Na+/Ca2+ exchange, Ca2+ binding, and electrogenic Ca2+ transport in plasma membranes of human placental syncytiotrophoblast.
To transfer a large amount of Ca2+ 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 microM and a Vmax of 0.8 nmol/min/mg. The Na+/Ca2+ exchange in BPM and the facilitated diffusion transporters in both BPM and microvillous membrane are electrogenic processes. Ca2+ binding in both BPM and microvillous membrane was affected by various monovalent cations and enhanced by Na+ more than by K+. In vivo, together with other sequestration mechanisms, Na+/Ca2+ exchange may play an important role in transsyncytial transfer and in regulating intracellular Ca2+, which is essential for a variety of physiologic mechanisms.
Kamath
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Edward Mallinckrodt Department of Pediatrics and Pathology, Washington University School of Medicine, St. Louis Children's Hospital, Missouri 63110.
Smith
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D001692:Biological Transport / Q000187:drug effects; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002132:Calcium Radioisotopes; D002414:Cations, Monovalent / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D005260:Female; D006801:Humans; D007700:Kinetics; D010920:Placenta / Q000378:metabolism; D011247:Pregnancy; D012964:Sodium / Q000378:metabolism*; D013997:Time Factors; D014327:Trophoblasts / Q000378:metabolism*
D002132:Calcium Radioisotopes; D002414:Cations, Monovalent; D000001:Calcimycin; D012964:Sodium; D002118:Calcium