Calcitonin gene-related peptide (CGRP) shares about 46% and 20% amino acid sequence homology with islet amyloid polypeptide (IAPP) and salmon calcitonin (sCT). We investigated whether these related peptides could cross-react with the specific binding of 125I-[His]hCGRP I to the CGRP receptor in hamster insulinoma cell membranes. A rapid dissociation of membrane bound 125I-[His]hCGRP I could be induced in the presence of 1 μM chicken CGRP (cCGRP). The specific 125I-[His]hCGRP I binding was inhibited by the related peptides and their half-maximal inhibitory concentrations (IC50) were: cCGRP (0.1 nM), rat CGRP I and human CGRP I and II (1.0–2.0 nM), fragment of hCGRP I (8–37) (150 nM), human IAPP (440 nM). The non-amidated form of hIAPP; human diabetes-associated peptide (hDAP) did not inhibit the binding of 125I-[His]hCGRP I and sCT was only effective at a high concentration (1 μM). Binding of 125I-[His]hCGRP I was dose dependently inhibited by guanosine-5′-O-(3-thiotriphosphate) or (GTPγS) and a 70% reduction of binding was obtained with 0.1 mM GTPγS. The IC50 value of cCGRP (0.1 nM) was increased 100-fold in the presence of 0.1 mM GTPγS. Human CGRP I and cCGRP at 2.5 μM did not stimulate the activity of hamster insulinoma cell membranes adenylate cyclase, while glucagon (1 μM) induced a 2-fold increase. Thus, specific CGRP receptors present in hamster β cells are associated with G protein (s) and IAPP can interact with these receptors. These results and the observation that cCGRP and hCGRP I did not influence adenylate cyclase activity provide further evidence for CGRP receptor subtypes.

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