PGE 2 reverses G s-mediated inhibition of platelet aggregation by interaction with EP 3 receptors, but adds to non-G s-mediated inhibition of platelet aggregation by interaction with EP4 receptors

摘要

Prostaglandin E 2 (PGE 2) has intriguing effects on platelet function in the presence of agents that raise cyclic adenosine 3′5′-monophosphate (cAMP). PGE 2 reverses inhibition of platelet aggregation by agents that stimulate cAMP production via a G s-linked receptor, but adds to the inhibition of platelet function brought about by agents that raise cAMP through other mechanisms. Here, we used the EP receptor antagonists DG-041 (which acts at the EP3 receptor) and ONO-AE3-208 (which acts at the EP4 receptor) to investigate the role of these receptors in mediating these effects of PGE 2. Platelet aggregation was measured in platelet-rich plasma obtained from healthy volunteers in response to adenosine diphosphate (ADP) using single platelet counting. The effects of a range of concentrations of PGE 2 were determined in the presence of (1) the prostacyclin mimetic iloprost, which operates through G s-linked IP receptors, (2) the cAMP PDE inhibitor DN9693 and (3) the direct-acting adenylate cyclase stimulator forskolin. Vasodilator-stimulated phosphoprotein (VASP) phosphorylation was also determined as a measure of cAMP. PGE 2 reversed the inhibition of aggregation brought about by iloprost; this was prevented in the presence of the EP3 antagonist DG-041, indicating that this effect of PGE 2 is mediated via the EP3 receptor. In contrast, PGE 2 added to the inhibition of aggregation brought about by DN9693 or forskolin; this was reversed by the EP4 antagonist ONO-AE3-208, indicating that this effect of PGE 2 is mediated via the EP4 receptor. Effects on aggregation were accompanied by corresponding changes in VASP phosphorylation. The dominant role of EP3 receptors circumstances where cAMP is increased through a G s-linked mechanism may be relevant to the situation in vivo where platelets are maintained in an inactive state through constant exposure to prostacyclin, and thus the main effect of PGE 2 may be prothrombotic. If so, the results described here further support the potential use of an EP3 receptor antagonist in the control of atherothrombosis.