Innervation and control of the heart of a gastropod,Rapana

Summary The innervation and control of the heart of a prosobranch mollusc,Rapana thomasiana, were studied. Acetylcholine was found to be an inhibitory neurotransmitter. Both serotonin and FMRFamide (Phe-Met-Arg-Phe-NH₂) showed excitatory effects on the heart; FMRFamide had greater inotropic and more regulatory chronotropic effects than serotonin. The effects of serotonin were blocked by methysergide, while the effects of FMRFamide and of stimulating the excitatory cardiac nerves were not blocked. Stimulation of circumesophageal ganglia elicited a slow enhancement of heart beat together with body movement. This enhancement was blocked by methysergide. Serotonin was considered to act at the heart as a local neurohormone. Although the mechanism of action of FMRFamide is still not yet clarified, it is possible that FMRFamide plays a physiological role as a cardioregulatory substance, as indicated by the physiological and histological findings.