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-faci ng) plasma membrane (BPM) of human placenta must be equipped with vari ous extrusion mechanisms. We studied one such mechanism, Na+/Ca2+ exch ange, as well as related membrane potential effects and binding proper ties of the two membranes. Na+/Ca2+ exchange was present in BPM and ab sent in microvillous (maternal-facing) membrane. Uptake and efflux of Ca2+ in BPM were enhanced by Na+ when it was present on the opposite s ide of the membranes. Na+-gradient-dependent Ca2+ uptake was saturable with a Km of 19 mu M and a V-max of 0.8 nmol/min/mg. The Na+/Ca2+ exc hange in BPM and the facilitated diffusion transporters in both BPM an d microvillous membrane are electrogenic processes. Ca2+ binding in bo th BPM and microvillous membrane was affected by various monovalent ca tions and enhanced by Na+ more than by K+. In vivo, together with othe r sequestration mechanisms, Na+/Ca2+ exchange may play an important ro le in transsyncytial transfer and in regulating intracellular Ca2+, wh ich is essential for a variety of physiologic mechanisms.