Rb⁺ and Cs⁺ permeation in NaK2K. (A) Single-channel traces of NaK2K at ±80 mV with a symmetrical 150 mM RbCl and its I-V curve (n = 5 measurements). Currents were recorded using giant liposome patch clamping. The same method was used in all other recordings. The arrows indicate the baseline. (B) Single-channel traces of NaK2K with 150 mM of a mixture of Rb⁺ and K⁺ and its I-V curve (n = 5 measurements). The channel is slightly selective for K⁺ with a reversal potential of ∼10 mV. (C) The I-V curves of NaK2K in the presence and absence of Cs⁺. The pipette solution contains 150 mM KCl. The bath solution contains 150 mM KCl (black trace), 135 mM KCl and 15 mM CsCl (blue trace), or 150 mM CsCl (red trace). For a better assessment of reversal potential, each I-V curve was obtained from the mean of 20 current traces recorded using a patch containing multiple channels with voltage ramps from −100 to 100 mV over a 400-ms duration. Error bars represent SEM.