(c) 1995 International Society for Neurochemistry

Abstract: Despite L-glutamine being the most abundant amino acid in CSF, the mechanisms of its transport at the choroid plexus have not been fully elucidated. This study examines the role of L-, A-, ASC-, and N-system amino acid transporters in L-[ ' 4C]glutamine uptake into isolated rat choroid plexus. In the absence of competing amino acids, approximately half the glutamine uptake was via a Na'-dependent mechanism. The Na'-inde-pendent uptake was inhibited by 2-amino-2-norbornane carboxylic acid, indicating that it is probably via an L-system transporter. Na'-dependent uptake was inhibited neither by the A-system substrate a-(methylamino)iso-butyric acid nor by the ASC-system substrate cysteine. It was inhibited by histidine, asparagine, and L-glutamate y-hydroxamate, three N-system substrates. Replace-ment of Na ' with Li ' had little effect on uptake, another feature of N-system amino acid transport. These data therefore indicate that N-system amino acid transport is present at the choroid plexus. The Vrnax and Km for gluta-mine transport by this system were 8.1 ± 0.3 nmol/mg/ min and 3.3 ± 0.4 mM, respectively. This system may play an important role in the control of CSF glutamine, particularly when the CSF glutamine level is elevated as i