Flavonoid potentiation of contractile responses in rat blood vessels

Certain bioflavonoids and phenolic compounds have long been known to enhance catecholamine responses, in vivo and in vitro. In the present studies the flavone, baicalein, potentiated nerve-stimulated contractions in vitro in rat tail and femoral artery isometric ring preparations. Inhibition of catecholamine reuptake with cocaine or catecholamine metabolism with tropolone and parglyine (monoamine oxidase and catecholamine-O-methyl transferase inhibitors, respectively) did not alter baicalein's ability to potentiate contractile responses to nerve stimulation. Baicalein (10(-5) M), the prototype flavone, also increased sensitivity to exogenous norepinephrine, serotonin, arginine vasopressin and to the noncatecholamine alpha-1 and alpha-2 adrenergic agonists, cirazoline and tramazoline. Structure-function studies indicated that flavone potentiation required three contiguous A or B ring hydroxylations. Several nonflavone phenol derivatives with three contiguous hydroxyls also potentiated nerve stimulation responses. As baicalein is a potent lipoxygenase inhibitor, comparisons were made between potentiating ability and lipoxygenase inhibitory activity in a series of flavonoids. There was no direct correlation between inhibition of 12-hydroxy-5,8,10,14-eicosatetraenoic acid levels in thrombin stimulated human platelets and potentiation of contractile responses in the femoral artery. Additionally, the specific substrate analog lipoxygenase inhibitor, 5,8,11-eicosatriynoic acid, and the cyclooxygenase inhibitor, ibuprofen, were nonpotentiating. Ibuprofen pretreatment did not alter the potentiating action of baicalein. It is concluded that flavonoids with three contiguous hydroxyls on either the A or B ring increase in vitro vascular responsiveness via a post-synaptic process, independent of cyclooxygenase, lipoxygenase, monoamine oxidase or catecholamine-O-methyl transferase activity.