In a final stage of activation, platelets become procoagulant because of the appearance of phosphatidylserine (PS) at the membrane outer surface. This PS exposure requires a rise in cytosolic [Ca2+]i, is accompanied by formation of membrane blebs, and stimulates the formation of thrombin from its precursor prothrombin. Here, we investigated whether thrombin, as a potent platelet agonist, can induce this procoagulant response in plasma-free platelets interacting with fibrin or fibrinogen through their integrin αIIbβ3 receptors. First, in platelets that were stimulated to spread over fibrin or fibrinogen surfaces with adrenaline, addition of thrombin and CaCl2 caused a potent Ca2+ signal that in about 30% of the cells was accompanied by exposure of PS. At low doses, integrin αIIbβ3 receptor antagonist (RGD peptide) inhibited platelet spreading as well as thrombin-evoked PS exposure. Second, in platelet-fibrinogen microaggregates that were preformed in the presence of adrenaline, thrombin/CaCl2 induced PS exposure and bleb formation of about 35% of the cells. Third, a potent, thrombin-dependent stimulation of prothrombinase activity was measured in platelet suspensions that were incubated with a fibrin clot. These results indicate that, in the absence of coagulating plasma, thrombin is a moderate inducer of the procoagulant response of platelets, once integrin αIIbβ3-mediated interactions are stimulated (by adrenaline) and CaCl2 is present.

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