Sodium-dependent, concentrative nucleoside transport in Walker 256 rat carcinosarcoma cells

Nucleoside transport in Walker 256 cells was reexamined using formycin B, a nonmetabolized analog of inosine. In the presence of dipyridamole to inhibit the equilibrative (facilitated diffusion) transporter previously described in these cells, the initial rate of uptake of 1 microM formycin B was 10-fold greater in Na(+)-containing medium than in Na(+)-free medium. In the presence of Na+ and dipyridamole the intracellular concentration of formycin B exceeded that in the medium within one min and was 6-fold greater than that of the medium by 5 min. Na(+)-dependent transport of formycin B was inhibited by low concentrations of inosine, but not thymidine. Furthermore, Na(+)-dependent transport of uridine, but not thymidine, was apparent in the presence of dipyridamole. These data indicate that Walker 256 cells have, in addition to the previously described equilibrative transporter, a concentrative nucleoside transporter. The specificity of this transporter appears to correspond to one of the two Na(+)-dependent transporters previously described in mouse intestinal epithelial cells.