N-System Amino Acid Transport at the Blood-CSF Barrier

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-[ 14 C]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 + -independent 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 Α-(methylamino)isobutyric acid nor by the ASC-system substrate cysteine. It was inhibited by histidine, asparagine, and l-glutamate Γ-hydroxamate, three N-system substrates. Replacement 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 V max and K max for glutamine transport by this system were 8.1 ± 0.3 nmol/mg/min and 3.3 ± 0.4 m M , respectively. This system may play an important role in the control of CSF glutamine, particularly when the CSF glutamine level is elevated as in hepatic encephalopathy.