Muscle Glycogen Content Modifies SR Ca2+ Release Rate in Elite Endurance Athletes

PURPOSE: The aim of the present study was to investigate the influence of muscle glycogen content on sarcoplasmic reticulum (SR) function and peak power output (Wpeak) in elite endurance athletes. METHODS: Fourteen highly trained male triathletes (VO2max = 66.5 ± 1.3 mL O2·kg·min), performed 4 h of glycogen-depleting cycling exercise (HRmean = 73% ± 1% of maximum). During the first 4 h of recovery, athletes received either water (H2O) or carbohydrate (CHO), separating alterations in muscle glycogen content from acute changes affecting SR function and performance. Thereafter, all subjects received CHO-enriched food for the remaining 20-h recovery period. RESULTS: Immediately after exercise, muscle glycogen content and SR Ca release rate was reduced to 32% ± 4% (225 ± 28 mmol·kg dw) and 86% ± 2% of initial levels, respectively (P < 0.01). Glycogen markedly recovered after 4 h of recovery with CHO (61% ± 2% of preexercise) and SR Ca release rate returned to preexercise level. However, in the absence of CHO during the first 4 h of recovery, glycogen and SR Ca release rate remained depressed, with the normalization of both parameters at the end of the 24 h of recovery after receiving a CHO-enriched diet. Linear regression demonstrated a significant correlation between SR Ca release rate and muscle glycogen content (P < 0.01, r = 0.30). The 4 h of cycling exercise reduced Wpeak by 5.5%-8.9% at different cadences (P < 0.05), and Wpeak was normalized after 4 h of recovery with CHO, whereas Wpeak remained depressed (P < 0.05) after water provision. Wpeak was fully recovered after 24 h in both the H2O and the CHO group. CONCLUSION: In conclusion, the present results suggest that low muscle glycogen depresses muscle SR Ca release rate, which may contribute to fatigue and delayed recovery of Wpeak 4 h postexercise.