Specificity of Cerebral Amino Acid Transport

Abstract

Recent work in a number of laboratories has shown that the passage of metabolites into and out of the brain occurs through a mechanism that differs from the mechanism governing the passage of foreign substances, including most drugs and dyes. It seems that diffusion, which plays a major role in the passage of foreign substances, has only an insignificant role in the passage of metabolites, which enter, and perhaps leave, the brain through mediated transport. It has been emphasized that these transport mechanisms, which determine the permeability properties of the brain towards metabolites, cannot be localized in a single membrane or in a single anatomical structure, but are most likely present in all membranes, with some differences existing between the properties of these membranes (Lajtha). Therefore, metabolite transport in the nervous system is considered a part of the brain barrier system. Although quantitative differences undoubtedly exist between brain and other organs, our knowledge is not sufficient to permit us to decide whether transport mechanisms present in brain are peculiar to this organ and are not present in other organs.