Osmotic regulation of ATA2 mRNA expression and amino acid transport SystemA activity

When porcine endothelial cells were exposed to hypertonicity, both the leve l of ATA2 (amino acid transporter 2) mRNA and activity of amino acid transp ort System A increased transiently, peaking after about 6 and 9 h, respecti vely. Cycloheximide, like actinomycin D, prevented both responses, showing that an earlier step also involves protein synthesis. Withdrawal of hyperto nicity after 6 h increased the rate of down regulation. These findings conf irm that ATA2 is a major isoform of System A and Show that changes in the e xpression of ATA2 mRNA precede both the induction and subsequent down regul ation of transport activity. (C) 2001 Academic Press.