Specificity and Sodium Dependence of the Active Nucleoside Transport System in Choroid Plexus

The transport of [ 3 H]deoxyuridine by the active nucleoside transport system into the isolated rabbit choroid plexus was measured in vitro under various conditions. Choroid plexuses were incubated in artificial CSF containing 1 μ M [ 3 H]deoxyuridine and 1 μ M nitroben‐zylthioinosine for 5 min under 95% O 2 ‐5% CO 2 at 37°C and the accumulation of [ 3 H]deoxyuridine measured. Ni‐trobenzylthioinosine was added to the artificial CSF at a concentration (1 μ M ) that did not inhibit the active nucleoside transport system but did inhibit the separate, saturable nucleoside efflux system. The active transport of deoxyuridine into the choroid plexus depended on Na + in the medium, as ouabain, substitution of Li + and cho‐line for Na + , and poly‐L‐lysine all inhibited deoxyuridine transport. Thiocyanate in place of chloride and penetrating sulfhydryl reagents also inhibited the active transport of deoxyuridine into choroid plexus. The active transport of deoxyuridine into choroid plexus, which is inhibited by naturally occurring ribo‐ and deoxyribonu‐cleosides (IC 50 = 7‐21 μ M ), was not inhibited (IC 50 > 150 μ M ) by nucleosides with certain alterations on the 2′, 3′, or 5’positions in D‐ribose or 2‐deoxy‐D‐ribose (e.g., adenine arabinoside, 3′‐deoxyadenosine, xylosyl‐adenosine); or the pyrimidine or purine rings (e.g., 6‐azauridine, xanthosine, 7‐methylinosine, or 8‐bromo‐adenosine). Other analogues were effective (IC 50 = 8‐26 μ M ; e.g., 5‐substituted pyrimidine nucleosides, 7‐deaza‐adenosine, 6‐mercaptoguanosine) or less effective (IC 50 = 46‐145 μ M ; e.g., 5‐azacytidine, 3‐deazauridine) inhibitors of deoxyuridine transport into the isolated choroid plexus.