Hydrosmotic salt effect in toad skin: urea permeability and glutaraldehyde fixation of water channels.

The "hydrosmotic salt effect" (HSE), the reversible dependence of skin osmotic water permeability upon the ionic concentration of the outer bathing solution, is known to induce the appearance of sucrose-impermeable pathways in the apical membrane of the outermost epithelial cell layer. Diffusional 14C-urea permeability, measured in the Jv = O condition to prevent solvent drag effects, indicates that the newly formed pathways induced by HSE are narrower than the size of the urea molecule, being therefore highly selective for water molecules. After mild glutaraldehyde (2% solution) fixation of the apical membrane structures, the water channels induced by the HSE are no longer affected by the ionic strength of the outer solution. This indicates that the channel-forming membrane protein can be fixed in different configurations with the water channels in the open or closed states.