Muscle triglyceride metabolism during exercise.

Skeletal muscle cell contains a considerable amount of triglycerides. The amount stored depends on the animal species as well as on muscle fiber composition. It is well documented that triglycerides in the fast-twitch red muscle and to a lesser extent in the slow-twitch muscle, but not those in the fast-twitch white muscle, are mobilized during prolonged exercise. Yet, little is known about the regulation of the metabolism of muscle triglycerides either at rest or during exercise. This is well reflected by the fact that an enzyme responsible for the hydrolysis of muscle triglycerides has not been identified. Mobilization of muscle triglycerides during exercise seems to be under both adrenergic and noradrenergic control. Accumulation of lactic acid and reduction in muscle pH are likely to be strong inhibitors of muscle triglyceride lipolysis. Reduction of carbohydrate availability accelerates mobilization of muscle triglycerides during exercise. The relationship between the plasma free fatty acids and muscle triglyceride metabolism seems to be complex. It has been proposed that most free fatty acids entering the muscle cell is esterified before being oxidized, but this is arguable for contracting skeletal muscles. It is suggested that most free fatty acids entering contracting high oxidative myocytes are transported directly to the mitochondria. A much lesser portion is likely esterified. It is proposed that triglycerides stored in the contracting muscle cell are mobilized when the delivery of the blood-borne-free fatty acids to the mitochondria is insufficient.     

Preparation and properties of human red-cell ankyrin.

We describe a procedure for the preparation of ankyrin from human red cells with a yield of 2-3 mg of protein from 30 ml of packed cells. This represents an improvement of an order of magnitude over the usual earlier procedure. Moreover, the product is, in our hands, much more stable against adsorption and proteolysis, and can in general be stored for at least 2 months at 4 degrees C without significant decrease in concentration and binding activity. The preparation depends on the release of the ankyrin-band-3 complex from the membrane cytoskeleton when intact cells are lysed in a medium containing concentrated Triton X-100. The complex is dissociated at high ionic strength, and the final purification is achieved by gel filtration in a medium containing 2 M-Tris or 0.6 M-NaBr. The ankyrin contains all the progression of components present in the intact membrane. All react with affinity-purified polyclonal anti-ankyrin antibodies, and all give widely similar patterns of peptides in partial proteolytic digests. The ankyrin is fully active, as judged by its capacity to bind to band-3-containing membrane vesicles and to Sepharose-coupled spectrin. All components bind to the membrane vesicles. Purified components 2.1 and 2.2, as well as the calmodulin-binding cytoskeletal constituent adducin, can be isolated in pure form by a single anion-exchange column step.     

Prior exercise delays the onset of acidosis during incremental exercise.

The effects of prior moderate- and prior heavy-intensity exercise on the subsequent metabolic response to incremental exercise were examined. Healthy, young adult subjects (n = 8) performed three randomized plantar-flexion exercise tests: 1) an incremental exercise test (approximately 0.6 W/min) to volitional fatigue (Ramp); 2) Ramp preceded by 6 min of moderate-intensity, constant-load exercise below the intracellular pH threshold (pHT; Mod-Ramp); and 3) Ramp preceded by 6 min of heavy-intensity, constant-load exercise above pHT (Hvy-Ramp); the constant-load and incremental exercise periods were separated by 6 min of rest. (31)P-magnetic resonance spectroscopy was used to continuously monitor intracellular pH, phosphocreatine concentration ([PCr]), and inorganic phosphate concentration ([P(i)]). No differences in exercise performance or the metabolic response to exercise were observed between Ramp and Mod-Ramp. However, compared with Ramp, a 14% (SD 10) increase (P < 0.01) in peak power output (PPO) was observed in Hvy-Ramp. The improved exercise performance in Hvy-Ramp was accompanied by a delayed (P = 0.01) onset of intracellular acidosis [Hvy-Ramp 60.4% PPO (SD 11.7) vs. Ramp 45.8% PPO (SD 9.4)] and a delayed (P < 0.01) onset of rapid increases in [P(i)]/[PCr] [Hvy-Ramp 61.5% PPO (SD 12.0) vs. Ramp 45.1% PPO (SD 9.1)]. In conclusion, prior heavy-intensity exercise delayed the onset of intracellular acidosis and enhanced exercise performance during a subsequent incremental exercise test.     

Re-evaluation of the structural integrity of red-cell glycoproteins during aging in vivo and nutrient deprivation.

Results presented in this paper show that removal of white-cell contaminations from human red blood cells by filtration through cellulose [Beutler, West & Blume (1976) J. Lab. Clin. Med. 88, 328-333] is a necessity whenever red cells are incubated at elevated temperatures or haemolysed after density separation. Omission of this precaution results in proteolysis of sialoglycoproteins in membranes from less-dense (young), but not dense (old), subpopulations. This proteolytic damage occurs during haemolysis of the cytoplasmic domain of glycophorin. A different type of proteolysis occurs if white-cell-contaminated red cells are incubated in the absence of glucose at elevated temperatures. Red cells release sialoglycopeptides. This process is stimulated by Ca2+ ions and is accompanied by the release of vesicles that differ from spectrin-free vesicles [Lutz, Liu & Palek (1977) J. Cell Biol. 73, 548-560]. This sialoglycopeptide release is dependent on white-cell contamination and is not required for the release of spectrin-free vesicles.     

Adaptations during familiarization to resistive exercise.

This study focused on adaptations during familiarization to resistive exercise. It was also determined if familiarization requires one or more sessions. Twenty-six sedentary, college-aged females were matched and randomly assigned to one of two groups. Measurements were obtained during the initial familiarization period (Group 1: 15 trials on 1 day, Group 2: 5 trials on each of three consecutive days), and during retention tests scheduled two weeks and 3 months after the first test session. Elbow flexion torque and surface electromyography (SEMG) of the biceps and triceps were monitored concurrently. There were no significant differences between groups for any of the criterion measures. There was a significant (p<0.05) increase (12.4 Nm, or 38.8%) in maximal isometric elbow flexion torque. Biceps (agonist) root-mean-square (RMS) SEMG exhibited a significant (p<0.05) increase of 95 microV (29%). Triceps (antagonist) RMS SEMG underwent alternating decreases then increases, and each change was significant (p<0.05). The ratio of biceps to triceps RMS SEMG was used to assess cocontraction, and it followed the same pattern of change as triceps RMS SEMG. We concluded that both groups responded in the same way to testing, regardless of the pattern of the first 15 contractions. The increase in maximal isometric elbow flexion torque was due to neural drive to the bicep (agonist). There was a low level of triceps (antagonist) cocontraction to provide joint stability, and it was adjusted throughout the duration of testing.     

Exogenous carbohydrate oxidation during ultraendurance exercise.

The purposes of this study were: 1) to obtain a measure of exogenous carbohydrate (CHO(Exo)) oxidation and plasma glucose kinetics during 5 h of exercise; and 2) to compare CHO(Exo) following the ingestion of a glucose solution (Glu) or a glucose + fructose solution (2:1 ratio, Glu+Fru) during ultraendurance exercise. Eight well-trained subjects exercised three times for 5 h at 58% maximum O2 consumption while ingesting either Glu or Glu+Fru (both delivering 1.5 g/min CHO) or water. The CHO used had a naturally high 13C enrichment, and five subjects received a primed continuous intravenous [6,6-2H2]glucose infusion. CHO(Exo) rates following the ingestion of Glu leveled off after 120 min and peaked at 1.24 +/- 0.04 g/min. The ingestion of Glu+Fru resulted in a significantly higher peak rate of CHO(Exo) (1.40 +/- 0.08 g/min), a faster rate of increase in CHO(Exo), and an increase in the percentage of CHO(Exo) oxidized (65-77%). However, the rate of appearance and disappearance of Glu continued to increase during exercise, with no differences between trials. These data suggest an important role for gluconeogenesis during the later stages of exercise. Following the ingestion of Glu+Fru, cadence (rpm) was maintained, and the perception of stomach fullness was reduced relative to Glu. The ingestion of Glu+Fru increases CHO(Exo) compared with the ingestion of Glu alone, potentially through the oxidation of CHO(Exo) in the liver or through the conversion to, and oxidation of, lactate.     

Transformation of nuclear morphology during cellular maturation.

Using in vivo labeling of mouse leukocytes with tritiated thymidine, cells in the neutrophilic series were studied to determine the change in their nuclear shape as a function of maturation level. Several morphologic shape parameters including perimeter and bending energy were used to quantify the distribution of the nuclear morphology in a given age cohort. The change in these distributions as a function of calender age level was determined. The two parameters named above were used to test the possibility of inferring the age from the quantitative morphology.     

Cardiopulmonary baroreflex is reset during dynamic exercise.

The purpose of this study was to examine the hypothesis that the operating point of the cardiopulmonary baroreflex resets to the higher cardiac filling pressure of exercise associated with the increased cardiac filling volumes. Eight men (age 26 +/- 1 yr; height 180 +/- 3 cm; weight 86 +/- 6 kg; means +/- SE) participated in the present study. Lower body negative pressure (LBNP) was applied at 8 and 16 Torr to decrease central venous pressure (CVP) at rest and during steady-state leg cycling at 50% peak oxygen uptake (104 +/- 20 W). Subsequently, two discrete infusions of 25% human serum albumin solution were administered until CVP was increased by 1.8 +/- 0.6 and 2.4 +/- 0.4 mmHg at rest and 2.9 +/- 0.9 and 4.6 +/- 0.9 mmHg during exercise. During all protocols, heart rate, arterial blood pressure, and CVP were recorded continuously. At each stage of LBNP or albumin infusion, forearm blood flow and cardiac output were measured. During exercise, forearm vascular conductance increased from 7.5 +/- 0.5 to 8.7 +/- 0.6 U (P = 0.024) and total systemic vascular conductance from 7.2 +/- 0.2 to 13.5 +/- 0.9 l.min(-1).mmHg(-1) (P < 0.001). However, there was no significant difference in the responses of both forearm vascular conductance and total systemic vascular conductance to LBNP and the infusion of albumin between rest and exercise. These data indicate that the cardiopulmonary baroreflex had been reset during exercise to the new operating point associated with the exercise-induced change in cardiac filling volume.     

Neurohumoral responses during prolonged exercise in humans.

This study examined neurohumoral alterations during prolonged exercise with and without hyperthermia. The cerebral oxygen-to-carbohydrate uptake ratio (O2/CHO = arteriovenous oxygen difference divided by arteriovenous glucose difference plus one-half lactate), the cerebral balances of dopamine, and the metabolic precursor of serotonin, tryptophan, were evaluated in eight endurance-trained subjects during exercise randomized to be with or without hyperthermia. The core temperature stabilized at 37.9 +/- 0.1 degrees C (mean +/- SE) in the control trial, whereas it increased to 39.7 +/- 0.2 degrees C in the hyperthermic trial, with a concomitant increase in perceived exertion (P < 0.05). At rest, the brain had a small release of tryptophan (arteriovenous difference of -1.2 +/- 0.3 micromol/l), whereas a net balance was obtained during the two exercise trials. Both the arterial and jugular venous dopamine levels became elevated during the hyperthermic trial, but the net release from the brain was unchanged. During exercise, the O2/CHO was similar across trials, but, during recovery from the hyperthermic trial, the ratio decreased to 3.8 +/- 0.3 (P < 0.05), whereas it returned to the baseline level of approximately 6 within 5 min after the control trial. The lowering of O2/CHO was established by an increased arteriovenous glucose difference (1.1 +/- 0.1 mmol/l during recovery from hyperthermia vs. 0.7 +/- 0.1 mmol/l in control; P < 0.05). The present findings indicate that the brain has an increased need for carbohydrates during recovery from strenuous exercise, whereas enhanced perception of effort as observed during exercise with hyperthermia was not related to alterations in the cerebral balances of dopamine or tryptophan.     

Regulation of muscle carbohydrate metabolism during exercise.

This review examines the mechanisms that regulate muscle carbohydrate metabolism during exercise. Muscle carbohydrate utilization is regulated primarily by two factors, namely, delivery of substrate to the glycolytic pathway either from glycogenolysis or from transport of extracellular glucose into the fibers, and formation of triosephosphate by phosphofructokinase. The regulation involves the integration of the glycolytic controls with other metabolic controls and the needs of the whole muscle in meeting the physiological demand. The controls operating in the glycolytic sequence in vivo appear to couple glycolytic recruitment to signals from the rate of energy demand, the TCA cycle state, and the mitochondrial redox state so as to satisfy the major regulatory goal of maintaining the supply of ATP for tension development.     

Neuromuscular fatigue during a long-duration cycling exercise.

The effects of prolonged cycling on neuromuscular parameters were studied in nine endurance-trained subjects during a 5-h exercise sustained at 55% of the maximal aerobic power. Torque during maximal voluntary contraction (MVC) of the quadriceps muscle decreased progressively throughout the exercise (P < 0.01) and was 18% less at the end of exercise compared with the preexercise value. Peak twitch torque, contraction time, and total area of mechanical response decreased significantly (P < 0.05) after the first hour of exercise. In contrast, changes in M-wave characteristics were significant only after the fourth hour of the exercise. Significant reductions (P < 0.05) in electromyographic activity normalized to the M wave occurred after the first hour for the vastus lateralis muscle but only at the end of the exercise for the vastus medialis muscle. Muscle activation level, assessed by the twitch interpolation technique, decreased by 8% (P < 0.05) at the end of the exercise. The results suggest that the time course is such that the contractile properties are significantly altered after the first hour, whereas excitability and central drive are more impaired toward the latter stages of the 5-h cycling exercise.     

Caffeine increases exogenous carbohydrate oxidation during exercise.

Both carbohydrate (CHO) and caffeine have been used as ergogenic aids during exercise. It has been suggested that caffeine increases intestinal glucose absorption, but there are also suggestions that it may decrease muscle glucose uptake. The purpose of the study was to investigate the effect of caffeine on exogenous CHO oxidation. In a randomized crossover design, eight male cyclists (age 27 +/- 2 yr, body mass 71.2 +/- 2.3 kg, maximal oxygen uptake 65.7 +/- 2.2 ml x kg(-1) x min(-1)) exercised at 64 +/- 3% of maximal oxygen uptake for 120 min on three occasions. During exercise subjects ingested either a 5.8% glucose solution (Glu; 48 g/h), glucose with caffeine (Glu+Caf, 48 g/h + 5 mg x kg(-1) x h(-1)), or plain water (Wat). The glucose solution contained trace amounts of [U-13C]glucose so that exogenous CHO oxidation could be calculated. CHO and fat oxidation were measured by indirect calorimetry, and 13C appearance in the expired gases was measured by continuous-flow IRMS. Average exogenous CHO oxidation over the 90- to 120-min period was 26% higher (P < 0.05) in Glu+Caf (0.72 +/- 0.04 g/min) compared with Glu (0.57 +/- 0.04 g/min). Total CHO oxidation rates were higher (P < 0.05) in the CHO ingestion trials compared with Wat, but they were highest when Glu+Caf was ingested (1.21 +/- 0.37, 1.84 +/- 0.14, and 2.47 +/- 0.23 g/min for Wat, Glu, and Glu+Caf, respectively; P < 0.05). There was also a trend (P = 0.082) toward an increased endogenous CHO oxidation with Glu+Caf (1.81 +/- 0.22 g/min vs. 1.27 +/- 0.13 g/min for Glu and 1.12 +/- 0.37 g/min for Wat). In conclusion, compared with glucose alone, 5 mg x kg(-1) x h(-1) of caffeine coingested with glucose increases exogenous CHO oxidation, possibly as a result of an enhanced intestinal absorption.     

Metabolic control of hepatic gluconeogenesis during exercise.

Prolonged exercise increased the concentrations of the hexose phosphates and phosphoenolpyruvate and depressed those of fructose 1,6-bisphosphate, triose phosphates and pyruvate in the liver of the rat. Since exercise increases gluconeogenic flux, these changes in metabolite concentrations suggest that metabolic control is exerted, at least, at the fructose 6-phosphate/fructose 1,6-bisphosphate and phosphoenolpyruvate/pyruvate substrate cycles. Exercise increased the maximal activities of glucose 6-phosphatase, fructose 1,6-bisphosphatase, pyruvate kinase and pyruvate carboxylase in the liver, but there were no changes in those of glucokinase, 6-phosphofructokinase and phosphoenolpyruvate carboxykinase. Exercise changed the concentrations of several allosteric effectors of the glycolytic or gluconeogenic enzymes in liver; the concentrations of acetyl-CoA, ADP and AMP were increased, whereas those of ATP, fructose 1,6-bisphosphate and fructose 2,6-bisphosphate were decreased. The effect of exercise on the phosphorylation-dephosphorylation state of pyruvate kinase was investigated by measuring the activities under conditions of saturating and subsaturating concentrations of substrate. The submaximal activity of pyruvate kinase (0.5 mM-phosphoenolpyruvate), expressed as percentage of Vmax., decreased in the exercised animals to less than half that found in the controls. These changes suggest that hepatic pyruvate kinase is less active during exercise, possibly owing to phosphorylation of the enzyme, and this may play a role in increasing the rate of gluconeogenesis.