Mechanism of ammonia secretion by cortical collecting ducts of rabbits
Knepper, M.A.; Good, D.W.; Burg, M.B.
American Journal of Physiology 247(5 Pt 2): F729-F738
1984
ISSN/ISBN: 0002-9513 PMID: 6093587 Document Number: 233690
The collecting duct system is a major site of ammonia addition to the tubule fluid. To study the mechanisms involved, total ammonia and total CO2 transport were studied in isolated, perfused cortical collecting ducts (CCD) from deoxycorticosterone- (DOC) treated rabbits. Perfusate and bath solutions contained 25 meq/l 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, CCD from DOC-treated rabbits was perfused with carbonic anhydrase (CA) (0.1 mg/ml). Without CA, there was net total ammonia secretion (-2.2 pmol .cntdot. min-1 .cntdot. mm-1) and net total CO2 secretion (-16.6 pmol .cntdot. min-1 .cntdot. mm-1). Luminal CA converted the net total ammonia secretion to net absorption (1.0 pmol .cntdot. min-1 .cntdot. mm-1) while the bicarbonate secretion persisted (-11.2 pmol .cntdot. min .cntdot. mm-1). Total ammonia secretion in these tubules occurs primarily by diffusion of NH3 and is dependent on luminal acid pH disequilibrium.