Sulfate influx across the rabbit ileal brush border membrane: Sodium and proton dependence,and substrate specificities

Summary In intact ileal mucosa, uptake of SO₄ across the brush border membrane requires the presence of Na and is saturable, withK1/2=1.3mm at 140mm Na (P.L. Smith, S.A. Orellana & M. Field, 1981.J. Membrane Biol.63:199–206). The present study examines the substrate specificities and transport stoichiometry of the Na-dependent SO₄ uptake process. The effects of variations in medium anion and cation composition on lumen-to-epithelium influx of SO₄ (J_me^SO4) were determined under short-circuit conditions.J_me^SO4 is inhibited by thiosulfate, but not by phosphate, methylsulfate, vanadate or taurocholate. Cl is weakly inhibitory. Uptake of SO₄ is poorly supported by Li, and is unaffected by K, indicating a specific dependence on Na. At low SO₄ concentration (0.22mm),J_me^SO4 is a hyperbolic function of medium Na concentration; the corresponding Hill plot is linear with a slope of 1.0, suggesting a transport stoichiometry of 1 Na: 1 SO₄. At high SO₄ concentration (6.7mm), the Na-dependent SO₄ velocity curve is sigmoidal and yields a Hill plot which is again linear but has a slope of 1.56, suggesting transport of more than 1 Na per SO₄. SO₄ uptake in presence of Na exhibits a dependence on medium pH. At 0.22mm SO₄ and 140mm Na,J_me^SO4 was doubled by lowering pH from 7.4 to 6.8. However, at 6.7mm SO₄ and 140mm Na, changing pH had no effect onJ_me^SO4 over the range 6.8 to 8.5. The pH dependence ofJ_me^SO4 at 6.7mm SO₄ was restored when medium Na was lowered to 3mm, suggesting that pH sensitivity is a function of the concentration of preformed NaSO₄^– ion pair. The results suggest that SO₄ influx across the ileal brush border occurs by electroneutral Na⁺/NaSO₄^– or Na⁺/H⁺/SO₄^2– cotransport, the former being favored by high concentrations of Na and SO₄.