A 67 kDa protein mediates pituitary adenylate cyclase-activating polypeptide and vasoactive intestinal peptide-stimulated insulin secretion in a hamster clonal beta-cell line.

ABSTRACT

Insulin secretion is regulated by neural and neurohormonal factors. The report of nerves releasing pituitary adenylate cyclase-activating polypeptide (PACAP)--a 38 amino acid peptide--in the endocrine pancreas, suggests it may be important in modulating insulin release. We therefore carried out receptor-binding studies on membranes prepared from the glucose-responsive clonal beta-cell line HIT-T15, and also examined the effects of PACAP38, PACAP27--a C-terminal truncated form of the peptide--and vasoactive intestinal peptide (VIP) on insulin and islet amyloid polypeptide (IAPP) release. We showed by chemical cross-linking that PACAP and VIP stimulate secretion from the clonal cells by binding to a receptor with a molecular weight of 67 kDa (n = 4). Binding was saturable when membranes were incubated with 125I-PACAP27 (Kd 1.2 +/- 0.2 nM; Bmax 415.7 +/- 35.3 fmol/mg; n = 4) or 125I-VIP (Kd 1.3 +/- 0.3 nM; Bmax 354.8 +/- 42.8 fmol/mg; n = 4). We also demonstrated an increase in glucose-stimulated insulin (PACAP27, 366.6 +/- 25.8%; PACAP38, 389.9 +/- 13.4%; VIP, 342.6 +/- 16.1% of control; all at 1 microM, P < 0.01 vs control) and IAPP release (PACAP27, 236.9 +/- 26.2%; PACAP38, 226.5 +/- 10.9%; VIP, 242.9 +/- 15.8% of control; all at 1 microM, P < 0.01 vs control). Incubation of the cells with these peptides, for a duration of 12 h, in the presence of 5.5 mM glucose, did not alter the expression of insulin or IAPP. These findings suggest that PACAP and VIP stimulate secretion of insulin and IAPP by binding to a 67 kDa protein on clonal beta-cells and do not alter the transcription of insulin and IAPP under the conditions tested.