Role of ET B and B 2 receptors in the ex vivo platelet inhibitory properties of endothelin and bradykinin in the mouse

We have developed a model to study the inhibitory properties of endogenous autacoids triggered by systemically‐administered vasoactive peptides, on platelet aggregation ex vivo in the mouse. Adenosine diphosphate (ADP) (0.5 – 10 μ M ) induces a concentration‐dependent aggregation of platelet‐rich plasma derived from C57BL/6 mice. Intravenously‐administered endothelin‐1 (0.01 – 1 nmol kg −1 ), the selective ET B agonist, IRL‐1620 (0.01 – 1 nmol kg −1 ) or bradykinin (1 – 100 nmol kg −1 ) significantly reduced in a dose‐dependent fashion the ADP‐induced platelet aggregation. The non‐selective cyclo‐oxygenase (COX) inhibitor, indomethacin, a selective COX‐2 inhibitor NS‐398 or the prostacyclin synthase inhibitor, tranylcypromine (10 mg kg −1 ), markedly reduced the inhibitory properties of endothelin‐1, whereas only a combination of both indomethacin, NS‐398 or tranylcypromine and L ‐NAME (10 mg kg −1 ) were required to abolish the response to bradykinin. An ET B ‐selective antagonist (BQ‐788) or knockout of the B 2 receptor gene (in B 2 knockout mice) abolishes the platelet inhibitory properties of endothelin‐1 and bradykinin, respectively. Our results suggest that intravenously‐administered endothelin‐1 and bradykinin, through ET B and B 2 receptor activation, respectively, inhibit platelet aggregation ex vivo in the mouse. The inhibitory properties of endothelin‐1 require the activation of COX‐2 and the subsequent generation of prostacyclin. In addition to the two previously mentioned factors, nitric oxide is required for the anti‐aggregatory effects of bradykinin.British Journal of Pharmacology (2001) 132 , 934–940; doi: 10.1038/sj.bjp.0703880