Nucleoside Transporter of Cerebral Micro vessels and Choroid Plexus

The nucleoside transporter of cerebral microvessels and choroid plexus was identified and characterized using [ 3 H]nitrobenzylthioinosine (NBMPR) as a specific probe. [ 3 H]NBMPR bound reversibly and with high affinity to a single specific site in particulate fractions of cerebral microvessels, choroid plexus, and cerebral cortex of the rat and the pig. The dissociation constants ( K D 0.1–0.7 n M ) were similar in the various tissue preparations from each species, but the maximal binding capacities ( B max ) were about fivefold higher in cerebral microvessels and choroid plexus than in the cerebral cortex. Nitrobenzylthioguano‐sine and dipyridamole were the most potent competitors for [ 3 H]NBMPR binding. Several naturally occurring nucleo‐sides displaced specific [ 3 H]NBMPR binding to cerebral microvessels in vitro, in a rank order that correlated well with their ability to cross the blood‐brain barrier in vivo. Adenosine analogues and theophylline were less effective in displacing [ 3 H]NBMPR binding than in displacing adenosine receptor ligands. Photoactivation of cerebral microvessels and choroid plexus bound with [ 3 H]NBMPR followed by solubilization and polyacrylamide gel electrophoresis labeled a protein(s) with a molecular weight of 60,000. These results indicate that cerebral microvessels and choroid plexus have a much higher density of the nucleoside transporter moiety than the cerebral cortex and that this nucleoside transporter has pharmacological properties and a molecular weight similar to those of erythrocytes and other mammalian tissues.