Effect of the nucleotides CMP and UMP on exhaustion in exercise rats

The aetiology of muscle fatigue has yet not been clearly established. Administration of two nucleotides, cytosine monophosphate (CMP) and uridine monophosphate (UMP), has been prescribed for the treatment of neuromuscular affections in humans. Patients treated with CMP/UMP recover from altered neurological functions and experience pain relief, thus the interest to investigate the possible effect of the drug on exhausting exercise. With such aim, we have determined, in exercised rats treated with CMP/UMP, exercise endurance, levels of lactate, glucose and glycogen, and the activity of several metabolic enzymes such as, creatine kinase (CK), lactate dehydrogenase (LDH), and aspartate aminotransferase (AST). Our results show that rats treated with CMP/UMP are able to endure longer periods of exercise (treadmill-run). Before exercise, muscle glucose level is significantly higher in treated rats, suggesting that the administration of CMP/UMP favours the entry of glucose in the muscle. Liver glycogen levels remains unaltered during exercise, suggesting that CMP/UMP may be implicated in maintaining the level of hepatic glycogen constant during exercise. Lactate dehydrogenase and aspartate aminotransferase activity is significantly lower in the liver of treated rats. These results suggest that administration of CMP/UMP enable rats to endure exercise by altering some metabolic parameters.     

Early muscular and metabolic adaptations to prolonged exercise training in humans

A short-term training program involving 2 h of daily exercise at 59% of peak O2 uptake (VO2max) repeated for 10-12 consecutive days was employed to determine the significance of adaptations in energy metabolic potential on alterations in energy metabolism and substrate utilization in working muscle. The initial VO2max determined before training on the eight male subjects was 53.0 +/- 2.0 (SE) ml.kg-1.min-1. Analysis of samples obtained by needle biopsy from the vastus lateralis muscle before exercise (0 min) and at 15, 60, and 99 min of exercise indicated that on the average training resulted (P less than 0.05) in a 6.5% higher concentration of creatine phosphate, a 9.9% lower concentration of creatine, and a 39% lower concentration of lactate. Training had no effect on ATP concentration. These adaptations were also accompanied by a reduction in the utilization in glycogen such that by the end of exercise glycogen concentration was 47.1% higher in the trained muscle. Analysis of the maximal activities of representative enzymes of different metabolic pathways and segments indicated no change in potential in the citric acid cycle (succinate dehydrogenase, citrate synthase), beta-oxidation (3-hydroxyacyl CoA dehydrogenase), glucose phosphorylation (hexokinase), or potential for glycogenolysis (phosphorylase) and glycolysis (pyruvate kinase, phosphofructokinase, alpha-glycerophosphate dehydrogenase, lactate dehydrogenase). With the exception of increases in the capillary-to-fiber area ratio in type IIa fibers, no change was found in any fiber type (types I, IIa, and IIb) for area, number of capillaries, capillary-to-fiber area ratio, or oxidative potential with training.(ABSTRACT TRUNCATED AT 250 WORDS)     

Activities of enzymes concerned with pyruvate and oxaloacetate metabolism in the heart and liver of developing sheep.

1. In order to assess whether the potential ability of heart ventricular muscle and liver to metabolise substrates such as alanine, aspartate and lactate varies as the sheep matures and its nutrition changes, the activities of the following enzymes were determined in tissues of lambs obtained at varying intervals between 50 days after conception to 16 weeks after birth and in livers from adult pregnant ewes: lactate dehydrogenase (EC 1.1.1.27), alanine aminotransferase (EC 2.6.1.2), pyruvate kinase (EC 2.7.1.40), pyruvate carboxylase (EC 6.4.1.1), phosphoenolpyruvate carboxykinase (GTP)(EC 4.1.1.32), malate dehydrogenase (EC 1.1.1.37), aspartate aminotransferase (EC 2.6.1.1) and citrate (si)-synthase (EC 4.1.3.7). 2. In the heart a most marked increase in alanine aminotransferase activity was found throughout development. During this period the activities of citrate (si)-synthase, lactate dehydrogenase and pyruvate carboxylase also increased. There were no substantial changes in the activities of aspartate aminotransferase, malate dehydrogenase or pyruvate kinase. Pyruvate kinase activities were five times greater in the heart compared with those found in the liver. No significant activity of phosphoenolpyruvate carboxykinase (GTP) was detected in heart muscle. 3. In the liver the activities of both alanine aminotransferase and aspartate aminotransferase increased immediately following birth although the activity of alanine aminotransferase was lower in livers of pregnant ewes than in any of the lambs. As with alanine aminotransferase the highest activities of lactate dehydrogenase were found during the period of postnatal growth. No marked changes were observed in malate dehydrogenase or citrate (si)-synthase activities during development. A small decline in pyruvate kinase activity occurred whilst the activities of pyruvate carboxylase and phosphoenolpyruvate carboxykinase (GTP) tended to rise during development.     

Divergent response of metabolite transport proteins in human skeletal muscle after sprint interval training and detraining

Skeletal muscle primarily relies on carbohydrate (CHO) for energy provision during high-intensity exercise. We hypothesized that sprint interval training (SIT), or repeated sessions of high-intensity exercise, would induce rapid changes in transport proteins associated with CHO metabolism, whereas changes in skeletal muscle fatty acid transporters would occur more slowly. Eight active men (22 +/- 1 yr; peak oxygen uptake = 50 +/- 2 ml.kg(-1).min(-1)) performed 4-6 x 30 s all-out cycling efforts with 4-min recovery, 3 days/wk for 6 wk. Needle muscle biopsy samples (vastus lateralis) were obtained before training (Pre), after 1 and 6 wk of SIT, and after 1 and 6 wk of detraining. Muscle oxidative capacity, as reflected by the protein content of cytochrome c oxidase subunit 4 (COX4), increased by approximately 35% after 1 wk of SIT and remained higher compared with Pre, even after 6 wk of detraining (P < 0.05). Muscle GLUT4 content increased after 1 wk of SIT and remained approximately 20% higher compared with baseline during detraining (P < 0.05). The monocarboxylate tranporter (MCT) 4 was higher after 1 and 6 wk of SIT compared with Pre, whereas MCT1 increased after 6 wk of training and remained higher after 1 wk of detraining (P < 0.05). There was no effect of training or detraining on the muscle content of fatty acid translocase (FAT/CD36) or plasma membrane associated fatty acid binding protein (FABPpm) (P > 0.05). We conclude that short-term SIT induces rapid increases in skeletal muscle oxidative capacity but has divergent effects on proteins associated with glucose, lactate, and fatty acid transport.     

Splanchnic and muscle fructose metabolism during and after exercise

Regional substrate exchange was studied in 12 healthy males during 90 min of bicycle exercise at 30% of maximal O2 consumption with a 20-min recovery. Six subjects received an intravenous fructose infusion (8.5 mmol/min) from 40 min of exercise to the end of recovery. Splanchnic glucose output, muscle glucose uptake, arterial glucose, and insulin were uninfluenced by the infusion. The respiratory exchange ratio rose to 0.93 +/- 0.04, and arterial free fatty acids fell by 50% (P less than 0.05). Fructose was taken up by splanchnic tissues (45% of administered load), leg muscle (28%), and resting muscle (28%). During infusion, arterial lactate and pyruvate rose two- to threefold, and these substrates were released from splanchnic tissues and taken up by exercising and resting muscle. Splanchnic release of lactate, pyruvate, and glucose accounted for 78% of fructose uptake at 90 min of exercise. Uptake of fructose, lactate, and pyruvate accounted for 55% and together with glucose for 103% of the total oxidative metabolism by exercising muscle. The regional fructose uptakes and lactate exchanges persisted throughout recovery. The present results indicate that fructose infusion during leg exercise 1) results in increased carbohydrate oxidation from fructose, lactate, and pyruvate in exercising muscle, 2) exerts a glycogenic effect in resting muscle and liver during exercise and in liver and muscle recovering from exercise, and 3) does not interfere with glucose metabolism, and that fructose transport into muscle differs from that of glucose.     

Phosphamidon, methylparathion and dichlorvos impact on tissue oxidative metabolism in penaeid prawn, Metapenaeus monoceros

Changes in oxidative metabolism of hepatopancreas and muscle tissues of penaeid prawn, Metapenaeus monoceros was studied, following its exposure to selected organophosphorous insecticides phosphamidon, dichlorovos and methylparathion. The OPI are found to inhibit the activity levels of acetylcholinesterase, succinate dehydrogenase, isocitrate dehydrogenase, pyruvate dehydrogenase, lactate dehydrogenase and cytochrome-c-oxidase and cause accumulation of acetylcholine in the hepatopancreas and muscle tissues. These changes in the activity levels of selected oxidative enzymes during insecticide exposure in these tissues of prawn indicates the shift in the metabolic emphasis from aerobic to anaerobic conditions and is interpreted as a functional adaptation to insecticide induced metabolic stress. These observed changes at cellular level pave way for successful survival of prawns in insecticide polluted environ.     

Short- and long-term enzymatic regulation secondary to metabolic insult in the rat retina

Changes in oxygen and/or glucose availability may result in altered levels of ATP production and amino acid levels, and alteration in lactic acid production. However, under certain metabolic insults, the retina demonstrates considerable resilience and maintains ATP production, and/or retinal function. We wanted to investigate whether this resilience would be reflected in alterations in the activity of key enzymes of retinal metabolism, or enzymes associated with amino acid production that may supply their carbon skeleton for energy production. Enzymatic assays were conducted to determine the activity of key retinal metabolic enzymes total ATPase and Na(+)/K(+)-ATPase, aspartate aminotransferase and lactate dehydrogenase. In vitro anoxia led to an increase in retinal lactate dehydrogenase activity and to a decrease in retinal aspartate aminotransferase activity, without significant changes in Na(+)/K(+)-ATPase activity. In vivo inhibition of glutamine synthetase resulted in a short-term significant decrease in retinal aspartate aminotransferase activity. An increase in retinal aspartate aminotransferase and lactate dehydrogenase activities was accompanied by altered levels of amino acids in neurons and glia after partial inhibition of glial metabolism, implying that short- and long-term up- and down-regulation of key metabolic enzymes occurs to supply carbon skeletons for retinal metabolism. ATPase activity does not appear to fluctuate under the metabolic stresses employed in our experimental procedures.     

Effects of two high-intensity intermittent training programs interspaced by detraining on human skeletal muscle and performance

The purpose of this study was to investigate the effects of repeated high-intensity intermittent training programs interspaced by detraining on human skeletal muscle and performances. First, nineteen subjects were submitted to a 15-week cycle ergometer training program which involved both continuous and high-intensity interval work patterns. Among these 19 subjects, six participated in a second 15-week training program after 7 weeks of detraining. Subjects were tested before and after each training program for maximal aerobic power and maximal short-term ergocycle performances of 10 and 90s. Muscle biopsy from the vastus lateralis before and after both training programs served for the determination of creatine kinase (CK), hexokinase, phosphofructokinase (PFK), lactate dehydrogenase (LDH), malate dehydrogenase, 3-hydroxyacyl-CoA dehydrogenase (HADH) and oxoglutarate dehydrogenase (OGDH) activities. The first training program induced significant increases in all performances and enzyme activities but not in CK. Seven weeks of detraining provoked significant decreases in maximal aerobic power and maximal 90s ergocycle performance. While the interruption of training had no effect on glycolytic enzyme markers (PFK and LDH), oxidative enzyme activities (HADH and OGDH) declined. These results suggest that a fairly long interruption in training has negligeable effects on glycolytic enzymes while a persistent training stimulus is required to maintain high oxidative enzyme levels in human skeletal muscle. The degree of adaptation observed after the second training program confirms that the magnitude of the adaptive response to exercise-training is limited.     

Rat brain slices oxidize glucose at high rates: a (13)C NMR study

Since glucose is the main cerebral substrate, we have characterized the metabolism of various ¹³C glucose isotopomers in rat brain slices. For this, we have used our cellular metabolomic approach that combines enzymatic and carbon 13 NMR techniques with mathematical models of metabolic pathways. We identified the fate and the pathways of the conversion of glucose carbons into various products (pyruvate, lactate, alanine, aspartate, glutamate, GABA, glutamine and CO₂) and determined absolute fluxes through pathways of glucose metabolism. After 60 min of incubation, lactate and CO₂ were the main end-products of the metabolism of glucose which was avidly metabolized by the slices. Lactate was also used at high rates by the slices and mainly converted into CO₂. High values of flux through pyruvate carboxylase, which were similar with glucose and lactate as substrate, were observed. The addition of glutamine, but not of acetate, stimulated pyruvate carboxylation, the conversion of glutamate into succinate and fluxes through succinate dehydrogenase, malic enzyme, glutamine synthetase and aspartate aminotransferase. It is concluded that, unlike brain cells in culture, and consistent with high fluxes through PDH and enzymes of the tricarboxylic acid cycle, rat brain slices oxidized both glucose and lactate at high rates.     

Use of β-Methylene-D,L-Aspartate to Assess the Role of Aspartate Aminotransferase in Cerebral Oxidative Metabolism

Several inhibitors of aspartate aminotransferase, a key enzyme of the malate-aspartate shuttle, were investigated for their effects on cerebral oxidative metabolism in vitro. beta-Methylene-D,L-aspartate (2 mM), aminooxyacetate (0.1 mM), and D,L-vinylglycine (20 mM) all significantly reduced the activity of aspartate aminotransferase and the rate of oxygen consumption of rat cerebral cortex slices respiring on glucose. In the presence of beta-methyleneaspartate, a one-to-one correlation was found between the degree of inhibition of tissue respiration and the degree of inhibition of transaminase activity. Slices of rat liver incubated in the presence of glucose and beta-methyleneaspartate showed a similar one-to-one relationship between inhibition of oxygen comsumption and inhibition of aspartate aminotransferase activity, whereas with rat kidney cortex slices, the inhibition of aspartate aminotransferase activity was greater than the inhibition of oxygen consumption. Structural analogs of beta-methyleneaspartate (D,L-beta-methyl-D,L-aspartate, gamma-methyl-D,L-glutamate, and alpha-methyl-D,L-didehydroglutamate) that did not inhibit the activity of aspartate aminotransferase similarly did not inhibit the rate of oxygen consumption by cerebral cortex slices. In the presence of beta-methyleneaspartate, pyruvate oxidation by cerebral cortex slices was inhibited to almost the same extent as was glucose oxidation, and the oxidation of succinate was decreased by approximately 20%. The artificial electron acceptor phenazine methosulfate (0.1 mM) only partially overcame the beta-methyleneaspartate-mediated inhibition of respiration with glucose as substrate. The content of ATP and phosphocreatine declined steadily in slices incubated with glucose and beta-methyleneaspartate. At 1 h the concentration of lactate and the lactate/pyruvate ratio, an indicator of the cytoplasmic redox state, increased threefold, whereas the concentrations of malate, citrate, and aspartate decreased. The findings are interpreted in the context of the hypothesis that enzymes common to the malate-aspartate shuttle and the tricarboxylic acid cycle are physically complexed in brain, so that inhibition of aspartate aminotransferase, a component of the complex, impedes the flow of carbon through both metabolic pathways.(ABSTRACT TRUNCATED AT 400 WORDS)     

Regulation of muscle pyruvate metabolism during exercise

Pyruvate dehydrogenase and phosphoenolpyruvate carboxykinase are important enzymes in the regulation of muscle pyruvate metabolism and their in vitro measured activities have been studied in muscle from rested and exercised rats. In addition, the muscle concentration of metabolic intermediates associated with pyruvate metabolism has been measured after exercise. Phosphoenolpyruvate concentration was decreased to less than half the value found in rested muscle but pyruvate concentration did not change. This suggests an increase in the in vivo rate of conversion of phosphoenolpyruvate to pyruvate. Concentrations of malate and aspartate increased two- to threefold which suggests that oxaloacetate concentration was also increased. An increase in oxaloacetate availability would increase acetyl CoA metabolism and therefore would increase pyruvate dehydrogenase activity in vivo. The basal activity of pyruvate dehydrogenase measured in vitro increased approximately twofold after 2 hr of exercise and returned to control values 5 min after the cessation of exercise. Total pyruvate dehydrogenase activity (activated to the maximal extent) was not changed by exercise. Muscle PEPCK activity was also increased during exercise suggesting an increased rate of conversion of oxaloacetate to pyruvate to provide net oxidation of oxaloacetate and other citric acid cycle intermediates. Results of this study demonstrate that the rates of formation and metabolism of pyruvate are increased during exercise.     

Exercise training reverses impaired skeletal muscle metabolism induced by artificial selection for low aerobic capacity

We have used a novel model of genetically imparted endurance exercise capacity and metabolic health to study the genetic and environmental contributions to skeletal muscle glucose and lipid metabolism. We hypothesized that metabolic abnormalities associated with low intrinsic running capacity would be ameliorated by exercise training. Selective breeding for 22 generations resulted in rat models with a fivefold difference in intrinsic aerobic capacity. Low (LCR)- and high (HCR)-capacity runners remained sedentary (SED) or underwent 6 wk of exercise training (EXT). Insulin-stimulated glucose transport, insulin signal transduction, and rates of palmitate oxidation were lower in LCR SED vs. HCR SED (P < 0.05). Decreases in glucose and lipid metabolism were associated with decreased β?-adrenergic receptor (β?-AR), and reduced expression of Nur77 target proteins that are critical regulators of muscle glucose and lipid metabolism [uncoupling protein-3 (UCP3), fatty acid transporter (FAT)/CD36; P < 0.01 and P < 0.05, respectively]. EXT reversed the impairments to glucose and lipid metabolism observed in the skeletal muscle of LCR, while increasing the expression of β?-AR, Nur77, GLUT4, UCP3, and FAT/CD36 (P < 0.05) in this tissue. However, no metabolic improvements were observed following exercise training in HCR. Our results demonstrate that metabolic impairments resulting from genetic factors (low intrinsic aerobic capacity) can be overcome by an environmental intervention (exercise training). Furthermore, we identify Nur77 as a potential mechanism for improved skeletal muscle metabolism in response to EXT.     

Lipogenesis in rat and guinea-pig isolated epididymal fat-cells

Fat-cells were prepared from rat and guinea-pig epididymal adipose tissue and compared on the basis of the intracellular distributions and activities of enzymes and with respect to their utilization of various U-(14)C-labelled substrates for lipogenesis. 1. Compared with the rat, guinea-pig extramitochondrial enzyme activities differed in that aconitate hydratase, alanine aminotransferase, ATP-citrate lyase, lactate dehydrogenase, NAD-malate dehydrogenase, NADP-malate dehydrogenase and phosphoenolpyruvate carboxykinase activities were appreciably lower, whereas aspartate aminotransferase, glucose 6-phosphate dehydrogenase, NADP-isocitrate dehydrogenase and 6-phosphogluconate dehydrogenase activities were appreciably higher. Mitochondrial activities of citrate synthase, NADP-isocitrate dehydrogenase and pyruvate carboxylase were appreciably lower, whereas mitochondrial activities of aspartate aminotransferase, glutamate dehydrogenase, NAD-malate dehydrogenase and phosphoenolpyruvate carboxykinase were higher in the guinea pig compared with the rat. 2. In general guinea-pig fat-cells incorporated acetate and lactate into fatty acids more readily than rat fat-cells, whereas rat fat-cells incorporated glucose and pyruvate more readily than guinea-pig fat-cells. 3. Acetate stimulated the incorporation of glucose into fatty acids in rat fat-cells, but had no appreciable effect upon this process in guinea-pig fat-cells. Acetate greatly decreased the incorporation of lactate into fatty acids in cells from both species. 4. Lactate/pyruvate ratios produced by incubation of guinea-pig cells with glucose+insulin were very low compared with those found with rat cells under the same conditions. 5. With glucose (+insulin) or with glucose+acetate (+insulin) as substrates guinea-pig cells produced enough NADPH by the hexose monophosphate pathway to satisfy the NADPH requirements of lipogenesis. In rat fat-cells under the same conditions, hexose monophosphate-pathway NADPH provision was not sufficient to meet the requirements of lipogenesis. 6. These results are discussed, particularly in relationship to the disposition of cytosolic reducing equivalents in the cells.     

Oxidative removal of lactate after strenuous exercise

Metabolic fate of lactate after strenuous exercise which lasted 2-3 min was investigated in rats and mice. 14C-labeled lactate or glucose was injected into the aorta of rats through an catheter. 14C-glucose was injected intraperitoneally into the mice after supramaximal exercise. The mice ran twice with a 4 hr interval to investigate muscle 14C-lactate metabolism which was produced from muscle 14C-glycogen. A great deal of blood and muscle 14C-lactate was expired as 14CO2 after the exercise. The results indicate that oxidative removal is the major fate of lactate metabolism after strenuous exercise and that blood glucose is the major substrate for muscle glycogen resynthesis. Light intensity exercise after strenuous exercise (active recovery) enhances oxidative removal of blood and muscle lactate. Gluconeogenesis from lactate to glycogen within the skeletal muscle is not a major pathway of muscle lactate metabolism, while high intensity training can activate this pathway.     

Alanine metabolism in skeletal muscle in tissue culture.

Alanine production by skeletal muscle in tissue culture was studied using an established myogenic line (L6) of rat skeletal muscle cells. Correlation analyses were performed on rates of metabolism of alanine, glucose, lactate and pyruvate over incubation periods up to 96 h. Alanine production did not correlate significantly with glucose utilization (r = 0.24, P less than 0.20). Alanine production, however, did correlate with lactate production (r = 0.72, P less than 0.0005) as well as medium (r = 0.50, P less than 0.025) and intracellular (r = 0.85, P less than 0.0005) pyruvate concentrations. The intercepts of the latter two correlation analyses indicated that when medium or cell pyruvate fell below 0.28 mM or 1 nmol/mg protein, respectively, net alanine consumption occurred. Alanine synthesis also correlated (r = 0.71, P less than 0.0005) with the percent change in the cell mass action ratio for the sum of the alanine and aspartate aminotransferase reactions, i.e., [alanine] [malate]/[aspartate] [lactate]. These results suggest that alanine production is not necessarily linked to the rate of glucose utilization but rater to pyruvate overflow above a critical intracellular level; under conditions of pyruvate overflow, alanine synthesis is driven by the tendency to establish equilibrium between metabolites of the linked amino acid transaminases in skeletal muscle.     

Ontogenesis of catabolic and energy metabolism capacities during the embryonic development of spotted wolffish (Anarhichas minor)

The catabolic and energy metabolism capacities during spotted wolffish (Anarhichas minor) embryogenesis were investigated. We assessed the embryo's ability to catabolize proteins (trypsin-like proteases) and lipids (triglyceride lipase) and examined the development of metabolic capacities using enzymatic assays: ability to use carbohydrates (pyruvate kinase), amino acids (aspartate aminotransferase) and fatty acids (hydroxyacyl-CoA dehydrogenase) for energy production, and aerobic (citrate synthase) and anaerobic (lactate dehydrogenase) energy production. Functional enzymatic systems were detected from the eyed stage (350 degree-days), except for fatty acids, which was detected from 540 degree-days. To compare the development of 1) aerobic and anaerobic pathways and 2) the capacity to mobilize the different energy substrates, enzymatic ratios were calculated. Anaerobic capacity appeared to increase at a significantly higher rate than the aerobic capacity. Ratios revealing the relative capacity to use specific energy substrates showed a significantly slower increase during development in the capacity to use carbohydrates than amino acids and fatty acids. The end of embryogenesis was characterized by a significant decrease in the use of carbohydrates for aerobic energy production but an increasing capacity to use amino acids. Egg survival as affected by the variability in metabolic parameters is discussed.     

Glucose and pyruvate metabolism in severe chronic obstructive pulmonary disease

The mechanisms leading to weight loss in patients with chronic obstructive pulmonary disease (COPD) are poorly understood but may involve alterations in macronutrient metabolism. Changes in muscle oxidative capacity and lactate production during exercise suggest glucose metabolism may be altered in COPD subjects. The objective of this study was to determine differences in the rates of glucose production and clearance, the rate of glycolysis (pyruvate production), and oxidative and nonoxidative pyruvate disposal in subjects with severe COPD compared with healthy controls. The in vivo rates of glucose production and clearance were measured in 14 stable outpatients with severe COPD (seven with low and seven with preserved body mass indexes) and 7 healthy controls using an intravenous infusion of [(2)H(2)]glucose. Additionally, pyruvate production and oxidative and non-oxidative pyruvate disposal were measured using intravenous infusions of [(13)C]bicarbonate and [(13)C]pyruvate. Endogenous glucose flux and glucose clearance were significantly faster in the combined COPD subjects (P = 0.002 and P < 0.001, respectively). This difference remained significant when COPD subjects were separated by body mass index. Pyruvate flux and oxidation were significantly higher in the combined COPD subjects than controls (P = 0.02 for both), but there was no difference in nonoxidative pyruvate disposal or plasma lactate concentrations between the two groups. In subjects with severe COPD, there are alterations in glucose metabolism leading to increased glucose production and faster glucose metabolism by glycolysis and oxidation compared with controls. However, no difference in glucose conversion to lactate via pyruvate reduction is observed.     

Adaptations in skeletal muscle exercise metabolism to a sustained session of heavy intermittent exercise

The purpose of this study was to investigate the hypothesis that a single, extended session of heavy exercise would be effective in inducing adaptations in energy metabolism during exercise in the absence of increases in oxidative potential. Ten healthy males [maximal aerobic power (VO(2 peak)) = 43.4 +/- 2.2 (SE) ml x kg(-1) x min(-1)] participated in a 16-h training session involving cycling for 6 min each hour at approximately 90% of maximal oxygen consumption. Measurements of metabolic changes were made on tissue extracted from the vastus lateralis during a two-stage standardized submaximal cycle protocol before (Pre) and 36-48 h after (Post) the training session. At Pre, creatine phosphate (PCr) declined (P < 0.05) by 32% from 0 to 3 min and then remained stable until 20 min of exercise at 60% VO(2 peak) before declining (P < 0.05) by a further 35% during 20 min of exercise at 75% VO(2 peak). Muscle lactate (mmol/kg dry wt) progressively increased (P < 0.05) from 4.59 +/- 0.64 at 0 min to 17.8 +/- 2.7 and 30.9 +/- 5.3 at 3 and 40 min, respectively, whereas muscle glycogen (mmol glucosyl units/kg dry wt) declined (P < 0.05) from a rest value of 360 +/- 24 to 276 +/- 31 and 178 +/- 36 at similar time points. During exercise after the training session, PCr and glycogen were not as depressed (P < 0.05), and increases in muscle lactate were blunted (P < 0.05). All of these changes occurred in the absence of increases in oxidative potential as measured by the maximal activities of citrate synthase and malate dehydrogenase. These findings are consistent with other studies, namely, that muscle metabolic adaptations to regular exercise are an early adaptive event that occurs before increases in oxidative potential.     

Amino acid sequence of the trypsin-generated C3d fragment from human complement factor C3.

Energy metabolism in proliferating cultured rat thymocytes was compared with that of freshly prepared non-proliferating resting cells. Cultured rat thymocytes enter a proliferative cycle after stimulation by concanavalin A and Lymphocult T (interleukin-2), with maximal rates of DNA synthesis at 60 h. Compared with incubated resting thymocytes, glucose metabolism by incubated proliferating thymocytes was 53-fold increased; 90% of the amount of glucose utilized was converted into lactate, whereas resting cells metabolized only 56% to lactate. However, the latter oxidized 27% of glucose to CO2, as opposed to 1.1% by the proliferating cells. Activities of hexokinase, 6-phosphofructokinase, pyruvate kinase and aldolase in proliferating thymocytes were increased 12-, 17-, 30- and 24-fold respectively, whereas the rate of pyruvate oxidation was enhanced only 3-fold. The relatively low capacity of pyruvate degradation in proliferating thymocytes might be the reason for almost complete conversion of glucose into lactate by these cells. Glutamine utilization by rat thymocytes was 8-fold increased during proliferation. The major end products of glutamine metabolism are glutamate, aspartate, CO2 and ammonia. A complete recovery of glutamine carbon and nitrogen in the products was obtained. The amount of glutamate formed by phosphate-dependent glutaminase which entered the citric acid cycle was enhanced 5-fold in the proliferating cells: 76% was converted into 2-oxoglutarate by aspartate aminotransferase, present in high activity, and the remaining 24% by glutamate dehydrogenase. With resting cells the same percentages were obtained (75 and 25). Maximal activities of glutaminase, glutamate dehydrogenase and aspartate aminotransferase were increased 3-, 12- and 6-fold respectively in proliferating cells; 32% of the glutamate metabolized in the citric acid cycle was recovered in CO2 and 61% in aspartate. In resting cells this proportion was 41% and 59% and in mitogen-stimulated cells 39% and 65% respectively. Addition of glucose (4 mM) or malate (2 mM) strongly decreased the rates of glutamine utilization and glutamate conversion into 2-oxoglutarate by proliferating thymocytes and also affected the pathways of further glutamate metabolism. Addition of 2 mM-pyruvate did not alter the rate of glutamine utilization by proliferating thymocytes, but decreased the rate of metabolism beyond the stage of glutamate significantly. Formation of acetyl-CoA in the presence of pyruvate might explain the relatively enhanced oxidation of glutamate to CO2 (56%) by proliferating thymocytes.