Calcium handling by the sarcoplasmic reticulum during oscillatory contractions of skinned skeletal muscle fibres.

Isometric ATP consumption and force were investigated in mechanically skinned fibres from iliofibularis muscle of Xenopus laevis. Measurements were performed at different [Ca2+], in the presence and absence of caffeine (5 nM). In weakly Ca2+-buffered solutions without caffeine, spontaneous oscillations in force and ATPase activity occurred. The repetition frequency was [Ca2+]-and temperature-dependent. The Ca2+ threshold (+/- SEM) for the oscillations corresponded to a pCa of 6.5 +/- 0.1. The maximum ATP consumption associated with calcium uptake by the sarcoplasmic reticulum (SR) reached during the oscillations was similar to the activity under steady-state conditions at saturating calcium concentrations in the presence of caffeine. Maximum activity was reached when the force relaxation was almost complete. The calculated amount of Ca2+ taken up by the SR during a complete cycle corresponded to 5.4 +/ 0.4 mmol per litre cell volume. In strongly Ca2+-buffered solutions, caffeine enhanced the calcium sensitivity of the contractile apparatus and, at low calcium concentrations, SR Ca uptake. These results suggest that when the SR is heavily loaded by net Ca uptake, there is a massive calcium-induced calcium release. Subsequent net Ca uptake by the SR then gives rise to the periodic nature of the calcium transient.