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

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 V max , 0.011 μmol/ml for K m , and 1.8 × 10 −4 ml/s/g for K d during perfusion with fluid that did not contain competing amino acids. d‐Phenylala‐nine competitively inhibited l‐phenylalanine transport with a K i ∼ 10‐fold greater than the K m for l‐phenylalanine. There were no significant regional differences in K m ,K d , or K i , whereas V max 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 K m 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/ K m ) for L‐phenylalanine is three‐ to 12‐fold greater than previously estimated in either awake or anesthetized animals.