A pertussis-toxin-sensitive protein controls exocytosis in chromaffin cells at a step distal to the generation of second messengers

Abstract

The role of GTP-binding proteins (G-proteins) in the secretory process in chromaffin cells was investigated by studying the effects of pertussis toxin (PTX) on catecholamine release and generation of various second messengers. PTX was found to stimulate the catecholamine secretion induced by nicotine, 59 mM-K+ or veratridine. PTX also potentiated Ca2(+)-evoked catecholamine release from permeabilized chromaffin cells, suggesting that PTX substrate(s) regulate the exocytotic machinery at a step distal to the rise in intracellular Ca2+. We have investigated the possible intracellular pathways involved in the stimulation of secretion by PTX. PTX did not modify the translocation of protein kinase C (PKC) to membranes in intact or permeabilized cells; in addition, neither inhibitors nor activators of PKC had any effect on catecholamine release induced by PTX. Thus it seems unlikely that the effect of PTX on secretion is mediated by activation of PKC. The effect of PTX is also cyclic AMP-independent, as PTX did not change cytoplasmic cyclic AMP levels. The relationship between PTX treatment and arachidonic acid release was also examined. We found that an increase in cytoplasmic arachidonic acid concentration enhanced Ca2(+)-evoked catecholamine release in permeabilized cells, but arachidonic acid did not mimic the effect of PTX on the Ca2(+)-dose-response curve for secretion. Furthermore, PTX did not significantly modify the release of arachidonic acid measured in resting or stimulated chromaffin cells, suggesting that the stimulatory effect of PTX on secretion is not mediated by an activation of phospholipase A2. Taken together, these results suggest that PTX may modulate the intracellular machinery of secretion at a step distal to the generation of second messengers. In alpha-toxin-permeabilized cells, full retention of the PTX-induced activation of secretion was observed even 30 min after permeabilization. In contrast, when chromaffin cells were permeabilized with streptolysin-O (SLO), there was a marked progressive loss of the PTX effect. We found that SLO caused the rapid leakage of three G-protein alpha-subunits which are specifically ADP-ribosylated by PTX. We propose that a PTX-sensitive G-protein may play an inhibitory role in the final stages of the Ca2(+)-evoked secretory process in chromaffin cells.