Deoxycytidine Transport and Metabolism in Choroid Plexus

Abstract

In vitro, the transport into and release of [3H]deoxycytidine from the isolated [rabbit] choroid plexus, the anatomical locus of the blood-CSF barrier, were studied separately. By use of the ability of nitrobenzylthioinosine (NBTI) to inhibit deoxycytidine efflux from the choroid plexus, the transport of 1 .mu.M [3H]deoxycytidine into choroid plexus at 37.degree. C was measured. Deoxycytidine was transported into choroid plexus against a concentration gradient by a saturable process that depended on intracellular energy production, but not intracellular binding or metabolism. The Michaelis-Menten constant (KT) for the active transport of deoxycytidine into choroid plexus was 15 .mu.M. The active transport system for deoxycytidine was inhibited by naturally occurring nucleosides and deoxynucleosides, but not by 1 mM probenecid and 2-deoxyribose or 100 .mu.M cytosine and cytosine arabinoside. With < 1 .mu.M [3H]deoxycytidine in the medium, the choroid plexus accumulated [3H]deoxycytidine against a concentration gradient. However, .apprx. 50% of the [3H]deoxycytidine was phosphorylated to [3H]deoxycytidine nucleotides at a low extracellular [3H]deoxycytidine concentration (6 nM) in 15-min incubations. This accumulation process depended, in part, on saturable intracellular phosphorylation. Evidently, the choroid plexus contains an active nucleoside transport system of low specificity for deoxynucleosides and ribonucleosides, and a separate, saturable efflux system for deoxynucleosides which is very sensitive to inhibition by NBTI.