Fig. 7.

Reduced sodium self-inhibition is pivotal to ENaC activity at high extracellular Na⁺ concentrations. (A) Representative transmembrane current (I_M) traces showing sodium self-inhibition (SSI), determined with guinea pig αβγ-ENaC expressing oocytes, with and without prior incubation with chymotrypsin (2 µg/ml in NMDG-ORS for 5 min). Application of amiloride (100 µM) is represented by black bars (a) and [Na⁺] is represented by white (90 mM) and gray (1 mM) bars. The perfusion speed was 12 ml/min. (B) The percentage of SSI was plotted for guinea pig αβγ-ENaC with and without prior incubation with chymotrypsin (Student’s unpaired t-test). SSI was calculated as (ΔI_M peak−ΔI_M 3 min)/ΔI_M peak×100, where ΔI_M peak=I_M under 90 mM [Na⁺] peak −I_M under 1 mM [Na⁺], and ΔI_M 3 min=I_M after 3 min under 90 mM [Na⁺]−I_M under 1 mM [Na⁺]. (C) Representative I_M traces for guinea pig αβγ-ENaC expressing oocytes with and without prior incubation with chymotrypsin (2 µg/ml for 5 min) across a range of extracellular Na⁺ concentrations ([Na⁺]), gray-shaded boxes). (D) The I_M from experiments shown in panel (C) were plotted against the extracellular [Na⁺] and fitted to the Michaelis–Menten equation allowing the estimation of the maximum I_M (V_max) and the [Na⁺] at which half of V_max is reached (K_M). (E) The V_max values of guinea pig αβγ-ENaC with and without prior incubation with chymotrypsin (Student’s unpaired t-test with Welch’s correction). (F) The K_M values of guinea pig αβγ-ENaC with and without prior incubation with chymotrypsin (Mann–Whitney U test). Numbers in parentheses indicate (n).