Requirement of Na+ and K+ for the action of antidiuretic hormone on water permeability.

Previous reports have shown that in the toad bladder the absence of serosal Na+ or K+ inhibits the action of antidiuretic hormone (ADH) on the osmotic permeability (Pf) but not on diffusional permeability (PDW) to water. This dissociation could be due to unstirred layers with low PDW, precluding detection of changes in the PDW of the cells. When serosal Na+ was replaced by choline, Pf (micrometer/s) was inhibited from 201 to 65. In the same conditions, if appropriate corrections were made to allow for the PDW of unstirred layers in the bulk phase and stroma, the PDW (micrometers/s) of the cellular pathway decreased from 19.8 to 15.9. The Pf/PDW ration then became 10 in the presence of serosal Na+ and 11 in its absence. When serosal K+ was deleted Pf decreased from 197 to 127 and PDW (corrected for unstirred layers) from 19.8 to 13.1 The Pf/PDW ratio was 10, both in the presence and absence of serosal K+. In conclusion, it is impossible to estimate the effect of any given experimental manipulation on the ADH-induced increase in PDW and compare it with the effect of Pf unless attention is paid to the restrictions upon measurements of diffusion imposed by unstirred layers. The deletion of Na+ or K+ from the serosa inhibits the effect of ADH on Pf and PDW to the same extent. Therefore, their presence in the serosa is essential for a full effect of ADH on the permeability to water of the apical membrane.