Hypoxanthine enters human vascular endothelial cells (ECV 304) via the nitrobenzylthioinosine-insensitive equilibrative nucleoside transporter

The transport properties of the nucleobase hypoxanthine were examined in the human umbilical vein endothelial cell line ECV 304. Initial rates of hypoxanthine influx were independent of extracellular cations: replacement of Na⁺ with Li⁺, Rb⁺, N-methyl-D-glucamine or choline had no significant effect on hypoxanthine uptake by ECV 304 cells. Kinetic analysis demonstrated the presence of a single saturable system for the transport of hypoxanthine in ECV 304 cells with an apparent K(m) of 320 ± 10 μM and a V(max) of 5.6 ± 0.9 pmol/10⁶ cells per s. Hypoxanthine uptake was inhibited by the nucleosides adenosine, uridine and thymidine (apparent K(i) 41 ± 6, 240 ± 27 and 59 ± 8 μM respectively) and the nucleoside transport inhibitors nitrobenzylthioinosine (NBMPR), dilazep and dipyridamole (apparent K(i) 2.5 ± 0.3, 11 ± 3 and 0.16 ± 0.006 μM respectively), whereas the nucleobases adenine, guanine and thymine had little effect (50% inhibition at > 1 mM). ECV 304 cells were also shown to transport adenosine via both the NBMPR-sensitive and -insensitive nucleoside carriers. Hypoxanthine specifically inhibited adenosine transport via the NBMPR-insensitive system in a competitive manner (apparent K(i) 290 ± 14 μM). These results indicate that hypoxanthine entry into ECV 304 endothelial cells is mediated by the NBMPR-insensitive nucleoside carrier present in these cells.