The action of 5-hydroxytryptamine onMytilussmooth muscle

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Abstract
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. 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. 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. In muscle bundles, the effective membrane resistance, Reff., decreased from 45-60 to 23-35 Md as 5-HT concentration was increased. It is suggested that 5-HT may facilitate spike discharge by lowering the internal free Ca2 concentration.