Vasorelaxant effect of total flavones from Dendranthema morifolium on rat thoracic aorta

To investigate the vasorelaxant effect of total flavones from Dendranthema morifolium (Ramat.) Tzvel. cv. Hangju (FDM), tension was recorded from rat thoracic aortic rings. FDM completely relaxed, in a dose-dependent manner, the contractions induced by either phenylephrine (PE) or a high concentration of KCl (60 mM) in rings with intact endothelium. Mechanical removal of the endothelium did not significantly modify the vasorelaxant effect of FDM. In endothelium-denuded aortic rings depolarized by 60 mM KCl, FDM inhibited Ca²⁺-induced contraction. FDM also reduced the transient contraction elicited by PE in Ca²⁺-free medium, but had no effect on active phorbol ester-induced contraction. Pretreatment of endothelium-denuded aorta with propranolol, a beta-adrenoceptor antagonist, significantly attenuated the relaxant effect of FDM. These results indicate that FDM induces an endothelium-independent relaxation in rat aortic rings. The mechanisms may include the activation of beta-adrenergic receptors, reduction in Ca²⁺ influx through the voltage-dependent and receptor-operated channels, and inhibition of intracellular Ca release in vascular smooth muscle cells.