Differential Effects of Acute and Extended Infusions of Glucagon-Like Peptide-1 on First- and Second-Phase Insulin Secretion in Diabetic and Nondiabetic Humans

Abstract

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 (1,952 ± 512 vs. 8,072 ± 1,664 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. 1,141 ± 409 pmol/l · min, P < 0.05), and augmented SPIR significantly (1,952 ± 512 vs. 4,026 ± 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 (Kg 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 (9,439 ± 2,885 vs. 31,553 ± 11660 pmol/l · min, P < 0.001) with less effect on FPIR (3,221 ± 918 vs. 4,917 ± 1,614 pmol/l · min, P = 0.075), while the 3-h preinfusion of GLP-1 significantly increased both FPIR (3,221 ± 918 vs. 7,948 ± 2,647 pmol/l · min, P < 0.01) and SPIR (9,439 ± 2,885 vs. 21,997 ± 9,849 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 β-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.