Altered sarcoplasmic reticulum function in rat diaphragm after high-intensity exercise

The present study examined the effects of acute high-intensity exercise on Ca(2+) uptake and release rates and Ca(2+)-adenosine triphosphatase (ATPase) activity of the sarcoplasmic reticulum (SR) from the costal diaphragm. The rats were run on a treadmill at an estimated requirement of 100% of maximal O2 consumption until fatigued (average time to exhaustion: 4.79 min). Muscle lactate and inorganic phosphate after exercise were increased by 65% (P < 0.05) and 35% (P < 0.05), respectively. With exercise, Ca(2+) uptake and release, which were detected in homogenates using the Ca(2+) fluorescent dye indo-1, were decreased by 24% (P < 0.05) and 22% (P < 0.05), respectively. The reduction in Ca(2+) uptake was paralleled by decreased activity of SR Ca(2+)-ATPase in both the absence and presence of Ca(2+) ionophore. These findings demonstrate that, in the diaphragm as well as in the locomotor muscles that have been explored in previous studies, the attenuations of the SR function is brought about by acute high-intensity exercise. These changes in the SR of the diaphragm may contribute, at least in part, to deteriorations in exercise tolerance and work productivity resulting from repetitive physical activities.