Glucagon-like peptide 1 (GLP-1) is a natural enteric incretin hormone, which is a potent insulin secretogogue in vitro and in vivo in humans. Its effects on overnight glucose concentrations and the specific phases of insulin response to glucose and nonglucose secretogogues in subjects with NIDDM are not known. We compared the effects of overnight intravenous infusion of GLP-1 (7-36) amide with saline infusion, on overnight plasma concentrations of glucose, insulin, and glucagon in eight subjects with NIDDM. The effects on basal (fasting) β-cell function and insulin sensitivity were assessed using homeostasis model assessment (HOMA) and compared with seven age- and weight-matched nondiabetic control subjects. The GLP-1 infusion was continued, and the first- and second-phase insulin responses to a 2-h 13 mmol/l hyperglycemic clamp and the insulin response to a subsequent bolus of the nonglucose secretogogue, arginine, were measured. These were compared with similar measurements recorded after the overnight saline infusion and in the control subjects who were not receiving GLP-1. The effects on stimulated β-cell function of lowering plasma glucose per se were assessed by a separate overnight infusion of soluble insulin, the rate of which was adjusted to mimic the blood glucose profile achieved with GLP-1. Infusion of GLP-1 resulted in significant lowering of overnight plasma glucose concentrations compared with saline, with mean postabsorptive glucose concentrations (2400–0800) of 5.6 ± 0.8 and 7.8 ± 1.4 mmol/l, respectively (P < 0.0002). Basal β-cell function assessed by HOMA was improved from geometric mean (1 SD range), 45% β (24–85) to 91% β (55–151) by GLP-1 (P < 0.0004). First-phase incremental insulin response to glucose was improved by GLP-1 from 8 pmol/l (-–33) to 116 pmol/l (12–438) (P < 0.005), second-phase insulin response to glucose from 136 pmol/l (53–352) to 1,156 pmol/l (357–3,748) (P < 0.0002), and incremental insulin response to arginine from 443 pmol/l (172–1,144) to 811 pmol/l (272–2,417) (P < 0.002). All responses on GLP-1 were not significantly different from nondiabetic control subjects. Reduction of overnight glucose by exogenous insulin did not improve any of the phases of stimulated β-cell function. Prolonged intravenous infusion of GLP-1 thus significantly lowered overnight glucose concentrations in subjects with NIDDM and improved both basal and stimulated β-cell function to nondiabetic levels. It may prove to be a useful agent in the reduction of hyperglycemia in NIDDM.

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