Abstract
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.
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