Ionic exchanges in isolated and open-circuited toad skin

Summary Net influxes of Na and Cl and effluxes of K and H (J _Na,J _Cl,J _K andJ _H) and volume flowJ _v across isolated open-circuited toad skins were measured using rotating chambers and a small volume of external solution, the ion fluxes being determined by chemical analysis of the external solution, in the range of 0.2 to 5.0mm external Na concentration. In this concentration range, with skin potential varying with (Na) _e ,J _Na is a linear function of the Na electrochemical potential difference across the skin, as expected on irreversible thermodynamic grounds. TheL _Na coefficient calculated asJ _Na/ _Na is equal to 5.5×10^–12 mole² joule^–1 cm^–2 min^–1, which is similar to values obtained for the same species in the short-circuited state and in higher ranges of (Na) _e . A positive correlation is observed betweenJ _Na andJ _K whenJ _Na varied with (Na) _e and also whenJ _Na varies in randomly selected skins. Antidiuretic hormone stimulatesJ _Na,J _K andJ _v in the range of 0.2 to 5.0mm (Na) _e and lowers the Na concentration in the equivalent solution absorbed by the skin (calculated asJ _Na/J _v ). Substitution of external Cl by SO₄ has no effect onJ _Na,J _K andJ _H and also in the skin potential in the range of (Na) _e studied. Substitution of external Na by K abolishesJ _Cl and reverses the skin polarity, the external solution now being positive to the internal one. Na removal from the external solution also reducesJ _K almost to zero.J _H is significantly reduced in this condition; however, a basal secretion still persists with (Na) _e equal to zero. The results of these experiments can be tentatively interpreted in terms of electrical coupling between ion fluxes, since only the procedures that result in alterations of skin potential are followed by changes in the rates of ion transport. The existence of other coupling mechanisms cannot be ruled out.