AJP - Renal Physiology, Vol 253, Issue 3 563-F575, Copyright © 1987 by American Physiological Society
A model of osmotic and hydrostatic pressure effects on volume absorption in the proximal tubule
J. C. Williams Jr and J. A. Schafer
A computer model of the proximal tubule of the rabbit is described in which
the tubule is treated as a single cylindrical barrier to the flow of solute
and water between lumen and bath, and volume absorption is assumed to be
driven exclusively by hydrostatic and osmotic pressure differences across
this barrier. The model mimics the function of the tubule in two in vitro
preparations: in simulations of the isolated tubule perfused under oil, the
model correctly describes the solute concentration gradients that exist
between the perfusate and absorbate and predicts differences in solute
concentrations among absorbate droplets on the same tubule if luminal
concentration becomes limiting. This prediction was tested experimentally
with glucose and found to be correct. In simulations of the tubule perfused
in an aqueous bath, the role of transmural hydrostatic pressure was
explored; it is predicted that, at normal rates of in vitro perfusion
(approximately 10 nl/min), increases in pressure have very little effect on
volume absorption but can greatly alter the osmotic differences present
across the wall of the tubule, especially with high values of osmotic water
permeability. At high rates of perfusion, the ability of the tubule to
produce a lumen hypotonic to the bath is reduced, but the direct effects of
pressure on volume absorption become more apparent, resulting in relatively
little effect of perfusion rate on volume absorption if the osmotic water
permeability is sufficiently high. A similar relationship was seen
experimentally. In all, this simple model provides a good prediction of
function in isolated perfused tubules without any assumptions of hypertonic
compartments within the epithelium.
