Ca2+ transport through the brush border membrane of human placenta syncytiotrophoblasts.
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.micrograms-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 greater than H+ in cis-position) favored Ca2+ transport, suggesting a H+/Ca2+ exchange mechanism. Finally, in contrast to the basal plasma membrane, the brush border membrane did not show any ATP-dependent Ca2+ transport activity.
Brunette
M G
MG
Maisonneuve-Rosemont Hospital, Montréal (Qué.), Canada.
Leclerc
M
M
D000255:Adenosine Triphosphate / Q000494:pharmacology; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism / Q000493:pharmacokinetics*; D002132:Calcium Radioisotopes; D002135:Calcium-Binding Proteins / Q000378:metabolism; D004594:Electrophysiology; D005260:Female; D006801:Humans; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D008274:Magnesium / Q000494:pharmacology; D008564:Membrane Potentials / Q000502:physiology; D008871:Microvilli / Q000378:metabolism; D009543:Nifedipine / Q000494:pharmacology; D010920:Placenta / Q000378:metabolism* / Q000648:ultrastructure; D011247:Pregnancy; D012680:Sensitivity and Specificity; D012964:Sodium / Q000494:pharmacology; D014327:Trophoblasts / Q000378:metabolism* / Q000648:ultrastructure; D014700:Verapamil / Q000494:pharmacology
D002132:Calcium Radioisotopes; D002135:Calcium-Binding Proteins; D000255:Adenosine Triphosphate; D012964:Sodium; D014700:Verapamil; D008274:Magnesium; D009543:Nifedipine; D002118:Calcium