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

Abstract— The transport into and release of tritium labeled vitamin B₆ ([³H]B₆) from rabbit brain slices and isolated choroid plexuses were studied. In vitro, both brain slices and choroid plexus concentrated [³H]B₆ by an energy dependent uptake system when [³H]pyridoxine (PIN) was added to the incubation medium. Most of the [³H] within the tissues was phosphorylated [³H]B₆. In each tissue, the nonphosphorylated vitamers inhibited the uptake of [³H]PIN from the medium significantly more than the phosphorylated vitamers. The concentrations of the nonphosphorylated B₆ vitamers necessary to inhibit brain and choroid plexus uptake of [³H]PIN from the medium by 50% were approx 0.4 μm and 5–10μm respectively after a 30 min incubation. Both brain slices and choroid plexus readily released (46 and 56% respectively in 30 min) previously accumulated [³H]B₆ into artificial CSF. However, brain slices released only nonphosphorylated [³H]B₆, whereas the choroid plexus released predominantly phosphorylated [³H]B₆. Addition of unlabeled PIN to the release media significantly increased the percentage of [³H]B₆ released by both brain slices and choroid plexus. The results of these in vitro studies provide evidence that: (1) both brain slices and chloroid plexus possess specific uptake and release mechanisms for B₆, and (2) these mechanisms tend to regulate intracellular B₆ levels. These studies also suggest that the choroid plexus serves as a locus for the transfer of B₆ from blood to CSF and is the source of most of the phosphorylated B₆ in CSF.