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 K-m 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 Nac-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 brush-border membrane Na+-sulphate cotransport
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