Expression of rat renal sodium/phosphate cotransporter in Xenopus laevis oocytes.

In an attempt to identify the renal Na+/Pi cotransporter, Xenopus laevis oocytes were used to express mRNA isolated from the renal cortex of rat kidney. Na(+)-dependent uptake of Pi in oocytes, injected with mRNA, resulted in an increase of 2-4-fold as compared to oocytes injected with water. Both the new expressed and endogenous Na(+)-dependent Pi uptake activity were inhibited with 2 mM phosphonoformic acid (PFA). Expression of Pi uptake into oocytes was dose-dependent with the amount of mRNA injected. When mRNA was fractionated on a sucrose gradient, a mRNA fraction of 2.5 kilobases expressed the Na+/Pi cotransport activity in oocytes. This fraction resulted in a 6-fold stimulation of Na(+)-dependent Pi transport when compared to oocytes injected with water. The Km and Vmax for Na(+)-dependent Pi uptake were 0.18 mM and 118 pmol/oocyte per 30 min, respectively.