Active Na transport across frog skin was separated from passive Na movement utilizing urea influx as a measure of passive (shunt) permeability. In this manner, the response of the overall active Na transport system to an applied potential was determined over a range from +200 mV to -100 mV. Active Na transport displays saturation as a function of applied potential, and both the level of saturation and the potential at which it is achieved are functions of the Na concentration in the external solution. The saturation with potential appears to involve a different step in the transport process than the saturation of Na flux as a function of external Na concentration. The observations can be qualitatively described by either a one-barrier or two-barrier model of the Na transport system.

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