Osmotic Behaviour of the Epithelial Cells of Frog Skin

The osmotic behaviour of the frog skin epithelium has been investigated by microscopic measurements of volume under different experimental conditions. Simultaneous measurements of potential and short-circuit current were made. The effects of changes in tonicity of both bathing solutions and of ionic replacements (K+ for Na+ ***SO=4 for Cl-) were studied. Furthermore the active sodium transport was stimulated by short-circuiting and by application of antidiuretic hormone, and inhibited by low pH and by g-strophanthin. The following conclusions could be drawn: 1) The outward facing boundary of the epithelium is permeable to Na+ and Cl-, but impermeable to K+ and SO=4. 2) The inward facing boundary is permeable to K+ and Cl- but practically impermeable to Na+ and SO=4. 3) The outward facing boundary is much less permeable to water than is the inward facing one. 4) Application of antidiuretic hormone to the inside bathing solution increases the water permeability of the outward facing boundary whereas the inward facing membrane is unaffected. 5) Inhibition of the active sodium transport by either g-strophanthin or by low pH in the inside bathing solution was accompanied by a pronounced decrease in the passive ion permeabilities. 6) The results strongly indicate that potassium is transported actively from the inside bathing solution into the epithelium.