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

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Abstract
The content and distribution of Na in the epithelium of the frog skin (Leptodactylus ocelleatus) were studied. The inulin space, the 22Na exchange, and the amount of water and Na were measured in samples of connective tissue. The necessary assumptions generally made to calculate the Na and water contents of the epithelial cells as the difference between the total content in the tissue and the amount contained in the inulin space are not valid in the frog skin. The mean concentration of Na in the epithelium was obtained from direct measurements of Na and water in samples of epithelium. To measure the water content of the epithelium a new technique was developed. When the skin is bathed with Ringer solution containing 115 mM-Na on both sides, the mean concentration of Na in the epithelium is 79 mM. When the concentration of Na in the Ringer is 1 mM the mean concentration in the epithelium is 25 mM. When the skin is bathed with Ringer 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 Na in the epithelium is 52 mM. The compartmentahzation of Na was studied by comparing the Na content and the degree of exchange with 22Na in the bathing solutions The skins were exposed to Ringer solutions with different concentrations of Na, and 22Na on one or both sides. The epithelium has a compartment of Na 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 hundred fold change in the concentration of the bathing solution. Sodium exchangeable in 40-80 nun. seems to be contained in 2 different compartments (a) a large one that contains fixed Na is mainly connected to the inside, and does not appear to participate directly in Na 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 Na 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 Na in the epithelium. The results are interpreted on the basis of a model recently proposed by Cereijido and Rotunno. The major feature of this model is that the Na transporting compartment is confined to the plasma membrane of the epithelial cells.