Figure 13. I _Nab is more permeant to Na⁺ than to K⁺ .

Ionic selectivity of I _Nab was assessed by changing intracellular K⁺ and Na⁺ concentrations (K⁺ concentrations 25, 50, 130 and 140 mM and Na⁺ concentrations 125, 100, 20, 10). The ionic concentrations highlighted in a grey filled rectangle correspond to the condition of a low (0.5 mM) intracellular Na⁺ concentration, which was used in experiments described in Fig. 6. Under each ionic condition, I _Nab was recorded through a voltage ramp command (−110 to −50 mV, 3 s duration). A, the E _rev value of I _Nab was calculated, for each recording condition, from a linear extrapolation applied to the subtracted currents. g _Na/g _K ratios were then calculated as described in Methods. B, robust correlation of E _rev over E _Na (plotted with the confidence band at 95%), with the restriction of [Na⁺]_I ≥ 10 mM (Ba) and of g _Na/g _K over E _Na (also plotted with the confidence band at 95%, Bb). Data were fitted with a linear regression (r ² = 0.985 and 0.916, respectively). Note that under physiological conditions (i.e. E _Na = +55 mV), g _Na/g _K is close to 2, indicating that I _Nab is about twice as permeant to Na⁺ as to K⁺. [Color figure can be viewed at wileyonlinelibrary.com]