PERMEABILITY OF TIME-DEPENDENT K+ CHANNEL IN GUINEA-PIG VENTRICULAR MYOCYTES TO CS+, NA+, NH4(+), 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+ > NH4+ = 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.