Block by Cs of K current iK1 and of carbachol induced K current iCch in frog atrium.

1. In frog atrium, Cs ions block both the inward rectifier iK1 and the carbachol induced K current iCch. 2. Both iK1 and iCch display a high affinity for Cs with a K0.5 of 4 X 10(-5) M for iK1 and of 8 X 10(-5) M for iCch at V = -50 mV. 3. Block of both iK1 and iCch is strongly voltage dependent. When fitted by the block model of Woodhull (1973), delta is greater than 1 for the two currents. 4. From these similarities, action of Cch on frog atrium K permeability could be interpreted as a modification of iK1.