PH-DEPENDENT AND VOLTAGE-DEPENDENT CONDUCTANCES IN TOAD SKIN

The present study focuses on two closely related topics on ion conduct ance in toad skins: (i) the interaction of apical protons with the api cal voltage-dependent Cl--activated channels of the mitochondriarich c ells, and (ii) the description and characterization of a novel subject , a voltage-dependent H+-activated conductance. The Cl- conductance (G (Cl)) is activated by tissue hyperpolarization (which leads to apical membrane depolarization) and the presence of Cl- ions in the apical so lution. Increasing apical proton concentration (from pH 8 to pH 4) imp airs the process of activation of the Cl- conductive pathway, slowing the kinetics of I-t activation and reducing the steady-stage values of G(t) and I-t. This effect is markedly voltage-dependent since no effe ct is seen at V-t = -100 mv and is fully present at -50 mV. The voltag e-dependence of the pH effect suggests that the critical protonation s ites of the apical Cl- channels are not freely exposed to the apical s olution but dwell within the membrane electric field. An also coherent interpretation is that titration of apical proton binding sites affec ts the gating of the voltage-dependent Cl- channels, shifting the cond uctance-vs.-voltage curve to more negative clamping potentials. Tissue conductance in the absence of apical Cl- ions can be importantly affe cted by the pH of the apical solution (pH(a)), the effect being marked ly dependent on the clamping potential. Generally speaking, the effect of rising apical proton concentration can be conspicuous at negative clamping potentials, while at positive potentials changes in tissue co nductance were never observed. For a clamping potential of -100 mV, a turning point somewhere between pH(a) = 4 and pH(a) = 3 was observed. Apical acidification to pH 4 has no effect upon tissue conductance whi le apical acidification to pH 3 leads to a marked, slow and reversible increase of tissue conductance, A striking similitude exists between the voltage-dependent Cl--gated conductance and the voltage-dependent proton-gated conductance regarding: (i) slow time courses of activatio n and deactivation, (ii) requirement for a negative clamping potential and the presence of a specific ion species in the apical solution for activation to take place, (iv) instantaneous ohmic behavior, and (v) steady-state rectification. However, so far the results do not permit one to conclude definitely that the voltage-dependent Cl--gated conduc tance and the voltage-dependent proton-gated conductance share a commo n pathway.