TBXA2R and atrial fibrillation: In order to investigate the hypothesis that the extract has an α1-adrenergic receptor antagonist effect, H. sabdariffa AF (5 mg/mL) was tested on mesenteric artery rings pre-contracted by U46619 (3 μM), an agonist of the thromboxane A2 receptor, which is an another Gq pathway coupled receptor.