaction catalysed by creatine kinase, and/or AMP deaminase activity) and therefore NH3 production would be more evident, if measured in mixed muscle or plasma, the greater the percentage and/or area of type II fibres. This suggestion is supported by the results of human studies in which a positive relationship has been observed between the percentage of type II fibres and the accumulation of NH3 in muscle (Katz et al. 1986) and plasma (Dudley et al. 1983). It is possible that a difference in NH3 accumulation may have been observed between treatments if the exercise protocol utilised in the present investigation had involved higher exercise intensity, shorter recovery periods and therefore a greater degree of metabolic stress. The primary stimulus for adenine nucleotide loss during exercise appears to be a high ATP turnover rate coupled with low phosphocreatine levels (Katz et al. 1986). Using an exercise protocol in which subjects cycled at 40% and 75% of 1202ma for 5 min each, followed by exercise to exhaustion at of gozmax [4.8 (0.6) rain], Sahlin and coworkers (1989) reported that phosphocreatine and ATP levels in vastus lateralis were reduced by 21% and 23% respectively at exhaustion. However, much greater reductions in the concentrations of phosphocreatine and ATP (65% and 42 70, respectively) were observed by Boobis et al. (1983) following 30 s of "all out" exercise. Furthermore, McCartney et al. (1986) reported reductions in the concentrations of phosphocreatine and ATP of 91% and 44% respectively immediately after the second of four "all out" 30-s bouts on an isokinetic cycle ergometer, each of which was followed by 4 min recovery. It appears from these results that greater reductions in muscle ATP and phosphocreatine concentrations can occur when one or more 30 s "all out" exercise bouts is performed than when exercise at 100% ~ro2max is performed to the point of exhaustion. Perhaps the metabolic alkalosis did not influence plasma NH3 accumulation in the present investigation because the reductions in phosphocreatine and in intramuscular ph were not of sufficient magnitude to substantially limit ATP turnover. However, our results clearly suggest that metabolic alkalosis has no measurable effect on plasma NH3 accumulation during intense submaximal and maximal (100% IZOzm~ ) exercise in man. An alternative explanation for the divergent findings between the investigation of Greenhaff et al. (1991) and the present investigation is that the degree of metabolic alkalosis induced in the investigation of Greenhaff et al. (1991) was probably much larger than that of the present investigation. A direct comparison of the acid-base values from this investigation with those attained in the investigation of Greenhaff et al. (1991) is difficult as we sampled arterialized venous blood while venous blood was sampled by Greenhaff et al. (1991). However, in the present investigation 3.6 mmol.kg body mass -1 (0.3 g.kg -1) NaHCO3 was administered orally while in the investigation of Greenhaff et al. (1991) 7.1 mmol.kg body mass- 1 (0.6 g' kg- 1) was administered via a nasogastric tube. As a result, the degree of metabolic alkalosis and therefore the muscle to blood lactate/h gradient in the investigation of Greenhaff et al. (1991) would have been much greater than that in the present investigation. Although the change in ph after the exercise bout at 90% of I202m~ in the present study was greater on treatment T than P, suggesting greater H efflux and possibly greater lactate production, the possibility remains that the degree of alkalosis induced in the present investigations was not of sufficient magnitude to influence intramuscular acid-base status and therefore NH3 production. It must be pointed out, however, that the amount of NaHCO3 administered in the present investigation was similar to that administered in other investigations where effects were observed on both exercise capacity and the plasma lactate concentration (Sutton et al. 1981). Some of the NH3 produced during intense contraction is removed from the muscle as the a-amino group of alanine: the pathway involves reductive amination of a-ketoglutarate to glutamate and a subsequent transamination reaction involving pyruvate (Katz et al. 1986). Therefore, it was of interest in the present investigation to evaluate the possible interactive effects of alkalosis and intense exercise on blood alanine accumulation. The finding that the blood alanine concentration was similar in alkalotic and control conditions suggests that the disposal of NH3 in the form of alanine was not measurably affected by metabolic alkalosis under the conditions of this experiment. Interestingly, the peak NH3 concentration immediately after exercise to exhaustion at 100% of 1202ma in the present investigation, approximately 130 gmol.1-1, was similar to and occurred at the same point in time as that reported by Babij et al. (1983). In that investigation, as in the present investigation, incremental exercise of increasing intensity was performed prior to the exhaustive bout at 100% of 1202m~,. In addition, our data for the peak blood alanine response, about 0.4 mmol.1-1, agree well with those reported by Greenhaff et al. (1988) (who also used arterialized venous blood) in which subjects exercised to exhaustion at 100% of I Many investigations have examined the possible ergogenic benefits of ingesting NaHCO3, but the results have been equivocal even when an identical exercise protocol was used (Goldfinch et al. 1988; Kindermann et al. 1977). It is not clear why performance was not enhanced in the present investigation, but it is possible that the

6 54 difference in acid-base status between treatments prior to the 100% bout, although statistically significant, was not great enough to enhance performance. Whether endurance capacity would have been enhanced had the bout at of VO2max been carried out to the point of exhaustion remains a matter of speculation, but the greater change in ph following this bout suggests that the lactate/h + efflux and/or production was greater on treatment T than P. Nonetheless, the results indicate that metabolic alkalosis does not measurably affect NH3 accumulation during intense submaximal and maximal (100% of ~'ro2max ) exercise in man. References Babij P, Matthews SM, Rennie MJ (1983) Changes in blood ammonia, lactate and amino acids in relation to workload during bicycle ergometer exercise in man. Eur J Appl Physiol 50 : Bockman E, McKenzie J (1983) Tissue adenosine content in active soleus and gracilis muscles of cats. Am J Physiol 244:H522- H599 Boobis LH, Williams C, Wootton SA (1983) Influence of sprint training on muscle metabolism during brief maximal exercise in man. J Physiol (Lond) 342:36P-37P Costill DL, Verstappen F, Kuipers H, Janssen E, Fink W (1984) Acid-base balance during repeated bouts of exercise: influence of HCOj. Int J Sports Med 5: Dill DB, Costill DL (1974) Calculation of percentage changes in volumes of blood, plasma, and red cells in dehydration. J Appl Physiol 37 : Dudley GA, Staron RS, Murray TF, Hagerman RC, Luginbuhl A (1983) Muscle fiber composition and blood ammonia levels after intense exercise in humans. J Appl Physiol 54: Essen B, Jansson E, Henriksson J, Taylor AW, Saltin B (1975) Metabolic characteristics of fibre types in human skeletal muscle. Acta Physiol Scand 95 : Forster HV, Dempsey JA, Thomson J, Vidruk E, DoPico GA (1972) Estimation of arterial PO2, PCOz, ph, and lactate from arterialised venous blood. 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Granta Editions Cambridge, UK, pp Spriet LL, Lindinger MI, McKelvie RS, Heigenhauser GFJ, Jones NL (1989) Muscle glycogenolysis and H concentration during maximal intermittent cycling. J Appl Physiol 66:8-13 Sutton JR, Jones NL, Toews CJ (1981) Effect of ph on muscle glycolysis during exercise. Clin Sci 61: Winder WW, Terjung RL, Baldwin KM, Holloszy JO (1974) Effect of exercise on AMP deaminase and adenylosuccinate in rat skeletal muscle. Am J Physiol 227: