Expression of rabbit ileal N3 Na+/nucleoside cotransport activity in Xenopus laevis oocytes

To determine if the Na+/nucleoside cotransport activity in the distal rabbit intestine has either purine-selective (N1) or broad (N3) substrate specificity, Na(+)-dependent inosine uptake was expressed in Xenopus laevis oocytes. The rate of expressed Na(+)-stimulated inosine uptake saturated with increasing inosine concentration (apparent K(m) = 58.2 +/- 13.0 microM), was insensitive to inhibition by 6-(4-nitrobenzyl)thio-9-beta-D-ribofuranosylpurine, and was partially inhibited by phloridzin. Na(+)-dependent uptake was inhibited by guanosine (IC50 = 7.1 microM) and thymidine (IC50 = 5.5 microM). The Na+/nucleoside cotransport activity expressed by rabbit ileal mRNA in the Xenopus oocyte expression system is most characteristic of the N3 subclass of this family of transport proteins.