Iioka H, Moriyama I, Amasaki M, Itoh K, Hino K, Ichijo M
Nihon Sanka Fujinka Gakkai Zasshi. 1985 Dec;37(12):2675-80.
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.
采用快速过滤技术，研究了从人胎盘滋养层分离的微绒毛（刷状缘）膜囊泡对磷酸盐的摄取。微绒毛膜囊泡表现出将磷酸盐摄取到具有渗透活性的囊泡内空间，并且用磷酸盐预孵育增加了磷酸盐向囊泡内的摄取。这些发现表明，胎盘滋养层微绒毛膜对磷酸盐的摄取代表了向膜囊泡的转运。在存在钠电化学梯度（囊泡外大于囊泡内）的情况下，囊泡对磷酸盐的摄取呈现过冲现象。在pH 6.0时，钠依赖性磷酸盐摄取比在pH 8.0时观察到的摄取高约两倍。钠依赖性磷酸盐转运的初始速率相对于磷酸盐浓度呈现饱和动力学：计算得出表观Km为0.28 mM，在20秒内Vmax为330 pmol/mg蛋白质。这些结果表明，磷酸盐跨微绒毛膜的转运是由载体介导的。用不同阴离子（SCN-、Cl-、葡萄糖酸盐-）和离子载体（缬氨霉素）进行的实验表明，在pH 7.4时，存在Na+梯度时的磷酸盐转运几乎与囊泡膜上的电势无关。得出的结论是，胎盘滋养层微绒毛膜含有一种电中性的Na+/磷酸盐共转运系统。
