Influence of Transepithelial Potential Difference on the Sodium Uptake at the Outer Surface of the Isolated Frog Skin

The unidirectional uptake of sodium across the outer surface of the isolated frog skin (J (12) (Na)) was measured in the presence of transepithelial potential difference (Δ(ψ)) ranging from +100 to -100 mV. With a sodium concentration of 115 mM in the bathing solutions J (12) (Na) increases significantly when the spontaneous Δ(ψ) is reduced to zero by short-circuiting the skin. With an Na concentration of 6 mM a progressive increase J (12) (Na) can be observed when Δ(ψ) is decreased in several steps from +100 to -100 mV (serosal side positive and negative, respectively). The observed change J (12) (Na) amounts to a fraction only of that predicted from the shift in Δ(ψ). The results suggest that under open circuit conditions the potential step across the outside surface is at most one half of Δ(ψ) and that the resistance across the outside and inside barrier of the skin is ohmic. This is in agreement with measurements of intracellular potentials in the frog skin and with resistance measurements carried out in the toad skin. The data strongly support the view that the saturating component of J (ψ) proceeds via a charged carrier system. Exposure to negative values of Δ(ψ) of 50 mV or more for times of 24 min or more result in a marked reduction of J (12) (Na) which shows only partial or no reversibility.