Function of skeletal muscle sarcoplasmic reticulum vesicles with exercise

In this study the response of sarcoplasmic reticulum (SR) to prolonged moderate-intensity exercise was examined in highly purified native vesicles isolated from rat gastrocnemius muscle. Maximal Ca(2+)-dependent ATP hydrolysis was reduced by 12.6% within 2 min after the onset of exercise. The reduction in Ca(2+)-dependent adenosinetriphosphatase activity progressed to 18% at 30 min of exercise and was maintained throughout the subsequent 90-100 min of exercise. Oxalate stimulation of unidirectional Ca2+ transport (Ca2+ loading) was unaffected by exercise. However, in the absence of anion stimulation, steady-state Ca2+ uptake (bidirectional flux) was 51.2 +/- 7.3 nmol Ca2+/mg SR after exercise compared with 36.2 +/- 2.5 nmol Ca2+/mg SR for the control period (P < 0.05). Anion-induced Ca2+ release increased from a control value of 33.9 +/- 4.3 to 55.9 +/- 9.8 nmol Ca2+/mg SR after exercise (P < 0.05). The mechanistic basis for the increase in apparent Ca(2+)-ATP coupling is unclear, although the early onset of the changes suggests the potential for functional adaptation for the SR in response to increased contractile activity.