Na+-K+-ATPase is distributed to microvillous and basal membrane of the syncytiotrophoblast in human placenta

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Abstract
Despite its importance for placental function, syncytiotrophoblast Na+-K+-ATPase has not been studied in detail. We purified syncytiotrophoblast microvillous (MVM) and basal (BM) membranes from full-term human placenta. Western blotting with isoform-specific antibodies demonstrated the presence of the α1-subunit, but not the α2- or α3-subunits, in MVM and BM. Relative density per unit membrane protein in BM was 48 ± 1% (mean ± SE, n = 4, P < 0.02) of that in the MVM. The activity of Na+-K+-ATPase was lower in BM (1.4 ± 0.14 μmol · mg−1· min−1, n = 8, P < 0.02) than in MVM (3.9 ± 0.25 μmol · mg−1· min−1). Immunocytochemistry confirmed the distribution of Na+-K+-ATPase to MVM and BM. These findings suggest that the syncytiotrophoblast represents a type of transporting epithelium different from the classical epithelia found in the small intestine and kidney, where Na+-K+-ATPase is confined to the basolateral membrane only. This unique polarization of the Na+pump does not, however, preclude a net transcellular transport of Na+to the fetus.