R666G K⁺ and Na⁺ steady-state gating pore currents. The permeability of the R666G gating pore for different monovalent cations was tested. The transmembrane cation gradient was manipulated as described in the insets, such that either cation was present exclusively in the internal or external compartments, or was distributed nearly symmetrically in both compartments. The normalized, leak-corrected I-V relationships of resultant K⁺ (A and B) and Na⁺ (C) gating pore currents are shown. The different colors in A and C represent current recordings made in different ionic gradients, with the legend in each figure inset. The number of samples recorded for each condition is also denoted in parenthesis. Very small outward currents were seen when either Na⁺ or K⁺ were present exclusively on the inside, despite significant outward driving force for both cations. In external solutions consisting of 10 mM K⁺ and 105 mM NMDG, low-amplitude K⁺ currents were observed (note the normalized current scale in B), indicating that NMDG does not block the pore from outside and therefore cannot account for the lack of outward currents. Furthermore, inward current amplitudes were nearly equivalent whenever K⁺ or Na⁺ was present on the outside, despite significant changes to the overall driving force due to differences in the internal cation concentration. Overlying the data are curves from the permeation model depicted in Fig. 6 and described (see Results), representing the best parameter fits listed in Table I. Solid lines represent currents from the full model, which includes voltage-dependent accessibility of the R666G gating pore due to movement of the DIIS4 voltage sensor, whereas the dotted lines demonstrate the predicted open-channel voltage dependence of currents flowing through an R666G gating pore in the absence of gating.