Effects of vasoactive intestinal polypeptide (VIP), secretin and gastrin on insulin secretion in the mouse

The in vivo effects of vasoactive intestinal polypeptide (VIP), secretin and two different molecular forms of gastrin, gastrin 17 and pentagastrin, on basal and stimulated insulin secretion have been investigated in the mouse. All these peptides induced a moderate dose-dependent increase in basal insulin secretion. The different polypeptides showed complex effects on insulin release stimulated by glucose, the cholinergic agonist carbachol or the beta-adrenergic agonist L-isopropylnoradrenaline (L-IPNA), these effects being dependent on the nature of the secretagogue. VIP and secretin both potentiated glucose-induced insulin release. Secretin inhibited insulin secretion induced by carbachol and L-IPNA, whereas VIP potentiated L-IPNA-induced insulin secretion and had no influence on the effect of carbachol. Gastrin 17 and pentagastrin did not affect glucose- or carbachol-induced insulin release, whereas they inhibited L-IPNA-induced insulin secretion. The results suggest that VIP, secretin and gastrin display their effects on insulin secretion through different mechanisms. The results indirectly suggest the existence of separate insulin secretory pathways which operate differently, or at least partly differently, after glucose stimulation, cholinergic stimulation, and beta-adrenergic stimulation.