Potassium channels in cultured bovine adrenal chromaffin cells.

K channels of bovine adrenal chromaffin cells were studied using patch‐clamp techniques. Whole‐cell K currents measured near +10 mV were much larger in 1 mM‐external Ca than in Ca‐free saline. Noise analysis suggested that this Ca‐dependent current was carried by a large unitary conductance channel, called BK channel, which was previously described in inside‐out patches (Marty, 1981). The Ca‐dependent K current near +10 mV declined with time due to 'run‐down' of Ca channels. At the same time, a fraction of the outward current observed above +50 mV was also eliminated. This outward current component probably represents K efflux through Ca channels. Whole‐cell Ca‐dependent K currents were studied using various Ca buffers. EGTA buffers were surprisingly inefficient: in order to block the current entirely, it was necessary to use an isotonic EGTA solution and to increase internal pH. 1,2‐bis(o‐aminophenoxy)ethane‐N,N,N',N'‐tetraacetic acid (BAPTA) was at least five times more efficient than EGTA. In isolated patches three types of single‐channel K currents were observed. Under normal ionic conditions (140 mM‐K inside, 140 mM‐Na outside), the unitary conductances measured between ‐20 and +40 mV were 96 pS, 18 pS and 8 pS. The 96 pS channels are the Ca‐dependent BK channels. 18 pS and 8 pS channels were both activated and then inactivated by membrane depolarization. Both displayed complex kinetics; single‐channel currents were grouped in bursts. Activation and inactivation kinetics were faster for the 18 pS channel (therefore termed FK channel, for fast K channel) than for the 8 pS channel (SK channel, for slow or small amplitude channel). The voltage dependence of opening probability was steeper for the FK channel as compared to the SK channel.