Different Requirements for Intrinsic Factor-Xa Forming Activity and Platelet Factor 3 Activity and Their Relationship of Platelet Aggregation and Release

Platelets provide coagulant activity in part by making available membrane phospholipids which promote the reactions of: (1) factors XIa, VIII, IX and calcium to activate factor X (intrinsic factor-Xa forming activity or XaFA); and, (2) factors Xa, V and calcium to activate prothrombin (platelet factor 3 activity or PF3A). We have studied the availability of XaFA and PF3A, platelet aggregation and [¹⁴C]-5HT release in washed platelet suspensions, incubated with collagen.XaFA developed rapidly (100% activity at 15-30 sec) well before maximum [¹⁴C]-5HT release (2-4 min) and decayed to <10% at 5-10 min.PF3A developed slowly (100% activity after 20 min stirring with collagen) well after maximal aggregation and release. Maximal XaFA availability occurred in unstirred platelet suspensions incubated with collagen and not undergoing aggregation or release, and was not enhanced by stirring, whereas PF3A did not become available unless platelet suspensions were stirred with resultant aggregation and release.PF3A, XaFA, aggregation and release were inhibited by pretreating platelet suspensions with p-chloro-mercuribenzene sulfonate, concanavalin A or a combination of 2-deoxyglucose and antimycin A, whereas pretreatment with indomethacin inhibited aggregation, release and PF3A availability but not XaFA availability. These results indicate that the biochemical determinants of XaFA and PF3A are different.PF3A developed only when aggregation and release occurred whereas XaFA was independent of aggregation and release.