Pharmacological effects of selected flavonoids on rat isolated ileum: structure-activity relationship.

1. Eleven selected flavonoids were studied to evaluate their effects on the rat isolated ileum and to determine their structure-activity relationships. 2. The flavonoids rutin and 3',5,7-trihydroxy-4' methoxyflavone-7-rutinoside, which have a sugar moiety (O-rha-glu), had no significant effect on the ileum, indicating that the presence of sugar substitution reduces the biological activity of the flavonoids. 3. Nine other flavonoids caused inhibition of tonic and phasic contractions of the ileum with the following order of potency from highest to lowest: galangin, quercetin, chrysin, xanthomicrol, flavone, naringenin, fisetin, morin, and flavanone. 4. Flavones were more potent than flavanones, indicating that the double bond at carbon 2-3 increases the potency of the flavonoid. 5. Galangin, quercetin, chrysin, and xanthomicrol, which have hydroxyl substituents on carbon 3 and/or 5, showed higher potency than flavone, indicating that such hydroxyl groups are essential for the activity. 6. Galangin was more potent than quercetin, morin, and fisetin, suggesting that the hydroxyl substituents on ring B attenuate the potency. 7. Quercetin caused more potent relaxation of the ileum than morin, suggesting that the presence of a hydroxyl group at C-2' of ring B attenuates the myolytic activity.