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

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Abstract
Two types of Cl current response to a step-wise hyperpolarization of the toad [B. bufo] skin is demonstrated. An instantaneous response observed immediately on voltage change, and a subsequent slow response, the time course of which is sigmoidal are seen. The slow response is due to an increase of a transcellular conductance which is specific to Cl 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 Cl current reveal that the Cl pathway has maximum conductance for V .apprxeq. =80 mV (outside of the skin being negative) and approaches a non-conducting state for V > 0 mV. This strong outward going rectification is a steady-state phenomenon. In skins hyperpolarized for a few minutes, the instaneous I-V curves show that the chloride pathway in the conducting state allows a large inward Cl current (outward Cl flux) to pass in the voltage range 40 mV > V > 0 mV. Calculations based on a 3-compartment model indicate that the strong steady-state Cl current rectification cannot be obtained if only the intracellular chloride concentration and the membrane potentials are allowed to vary (Goldman-rectification). The permeability of the Cl 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.

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