Functional and structural demonstration of the presence of Ca-ATPase (PMCA) in both microvillous and basal plasma membranes from syncytiotrophoblast of human term placenta
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It is known that human syncytiotrophoblast (hSCT) actively transports more than 80% of the Ca2þ that goes from maternal to fetal circulation. Transepithelial transport of Ca2þ is carried out through channels, transporters and exchangers located in both microvillous (MVM) and basal (BM) plasma membranes. The plasma membrane Ca-ATPase (PMCA) is the most important mechanism of Ca2þ homeostasis control in the human placenta. In this work, we reexamined the distribution of PMCA in isolated hSCT of term placenta. The PMCA activity was determined in isolated hSCT plasma membranes. A partial characterization of the PMCA activity was performed, including an evaluation of the sensitivity of this enzyme to an in vitro induced lipid peroxidation. Expression of the PMCA in hSCT plasma membranes and tissue sections was investigated using Western blots and immunohistochemistry, respectively. Our study demonstrates, for the first time, a correlation between the activity and structural distribution of PMCA in both MVM and BM of hSCT. It also demonstrates a higher PMCA activity and expression in MVM as compared to BM. Finally, PMCA4 seems to be preferentially distributed in both hSCT plasma membranes, while PMCA1 is shown to be present in the hSCT homogenate. However, the membrane fractions did not show any PMCA1 labeling. Our results must be taken into account in order to propose a new model for the transport of calcium across the hSCT.