1. The inward facing membranes of in vitro frog skin epithelium were depolarized with solutions of high K concentration. The electrical properties of the epithelium are then expected to be governed by the outward facing, Na-selective membrane.
2. In this state, the transepithelial voltage (V) was clamped to zero and step-changes of Na activity in the outer solution ((Na)o) were performed with a fast-flow chamber at constant ionic strength, while the short-circuit current was recorded.
3. At pre-selected times after a step-change of (Na)o the current response (I) to a fast voltage staircase was recorded. This procedure was repeated after blocking the Na channels with amiloride to obtain the current—voltage curve of transmembrane and paracellular shunt pathways. The current—voltage curve of the Na channels was computed by subtracting the shunt current from the total current.
4. The instantaneous INa—V curve thus obtained at a given (Na)o could easily be fitted with the constant field equation in the range between -50 and zero mV. This fit yielded approximate estimates of PNa, the Na— permeability of the Na-selective membrane (at this (Na)o) and the cellular Na activity, (Na)c. As residual properties of the serosal membrane were ignored the computed values are expected to underestimate the true ones.
5. At constant (Na)c, the steady-state value of 1/PNa increases linearly with (Na)o. Error analysis and the effect of drugs show that the dependence is not due to the residual properties of the inward facing membranes but reflects the true behaviour of PNa.
6. The steady-state PNa at a given (Na)o is smaller than the transient PNa observed right after a stepwise increase of (Na)o to this value. The time constant of PNa-relaxation is in the order of seconds.
7. In conclusion, Na transport through open Na-selective channels of the outward facing membrane of the stratum granulosum cells can be described as an electrodiffusion process which as such does not saturate with increasing (Na)o. However, when added to the outer border of the membrane Na causes a decrease of PNa within several seconds. It is considered that binding of Na results in closure of Na channels.