Relationship between Osmotic Reactions and Active Sodium Transport in the Frog Skin Epithelium

The osmotic behaviour of the skin epithelium of the brown frog (R. temp.) has been investigated by microscopic measurement of thickness under different experimental conditions. Simultaneous measurements of skin potential and short‐circuit current were made. Epithelial swelling was induced by hypotonicity of the inside but not the outside bathing solution, whereas shrinkage was brought about by hypertonicity of either inside or outside bathing solutions, although the tonicity of the inside bathing solution dominates the osmotic reponse. Shrinkage can also be induced by lowering the potassium concentration of the inside solution. Complex shrinkage and swelling reactions are produced by addition of diffusible substances like urea to either bathing solution. In practically every instance shrinkage leads to inhibition of the short‐circuit current (and thus of the active sodium transport), whereas swelling of the epithelium gives rise to increased active sodium transport. The skin potential is related to swelling and shrinkage in a more involved fashion. In general hypertonicity of the outside bathing solution leads to a violent drop in skin potential. The observations indicate that hypertonicity of the outside solution increases the permeability of the outer epithelial boundary to passively diffusing electrolytes and to water.