FURTHER CHARACTERIZATION OF THE SODIUM-DEPENDENT NUCLEOSIDE TRANSPORTER (N3) IN CHOROID-PLEXUS FROM RABBIT

The Na+/nucleoside cotransporter in rabbit choroid plexus differs from Na+/nucleoside cotransporters in other tissues in terms of substrate selectivity and stoichiometry. The overall goal of this study was to f urther characterize the kinetics of this system (N3). Choroid plexus t issue slices obtained from rabbit brain were depleted of ATP and treat ed with valinomycin and K+. Na+/thymidine uptake at 30 s in the presen ce of an inside negative potential difference was significantly greate r than in the absence of a potential difference. Na+/thymidine uptake was not significantly affected by replacing chloride with either thioc yanate or sulfate. The K-m of Na+/guanosine uptake was 149, 85.2 and 3 0.5 mu M in the presence of a 25, 50 and 100 mM Na+ gradient, respecti vely, whereas the V-max was unaffected, suggesting that Na+ binds firs t to the cotransporter, then, the nucleoside. Therapeutically relevant base-modified nucleoside analogs, 5-fluorouridine, 2-chloroadenosine and 5-iododeoxyuridine, significantly inhibited Na+/thymidine uptake w ith IC,, values (mean +/- S.E.) of 12.0 +/- 2.3, 21.3 +/- 2.2 and 24.4 +/- 2.1 mu M, respectively, whereas nucleoside analogs structurally m odified on the ribose ring, 3'-azidothymidine, dideoxyinosine and dide oxycytidine (100 mu M) did not. These studies suggest that Na+/nucleos ide cotransport in the choroid plexus is electrogenic and is not depen dent on chloride. This cotransporter, which is present in choroid plex us but not in renal brush-border membrane vesicles from rabbit, may pl ay a role in the disposition of clinically relevant base-modified nucl eoside analogs into and out of the brain.