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

Abstract

 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 con- centrations (≤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 ≈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.