effects of reduced muscle glycogen on excitation–contraction coupling in rat fast-twitch muscle: a glycogen removal study

The aim of this study was to investigate the effects of an enzymatic removal of glycogen on excitation–contraction coupling in mechanically skinned fibres of rat fast-twitch muscles, with a focus on the changes in the function of na+–k+-pump and ryanodine receptor (ryr). glycogen present in the skinned fibres and binding to microsomes was removed using glucoamylase (ga). exposure of whole muscle to 20 u ml−1 ga for 6 min resulted in a 72% decrease in the glycogen content. six minutes of ga treatment led to an 18 and a 22% reduction in depolarization- and action potential-induced forces in the skinned fibres, respectively. there was a minor but statistically significant increase in the repriming period, most likely because of an impairment of the na+–k+-pump function. ga treatment exerted no effect on the maximum ca2+ release rate from the ryr in the microsomes and the myofibrillar ca2+ sensitivity in the skinned fibres. these results indicate that reduced glycogen per se can decrease muscle performance due to the impairment of sr ca2+ release and suggest that although na+–k+-pump function is adversely affected by reduced glycogen, the extent of the impairment is not sufficient to reduce ca2+ release from the sarcoplasmic reticulum. this study provides direct evidence that glycogen above a certain amount is required for the preservation of the functional events preceding ca2+ release from the sarcoplasmic reticulum.