Sodium-dependent and Equilibrative Nucleoside Transport Systems in L1210 Mouse Leukemia Cells: Effect of Inhibitors of Equilibrative Systems on the Content and Retention of Nucleosides

Abstract

The presence of 10 µm dipyridamole in incubation media of L1210/C2 cells decreased initial rates of zero-trans influx of formycin B (FB, 50 µm), a poorly metabolized inosine analogue, from 4.84 pmol/µl cell water/s to 0.87 pmol/µl cell water/s. However, after a 5-min interval of uptake, free FB levels in dipyridamole-treated cells were 165 pmol/µl cell water, 2.3-fold greater than in dipyridamole-free cultures. This indicated the presence of a concentrative, dipyridamole-insensitive nucleoside transport (NT) system in L1210 cells, in addition to the equilibrative NT systems known to be expressed in these cells. The concentrative system was demonstrable only in the presence of NT inhibitors and required extracellular Na⁺. The presence of 8 µm 6-[(4-nitrobenzyl)thio]-9-β-d-ribofuranosylpurine or 15 µm dilazep also induced an accumulation of free FB above steady-state levels, although of a lesser magnitude than that observed with dipyridamole. It appears that NT inhibitors induced nucleoside accumulation by inhibiting bidirectional nucleoside movements mediated by the equilibrative component of nucleoside transport in L1210/C2 cells without interfering with inward FB fluxes mediated by the Na⁺-dependent transporter. The presence of NT inhibitors also enhanced the cellular accumulation and retention of arabinosyladenine and its 5′-triphosphate in these cells. The increased cellular accumulation of 9-β-d-arabinofuranosyladenine and 9-β-d-arabinofuranosyladenine triphosphate by dipyridamole was associated with enhanced antiproliferative activity of 9-β-d-arabinofuranosyladenine towards the leukemia cells.