[K⁺]_out dependence of sKir currents reconstructed using Na⁺ block in competition with external K⁺. An increase in external K⁺ ions surmounts the fast-pore block of sKir channels by external Na⁺, thereby increasing both the inward and outward currents. The channel blockade by external Na⁺ shows a weak voltage dependence. Blockade of sKir channels by cytoplasmic polyamines (spermine), which causes their inward rectification, depends on V – E_K when the [K⁺]_out is changed. Shown are theoretical currents calculated at 3.5 mM (blue), 10 mM (green), and 40 mM (magenta) [K⁺]_out levels in the absence of both external Na⁺ and cytoplasmic polyamines (dashed lines), in the presence of 140 mM external Na⁺ and no cytoplasmic polyamines (continuous lines with faint colors), and in the presence of 140 mM external Na⁺ and cytoplasmic polyamines (continuous lines with bold colors). Theoretical currents at 120 mM external Na⁺ (in the presence of polyamines) are also superimposed at each [K⁺]_out (gray), but are only partly visible. Current amplitudes are given in arbitrary units. See Materials and methods for the details of calculations.