Polarity of the blood-brain barrier: neutral amino acid transport into isolated brain capillaries.

Capillary endothelial cells isolated from rat brain exhibit Na+-dependent uptake of the neutral amino acid analog alpha-(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 antiluminal plasma membrane appear to be functionally distinct. This polarity should permit brain capillary endothelial cells to actively regulate the internal milieu of the brain.