PANCREASTATIN INHIBITS INSULIN-SECRETION IN RINM5F CELLS THROUGH OBSTRUCTION OF G-PROTEIN MEDIATED, CALCIUM-DIRECTED EXOCYTOSIS

To elucidate the regulatory pathway through which pancreastatin inhibi ts insulin secretion, RINm5F insulinoma cells were challenged with phy siological and pharmacological probes known to stimulate insulin relea se through different mechanisms. Utilizing the electrophysiological te chnique of capacitance measurements as a correlate to exocytosis, panc reastatin was found to significantly diminish maximum capacitance chan ges evoked by glyceraldehyde, an effect which was attenuated in pertus sis toxin-treated cells. In static incubations of this cell line, panc reastatin significantly inhibited insulin secretion stimulated by glyc eraldehyde, carbachol and A23187, secretagogues known to directly elev ate beta-cell cytosolic Ca2+. This peptide also inhibited insulin secr etion stimulated by phorbol myristate acetate (PMA), but only at incub ation times less than or equal to 15 min. It was without effect on ins ulin secretion stimulated by mastoparan and longer incubations (30 min ) with PMA, where the secretory mechanisms are not necessarily Ca2+-de pendent. Additionally. pancreastatin had no effect on carbachol-genera ted inositol phosphate accumulation but inhibited simultaneously stimu lated insulin secretion. All inhibitory effects of pancreastatin were pertussis toxin sensitive. These results suggest that pancreastatin in hibits insulin secretion in RINm5F cells through a G-protein regulated mechanism at a control point involved in the Ca2+-directed exocytotic machinery, a feature shared by other physiologic inhibitors of insuli n secretion.