Differential effects of acute and extended infusions of glucagon-like peptide-1 on first- and second-phase insulin secretion in diabetic and nondiabetic humans.

OBJECTIVE: The purpose of this study was to determine whether an extended infusion of the incretin hormone glucagon-like peptide 1 (GLP-1) has a greater effect to promote insulin secretion in type 2 diabetic subjects than acute administration of the peptide. RESEARCH DESIGN AND METHODS: Nine diabetic subjects and nine nondiabetic volunteers of similar age and weight were studied in identical protocols. First-phase insulin release (FPIR; the incremental insulin response in the first 10 min after the intravenous glucose bolus) and second-phase insulin release (SPIR; the incremental insulin response from 10-60 min after intravenous glucose) were measured during three separate intravenous glucose tolerance tests (IVGTTs): 1). without GLP-1 (control); 2). with acute administration of GLP-1 as a square wave starting just before glucose administration; and 3). with an extended infusion of GLP-1 for 3 h before and during the IVGTT.
RESULTS: In the subjects with diabetes, FPIR was severely impaired-a defect that was only modestly improved by acute administration of GLP-1 (197 +/- 97 vs. 539 +/- 218 pmol/l. min, P < 0.05), while SPIR was substantially increased (1952 +/- 512 vs. 8072 +/- 1664 pmol/l. min, P < 0.05). In contrast, the 3-h preinfusion of GLP-1 normalized fasting hyperglycemia (7.9 +/- 0.5 vs. 5.2 +/- 0.6, P < 0.05), increased FPIR by 5- to 6-fold (197 +/- 97 vs. 1141 +/- 409 pmol/l. min, P < 0.05), and augmented SPIR significantly (1952 +/- 512 vs. 4026 +/- 851 pmol/l. min, P < 0.05), but to a lesser degree than the acute administration of GLP-1. In addition, only the 3-h GLP-1 preinfusion significantly improved intravenous glucose tolerance (K(g) control 0.61 +/- 0.04, acute infusion 0.71 +/- 0.04, P = NS; 3-h infusion 0.92 +/- 0.08%/min, P < 0.05). These findings were also noted in the nondiabetic subjects in whom acute administration of GLP-1 significantly increased SPIR relative to the control IVGTT (9439 +/- 2885 vs. 31553 +/- 11660 pmol/l. min, P < 0.001) with less effect on FPIR (3221 +/- 918 vs. 4917 +/- 1614 pmol/l. min, P = 0.075), while the 3-h preinfusion of GLP-1 significantly increased both FPIR (3221 +/- 918 vs. 7948 +/- 2647 pmol/l. min, P < 0.01) and SPIR (9439 +/- 2885 vs. 21997 +/- 9849 pmol/l. min, P < 0.03).
CONCLUSIONS: Extended administration of GLP-1 not only augments glucose-stimulated insulin secretion, but also shifts the dynamics of the insulin response to earlier release in both diabetic and nondiabetic humans. The restitution of some FPIR in subjects with type 2 diabetes is associated with significantly improved glucose tolerance. These findings demonstrate the benefits of a 3-h infusion of GLP-1 on beta-cell function beyond those of an acute insulin secretagogue, and support the development of strategies using continuous or prolonged GLP-1 receptor agonism for treating diabetic patients.