Single-Dose Metformin Enhances Bile Acid-Induced Glucagon-Like Peptide-1 Secretion in Patients With Type 2 Diabetes

Context: Despite a position as the first-line pharmacotherapy in type 2 diabetes, the glucose-lowering mechanisms of metformin remain to be fully clarified. Gut-derived modes of action, including suppression of bile acid reabsorption and a resulting increase in glucagon-like peptide-1 (GLP-1) secretion, have been proposed. Objective: The aim of this study was to assess the GLP-1 secretory and glucometabolic effects of endogenously released bile, with and without concomitant single-dose administration of metformin in patients with type 2 diabetes. Design: Randomized, placebo-controlled, and double-blinded crossover study. Setting: This study was conducted at Center for Diabetes Research, Gentofte Hospital, Denmark. Patients: Fifteen metformin-treated patients with type 2 diabetes; all participants completed the study. Interventions: Four experimental study days in randomized order with administration of either 1500 mg metformin or placebo in combination with intravenous infusion of cholecystokinin (0.4 pmol x kg(-1) x min(-1)) or saline. Main Outcome Measure: Plasma GLP-1 excursions as measured by baseline-subtracted area under the curve. Results: Single-dose metformin further enhanced bile acid-mediated induction of GLP-1 secretion (P = 0.02), whereas metformin alone did not increase plasma GLP-1 concentrations compared with placebo (P = 0.17). Metformin, both with (P = 0.02) and without (P = 0.02) concomitant cholecystokinin-induced gallbladder emptying, elicited reduced plasma glucose excursions compared with placebo. No GLP1-mediated induction of insulin secretion or suppression of glucagon was observed. Conclusions: Metformin elicited an enhancement of the GLP-1 response to cholecystokinin-induced gallbladder emptying in patients with type 2 diabetes, whereas no derived effects on insulin or glucagon secretion were evident in this acute setting.