Failure of glutamate dehydrogenase system to predict oxygenation state of human skeletal muscle

In a recent study, the total tissue contents of glutamate (Glu), ammonium (NH+4), and 2-oxoglutarate (2-OG) were used to estimate changes in the mitochondrial redox state ([NAD+]/[NADH]) of contracting skeletal muscle with intact circulation [Am. J. Physiol. 253 (Cell Physiol. 22): C263-C268, 1987]. These metabolites participate in the glutamate dehydrogenase (GDH) reaction, which, based on a number of assumptions, theoretically enables calculation of the mitochondrial redox state as follows (brackets indicate concentrations): [NAD+]/[NADH] = ([NH+4] [2-OG])/[( Glu]Kapp), where Kapp is the apparent equilibrium constant for GDH. The purpose of this study was to determine whether changes in the total tissue contents of Glu, NH+4, and 2-OG could be used to predict a reduction of the mitochondrial redox state in anoxic skeletal muscle. Anoxia was induced in the quadriceps femoris muscle by 10 min of circulatory occlusion (low metabolic rate) and isometric contraction to fatigue (high metabolic rate). The mean (+/- SE) value for the metabolite ratio ([NH+4][2-OG]/[Glu]) at rest was 6 +/- 3 mmol/kg dry wt (x 10(-4]. No significant change occurred after circulatory occlusion (4 +/- 2 x 10(-4); P greater than 0.05), whereas an almost 60-fold increase was observed after isometric contraction (P less than 0.05). Because the muscle was anoxic under both conditions, a significant decrease in the metabolite ratio should have occurred. These data demonstrate that changes in total tissue contents of Glu, NH+4, and 2-OG cannot be used to estimate changes in the redox and oxygenation state of mitochondria in intact human skeletal muscle.

Influence of reduced glycogen level on glycogenolysis during short-term stimulation in man

The relationship between muscle glycogen concentration and the rate of glycogen breakdown during short, intense contraction has been investigated in man. Prior to the experiment, muscle glycogen content was manipulated by a combination of exercise and diet, and varied from 155 +/- 19 to 350 +/- 25 mmol kg-1 dry muscle (36-81 mmol kg-1 wet wt). The quadriceps femoris muscle was stimulated electrically at a frequency of 20 Hz for 1 min. The blood flow to the leg was occluded during the experiment and muscle biopsies were taken before and after 10, 30 and 60 s stimulation. Force development and glycogenolytic rate were maintained constant during electrical stimulation and similar in all conditions, irrespective of the initial glycogen concentration. The phosphorylase a fraction was increased after 10 s stimulation, but returned to the initial values at the end of the stimulation. Muscle ATP was unaltered during the first 30 s stimulation, but decreased thereafter. The decrease in ATP was accompanied by a stoichiometric increase in inosine monophosphate. Phosphocreatine decreased during stimulation and was almost depleted at the end of stimulation. Muscle lactate and glucose 6-phosphate (Glu 6-P) increased during stimulation. None of these changes was significantly affected by the reduced glycogen contents. It is concluded that the rate of muscle glycogen breakdown is not affected by the initial glycogen level in the range of 155 +/- 19 to 350 +/- 25 mmol kg-1 dry muscle.

Muscle glucose metabolism during exercise

Substrate utilization during exercise is determined by the intensity and duration of exercise as well as by training status and the availability of substrate. The major energy source at rest and during low intensity exercise is fat oxidation while oxidation of carbohydrates and anaerobic energy utilization predominates at higher intensities. Hypoxia will induce a similar change in substrate utilization as an increase of the exercise intensity and it is suggested that O2 availability is an important determinant of muscle metabolism during exercise. Muscle glucose uptake increases during exercise and can reach values that are 30-50 times higher than at rest. The increase in muscle glucose uptake is related to the work load and the duration of the exercise, and corresponds to the output of glucose from the liver. Animal experiments have shown that muscle contraction can induce an increase in glucose uptake in the absence of insulin. Studies in man have shown that insulin and exercise at low intensity have a synergistic effect on glucose uptake, possibly due to an increased insulin inflow to the muscle. Intracellular glucose concentration is low at rest but can under certain conditions such as intensive exercise and hypoxia increase to maximum values of 3-4 mmol/l of intracellular water which demonstrates that glucose uptake under certain conditions can be in excess of the utilization of glucose.

Adenine nucleotide depletion in human muscle during exercise: causality and significance of AMP deamination

The predominant route for adenine nucleotide catabolism in skeletal muscle is deamination of AMP to inosine monophosphate (IMP) and ammonia (NH3). Deamination of AMP is enhanced during exercise when the capacity to rephosphorylate ADP is impaired. Thus, in human muscle the formation of IMP (NH3) during exercise is augmented under the following conditions (1) at high intensities, (2) during beta-adrenoceptor blockade, (3) during hypoxia, (4) after detraining, and (5) at low glycogen levels. The formation of IMP is related to the metabolic stress (as indicated by the degree of phosphocreatine breakdown and lactate accumulation), the rate of ATP turnover, and the fiber type composition. During maximal exercise at 100% of VO2max or sustained isometric contractions to fatigue, about 15% of the adenine nucleotide (AN) pool is degraded through deamination of AMP to IMP. It is suggested that the stimulus for increased AMP deamination is increased transient levels of ADP and AMP in the contracting muscle fiber. Deamination of AMP to IMP and NH3 provides a sink for ADP, whereby the ATP/ADP ratio and the phosphorylation potential are kept high, which may be essential for the continuation of the contraction process. This implies that the relative levels of the adenine nucleotides are more important for maintenance of adequate cellular function than the absolute concentration of ATP.

Muscle carnitine metabolism during incremental dynamic exercise in humans

The changes in muscle content of carnitine and acetylcarnitine have been studied during incremental dynamic exercise. Six subjects exercised for 10 min on an ergometer at 40 and 75% of their maximal oxygen uptake (VO2 max) and to fatigue at 100% of VO2 max (about 4 min). Muscle samples were taken from the quadriceps femoris muscle at rest and after exercise. Muscle content of free carnitine was (means +/- SE) 15.9 +/- 1.7 mmol kg-1 d.wt (dry weight) at rest and remained unchanged after exercise at low intensity but decreased to 5.9 +/- 0.6 and 4.6 +/- 0.5 mmol kg-1 d.wt after exercise at 75 and 100% of VO2 max respectively. Acetylcarnine content at rest was 6.9 +/- 1.9 mmol kg-1 d.wt and increased during exercise in correspondence with the decrease in free carnitine. Muscle content of pyruvate and lactate was unchanged after exercise at 40% of VO2 max but increased at the higher intensities. The parallel increases in acetylcarnitine, pyruvate and lactate indicate that formation of acetylcarnitine is augmented when the availability of glycolytic three-carbon metabolites is high and is consistent with the idea that acetylcarnitine provides a sink for pyruvate and acetyl CoA. This could be of importance for the maintenance of an adequate level of CoA and thus function of the tricarboxylic acid cycle.

Localization of rate-limiting defect for glucose disposal in skeletal muscle of insulin-resistant type I diabetic patients

We searched for metabolic crossover points in muscle glucose metabolite profiles during maintenance of matched glucose fluxes across forearm muscle in insulin-resistant type I (insulin-dependent) diabetic patients and nondiabetic subjects. To classify subjects as insulin sensitive or insulin resistant, whole-body and forearm glucose disposal, oxidative and nonoxidative glucose disposal (indirect calorimetry), and glycogen synthesis (muscle glycogen content in needle biopsies) were measured under euglycemic conditions at two insulin concentrations. Whole-body and forearm muscle glucose disposal were significantly reduced in diabetic patients compared with control subjects. The reduction in total glucose disposal was due to similar relative reductions in oxidative and nonoxidative glucose disposal, pointing toward rate limitation early in glucose metabolism. The defect in nonoxidative glucose disposal was at least partly due to a defect in muscle glycogen synthesis, because muscle glycogen content failed to increase in response to an increase in the plasma insulin concentration in the diabetic patients. The most-insulin-resistant type 1 diabetic patients were restudied under conditions where, by glucose mass action, whole-body glucose disposal was forced to be similar to that in the control subjects. Matching glucose fluxes in the two groups resulted in similar rates of forearm and whole-body oxidative and nonoxidative glucose disposal and muscle glycogen synthesis, but it did not result in accumulation of free intracellular glucose, glucose-6-phosphate, glucose-1-phosphate, fructose-6-phosphate, or lactate in muscle. These data imply that the rate-limiting defect for glucose disposal in skeletal muscle of type I diabetic patients is at the level of glucose transport.

Relationship of contraction capacity to metabolic changes during recovery from a fatiguing contraction

The relationship between changes in muscle metabolites and the contraction capacity was investigated in humans. Subjects (n = 13) contracted (knee extension) at a target force of 66% of the maximal voluntary contraction force (MVC) to fatigue, and the recovery in MVC and endurance (time to fatigue) were measured. Force recovered rapidly [half-time (t 1/2) less than 15 s] and after 2 min of recovery was not significantly different (P greater than 0.05) from the precontraction value. Endurance recovered more slowly (t 1/2 approximately 1.2 min) and was still significantly depressed after 2 and 4 min of recovery (P less than 0.05). In separate experiments (n = 10) muscle biopsy specimens were taken from the quadriceps femoris muscle before and after two successive contractions to fatigue at 66% of MVC with a recovery period of 2 or 4 min in between. The muscle content of high-energy phosphates and lactate was similar at fatigue after both contractions, whereas glucose 6-phosphate was lower after the second contraction (P less than 0.05). During recovery, muscle lactate decreased and was 74 and 43% of the value at fatigue after an elapsed period of 2 and 4 min, respectively. The decline in H+ due to lactate disappearance is balanced, however, by a release of H+ due to resynthesis of phosphocreatine, and after 2 min of recovery calculated muscle pH was found to remain at a low level similar to that at fatigue.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenine nucleotide degradation in human skeletal muscle during prolonged exercise

Eight healthy men cycled at a work load corresponding to approximately 70% of maximal O2 uptake (VO2max) to fatigue (exercise I). Exercise to fatigue at the same work load was repeated after 75 min of rest (exercise II). Exercise duration averaged 65 and 21 min for exercise I and II, respectively. Muscle (quadriceps femoris) content of glycogen decreased from 492 +/- 27 to 92 +/- 20 (SE) mmol/kg dry wt and from 148 +/- 17 to 56 +/- 17 (SE) mmol/kg dry wt during exercise I and II, respectively. Muscle and blood lactate were only moderately increased during exercise. The total adenine nucleotide pool (TAN = ATP + ADP + AMP) decreased and inosine 5'-monophosphate (IMP) increased in the working muscle during both exercise I (P less than 0.001) and II (P less than 0.01). Muscle content of ammonia (NH3) increased four- and eight-fold during exercise I and II, respectively. The working legs released NH3, and plasma NH3 increased progressively during exercise. The release of NH3 at the end of exercise II was fivefold higher than that at the same time point in exercise I (P less than 0.001, exercise I vs. II). It is concluded that submaximal exercise to fatigue results in a breakdown of the TAN in the working muscle through deamination of AMP to IMP and NH3. The relatively low lactate levels demonstrate that acidosis is not a necessary prerequisite for activation of AMP deaminase. It is suggested that the higher average rate of AMP deamination during exercise II vs. exercise I is due to a relative impairment of ATP resynthesis caused by the low muscle glycogen level.

Effect of hypoxia on glucose metabolism in human skeletal muscle during exercise

The effect of respiratory hypoxia on muscle glucose metabolism during short-term dynamic exercise has been investigated. Eight men cycled for 5 min at 120 +/- 6 W (mean +/- SE), which corresponded to 50% of maximal O2 uptake during normoxia (N), breathing air (N) on one occasion and 11% O2 (hypoxia-H) on the other. Biopsies were taken from the quadriceps femoris muscle before and after exercise. Oxygen uptake during exercise was not affected by H. The arterial blood glucose concentration during N exercise remained constant, but increased from 4.62 +/- 0.11 mmol l(-1) at rest to 5.22 +/- 0.19 mmol l-1 at the end of H exercise (P less than 0.05 vs N exercise). The intracellular glucose content at rest was low and did not change during N exercise, but was four times higher after exercise during H vs N (P less than 0.01). Glucose 6-P increased under both conditions but significantly more during H (P less than 0.01), while glucose 1,6-P2 was not significantly different between treatments either at rest or after exercise. It is concluded that: (1) glucose uptake by skeletal muscle during short-term exercise. It is concluded that: (1) glucose uptake by skeletal muscle during short-term exercise during H is not associated with a stoichiometric glucose utilization; (2) the inhibition of hexokinase during H (evidenced by increase in muscle glucose) is due primarily to the increase in glucose 6-P; and (3) glucose 1,6-P2 is of minor importance for the regulation of contraction-mediated flux through hexokinase in human skeletal muscle.

Formation of inosine monophosphate (IMP) in human skeletal muscle during incremental dynamic exercise

The influence of exercise intensity on the accumulation of inosine monophosphate (IMP) in human skeletal muscle has been investigated. Ten men cycled at workloads corresponding to 40%, 75% and 100% of their maximal oxygen uptake (VO2 max). Muscle IMP was below the detection limit (less than 0.01 mmol kg-1 dry wt) at rest and after exercise at 40% of VO2 max, but increased to 0.26 +/- 0.06 (mean +/- SEM) and 3.50 +/- 0.51 mmol kg-1 dry wt after exercise at 75% and 100% of VO2 max respectively. Accumulation of IMP corresponded to a similar decrease in the total adenine nucleotide content. The muscle content of IMP was positively related to lactate and negatively related to phosphocreatine (PCr). IMP was formed in both fibre types, but the IMP content at fatigue was about twice as high in type II fibres as in type I fibres. It was concluded that the IMP content of human skeletal muscle is very low at rest and after low-intensity exercise, but increases after moderate and high-intensity exercise. In contrast to rat muscle, where deamination of AMP predominantly occurs in the fast-twitch muscle fibres, IMP is formed during exercise in both fibre types in human muscle. Accumulation of IMP appears to reflect an imbalance between the rate of utilization and the rate of regeneration of ATP.

Hypoxaemia increases the accumulation of inosine monophosphate (IMP) in human skeletal muscle during submaximal exercise

The effect of hypoxaemia on the muscle content of inosine monophosphate (IMP) during short-term, low-intensity exercise has been investigated. Six men cycled twice for 5 min at 120 +/- 6 W (mean +/- SE), which corresponded to approximately 50% of their maximal normoxic O2 uptake, breathing air (N) on one occasion and 11% O2 in N2 (H) on the other. Oxygen uptake at the end of the exercise period was similar between treatments. No significant difference was observed between H and N in the muscle metabolite contents at rest. Muscle content of phosphocreatine (PCr) decreased and lactate increased during exercise. Post-exercise PCr during H was 80% of the value during N (P greater than 0.05) and post-exercise muscle lactate was fourfold higher during H than during N (P less than 0.001). Post-exercise muscle content of ADP was significantly higher during H than during N (P less than 0.01), while ATP and AMP remained constant under both H and N exercise (P greater than 0.05 H vs N). IMP was not detectable in pre-exercise muscle samples (less than 0.01 mmol kg-1 dry wt) but increased during N exercise (0.03 +/- 0.01 mmol kg-1 dry wt, wt, P less than 0.05) and even more during H exercise (0.16 +/- 0.05 mmol kg-1 dry wt, P less than 0.05, H vs N). Post-exercise IMP was negatively related to PCr (r = -0.90) and positively related to lactate (r = 0.88). It is concluded that hypoxaemia results in an enhanced accumulation of IMP during submaximal exercise and that the IMP level is related to the degree of anaerobic energy utilization.

Oxygen deficit is not affected by the rate of transition from rest to submaximal exercise

Five subjects cycled on an ergometer at power outputs corresponding to 20, 40, 60 and 80% of their maximal oxygen uptake (VO2 max). On one occasion the transition from rest to work was direct (D), while on the other occasion the power output was increased slowly (S) in a stepwise manner for 6-15 min prior to exercise at the predetermined intensity. Oxygen uptake (VO2) was measured, and O2 deficit and O2 debt were calculated. Oxygen deficit increased with the exercise intensities, the peak values being 2.1 +/- 0.2 and 1.9 +/- 0.1 litres (mean +/- SEM) at 80% of VO2 max after D and S respectively. No significant difference was observed in O2 deficit or O2 debt between D and S at any exercise intensity (P less than 0.05). The O2 debt was similar to the O2 deficit at 20, 40 and 60% of VO2 max but lower than the O2 deficit (P less than 0.05) at 80% of VO2 max. Femoral venous blood lactate remained unchanged at 20% of VO2 max but increased at the higher exercise intensities, reaching peak values of 7.6 +/- 0.6 and 7.4 +/- 1.1 mmol l-1 at 80% of VO2 max after D and S respectively. Blood lactate was not significantly different between D and S at any exercise intensity (P greater than 0.05). It is concluded that O2 deficit, O2 debt and blood lactate are not affected by the rate of transition from rest to submaximal exercise. The data contradict the hypothesis that O2 deficit is caused by an inadequate O2 transport at the onset of exercise.

Glucose formation in human skeletal muscle. Influence of glycogen content

Eight men exercised at 66% of their maximal isometric force to fatigue after prior decrease in the glycogen store in one leg (low-glycogen, LG). The exercise was repeated with the contralateral leg (control) at the same relative intensity and for the same duration. Muscle (quadriceps femoris) glycogen content decreased in the LG leg from 199 +/- 17 (mean +/- S.E.M.) to 163 +/- 16 mmol of glucosyl units/kg dry wt. (P less than 0.05), and in the control leg from 311 +/- 23 to 270 +/- 18 mmol/kg (P less than 0.05). The decrease in glycogen corresponded to a similar accumulation of glycolytic intermediates. Muscle glucose increased in the LG leg during the contraction, from 1.8 +/- 0.1 to 4.3 +/- 0.6 mmol/kg dry wt. (P less than 0.01), whereas no significant increase occurred in the control leg (P greater than 0.05). It is concluded that during exercise glucose is formed from glycogen through the debranching enzyme when muscle glycogen is decreased to values below about 200 mmol/kg dry wt.

Effect of freeze-drying on measurements of pH in biopsy samples of the middle gluteal muscle of the horse: comparison of muscle pH to the pyruvate and lactate content

Muscle biopsies taken after exercise, in comparison to those at rest, contain increased amounts of blood and this is a particular problem in studies of the horse. The inclusion of blood in muscle will introduce an upward bias in values of pH measured in muscle homogenates. In an attempt to control this, muscle biopsy samples of the middle gluteal from Thoroughbred horses were freeze-dried and dissected free of blood before determination of pH. Following exercise, muscle pH measured after freeze-drying was similar to that measured in homogenates prepared from frozen samples. In contrast, freeze-drying appeared to increase muscle pH in samples taken at rest. This was probably the result of loss of CO2 during freeze-drying. Muscle pH determined in freeze-dried samples taken at rest and after exercise was linearly related to pyruvate and lactate content (P less than 0.001). It is concluded that muscle samples taken after exercise can be freeze-dried and dissected free from blood before determination of pH, whereas this procedure will cause an alkaline shift in samples taken at rest.

Hyperammoniemia during prolonged exercise: an effect of glycogen depletion?

Eight healthy men exercised to exhaustion on a cycle ergometer at a work load of 176 +/- 9 (SE) W corresponding to 67% (range 63-69%) of their maximal O2 uptake (exercise I). Exercise of the same work load was repeated after 75 min of recovery (exercise II). Exercise duration (range) was 65 (50-90) and 21 (14-30) min for exercise I and II, respectively. Femoral venous blood samples were obtained before and during exercise and analyzed for NH3 and lactate. Plasma NH3 was 12 +/- 2 and 19 +/- 6 mumol/l before exercise I and II, respectively and increased during exercise to exhaustion to peak values of 195 +/- 29 (exercise I) and 250 +/- 30 (exercise II) mumol/l, respectively. Plasma NH3 increased faster during exercise II compared with exercise I and at the end of exercise II was threefold higher than the value for the corresponding time of exercise I (P less than 0.001). Blood lactate increased during exercise I and after 20 min of exercise was 3.7 +/- 0.4 mmol/l and remained unchanged until exhaustion. During exercise II blood lactate increased less than during exercise I. It is concluded that long-term exercise to exhaustion results in large increases in plasma NH3 despite relatively low levels of blood lactate. It is suggested that the faster increase in plasma NH3 during exercise II (vs. exercise I) reflects an increased formation in the working muscle that may be caused by low glycogen levels and impairment of the ATP resynthesis.

Propranolol enhances adenine nucleotide degradation in human muscle during exercise

Eight healthy men cycled to exhaustion [4.1 +/- 0.3 (SE) min] during beta-adrenoceptor blockade (beta B) with propranolol. The exercise was repeated on another day with the same power output and duration but without propranolol (control). The total adenine nucleotide (TAN) content in muscle (quadriceps femoris) decreased during exercise, and the decrease was more pronounced during beta B (delta TAN = 4.8 +/- 1.0 mmol/kg dry wt) than during control (delta TAN = 2.8 +/- 0.9; P less than 0.01, beta B vs. control). The decrease in TAN corresponded with a similar increase in inosine 5'-monophosphate (IMP). The increase in IMP was more pronounced during beta B (delta IMP = 5.1 +/- 1.2 mmol/kg dry wt) than during control (delta IMP = 2.8 +/- 0.7; P less than 0.05, beta B vs. control). Similarly, the increase in the content of NH3 in muscle was twice as high during beta B vs. control (P less than 0.01). The increase in muscle lactate and the decrease in phosphocreatine during exercise were similar between treatments, but postexercise hexose phosphates were approximately twofold higher (P less than 0.05) during control than during beta B. It is concluded that beta B enhances the degradation of TAN and the production of NH3 and IMP in muscle during intense exercise. This indicates that the imbalance between the rates of utilization and resynthesis of ATP is more pronounced during beta B possibly because of a decreased O2 transport to the contracting muscle and a diminished activation of glycolysis by the hexose phosphates.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxygen deficit at the onset of submaximal exercise is not due to a delayed oxygen transport

Six subjects cycled on two occasions for 10 min at power output of 188 +/- 11 W (means +/- SEM), which corresponded to 70 +/- 2% of their maximal oxygen uptake (VO2 max). The exercise intensity was either increased gradually in a stepwise manner over about 15 min (slow transition-S), or increased directly (direct transition-D) to the predetermined power output. Muscle samples from the quadriceps femoris muscle were taken at rest and immediately after exercise in both trials. During exercise with both D and S muscle lactate increased approximately 10 times (P less than 0.01), phosphocreatine decreased about 50% (P less than 0.01) and ADP increased about 20% (P less than 0.05). There were no significant differences between S and D (P greater than 0.05). Furthermore, blood lactate, O2 deficit, O2 debt, and the calculated increase in muscle content of inorganic phosphate (Pi) were all similar between D and S (P greater than 0.05). It is concluded that the O2 deficit and the anaerobic energy utilization is not affected by the rate of transition from rest to exercise. Consequently, the O2 deficit at the onset of exercise is not due to a delay in O2 transport, but may be due to a limited peripheral O2 utilization as a result of metabolic adjustments at the cellular level. Increases in ADP and Pi are suggested to be primary metabolic regulators which activate both aerobic and anaerobic energy production resulting in the O2 deficit.

Regulation of lactic acid production during exercise

Lactic acid accumulates in contracting muscle and blood beginning at approximately 50-70% of the maximal O2 uptake, well before the aerobic capacity is fully utilized. The classical explanation has been that part of the muscle is O2 deficient and therefore lactate production is increased to provide supplementary anaerobically derived energy. Currently, however, the predominant view is that lactate production during submaximal dynamic exercise is not O2 dependent. In the present review, data and arguments in support of and against the hypothesis of O2 dependency have been scrutinized. Data underlying the conclusion that lactate production during exercise is not O2 dependent were found to be 1) questionable, or 2) interpretable in an alternative manner. Experiments in human and animal muscles under various conditions demonstrated that the redox state of the muscle is reduced (i.e., NADH is increased) either before or in parallel with increases in muscle lactate. Based on experimental data and theoretical considerations, it is concluded that lactate production during submaximal exercise is O2 dependent. The amount of energy provided through the anaerobic processes during steady-state submaximal exercise is, however, low, and the role of lactate formation as an energy source is of minor importance. It is proposed that the achievement of increased aerobic energy formation under conditions of limiting O2 availability requires increases of ADP, Pi, and NADH and that the increases in ADP (and therefore AMP via the adenylate kinase equilibrium) and Pi will stimulate glycolysis, and the resulting increase in cytosolic NADH will shift the lactate dehydrogenase equilibrium toward increased lactate production.

Carbohydrate metabolism in human skeletal muscle during exercise is not regulated by G-1,6-P2

Glucose 1,6-bisphosphate (G-1,6-P2) is a potent activator of phosphofructokinase (PFK) and an inhibitor of hexokinase in vitro. It has been suggested that increases in G-1,6-P2 are a main means by which PFK can achieve significant catalytic function in vivo despite falling pH and that increases in G-1,6-P2 will inhibit hexokinase in vivo. The purpose of the present study was to determine whether contraction-induced changes in flux through PFK and hexokinase are associated with changes in G-1,6-P2 in skeletal muscle. Ten men performed bicycle exercise for 10 min at 40 and 75% of maximal O2 uptake (VO2max) and to fatigue [4.8 +/- 0.6 (SE) min] at 100% VO2max. Biopsies were obtained from the quadriceps femoris muscle at rest and after each work load and analyzed for G-1,6-P2. G-1,6-P2 averaged 111 +/- 13 mumol/kg dry wt at rest and 121 +/- 16, 123 +/- 15, and 123 +/- 11 mumol/kg dry wt after the low-, moderate-, and high-intensity exercise bouts, respectively (P less than 0.05 for all means vs. rest). Flux through PFK was estimated to increase exponentially as the exercise intensity increased and muscle pH decreased at the higher work loads, whereas flux through hexokinase was estimated to increase during exercise at 40 and 75% VO2max but decrease sharply at 100% VO2max. These data demonstrate that flux through neither PFK nor hexokinase is mediated by changes in G-1,6-P2 in human skeletal muscle during short-term dynamic exercise.

Gastrin/CCK-like immunoreactivity in Hatschek's groove of Branchiostoma lanceolatum (Cephalochordata)

Gastrin/CCK-like immunoreactivity was demonstrated in a specific group of epithelial cells in Hatschek's groove. Positive reactions were obtained with antisera directed against the C-terminal amino acid sequence common to mammalian CCK and gastrin, but not with antisera against the midportions of these hormones. No immunoreactivity was obtained with antisera against peptides from hypophysis, neuroendocrine peptides in the gastrointestinal tract, islet hormones, calcitonin, NSE, and S-100. The function of the gastrin/CCK-like peptide found in Hatschek's groove is discussed.

NADH content in type I and type II human muscle fibres after dynamic exercise

The effect of dynamic exercise on the NADH content of human type I (slow-twitch) and II (fast-twitch) muscle fibres was investigated. Muscle biopsy samples were obtained from the quadriceps femoris of seven healthy subjects at rest and after bicycle exercise at 40, 75 and 100% of the maximal oxygen uptake [VO2(max.)]. At rest and after exercise at 100% VO2(max.), muscle NADH content was significantly higher (P less than 0.05) in type I than in type II fibres. After exercise at 40% VO2(max.), muscle NADH decreased in type I fibres (P less than 0.01), but was not significantly changed in type II fibres. After exercise at 75 and 100% VO2(max.), muscle NADH increased above the value at rest in both type I and II fibres (P less than 0.05). Muscle lactate was unchanged at 40% VO2(max.), but increased 20- and 60-fold after exercise at 75 and 100% VO2(max.) respectively. The finding that NADH decreased only in type I fibres at 40% VO2(max.) supports the idea that type I is the fibre type predominantly recruited during low-intensity exercise. The increase of NADH in both fibre types after exercise at 75% and 100% VO2(max.) suggests that the availability of oxygen relative to the demand is decreased in both fibre types at high exercise intensities.

Effect of decreased oxygen availability on NADH and lactate contents in human skeletal muscle during exercise

Eight men cycled for 5 min at 120 +/- 6 W (mean +/- SE) at which O2 uptake was 50% of its maximal normoxic value, breathing room air (21% O2; normoxia) on one occasion and 11% O2 in N2 (respiratory hypoxia/hypoxic--Resp. Hx.) on the other. Biopsies were taken from the quadriceps femoris muscle. Oxygen uptake during exercise was not significantly different between Resp. Hx (1.59 +/- 0.08 1 min-1) and normoxia (1.55 +/- 0.08 1 min-1). At rest, muscle lactate was the same under both conditions but was four times higher after Resp. Hx (33.2 +/- 5.2 mmol kg-1 dry wt) than normoxic cycling (8.6 +/- 1.0 mmol kg-1 dry wt; P less than 0.01). The muscle lactate/pyruvate (which is proportional to cytosolic NADH/NAD) was significantly higher after Resp. Hx.(76 +/- 19) than after normoxic cycling (26 +/- 2; P less than 0.05). At rest, analytically determined NADH averaged 0.14 +/- 0.02 mmol kg-1 dry wt under both conditions. However, exercise during Resp. Hx. resulted in a significantly higher NADH content (0.17 +/- 0.01) than exercise during normoxia (0.12 +/- 0.01; P less than 0.01). Indirect evidence indicates that the difference in muscle NADH reflects a difference in the mitochondrial redox state (Sahlin & Katz 1986). The increased muscle NADH during Resp. Hx. therefore indicates a relative lack of O2 at the cellular level (muscle hypoxia). It is suggested that the increased lactate production during Resp. Hx. is a consequence of the cellular adaptation to muscle hypoxia (i.e. increases in cytosolic ADP, AMP, Pi and NADH).

Redox state and lactate accumulation in human skeletal muscle during dynamic exercise

The relationship between the redox state and lactate accumulation in contracting human skeletal muscle was investigated. Ten men performed bicycle exercise for 10 min at 40 and 75% of maximal oxygen uptake [VO2(max.)], and to fatigue (4.8 +/- 0.6 min; mean +/- S.E.M.) at 100% VO2(max.). Biopsies from the quadriceps femoris muscle were analysed for NADH, high-energy phosphates and glycolytic intermediates. Muscle NADH was 0.20 +/- 0.02 mmol/kg dry wt. of muscle at rest, and decreased to 0.12 +/- 0.01 (P less than 0.01) after exercise at 40% VO2(max.), but no change occurred in the [lactate]/[pyruvate] ratio. These data, together with previous results on isolated cyanide-poisoned soleus muscle, where NADH increased while [lactate]/[pyruvate] ratio was unchanged [Sahlin & Katz (1986) Biochem. J. 239, 245-248], suggest that the observed changes in muscle NADH occurred within the mitochondria. After exercise at 75 and 100% VO2(max.), muscle NADH increased above the value at rest to 0.27 +/- 0.03 (P less than 0.05) and 0.32 +/- 0.04 (P less than 0.001) mmol/kg respectively. Muscle lactate was unchanged after exercise at 40% VO2(max.), but increased substantially at the higher work loads. At 40% VO2(max.), phosphocreatine decreased by 11% compared with the values at rest, and decreased further at the higher work loads. The decrease in phosphocreatine reflects increased ADP and Pi. It is concluded that muscle NADH decreases during low-intensity exercise, but increases above the value at rest during high-intensity exercise. The increase in muscle NADH is consistent with the hypothesis that the accelerated lactate production during submaximal exercise is due to a limited availability of O2 in the contracting muscle. It is suggested that the increases in NADH, ADP and Pi are metabolic adaptations, which primarily serve to activate the aerobic ATP production, and that the increased anaerobic energy production (phosphocreatine breakdown and lactate formation) is a consequence of these changes.