Na+/Ca2+ Exchange, Ca2+ Binding, and Electrogenic Ca2+ Transport in Plasma Membranes of Human Placental Syncytiotrophoblast

Abstract

To transfer a large amount of CaZ+ to the fetus, the basal (fetal-facing) plasma membrane (BPM) of human placenta must be equipped with various extrusion mechanisms. We studied one such mechanism, Na+/Ca2+ exchange, as well as related membrane potential effects and binding properties of the two membranes. Na+/Ca2+ exchange was present in BPM and absent in microvillous (maternal-facing) membrane. Uptake and efflux of Ca2+ in BPM were enhanced by Na+ when it was present on the opposite side of the membranes. Na+-gradient-dependent Ca2+ uptake was saturable with a Km of 19 FM and a V , , of 0.8 nmol/min/mg. The Na+/Ca2+ exchange in BPM and the facilitated difhsion transporters in both BPM and miThe human placental syncytiotrophoblast is the major bamer to the passage of ca2+ ions from the mother to the fetus (1). specific transport mechanisms in the microvillous (maternal-facing) and basal (fetal-facing) membranes of syncytiotrophoblast mediate the transfer of ca2+ across the placenta. While providing for the transplacental passage of large amounts of Ca2+, the syncytiotrophoblast must also maintain intrasyncytial free ca2+ concentrations several orders of magnitude lower than those in the plasma or extracellular fluid (2). To achieve this, one or both of its plasma membranes must possess energydependent extrusion mechanisms such as an ATPdependent Ca2+ pump or an ~a + /~a~+ exchange mechanism. The higher concentration of ca2+ in the fetal circulation (1) suggests the presence of such extrusion mechanisms in the basal membrane.Several investigators have attempted to characterize ATP-dependent ca2+ transport mechanisms of mem- branes from human placenta and other cell types (3-7). Our laboratory (3) has demonstrated ATP-dependent Ca2+ uptake in the basal membranes at nanomolar concentrations of free ca2+. We (8, 9) and others (10,11) have demonstrated the presence of facilitated diffusion transporters in both BPM and MVM.We now report the presence of an ~a + /~a~+ extrusion mechanism in basal membrane isolated from term human placentas. We investigated the effect of Na+ on ca2+ entry and binding in the microvillous and the basal membranes and also the effect of membrane potential on plasma membrane ca2+ transfer processes.