Characterization of calcium transport by basal plasma membranes from human placental syncytiotrophoblast.

We have studied the mechanisms involved in calcium (Ca2+) transport through the basal plasma membranes (BPM) of the syncytiotrophoblast cells from full-term human placenta. These purified membranes were enriched 25-fold in Na+/K(+)-adenosine triphosphate (ATPase), 37-fold in [3H] dihydroalprenolol binding sites, and fivefold in alkaline phosphatase activity compared with the placenta homogenates. In the absence of ATP and Mg2+, a basal Ca2+ uptake was observed, which followed Michaelis-Menten kinetics, with a Km Ca2+ of 0.18 +/- 0.05 microM and Vmax of 0.93 +/- 0.11 nmol/mg/min. The addition of Mg2+ to the incubation medium significantly decreased this uptake in a concentration-dependent manner, with a maximal inhibition at 3 mM Mg2+ and above. The Lineweaver-Burk plots of Ca2+ uptake in the absence and in the presence of 1 mM Mg2+ suggest a noncompetitive type of inhibition. Preloading the BPM vesicles with 5 mM Mg2+ had no significant effect on Ca2+ uptake, eliminating the hypothesis of a Ca2+/Mg2+ exchange mechanism. This ATP-independent Ca2+ uptake was not sensitive to 10(-6) M nitrendipine nor to 10(-4) M verapamil. An ATP-dependent Ca2+ transport was also detected in these BPM, whose Km Ca2+ was 0.09 +/- 0.02 microM and Vmax 3.4 +/- 0.2 nmoles/mg/3 min. This Ca2+ transport requires Mg2+, the optimal concentration of Mg2+ being approximately 1 mM. Preincubation of the membrane with 10(-6) M calmodulin strongly enhanced the initial ATP-dependent Ca2+ uptake. Finally, no Na+/Ca2+ exchange process could be demonstrated.