Barium and Calcium Stimulate Secretion from Digitonin‐Permeabilized Bovine Adrenal Chromaffin Cells by Similar Pathways

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