title: 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-[14C]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 alpha-(methylamino)isobutyric acid nor by the ASC-system substrate cysteine. It was inhibited by histidine, asparagine, and L-glutamate gamma-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 Vmax and Km for glutamine 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 in hepatic encephalopathy.
authors.affiliation: Department of Surgery, University of Michigan, Ann Arbor 48109-0532, USA.
authors.forename: R F
authors.initials: RF
authors.lastname: Keep
authors.forename: J
authors.initials: J
authors.lastname: Xiang
mesh_terms: D000596:Amino Acids / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport; D001812:Blood-Brain Barrier; D002555:Cerebrospinal Fluid / Q000502:physiology*; D002831:Choroid Plexus / Q000378:metabolism; D005973:Glutamine / Q000493:pharmacokinetics; D007700:Kinetics; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000502:physiology
chemical_list: D000596:Amino Acids; D005973:Glutamine; D012964:Sodium