Background— Among the prostanoids, thromboxane (TX) A₂ is a potent stimulator of platelets, whereas prostaglandin (PG) I₂ inhibits their activation. The roles of PGE₂ in the regulation of platelet function have not been established, however, and the contribution of PGE₂ in hemostasis and thromboembolism is poorly understood. The present study was intended to clarify these roles of PGE₂ by using mice lacking the PGE₂ receptor subtype 3 (EP₃^−/− mice).

Methods and Results— Expression of mRNAs for EP₃ in murine platelets was confirmed by quantitative reverse transcription-polymerase chain reaction. PGE₂ and AE-248, a selective EP₃ agonist, showed concentration-dependent potentiation of platelet aggregation induced by U46619, a TXA₂ receptor agonist, although PGE₂ alone could not induce aggregation. PGE₂ and AE-248 increased cytosolic calcium ion concentration ([Ca²⁺]_i), and AE-248 inhibited the forskolin-induced increase in cytosolic cAMP concentration ([cAMP]_i), suggesting G_i coupling of EP₃. The potentiating effects of PGE₂ and AE-248 on platelet aggregation along with their effects on [Ca²⁺]_i and [cAMP]_i were absent in EP₃^−/− mice. In vivo, the bleeding time was significantly prolonged in EP₃^−/− mice. Moreover, when mice were challenged intravenously with arachidonic acid, mortality and thrombus formation in the lung were significantly reduced in EP₃^−/− mice.

Conclusions— PGE₂ potentiated platelet aggregation induced by U46619 via EP₃ by increasing [Ca²⁺]_i, decreasing [cAMP]_i, or both. This potentiating action of PGE₂ via EP₃ is essential in mediating both physiological and pathological effects of PGE₂ in vivo.