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Muscle metabolic, SR Ca²⁺-cycling responses to prolonged cycling, with and without glucose supplementation

Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada

Submitted 19 December 2006 ; accepted in final form 28 September 2007

This study investigated the effects of prolonged exercise, with and without glucose supplementation, on metabolism and sarcoplasmic reticulum (SR) Ca²⁺-handling properties in working vastus lateralis muscle. Fifteen untrained volunteers [peak O₂ consumption (O_2peak) = 3.45 ± 0.17 l/min; mean ± SE] cycled at 60% O_2peak on two occasions, during which they were provided with either an artificially sweetened placebo beverage (NG) or a 6% glucose (G) beverage (1.00 g carbohydrate/kg body mass). Beverage supplementation started at 30 min of exercise and continued every 15 min thereafter. SR Ca²⁺ handling, metabolic, and substrate responses were assessed in tissue extracted from the vastus lateralis at rest, after 30 min and 90 min of exercise, and at fatigue in both conditions. Plasma glucose during G was 15–23% higher (P < 0.05) than those observed during NG following 60 min of exercise until fatigue. Cycle time to fatigue was increased (P < 0.05) by 19% during G (137 ± 7 min) compared with NG (115 ± 6 min). Prolonged exercise reduced (P < 0.05) maximal Ca²⁺-ATPase activity (–18.4%), SR Ca²⁺ uptake (–27%), and both Phase 1 (–22.2%) and Phase 2 (–34.2%) Ca²⁺-release rates during NG. The exercise-induced reductions in SR Ca²⁺-cycling properties were not altered during G. The metabolic responses to exercise were all unaltered by glucose supplementation, since no differences in respiratory exchange ratios, carbohydrate and lipid oxidation rates, and muscle metabolite and glycogen contents were observed between NG and G. These results indicate that the maintenance of blood glucose homeostasis by glucose supplementation is without effect in modifying the muscle metabolic, endogenous glycogen, or SR Ca²⁺-handling responses.

Ca²⁺ regulation; glucose supplementation; human skeletal muscle; metabolism

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