Background—ADP plays a key role in hemostasis, acting through 2 platelet receptors: the P2Y₁ receptor and an unidentified P2 receptor, called P2cyc, coupled to adenylyl cyclase inhibition, which is the target of the antiplatelet drug clopidogrel. We showed that the P2Y₁ receptor is an essential cofactor in thrombotic states induced by intravenous infusion of collagen and epinephrine. The aim of the present study was to assess the role of this receptor in thrombin-dependent tissue factor–induced thromboembolism.

Methods and Results—Human thromboplastin was injected intravenously into wild-type or P2Y₁-deficient mice, and the effects on platelet count and mortality were determined and plasma thrombin–antithrombin III (TAT) complexes were quantified. P2Y₁-deficient mice were resistant to the thromboembolism induced by injection of thromboplastin. Whereas the platelet count decreased sharply in wild-type mice, there was no significant drop in platelets in P2Y₁-knockout mice. The platelet consumption in wild-type mice was probably due to thrombin generation, because it was abolished by hirudin. Thromboplastin also led to a rise in TAT complexes in plasma, again reflecting thrombin formation. This effect, however, was less important in P2Y₁-knockout mice than in wild-type mice, indicating that less thrombin was generated in the absence of P2Y₁. Similar results were obtained after intravenous administration of N⁶-methyl-2′-deoxyadenosine-3′:5′-bisphosphate, a selective antagonist of the P2Y₁ receptor, to wild-type mice.

Conclusions—Our results demonstrate a role of the P2Y₁ receptor in thrombotic states involving thrombin generation and provide further evidence for the potential relevance of this receptor as a target for antithrombotic drugs.