Rb+, Cs+ ions and the inwardly rectifying K+ channels in guinea-pig ventricular cells

Rb+ and Cs+ ion permeability and the effects of these ions from the inside on the inwardly rectifying K+ channel were studied in guinea-pig ventricular cells. A total substitution of either Rb+ or Cs+ for external K+ in the outside-out configuration of the patch-clamp technique abolished the inward current. The outward current carried by K+ was recorded. The unitary amplitude was reduced to about half of the control value with Rb+ but was not changed with Cs+. Internal Rb+ and Cs+, at a concentration of 10-40 mM, reduced the unitary amplitude of the outward current. No substate behaviour was observed. The reversal potential was +18 mV after replacing 105 mM internal K+ with Rb+ at 150 mM external K+. This value gives a permeability ratio of Rb+ to K+ of 0.27. Under a total substitution of Rb+ or Cs+ for internal K+, the outward currents were not measurable. Cs+ induced flickering in the inward current carried by K+. It is thus concluded that Rb+ and Cs+ ions are not measurably permeant at the single-channel level but permit K+ permeation in place of external K+ and that internal Rb+ and Cs+ produce a voltage-dependent block of the channel with fast kinetics.