title: Ionic permeation and blockade in Ca2+-activated K+ channels of bovine chromaffin cells.
abstract: Single channel currents through Ca2+-activated K+ channels of bovine chromaffin cells were measured to determine the effects of small ions on permeation through the channel. The channel selects strongly for K+ over Na+ and Cs+, and Rb+ carries a smaller current through the channel than K+. Tetraethylammonium ion (TEA+) blocks channel currents when applied to either side of the membrane; it is effective at lower concentrations when applied externally. Millimolar concentrations of internal Na+ reduce the average current through the channel and produce large fluctuations (flicker) in the open channel currents. This flickery block is analyzed by a new method, amplitude distribution analysis, which can measure block and unblock rates in the microsecond time range even though individual blocking events are not time-resolved by the recording system. The analysis shows that the rate of block by Na+ is very voltage dependent, but the unblock rate is voltage independent. These results can be explained easily by supposing that current flow through the channel is diffusion limited, a hypothesis consistent with the large magnitude of the single channel current.
mesh_terms: D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002417:Cattle; D002586:Cesium / Q000494:pharmacology; D002838:Chromaffin System / Q000378:metabolism*; D004058:Diffusion; D004553:Electric Conductivity; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism*; D008433:Mathematics; D011188:Potassium / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism / Q000494:pharmacology; D013757:Tetraethylammonium Compounds / Q000494:pharmacology
chemical_list: D007473:Ion Channels; D013757:Tetraethylammonium Compounds; D002586:Cesium; D012964:Sodium; D011188:Potassium; D002118:Calcium