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 K_m values of 0.17 ± 0.03 and 2.98 ± 0.17 mM Ca²⁺, and V_max 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.