Antiaggregatory effects of flavonoids in vivo and their influence on lipoxygenase and cyclooxygenase in vitro.

Quercetin, rutin and troxerutin were found to inhibit platelet aggregation on collagen strip superfused with blood of anesthetized cats. Quercetin was the most potent acting at the dose of 1 micrograms/kg. Its effect was shortlasting. Troxerutin was a weak inhibitor of platelet aggregation and its effect was delayed. Quercetin inhibited in 50% 15-lipoxygenase and 12-lipoxygenase in vitro at the concentration of 1.3 microm and 13 microM respectively. It stimulated cyclooxygenase when 100 microM of arachidonic acid was applied. Quercetin inhibited cyclooxygenase in the presence of 1.6 microM of substrate. Rutin was a weaker inhibitor of lipoxygenase. Troxerutin was inactive in all experiments in vitro. It is concluded that unusually strong effect of quercetin in vivo can be explained neither by its influence on cyclooxygenase nor on lipoxygenase because the effects in vitro were observed in much higher concentrations.