Evaluation of the role of inositol trisphosphate in IgE-dependent exocytosis.

A close correlation exists between inhibition by 12-O-tetradecanoylphorbol 13-acetate (TPA) of inositol trisphosphate (InsP3) formation and the rise in internal Ca2+ concentrations in IgE-stimulated rat basophilic leukemia (RBL-2H3) cells. Inhibition of both processes is dose-dependent, with half-maximal and maximal inhibition occurring at 1.5 and 10 ng of TPA/ml respectively. At a similar range of concentrations TPA does not inhibit, but rather enhances, IgE-dependent secretion. When added to antigen-activated cells. EGTA immediately abrogates secretion and stimulates InsP3 production. In contrast, EGTA has only a small inhibitory effect on IgE-induced secretion from TPA-activated cells. In antigen-activated cells, EGTA partially inhibits InsP1 formation, suggesting that, unlike InsP3, InsP1 may in part be formed directly from phosphatidylinositol in a Ca2(-)-dependent manner. Together, these findings suggest that under physiological conditions the stimulated formation of InsP3 is insufficient for triggering secretion, and that Ca2+ influx is essential. Moreover, InsP3 formation is not obligatory for IgE-mediated exocytosis, provided that the cells are activated by TPA. Secretion from TPA-activated cells, which is independent of InsP3 formation and the rise in internal Ca2+, does not require the presence of external Ca2+, implying that the presence of external Ca2+ during IgE-induced secretion is required for producing the Ca2+ signal and not for exocytosis per se.