ATP-dependent Ca2+ uptake in brush border membranes of human term placenta

In this communication, an ATP-dependent Ca2+ uptake was characterized in term human placental brush border membranes. Multiple freeze-thaw cycles greatly diminished uptake activity by 90 per cent, indicating an intravesicular accumulation of Ca2+. Kinetic studies indicate an apparent Km for Ca2+ of 0.22 +/- 0.04 microM and a Vmax of 441 +/- 137 pmoles/min/mg protein at 37 degrees C. The uptake was shown to have an optimum pH between 7.0 and 7.2, and was unaffected by the addition of oxalate, characteristics which are consistent with a plasma membrane origin of uptake. The process was temperature-dependent with a Q10 of 1.11 and was significantly inhibited (50 per cent) by 100 microM concentrations of trifluoperazine and vanadate. The characteristics of placental brush border Ca2+ uptake are similar to those of other Ca2+ uptake systems known to regulate intracellular calcium concentrations. By analogy, we suggest a similar role for this process in the maintenance of the low intracellular Ca2+ levels necessary for placental syncytiotrophoblast viability.