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 X 10(-4) mumol/s/g for Vmax, 0.011 mumol/ml for Km, and 1.8 X 10(-4) ml/s/g for KD during perfusion with fluid that did not contain competing amino acids. D-Phenylalanine competitively inhibited L-phenylalanine transport with a Ki approximately 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 approximately 20-fold, to 0.21 mumol/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.
D000596:Amino Acids / Q000097:blood; D000818:Animals; D000983:Antipyrine / Q000031:analogs & derivatives / Q000378:metabolism; D001692:Biological Transport; D001812:Blood-Brain Barrier; D001921:Brain / Q000098:blood supply / Q000378:metabolism*; D002199:Capillary Permeability; D002421:Caudate Nucleus / Q000378:metabolism; D006624:Hippocampus / Q000378:metabolism; D007700:Kinetics; D008297:Male; D010296:Parietal Lobe / Q000378:metabolism; D010477:Perfusion; D010649:Phenylalanine / Q000097:blood / Q000378:metabolism*; D051381:Rats; D012965:Sodium Chloride; D012996:Solutions; D013395:Sucrose
D000596:Amino Acids; D012996:Solutions; D012965:Sodium Chloride; D010649:Phenylalanine; D013395:Sucrose; D000983:Antipyrine; C008135:iodoantipyrine