Permeability of time-dependent K⁺ channel in guinea pig ventricular myocytes to Cs⁺, Na⁺, NH⁺₄, and Rb⁺

Currents through time-dependent K⁺ channels (also referred to as I(K) or the delayed rectifier) were studied with the whole cell patch-clamp technique in isolated guinea pig ventricular myocytes. I(K) measurements were restricted to the examination of deactivation tail currents. Substitution of various monovalent cations for external K⁺ produced shifts of the reversal potential of I(K). These shifts were used to calculate permeability ratios relative to K⁺. The permeability sequence for the I(K) channels was K⁺ = Rb⁺ > NH⁺₄ = Cs⁺ > Na⁺. Time-dependent outward currents were also examined when the myocytes were dialyzed with Cs⁺ instead of K⁺. A sizeable time-dependent outward current, quite similar to that seen with K⁺ dialysis, was demonstrated. This current was primarily carried by intracellular Cs⁺, as the reversal potential of the current shifted 46 mV per 10-fold change of external Cs⁺ concentration. The significance of Cs⁺ permeation through I(K) channels is discussed with respect to the common use of Cs⁺ in isolating other currents.