SMALL-CONDUCTANCE CA-2(+)-ACTIVATED K+ CHANNELS IN BOVINE CHROMAFFIN CELLS

Simultaneous whole-cell patch-clamp and fura-2 fluorescence [Ca2+]i measurements were used to characterize Ca2+-activated K+ currents in cultured bovine chromaffin cells. Extracellular application of histamine (10 muM) induced a rise of [Ca2+]i concomitantly with an outward current at holding potentials positive to -80 mV. The activation of the current reflected an increase in conductance, which did not depend on membrane potential in the range -80 mV to -40 mV. Increasing the extracellular K+ concentration to 20 mM at the holding potential of -78 mV was associated with inwardly directed currents during the [Ca2+]i elevations induced either by histamine (10 muM) or short voltage-clamp depolarizations. The current reversal potential was close to the K+ equilibrium potential, being a function of external K+ concentration. Current fluctuation analysis suggested a unit conductance of 3 - 5 pS for the channel that underlies this K+ current. The current could be blocked by apamin (1 muM). Whole-cell current-clamp recordings showed that histamine (10 muM) application caused a transient hyperpolarization, which evolved in parallel with the [Ca2+]i changes. It is proposed that a small-conductance Ca2+-activated K+ channel is present in the membrane of bovine chromaffin cells and may be involved in regulating catecholamine secretion by the adrenal glands of various species.