Muscle power and metabolism in maximal intermittent exercise

Muscle power and the associated metabolic changes in muscle were investigated in eight male human subjects who performed four 30-s bouts of maximal isokinetic cycling at 100 rpm, with 4-min recovery intervals. In the first bout peak power and total work were (mean +/- SE) 1,626 +/- 102 W and 20.83 +/- 1.18 kJ, respectively; muscle glycogen decreased by 18.2 mmol/kg wet wt, lactate increased to 28.9 +/- 2.7 mmol/kg, and there were up to 10-fold increases in glycolytic intermediates. External power and work decreased by 20% in both the second and third exercise periods, but no further change occurred in the fourth bout. Muscle glycogen decreased by an additional 14.8 mmol/kg after the second exercise and thereafter remained constant. Muscle adenosine triphosphate (ATP) was reduced by 40% from resting after each exercise period; creatine phosphate (CP) decreased successively to less than 5% of resting; in the recovery periods ATP and CP increased to 76 and 95% of initial resting levels, respectively. Venous plasma glycerol increased linearly to 485% of resting; free fatty acids did not change. Changes in muscle glycogen, lactate, and glycolytic intermediates suggested rate limitation at phosphofructokinase during the first and second exercise periods, and phosphorylase in the third and fourth exercise periods. Despite minimal glycolytic flux in the third and fourth exercise periods, subjects generated 1,000 W peak power and sustained 400 W for 30 s, 60% of the values recorded in the first exercise period.(ABSTRACT TRUNCATED AT 250 WORDS)     

Metabolic adaptation in phosphorylase kinase deficiency. Changes in metabolite concentrations during tetanic stimulation of mouse leg muscles.

1. Glycogen, nucleotides and glycolytic intermediates and products were measured before and during tetanus in the hamstrings-muscle groups of normal (C3H) and phosphorylase kinase-deficient (ICR/IAn) mice. 2. Phosphorylase kinase-deficient muscles contained 3-4-fold more glycogen and sustained a larger (approx. 2-fold), more rapid (11 +/- 2 ng/s faster) and more prolonged glycogenolysis during 120s tetanus despite their lack of phosphorylase a. 3. No significant change in total adenine nucleotide contents occurred during tetanus in either strain, but there was a 60-100-fold rise in IMP concentration to approx. 2mM in both strains. The initial rate of IMP formation was 6-fold more rapid (112 nmol/s per g) in phosphorylase kinase-deficient muscle. 4. Adenylosuccinate content rose to 36 nmol/g in phosphorylase kinase-deficient muscle and to 9 nmol/g in normal muscle at 45s tetanus, but then fell. 5. In phosphorylase kinase-deficient muscle, glucose 6-phosphate, a powerful phosphorylase inhibitor, was 56% of that in normal muscle. 6. The mass-action ratio of the phosphoglucomutase-catalysed reaction [glucose 6-phosphate]/[glucose 1-phosphate] was markedly lower than Keq. (approx. 17) in relaxed muscle of both strains (approx. 5-7), but rose significantly during tetanus to the value for Keq. 7. The data for IMP satisfy the criteria put forward by Rahim, Perrett & Griffiths [(1976) FEBS Lett. 69, 203-206] for a nucleotide activator of phosphorylase b: it should be present at a higher concentration in phosphorylase kinase-deficient muscle, its concentration should rise during muscle work, and it should attain a concentration comparable with its activation constant for phosphorylase b.     

Effects of ischaemia on content of metabolites in rat liver and kidney in vivo

1. The time-course of changes in content of intermediates of glycolysis in rat liver and kidney cortex after severance of blood supply was investigated. 2. The decline in content of ATP was more rapid in kidney (1.7-0.5mumol/g in 30s) than in liver (2.7-1.6mumol/g in 60s). In both tissues AMP and P(i) accumulated. 3. Net formation of lactate was 1.7mumol/g during the second minute of ischaemia in liver from well-fed rats, 1.1mumol/g in liver from 48h-starved rats, and about 1.0mumol/g during the first 30s of ischaemia in kidney. Net formation of alpha-glycerophosphate was rapid, especially in liver. 4. In kidney the concentration of beta-hydroxybutyrate rose, but that of alpha-oxoglutarate and acetoacetate decreased. 5. In both organs the concentrations of fructose diphosphate and triose phosphates increased during ischaemia and those of other phosphorylated C(3) intermediates decreased. 6. The concentration of the hexose 6-phosphates rose rapidly during the first minute of ischaemia in liver, but decreased during renal ischaemia. 7. In kidney the content of glutamine fell after 2min of ischaemia, and that of ammonia and glutamate rose. 8. The redox states of the cytoplasmic and mitochondrial NAD couple in kidney cortex were similar to those in liver. 9. The regulatory role of glycogen phosphorylase, pyruvate kinase and phosphofructokinase is discussed in relation to the observed changes in the concentrations of the glycolytic intermediates.     

Regulatory steps of glycolysis during environmental anoxia and muscular work in the cockle,Cardium tuberculatum: control of low and high glycolytic flux

Summary Concentrations of glycolytic intermediates, end products of anaerobic metabolism and the adenylates have been determined in the foot muscle and in the whole soft body tissue of the cockle,Cardium tuberculatum, after anoxic incubation and after the performance of vigorous escape movements. Comparison of the mass action ratios (MAR) with the equilibrium constants (Keq) showed that the reactions catalyzed by glycogen phosphorylase, hexokinase, phosphofructokinase (PFK) and pyruvate kinase (PK) were displaced from equilibrium under all physiological situations investigated.Changes in the levels of the glycolytic intermediates showed that activation of phosphofructokinase is largely responsible for the 100-fold increase of glycolytic flux in the foot muscle during exercise.Analysis of the whole soft body tissue showed that PFK is also involved in reduction of the glycolytic flux during anoxia, but a more pronounced change in the MAR occurs for PK, indicating that PK is strongly inhibited under these conditions.Differences in the regulation of glycolysis in muscular and non-muscular tissues can be related to changes in metabolite levels and to tissue-specific forms of pyruvate kinase with different regulatory properties.     

ATP utilization and force during intermittent and continuous muscle contractions

Energy utilization and force generation under anaerobic conditions were studied in electrically stimulated quadriceps femoris muscle of four volunteers. To investigate the effects of intermittent vs. continuous stimulation one leg was stimulated intermittently and the other continuously during 50 s. The same initial force was produced, and biopsy samples were obtained before the stimulation and after 10, 20, and 50 s and analyzed for energy-rich phosphagens, glycolytic intermediates, and phosphorylase. The ATP utilization and glycolysis were greater during intermittent contraction, but glycogenolysis was equal. ATP content decreased to lower values after intermittent contraction (16.4 compared with 19.6 mmol/kg dry muscle after continuous contraction). Force generation was well preserved during continuous contraction but successively decreased after 20 s of intermittent stimulation down to 50% of initial at end of work. The energy cost per unit work was greater during intermittent stimulation and increased with contraction time, whereas it decreased with time during continuous stimulation. The decrease in force generation in intermittent exercise is suggested to be due to the higher energy cost for contraction resulting in greater changes in the intracellular environment with lower ATP and increased H+ and Pi. These changes would decrease both activation of the contractile system and the cross-bridge turnover rate resulting from activation.     

Effects of increased heart work on glycolysis and adenine nucleotides in the perfused heart of normal and diabetic rats

1. In the isolated perfused rat heart, the contractile activity and the oxygen uptake were varied by altering the aortic perfusion pressure, or by the atrial perfusion technique (;working heart'). 2. The maximum increase in the contractile activity brought about an eightfold increase in the oxygen uptake. The rate of glycolytic flux rose, while tissue contents of hexose monophosphates, citrate, ATP and creatine phosphate decreased, and contents of ADP and AMP rose. 3. The changes in tissue contents of adenine nucleotides during increased heart work were time-dependent. The ATP content fell temporarily (30s and 2min) after the start of left-atrial perfusion; at 5 and 10min values were normal; and at 30 and 60min values were decreased. ADP and AMP values were increased in the first 15min, but were at control values 30 or 60min after the onset of increased heart work. 4. During increased heart work changes in the tissue contents of adenine nucleotide and of citrate appeared to play a role in altered regulation of glycolysis at the level of phosphofructokinase activity. 5. In recirculation experiments increased heart work for 30min was associated with increased entry of [(14)C]glucose (11.1mm) and glycogen into glycolysis and a comparable increase in formation of products of glycolysis (lactate, pyruvate and (14)CO(2)). There was no major accumulation of intermediates. Glycogen was not a major fuel for respiration. 6. Increased glycolytic flux in Langendorff perfused and working hearts was obtained by the addition of insulin to the perfusion medium. The concomitant increases in the tissue values of hexose phosphates and of citrate contrasted with the decreased values of hexose monophosphates and of citrate during increased glycolytic flux obtained by increased heart work. 7. Decreased glycolytic flux in Langendorff perfused hearts was obtained by using acute alloxan-diabetic and chronic streptozotocin-diabetic rats; in the latter condition there were decreased tissue contents of hexose phosphates and of citrate. There were similar findings when working hearts from streptozotocin-diabetic rats with insulin added to the medium were compared with normal hearts. 8. The effects of insulin addition or of the chronic diabetic state could be explained in terms of an action of insulin on glucose transport. Increased heart work also acted at this site, but in addition there was evidence for altered regulation of glycolysis mediated by changes in tissue contents of adenine nucleotides or of citrate.     

Glycogen phosphorylase and pyruvate dehydrogenase transformation in white muscle of trout during high-intensity exercise

We examined the regulation of glycogen phosphorylase (Phos) and pyruvate dehydrogenase (PDH) in white muscle of rainbow trout during a continuous bout of high-intensity exercise that led to exhaustion in 52 s. The first 10 s of exercise were supported by creatine phosphate hydrolysis and glycolytic flux from an elevated glycogenolytic flux and yielded a total ATP turnover of 3.7 micromol x g wet tissue(-1) x s(-1). The high glycolytic flux was achieved by a large transformation of Phos into its active form. Exercise performed from 10 s to exhaustion was at a lower ATP turnover rate (0.5 to 1.2 micromol x g wet tissue(-1) x s(-1)) and therefore at a lower power output. The lower ATP turnover was supported primarily by glycolysis and was reduced because of posttransformational inhibition of Phos by glucose 6-phosphate accumulation. During exercise, there was a gradual activation of PDH, which was fully transformed into its active form by 30 s of exercise. Oxidative phosphorylation, from PDH activation, only contributed 2% to the total ATP turnover, and there was no significant activation of lipid oxidation. The time course of PDH activation was closely associated with an increase in estimated mitochondrial redox (NAD(+)-to-NADH concentration ratio), suggesting that O2 was not limiting during high-intensity exercise. Thus anaerobiosis may not be responsible for lactate production in trout white muscle during high-intensity exercise.     

Tissue glycolytic potentials of penaeid prawn, Metapenaeus monoceros during methylparathion, carbaryl and aldrin exposure.

Changes in midgut gland and muscle tissue glycolytic potentials of penaeid prawn, Metapenaeus monoceros following exposure to methylparathion, carbaryl and aldrin were studied. A decrease in total carbohydrates, glycogen and pyruvate and an increase in lactate levels were observed. An increase in phosphorylase 'a' and aldolase activity levels suggested increased formation of trioses during selected insecticide toxicity. The decrease in the lactate dehydrogenase activity levels and an increase in the lactate content indicative of reduced mobilization of pyruvate into the citric acid cycle. During selected insecticide exposure, the prawn tissues adopting some sort of regulatory pathways such as enhanced glycolysis and a shift of metabolic emphasis from aerobiosis to anaerobiosis. All the above mentioned changes were more pronounced in the midgut gland compared to muscle tissue. The per cent decrease or increase are more under aldrin exposure followed by carbaryl and methylparathion exposure. The selected insecticides in the present investigation, though belongs to different representative groups, but appears to be neurotoxic leading to disturbances in the carbohydrate catabolism.     

Human muscle metabolism during sprint running

Biopsy samples were obtained from vastus lateralis of eight female subjects before and after a maximal 30-s sprint on a nonmotorized treadmill and were analyzed for glycogen, phosphagens, and glycolytic intermediates. Peak power output averaged 534.4 +/- 85.0 W and was decreased by 50 +/- 10% at the end of the sprint. Glycogen, phosphocreatine, and ATP were decreased by 25, 64, and 37%, respectively. The glycolytic intermediates above phosphofructokinase increased approximately 13-fold, whereas fructose 1,6-diphosphate and triose phosphates only increased 4- and 2-fold. Muscle pyruvate and lactate were increased 19 and 29 times. After 3 min recovery, blood pH was decreased by 0.24 units and plasma epinephrine and norepinephrine increased from 0.3 +/- 0.2 nmol/l and 2.7 +/- 0.8 nmol/l at rest to 1.3 +/- 0.8 nmol/l and 11.7 +/- 6.6 nmol/l. A significant correlation was found between the changes in plasma catecholamines and estimated ATP production from glycolysis (norepinephrine, glycolysis r = 0.78, P less than 0.05; epinephrine, glycolysis r = 0.75, P less than 0.05) and between postexercise capillary lactate and muscle lactate concentrations (r = 0.82, P less than 0.05). The study demonstrated that a significant reduction in ATP occurs during maximal dynamic exercise in humans. The marked metabolic changes caused by the treadmill sprint and its close simulation of free running makes it a valuable test for examining the factors that limit performance and the etiology of fatigue during brief maximal exercise.     

Studies with a reconstituted muscle glycolytic system. The rate and extent of creatine phosphorylation by anaerobic glycolysis

A mixture of purified muscle glycolytic enzymes was reconstituted and the mixture shown to behave in a fashion analogous to that occurring in vivo. Glycolysis leads to ATP production in muscle and results in the phosphorylation of creatine. The extent of this phosphorylation by anaerobic glycolysis was shown to depend to a small extent on the relative proportions of available P(i) and creatine initially, but more importantly on the first step in glycolysis, in this case the enzyme phosphorylase. With less than 0.1% of the phosphorylase in the a form, only about one-third of the creatine was phosphorylated in 30min, whereas with 4% or more of phosphorylase a, 90% of the creatine was phosphorylated within this time. Inclusion of an adenosine triphosphatase decreased the steady-state concentration of phosphocreatine in the system. Calculations of the theoretical concentrations of ADP and AMP showed that phosphorylase b was almost inactive even in the presence of 9mum-AMP, because of ATP inhibition. With phosphorylase a present, glycolysis was able to continue at least until the calculated concentration of MgADP(-) was only 7mum, and AMP in the sub-mumolar range. The relation of these values to measured concentrations of nucleotides and to phosphorylase a percentages in intact muscle is discussed.     

The regulation of glycolysis in perfused locust flight muscle

Concentrations of glycolytic intermediates, amino acids and possible regulator substances were measured in extracts from locust thoracic muscles perfused under different conditions. The conversion of [(14)C]glucose into intermediates and CO(2) by muscle preparations was also followed. When muscles perfused with glucose were made anaerobic changes in metabolite concentrations occurred that could be accounted for by an activation of phosphofructokinase and pyruvate kinase. When butyrate and glucose were present in the perfusion medium the rate of glycolytic flux was lower than with glucose alone, and the aldolase reaction appeared to be inhibited. When butyrate alone was supplied to the muscle the concentrations of most glycolytic intermediates were similar to those found when glucose was supplied. Iodoacetate caused changes in concentrations of intermediates that appeared to result from inhibition of glyceraldehyde 3-phosphate dehydrogenase. Fluoroacetate-poisoned muscles showed a high citrate concentration, but no obvious site of inhibition by citrate was apparent in the glycolytic pathway. Mechanisms for control of glycolysis in locust flight muscle are discussed and related to the known properties of isolated enzymes. It is proposed that trehalase, hexokinase, phosphofructokinase, aldolase, and pyruvate kinase may be control enzymes in this tissue.     

Possible regulatory interactions between compartmentalized glycolytic systems during initiation of glycolylsis in ascites tumor cells

Changes were measured in the rates of respiration and in the levels of glycolytic intermediates during the first 5 min after addition of 1.6 mM glucose to a suspension (5%, v/v) of respiring Ehrlich ascites carcinoma cells incubated in an isotonic 50 mM tris(hydroxymethyl)methylglycine buffer (pH 7.4) at 38 °C. The rates of accumulation of lactate and glycolytic intermediates were used to calculate the in vitro velocities of glycolytic enzymes.The initial velocities of hexokinase (EC 2.7.1.1), fructose-6-phosphate kinase (EC 2.7.1.11) and lactate dehydrogenase (EC 1.1.1.27) in μmoles glucose equivalents/ ml cells per min were 14, 11 and 4, respectively. The velocities of the two kinases fell sharply to less than 5 between 5 and 10 s, while the velocity of the dehydrogenase declined gradually over the first minute. The initial burst of activity in the kinases, which lasted for about 8 s, was associated with a rapid accumulation of phosphate ester and a negative net ATP generation by glycolysis. The accumulation of phosphate ester is almost exactly matched by the generation of ATP by the “tail end” of glycolysis (triose-P to lactate) in this period. After this time (10–25 s) the rate of oxidative phosphorylation calculated as six times the rate of O₂ consumption, is nearly identical to the combined rate of ATP utilization by hexokinase and fructose-6-phosphate kinase. As observed previously, oxamate (42 mM) blocked lactate dehydrogenase but did not depress the rate of phosphate ester accumulation.These various observations and correlations can be interpreted in terms of a dual glycolytic system. The accumulation of phosphate ester during the first 8 s is attributed to the operation of a partial glycolytic system, System B, which includes only the first three or four enzymes of glycolysis, and which draws upon an ATP pool (Pool I) previously employed in assorted cytoplasmic phosphorylations. The ADP generated by System B is rephosphorylated by and regulates the rate of a complete glycolytic system A, which converts glucose to lactate with little intermediate accumulation. The tail end of System A generates a new pool of ATP (Pool II) and controls the rate of glucose input through its head end, which is supplied by ATP being produced by oxidative phosphorylation. This scheme of interlocking controls is transient and alters after 8 s, when System B slows to a stop.     

Phosphocreatine recovery kinetics following low- and high-intensity exercise in human triceps surae and rat posterior hindlimb muscles

Previous studies have suggested the recovery of phosphocreatine (PCr) after exercise is at least second-order in some conditions. Possible explanations for higher-order PCr recovery kinetics include heterogeneity of oxidative capacity among skeletal muscle fibers and ATP production via glycolysis contributing to PCr resynthesis. Ten human subjects (28 +/- 3 yr; mean +/- SE) performed gated plantar flexion exercise bouts consisting of one contraction every 3 s for 90 s (low-intensity) and three contractions every 3 s for 30 s (high-intensity). In a parallel gated study, the sciatic nerve of 15 adult male Sprague-Dawley rats was electrically stimulated at 0.75 Hz for 5.7 min (low intensity) or 5 Hz for 2.1 min (high intensity) to produce isometric contractions of the posterior hindlimb muscles. [(31)P]-MRS was used to measure relative [PCr] changes, and nonnegative least-squares analysis was utilized to resolve the number and magnitude of exponential components of PCr recovery. Following low-intensity exercise, PCr recovered in a monoexponential pattern in humans, but a higher-order pattern was typically observed in rats. Following high-intensity exercise, higher-order PCr recovery kinetics were observed in both humans and rats with an initial fast component (tau < 15 s) resolved in the majority of humans (6/10) and rats (5/8). These findings suggest that heterogeneity of oxidative capacity among skeletal muscle fibers contributes to a higher-order pattern of PCr recovery in rat hindlimb muscles but not in human triceps surae muscles. In addition, the observation of a fast component following high-intensity exercise is consistent with the notion that glycolytic ATP production contributes to PCr resynthesis during the initial stage of recovery.     

Muscle glycogenolysis and H+ concentration during maximal intermittent cycling

The relationships between muscle glycogenolysis, glycolysis, and H+ concentration were examined in eight subjects performing three 30-s bouts of maximal isokinetic cycling at 100 rpm. Bouts were separated by 4 min of rest, and muscle biopsies were obtained before and after bouts 2 and 3. Total work decreased from 20.5 +/- 0.7 kJ in bout 1 to 16.1 +/- 0.7 and 13.2 +/- 0.6 kJ in bouts 2 and 3. Glycogenolysis was 47.2 and 15.1 mmol glucosyl U/kg dry muscle during bouts 2 and 3, respectively. Lower accumulations of pathway intermediates in bout 3 confirmed a reduced glycolytic flux. In bout 3, the work done represented 82% of the work in bout 2, whereas glycogenolysis was only 32% of that in bout 2. Decreases in ATP and phosphocreatine contents were similar in the two bouts. Muscle [H+] increased from 195 +/- 12 to 274 +/- 19 nmol/l during bout 2, recovered to 226 +/- 8 nmol/l before bout 3, and increased to 315 +/- 24 nmol/l during bout 3. Muscle [H+] could not be predicted from lactate content, suggesting that ion fluxes are important in [H+] regulation in this exercise model. Low glycogenolysis in bout 3 may be due to an inhibitory effect of increased [H+] on glycogen phosphorylase activity. Alternately, reduced Ca2+ activation of fast-twitch fibers (including a possible H+ effect) may contribute to the low overall glycogenolysis. Total work in bout 3 is maintained by a greater reliance on slow-twitch fibers and oxidative metabolism.     

Control of glycolysis in the posterior adductor muscle of the sea musselMytilus edulis

Summary Concentrations of glycolytic intermediates, lactate, adenine nucleotides, inorganic phosphate, phosphoarginine and citrate have been estimated after various periods of valve closure (Table 1 and Fig. 1). Mass action ratios of enzyme steps involved in the metabolism of these components are compared with their equilibrium constants. This reveals glycogen phosphorylase, phosphofructokinase, hexosediphosphatase and pyruvate kinase catalyze non-equilibrium reactions. The first three enzymes possess relatively low activities (Table 2).From the changes in concentrations of the glycolytic intermediates it is concluded that phosphofructokinase controls the carbon flow during the first hours after valve closure, whereas later on the rate of conversion of phosphoenolpyruvate is determining this flow. In skeletal muscle phosphofructokinase controls the carbon flow during the whole period of exercise.The concentrations of ADP, AMP and inorganic phosphate increase, whereas the concentrations of ATP, phosphoarginine and citrate decrease during valve closure (Table 1 and Fig. 2). In contrast to skeletal muscle, these changes do not result in a strong increase in the glycolytic flux.There is a much greater potential for ATP hydrolysis by the myofibrillar ATPase system than is actually realized by the adductor muscle during valve closure.     

Glucokinase regulatory protein is associated with mitochondria in hepatocytes

The association of glucokinase with liver mitochondria has been reported [Danial et al. (2003) BAD and glucokinase reside in a mitochondrial complex that integrates glycolysis and apoptosis. Nature 424, 952-956]. We confirmed association of glucokinase immunoreactivity with rat liver mitochondria using Percoll gradient centrifugation and demonstrated its association with the 68 kDa regulatory protein (GKRP) but not with the binding protein phosphofructokinase-2/fructose bisphosphatase-2. Substrates and glucagon induced adaptive changes in the mitochondrial glucokinase/GKRP ratio suggesting a regulatory role for GKRP. Combined with previous observations that GKRP overexpression partially inhibits glycolysis [de la Iglesia et al. (2000) The role of the regulatory protein of glucokinase in the glucose sensory mechanism of the hepatocyte. J. Biol. Chem. 275, 10597-10603] these findings suggest that there may be distinct glycolytic pools of glucokinase.     

Effect of hindlimb unweighting on anaerobic metabolism in rat skeletal muscle.

Female Sprague-Dawley rats (250 g) were hindlimb suspended for 14 days, and the effects of hindlimb unweighting (HU) on skeletal muscle anaerobic metabolism were investigated and compared with nonsuspended controls (C). Soleus (SOL), plantaris (PL), and red and white portions of the gastrocnemius (RG, WG) were sampled from resting and stimulated limbs. Muscle atrophy after HU was 46% in SOL, 22% in PL, and 24% in the gastrocnemius compared with nonsuspended C animals. The muscles innervated by the sciatic nerve were stimulated to contract with an occluded circulation for 60 s with trains of supramaximal impulses (100 ms, 80 Hz) at a train rate of 1.0 Hz. Peak tension development by the gastrocnemius-PL-SOL muscle group was similar in HU and C animals (13.0 +/- 1.2, 12.2 +/- 0.8 N/g wet muscle). Occlusion of the circulation before stimulation created a predominantly anaerobic environment, and in situ glycogenolysis and glycolysis were estimated from accumulations of glycolytic intermediates. Total glycogenolysis and glycolysis were higher in the RG muscle of HU animals (74.6 +/- 3.3, 58.1 +/- 1.1) relative to C (57.1 +/- 4.6, 46.1 +/- 2.9 mumol glucosyl units/g dry muscle). Consequently, total anaerobic ATP production was also increased (HU, 251.3 +/- 1.1; C, 204.6 +/- 8.9 mumol ATP/g dry muscle). Total ATP production, glycogenolysis, and glycolysis were unaffected by HU in SOL, PL, and WG muscles. The enhanced glycolytic activity in RG after HU may be attributed to a shift in the metabolic profile from oxidative to glycolytic in the fast oxidative-glycolytic fiber population.     

Fructose 2,6-bisphosphate in rat skeletal muscle during contraction.

Fructose 2,6-bisphosphate and several glycolytic intermediates were measured in two rat muscles, extensor digitorum longus and gastrocnemius, which were electrically stimulated in situ. Both the duration and the frequency of stimulation were varied to obtain different rates of glycolysis. There was no relationship between fructose 2,6-bisphosphate content and the increase in tissue lactate in contracting muscle. However, in gastrocnemius stimulated at low frequencies (less than or equal to 5 Hz), there was a 2-fold increase in fructose 2,6-bisphosphate at 10s, followed by a return to basal values, whereas lactate increased only after 1 min of contraction. The concentrations of hexose 6-phosphates, fructose 1,6-bisphosphate and triose phosphates were all increased during the 3 min stimulation. During tetanus (frequencies greater than or equal to 10 Hz) fructose 2,6-bisphosphate was not increased, whereas glycolysis was maximally stimulated and resulted in an accumulation of tissue lactate, mostly from glycogen. The concentrations of hexose 6-phosphate increased continuously during the 1 min tetanus, whereas fructose 1,6-bisphosphate was increased at 10s and then decreased progressively. It therefore appears that fructose 2,6-bisphosphate does not play a role in the stimulation of glycolysis during tetanus; it may, however, be involved in the control of glycolysis when the muscles are stimulated at low frequencies for short periods of time.