Brain microvessels take up large neutral amino acids in exchange for glutamine. Cooperative role of Na+-dependent and Na+-independent systems.

Some regulatory aspects of neutral amino acid transport were investigated in isolated brain microvessels, an in vitro model of the blood-brain barrier. Preloading of the microvessels with glutamine stimulated the subsequent uptake of other neutral amino acids by way of the Na+-independent L system, but had no effect on the uptake of either basic or acidic amino acids. Moreover, this stimulation was abolished when the loading step was carried out in the absence of Na+ ions or in the presence of a high concentration of alpha-methylaminoisobutyric acid, indicating that the microvessels were able to concentrate glutamine via the A system of amino acid transport. Since the presence of the A system of neutral amino acid transport has not been detected in studies of blood-brain transport performed in vivo, the A system is probably associated with the antiluminal side of brain microvessels. Our results indicate, therefore, that the concentrative Na+-dependent A system and the exchanging Na+-independent L system can cooperate in the uptake of the large neutral hydrophobic amino acids. Such a cooperation may be relevant in the pathogenesis of some neurological disturbances such as hepatic encephalopathy, in which brain glutamine concentration is unusually high.