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

Oct 6, 2007Journal of applied physiology (Bethesda, Md. : 1985)

Muscle energy use and calcium handling during long cycling with and without extra glucose

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Abstract

Plasma glucose levels were 15-23% higher during glucose supplementation compared to placebo after 60 minutes of exercise.

Cycle time to fatigue increased by approximately 19% during glucose supplementation (137 ± 7 min) compared to placebo (115 ± 6 min).
Prolonged exercise reduced maximal Ca(2+)-ATPase activity by 18.4%, SR Ca(2+) uptake by 27%, and both Phase 1 and Phase 2 Ca(2+)-release rates during placebo.
The exercise-induced reductions in SR Ca(2+)-cycling properties were not altered by glucose supplementation.
Metabolic responses, including respiratory exchange ratios and oxidation rates, showed no differences between glucose and placebo conditions.

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This study investigated the effects of prolonged exercise, with and without glucose supplementation, on metabolism and sarcoplasmic reticulum (SR) Ca(2+)-handling properties in working vastus lateralis muscle. Fifteen untrained volunteers [peak O(2) consumption (Vo(2peak)) = 3.45 +/- 0.17 l/min; mean +/- SE] cycled at approximately 60% Vo(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 Ca(2+) 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 approximately 19% during G (137 +/- 7 min) compared with NG (115 +/- 6 min). Prolonged exercise reduced (P < 0.05) maximal Ca(2+)-ATPase activity (-18.4%), SR Ca(2+) uptake (-27%), and both Phase 1 (-22.2%) and Phase 2 (-34.2%) Ca(2+)-release rates during NG. The exercise-induced reductions in SR Ca(2+)-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 Ca(2+)-handling responses.

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

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