Transport and binding in calcium uptake by microvillous membrane of human placenta
Kamath, S.G.; Kelley, L.K.; Friedman, A.F.; Smith, C.H.
American Journal of Physiology 262(3 Pt 1): C789-C794
1992
ISSN/ISBN: 0002-9513 PMID: 1550216 Document Number: 388983
Calcium entry across the microvillous membrane of the human placental syncytiotrophoblast is the first step in the transfer of this important nutrient to the fetus. Calcium uptake by isolated microvillous membrane was time dependent. Equilibrium uptake was very much greater than could be explained by equilibration of the vesicles space with medium, indicating that calcium is bound to internal sites. Addition of the ionophore A23187 greatly increased the rates of influx and efflux, indicating that transport across the plasma membrane is rate limiting in entry. Concentrations dependence data for calcium transport at 4 s fit well to a Michaelis-Menten equation having two saturable sites and diffusion. Calcium entry by both transporters was unaffected by calcium channels blockers but was strongly inhibited by the group II metals. The distinct inhibition constant values for strontium inhibition provided additional evidence for two transporters. Calcium binding fit well to a single-site model saturable in the micromolar range. In vivo the saturable transport processes may mediate calcium entry into the syncytiotrophoblast and binding may regulate concentrations within the placental microvilli.