Figure 4.

Acidic pH elicits a reversible, consistent, and Na⁺-dependent activation of Xenopus δβγ-ENaC. a, representative current trace of an oocyte expressing Xenopus δβγ-ENaC. Repetitive acidification of the extracellular milieu (from pH 7.4 to pH 6.0 for 30 s) reversibly increases I_M (a = amiloride, 100 μm). b, normalized I_M fractions sensitive to pH 6.0 ((I_{M, pH 6.0} − I_{M, pH 7.4})/I_{M, pH 6.0}) from recordings as depicted in a. Acid-sensitive current fractions do not decline but rather increase over time (repeated-measures one-way ANOVA, F = 57.8, p < 0.0001; Tukey's multiple comparisons test). c, assessment of repetitive SSI in an oocyte expressing Xenopus δβγ-ENaC. SSI was defined as the decline in I_M at 3 min after increasing extracellular [Na⁺] from 1 to 90 mm. d, Na⁺ self-inhibition values from recordings as depicted in c. Current fractions lost to SSI ((I_{M, peak} − I_{M, 3 min})/I_{M, peak}) are constant over time (repeated-measures one-way ANOVA, F = 0.8537, p = 0.4313; Tukey's multiple comparisons test). e, Na⁺ dependence of acid-induced ENaC activation was examined through determination of the pH EC₅₀ (pH 8.5–6.0; pH 0.5 increments) under different Na⁺ concentrations (3–90 mm). Fractional channel activation was calculated as the difference between I_M at a given pH (I_{M, x}) and the minimal I_M (I_{M, min}) in relation to the maximally observed difference in I_M (Δ_max I_M) in the respective recording ((I_{M, x} − I_{M, min})/Δ_max I_M)). f, alkaline-shift of the pH EC₅₀ in the presence of reduced [Na⁺] indicates that less protons are needed for acid-induced channel activation under low [Na⁺] (one-way ANOVA, F = 38.51, p < 0.0001; Tukey's multiple comparisons test). g, extracellular acidification (from pH 7.4 to pH 6.0) still activates Xenopus δβγ-ENaC after maximal rundown of currents.