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

1 Dipartimento di Medicina Sperimentale, Sezione di Patologia Molecolare e Immunologia, Università degli Studi di Parma, 43100 Parma; 2 Centro Substrati Cellulari, Istituto Zooprofilattico Sperimentale, 25125 Brescia, Italy; and 3 School of Biological Sciences, University of Sussex, Brighton BN1 9QG, United Kingdom We studied the responses to hypertonicity of cultured endothelial cells from swine pulmonary arteries. In 0.5 osmol/kgH 2 O medium, initial cell shrinkage 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 ~6 h. The subsequent 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 ~24 h. Inhibitors of transcription or translation prevented induction of both transport activities. The increased expression of BGT-1, which involved activation of "tonicity-responsive 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 suggests it has a physiological role. membrane transport; osmolyte; regulatory volume increase; volume