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 IK or the delayed rectifier) were studied with the whole cell patch-clamp technique in isolated guinea pig ventricular myocytes. IK 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 IK. These shifts were used to calculate permeability ratios relative to K+. The permeability sequence for the IK channels was K+ = Rb+ greater than NH4+ = Cs+ greater than 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 IK channels is discussed with respect to the common use of Cs+ in isolating other currents.