PROTEIN-KINASE-C AS A SIGNAL FOR EXOCYTOSIS

We have studied signaling mechanisms that stimulate exocytosis and lut einizing hormone secretion in isolated male rat pituitary gonadotropes , As judged by reverse hemolytic plaque assays, phorbol-12-myristate-1 3-acetate (PMA) stimulates as many gonadotropes to secrete as does gon adotropin-releasing hormone (GnRH), However, PMA and GnRH use differen t signaling pathways, The secretagogue action of GnRH is not very sens itive to bisindolylmaleimide I, an inhibitor of protein kinase C, but is blocked by loading cells with a calcium chelator, 1,2-bis-(2-aminop henoxy) ethane-N,N,N',N'-tetraacetic acid. The secretagogue action of PMA is blocked by bisindolylmaleimide I and is not very sensitive to t he intracellular calcium chelator, GnRH induces intracellular calcium elevations, whereas PMA does not. As judged by amperometric measuremen ts of quantal catecholamine secretion from dopamine-or serotonin-loade d gonadotropes, the secretagogue action of PMA develops more slowly (i n several minutes) than that of GnRH. We conclude that exocytosis of s ecretory vesicles can be stimulated independently either by calcium el evations or by activation of protein kinase C.