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

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Abstract
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 triphosphatase (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 μM 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 μM 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.