Effects of ethanol on amino acid transport in basolateral liver plasma membrane vesicles

Abstract

Ethanol has been reported to inhibit hepatocellular processes such as gluconeogenesis and protein synthesis that depend, in part, on amino acid uptake. Since previous studies in cultured hepatocytes indicate that ethanol may have a direct and selective inhibitory effect on amino acid transport, the effects of ethanol on amino acid uptake into basolateral (sinusoidal) rat liver plasma membrane (blLPM) vesicles were examined. Uptake of [3H]alanine, [3H]leucine, and [35S]cysteine was measured by a rapid Millipore filtration technique in the presence of inwardly directed Na+ and K+ gradients and under tetramethylammonium (TMA+)- and Na+-equilibrated conditions. Ethanol preincubation produced a concentration-dependent inhibition of Na+-dependent alanine and cysteine uptake; no effect was observed on either Na+-independent alanine and cysteine uptake or Na+-independent leucine transport. Ethanol had no effect on L-alanine transport under Na+-equilibrated conditions; however, initial rates of 22Na flux were enhanced in the presence of ethanol. On the basis of differences in 2-(methylamino)isobutyrate and L-cysteine sensitivity, ethanol inhibition of Na+-dependent alanine transport in blLPM vesicles largely but not exclusively corresponded to the hormone-responsive system A for amino acid transport described in isolated hepatocytes. Kinetic analysis showed that ethanol treatment resulted in an alteration in the apparent maximum velocity of reaction (Vmax) of Na+-dependent alanine transport without affecting the apparent Km for alanine. The inhibitory effects of ethanol on the time course of Na+-dependent alanine uptake were reversible.(ABSTRACT TRUNCATED AT 250 WORDS)