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

1993; Wiley; Volume: 336; Issue: 1 Linguagem: Inglês

10.1016/0014-5793(93)81606-z

1873-3468

Ludolf von Rüden, Antonio G. Garcı́a, Manuela G. López,

Neuroscience and Neural Engineering

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.