Evidence for neural contribution to islet effects of DPP-4 inhibition in mice

It has been suggested that neural mechanisms may contribute to effects of the incretin hormones, and, therefore, also to the effects of dipeptidyl peptidase (DPP-4) inhibition. We therefore examined whether muscarinic mechanisms are involved in the stimulation of insulin secretion by DPP-4 inhibition. Fasted, anesthetized mice were given intraperitoneal saline or the muscarinic antagonist atropine (5mg/kg) before duodenal glucose (75mg/mouse), with or without the DPP-4 inhibitor NVPDPP728 (0.095mg/mouse), or before intravenous glucose (0.35g/kg) with or without co-administration with GLP-1 or glucose-dependent insulinotropic polypeptide (GIP) (both 3nmol/kg). Furthermore, isolated islets were incubated (1h) in 2.8 and 11.1mM glucose, with or without GIP or GLP-1 (both 100nM), in the presence or absence of atropine (100µM). Duodenal glucose increased circulating insulin and this effect was potentiated by DPP-4 inhibition. The increase in insulin achieved by DPP-4 inhibition was reduced by atropine by approximately 35%. Duodenal glucose also elicited an increase in circulating intact GLP-1 and GIP and this was augmented by DPP-4 inhibition, but these effects were not affected by atropine. Atropine did also not affect the augmentation by GLP-1 and GIP on glucose-stimulated insulin secretion from isolated islets. Based on these findings, we suggest that muscarinic mechanisms contribute to the stimulation of insulin secretion by DPP-4 inhibition through neural effects induced by GLP-1 and GIP whereas neural effects do not affect the levels of GLP-1 or GIP or the islet effects of the two incretin hormones.