Specific gastric inhibitory polypeptide (GIP) receptors were characterized in human benign insulinoma plasma membranes employing [mono-[125I]iodo-Tyr10]-GIP (125I-GIP) as the radioligand. GIP 1-42 inhibited 125I-GIP binding with an IC50 value of 10(-9) M. Scatchard analysis showed two classes of binding sites: a high-affinity site (Kd = 2.23 x 10(-10) M; Bmax = 24 fmol/mg protein) and a low-affinity site (Kd = 8.39 x 10(-9) M; Bmax = 118 fmol/mg protein). A synthetic replicate of human GIP 1-31 inhibited 125I-GIP binding with an IC50 value of 10(-8) M. The GIP binding sites of human insulinoma were coupled to adenylate cyclase stimulation. GIP 1-31 regulated the adenylate cyclase activity to the same extent as GIP 1-42. The concentrations of GIP required for maximal activity ranged from 10(-9) to 10(-8) M for either GIP 1-42 or GIP 1-31. The existence of functional GIP receptors in human insulinoma substantiates our recent reports demonstrating the presence of GIP binding sites in transplantable hamster insulinoma and indicates that GIP could exert a direct control of the beta-cell function in humans through a purely endocrine pathway.

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Studies were conducted to explore the effects of vasoactive intestinal peptide (VIP), histamine, somatostatin-14 and -28 (S-14 and S-28), and prostaglandin E2 (PGE2) on cAMP production in gastric glands isolated from the guinea pig. VIP (EC50 = 5 X 10(-10) M) and PGE2 (EC50 = 10(-8) M) induced cAMP accumulation in glands isolated by means of EDTA from the fundus or antrum. The structurally related peptides PHI (peptide with N-terminal histidine and C-terminal isoleucine amide) and secretin also increased cAMP production in the system, but with 200 to 2000 times lower potencies than VIP. Combinations of VIP with PHI or secretin do not produce additive stimulation, indicating that PHI or secretin interact with the receptor-cAMP system highly sensitive to VIP. Histamine was about 10 times more potent in fundus (EC50 = 10(-5) M) than in antrum (EC50 = 9 X 10(-5) M) and did not produce any stimulation in enterocytes isolated from the upper part of the duodenum. Complete inhibitions caused by the H2 receptor antagonist cimetidine (Ki = 0.15-0.16 X 10(-6) M) (Ki is the inhibition constant) or by the H1 receptor antagonist diphenhydramine (DPH) (Ki = 13-17 X 10(-6) M) indicate that H interacts with typical H2 receptors mediating adenylate cyclase activation in fundic (Ka = 10(-5) M) (Ka is the association constant) or antral membranes (Ka = 3 X 10(-5) M). In fundus, S-14 inhibited partially (about 60%) cAMP production evoked by H or by its H1 or H2 agonists. The kinetics and the inhibitory potencies (2 X 10(-8) M) or efficacies of S-14 and -28 were identical. No effect of S-14 was found on basal or on cAMP production induced by VIP or PGE2 in either fundic or antral glands or by H in antral glands. The results support the hypothesis of a regulatory role for VIP and/or secretin in mucous and/or peptic secretions via a cAMP-dependent mechanism in gastric mucosa in mammals. Second, we propose that S-14 as well as S-28 may inhibit gastric acid secretion by a direct and selective control of H-induced cAMP production in parietal cells, through a common recognition site (receptor?) distinct from the H2 receptor. Third, not only parietal cells, but also nonparietal cells of the antrum possess an H2 receptor-cAMP system. This finding could be related to the in vivo regulation by cimetidine of endocrine (somatostatin) and exocrine (pepsin) secretions by the stomach.

Abstract. The binding of vasoactive intestinal peptide (VIP) and its effect on adenosine 3′:5′-monophosphate (cyclic AMP) production were measured in isolated rat intestinal epithelial cells from duodenum, jejunum, ileum, coecum, colon and rectum. In every segment tested, VIP binding and VIP-induced cyclic AMP production exhibited a similar pattern with two populations of VIP binding sites and similar magnitude of cyclic AMP production. Continuous displacement of the binding of 125I-labelled VIP and stimulation of cyclic AMP production were observed in the range of 10−10–10−7 m VIP concentrations. Fifty per cent inhibition of the initial binding of tracer was obtained with 2.2 to 3.2 nm VIP. The Km of stimulation of cyclic AMP production was 1.2 to 2.6 nm VIP. Chicken VIP and porcine secretin showed a 4-fold higher and a 100-fold lower affinity than porcine VIP for binding sites, respectively. The same was true for the stimulation of cyclic AMP production. The existence of receptors highly sensitive to VIP in the separate segments of the bowel corresponds to the presence of high concentration of VIP at the vicinity of epithelial cells and strongly suggests that VIP is active in regulating different intestinal functions.