Net calcium fluxes in anterior byssus retractor muscle with phasic and catch contraction

Calcium in the artificial seawater bathing whole Mytilus anterior byssus retractor muscles (ABRM) was measured by a specific Ca electrode under various conditions of activation, catch, and catch relaxation. Activation in response to ACh was associated with uptake of Ca by the muscles. Phasic contractions produced a small Ca uptake; catch contractions produced a larger and sustained Ca uptake. After tension relaxation, the muscle lost an amount of Ca roughly equal to that gained. Catch relaxation by 5-hydroxytryptamine (5HT) was associated with Ca release. ACh at identical concentrations, applied to the muscle for increasingly longer times, produced increasing amounts of Ca uptake. Regardless of the previous gain of Ca by the muscle, 5HT applied for a constant interval caused release of the same amount of Ca. A model for the Ca control system in ABRM based on this and previously obtained 45Ca efflux data is proposed.