A combination of the methods described by Schultz et al. (6) and by Ussing and Zerahn (9) was used to measure directly the unidirectional uptake of sodium from the outside solution into the frog skin, under shortcircuit conditions. The sodium uptake was determined at six sodium concentrations ranging from 3.4 to 114 m. NaCI was replaced by choline chloride in the solutions bathing both sides of the skin. Sodium uptake is not a linear function of sodium concentration but appears to be composed of two components, a saturating one and one that varies linearly with concentration. The sodium uptake is inhibited by the addition of lithium to the outside solution. The effect appears to be primarily on the saturating component and has the characteristics of competitive inhibition. In addition, lithium uptake by the skin is inhibited by sodium. The effects of lithium cannot be ascribed to changes in electrical potential difference. Measurements with microelectrodes indicate that under short-circuit condition there is no change in the intracellular potential when lithium chloride is added to the outside solution.The functional model for Na transport across frog skin proposed by KoefoedJohnsen and Ussing (1) has frequently served as a basis for consideration of transport across other epithelial tissues. One of the important features of this model was that entry of Na into the skin from the outside solution is due to simple diffusion. This concept was based primarily on the observation that in open-circuit conditions the outer surface of the skin behaved like a Na electrode. However, some recent observations are rather difficult to explain in terms of simple diffusion of Na across the outer surface. For example, Cereijido et al. (2) concluded from indirect estimates of rate coefficients for Na movement that the apparent Na permeability of the outer barrier decreased with increasing Na concentration. On the basis of studies of Na transport by skins bathed on the outside with dilute solution containing I nm NaCl, Biber et al. (3) suggested that if all epithelial Na were in a single 83

The unidirectional sodium uptake at the outer surface of the frog skin was measured by the method described by Biber and Curran (8). With bathing solutions containing 6 mM NaC1l there is a good correlation between sodium uptake and short-circuit current (SCC) measured simultaneously except that the average uptake is about 40 % higher than the average SCC. The discrepancy between uptake and SCC increases approximately in proportion to an increase in sodium concentration of the bathing solutions. Amiloride inhibits the unidirectional sodium uptake by 21 and 69% at a sodium concentration of 115 and 6 mM, respectively. This indicates that amiloride acts on the entry step of sodium but additional effects cannot be excluded. The sodium uptake is not affected by 10 -4 M ouabain at a sodium concentration of 115 mM but is inhibited by 40 % at a sodium concentration of 6 mM. Replacement of air by nitrogen leads to a 40 % decrease of sodium uptake at a sodium concentration of 6 mM. The results support the view proposed previously (8) that the sodium uptake is made up of two components, a linear component which is, essentially, not involved in transepithelial movement of sodium and a saturating component which reflects changes in transepithelial transport. Amiloride, seems largely to affect the saturating component.

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Isolated frog skin was bathed with a dilute solution containing 1 mM NaCl on the outside and with normal Ringer's solution on the inner surface. Net Na flux was determined by simultaneous measurement of unidirectional fluxes with Na 22 and Na 24 and intracellular electrical potentials were examined with microelectrodes. There was a net inward transport of Na under both open-circuit and short-circuit conditions. The short-circuit current was approximately 15% greater than the net Na flux; the discrepancy could be accounted for by a small outward flux of CI. The electrical potential profile did not differ greatly from that observed in skins bathed on the outside with normal Ringer's solution. Under open-circuit conditions, there were usually several potential steps and under short-circuit conditions the cells were negative relative to the bathing solutions. Estimates of epithelial Na concentrations utilizing radioactive Na suggested that if all epithelial Na were in a single compartment, an active entry step would be necessary to allow a net inward Na transport. The results could also be explained by a series arrangement of Na compartments without necessarily postulating an active Na entry. The behavior of the potential profile suggested that this latter alternative was more likely.The active transport of Na across isolated frog skin has been examined extensively, but in the majority of studies the outer surface of the skin has been bathed with solutions containing relatively high (115 mM) concentrations of Na. Variation in rates of Na transport and in electrical properties of the skin with changes in Na concentration has also been studied, but in most cases constant anion concentration was maintained by using other cations to substitute for Na. However, the early data of Krogh (1) indicated that intact frogs could take up Na through the skin from solutions containing only 10 6 ‫--‬ M NaCl. Since the model of the Na transport system proposed by Koefoed-Johnsen and Ussing (2) suggests that Na entry into the skin cells is a 1161 on May 12, 2018 jgp.rupress.org Downloaded from