Aggregation of IgE receptors on RBL-2H3 cells with oligomers of IgE and activation of regulatory GTP binding proteins by sodium fluoride induced substantial release of inositol phosphates whereas the ionophores A23187 and the Na+ ionophore, monensin, induced only limited release of the sugar phosphates. All of the above reagents stimulated histamine released to a variable extent with monensin inducing the least release. Oligomers of IgE and A23187 caused large increases in concentration of cytosol Ca2+ (Ca2+)i, sodium fluoride produced a slow but progressive increase in (Ca2+)i over the course of 30 min. Monensin produced no perturbation in the levels of (Ca2+)i. In contrast to the above agents the phorbol ester, PMA, elicited neither stimulatory nor secretory responses in 2H3 cells. Of the various combinations of drugs tested the most marked effect was the enhancement of breakdown of phospholipids and calcium signal when sodium fluoride and monensin were tested in combination and enhancement of histamine release when cells were exposed to combination of PMA and A23187. Also stimulation of breakdown of phospholipids by both oligomers and sodium fluoride was markedly reduced in cells exposed to short-term exposure to PMA. The secretory response to sodium fluoride and A23187 but to higher oligomer was enhanced markedly by short-term response to PMA. In contrast to short-term exposure to PMA, after prolonged exposure (20 hrs) to PMA both the inhibitory effects on release of inositol phosphates and synergistic effects on secretion of histamine were lost. These data suggested that protein kinase C exerted both stimulatory and inhibitory effects on signal transduction and that its modulatory actions were lost with long- term exposure to PMA. Tests with polyclonal antibodies against protein kinase C indicated that long term treatment of cells with PMA resulted in disappearance of protein kinase C from both cytosol and membrane.
