Fig. 9. Substantial sodium current carried by S177W but not N94Y–S177W G protein-coupled inwardly rectifying potassium 3.2 (GIRK2) channels, with or without the D228N mutation of the intracellular Na⁺-binding site. (A) Representative current traces from Xenopus oocytes were elicited with voltage pulses from +40 to –150 mV (in 10-mV increments) in 90 mM Na⁺ (Left) and 90 mM K⁺ (Right) bath solutions from a holding potential of 0 mV. (B) Average ratio of currents (at –100 mV) (Left) and average permeability ratio (Right) determined in Na⁺ and K⁺ solutions for S177T, S177W, and N94Y–S177W confirmed that the N94Y mutation reduced I_Na/I_K and P_Na/P_K. Comparable values of I_Na/I_K and P_Na/P_K were obtained with or without the D228N mutation. (C) N94Y suppressed the large Na⁺ current due to the S177W mutation. Two-electrode voltage-clamp currents were recorded from oocytes expressing various mutant channels, and the mean ± SEM were plotted as I–V curves, in 90 mM Na⁺ (filled circles) and in 90 mM K⁺ (open triangles).