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Since the AT 2 -receptor (AT 2 R) agonist C21 has structural similarity to the AT 1 -receptor antagonists Irbesartan and Losartan, which are antagonists not only at the AT 1 R, but also at thromboxane TP-receptors, we tested the hypothesis that C21 has TP-receptor antagonistic properties as well. Isolated mouse mesenteric arteries from C57BL/6 J and AT 2 R-knockout mice (AT 2 R -/y ) were mounted in wire myographs, contracted with either phenylephrine or the thromboxane A 2 (TXA 2 ) analogue U46619, and the relaxing effect of C21 (0.1 nM - 10 µM) was investigated. The effect of C21 on U46619-induced platelet aggregation was measured by an impedance aggregometer. Direct interaction of C21 with TP-receptors was determined by an β-arrestin biosensor assay. C21 caused significant, concentration-dependent relaxations in phenylephrine- and U46619-contracted mesenteric arteries from C57BL/6 J mice. The relaxing effect of C21 was absent in phenylephrine-contracted arteries from AT 2 R -/y mice, whereas it was unchanged in U46619-contracted arteries from AT 2 R -/y mice. C21 inhibited U46619-stimulated aggregation of human platelets, which was not inhibited by the AT 2 R-antagonist PD123319. C21 reduced U46619-induced recruitment of β-arrestin to human thromboxane TP-receptors with a calculated K i of 3.74 µM. We conclude that in addition to AT 2 R-agonistic properties, C21 also acts as low-affinity TP-receptor antagonist, and that - depending on the constrictor - both mechanisms can be responsible for C21-induced vasorelaxation. Furthermore, by acting as a TP-receptor antagonist, C21 inhibits platelet aggregation. These findings are important for understanding potential off-target effects of C21 in the preclinical and clinical context and for the interpretation of C21-related myography data in assays with TXA 2 -analogues as constrictor., Competing Interests: Conflicts of interest Lena Lindblad was an employee of Vicore Pharma, Gothenburg, Sweden, by the time of the study., (Copyright © 2023 The Authors. Published by Elsevier Inc. All rights reserved.)