Glucagon-Like Peptide 1(7-36) Amide Stimulates Exocytosis in Human Pancreatic β-Cells by Both Proximal and Distal Regulatory Steps in Stimulus-Secretion Coupling

The effect of glucagon-like peptide 1(7–36) amide [GLP-l(7–36) amide] on membrane potential, wholecell ATP-sensitive potassium channel (KATP) and Ca²⁺ currents, cytoplasmic Ca²⁺ concentration, and exocytosis was explored in single human β-cells. GLP-1(7–36) amide induced membrane depolarization that was associated with inhibition of whole-cell KATP current. In addition, GLP-l(7–36) amide (and forskolin) produced greater than fourfold potentiation of Ca²⁺-dependent exocytosis. The latter effect resulted in part (40%) from acceleration of Ca²⁺ influx through voltage-dependent (L-type) Ca²⁺ channels. More importantly, GLP- 1(7–36) amide (via generation of cyclic AMP and activation of protein kinase A) potentiated exocytosis at a site distal to a rise in the cytoplasmic Ca²⁺ concentration. Photorelease of caged cAMP produced a two- to threefold potentiation of exocytosis when the cytoplasmic Ca²⁺ concentrations were clamped at >170 nmol/1. The effect of GLP-1(7–36) amide was antagonized by the islet hormone somatostatin. Similar effects on membrane potential, ion conductances, and exocytosis were observed with glucose-dependent insulinotropic polypeptide (GIP), the second major incretin. The present data suggest that the strong insulinotropic action of GLP-1(7–36) amide and GIP in humans results from its interaction with several proximal as well as distal important regulatory steps in the stimulus-secretion coupling.