PROTEIN-KINASE A-DEPENDENT STIMULATION OF EXOCYTOSIS IN MOUSE PANCREATIC BETA-CELLS BY GLUCOSE-DEPENDENT INSULINOTROPIC POLYPEPTIDE

The mechanisms by which glucose-dependent insulinotropic polypeptide ( GIP) stimulates insulin secretion were investigated by measurements of whole-cell Ca2+ currents, the cytoplasmic Ca2+ concentration, and cel l capacitance as an indicator of exocytosis in individual mouse pancre atic beta-cells maintained in short-term culture. GIP produced a 4.2-f old potentiation of depolarization-induced exocytosis. This stimulatio n of exocytosis was not associated with a change in the whole-cell Ca2 +-current, and there was only a small increase (30%) in the cytoplasmi c Ca2+ concentration [intercellular free Ca2+([Ca2+](i))]. The stimula tory effect of GIP on exocytosis was blocked by pretreatment with the specific protein kinase A (PKA) inhibitor Rp-8-Br-cAMPS. Glucagon-like peptide-I (7-36) amide (GLP-I) stimulated exocytosis (90%) in the pre sence of a maximal GIP concentration (100 nmol/l). Replacement of GLP- I with forskolin produced a similar stimulatory action on exocytosis. These effects of GLP-I and forskolin in the presence of GIP did not in volve a change in the whole-cell Ca2+-current or [Ca2+](i). GIP was in effective in the presence of both forskolin and the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX). Under the same experimental c onditions, the protein kinase C (PKC)-activating phorbol ester 4-phorb ol 12-myristate 13-acetate (PMA) stimulated exocytosis (60%). Collecti vely, our data indicate that the insulinotropic hormone GIP stimulates insulin secretion from pancreatic p-cells, through the cAMP/PKA signa ling pathway, by interacting with the secretory machinery at a level d istal to an elevation in [Ca2+](i).