Receptor-mediated hydrolysis of inositol phospholipids in 2H3 cells, unlike that in other secretory cells, is dependent on extracellular Ca2+. Several lines of evidence, however, suggest that this hydrolysis may mediate the influx of Ca2+ across the plasma membrane. Studies with covalently cross-linked oligomers of IgE, for example, have shown that such hydrolysis and the increase in cytosol (Ca2+)i are closely correlated with the number of IgE receptors aggregated and that with saturating concentrations of oligomer the generation of these early signals exceeds that required for maximal secretion of histamine. Excess signalling capacity was evident also in studies with monoclonal anti-DNP IgE and DNP24-BSA (1 mole bovine serum albumin conjugated with 24 moles of dinitrophenol). Furthermore, the hydrolysis of phospholipids appeared to be a consequence of receptor aggregation and not of Ca2+ mobilization. For example, disaggregation of receptors by displacement of DNP24BSA with DNP-lysine resulted in abrupt cessation of hydrolysis and secretion. Antigen stimulated hydrolysis of phospholipds, but not secretion, became increasingly less dependent on external Ca2+ with time. Finally, analysis of antigen stimulated cells by HPLC revealed multiple isomers of the insoitol phosphates but correlations in the pattern of hydrolysis and the increase in (Ca2+)i were established.
