Ethanol stimulates leucine uptake by rat fetal hepatocytes via trans-stimulation

Prior studies showed that exposure of cultured rat fetal hepatocytes to ethanol increased sodium-independent transport of α-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 β-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 K(m) was in the 0.5 mM range, whereas the high-affinity K(m) was 2% of that value. Under optimal growth conditions, ~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 increased concentrations in Leu.