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Phenylalanine Transport Across the Blood-Brain Barrier as Studied with the In Situ Brain Perfusion Technique

Authors

  • Seiji Momma,

    1. Laboratory of Neurosciences, National Institute on Aging, National Institutes of Health, Bethesda, Maryland, U.S.A.
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  • Masaki Aoyagi,

    1. Laboratory of Neurosciences, National Institute on Aging, National Institutes of Health, Bethesda, Maryland, U.S.A.
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  • Stanley I. Rapoport,

    1. Laboratory of Neurosciences, National Institute on Aging, National Institutes of Health, Bethesda, Maryland, U.S.A.
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  • Quentin R. Smith

    Corresponding author
    1. Laboratory of Neurosciences, National Institute on Aging, National Institutes of Health, Bethesda, Maryland, U.S.A.
      Address correspondence and reprint requests to Dr. Q. R. Smith at Laboratory of Neurosciences, National Institute on Aging, NIH, Building 10, Room 6C103, Bethesda, MD 20892, U.S.A.
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Address correspondence and reprint requests to Dr. Q. R. Smith at Laboratory of Neurosciences, National Institute on Aging, NIH, Building 10, Room 6C103, Bethesda, MD 20892, U.S.A.

Abstract

Abstract: Unidirectional L-phenylalanine transport into six brain regions of pentobarbital-anesthetized rats was studied using the in situ brain perfusion technique. This technique allows both accurate measurements of cerebrovascular amino acid transport and complete control of perfusate amino acid composition. l-Phenylalanine influx into the brain was sodium independent and could be described by a model with a saturable and a nonsaturable component. Best-fit values for the kinetic constants in the parietal cortex equaled 6.9 × 10−4 Mmol/s/g for Vmax, 0.011 μmol/ml for Km, and 1.8 × 10−4 ml/s/g for Kd during perfusion with fluid that did not contain competing amino acids. d-Phenylala-nine competitively inhibited l-phenylalanine transport with a Ki∼ 10-fold greater than the Km for l-phenylalanine. There were no significant regional differences in Km,Kd, or Ki, whereas Vmax was significantly greater in the cortical lobes than in the other brain regions. l-Phenylalanine influx during plasma perfusion was only 30% of that predicted in the absence of competing amino acids. Competitive inhibition increased the apparent Km during plasma perfusion by ∼20-fold, to 0.21 pmol/ml. These data provide accurate new estimates of the kinetic constants that describe L-phenylalanine transport across the blood-brain barrier. In addition, they indicate that the cerebrovascular transfer site affinity (1/Km) for L-phenylalanine is three- to 12-fold greater than previously estimated in either awake or anesthetized animals.

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