A Na(+)-dependent nucleoside transporter in microglia.

In the central nervous system, HIV-1 has a defined tropism for brain macrophages and microglia. Nucleoside analog drugs such as zidovudine improve the clinical and neuropsychological functions in HIV-demented patients. Multiple carrier-mediated transport systems can play an important role in the membrane permeation of nucleosides and nucleoside analog drugs in a number of cells. The purpose of this project was to characterize the uptake properties of the pyrimidine nucleoside probe thymidine by a continuous rat microglia cell line (MLS-9) grown as a monolayer on an impermeable substratum. Approximately 50% of thymidine (10 microM) uptake by the monolayer cells was found to be Na(+) dependent. Kinetics of specific thymidine uptake showed a single saturation system (K(m) = 44 microM at 37 degrees C) and a Na(+)/thymidine stoichiometry of 2:1. Pyrimidine and purine nucleoside probes (50 microM) exerted a competitive inhibitory effect on specific thymidine uptake with K(i) values of 40, 38, 45, and 39 microM for adenosine, uridine, guanosine, and cytidine, respectively. In addition, nucleoside analog drugs significantly decreased specific thymidine uptake, with IC(50) values of 135.1 microM for abacavir and 0.6 microM for zidovudine, which inhibited in a noncompetitive manner. These results suggest that a Na(+)-dependent nucleoside transport system is present in rat microglia and that long-range interactions between antiretroviral nucleoside analog drugs and the nucleoside substrates may occur at the transporter sites.