Voltage dependence of proton block of single-channel conductance. (A and B) Representative single-channel currents recorded at two extracellular pHs with different voltages ranging from −100 to +100 mV. (A) pH 7.4 and (B) pH 5.5, recorded with symmetrical 150-mM KCl solutions. (C) Single-channel i-V curves at different values of pHos as indicated. The smooth lines were polynomial fits. (D; top plot) Ratio of the unitary current amplitudes at each pHo (7.4, 6.5, 5.5, and 4.5) to the amplitudes at pH 8.5 plotted as a function of voltage. The dashed lines represent the best fit to a Woodhull model, which corresponds to an electrical distance d = 0.5 for the binding site from the bulk solution, an apparent valence z = 1, a dissociation constant pKd,0 = 5.3 at 0 mV, and a pseudo Hill coefficient n = 0.4. The model was globally fit to the unitary current amplitudes at different pH values. (Bottom plot) pKd of proton inhibition estimated from the Hill equation fit at each voltage.
