Capillary endothelial cells isolated from rat brain exhibit Na +-dependent uptake of the neutral amino acid analog α-(methylamino)isobutyric acid. Since studies in vivo demonstrate that this transport system is not present on the blood side of brain capillaries we conclude that Na+-dependent neutral amino acid transport is located on the brain side. Therefore, the luminal plasma membrane and the anti-luminal plasma membrane appear to be functionally distinct. This polarity should permit brain capillary endothelial cells to actively regulate the internal milieu of the brain.
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