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In humans, acute aerobic exercise elicits a sustained postexercise vasodilation within previously active skeletal muscle. This response is dependent on activation of histamine H1 and H2 receptors, but the source of intramuscular histamine remains unclear. We tested the hypothesis that interstitial histamine in skeletal muscle would be increased with exercise and would be dependent on de novo formation via the inducible enzyme histidine decarboxylase and/or mast cell degranulation. Subjects performed 1 h of unilateral dynamic knee-extension exercise or sham (seated rest). We measured the interstitial histamine concentration and local blood flow (ethanol washout) via skeletal muscle microdialysis of the vastus lateralis. In some probes, we infused either α-fluoromethylhistidine hydrochloride (α-FMH), a potent inhibitor of histidine decarboxylase, or histamine H1/H2-receptor blockers. We also measured interstitial tryptase concentrations, a biomarker of mast cell degranulation. Compared with preexercise, histamine was increased after exercise by a change (Δ) of 4.2 ± 1.8 ng/ml (P < 0.05), but not when α-FMH was administered (Δ-0.3 ± 1.3 ng/ml, P = 0.9). Likewise, local blood flow after exercise was reduced to preexercise levels by both α-FMH and H1/H2 blockade. In addition, tryptase was elevated during exercise by Δ6.8 ± 1.1 ng/ml (P < 0.05). Taken together, these data suggest that interstitial histamine in skeletal muscle increases with exercise and results from both de novo formation and mast cell degranulation. This suggests that exercise produces an anaphylactoid signal, which affects recovery, and may influence skeletal muscle blood flow during exercise. NEW & NOTEWORTHY Blood flow to previously active skeletal muscle remains elevated following an acute bout of aerobic exercise and is dependent on activation of histamine H1 and H2 receptors. The intramuscular source of histamine that drives this response to exercise has not been identified. Using intramuscular microdialysis in exercising humans, we show both mast cell degranulation and formation of histamine by histidine decarboxylase contributes to the histamine-mediated vasodilation that occurs following a bout of aerobic exercise. [ABSTRACT FROM AUTHOR]

This investigation evaluated the physiological impact of different dietary protein intakes on skeletal muscle protein synthesis postexercise in endurance runners. Five endurance-trained, male runners participated in a randomized, crossover design diet intervention, where they consumed either a low (0.8 g/kg; LP)-, moderate (1.8 g/kg; MP)-, or high (3.6 g/kg; HP)-protein diet for 4 wk. Diets were designed to be eucaloric with carbohydrate, fat, and protein approximating 60, 30, and 10%; 55, 30, and 15%; and 40, 30, and 30% for LP, MP, and HP, respectively. Substrate oxidation was assessed via indirect calorimetry at 3 wk of the dietary interventions. Mixed-muscle protein fractional synthetic rate (FSR) was measured after an endurance run (75 rain at 70% VO2 peak) using a primed, continuous infusion of [²H5]phenylalanine. Protein oxidation increased with increasing protein intake, with each trial being significantly different from the other (P « 0.01). FSR after exercise was significantly greater for LP (0.083%/h) and MP (0.078%/h) than for HP (0.052%/h; P « 0.05). There was no difference in FSR between LP and MP. This is the first investigation to establish that habitual dietary protein intake in humans modulates skeletal muscle protein synthesis after an endurance exercise bout. Future studies directed at mechanisms by which level of protein intake influences skeletal muscle turnover are needed. [ABSTRACT FROM AUTHOR]

We have tested the hypothesis that transit through the interstitial fluid, rather than across cell membranes, is rate limiting for amino acid uptake from blood into muscle in human subjects. To quantify muscle transmembrane transport of naturally occurring amino acids, we developed a novel 4-pool model that distinguishes between the interstitial and intracellular fluid compartments. Transport kinetics of phenylalanine, leucine, lysine, and alanine were quantified using tracers labeled with stable isotopes. The results indicate that interstitial fluid is a functional compartment insofar as amino acid kinetics are concerned. In the case of leucine and alanine, transit between blood and interstitial fluid was potentially rate limiting for muscle amino acid uptake and release in the postabsorptive state. For example, in the case of leucine, the rate of transport between blood and interstitial fluid compared with the corresponding rate between interstitial fluid and muscle was 247 ± 36 vs. 610 ± 95 nmol·min-1·100 ml leg-1, respectively (P < 0.05). Our results are consistent with the process of diffusion governing transit from blood to interstitial fluid without selectivity, and of specific amino acid transport systems with varying degrees of efficiency governing transit from interstitial fluid to muscle. These results imply that changes in factors that affect the transit of amino acids from blood through interstitial fluid, such as muscle blood flow or edema, could play a major role in controlling the rate of muscle amino acid uptake. [ABSTRACT FROM AUTHOR]

Investigates the relationship between age and leg blood flow responses during submaximal cycle ergometry in health normally active men. Oxygen consumption; Mean arterial pressure; Cardiac output; Blood oxygen content; Plasma catecholamine levels.

The inotropic and toxic effects of cardiac steroids are thought to result from Na[sup +]-K[sup +]ATPase inhibition, with elevated intracellular Na[sup +](Na[sup +, sub i])causing increased intracellular Ca[sup 2+](Ca[sup 2+, sub i]) via Na-Ca exchange. We studied the effects of ouabain on cat ventricular myocytes in Na[sup +]-free conditions where the exchanger is inhibited. Cell shortening and Ca[sup 2+, sub i] transients (with fluo 4-AM fluorescence) were measured under voltage clamp during exposure to Na[sup +]-free solutions [LiC1 or N-methyl-Dglucamine (NMDG) replacement]. Ouabain enhanced contractility by 121 ± 55% at 1 µmol/1 (n = 11) and 476 ± 159% at 3 µmol/l (n = 8) (means ± SE). Ca[sup 2+, sub i] transient amplitude was also increased. The inotropic effects of ouabain were retained even after pretreatment with saxitoxin (5 µmol/1) or changing the holding potential to -40 mV (to inactivate Na[sup +] current). Similar results were obtained with both Li[sup +] and NMDG replacement and in the absence of external K[sup +], indicating that ouabain produced positive inotropy in the absence of functional Na-Ca exchange and Na[sup +]-K[sup +]-ATPase activity. In contrast, ouabain had no inotropic response in rat ventricular myocytes (10-100 µmol/l). Finally, ouabain reversibly increased Ca[sup 2+] overload toxicity by accelerating the rate of spontaneous after contractions (n = 13). These results suggest that the cellular effects of ouabain on the heart may include actions independent of Na[sup +]-K[sup +]-ATPase inhibition, Na-Ca exchange, and changes in Na[sup +, sub i]. [ABSTRACT FROM AUTHOR]

Presents a study that examined the time course changes in serum insulin concentrations in food-deprived rats after an oral bolus of leucine. Examination of the contribution of changes in serum concentrations of insulin to the increase in protein synthesis; Circulating insulin concentrations in food-deprived rats infused with somatostatin and leucine; Chemical essential for the enhanced rates of protein synthesis observed in skeletal muscle after food intake.

Examines the effects of dynamic one-legged knee extension exercise on mean blood velocity (MBV) and muscle interstitial metabolite concentrations in healthy young people. Examination of blood flow changes during exercise using Doppler ultrasound; Influence of exercise intensity on MBV and interstitial potassium and adenosine; Effect of exercise on the temporal pattern of response for MBV and interstitial adenosine.

Deals with a study which examined the effects of adenosine perfusion of the isolated triceps surae muscle group on the interstitial adenosine concentrations as well as heart rate and blood pressure responses. Methods; Results; Discussion.

Provides information on a study which evaluated an approach for determining skeletal muscle-specific glucose flux using radioactive stereoisomers and the microdialysis technique. Methods; Results; Discussion.

Focuses on a study that used microdialysis technique to measure the interstitial concentrations of several putative stimulators of the exercise pressor reflex during intermittent static quadriceps exercise in humans. Increase in muscle sympathetic nerve activity; Methodology of the study; Results and discussion; Conclusion.

Examines the potential mechanism contributing to the inhibition of protein synthesis in skeletal muscle and heart after administration of tumor necrosis factors-α in rats. Reason behind reduction in synthesis of microfibrillar and sarcoplasmic proteins; Study of eukaryotic initiation factors and elongation factors; Relation between heart and skeletal muscle protein synthesis and messenger ribonucleic acid translational efficiency.