Reference: Biol. Bull. 162: 311-332. (June, 1982) A SURVEY OF THE RESPONSES OF BIVALVE HEARTS TO THE MOLLUSCAN NEUROPEPTIDE FMRFAMIDE AND TO 5-HYDROXYTRYPTAMINE S. D. PAINTER' AND MICHAEL J. GREENBERG 2 Department of Biological Science. Florida State University, Tallahassee, Florida 32306, USA ABSTRACT Ventricles from 50 species of bivalved molluscs were surveyed for their me-chanical responses to the molluscan neuropeptide FMRFamide (Phe-Met-Arg-Phe-NH 2 ) and to 5-hydroxytryptamine (5HT). Both were predominantly cardioexci-tatory, but neither was exclusively so. FMRFamide was inhibitory or weakly ex-citatory more often than 5HT, and such effects were most common in the subclasses Paleoheterodonta and Heterodonta. In contrast, 5HT was only rarely inhibitory or even weakly excitatory, and such effects were most common in the subclass Pteriomorphia. The responses to FMRFamide or 5HT were strikingly uniform in some bivalve families, but characteristically diverse in others. Thus, FMRFamide is neither a general cardioexcitor nor a general serotonomimetic agent. INTRODUCTION The molluscan neuropeptide FMRFamide (Phe-Met-Arg-Phe-NH 2 ), isolated from the clam Macrocallista nimbosa, increases the force and frequency of beat of isolated Macrocallista or Mercenaria mercenaria ventricles. These actions are identical to those of 5-hydroxytryptamine (5HT; serotonin) (Price and Greenberg, 1977). Both FMRFamide and 5HT stimulate adenylate cyclase activity and elevate cyclic adenosine monophosphate (cAMP) levels in these hearts (Higgins, 1977; Higgins et al., 1978), even though the two agonists act at pharmacologically dis-tinguishable receptor sites (Price and Greenberg, 1977). These observations sug-gested that FMRFamide might function as a long distance, long duration sero-tonomimetic agent (Price and Greenberg, 1977). To test this hypothesis, we began to survey the effects of FMRFamide and 5HT on the mechanical activity of hearts from many species of bivalved molluscs. It soon became clear that FMRFamide is neither an unswerving mimic of 5HT nor an inevitable cardioexcitor. These general conclusions and representative data have been presented to the American Society of Zoologists (Painter et al., 1979) and reviewed (Greenberg and Price, 1979, 1980; Greenberg et al., 1982), but the survey itself, now completed, has never been published. These primary data are pre-sented here. Received 20 February 1981; accepted 12 March 1982. Abbreviations: FMRFamide, Phe-Met-Arg-Phe-NH 2 (from the one letter abbreviations for amino acids approved by the IUPAC-IUB Commission on Biochemical Nomenclature, and after Price and Greenberg, 1977); 5HT, 5-hydroxytryptamine; DRP, dose-response profile; SRI, species response index. ' Present address: The Marine Biomedical Institute, University of Texas Medical Branch, 200 University Blvd., Galveston, Texas 77550. 2 Present address: C. V. Whitney Marine Lab, University of Florida, Rt. 1, Box 121, St. Augustine, Florida 32084. 311
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