Thymidine accumulation by choroid plexus in vitro

Abstract

In vitro, the accumulation and release of [ methyl- 3H]thymidine ([ 3H ]thymidine) by the isolated choroid plexus, the anatomical locus of the blood-cerebrospinal fluid barrier, was studied. With concentrations of [ 3H ]thymidine in the medium of 1.0 μ m (or greater), the choroid plexus accumulated [ 3H ]thymidine against a concentration gradient by a process that depended on intracellular energy production but did not depend on intracellular binding or metabolism of the [ 3H ]thymidine. This transport process was inhibited (although differentially) by various nucleosides and low temperatures but not by 2-deoxyribose or pyrimidine bases. With concentrations of less than 1.0 μ m [ 3H ]thymidine in the medium, the choroid plexus accumulated [ 3H ]thymidine against a concentration gradient. However, the majority of the [ 3H ]thymidine within the choroid plexus was metabolized to [ 3H ]thymidine nucleotides at low extracellular [ 3H ]thymidine concentrations (3 n m). This accumulation process depended, in large part, on saturable intracellular phosphorylation. Thymidine was the principal form released from choroid plexuses that had been incubated for various times in media containing concentrations of thymidine from 3 to 1.0 m m. The release of thymidine from choroid plexus was depressed by cold temperatures and a very high (2.56 mmol/kg) intracellular thymidine concentration.