AB ST R A C T The effects of changes in Na concentration of the bathing solutions on some transport and permeability properties of the isolated frog skin have been examined. Rate coefficients for unidirectional Na movements across the two major barriers in the skin have been estimated as functions of Na concentration. The results indicate that the "apparent Na permeability" of the outer barrier of the skin decreases markedly when Na concentration in the outer solution is increased from 7 to 115 mM.The observed saturation of rate of Na transport with increasing Na concentration can be ascribed, in part, to this permeability change rather than to saturation of the transport system itself. Unidirectional C1 flux across the short-circuited skin was not significantly altered by an increase in Na concentration from 30 to 115 mM suggesting that the changes in membrane properties are relatively specific for the Na ion. The results also suggest that the movement of Na across the outer membrane may not be due entirely to simple passive diffusion of free Na ions.In studying the effects of Ca and antidiuretic hormone on Na transport across isolated frog skin, Curran et al. (1) obtained evidence indicating that the rate of active transport is strongly dependent on the effective Na permeability of the outward facing membrane of the transporting cells. On the basis of their results, the saturation of the rate of Na transport with increasing Na concentration in the outside bathing solution (2, 3) cannot be ascribed solely to saturation of the active transport mechanism, assuming it is located at the inner membrane. They suggested that this saturation phenomenon might be associated with the outer membrane, and that the properties of this barrier might be altered by changes in Na concentration. The present experiments were carried out to examine this possibility in more detail by studying the influence of Na concentration on some of the ion transport and permeability properties of the isolated frog skin. The relationship between 879

A method has been developed for determining unidirectional Na fluxes across the two faces of the transporting cells in the frog skin. The method has been used to investigate the location of the sites at which Ca and anti-diuretic hormone act to alter the rate of active Na transport across the skin. The results have indicated that the primary effect of both agents is on the Na permeability of the outward facing membrane of the cells. Ca decreases and the hormone increases permeability of this barrier. Neither agent appears to have a direct effect on the active transport system itself assuming that it is located at the inner membrane of the cells. The rate of active Na transport is altered as a result of changes in the size of the Na pool in the cells which occur because of changes in the rate of Na entry through the outer membrane. Thus, the results indicate that the Na permeability of the outer membrane plays an important role in controlling the rate of net active Na transport across the skin.

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