Endothelium-independent Vasorelaxant Effect of the Phyto-oestrogen Biochanin A on Rat Thoracic Aorta.

Epidemiological data suggest that the incidence of cardiovascular disease is reduced in people who have a high intake of phytoestrogens. The plant-derived estrogen biochanin A is known to cause vasodilation, but its mechanisms of action remain unclear. This study was undertaken to investigate the effects and mechanisms of biochanin A on rat thoracic aorta. Isolated aortic rings were suspended in individual organ baths and isometric tension was measured. Biochanin A induced significant relaxation in rings with or without endothelium. Contractile responses induced by phenylephrine (PE), KCl and CaCl<inf>2</inf>were antagonized by 10^-7~10^-4mol/L biochanin A. The transient contraction elicited by PE was significantly attenuated by 10^-5mol/L biochanin A in Ca²⁺-free medium. The relaxant effect of biochanin A was significantly inhibited by pretreatment with the K⁺channel antagonists tetraethylammonium and glibenclamide in endothelium-denuded aorta. We conclude that biochanin A induces an endothelium-independent relaxation in rat aortic rings. The underlying mechanism may involve the blockage of Ca²⁺entry through both voltage-dependent and receptor-operated Ca²⁺channels, the inhibition of intracellular Ca²⁺release, and the activation of large-conductance Ca²⁺-activated K⁺channels and ATP-sensitive K⁺channels.