Uptake of L‐[14C]Proline by Isolated Rat Brain Capillaries

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Abstract
Rat brain capillaries exhibit concentrative uptake of l‐proline. The uptake is mediated by two saturable systems, one with a Km of 0.11 mM and another with a Km of 5.9 mM. Entry also occurred by diffusion, especially at high substrate concentrations. The saturable high‐affinity system is sodium‐dependent, with a Km for sodium of 36 mM. Proline uptake is not inhibited by lysine, but is inhibited by phenylalanine, glycine and leucine. α‐Methylaminoisobutyric acid (MeAIB), a model for sodium‐requiring transport systems, is a competitive inhibitor of the low‐Km system. b‐2‐Aminobicyclo‐[2, 2, 1]‐heptane‐2‐carboxylic acid (BCH), a model for nonsodium‐dependent transport, however, also inhibited proline uptake.