[Study on the mechanism of placental transport of phosphate (using human placental microvillous (brush border) membrane vesicles)].

Using the rapid filtration technique, the uptake of phosphate into microvillous (brush border) membrane vesicles isolated from human placental trophoblast was investigated. The microvillous membrane vesicles exhibit the uptake of phosphate into an osmotically reactive intravesicular space and preincubation with the phosphate increased the uptake of phosphate into the vesicles. These findings indicate that the uptake of phosphate by placental trophoblast microvillous membranes represents transport into membrane vesicles. In the presence of sodium electrochemical gradient (extravesicular greater than intravesicular) the uptake of phosphate by vesicles shows an overshoot phenomenon. Sodium-dependent phosphate uptake is about two times higher at pH 6.0 as in the uptake observed at pH 8.0. The initial rate of sodium-dependent phosphate transport exhibited saturation kinetics with respect to phosphate concentration: An apparent Km of 0.28 mM and Vmax of 330 pmol/mg protein in 20 seconds were calculated. These results indicate that the transport of phosphate across the microvillous membranes is carrier mediated. Experiments with different anions (SCN-, Cl-, gluconate-) and ionophore (valinomycin) showed that at pH 7.4 phosphate transport in the presence of a Na+ gradient is almost independent of electrical potential across the vesicle membranes. It was concluded that placental trophoblast microvillous membranes contain an electroneutral Na+/phosphate co-transport system.