Preincubation of human platelet rich plasma with PGI2 in a concentration preventing collagen induced platelet aggregation abolished also platelet factor 3 availability brought about by collagen. Following PGI2 pretreatment no second wave aggregation could be elicited by ristocetin. However, primary aggregation as well as platelet factor 3 activity were only partially inhibited in this case and the inhibitory action of PGI2 was not increased by raising its concentration. Similarly, marked but not complete inhibition of platelet factor 3 availability was obtained when kaolin was used as activating agent.
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