ATP INDEPENDENT CALCIUM-TRANSPORT AND BINDING BY BASAL PLASMA-MEMBRANE OF HUMAN PLACENTA

The transport of large amounts of Ca2+ by the plasma membranes of human placental syncytiotrophoblast is essential to the mineralization of the growing fetal skeleton. We have investigated transport by the basal (fetal-facing) plasma membrane (BPM). Ca2+ was taken up by purified BPM vesicles in a time-dependent manner and equilibrium attained in approximately 60 min. The apparent equilibrium space was many fold higher than that determined using other substrates (e.g. leucine), suggesting that Ca2+ is concentrated or bound within the vesicles. The more rapid uptake and exit in the presence of A23187 indicates that membrane transport is rate limiting and that Ca2+ is internalized within the membrane space. The initial rate of uptake was approximated by measurement during the first 2 s of incubation. Concentration dependence data were fit to a Michaelis-Menten model with one saturable site and diffusion (Km=12 μm; Vmax=4 nmol/min/mg; KD =39 nmol/min/mg/mm). Saturable Ca2+ binding (Kd=16 μm; Bmax=3.4 nmol/mg) was of lower capacity than previoiusly observed for microvillous membrane. © 1994, W.B. Saunders Company Ltd. All rights reserved.