Modulation of Crayfish Hearts by FMRFamide-related Peptides.

The present study examines effects of FMRFamide-related peptides (FaRPs) on crayfish heart. Lobster peptides F1 (TNRNFLRFamide) and F2 (SDRNFLRFamide) increase the rate and amplitude of heart beat in hearts isolated from Procambarus clarkii. Thresholds for these effects were between 10-10 and 10-9 M for F2 and between 10-9 and 10-8 M for F1. FMRFamide and FLRFamide elicited similar cardioexcitatory effects, but at thresholds of approximately 10-7 M. Thus, the aminoterminal extensions "TNRN" and "SDRN" enhance the excitatory actions of FMRFamide and FLRFamide. SchistoFLRFamide (PDVDHVFLRFamide) and leucomyosuppressin (pQDVDHVFLRFamide) markedly decrease the rate of cardiac contractions at 10-9 to 10-8 M and can suppress the cardiac rhythm for one minute or more at 10-7 M. The amino-terminal extensions of these two peptides, therefore, are necessary for inhibition of heart rate. Both of these peptides cause an initial reduction in contraction amplitude, but contractions subsequently increase in the presence of schistoFLRFamide. Thus, crayfish hearts are sensitive to several FMRFamide-related peptides, but the sites and mechanisms of action remain to be determined.