The effect of antidiuretic hormone on Na movement across frog skin

1. The effect of antidiuretic hormone (ADH) on the movement and distribution of Na was studied. This was done using three different approaches: (a) the measurement of Na and (22)Na in slices of epithelium of skins which were exposed to Ringer of varied composition containing (22)Na, (b) the measurement of the influx of Na from the outer to the inner bathing solution with (22)Na added to the outside, and (c) the use of a recently introduced technique which permits the direct evaluation of the flux from the outer solution --> epithelium, (J(OT)), i.e. the flux across the barrier which is generally regarded as the site of ADH activity.2. ADH increased the influx from the outer to the inner bathing solution of Na (50%) not only when the concentration of Na on the outside was 115 mM (i.e. higher than in the epithelium) but even when the concentration was 1 mM (67%).3. When the skin was bathed with 1mM-Na Ringer on the outside, ADH increased the unidirectional Na flux J(OT) by 56% (Rana pipiens) and 71% (Leptodactylus ocellatus). When the concentration was 115 mM a small increase (17%) was observed in paired skins of R. pipiens. Under this condition no change was observed in L. ocellatus.4. The amount of epithelial sodium which is labelled by (22)Na added to the outside was taken to reflect the amount of Na involved in Na transport across the epithelium. Depending on whether the concentration of Na on the outside was high (115 mM) or low (1 mM), ADH produced an increase, or a decrease, of both the total Na content and the amount of (22)Na exchanged.5. When the concentration of Na on the outside was low, ADH increased the total influx and J(OT) in spite of the fact that it lowers the total Na content and does not affect the exchangeable pool of Na. This observation is inconsistent with the view that the effect of ADH is due to the fact that the increased permeability of the outer barrier allows more Na into the cell, and that the resulting increase of Na concentration in the cytoplasm accelerates the Na pumps at the inner side of the cells.6. It is concluded that ADH speeds up Na movements at the outward facing barrier, and that this exchange which facilitates the penetration of Na into a transporting compartment produces also a gain or a loss of Na in compartments not directly involved in Na transport across the epithelium. One compartment which is not involved in Na transport might be the cytoplasm of the epithelial cells.