APStracts 2:0019F, 1995.

Inner medullary external osmotic driving force in a 3-d model of the renal concentrating mechanism . Wexler, S. Randall Thomas Anthony S. INSERM U.323, Necker Faculty of Medicine, 75730 Paris FRANCE, Dept. of Mechanical Engineering, University of Delaware, Newark,DE 19716

APStracts 2:0019F, 1995.

The mechanism by which the renal medulla establishes and maintains a gradient ofosmolarity along the cortico-medullary axis, especially in the inner medulla where there is no active transmural flux out of the ascending limbs of Henle, remains a source of controversy. We show here that if realistic values of urea permeability in the inner medullary descending limbs and water permeability in the upper inner medullary section of the collecting ducts are taken into account, even a model including the 3-dimensional, vascular bundle structures (Wexler, A. S., R. E. Kalaba, & D. J. Marsh. Am. J. Physiol. 260 (Renal Fuid Electrolyte Physiol. 29): F368--F383, 1991, referred to hereafter as WKM) fails to explain the experimentally observed inner medullary osmolality gradient. We show here that this failure can be overcome by application of an external osmotic driving force, an idea recently revived by Jen & Stephenson (Bull. Math. Biol. 56(3): 491 -514, 1994) in the context of a single- solute, single loop central core model. We show that inclusion of such an external driving force with a value equivalent to at least 100 mOsm of inner medullary interstitial osmolytes in the 3D model of WKM accounts for a physiological osmolality gradient even in the face of realistic permeability values. Furthermore, inclusion of the external driving force makes the model less dependent on the positions of descending and ascending limbs of Henle with respect to the collecting ducts. In an effort to assess whether there is any experimental basis for osmolytes, we show that a significant amount of extra inner medullary interstitial osmolytes is plausible, based on extrapolation from existing experimental data.

Received 7 November 1994; accepted in final form 8 February 1995.
APS Manuscript Number F399-4.
Article publication pending Am. J. Physiol. (Renal Fluid Electrolyte
Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 February 1995.