Calcium-independent Phosphoinositide Breakdown in Rat Basophilic Leukemia Cells. Evidence for an Early Rise in Inositol 1,4,5-trisphosphate Which Precedes the Rise in Other Inositol Phosphates and in Cytoplasmic Calcium

Aggregation of the receptor with high affinity for immunoglobulin E (IgE) in rat basophilic leukemia cells leads to a calcium-dependent and a calcium-independent hydrolysis of phosphoinositides. The increase in the levels of inositol phosphates induced in the absence of calcium is only 25% of that observed with 1 mM Ca2+. The inositol phosphates reach a new steady state level 2 min after stimulation in EGTA, whereas with calcium they continue to increase up to 15 min. A similar response is observed when the receptors are aggregated due to the interaction of bound IgE with antigen or with anit-IgE, or by the binding of IgE cross-linked chemically. The antigen-mediated response is inhibited by hapten and disruption of such antigen-antibody aggregates late after stimulation leads to a rapid decline in the levels of the inositol phosphates to basal values. Separation of the inositol phosphates by Dowex columns shows that there is a fast rise in inositol trisphosphate which peaks at 15 s and slowly declines to a lower plateau within 2 min. Analysis by high pressure liquid chromatography reveals a 5-fold increase in the levels of inositol 1,4,5-trisphosphate in less than 10 s after stimulation, which precedes any major change in the other inositol phosphates. Aggregation of the receptor in the absence of external calcium induces a transient increase in cytoplasmic calcium which reaches a maximum of approximately 25 nM over basal levels after activation. The onset of the rise in Ca2+ lags after the initial rise in the inositol 1,4,5-trisphosphate.