Ca2+-activated K+ channels in the apical membrane of Necturus choroid plexus

The properties of Ca2+-activated K+ channels in the apical membrane of the Necturus choroid plexus were studied using single-channel recording techniques in the cell-attached and excised-patch configurations. Channels with large unitary conductances clustered around 150 and 220 pS were most commonly observed. These channels exhibited a high selectivity for K+ over Na+ and K+ over Cs+. They were blocked by high cytoplasmic Na+ concentrations (110 mM). Channel activity increased with depolarizing membrane potentials, and with increasing cytoplasmic Ca2+ concentrations. Increasing Ca2+ from 5 to 500 nM, increased open probability by an order of magnitude, without changing single-channel conductance. Open probability increased up to 10-fold with a 20-mV depolarization when Ca2+ was 500 nM. Lowering intracellular pH one unit, decreased open probability by more than two orders of magnitude, but pH did not affect single-channel conductance. Cytoplasmic Ba2+ reduced both channel-open probability and conductance. The sites for the action of Ba2+ are located at a distance more than halfway through the applied electric field from the inside of the membrane. Values of 0.013 and 117 mM were calculated as the apparent Ba2+ dissociation constants (KD(0 mV] for the effects on probability and conductance, respectively. TEA+ (tetraethylammonium) reduced single-channel current. Applied to the cytoplasmic side, it acted on a site 20% of the distance through the membrane, with a KD(0 mV) = 5.6 mM. A second site, with a higher affinity, KD(0 mV) = 0.23 mM, may account for the near total block of channel conductance by 2 mM TEA+ applied to the outside of the membrane. It is concluded that the channels in Necturus choroid plexus exhibit many of the properties of "maxi" Ca2+-activated K+ channels found in other tissues.