Cycles and separations in a model of the renal medulla.
Randall, S., Thomas.
INSERM U. 467, Necker Faculty of Medicine, 156, rue de Vaugirard,
75730 Paris Cedex 15 France.
APStracts 5:0117F, 1998.
This study gives the first quantitative analysis of net steady state
transmural fluxes of water, urea and NaCl in a numerical model of the
rat renal medulla in antidiuresis, revealing the model's predictions
of water, urea, and NaCl cycling patterns. These predictions are
compared both to in vivo micropuncture data from the literature and
to earlier qualitative proposals (e.g., Lemley, K. V. and W. Kriz.
Kidney Int. 31: 538-548, 1987) of cycling and exchange patterns based
on medullary anatomy and available permeability and transport
parameter measurements. The analysis is based on our most recent 3-D
model (Wang, X. et al. Amer. J. Physiol. 274 (Renal 43): F413-F424,
1998). In general agreement with earlier proposed patterns, this
analysis predicts: 1) important water short-circuiting from
descending structures to ascending vasa recta in most medullary
regions, 2) massive urea recycling in the upper inner medulla, 3) a
progressive increase of the ratio of urea to total osmoles along the
cortico-papillary axis, 4) urea dumped from the collecting ducts (CD)
into the deep papilla is returned to the cortex essentially via outer
medullary short vasa recta, bearing witness to a shift from the long
descending limbs and vasa recta of the inner medulla (IM) to short
structures in the outer medulla (OM). The analysis also shows that
the known radial heterogeneity of the inner stripe (IS) implies
unequal osmolalities in long descending limbs, vasa recta, and CDs
entering the IM across the OM/IM border and explains the model's
unorthodox osmolality profile along the CD. In conflict with
micropuncture evidence of a doubling of urea flow in superficial
Henle's loops (SHL) between the end proximal and early distal tubule
(Armsen, T. and H. W. Reinhardt. Pfl[umlaut]ugers Archiv 326: 270
-280, 1971), the model predicts net urea loss from SHL due to the
model's inclusion of non-negligible measured urea permeability of
medullary thick ascending limbs (Knepper, M. A. Amer. J. Physiol. 245
(Renal 14): F634-F639, 1983). We present a suite ofadjusted model
permeabilities that improves agreement with the micropuncture data on
this point. In conclusion, this modeling analysis of solute and water
recycling serves as a quantitative check on qualitative propositions
in the literature and allows closer critical comparison of model
behavior with published experimental results than was heretofore
possible.
Received 13 March 1998; accepted in final form 26 June 1998.
APS Manuscript Number F64-8.
Article publication pending Am. J. Physiol. (Renal Physiology).
ISSN 1080-4757 Copyright 1998 The American Physiological Society.
Published in APStracts on 30 July 1998