In an attempt to clarify the mechanism of blood flowing in the placental intervillous circulation, we investigated the possibility as to whether the placenta might have an inhibitory effect of platelet aggregation.Materials and Methods : Fresh, full-term placental extracts were prepared by washing several times in 0.01 M Tris at 4°C to remove contaminating blood and tissue fluids. Following centrifugation at 3, 000 r. p. m. for 15 minutes, the supernatant fluid was taken and centrifuged again. PRP was prepared by standard techniques. The extract was added to PRP and incubated at 37°C for 3 minutes. The effects of placental extracts on platelet aggregation were measured by using Lumi-aggregometer with inducers such as ADP, collagen, epinephrine and ristocetin.In addition, the inhibitory effects of placental lipids, placental proteins and PGI2 on platelet aggregation were investigated by the same method.Results : 1) The placental extracts inhibited not only the secondary platelt aggregation induced by ADP, but also the release reaction of ATP induced by collagen, epinephrine and ristocetin.2) The tissue extracts obtained from maternal side and central portion of the placenta also exhibited the strong inhibitory effect of platelt aggregation, but the umbilical cord vessels had little inhibitory effect on secondary aggregation.3) Tissue extracts of hydatidif orm mole showed not only a strong inhibitory effect of secondary aggregation, but also inhibited release reaction by ATP. 4) The inhibitory effect by placental

Key words:human placental extracts, placental protein, inhibition and acceleration of platelet aggregation, ATP release, supernatant fractionIn the preliminary repory, we have demonstrated that the supernatant obtaind by centrifugation at 10,000 xg from human placental extracts shows a potent inhibitory effect on platelet aggregation, and that placental protein (PP 5; Bohn, 1977) has also an inhibitory effect of platelet aggregation as well. In this report, the two contrary effects on platelet aggregation were found in the process of the purification of platelet antiaggregatory substances from human placental extracts.Results; 1) The supernatant obtained at 10, 000xg for 15 min, and the precipitate obtained at 104,000xg for 60 min. from human placental extracts showed the strong inhibitory effects on platelet aggregation which was prominent in the precipitate of microsomal fraction. 2) A definite fraction of the supernatant obtaind at 104,000xg for 60 min, after gel-filtrayion showed an acceleration of platelet aggregation. 3) Although PP5 was demonstrated in the supernatants at 10,000xg for 15 min. and at 104,000xg for 60 min. by the use of an immunoperoxidase technique, PP5 was not found in the precipitate at 104,000xg for 60 min.