ATP-dependent calcium transport across basal plasma membranes of human placental trophoblast.

As a first step in understanding the cellular basis of maternal-fetal calcium transfer, we examined the characteristics of calcium uptake by a highly purified preparation of the syncytiotrophoblast basal (fetal facing) plasma membrane. In the presence of nanomolar concentrations of free calcium, basal membranes demonstrated substantial ATP-dependent calcium uptake [K0.5 = 119 nM, maximum velocity (Vmax) = 2 nM X min-1 X mg-1]. This uptake required magnesium, was not significantly affected by Na+ or K+ (50 mM), or sodium azide (10 mM). Intravesicular calcium was rapidly and completely released by the calcium ionophore A23187. Calcium transport was significantly stimulated by the calcium-dependent regulatory protein calmodulin. Placental membrane fractions enriched in endoplasmic reticulum (ER) and mitochondria also demonstrated ATP-dependent calcium uptake. In contrast to basal membrane, mitochondrial calcium uptake was completely inhibited by azide. The rate of calcium uptake by the ER was only 20% of that of basal membranes. We conclude that the placental basal plasma membrane possesses a high-affinity calcium transport system similar to that found in plasma membranes of a variety of cell types. This transporter is situated to permit it to function in vivo in maternal-fetal calcium transfer.