AJP - Renal Physiology, Vol 259, Issue 6 961-F970, Copyright © 1990 by American Physiological Society
Phosphate-dependent glutaminase activity in rat renal cortical and medullary tubule segments
P. A. Wright and M. A. Knepper
Laboratory of Kidney and Electrolyte Metabolism, National Institutes of Health, Bethesda, Maryland 20892.
To determine whether local production of ammonium by medullary renal tubule
segments may contribute to medullary ammonium accumulation, we measured
activities of phosphate-dependent glutaminase (PDG) in microdissected
tubule segments from rat medulla and cortex. PDG activities were very low
in medullary loop of Henle segments but surprisingly high in inner
medullary collecting duct (IMCD). In cortex, PDG levels were highest in
distal convoluted tubule and cortical thick ascending limb, but substantial
levels were also found in proximal segments, as reported previously. To
determine effects of acid loading and alkali loading on PDG activity, 0.28
M NH4Cl (acid) or 0.28 M NaHCO3 (alkali) was added to rats' drinking water
for 7 days. PDG activities in medullary segments were not affected by acid
or alkali intake. Acid intake by rats increased PDG activity in S1 and S2
proximal convoluted tubules severalfold but did not affect the other
cortical segments. We conclude that medullary loop of Henle segments
probably contribute relatively little to medullary ammonium accumulation
because of their low activities. The high PDG activity in IMCD suggests
that ammonium could be produced and secreted by this segment. However,
because total tubule length of IMCD is very low compared with proximal
tubules, it appears unlikely that IMCD contributes substantially to overall
renal ammonium production. PDG activity is regulated only in S1 and S2
proximal tubules, consistent with the view that the proximal tubule is the
major site of regulation of renal ammonium production.
