Effects of flavonoids on rat aortic smooth muscle contractility: structure-activity relationships.

1. Flavonoids produced a concentration-dependent relaxation of the contractile responses induced by noradrenaline, KCl, or phorbol 12-myristate-13-acetate in rat aortic rings. Only the flavonoid with three contiguous hydroxyls in B rings (myricetin), at low concentrations, potentiates the contractions evoked by these agonists. 2. The relaxant effects of flavanone on the noradrenaline-induced contractions were potentiated by isoprenaline and those of morin, chrysin, flavanone, and naringenin by sodium nitroprusside. 3. Several mechanisms are implicated in the vasodilatory effects of flavonoids: inhibition of protein kinase C; inhibition of cyclic nucleotide phosphodiesterases; and/or decreased Ca2+ uptake.