Protein kinase C regulation of the receptor-coupled calcium signal in histamine-secreting rat basophilic leukaemia cells

It has been proposed that protein kinase C mediates cellular responses evoked by external stimuli, leading to alterations in internal free calcium concentrations^1-4. We have shown previously that histamine-secreting rat basophilic leukaemia cells (RBL-2H3), which degranulate on aggregation of the receptors for immunoglobulin IgE⁵, contain a Ca²⁺- and phospholipid-dependent protein kinase (kinase C)^6,7. The partially purified enzyme is activated directly by the tumour-promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate(TPA). In intact RBL cells, TPA potentiates histamine release induced⁶ by the Ca²⁺-ionophore A23187 (similar to the synergy reported for platelets¹, neutrophils ² and rat peritoneal mast cells⁸). Although TPA at concentrations below 15 nM synergizes with the antigen, higher TPA concentrations inhibit secretion⁶. This selective inhibition suggested that kinase C is involved in both the activation and termination of the secretory process. To examine this possibility, we have determined the effect of TPA on changes in free cytosolic Ca²⁺ concentration during antigen-induced release. We report here that TPA completely blocks the increase in Ca²⁺ concentration induced by antigen. Our results strongly suggest that protein kinase C is involved in the regulation of receptor-dependent Ca²⁺ signalling.