Effect of endurance training and acute exercise on sarcoplasmic reticulum function in rat fast- and slow-twitch skeletal muscles.

Following 10 weeks of endurance training and in age-matched sedentary rats, sarcoplasmic reticulum (SR) Ca(2+)-uptake, Ca(2+)-release, and Ca(2+)-stimulated adenosinetriphosphatase (ATPase) activity were examined in homogenates of the plantaris and soleus muscles from rats subjected to moderate-intensity treadmill running to exhaustion. In order to examine the effects of acute exercise and/or training on SR Ca(2+)-handling capacity, comparisons between exhausted and non-exercised rats and between trained and untrained rats were performed. Our data confirm that Ca(2+)-sequestration by the SR from fast-twitch muscles is depressed after training. Immediately after exhaustive running, decreases in SR function occurred in both muscles, but were more pronounced in the soleus. In the plantaris, reductions in SR Ca(2+)-uptake rate and Ca(2+)-ATPase activity were observed in untrained rats only, while in the soleus they were adversely affected irrespective of training status. Although the average run time to exhaustion varied markedly between untrained and trained animals (untrained: 253.0 min; trained: 559.4 min), no differences existed with regard to the magnitude of decreases in SR function in the soleus after exercise. The mean rate of decline in SR Ca(2+)-handling capacity during acute exercise, as estimated from the run time and the extent of the decline, was more than twofold higher in untrained than in trained soleus. From the present study, it is unclear whether there exists a causal relationship between muscular fatigue and SR function because the run time to exhaustion was not significantly correlated with any of parameters indicative of SR Ca(2+)-handling capacity, but suggested that endurance training may be capable of delaying a progression of the deterioration in SR function that occurs during exercise.