The action of 5-hydroxytryptamine on Mytilus smooth muscle.

1. In the nerve-muscle preparation, where catch was characteristically minimal, 5-hydroxytryptamine (5-HT) had no effect on resting membrane potential, junction potentials, spikes or contraction.2. In muscle bundles, where catch was prominent, 5-HT did not change membrane potentials, but prolonged junction potentials and lowered the threshold for spike discharge and contraction.3. In muscle bundles, exposed to high concentrations of 5-HT, depolarization evoked repetitive spikes, while in low 5-HT, spikes were seldom fired even with much greater depolarization.4. In muscle bundles, the effective membrane resistance, R(eff.), decreased from 45-60 to 23-35 MOmega as 5-HT concentration was increased.5. It is suggested that 5-HT may facilitate spike discharge by lowering the internal free Ca(2+) concentration.