Block by Cs of K currentiK1 and of carbachol induced K currentiCch in frog atrium

1. In frog atrium, Cs ions block both the inward rectifieriK1 and the carbachol induced K currentiCch. 2. BothiK1 andiCch display a high affinity for Cs with a K0.5 of 4×10−5 M forik1 and of 8×10−5 M foriCch atV=−50 mV. 3. Block of bothiK1 andiCch is strongly voltage dependent. When fitted by the block model of Woodhull (1973), δ is >1 for the two currents. 4. From these similarities, action of Cch on frog atrium K permeability could be interpreted as a modification ofiK1.