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

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Abstract
To evaluate the contribution of NH4+ movement to net ammonia flux across the turtle urinary bladder, studies were designed to determine the individual permeabilities of NH3 and NH4+. The effect of varying the NH4+ concentration gradient was examined while holding that for NH3 constant or the opposite maneuver on the unidirectional flux of ammonia. 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 NH4+ permeability (4.9 .times. 10-6 cm .cntdot. s) and the NH3 permeability (2.6 .times. 10-4 cm/s) was estimated. In the absence of H+ transport the unidirectional fluxes of NH4+ 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 NH4+ 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 NH4+ flux to the same extent as that produced by acetazolamide. The turtle bladder may have a significant passive permeability to NH4+. There also is apparent coupling between H+ transport and ammonia secretion. This coupling may result from titration of NH4+ in the serosal solution to NH3 by alkali generated behind the H+ pump and the diffusion of NH3 into the mucosal solution.

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