Figure 3.
Xenopus δβγ-ENaC is activated by extracellular acidification. a, recordings of IM in oocytes expressing Xenopus αβγ- or δβγ-ENaC. Amiloride-sensitive (a, amiloride, 100 μm) currents mediated by δβγ- but not αβγ ENaC are dose-dependently increased by a stepwise acidification of the extracellular solution (pH 8.0–6.0; pH 0.2 increments). b, dose-response curves of acid-mediated activation of Xenopus ENaC isoforms. Normalized (IM, pHx/IM, pH 7.4) current levels were fit to a sigmoidal dose-response function with variable slope (Hill-slope of Xenopus δβγ-ENaC: 1.6 ± 0.4). Compared with pH 7.4, alkaline conditions decrease channel activity to a minimal factor (min.) of 0.5 ± 0.2, whereas acidification enhances activity of δβγ-ENaC by a maximal factor (max.) of 3.9 ± 0.2. c and d, maximal acid-induced activation of currents mediated by human αβγ-ENaC (max.: 1.03 ± 0.01) or δβγ-ENaC (max.: 1.05 ± 0.01) is considerably reduced with respect to Xenopus δβγ-ENaC.