Multisite regulation of insulin secretion by cAMP-increasing agonists: evidence that glucagon-like peptide 1 and glucagon act via distinct receptors

The mechanisms by which glucagon-like peptide 1(7-36)amide (GLP-1[7-36]amide) potentiates insulin secretion were investigated by measurements of whole-cell K+ and Ca2+ currents, membrane potential, the cytoplasmic Ca2+ concentration ([Ca2+](i)) and exocytosis in mouse pancreatic B-cells. GLP-1(7-36)amide (10 nM) stimulated glucose-induced (10 mM) electrical activity in intact pancreatic islets. The effect was manifested as a 34% increase in the duration of the bursts of action potentials and a corresponding 28% shortening of the silent intervals. GLP-1(7-36)amide had no effect on the electrical activity at subthreshold glucose concentrations (less than or equal to 6.5 mM). In cultured B-cells, GLP-1(7-36)amide produced a decrease of the whole-cell ATP-sensitive K+ (K-ATP) conductance remaining at 5 mM glucose by approximate to 30%. This effect was associated with membrane depolarization and the initiation of electrical activity. GLP-1(7-36)amide produced a protein-kinase-A- (PKA-) and glucose-dependent fourfold potentiation of Ca2+-induced exocytosis whilst only increasing the Ca2+ current marginally. The stimulatory action of GLP-1(7-36)amide on exocytosis was mimicked by the pancreatic hormone glucagon and exendin-4, a GLP-1 receptor agonist. Whereas the stimulatory action of GLP-1(7-36)amide could be antagonized by exendin-(9-39), this peptide did not interfere with the ability of glucagon to stimulate exocytosis. We suggest that GLP-1(7-36)amide and glucagon stimulate insulin secretion by binding to distinct receptors. The GLP-1(7-36)amide-induced stimulation of electrical activity and Ca2+ influx can account for (maximally) a doubling of insulin secretion. The remainder of its stimulatory action results from a cAMP/PKA-dependent potentiation of Ca2+-dependent exocytosis exerted at a stage distal to the elevation of [Ca2+](i).