Induction of BGT-1 and amino acid System A transport activities in endothelial cells exposed to hyperosmolarity

We studied the responses to hypertonicity of cultured endothelial cells fro m swine pulmonary arteries. In 0.5 osmol/kgH(2)O medium, initial cell shrin kage was followed by a regulatory volume increase (RVI), complete after 1 h , concomitant with an increase in cellular K+ content. Then the activity of amino acid transport System A increased, accompanied by an accumulation of ninhydrin-positive solutes (NPS), reaching a peak at similar to6 h. The su bsequent decline in System A activity was paralleled by an induction of the betaine-GABA transporter (BGT-1), detected as increases of BGT-1 mRNA and of transport activity, which peaked at similar to 24 h. Inhibitors of trans cription or translation prevented induction of both transport activities. T he increased expression of BGT-1, which involved activation of "tonicity-re sponsive enhancer," was inhibited by 5 mM extracellular betaine. Cellular K + concentration gradually declined after the accumulation of NPS and during the induction of BGT-1. This very effective adaptation to hypertonicity su ggests it has a physiological role.