The question as to whether the homologous peptides CGRP and IAPP can regulate insulin secretion in RINm5F cells was addressed. Chicken CGRP displayed a reproducible inhibitory effect on insulin secretion within 0.1 and 1 nM concentrations and a stimulatory effect at higher concentrations. The maximal stimulatory effects on insulin secretion were obtained with 1.0 μM of chicken CGRP (cCGRP), human α-CGRP (h α-CGRP) and human IAPP (hIAPP) which caused 246 ± 22, 302 ± 63 and 224 ± 14 percent increases of control levels, respectively ( p < 0.001). Similarly, maximal accumulations of cAMP were obtained with 1.0 μM of cCGRP, h α-CGRP and hIAPP with the respective percent increases of control levels of 587 ± 24, 436 ± 41 and 410 ± 25 ( p < 0.005). Thus the stimulatory effects on insulin secretion in RINm5F cells by cCGRP, h α-CGRP and hIAPP appear to be mediated by the cAMP pathway. Chicken CGRP, the most potent peptide tested, displayed a correlated dose response stimulation of intracellular cAMP and insulin release within the concentration range of 10–1000nM. The EC 50 values of cCGRP for cAMP accumulation and insulin release were similar (20nM and 10 nM respectively). The stimulatory effect of IAPP on cAMP was not additive with that of cCGRP suggesting that IAPP action was mediated by CGRP receptors. This hypothesis was further sustained by a preferential inhibition of 125 I[His]h α-CGRP binding to RINm5F cells by cCGRP as compared to IAPP. We conclude that CGRP and IAPP, through a direct action on a chicken CGRP preferring receptor present in β cells, stimulated insulin by a cAMP mediated pathway.
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 125 I-[His]hCGRP I to the CGRP receptor in hamster insulinoma cell membranes. A rapid dissociation of membrane bound 125 I-[His]hCGRP I could be induced in the presence of 1 μM chicken CGRP (cCGRP). The specific 125 I-[His]hCGRP I binding was inhibited by the related peptides and their half-maximal inhibitory concentrations (IC 50 ) 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 125 I-[His]hCGRP I and sCT was only effective at a high concentration (1 μM). Binding of 125 I-[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 IC 50 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.