Properties of a conductive cellular chloride pathway in the skin of the toad (Bufo bufo)

Two types of chloride current response to a step-wise hyperpolarization of the toad skin is demonstrated: (1) An "instantaneous" response observed immediately upon voltage change, and (2) a subsequent slow response, the time course of which is sigmoidal. The slow response is due to an increase of a transcellular conductance which is specific to chloride ions. The time constant of the conductance increase is dependent on the amplitude of the transepithelial voltage displacement, the smallest time constants are obtained for the highest amplitudes and are in the order of 30 s. The voltage dependences of the steady-state conductance and the steady-state chloride current reveal that the chloride pathway has maximum conductance for V approximately -80 mV (outside of the skin being negative) and approaches a non-conducting safe for V greater than 0 mV. This strong outward going rectification is a steady-state phenomenon: In skins hyperpolarized for a few minutes, the "instantaneous" I-V curves show that the chloride pathway in the conducting state allows a large inward chloride current (outward chloride flux) to pass in the voltage range 40 mV greater than V greater than 0 mV. Calculations based on a three-compartment model indicate that the strong steady-state chloride current rectification cannot be obtained if only the intracellular chloride concentration and the membrane potentials are allowed to vary ("Goldman-rectification"). It is suggested, therefore, that the premeability of the chloride pathway varies reversibly with the transepithelial potential difference. The variable which controls the chloride permeability may be a membrane potential or the concentration of an intracellular ion.