Characterization of binding sites for calcitonin gene-related peptide in the mollusc gill

Target organs for calcitonin gene-related peptide (CGRP) were investigated in Pecten maximus using 125I-labelled human CGRP. CGRP was shown to interact specifically with mantle and gill tissue. Receptor studies using branchial membrane preparations indicated that the binding was time dependent. Scatchard analysis of binding data showed that there was a single class of binding sites. The affinity constant was found to be 0.7.10(8) M-1 and the number of binding sites 2600.10(8)/mg of protein. Salmon CT inhibited the binding of 125I-labelled CGRP to branchial membranes with a lesser efficiency than that of the unlabelled hormone. A 40% inhibition of the 125I-labelled CGRP binding was observed in the presence of 2.6 and 26 nM CGRP and salmon CT, respectively. In addition, 200 nM human CGRP inhibited 25 and 10% of the basal branchial and mantle adenylate cyclase activity, respectively. These data suggest that CGRP participates in the regulation of the branchial function in molluscs probably via a vasoconstrictor role.