Inhibition of single-channel conductance of Kir2.1 by external Na⁺. (A) Single-channel currents recorded at different [K⁺]_out levels (150, 50, and 20 mM) in the absence and presence of Na⁺ substitution ([K⁺]_out + [Na⁺]_out = 150 mM). Currents were recorded in the cell-attached mode from L cells stably expressing Kir2.1. Shown on the top are typical whole-cell currents recorded from Kir2.1-expressing L cells immersed in Tyrode solution at membrane potentials between −130 and +40 mV. The membrane potentials (means ± SD) of the cells (V_cell) measured by escin perforation were −47 ± 3 mV (n = 5); patch membrane potentials (V_P) indicated near the current traces were calculated by assuming the membrane potential of the cells was −50 mV. Dashed lines superimposed on current traces indicate the closed-state level. (B) i–V_P relationships obtained from cell-attached patches at different [K⁺]_out in the absence (left) and presence (right) of Na⁺ substitution. Relationships are fitted with straight lines. (C) [K⁺]_out dependence of the single-channel conductance in the presence and absence of Na⁺ substitution shown on double-logarithmic coordinates. Error bars represent SD. The slope of the dashed line is 0.41.