EXPRESSION OF A HUMAN RENAL SODIUM NUCLEOSIDE COTRANSPORTER IN XENOPUS-LAEVIS OOCYTES

In this study, Xenopus laevis oocytes injected with poly(A)(+) RNA (mR NA) isolated from human kidney were used to express a Na+-nucleoside c otransporter. Na+-stimulated [H-3]thymidine uptake was enhanced 2-3-fo ld in oocytes injected with 50 ng poly(A)(+) RNA and 4-5-fold in oocyt es injected with 20 ng of a size-fractionated human renal cortex mRNA fragment (2-3 kb) in comparison with water-injected oocytes. Na+-depen dent thymidine uptake in oocytes injected with the 2-3 kb mRNA fragmen t was inhibited significantly by thymidine and guanosine but not by fo rmycin B, consistent with the N4 Na+-nucleoside cotransporter. The K-m (28 mu M) of Na+-dependent thymidine uptake in the oocytes injected w ith the 2-3 kb mRNA fragment was similar to the K-m (27 mu M) of Na+-d ependent thymidine uptake obtained in human renal brush border membran e vesicles. These data suggest for the first time that a Na+-nucleosid e cotransporter from human kidney can be expressed in X. laevis oocyte s.