Abstract
SO4(2-) transport by microvillous brush border membrane vesicles prepared from normal-term human placenta has been studied using an ion-exchange column assay. The uptake of this anion is time dependent and at 20 degrees C reaches a point of equilibrium by 60 min. 4,4'-Diisothiocyanostilbene-2,2'-disulphonate (DIDS) and 4-acetamido-4'-isothiocyanostilbene-2-2'-disulphonate (SITS) (at 10(-4) M) were found to inhibit approximately 80% of uptake. The concentration of DIDS producing half-maximal inhibition was approximately 10(-5) M. In contrast furosemide and probenecid were weak inhibitors of SO4(2-) influx. The anions Cl-, I-, thiocyanate (SCN-) and gluconate (cis-side) caused minimal inhibition of SO4(2-) influx. Salicylate (10 mM) produced 47% inhibition. The divalent anions thiosulphate, tungstate and unlabelled SO4(2-) (10 mM) inhibited the uptake of SO4(2-) (at 1 mM) to the same extent as DIDS. The vesicle membrane potential was altered by varying external K+ concentration using valinomycin. The DIDS-sensitive SO4(2-) influx was not affected by changes in membrane potential. A novel method has been developed for studying solute efflux from pre-loaded vesicles. Both DIDS and a reduction in temperature were effective inhibitors of SO4(2-) efflux. These results are discussed.