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.