AJP - Renal Physiology, Vol 259, Issue 3 402-F407, Copyright © 1990 by American Physiological Society
Contribution of luminal ammoniagenesis to proximal tubule ammonia appearance in the rat
E. E. Simon, C. Merli, J. Herndon and L. L. Hamm
Department of Medicine, Jewish Hospital of St. Louis at Washington University, Missouri.
The contribution of luminal ammoniagenesis in the late proximal convolute
tubule (PCT) via phosphate-independent glutaminase
[gamma-glutamyltransferase (gamma-GT)] remains controversial. If this
pathway is important, it must rely on glutamine secretion, because filtered
glutamine is reabsorbed in the early PCT. The contribution of gamma-GT to
luminal ammoniagenesis was tested by use of in vivo microperfusion in
conjunction with a new microfluorometric assay for glutamate. We first
confirmed that aspartate completely blocked glutamate uptake in the PCT.
Furthermore, the gamma-GT inhibitor acivicin completely eliminated
glutamate entry, showing that passive glutamate entry was negligible. Thus
the accumulation of glutamate can be used as an estimate of luminal
glutamine deamidation. L-Phenylalanine was used to inhibit glutamine loss,
and hippurate was used to stimulate gamma-GT activity; therefore luminal
glutamine conversion to glutamate was promoted. Perfusing the tubule at 30
nl/min with a solution containing 10 mM each of hippurate, phenylalanine,
and aspartate resulted in a glutamate delivery of 1.08 +/- 0.12
pmol.min-1.mm-1. Ammonia appearance was 10-fold higher, averaging 11.5 +/-
1.3 pmol.min-1.mm-1 under these same conditions. Thus the luminal
conversion of glutamine to glutamate via gamma-GT is a small component of
total ammoniagenesis in this segment.
