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

Abstract

The unidirectional uptake of sodium across the outer surface of the isolated frog skin (J12Na) was measured in the presence of transepithelial potential difference (Δψ) ranging from +100 to -100 mV. With a sodium concentration of 115 mM in the bathing solutions J12Na increases significantly when the spontaneous Δψ is reduced to zero by short-circuiting the skin. With an Na concentration of 6 mM a progressive increase J12Na can be observed when Δψ is decreased in several steps from +100 to -100 mV (serosal side positive and negative, respectively). The observed change J12Na amounts to a fraction only of that predicted from the shift in Δψ. The results suggest that under open circuit conditions the potential step across the outside surface is at most one half of Δψ 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 Jψ proceeds via a charged carrier system. Exposure to negative values of Δψ of 50 mV or more for times of 24 min or more result in a marked reduction of J12Na which shows only partial or no reversibility.