title: Sulfate transport in apical membrane vesicles isolated from tracheal epithelium.
abstract: Sulfate uptake in apical membrane vesicles isolated from bovine tracheal epithelium is shown to occur into an osmotically sensitive intravesicular space, via a carrier-mediated system. This conclusion is based on three lines of evidence: 1) saturation kinetics; 2) substrate specificity; and 3) inhibition by the anion transport inhibitors SITS and DIDS. The affinity of the transport system is highest in low ionic strength media (apparent Km = 0.13 mM) and decreases in the presence of gluconate (apparent Km = 0.68 mM). Chloride appears to cis-inhibit sulfate uptake and to trans-stimulate sulfate efflux. Cis-inhibition and trans-stimulation studies with a variety of anions indicate that this exchange system may be shared by HCO3-, S2O3(2-), SeO4(2-), and MoO4(2-) but not by H2PO4- or HAsO4(2-). Studies indicate that protons may play two distinct roles in sulfate transport in this system. 1) Their possible modifier role is suggested by the fact that protons affect SO2-4 transport in an uncompetitive manner. 2) The possibility that the proton gradient may act as an energy source for a secondary active transport is indicated by the fact that the imposition of a proton gradient stimulates a transient movement of sulfate in to the tracheal apical membrane vesicle, against its concentration gradient, causing an "overshoot" phenomenon. Our studies show that the carrier-mediated system can function in the absence of chloride. The overshoot observed in the presence of a proton gradient (OH- gradient) indicates that under those conditions the mechanism of transport may be a SO4(2-)-OH- exchange. The fact that chloride cis-inhibits and trans-stimulates SO4(2-) transport indicates that SO2-4 uptake may also occur via a SO4(2-)-Cl- exchange. Studies carried out so far do not enable us to conclude unequivocally whether the tracheal apical membrane system displays two distinct carrier activities (SO4(2-)-Cl-; SO4(2-)-OH-) or one anion exchanger, which like the erythrocyte anion exchanger, may interact with SO4(2-), Cl-, and H+. The fact that the anion transport inhibitors DIDS and SITS inhibit SO4(2-) transport in the presence or absence of chloride suggests that the latter possibility may be the case.
mesh_terms: D000818:Animals; D000838:Anions / Q000037:antagonists & inhibitors / Q000378:metabolism / Q000494:pharmacology; D001692:Biological Transport; D002412:Cations / Q000494:pharmacology; D002417:Cattle; D002462:Cell Membrane / Q000378:metabolism; D004848:Epithelium / Q000378:metabolism; D066298:In Vitro Techniques; D007700:Kinetics; D009994:Osmolar Concentration; D013431:Sulfates / Q000378:metabolism*; D013997:Time Factors; D014132:Trachea / Q000378:metabolism*
chemical_list: D000838:Anions; D002412:Cations; D013431:Sulfates