FUNCTION OF SKELETAL-MUSCLE SARCOPLASMIC-RETICULUM VESICLES WITH EXERCISE

In this study the response of sarcoplasmic reticulum prolonged moderat e-intensity exercise was examined in highly purified native vesicles i solated from rat gastrocnemius muscle. Maximal Ca2+-dependent ATP hydr olysis was reduced by 12.6% within 2 min after the onset of exercise. The reduction in Ca2+-dependent adenosinetriphosphatase activity progr essed to 18% at 30 min of exercise and was maintained throughout the s ubsequent 90-100 min of exercise. Oxalate stimulation of unidirectiona l Ca2+ transport (Ca2+ loading) was unaffected by exercise. However, i n the absence of anion stimulation, steady-state Ca2+ uptake (bidirect ional flux) was 51.2 +/- 7.3 nmol Ca2+/mg SR after exercise compared w ith 36.2 +/- 2.5 nmol Ca2+/mg SR for the control period (P < 0.05). An ion-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 mecha nistic basis for the increase in apparent Ca2+-ATP coupling is unclear , although the early onset of the changes suggests the potential for f unctional adaptation for the SR in response to increased contractile a ctivity.