Inhibition of platelet aggregation by placental extracts

Extracts of human placenta and of human umbilical cord vessels were examined for anti-platelet aggregating activity. Placental extracts progressively inhibited aggregation induced by ADP but had little effect on that produced by adrenaline, collagen or ristocetin. In contrast, umbilical vessel extracts and PGI₂ had an almost immediate inhibitory effect on aggregation induced by all four reagents. The effect of placental extracts was shown to be mediated through the degradation of ADP and was mimicked by purified human placental alkaline phosphatase. Since human placental alkaline phosphatase activity is known to be low in pregnancies associated with toxaemia, an alteration in this ADPase activity of the placenta may contribute to the pathogenesis of disseminated intravascular coagulation in eclampsia of pregnancy.