NIACIN AND NIACINAMIDE ACCUMULATION BY RABBIT BRAIN SLICES AND CHOROID PLEXUS IN VITRO

Abstract

Abstract The transport into and release of 14 C‐labeled niacin and niacinamide from rabbit brain slices and isolated choroid plexuses were studied. In vitro , both brain slices and choroid plexus concentrated 14 C by specific, energy‐dependent mechanisms when [ 14 C]niacinamide was added to the incubation medium. The saturable accumulation velocities, which were linear for 30 min, depended, in part, on incorporation of the [ 14 C]niacinamide into NAD. The X T and Y max for 14 C accumulation with [ 14 C]niacinamide in the medium by brain slices and choroid plexus were 0.80 μM and 1.45 μmolkg −1 (30 min) −1 , and 0.23 μM and 18.6 μmol kg −1 (30 min) −1 respectively. In vitro , the choroid plexus, unlike brain slices, vigorously concentrated 14 C by a separate, specific energy‐dependent process when 14 C niacin was added to the incubation medium. The saturable accumulation velocity, which was linear for 30 min, depended completely on the metabolism of [ 14 C]niacin. The K T and Y max for 14 C accumulation by choroid plexus with [ 14 C]niacin in the medium were 18.1 μM and 439 μmol kg −1 (30 min) −1 respectively. Whether preincubated in [ 14 C]niacin or [ 14 C]niacinamide, choroid plexus released predominantly [ 14 C]niacinamide.