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

Abstract

Dialysis of Ba 2+ 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, Ba 2+ linearly increased both the rate and the total amount of exocytosis. In undamped cells also, extracellular Ba 2+ induced the release of catecholamines, as assayed with a carbon-fibre electrode in the amperometric mode. Additionally, extracellular application of Ba 2+ increased the apparent internal Ca 2+ concentration ([Ca 2+] app) in fura-2-loaded chromaffin cells. These observations were made both in the presence and absence of external Ca 2+ (Ca 2+ 0), as well as after depletion of the intracellular Ca 2+ stores with ionomycin. Under current-clamp conditions, Ba 2+ induced pronounced depolarization of the cells. These results are compatible with the following conclusions: by blocking K + channels, Ba 2+ causes depolarization of chromaffin cells. This results in opening of voltage-gated Ca 2+ channels and Ba 2+ entry into the cytosol. Ba 2+ then directly triggers exocytotic events, although it induces exocytosis only at concentrations more than a 100-fold higher than Ca 2+. Various effects contribute to the generally observed greater secretory responses with Ba 2+ as compared with Ca 2+; these are the depolarizing effects of extracellular Ba 2+, its greater entry through non-inactivating Ca 2+ channels and its poor intracellular buffering largely arising from its weak affinity for plasmalemmal Ca 2+ extrusion mechanisms. In some cases, Ba 2+ additionally induces release of Ca 2+ from internal stores, as evidenced by its effect on fura-2 fluorescence at different wavelengths.