Relation of exocytosis and Ca-activated K current during Ca release from intracellular stores in individual rat chromaffin cells

Measurement of the change in cell membrane capacitance (C_m) along with the change in I_K(Ca) was used to investigate the effects of bradykinin and caffeine on the secretory process in rat adrenal chromaffin cells. In a Ca²⁺-free external solution, bradykinin (100 nM) caused a transient increase in C_m with a concurrent change in I_K(Ca). Extracellular application of neomycin as an inhibitor of phospholipase C activity reversibly inhibited the bradykinin-activated event, implying an IP₃-mediated increase of submembrane-free Ca²⁺. The increases in C_m and I_K(Ca) caused by bradykinin were transient even with the sustained application of bradykinin. Caffeine also caused exocytosis in the Ca²⁺-free solution, and this was irreversibly blocked by ryanodine (1 μM) in a use-dependent manner. Caffeine-sensitive intracellular Ca²⁺ stores were also depleted in several seconds and recovered by an influx of external Ca²⁺. The sequential application of bradykinin and caffeine showed that these are likely to activate Ca²⁺ release from the same or distinct but rapidly equilibrating intracellular Ca²⁺ stores. The single cell assay of exocytosis and the increase in I_K(Ca) revealed cell-to-cell variability in bradykinin- and caffeine-induced exocytotic response. Our results suggest that Ca²⁺ release from intracellular stores potentially increases submembrane Ca²⁺ concentration and modulates simultaneously two submembrane Ca²⁺-dependent processes, exocytosis and I_K(Ca), in rat adrenal chromaffin cells.