Carrier-mediated sulfate transport in human ureteral epithelial cells cultured in serum-free medium

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Abstract
Sulfate transport studies were carried out in secondary cultures of epithelial cells isolated from the human ureter. Results demonstrate the presence of carrier-mediated SO4(2-) transport as supported by three lines of evidence: 1) saturation kinetics, 2) substrate specificity, and 3) inhibition by the anion transport inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). The DIDS-insensitive component of SO4(2-) transport was markedly lower than the DIDS-sensitive component and was not affected by changes in extracellular pH (pHo) or Cl- concentration. The mechanism of this DIDS-insensitive component is not clear. The DIDS-sensitive component of SO4(2-) uptake was a saturable function of the extracellular sulfate concentration ([SO4(2-)]o). Increasing the extracellular chloride concentration ([Cl-]o) inhibited DIDS-sensitive SO4(2-) uptake competitively. Taken together with the fact that increasing [Cl-]o stimulated SO4(2-) efflux, these results suggest that SO4(2-) uptake in uroepithelial cells occurs via SO4(2-)-Cl- anion exchange. Cis-inhibition studies with a variety of anions indicate that this anion-exchange system may be shared by S2O3(2-) and MoO4(2-) but not by NO3- and H2PO4-. SO4(2-) uptake was stimulated at decreasing pHo with a pK approximately 7.4. Decreasing pHo from 7 to 6 lowered the apparent Michaelis constant significantly but had no significant effect on kcat, suggesting that protons may increase the affinity of the SO4(2-) transporter for SO4(2-). SO4(2-) efflux was inhibited at low pHo and was stimulated by increasing [Cl-]o. This study is the first to demonstrate an ion transport process in epithelial cell cultures isolated from the human ureter. In contrast to epithelial cells from the upper urinary tract, no Na(+)-dependent SO4(2-) transport could be demonstrated in these lower urinary tract epithelial cells. In conclusion, the major mechanism for SO4(2-) transport in ureteral epithelial cells is a carrier-mediated, DIDS-sensitive, pHo-sensitive SO4(2-)/Cl- anion-exchange mechanism. These studies suggest that varying [SO4(2-)]o and [Cl-]o or pHo in the ureteral lumen will affect SO4(2-) influx and efflux and may influence the size of the intracellular pool of SO4(2-) available for macromolecular sulfation in these cells.