Ca(2+)-transport across brush border and basal surfaces of human term [correction of terms] placenta

Brush border microvillous (BBM) and basal cell membranes (BCM) were isolated from syncytiotrophoblast of human term placenta by homogenization, sonication, prolonged stirring and differential centrifugation. Uptake of 45Ca(2+)-CaCl2 in membrane vesicles in morpholino propane sulphonic acid (MOPS) buffer was studied up to 60 min. Maximum uptake of the radioisotope was recorded at 10 and 15 min of incubation of the BBM and BCM, respectively. Radioisotopic uptake was also dependent on the Ca(2+)-concentration, linear up to 3 mu mole and then assuming hyperbolic substrate saturation kinetics. The Lineweaver-Burk transformation of the data gave Kt value for BBM and BCM, 0.85 and 1.08 microM, respectively while the Vmax of uptake (Jmax) in the same were 105.26 and 188.68 pmole Ca2+/microgram protein/20 min. Ca(2+)-Uptake in placental BBM and BCM vesicles was inhibited by two Ca(2+)-channel blockers, nifedipine and verapamil to as much as 50% while Ca(2+)-ionophore A23187 enhanced the uptake process significantly.