BARIUM AND CALCIUM STIMULATE SECRETION FROM DIGITONIN-PERMEABILIZED BOVINE ADRENAL CHROMAFFIN CELLS BY SIMILAR PATHWAYS

We compared the characteristics of secretion stimulated by EGTA-buffered Ba2+- and Ca2+-containing solutions in digitonin-permeabilized bovine adrenal chromaffin cells. Half-maximal secretion occurred at approximately 100-mu-M Ba2+ or 1-mu-M Ca2+. Ba2+-stimulated release was not due to release of sequestered intracellular Ca2+ because at a constant free Ba2+ concentration, increasing unbound EGTA did not diminish the extent of release due to Ba2+. The maximal extents of Ba2+- and Ca2+-dependent secretion in the absence of MgATP were identical. MgATP enhanced Ba2+-induced secretion to a lesser extent than Ca2+-induced secretion. Half-maximal concentrations of Ba2+ and Ca2+, when added together to cells, yielded approximately additive amounts of secretion. Maximal concentrations of Ba2+ and Ca2+ when added together to cells for 2 or 15 min were not additive. Tetanus toxin inhibited Ba2+- and Ca2+-dependent secretion to a similar extent. Ba2+, unlike Ca2+, did not activate polyphosphoinositide-specific phospholipase C. These data indicate that (1) Ba2+ directly stimulates exocytosis, (2) Ba2+-induced secretion is stimulated to a lesser extent than Ca2+-dependent secretion by MgATP, (3) Ba2+ and Ca2+ use similar pathways to trigger exocytosis, and (4) exocytosis from permeabilized cells does not require activation of polyphosphoinositide-specific phospholipase C.