Active sulfate secretion by the intestine of winter flounder is through exchange for luminal chloride.

SO4(2-) transport by winter flounder intestine in Ussing chambers was characterized. With 50 mM SO4(2-) (physiological level) bathing the lumen, net absorption (lumen to blood) dominated. Under short-circuited conditions, 1 mM SO4(2-) on both sides, net active SO4(2-) secretion occurred (8.55 +/- 0.96 nmol. cm(-2). h(-1)). NaCN (10 mM), ouabain (10(-4) M), and luminal DIDS (0.2 mM) inhibited net secretion. Removal of luminal Cl- and HCO3- together (Cl--HCO3-) or Cl- alone blocked net secretion, whereas removal of luminal HCO3- alone increased net secretion. SO4(2-) uptake into foregut brush-border membrane vesicles was stimulated by a trans-Cl- gradient (in > out) and unaffected by a trans-HCO3- gradient (in > out). Short-circuiting with K+ (in = out) and valinomycin had no effect on Cl--stimulated SO4(2-) uptake, suggesting electroneutral exchange. Satiety (i.e., full stomach) stimulated the unidirectional absorptive flux, eliminating net secretion. It was concluded that the intestine is a site of SO4(2-) absorption in marine teleosts and that active SO4(2-) secretion is in exchange for luminal Cl-.