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

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 alpha(1)-subunit, but not the alpha(2)- or alpha(3)-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 mu mol . mg(-1) . min(-1), n = 8, P < 0.02) than in MVM (3.9 +/- 0.25 mu mol . 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.