The Frog Skin Potential

For many years the isolated surviving frog skin has been among the favorite objects of electrophysiologists and students of ion transport. This interest has turned out to be rather rewarding. Not only is the frog skin preparation interesting in its own rights, but there is mounting evidence that it can serve as a model system for the study of properties found in many epithelial and glandular tissues. In fact, certain features like the active sodium transport which is so conveniently studied in the frog skin, seem to be characteristic of most animals cells and of many plant cells as well. Since the time of Du Bois Reymond (1848) it has been known that the isolated frog skin maintains a potential difference between its outside and inside, the inside solution becoming sometimes more than 100 my positive relative to the outside. In the beginning of this century Galeotti (1904, 1907) demonstrated that the maintenance of the potential depends on the presence of sodium ions (or lithium ions), and he proposed that the potential arises from the fact that the skin is more permeable for these ions in the inward direction than in the outward direction. As this seems to violate the second law of thermodynamics, the theory was, however, discarded by his contemporaries. Nevertheless, he was not far from the truth as will appear from the following. Besides maintaining the potential difference, even when in contact with identical sodium-containing solutions, the amphibian skin also has another striking property, namely that of transporting sodium chloride from the outside to the inside solution (Huf, 1935, Krogh, 1937, 1938). That the sodium transfer is due to active transport is evidenced by the fact that it can take place against the electric as well as the chemical potential gradient (Ussing, 1949a). The chloride transfer, on the other hand, always takes place down the electrochemical potential gradient. Indeed, the rat io between the inward and outward fluxes of chloride ions is exactly what would be predicted from the simple flux equation (Ussing 1949b) for an ion moving passively through the skin, influenced only by the combined effect of concentration and electric gradients (Koefoed-Johnsen, Levi, and Ussing, 1952).