Differential uptake of [³H]guanosine by nucleoside transporter subtypes in Ehrlich ascites tumour cells

Intracellular metabolism of [3H]guanosine was minimal (< 15%) during the first 22 s of incubation, and hence reasonable estimates of initial-rate influx kinetics could be derived by using metabolically active cells. Na(+)-dependent concentrative [3H]guanosine uptake was not observed. Data suggest that [3H]guanosine was accumulated primarily via the nitrobenzylthioguanosine (NBTGR)-sensitive subtype of facilitated nucleoside transporter. Incubation of cells with 100 nM-NBTGR significantly decreased the potency of guanosine as an inhibitor of [3H]uridine influx. The Vmax. for [3H]guanosine influx (9.2 pmol/s per microliters) was significantly lower than that for [3H]uridine influx (16 pmol/s per microliters). The Km for transporter-mediated [3H]guanosine influx determined in the presence of 100 nM-NBTGR was 16-fold higher (1780 microM) than that determined in its absence, whereas the Km for [3H]uridine influx was shifted by only 2-fold. In other respects, the cellular accumulations of [3H]guanosine and [3H]uridine were similar; both had Km values of approx. 140 microM for total mediated influx, and both were inhibited similarly by other nucleosides and transport inhibitors. These characteristics, and the fact that guanosine is an endogenous nucleoside, suggest that [3H]guanosine may prove useful as a poorly metabolized, relatively selective, substrate for study of the NBTGR-sensitive nucleoside transport systems of mammalian cells.