Pathways for Chloride and Sodium Transport across Toad Skin

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Abstract
The voltage dependencies of Na and Cl fluxes were investigated in the isolated toad skin. With Cl-Ringer''s on both sides Cl outflux varied very little with clamping voltage. The same was true for the influx of Cl at depolarizing voltages (.psi.i-.psi.o < 0 mV), whereas hyperpolarization led to a marked increase in this flux. A net Cl influx under short circuit conditions indicated active transport of Cl. The outflux of Cl was a saturable function of outside Cl concentration. The outflux of urea was hardly affected by raising outside Cl concentration, and the effect of varying outside nitrate concentration on Cl outflux was likewise small. Both influx and outflux could be inhibited partially by acetazolamide. The outflux of Na varied with the clamping voltage as would be expected for an ion transported by electrodiffusion, and amiloride had no effect on Na outflux, thus the Na outflux route is paracellular. In the absence of Cl in the outside solution a small outflux of Cl persists. Also this flux varied with the clamping voltage according to laws for electrodiffusion. The variation of the ratio of Na outflux to Cl outflux with clamping voltage indicated free passive diffusion of both these ions. A comparison of the outfluxes of Na and Cl in skins bathed with gluconate Ringer''s outside showed that the outflux route of these 2 ions was cation selective (PNa/PCl = 1.88). When the paracellular leak pathway of the skins was opened by exposing the outside to hyperosmolar urea solutions, the ratio of the transport numbers was TNa/TCl = 1.71. The roles of the 2 ions in determining the steady state current-voltage relationships were compared. At hyperpolarizing voltages most or all of the clamping current was carried by an inward Cl flux. By depolarization Na influx plays an increasing role with increasing depolarization. Under short circuit conditions active Cl transport contributed to the short circuit current.