Expression of rat ileal Na+-sulphate cotransport in Xenopus laevis oocytes: functional characterization

Small-intestinal sulphate absorption is a Na+-dependent process having its highest rate in the ileum; it involves brush-border membrane Na+-sulphate cotransport. Injection of rat ileal mRNA into Xenopus laevis oocytes induced Na+-dependent sulphate uptake in a dose-dependent manner, with no apparent effect on Na+-independent sulphate uptake. For mRNA-induced transport, the apparent Km value for sulphate interaction was 0.6±0.2 mM and that for sodium interaction was 25±2 mM (Hill coefficient: 2.3±0.3). mRNA-induced transport, was inhibited by thiosulphate, but not by phosphate or 4,4,′-diisothiocyanatostilbene-2,2′-disulphonic acid (DIDS). Using a rat renal Na+-sulphate cotransporter cDNA as a probe [NaSi-1; Markovich et al. (1993) Proc Natl Acad Sci USA 90:8073-8077], the highest hybridization signals (2.3 kb and 2.9 kb) were obtained in size fractions showing the highest expression of Na+-dependent sulphate transport in oocytes. Hybrid depletion experiments using antisense oligonucleotides (from the NaSi-1 cDNA sequence), provided further evidence that rat small-intestinal (ileal) Na+-sulphate cotransport is closely related to rat proximal-tubular brushborder membrane Na+-sulphate cotransport.