Rates of exchange of free amino acids between plasma and brain in mice

The rate of appearance of label in the brain in mice following the intraperitoneal or intravenous injection of tracer doses of amino acids was measured in short time periods (1-8 min). Amino acid flux varied between 2 and 10 nmol/min per g brain for the amino acids used. Defining half-life as the uptake of labeled amino acid amounting to 50% of endogenous levels, a short half-life (between 3 and 30 min) was found for the essential amino acids. The half-life of the nonessential amino acids varied between 2 and 24 h, depending on their level in brain. Flux (exchange) of an amino acid was increased when the level of amino acids belonging to the same transport class was increased by intracerebral injection. Protein-free diet resulted in decrease in some amino acids, increase in others; flux was altered parallel to changes in brain levels in animals on this diet. The stercospecificity of exchange and the substrate specificity of effects of altered brain amino acids indicate that exchange occurs via mediated transport. Mediated exchange was present in immature brain. Heteroexchange (flow of one amino acid causing the counterflow of a related amino acid) may play an important part in cerebral homeostasis.