VITAMIN B6 TRANSPORT IN THE CENTRAL NERVOUS SYSTEM: IN VIVO STUDIES

Abstract— The total concentrations of vitamin B₆ (B₆) in plasma, choroid plexus, CSF and brain of adult New Zealand white rabbits, measured fluorometrically, were 0.30, 15.10, 0.39 and 8.90 μ mol/l or kg respectively. The mechanisms by which B₆ enters and leaves brain, choroid plexus and CSF were investigated by injecting ³H]pyridoxine (PIN) intravenously, intraventricularly and intraarterially. ³H]PIN, with or without unlabelled PIN, was infused intravenously at a constant rate into conscious rabbits. At 150 min, ³H]B₆ readily entered CSF, choroid plexus and brain. The addition of 0.5 mmol/kg carrier PIN to the infusion solution depressed the relative entry of ³H]B₆ into CSF, choroid plexus and brain by about 80%. After intraventricular injection, ³H]PIN readily entered brain from CSF. The intraventricular injection of carrier PIN with ³H]PIN decreased the amount of ³H]B₆ in brain and also decreased the percentage of ³H]B₆ in CSF and brain that was phosphorylated. During one pass through the cerebral circulation, ³H]PIN (1 μ m ) was cleared from the circulation no more rapidly than mannitol. These results were interpreted as showing that the entry of B₆ from blood into CSF and presumably the extracellular space of brain and thence into brain cells involves one or more saturable transport and/or metabolic steps.