Interactions between thyroid hormone and tryptophan transport in
rat liver are modulated by thyroid status.
Kemp, Helen F., and Peter. M. Taylor.
Department of Anatomy & Physiology, University of Dundee,
Dundee, DD1 4HN, Scotland
APStracts 4:0014E, 1997.
We have identified both NEM-resistant (System T) and NEM-sensitive
(System L1) [3H]L-tryptophan transporters in sinusoidal membrane
vesicles (SMVs) from euthyroid, hypothyroid (propylthiouracil
-treated) and hyperthyroid (L-T3 -injected) rats. [125I]L-T3
associates with SMVs largely by surface binding. Kinetic
characteristics of tryptophan uptake and T3 binding (transporter /
receptor abundance and substrate affinity) are not significantly
affected by thyroid status. T3 and T4 inhibit NEM-resistant
tryptophan uptake in SMVs to an extent dependent upon the thyroid
status of the donor rat, increasing in the order hypothyroid <
euthyroid < hyperthyroid; the Ki for this inhibition (0.3 mM
T3) is equal to the Kd for T3 binding. Both T3 binding and T3
inhibition of tryptophan transport in SMVs are markedly reduced by
treatments (Triton X-100 or trypsin) which do not significantly
affect vesicle integrity or transport of tryptophan and glucose. T3 /
T4 transport at the liver-plasma interface may be facilitated by
direct interactions between hormone receptors and System T
transporter proteins. Modulation of such interactions may be
important for control of hepatic T4 / T3 turnover and aromatic amino
acid metabolism during altered thyroid status.
Received 9 October 1996; accepted in final form 6 January 1997.
APS Manuscript Number E504-6.
Article publication pending Am. J. Physiol. (Endocrinol. Metab.).
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 19 February 1997