Active Sodium Transport across the Frog Skin Epithelium and its Relation to Epithelial Structure

Abstract

AbstractThe electric potential difference between the inside and outside of an isolated frog skin (10‐170 millivolts, inside positive) is maintained by active sodium transport from the outside to the inside bathing solution. If the two solutions are connected by a circuit of zero effective resistance, the skin produces for many hours a current which consists entirely of sodium ions. The mechanism is highly selective, the only ion which can to some extent replace sodium being lithium.The rate of sodium transport largely determines the oxygen consumption of the skin, the transport of 18 sodium ions requiring an excess consumption of one molecule of oxygen.As first approximation the skin potential in the presence of a non‐penetrating anion behaves as if the outward facing boundary were a sodium‐selective membrane whereas the inward facing membrane were potassium selective. The metabolic process then would be to maintain a high potassium concentration and a low sodium concentration in the epithelium cells. Evidence is given that the maintenance of a high cellular potassium concentration is an active process, but it remains uncertain whether the active sodium transport and the active potassium uptake in the cells are coupled.The anatomical structure of the skin and its relation to the electrochemical properties arc briefly discussed. Finally an interesting deviation from the rule of identity between net sodium transport and short‐circuit current is presented. When the outside bathing medium is made hyperosmotic by addition of 100‐200 millimoles of urea, the skin becomes very leaky to ions as well as to non‐electrolytes. Even sucrose goes through. The transport of sucrose is asymmetric, so that more goes in than out, even though the osmotic water stream is going in the outward direction. A model is proposed for the coupling between the inward diffusion of urea and the anomalous inward transport of sucrose. The experiment emphasises the theses that only in case of identical bathing solutions can one be sure that an asymmetrical transport of a component is active.