Calcium fluxes accompanying the action of 5-hydroxytryptamine on mussel hearts

1. Isolated, superfused ventricles of Geukensia demissa demissa were soaked in artificial seawater containing radiocalcium (⁴⁵Ca). The efflux of ⁴⁵Ca into cold seawater was analyzed, and the effect of 5-hydroxytryptamine (5-HT) on the efflux was studied.2. The control ⁴⁵Ca efflux contained three identifiable components with the following time constants: τ₁ = 2.27 ± 0.30 min; τ₂ = 8.8 ± 1.1 min; and τ₃ = 133.1 ± 39.2 min.3. Excitation by 5-HT (10⁻⁶ M) caused an immediate systolic contracture and a transient increase in ⁴⁵Ca efflux. The source of this efflux was depleted by a single, 10 min episode of 5-HT excitation.4. When 5-HT was washed out, an efflux increase of long duration was observed. The time constant (9.3 ± 1.8 min) was identical to that of the second control component, τ₂.5. 5-HT caused excitation by decreasing the net calcium efflux, thereby increasing intracellular free calcium. No such change was associated with cardioinhibition by 5-HT.