Charge movement and gating pore currents in rNaV1.4-R666G channels. Oocytes expressing WT and R666G mutant channels were recorded using different bath solutions in which either K⁺ or NMDG was substituted as the predominant external cation (denoted at top of figure). For comparison, currents recorded from a mock-injected oocyte in K⁺-containing bath solution are also shown. Representative raw gating current traces are shown in A. Gating charge displacement was assessed by recording the nonlinear transient current responses elicited by 15-ms voltage steps from a prepulse potential of −160 mV (see voltage protocol in inset). Membrane potential commands (in mV) eliciting charge movement are denoted to the left of each trace. Scale bars for the current records in A are shown at the bottom right. In B, representative steady-state background current recordings are shown for the same oocytes whose gating currents are depicted in A. Steady-state currents were elicited by a 300-ms voltage command from a holding potential of −100 mV (see voltage protocol in inset). Scale bars for the current records in B are shown at the bottom left. The mean I-V relationships of the steady-state background currents from the oocyte recordings depicted in B are shown in C. The symbol legend is to the right. In K⁺-containing bathing solutions, oocytes expressing R666G mutant channels exhibit robust inward currents at voltages <+20 mV that are not seen in oocytes expressing comparable levels of WT channels under the same conditions (open circles are not fully visible beneath filled squares). Steady-state currents in R666G-expressing oocytes recorded in NMDG bath solution, however, are indistinguishable from the nonspecific membrane leak seen in mock-injected oocytes and oocytes expressing WT rNav1.4.