Platelet aggregation inhibiting activity of human placental chorioepithelial brush border membrane vesicles--the role of alkaline phosphatase.

Human placental chorioepithelial brush border membrane, which is in direct contact with maternal blood flow, has platelet aggregation inhibiting activity. In the present study, the mechanism of this action has been examined in relation to ADP (adenosine diphosphate) degrading activity and alkaline phosphatase activity of brush border membrane vesicles (BBMV). BBMV prepared from human early and term placental villi, inhibited platelet aggregation induced by ADP. BBMV had potent ADP degrading (ADPase) activity. ADP was quickly degraded by BBMV. ADP degrading activity of BBMV was not so different between early and term placenta. Alkaline phosphatase activity of late placental BBMV was about three times greater than that of early placental BBMV. On the other hand, ADP degrading activity of late placental BBMV was almost the same as that of early placental BBMV. Inhibiting activity of platelet aggregation induced by ADP and ADP degrading activity of BBMV, were not inhibited by levamisole (alkaline phosphatase inhibitor).