The comparison of the relaxant effects of two methoxylated flavones in rat aortic rings

The vascular effect of salvigenin (6-hydroxyapigenin 6,7,4′-trimethyl ether) (1), a natural flavone, was investigated in comparison with another flavone, 6-hydroxyluteolin 6,7,3′,4′-tetramethyl ether (2) in rat aotic rings. Cumulative addition of their increasing concentrations (10− 9-10− 4M) produced graded relaxations on rings precontracted with noradrenaline (10− 6 M) and KCl (40 mM). The maximal relaxations induced by flavones were similar, however, based on their pEC50 values salvigenin displayed a higher potency than 6-hydroxyluteolin 6,7,3′,4′-tetramethyl ether. Endothelium removal markedly reduced the relaxations to salvigenin while the responses to 6-hydroxyluteolin 6,7,3′,4′-tetramethyl ether were partially affected. In addition, a significant decrease was observed in maximal responsiveness and sensitivity to flavones in the presence of l-NOARG, a NO synthase inhibitor. The cyclooxygenase inhibitor indomethacin significantly inhibited the relaxations to salvigenin, but not altered the responses to 6-hydroxyluteolin 6,7,3′,4′-tetramethyl ether. Our results provide evidence that salvigenin is an effective flavone in causing vasorelaxation which appears to be mediated by endothelium derived NO and prostacyclin. Whereas, the other flavone, 6-hydroxyluteolin 6,7,3′,4′-tetramethyl ether induced relaxant responses are partially endothelium, presumably NO mediated.