The incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) are secreted postprandially and contribute importantly to postprandial glucose tolerance. In this study, we assessed the individual and combined contributions of endogenous GIP and GLP-1 to the postprandial changes in glucose and glucoregulatory hormones using the novel GIP receptor antagonist GIP(3-30)NH2 and the well-established GLP-1 receptor antagonist exendin(9-39)NH2. During 4-h oral glucose tolerance tests (75 g) combined with an ad libitum meal test, 18 healthy men received on four separate days in randomized, double-blinded order intravenous infusions of A) GIP(3-30)NH2 (800 pmol/kg/min) plus exendin(9-39)NH2 (0–20 min: 1,000 pmol/kg/min; 20–240 min: 450 pmol/kg/min), B) GIP(3-30)NH2, C) exendin(9-39)NH2, and D) saline, respectively. Glucose excursions were significantly higher during A than during B, C, and D, while glucose excursions during B were higher than during C and D. Insulin secretion (assessed by C-peptide/glucose ratio) was reduced by 37 ± 16% (A), 30 ± 17% (B), and 8.6 ± 16% (C) compared with D (mean ± SD). A and C resulted in higher glucagon levels and faster gastric emptying. In conclusion, endogenous GIP affects postprandial plasma glucose excursions and insulin secretion more than endogenous GLP-1, but the hormones contribute additively to postprandial glucose regulation in healthy individuals.
The study was funded by Gangstedfonden, the European Foundation for the Study of Diabetes, and Aase og Ejnar Danielsens fond.
Context The actions of both endogenous incretin hormones during a meal have not previously been characterized. Objective Using specific receptor antagonists, we investigated the individual and combined contributions of endogenous glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) to postprandial glucose metabolism, energy expenditure, and gallbladder motility. Design Randomized, double-blinded, placebo-controlled, crossover design. Setting On four separate days, four liquid mixed meal tests (1894 kJ) over 270 minutes (min). Patients or Other Participants Twelve healthy male volunteers. Interventions Infusions of the GIP receptor antagonist GIP(3–30)NH2 (800 pmol/kg/min), the GLP-1 receptor antagonist exendin(9–39)NH2 (0–20 min: 1000 pmol/kg/min; 20–270 min: 450 pmol/kg/min), GIP(3–30)NH2+exendin(9–39)NH2, or placebo/saline. Main Outcome Measure Baseline-subtracted area under the curve (bsAUC) of C-peptide. Results Infusion of GIP(3–30)NH2+exendin(9–39)NH2 significantly increased plasma glucose excursions (bsAUC: 261 ± 142 mmol/L × min) during the liquid mixed meals compared with GIP(3–30)NH2 (180 ± 141 mmol/L × min; P = 0.048), exendin(9–39)NH2 (171 ± 114 mmol/L × min; P = 0.046), and placebo (116 ± 154 mmol/L × min; P = 0.015). Correspondingly, C-peptide:glucose ratios during GIP(3–30)NH2+exendin(9–39)NH2 infusion were significantly lower than during GIP(3–30)NH2 (P = 0.0057), exendin(9–39)NH2 (P = 0.0038), and placebo infusion (P = 0.014). GIP(3–30)NH2 resulted in significantly lower AUCs for glucagon than exendin(9–39)NH2 (P = 0.0417). Gallbladder ejection fraction was higher during GIP(3–30)NH2 compared with placebo (P = 0.004). For all interventions, energy expenditure and respiratory quotient were similar. Conclusions Endogenous GIP and GLP-1 lower postprandial plasma glucose excursions and stimulate insulin secretion but only endogenous GIP affects gallbladder motility. The two incretin hormones potentiate each other’s effects in the control of postprandial glycemia in healthy men.
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