Muscle Glycogen Content Alters Time-Dependent Reductions in Sarcoplasmic Reticulum Ca2+-ATPase Activity During Exercise

Abstract

1219 PURPOSE: In this study, we investigated the hypothesis that muscle glycogen content alters the function of the sarcoplasmic reticulum (SR) Ca2+-ATPase during prolonged exercise in skeletal muscle. METHODS: Ten untrained males cycled at ∼65% VO2peak on two occasions following a standardized glycogen depeletion protocol, namely, after a 4-day low CHO diet (LCHO; 2234 kcal; 17% CHO; 60% Fat) and after a 4-day high CHO diet (HCHO; 3966 kcal; 66% CHO: 23% Fat). Maximal SR Ca2+-ATPase (Vmax) was assessed in homogenates prepared from tissue extracted from the vastus lateralis at rest, after 30 min of exercise and at a time corresponding to fatigue during LCHO (66 ± 6 min) in both LCHO and HCHO, and at fatigue in HCHO (103 ± 9 min). RESULTS: Pre-exercise glycogen content was lower ((P<0.01) by 37% in LCHO compared to HCHO. Muscle glycogen content was also lower (P<0.01) after 30 min, and at fatigue in LCHO, compared to the matching time points in HCHO. Glycogen content was depleted to similar levels at fatigue in LCHO compared to fatigue in HCHO. No differences in SR properties were observed at rest between diets. However, during exercise, reductions (P<0.05) in Vmax were observed by 30 min of exercise (−28%) and at fatigue during LCHO (−32%) in LCHO, but not for matched time points in HCHO (−1%, −10%, respectively). When a comparison was made at fatigue, it was observed that Vmax was reduced to similar levels in both LCHO (−32%) and HCHO (−26%). Neither the sensitivity (Ca50) nor the cooperatively (Hill coefficient) of the Ca2+-ATPase enzyme were affected by diet or by exercise. The efficiency of the Ca2+-ATPase enzyme to transport Ca2+ into the lumen of the SR per ATP hydrolyzed (coupling ratio) was also not altered by diet or by exercise. Additionally, passive Ca2+ leak out of the SR membrane, as assessed by comparing the ratio between Vmax in the presence and absence of 1 FM of Ca2+-ionophore A23187, was not altered by diet or exercise. CONCLUSION: These results indicate that a low muscle glycogen content can influence the time course of exercise-induced alterations in the SR Ca2+-ATPase enzyme, resulting in the reduction in Vmax, without altering the Ca2+ sensitivity of the SR Ca2+-ATPase enzyme or the efficiency of Ca2+-transport. Supported by NSERC (Canada).