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

We have studied the mechanisms involved in calcium (Ca 2+ ) 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 [ 3 H]dihydroalprenolol binding sites, and fivefold in alkaline phosphatase activity compared with the placenta homogenates. In the absence of ATP and Mg 2+ , a basal Ca 2+ uptake was observed, which followed Michaelis‐Menten kinetics, with a K m Ca 2+ of 0.18 ± 0.05 μM and V max of 0.93 ± 0.11 nmol/mg/min. The addition of Mg 2+ to the incubation medium significantly decreased this uptake in a concentration‐dependent manner, with a maximal inhibition at 3 mM Mg 2+ and above. The Lineweaver‐Burk plots of Ca 2+ uptake in the absence and in the presence of 1 mM Mg 2+ suggest a noncompetitive type of inhibition. Preloading the BPM vesicles with 5 mM Mg 2+ had no significant effect on Ca 2+ uptake, eliminating the hypothesis of a Ca 2+ /Mg 2+ exchange mechanism. This ATP‐independent Ca 2+ uptake was not sensitive to 10 −6 M nitrendipine nor to 10 −4 M verapamil. An ATP‐dependent Ca 2+ transport was also detected in these BPM, whose Km Ca 2+ was 0.09 ± 0.02 μM and V max 3.4 ± 0.2 nmoles/mg/3 min. This Ca 2+ transport requires Mg 2+ , the optimal concentration of Mg 2+ being approximately 1 mM. Preincubation of the membrane with 10 −6 M calmodulin strongly enhanced the initial ATP‐dependent Ca 2+ uptake. Finally, no Na + /Ca 2+ exchange process could be demonstrated.