Ammonia transport by the turtle bladder: relationship to H+ secretion

Abstract

The turtle bladder is apparently capable of transporting ammonia in the form of NH3 as well as NH+4. In the present study we examined the relationship of ammonia transport into the mucosal solution to H+ secretion and to chemical reactions in the unstirred layers. The relationship between ammonia transport and H+ secretion was examined before and after inhibition of H+ secretion by acetazolamide, SITS, and the putative inhibitor of the H+ pump dicyclohexylcarbodiimide. All these inhibitors caused a significant decrease in H+ secretion and led to a parallel decrease in the rate of ammonia transport with serosal pH at 6.4. Stimulation of H+ secretion by 1% CO2 increased both H+ secretion and ammonia transport at serosal pH 6.4. At serosal pH 6.4 the changes in H+ secretion were highly correlated with changes in ammonia transport. Ammonia transport at serosal pH 6.4 was electrogenic and inhibited by a low mucosal pH. In contrast, the diffusion of NH3 down an imposed concentration gradient was not correlated with changes in H+ secretion. Chemical reactions in the unstirred layers seem to influence ammonia transport in that increasing serosal nonvolatile buffer concentrations decreased the diffusion of NH3 down the concentration gradient from the serosal into the mucosal solution, probably by decreasing serosal NH3 concentration. Conversely, addition of uncouplers in the mucosal solution in an attempt to decrease the H+ concentration in the mucosal solution unstirred layer decreased the ammonia transport at serosal pH 6.4.