The molluscan neuropeptide FMRFamide: calcium-dependent release and blood levels in Macrocallista (Bivalvia).

Ganglia dissected from the marine bivalve, Macrocallista nimbosa, were pooled and incubated in a series of seawater media designed to promote or inhibit the release of the neuropeptide Phe-Met-Arg-Phe-NH2 (FMRFamide). The amount of peptide released, determined by radioimmunoassay (RIA), was enhanced two to seven times when the medium contained high potassium. FMRFamide released was reversibly inhibited when calcium was omitted from the high potassium medium. Immunoreactive FMRFamide was found in Macrocallista hemolymph in a concentration (10(-9)M) that excites isolated Macrocallista ventricles. Although the in vivo release sites have not yet been localized, these findings already show that the endogenous neuropeptide FMRFamide can be characterized as a neurohormone or as a neurotransmitter in Macrocallista.