Calcium- and guanine-nucleotide-dependent exocytosis in permeabilized rat mast cells. Modulation by protein kinase C

We have used a digitonin-permeabilized cell system to study the signal transduction pathways responsible for stimulus-secretion coupling in the rat peritoneal mast cell. Conditions were established for permeabilizing the mast cell plasma membrane without disrupting secretory vesicles. Exocytotic release of histamine from digitonin-permeabilized cells required a combination of micromolar concentrations of Ca2+ and the stable guanine nucleotide analogue guanosine 5′-[gamma-thio]triphosphate (GTP[S]), but was independent of exogenous ATP. In the presence of 40 microM-GTP[S], exocytosis was half-maximal at 1.3 microM-Ca2+ and maximal at 10 microM-Ca2+; GTP[S] alone (100 microM) had no effect on histamine release in the absence of added Ca2+. In the presence of 10 microM free Ca2+, 5 microM-GTP[S] was required for half-maximal exocytosis. To examine the possible role of protein kinase C (PKC) in exocytosis, we utilized 12-O-tetradecanoylphorbol 13-acetate (TPA) to activate PKC and studied its effect on histamine release from permeabilized mast cells. Cells that had been incubated with TPA (25 nM for 5 min) exhibited increased sensitivity to both GTP[S] and Ca2+. The PKC inhibitor staurosporine blocked the effect of TPA without inhibiting normal exocytosis in response to the combination of GTP[S] and Ca2+. In addition, down-regulation of mast-cell PKC by long-term TPA treatment (25 nM for 20 h) blocked the ability of the cells to respond to TPA and inhibited exocytosis in response to Ca2+ and GTP[S] by 40-50%. These results suggest that the sensitivity of the exocytotic machinery of the mast cell can be altered by PKC-catalysed phosphorylation events, but that activation of PKC is not required for exocytosis to occur.