Cerebral uptake of amino acids in human subjects and rhesus monkeys in vivo

In experiments with 11 human subjects and 12 rhesus monkeys given 14C‐labeled amino acids intravenously, blood samples were drawn simultaneously from the femoral artery (A) and the superior bulb of the internal jugular vein (V). Analyses of labeled and unlabeled free amino acids indicated that both A‐V and V‐A differences resulted with each amino acid. In no case were there consistent A‐V or V‐A differences (excepting with L‐[methyl‐14C] methionine where consistent A‐V differences resulted). However, in experiments with rhesus monkeys, from 0.2% to 2.3% of the injected 14C‐labeled amino acids was taken up by brains 90–120 minutes after injection. Therefor, it was suggested that, in humans and monkeys in vivo, small but significant amounts of amino acids enter the brain from blood over a period of time by a bidirectional exchange mechanism.