Ca2+-dependent K+ current and exocytosis in responses to caffeine and muscarine in voltage-clamped guinea-pig adrenal chromaffin cells

We characterized changes in membrane currents and the cytosolic Ca2+ concentration, [Ca2+](i), in response to caffeine, and compared them with those in response to muscarine using the perforated patch-clamp technique and fura-2 microfluorimetry in guinea-pig adrenal chromaffin cells. Catecholamine release from single voltage-clamped cells was monitored with amperometry using carbon microelectrodes. Caffeine produced a transient outward current (I-out) at holding potentials over -60 mV, increasing in amplitude with increasing the potentials. It also evoked a rapid increase of [Ca2+](i) at all potentials examined. The current-voltage relation revealed that the activation of K+ channels was responsible for the I-out evoked by caffeine. Both current and [Ca2+](i) responses were reversibly abolished by cyclopiazonic acid, an inhibitor of Ca2+-pump ATPase. At -30 mV, the caffeine-induced I-out, but not [Ca2+](i), was partly inhibited by either charybdotoxin or apamin. In the majority of cells tested, caffeine induced a larger I-out but a smaller [Ca2+](i) increase than muscarine. Caffeine and muscarine increased catecholamine release from voltage-clamped single cells concomitant with the transient increase of [Ca2+](i), and there was a positive correlation between them. These results indicate that caffeine activates Ca2+-dependent K+ channels and catecholamine secretion due to the release of Ca2+ from internal stores in voltage-clamped adrenal chromaffin cells of the guinea-pig. There seems to be a spatial difference between [Ca2+](i) increased by Ca2+ release from caffeine-sensitive stores and that released from muscarine (inositol 1,4,5-trisphosphate)-sensitive ones.