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

✍ Scribed by J. LaFond; M. LeClerc; M. G. Brunette

• Publisher: John Wiley and Sons
• Year: 1991
• Tongue: English
• Weight: 742 KB
• Volume: 148
• Category: Article
• ISSN: 0021-9541
• DOI: 10.1002/jcp.1041480103

✦ Synopsis

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]dihydroalprenoloI binding sites, and fivefold in alkaline phosphatase activity compared with the placenta homogenates. In the absence of ATP and Mg'+, a basal Ca2+ uptake was observed, which followed Michaelis-Menten kinetics, with a K , Ca2+ of 0.18 k 0.05 pM and V , , , of 0.93 2 0.1 1 nmol/mgimin. The addition of Mg2+ to the incubation medium significantly decreased this uptake in a concentrationdependent 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+iMg2+ exchange mechanism. This ATP-independent Ca2+ uptake was not sensitive to M verapamil. An ATP-dependent Ca2+ transport was also detected in these BPM, whose K, Ca2+ was 0.09 t-0.02 pM and V , , , 3.4 +. 0.2 nmolesimgi3 min. This Ca2+ transport requires Mg2+, the optimal concentration of MgL+ being approximately 1 mM. Preincubation of the membrane with lop6 M calmodulin strongly enhanced the initial ATP-dependent Ca2+ uptake. Finally, no Na+/Ca2+ exchange process could be demonstrated.