| Abstract: | Cells of the murine mast-cell clone MC9 grown in suspension culture were sensitized with an anti-DNP (dinitrophenol) IgE and subsequently prelabelled by incubating with 32P]Pi. Stimulation of these cells with DNP-BSA (bovine serum albumin) caused marked decreases in 32P]polyphosphoinositides (but not 32P]phosphatidylinositol) with concomitant appearance of 32P]phosphatidic acid. Whereas phosphatidylinositol monophosphate levels returned to baseline values after prolonged stimulation, phosphatidylinositol bisphosphate levels remained depressed. Stimulation of sensitized MC9 cells with DNP-BSA increased rates of incorporation of 32P]Pi into other phospholipids in the order: phosphatidylcholine greater than phosphatidylinositol greater than phosphatidylethanolamine. In sensitized cells prelabelled with 3H]inositol, release of inositol monophosphate, inositol bisphosphate and inositol trisphosphate, was observed after stimulation with DNP-BSA. When Li+ was added to inhibit the phosphatase activity that hydrolysed the phosphomonoester bonds in the sugar phosphates, greater increases were observed in all three inositol phosphates, particularly in inositol trisphosphate. The IgE-stimulated release of inositol trisphosphate was independent of the presence of extracellular Ca2+. In addition, the Ca2+ ionophore A23187 caused neither the decrease in 32P]polyphosphoinositides nor the stimulation of the release of inositol phosphates. These results demonstrate that stimulation of the MC9 cell via its receptor for IgE causes increased phospholipid turnover, with effects on polyphosphoinositides predominating. These data support the hypothesis that hapten cross-bridging of IgE receptors stimulates phospholipase C activity, which may be an early event in stimulus-secretion coupling of mast cells. The results with the Ca2+ ionophore A23187 indicate that an increase in intracellular Ca2+ alone is not sufficient for activation of this enzyme. |
