It is a well known fact that the isolated surviving frog skin, when in contact with Ringer solution on both dides, maintains for many hours a considerable potential difference between the inside and the outside, the inside being often more than 100 mV positive relative to the outside. I n previous papers the hypothesis was set forth that this potential difference was due mainly to an active and specific transport of Na ions inward, giving the inside solution a positive charge which in turn would attract C1 ions (and repel K ions), so t h a t the net result would be a transfer of XaC1 from the outside to the inside and to a lesser extent an exchange of outside Na against inside K. This view was based on the observation that the S a ion is the only ion so far studied which always moves from a lower to a higher electrochemical potential when the skin is alive and in contact with S a + containing solutions on both sides. This view has been strengthened recently by a study of the behaviour of the C1 ion in the isolated frog skin (KOEFOED JOHSSES, LEVI and USSING, 1951). It turned out that the C1 ion moves in an entirely passive way (compare CSSIKG, 1949 b) so that the electrochemical potential difference is the only force available for its transfer across the skin.However, even if the diffusion of C1 ions influences the skin POtential only in so far as it represents a short-circuit of the E. If. F. set up by the active transport of Na, the behaviour of the less abundant ions present in the system, notably those formed in the
Summary.1. The outside of the isolated frog skin in the absence of penetrating anions behaves over a wide range of concentrations as a sodium electrode, while the inward‐facing surface behaves like a potassium electrode.2. A theory concerning the origin of the frog skin potential is put forward based on these observations and the assumption that the active transport of sodium is located at the inward‐facing membrane of the epithelial cells.3. Reasons are given for the hypothesis that the active transport of sodium in reality is a forced exchange of sodium against potassium.
Summary. It is shown that for a free ion diffusing through a membrane the ratio between the flux in one direction and the simultaneous flux in the opposite direction is independent of the structure of the membrane. This ratio , which can be evaluated through tracer experiments, is equal to the ratio between the electrochemical activities of the ion on the two sides of the membrane. Deviations from the equation indicate that the ion does not diffuse in the free state only, but, in part at least, as a component of some other moving particle in the membrane. Complex formation for instance as a part of an active transport mechanism brings about deviations from the above equation. The diffusion of iodide through the isolated surviving frog skin is studied by means of the radioactive I131. I‐ diffuses inwards faster than outwards under all conditions studied. Nevertheless, no active transport of I‐ need be postulated since the potential difference across the skin is somewhat higher than required to explain the difference in diffusion rate. Both influx and outflux of I‐ show a negative correlation to the potential difference, so that high P. D. values are found only when the I‐ permeability is low.
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: The outward facing boundary of the epithelium is permeable to Na+ and Cl‐, but impermeable to K+ and SO=4. The inward facing boundary is permeable to K+ and Cl‐ but practically impermeable to Na+ and SO=4. The outward facing boundary is much less permeable to water than is the inward facing one. 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. 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. The results strongly indicate that potassium is transported actively from the inside bathing solution into the epithelium.
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