The effect of sodium concentration on the content and distribution of sodium in the frog skin.

1. The content and distribution of sodium in the epithelium of the frog skin (Leptodactylus ocellatus L.) have been studied.2. The inulin space, the (22)Na exchange, and the amounts of water and sodium were measured in samples of connective tissue. The results indicate that the necessary assumptions generally made to calculate the sodium and water contents of the epithelial cells as the difference between the total content in the tissue and the amounts contained in the inulin space are not valid in the frog skin.3. The mean concentration of sodium in the epithelium has been obtained from direct measurements of sodium and water in samples of epithelium. To measure the water content of the epithelium a new technique has been developed. When the skin is bathed with Ringer solution containing 115 mM-Na on both sides, the mean concentration of sodium in the epithelium is 79 mM. When the concentration of sodium in the Ringer is 1 mM the mean concentration in the epithelium is 25 mM. When the skin is bathed with Ringer with 1 mM-Na on the outside and 115 mM-Na on the inside-a situation which resembles the natural condition in the skin-the mean concentration of sodium in the epithelium is 52 mM.4. The compartmentalization of Na was studied by comparing the sodium content and the degree of exchange with (22)Na in the bathing solutions. In these experiments the skins were exposed to Ringer solutions with different concentrations of sodium, and (22)Na on one or both sides.5. The results indicate that the epithelium has a compartment of sodium which is not exchangeable in 40-80 min and whose size is not appreciably changed by a threefold change in the Na content in the epithelium and a hundredfold change in the concentration of the bathing solution.6. Sodium exchangeable in 40-80 min seems to be contained in two different compartments: (a) a large one that contains fixed sodium is mainly connected to the inside, and does not appear to participate directly in sodium transport across the frog skin; (b) a small one, that is bounded on the inside by a Na-impermeable barrier, and that seems to comprise the sodium involved in active transport. When the skin is bathed with Ringer solutions with 115 mM-Na on the inside and 1 mM-Na on the outside, the transporting compartment contains some 13% of the total sodium in the epithelium.7. The results are interpreted on the basis of a model recently proposed by Cereijido & Rotunno (1968). The major feature of this model is that the sodium transporting compartment is confined to the plasma membrane of the epithelial cells.