FMRFamide inhibition of a molluscan heart is accompanied by increases in cyclic AMP.

The molluscan neuropeptide FMRFamide (Phe-Met-Arg-Phe-NH2) inhibits the beat of isolated ventricles of the freshwater clam Lampsilis claibornensis. The decline in frequency and diastolic tone precedes the dose- and time-dependent increase in cAMP also produced by the peptide. Phosphodiesterase inhibitors (theophylline, 3-isobutyl-1-methylxanthine) and cAMP analogs also inhibit Lampsilis hearts, but only at high doses (10(-5)-10(-3)M). Again, the inhibition by theophylline is accompanied by a rise in cAMP. Nevertheless, the delayed increase in cAMP levels following FMRFamide treatment may suggest that cAMP is not mediating the onset of FMRFamide cardioinhibition.