The insulinotropic effect of exogenous glucagon-like peptide-1 is not affected by acute vagotomy in anaesthetized pigs

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Glucagon-like peptide 1 (GLP-1) is secreted from the gut in response to luminal stimuli and stimulates insulin secretion in a glucose-dependent manner. As a result of rapid enzymatic degradation of GLP-1 by dipeptidyl peptidase-4, a signalling pathway involving activation of intestinal vagal afferents has been proposed. We conducted two series of experiments in α-chloralose-anaesthetized pigs. In protocol I, pigs (n = 14) were allocated for either i.v. or i.a. (mesenteric) GLP-1 infusions (1 and 2 pmol kg−1 min−1, 30 min) while maintaining permissive glucose concentrations at 6 mmol l−1 by i.v. glucose infusion. The GLP-1 infusions were repeated after acute truncal vagotomy. In protocol II, pigs (n = 27) were allocated into six groups. Glucagon-like peptide 1 was infused i.v. or i.a. (mesenteric) for 1 h at 3 or 30 pmol kg−1 min−1. During the steady state (21 min into the GLP-1 infusion), glucose (0.2 g kg−1, i.v.) was administered over 9 min to stimulate β-cell secretion. Thirty minutes after the glucose infusion, GLP-1 infusions were discontinued. Following a washout period, the vagal trunks were severed in four of six groups (vagal trunks were left intact in two of six groups), whereupon all infusions were repeated. We found no effect of vagotomy on insulin or glucagon secretion during administration of exogenous GLP-1 in any experiment. We speculate that the effect of exogenous GLP-1 overshadowed any effect occurring via the vagus. Within dosage groups, total GLP-1 concentrations were similar, but intact GLP-1 concentrations were much lower when infused via the mesenteric artery because of extensive degradation of GLP-1 in the splanchnic bed. This demonstrates the effectiveness with which intestinal capillary dipeptidyl peptidase-4 protects the systemic circulation from intact GLP-1, consistent with a local role for GLP-1 involving activation of vagal pathways.

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