Leucine activates system a amino acid transport in l6 rat skeletal
muscle cells.
McDowell, Helen E., Graham R. Christie, Grant Stenhouse, and Harinder
S. Hundal.
Department of Anatomy & Physiology, The University of Dundee,
Dundee, DD1 4HN, Scotland, United Kingdom
APStracts 2:0211C, 1995.
In this study we present evidence showing that Leu is involved in the
up-regulation of System A amino acid transport activity in the L6 rat
skeletal muscle cell line. At Leu concentrations of > 0.05 mM the
uptake of Me-AIB, a paradigm System A substrate, was stimulated by up
to 50%. Kinetic analysis revealed that this stimulation was as a
result of an increase in the Vmax of Me-AIB uptake from 327 + 26
pmol.min/mg protein to 450 + 8 pmol.min/mg protein following
incubation of cells with Leu. No significant change in Km was
observed. System A activation was biphasic reaching an initial
plateau after 3 h with a second phase of activation being observed
after 5 h. The initial activation of System A transport occured by a
mechanism distinct from that activated by IGF-1 (3 nM) since the
effects of Leu and IGF-1 were additive. This activation was not due
to trans-stimulation since BCH, a specific System L substrate, did
not stimulate System A. Leu's keto acid, Ketoisocaproic acid,
prevented the activation of System A transport, whereas,
aminooxyacetate, a transaminase inhibitor, augmented the increase in
System A activity by Leu. Cycloheximide and Actinomycin D both
inhibited the Leu-induced increase in Me-AIB uptake. The present
results indicate that Leu or some cellular component regulated by it,
is capable of stimulating System A transport through control of DNA
transcription, possibly of a gene encoding a System A
Received 16 February 1995; accepted in final form 17 May 1995.
APS Manuscript Number C90-5.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 May 1995.