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. Received: 9 March 1995/Received after revision: 27 September 1995/Accepted: 2 January 1996