Mechanism of ammonia secretion by cortical collecting ducts of rabbits

The collecting duct system is a major site of ammonia addition to the tubule fluid. To study the mechanisms involved, we measured total ammonia and total CO₂ transport is isolated, perfused cortical collecting ducts (CCD) from deoxycorticosterone-(DOC) treated rabbits. Perfusate and bath solutions contained 25 meq/liter HCO₃ and 4 mM total ammonia. Net fluid transport was not significantly different from zero. Net secretion of total CO₂ occurred in all tubules (mean collected concentration, 44.2 mM). Despite bicarbonate secretion, there was net secretion of total ammonia (mean collected concentration, 6.4 mM). There was no detectable ammonia addition to the collected fluid when ammonia was excluded from the perfusate and bath, ruling out a major contribution from synthesis. Ouabain did not significantly affect net transport of total ammonia or total CO₂. To test the hypothesis that an acid pH disequilibrium may lower the luminal pH enough to drive ammonia secretion by nonionic diffusion, we perfused CCD from DOC-treated rabbits with carbonic anhydrase (CA) (0.1 mg/ml). Without CA, there was net total ammonia secretion (-2.2 pmol·min^-1·mm^-1). Luminal CA converted the net total ammonia secretion to net absorption (1.0 pmol·min^-1·mm^-1) while the bicarbonate secretion persisted (-11.2 pmol·min·mm^-1). We conclude that total ammonia secretion in these tubules occurs primarily by diffusion of NH₃ and is dependent on a luminal acid pH disequilibrium.