Ammonia transport by the turtle urinary bladder

Abstract

Ammonia transport across the turtle bladder was examined by adding NH4Cl to the serosal (S) or mucosal (M) solution. With appropriately fixed levels of pH and/or NH4Cl concentration the transepithelial flow of ammonia parallels the extracellular concentration of NH+4 while that of NH3 is kept constant and parallels the extracellular concentration of NH3 while that of NH+4 is kept constant. This suggests that NH+4 as well as NH3 traverses the bladder wall. The apparent S----M permeability to NH3 was 15-18 times greater than that to NH+4. At pH 6.4 in both S and M solutions, the net flow of ammonia was from S----M, but at pH 8.4 in the S and 6.4 in the M or vice versa ammonia transport was of the same magnitude in both directions. The relative permeability of the M membrane to NH+4 was less than that to Na and nearly the same as that to K. The relative permeability of the S membrane to NH+4 was greater than that to K. At S pH of 8.4 and M pH of 6.4, ammonia transport was a linear function of NH4Cl concentration. At S pH of 6.4 and M pH of 6.4, ammonia transport was a saturation function of NH4Cl concentration in that it was linear up to 5 mM and constant and maximal in excess of 7.5 mM. The net transport of methylamine directed from S to M was competitively inhibited by NH4Cl, suggesting that the two substances are transported through a common carrier system. At pH 6.4 in both S and M, the S addition of NH4Cl induced an increase in reverse short-circuit current, the magnitude of which approximated the chemically determined rate of ammonia transport. This means that ammonia transport at pH 6.4 is, at least in part, electrogenic due to the flow of ionic NH+4 through a transbladder conductive path. However, when the S pH was raised to 8.4 the increase in ammonia transport was not associated with an increase in current. The present study demonstrates that the turtle bladder is capable of transporting ammonia with different characteristics of NH+4 and NH3 transport.