AJP - Gastrointestinal and Liver Physiology, Vol 256, Issue 2 384-G389, Copyright © 1989 by American Physiological Society
Ethanol stimulates leucine uptake by rat fetal hepatocytes via trans-stimulation
G. I. Henderson, T. A. Frosto, D. W. Heitman and S. Schenker
Department of Medicine, University of Texas Health Science Center, San Antonio 78284.
Prior studies showed that exposure of cultured rat fetal hepatocytes to
ethanol increased sodium-independent transport of alpha-amino-isobutyric
acid and cycloleucine. Using leucine (Leu) as a probe, we now show that
this is a reflection of trans-stimulation of system L inward flux.
Transport of Leu was entirely sodium independent and
beta-2-aminobicyclo(2,2,1)-heptane-2-carboxylic acid inhibitable. Uptake
kinetics indicated two components, likely systems L1 and L2 reported for
the adult hepatocyte. The low-affinity Km was in the 0.5 mM range, whereas
the high-affinity Km was 2% of that value. Under optimal growth conditions,
approximately 65% of the Leu was transported by the latter system. Strong
bidirectional exchange was shown with Leu loading, stimulating initial Leu
uptake by 66%. Externally directed transport was enhanced 2.9 times against
5 x 10(-3) M Leu vs. no external Leu. A 24-h exposure to ethanol (2 mg/ml)
increased Leu uptake by up to 100%, an effect that could be mimicked by
arrested cell replication. Both enhanced rates could be reversed by amino
acid depletion, reflecting intracellular amino accrual that induced
trans-stimulation of Leu uptake. Enhanced uptake was also reproduced in
replicating cells by loading with increasing concentrations of Leu.
