Characteristics of NH4+ and NH3 transport across the isolated turtle urinary bladder

To evaluate the contribution of NH+4 movement to net ammonia flux across the turtle urinary bladder, studies were designed to determine the individual permeabilities of NH3 and NH+4. In these studies we examined the effect of varying the NH+4 concentration gradient while holding that for NH3 constant or the opposite maneuver on the unidirectional flux of ammonia. It was demonstrated that the changes in the unidirectional flux of ammonia was a linear function of the change in the concentration of the varied species. From the slope of these relationships we estimated the NH+4 permeability (4.9 X 10(-6) cm . s-1) and the NH3 permeability (2.6 X 10(-4) cm . s-1). In the absence of H+ transport the unidirectional fluxes of NH+4 were equal, and these fluxes changed in a predictable manner with the application of transepithelial electrical potentials. In the presence of H+ transport there was net NH+4 secretion due to an increase in the serosal-to-mucosal flux. Net secretion could be abolished by inhibiting H+ transport with acetazolamide. The addition of NH4Cl to the serosal solution resulted in a decrement in the rate of mucosal acidification. The subsequent addition of nonvolatile buffer to the serosal solution restored H+ transport and reduced the secretory NH+4 flux to the same extent as that produced by acetazolamide. We conclude that the turtle bladder has a significant passive permeability to NH+4. There also is apparent coupling between H+ transport and ammonia secretion. This coupling may result from titration of NH+4 in the serosal solution to NH3 by alkali generated behind the H+ pump and the diffusion of NH3 into the mucosal solution.