Blood–Brain Barrier Efflux Transport of Pyrimidine Nucleosides and Nucleobases in the Rat

The brain efflux index (BEI), a measurement of blood–brain barrier (BBB) efflux transport, was estimated at 15 s, 30 s, 1 min, 3 min and 10 min after intracerebral injection of [ 14 C]pyrimidines. An initial steep increase of the BEI values over time was observed for [ 14 ]uracil and [ 14 C]thymine, followed by a more moderate increase after 1 min. For the corresponding nucleosides, [ 14 C]uridine and [ 14 C]thymidine, the increase of BEI values over time was less steep and linear between 30 s and 3 min. The apparent BBB efflux clearances for [ 14 C]uridine, [ 14 C]thymidine, [ 14 C]uracil and [ 14 C]thymine were (μl/min/g): 95.2 ± 12.1, 125.3 ± 18.4, 290.4 ± 28 and 358.5 ± 32.5, respectively, which is at least several folds higher than the predicted BBB influx clearances of uridine, uracil and thymidine. Quick depletion of brain parenchyma from brain microvasculature has revealed that [ 14 C] radioactivity accumulated in brain microvessels after injection of nucleosides [ 14 C]thymidine and [ 14 C]uridine, but that was not observed when nucleobases, [ 14 C]thymine and [ 14 C]uracil, were injected. Reverse transcriptase-PCR revealed that the rat brain and liver (positive control) express dihydropyrimidine dehydrogenase, a key enzyme in pyrimidine nucleobase catabolism. Two bands representing spliced variants have been detected with the relative density of the bands (expressed relative to the density of glyceraldehyde3-phosphate dehydrogenase bands, mean ± SEM from 3 separate samples) 0.16 ± 0.06 and 0.04 ± 0.01 (brain) and 0.49 ± 0.1 and 0.07 ± 0.01 (liver). Overall, these results indicate that the net direction of pyrimidine BBB transport is the efflux transport; rapid BBB efflux transport and metabolic breakdown of pyrimidine nucleobases appear to be important for brain homeostasis.