The mechanism of Ba(2+)-induced exocytosis from single chromaffin cells

Dialysis of Ba2+ into voltage-clamped single bovine chromaffin cells produced a concentration-dependent increase in cell capacitance, reflecting an enhanced rate of exocytotic events. Between 0.1 and 1 mM, Ba2+ linearly increased both the rate and the total amount of exocytosis. In unclamped cells also, extracellular Ba2+ induced the release of catecholamines, as assayed with a carbon-fibre electrode in the amperometric mode. Additionally, extracellular application of Ba2+ increased the apparent internal Ca2+ concentration ([Ca2+]app) in fura-2-loaded chromaffin cells. These observations were made both in the presence and absence of external Ca2+ (Cao2+), as well as after depletion of the intracellular Ca2+ stores with ionomycin. Under current-clamp conditions, Ba2+ induced pronounced depolarization of the cells. These results are compatible with the following conclusions: by blocking K+ channels, Ba2+ causes depolarization of chromaffin cells. This results in opening of voltage-gated Ca2+ channels and Ba2+ entry into the cytosol. Ba2+ then directly triggers exocytotic events, although it induces exocytosis only at concentrations more than a 100-fold higher than Ca2+. Various effects contribute to the generally observed greater secretory responses with Ba2+ as compared with Ca2+; these are the depolarizing effects of extracellular Ba2+, its greater entry through non-inactivating Ca2+ channels and its poor intracellular buffering largely arising from its weak affinity for plasmalemmal Ca2+ extrusion mechanisms. In some cases, Ba2+ additionally induces release of Ca2+ from internal stores, as evidenced by its effect on fura-2 fluorescence at different wavelengths.