FIGURE 3.

Mutations in the acidic cleft alter the cation selectivity of ENaC inhibition. A, ENaC expressing oocytes were perfused for 1 min with each solution, after which currents were determined at varying holding potentials (−140 to 60 mV in 20-mV steps, 0.5 s/step). Amiloride (amil)-supplemented solutions were then perfused for 20 s, followed by measurement of currents at the different holding potentials. B, resultant I-V curves were fit with equation 1 or 2 to derive P_Na values in each solution (red, initial NMDG⁺; green, Na⁺; blue, Li⁺; black, K⁺; orange, final NMDG⁺). Measurements in NMDG⁺ were taken at the beginning and end of each experiment to estimate channel rundown (∼2–7%/min), which was used to adjust P_Na values for channel rundown. C, correlation between Na⁺ self-inhibition measured under a constant voltage clamp (Fig. 1B) and the ratio of Na⁺ permeabilities in Na⁺ and NMDG⁺. D, ratio of Na⁺ permeabilities measured in different bath solutions: Li⁺ versus Na⁺ (top panel) and K⁺ versus Na⁺ (bottom panel). *, p < 0.003 versus wild type (n = 40 for the wild type and 5–12 for mutants).