Dependence of chloride permeability on sodium in the isolated frog skin

By substituting impermeable SO4 ions for 90% of Cl, the response of the potential difference across an isolated frog skin to a tenfold reduction in Cl concentration bathing the external (epithelial) surface was measured while maintaining a constant composition of all other ions. In the presence of Na the external surface does not behave like a Cl electrode, presumably because of the presence of permeable Na ions. However, in the absence of Na (replaced by the relatively impermeable K or Mg ions) the potential response to a tenfold dilution in Cl is much smaller than in its presence. Measurements of Cl36 flux indicate that the Cl conductance is depressed when Na is withdrawn. The results suggest a dependence of the Cl permeability of the external barrier on Na ions.