Substrate and inhibitor specificity of anion exchangers on the brush border membrane of rabbit ileum

Summary

In previous studies we have found that several anions can be transported by an exchange process in rabbit ileal brush border membranes. We demonstrated exchanges of Cl for OH or HCO₃, SO₄ for OH, oxalate for OH, and oxalate for Cl. The purpose of these studies was to determine the number of distinct carriers mediating these exchanges. We utilized substrate and inhibitor specificity studies to distinguish between different anion exchange transporters. We conclude that SO₄∶OH and oxalate: OH exchange occur on the same carrier because: (i) pH-gradient stimulated transport of both¹⁴C-oxalate and³⁵SO₄ were equally sensitive tocis-inhibition by unlabeled SO₄ or oxalate; and (ii) both were inhibited equally by K. We conclude that oxalate: OH and oxalate: Cl exchanges occur on different carriers because: (i) Cl or SO₄ caused unequalcis-inhibition of these two exchanges; and (ii) as compared to oxalate: Cl exchange, oxalate: OH exchange was more sensitive to inhibition by probenecid and K and less sensitive to inhibition by bumetanide. Finally, we conclude that oxalate: Cl exchange and Cl∶HCO₃ exchange occur on different carriers because: (i) Cl∶HCO₃ exchange was almost completely insensitive tocis-inhibition by oxalate; and (ii) oxalate: Cl exchange was more sensitive to inhibition by DIDS and bumetanide than Cl∶HCO₃ exchange. Thus we have found that there are at least three separate anion exchangers on rabbit ileal brush border: (i) a Cl∶HCO₃ exchanger; (ii) a SO₄∶OH exchanger, which also transports oxalate; and (iii) an oxalate: Cl exchanger.