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 cell capacitance as an indicator of exocytosis in individual mouse pancreatic β-cells maintained in short-term culture. GIP produced a 4.2-fold potentiation of depolarization-induced exocytosis. This stimulation of exocytosis was not associated with a change in the whole-cell Ca2+-current, and there was only a small increase (30%) in the cytoplasmic Ca2+ concentration [intercellular free Ca2+([Ca2+]i)]. The stimulatory effect of GIP on exocytosis was blocked by pretreatment with the specific protein kinase A (PKA) inhibitor Rp-8-BrcAMPS. Glucagon-like peptide-I(7–36) amide (GLP-I) stimulated exocytosis (90%) in the presence of a maximal GIP concentration (100 nmol/1). 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 involve a change in the whole-cell Ca2+-current or [Ca2+]i. GIP was ineffective in the presence of both forskolin and the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX). Under the same experimental conditions, the protein kinase C (PKC)-activating phorbol ester 4-phorbol 12-myristate 13-acetate (PMA) stimulated exocytosis (60%). Collectively, our data indicate that the insulinotropic hormone GIP stimulates insulin secretion from pancreatic β-cells, through the cAMP/PKA signaling pathway, by interacting with the secretory machinery at a level distal to an elevation in [Ca2+]i.

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