SCP-RELATED PEPTIDES FROM BIVALVE MOLLUSKS - IDENTIFICATION, TISSUE DISTRIBUTION, AND ACTIONS

The SCPs(3) are a small peptide family, characterized in gastropods, a nd implicated in the control of the cardiovascular System and the musc les involved in feeding and gut motility. We aimed to determine the ma nifestation of this peptide family in the class Bivalvia. Acetone extr acts of whole bivalves were fractionated by high pressure liquid chrom atography (HPLC), and reactive peaks were identified by radioimmunoass ay (RIA). After purification, sequencing, and analysis by mass spectro scopy, three peptides were identified in the clam Mercenaria mercenari a: IAMSFYFPRMamide, AMSFYFPRMamide, and YFAFPRQamide(4). SCP-related p eptides from two other species were also sequenced: APKYFYFPRMamide an d SAFYFPRMamide from an oyster, Crassostrea virginica, and AMSFYFPRMam ide (identical to one of the clam peptides) from a cockle, Dinocardium robustum. The tissue distribution and pharmacological actions of the clam SCPs were determined in M. mevcenaria, as follows. The levels of peptide in extracts of 12 tissues were estimated by RIA. The largest c oncentrations of SCP occur in the palps and the visceral ganglia; the levels in the cerebral and pedal ganglia, the rectum, intestinal typhl osole, and gills were substantially lower; and the smallest amounts we re found in the heart and the style sac typhlosoles. Immunohistochemis try revealed many cell bodies in the periphery of the ganglia and fibe rs in the neuropil. Immunoreactive, varicose fibers also occur in the typhlosoles of the intestine and style sac, and in the rectum, gill, a nd palps. The atrioventricular valves, but not the atria or ventricle proper, contain immunoreactive fibers. Synthetic clam SCPs were assaye d on the rectum, the typhlosoles of the intestine and style sac, and t he ventricle, all isolated in an organ bath. At low to moderate doses, the SCPs relaxed the muscles of the rectum; higher doses had biphasic actions. The muscles of the intestinal and style sac typhlosoles were relaxed, and spontaneous rhythmicity was slowed by the SCPs. Most ven tricles were unresponsive. We conclude that the SCPs isolated in bival ves-though distinctive-are true homologs of those in gastropods. Moreo ver, the bivalve peptides also serve similar roles, controlling feedin g and digestion, and perhaps even cardioactivity.