CALCIUM HANDLING BY THE SARCOPLASMIC-RETICULUM DURING OSCILLATORY CONTRACTIONS OF SKINNED SKELETAL-MUSCLE FIBERS

Isometric ATP consumption and force were investigated in mechanically skinned fibres from iliofibularis muscle of Xenopus laevis. Measuremen ts were performed at different [Ca2+], in the presence and absence of caffeine (5 mM). In weakly Ca2+-buffered solutions without caffeine, s pontaneous oscillations in force and ATPase activity occurred. The rep etition frequency was [Ca2+]-and temperature-dependent. The Ca2+ thres hold (+/- 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 simila r to the activity under steady-state conditions at saturating calcium concentrations in the presence of caffeine. Maximum activity was reach ed when the force relaxation was almost complete. The calculated amoun t 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 solut ions, caffeine enhanced the calcium sensitivity of the contractile app aratus 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 b y the SR then gives rise to the periodic nature of the calcium transie nt. (C) Chapman & Hall Ltd.