Mechanism of vasorelaxation induced by 3'-hydroxy-5,6,7,4'-tetramethoxyflavone in the rats aortic ring assay.

Previous studies have demonstrated that 3'-hydroxy-5,6,7,4'-tetramethoxyflavone (TMF) content in Orthosiphon stamineus fractions correlate with its vasorelaxation activity. Even with the availability of previous studies, there is still very little information on the vasorelaxation effect of TMF, and few scientific studies have been carried out. Therefore, the present study was designed to investigate the vasorelaxation activity and mechanism of action of the TMF. The vasorelaxation activity and the underlying mechanisms of TMF were evaluated on thoracic aortic rings isolated from Sprague Dawley rats. TMF caused the relaxation of aortic rings with endothelium pre-contracted with phenylephrine. However, the vasorelaxant effect of TMF was significantly decreased in PE-primed endothelium-denuded and potassium chloride-primed endothelium-intact aortic rings. In the presence of Nω-nitro-L-arginine methyl ester, methylene blue, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, indomethacin, tetraethylammonium, 4-aminopyridine, barium chloride, atropine and propranolol, the relaxation stimulated by TMF was significantly reduced. TMF was also found to reduce Ca release from sarcoplasmic reticulum (via IPR) and block calcium channels (VOCC). The present study demonstrates the vasorelaxant effect of TMF involves NO/sGC/cGMP and prostacyclin pathways, calcium and potassium channels and muscarinic and beta-adrenergic receptors.