Exocytosis in permeabilised mast cells requires only that the concentrations of Ca2+ and GTP-gamma-S (the essential effectors) are elevated into the micromolar range of concentrations. These act through an unidentified Ca2(+)-binding protein and an uncharacterized G-protein (GE). There is no requirement for ATP in the final stages of the secretory pathway. However, mast cells permeabilised in the absence of ATP rapidly become refractory to stimulation due to a reduction in the affinity for the essential effectors. Here, we show that responsiveness may be restored by the addition of ATP. The characteristics of such ATP-dependent secretion have been examined. Preincubation (prior to permeabilization) of the cells with phorbol ester enhances affinity to Ca2+, and introduction of neomycin reduces Ca2+ affinity. AMG.C16, an ether-linked analogue of diglyceride, inhibits secretion in a manner which can be partially reversed by elevating the concentration of ATP. These observations indicate that while protein phosphorylation does not comprise a step in the triggering of exocytosis, a primed condition most likely involving a state of protein phosphorylation, and maintained by reactions catalysed by protein kinase C, is essential.
