An endogenous SCP-related peptide modulates ciliary beating in the gills of a venerid clam, Mercenaria mercenaria

The activities of both the lateral smd frontal cilia of Mercenaria mercenar ia were unaffected, either by the two endogenous SCP-related peptides AMSFY FPRM-amide and YFAFPRQamide, or by FMRFamide (all at 10(-6) M). Dopamine (D A) inhibited the lateral cilia; the mean EC50 was 2 x 10(-6) M. The peptide YFAFPRQamide-but neither AMSFYFPRMamide nor FMRFamide-antagonized the inhi bition induced by DA; this effect was dependent on both time and dose. At a DA concentration of 5 x 10(-7) M, the effect of YFAFPRQamide appeared with in 20 min and became maximal within 40-60 min; the mean EC50 at these times was 4.7 x 10(-11) M. If the concentration of DA was increased to 10(-6) M, the maximal effect of the peptide was delayed to 50 min, and the mean EC50 increased to 1.1 x 10(-7) M. Particle transport by the frontal cilia was i nhibited by 5-hydroxytryptamine (5HT); the mean EC50 was 5.7 x 10(-7) M Aga in, only YFAFPRQamide had an antagonistic effect on the 5HT-induced inhibit ion. At a 5HT concentration of 10(-6) M, the effects of YFAFPRQamide did no t appear until 45 min; the mean EC50 was 10(-6) M. When radioimmunoassayed with an SCP antiserum, the elution profile of a gill extract overlapped tho se of the SCP-related peptides that had previously been identified in extra cts of whole animals. These data suggest that all three SCP analogs occur i n the gill. Immunohistochemistry of the gill, carried out with a monoclonal antibody raised to SCPB, stained many varicose neuronal fibers. Most of th ese were associated with the gill musculature, but a sparse innervation of the filaments underlying the cilia was also observed. Some fluorescent nerv e cell bodies were also seen in the gill tissue. Our results are consistent with the hypothesis that YFAFPRQamide modulates branchial activities-muscu lar as well as ciliary-that are associated with feeding.