Hypoxanthine transport in mammalian cells: Cell type-specific differences in sensitivity to inhibition by dipyridamole and uridine

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Abstract
We have measured by rapid kinetic techniques the zero-trans influx of hypoxanthine in various cell lines and its sensitivity to inhibition by uridine, dipyridamole, nitrobenzylthioinosine and nitrobenzylthiopurine. The results and those reported earlier divided the cells into two distinct groups. In mouse P388, L1210 and L929 cells uridine and hypoxanthine had little effect on the transport of each other, supporting the view that nucleosides and hypoxanthine are transported by different carriers. In these cells, hypoxanthine transport was also uniquely resistant to inhibition by dipyridamole (IC50 (50% inhibition dose) >30μM). In Novikoff and HTC rat hepatoma, Chinese hamster ovary and Ehrlich ascites tumor cells, on the other hand, hypoxanthine and uridine inhibited the transport of each other about 50% at a concentration corresponding to the Michaelis-Menten constant of their transport, and hypoxanthine transport was strongly inhibited by dipyridamole (IC50=100 to 400nM). Although these results are compatible with the view that nucleosides and hypoxanthine are transported by a common carrier in these cells, this conclusion is not supported by the finding that uridine transport is strongly inhibited in some of these cell lines, as in first group of cells, by nitrobenzylthioinsine, whereas hypoxanthine transport is highly resistant in all cell lines tested. In contrast, the transport of both substrates is highly resistant to inhibition by nitrobenzylthiopurine. The Michaelis-Menten constants for uridine transport are about the same in all cell lines. The Michaelis-Menten constants for hypoxanthine transport are similar to those for uridine transport in some cell lines, but are much higher in others. This difference is unrelated to the sensitivity of uridine and hypoxanthine transport to inhibition by each other or dipyridamole.

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