Ca2+-dependent K+ currents induced by muscarinic receptor activation in guinea pig adrenal chromaffin cells.

The characteristics of the outward current (I(out)) induced by muscarine were examined by using the whole-cell patch-clamp technique with a K+-containing pipette solution in combination with fura-2 microfluorometry in guinea pig chromaffin cells. Muscarine caused a transient increase in the cytosolic Ca2+ concentration ([Ca2+]i) and activated an I(out) with a reversal potential close to a K+ equilibrium potential. Under symmetric K+ conditions, muscarine produced a transient inward current and an increase in [Ca2+]i at -60 mV. At -15 mV, apamin and charybdotoxin, respective SK and BK channel blockers, decreased the I(out) but scarcely affected the [Ca2+]i response to muscarine. Muscarine produced an I(out) and an increase in [Ca2+]i even after a removal of external Ca2+ and in the presence of Co2+, indicating that these responses are mediated by Ca2+ release from intracellular stores. The I(out) evoked by the Ca2+ release was much smaller than that evoked by the voltage-dependent Ca2+ influx, even when similar [Ca2+]i changes assessed by fura-2 microfluorometry occurred. Inositol 1,4,5-trisphosphate (InsP3) applied intracellularly and the photolysis of caged InsP3 each evoked current changes similar to those induced by muscarine. These results indicate that the I(out) evoked by muscarinic stimulation is mediated by Ca2+-dependent K+ channels (probably BK and SK channels), which are activated by Ca2+ released from intracellular Ca2+ stores in guinea pig chromaffin cells.