Ca2+ transport through the brush border membrane of human placenta syncytiotrophoblasts

Abstract

The calcium (Ca2+) uptake by brush border membrane vesicles isolated from fresh human placentas has been characterized. This process was saturable and time- and concentration-dependent. It exhibited a double Michaelis–Menten kinetics, with apparent Km values of 0.17 ± 0.03 and 2.98 ± 0.17 mM Ca2+, and Vmax values of 0.9 ± 0.13 and 2.51 ± 0.45 pmol∙μg−1∙5 s−1. It was not influenced by the presence of Na+ or Mg2+ in the incubation medium. It was not increased by K+ or anion diffusion potentials, inside negative. At a steady state of 1 mM Ca2+ uptake, a large proportion (approximately 94%) of the Ca2+ was bound to the internal surface of the membranes. Preincubation of these membrane vesicles with voltage-dependent Ca2+ channel blockers (nifedipine and verapamil) had no influence on Ca2+ uptake. However, this uptake was very sensitive to pH. In the absence of a pH gradient, the Ca2+ uptake increased with alkalinity. When the intravesicular pH was kept constant while the pH of the incubation medium was increased, Ca2+ uptake was also stimulated by alkaline pH. In contrast, when the pH of the incubation medium was kept constant and the intravesicular pH was progressively increased, Ca2+ uptake was diminished with alkaline pH. Therefore, H+ gradient (H+ in trans-position > H+ in cis-position) favored Ca2+ transport, suggesting a H+/Ca2+ exchange mechanism. Finally, in contrast to the basal plasma membrane, the brash border membrane did not show any ATP-dependent Ca2+ transport activity.Key words: calcium transport, brush border membranes, placenta.