Non-traditional effects of protein binding and hematocrit on uptake
of indocyanine green by perfused rat liver.
Ott, Peter, and Richard A. Weisiger.
Medical Department A, Rigshospitalet, 2100, Copenhagen, Department
of Medicine, University of California San Francisco, 94143-0538
California
APStracts 3:0094G, 1996.
We used a novel parameter-free approach to study the role of protein
binding in the hepatic clearance of indocyanine green (ICG) from
reconstituted pig blood by perfused rat liver. Either perfusate total
plasma protein concentration or hematocrit were changed. By analyzing
protein concentration ratios or plasma volume ratios relative to
ratios of intrinsic hepatic clearance of ICG (K), it was possible to
evaluate current models of hepatic uptake of protein bound ligands
without precise knowledge of some of the model parameters. A four
-fold increase in the total plasma protein concentration produced only
a 36% decrease K. This was substantially less than predicted by the
traditional model, where K is proportional to the free concentration
of ligand. As an unstirred water layer effect could not alone account
for the observations, the effects of binding disequilibrium in the
sinusoids or uptake directly from the bound pool had to be
considered. To discriminate, hematocrit was increased from 15 to 29%,
causing a 20% decrease in the sinusoidal plasma volume. A significant
reduction in K strongly suggested a sinusoidal binding disequilibrium
effect. The dissociation rate constant predicted by this model was
confirmed by in vitro measurement, further supporting this
interpretation. The simple experimental design and its parameter-free
evaluation provide a new tool for investigating the hepatic uptake of
protein bound ligands.
Received 22 June 1995; accepted in final form 18 April 1996.
APS Manuscript Number G275-5.
Article publication pending Am. J. Physiol. (Gastrointest. Liver
Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 8 May 96