Expression and role of calcium-ATPase pump and sodium-calcium exchanger in differentiated trophoblasts from human term placenta

Although placental transfer of maternal calcium (Ca(2+)) is a crucial process for fetal development, the biochemical mechanisms are not completely elucidated. Especially, mechanisms of syncytiotrophoblast Ca(2+) extrusion into fetal circulation remain to be established. In the current study we have investigated the characteristics of Ca(2+) efflux in syncytiotrophoblast-like structure originating from the differentiation of cultured trophoblasts isolated from human term placenta. Time-courses of Ca(2+) uptake by differentiated human trophoblasts displayed rapid initial entry (initial velocity (V(i)) of 8.82 +/- 0.86 nmol/mg protein/min) and subsequent establishment of a plateau. Ca(2+) efflux studies with (45)Ca(2+)-loaded cells also showed rapid decline of cell-associated (45)Ca(2+) with a V(i) of efflux (V(ie)) of 8.90 +/- 0.96 nmol/mg protein/min. Expression of membrane systems responsible for intracellular Ca(2+) extrusion from differentiated human trophoblast were investigated by RT-PCR. Messenger RNAs of four known isoforms of PMCA (PMCA 1-4) were detected. Messenger RNAs of two cloned human NCX isoforms (NCX1 and NCX3) were also revealed. More specifically, both splice variants NCX1.3 and NCX1.4 were amplified by PCR with total RNA of differentiated human trophoblast cells. Ca(2+) flux studies in Na-free incubation medium indicated that NCX played a minimal role in the cell Ca(2+) fluxes. However, erythrosine B (inhibitor of PMCA) time- and dose-dependently increased cell associated (45)Ca(2+) suggesting a principal role of plasma membrane Ca(2+)-ATPase (PMCA) in the intracellular Ca(2+) extrusion of syncytiotrophoblast-like structure originating from the differentiation of cultured trophoblast cells isolated from human term placenta.