Effect of external Na⁺ on SBFI fluorescence ratio and background currents in WT and V876E fibers. (A) Simultaneous recordings of SBFI fluorescence ratio (upper trace) and membrane currents (lower trace) in response to a change of the external solution from a Na⁺-free NMDG containing solution to a 140 mM Na⁺–containing solution in a V876E fiber held at −80 mV and stimulated by 50-ms duration voltage pulses given to −90 mV at a frequency of 0.5 Hz. (B) Mean and SEM of SBFI fluorescence ratio as a function of time in response to external Na⁺ readmission in WT and in V876E fibers. In each cell, values of fluorescence ratio have been normalized to the value of the last data point measured before Na⁺ readmission. (C) Mean rate of change in SBFI fluorescence ratio measured during the first 30 s of exposure of the cell to external Na⁺. The number of fibers tested is indicated above each histogram bar. Data are given as means ± SEM. P-value was 0.0043 (**).