Evidence that the angiotensin at 2-receptor agonist compound 21 is also a low affinity thromboxane TXA2-receptor antagonist

Objective: The objective of this study was to test whether Compound 21 (C21), a high-affinity, non-peptide angiotensinAT2-receptor agonist, is also an antagonist of thromboxane A2 (TXA2) receptors thus reducing both vasoconstriction and platelet aggregation. Design and method: Binding of C21 to the TXA2 receptor was determined by TBXA2R Arrestin Biosensor Assay. Mouse mesenteric arteries were mounted in wire myographs, and responses to increasing concentrations of C21 (1nM- 10muM) were recorded during submaximal contractions with 0.1muM U46619 (TXA2 analogue) or 1muMphenylephrine. To control forAT2-receptor specificity, arteries were pre-incubated with the AT2-receptor antagonist PD123319 (10muM), or mesenteric arteries from AT2-receptor knock-out (AT2R-/y) mice were used. An inhibitory effect of C21 (100nM - 10muM) on U46619 (0,3muM) induced platelet aggregation was examined in whole human blood. Results: In TXAR2 binding, C21 had an IC50 of 8,6muM. C21 significantly reduced U46619-induced contractions in mesenteric arteries of C57BL/6 and also of AT2R- /y mice, and PD123319 was not able to block this effect, indicating that the vasorelaxant effect was AT2-receptor independent and rather due to TXA2 blockade. C21 also reduced phenylephrine-induced vasoconstriction, but this effect was abolished by PD123319 pointing to an AT2-dependent effect. U46619-induced, but not ADP-induced platelet aggregation was dose-dependently and significantly (at 10muM) inhibited by C21, again suggesting a TXA2-antagonistic effect of C21. Conclusions: Apart from being a high affinity angiotensin AT2-receptor agonist, C21 is also a low affinity antagonist of the TXA2 receptor. By blocking the TXA2 receptor it inhibits TXA2-mediated vasoconstriction and platelet aggregation.