Prostacyclin (PGI2) inhibits the development in human platelets of ADP and arachidonic acid-induced shape change and procoagulant activity

Platelets which change shape from discs to spheres concomitantly develop platelet procoagulant activity which is independent of and precedes aggregation or the release reaction. Since prostacyclin (PGI₂) is known to be potent inhibitor of platelet aggregation and releae, the effect of PGI₂ on platelet shape change and the development of platelet procoagulant activity was measured. Platelet shape change (percent discs and spheres) was assayed by a light transmission technique. Platelet procoagulant activity was assayed using recalcified clotting times measured concurrently (by aggregometry) with platelet shape assays. PGI₂ inhibited the development of platelet shape change and procoagulant activity induced by the addition of ADP (0.7 μM); the 50% inhibitory dose of PGI₂ was 2 nM. PGI₂ also inhibited arachidonic acid (0.3–1.2 mM) induced platelet shape change and procoagulant activity; the 50% inhibitory dose of PGI₂ was 2.3 nM. Thus, physiologic concentrations of PGI₂ inhibit platelet shape change and prevent the development of sphering associated procoagulant activity.