The mechanism of coupled transport of sodium and chloride in isolated urinary bladder of the trout.

1. Transepithelial fluxes of Na and Cl were measured in dissected trout bladders mounted in Ussing chambers. The mucosa-to-serosa fluxes appeared to comprise a linear and a saturating (Km = 8 mM) component as a function of the luminal concentration of the ion. The over-all permeability for Cl was higher than for Na. Removal of either ion from the mucosal side induced the disappearance of the net flux of the co-ion even when the substituting ions were not impermeant. This indicated a complete and specific mutual dependence between the Na and Cl transports. A large part (30-50%) of Cl exchanges was ascribed to an exchange-diffusion process. 2. In order to localize the site of ion copuling, the apical uptakes of Na and Cl were measured by short-term exposure (2 min) to radioisotopes. The apical fluxes were similar to the transepithelial fluxes and therefore appeared to be the limiting factor for ion transfer. Ionic replacements on the mucosal side showed that the coupling between Na and Cl and Cl/Cl exchange occurs at this step.