EVIDENCE AGAINST A CYTOCHROME P450-DERIVED REACTIVE OXYGEN SPECIES ASTHE MEDIATOR OF THE NITRIC OXIDE-INDEPENDENT VASODILATOR EFFECT OF BRADYKININ IN THE PERFUSED HEART OF THE RAT

The coronary vasodilator effect of bradykinin (BK) in the rat is indep endent of NO but dependent on activation of phospholipases with involv ement of cytochrome P450 mono-oxygenase (P450) and stimulation of Ca+-activated K+ channels, implicating an unidentified hyperpolarizing fa ctor generated via P450 metabolism of arachidonic acid (AA). Because P 450 activity also generates free radicals, such as superoxide, which c an lead to the formation of hydrogen peroxide and hydroxyl radicals, w hich are vasoactive, we addressed the contribution of superoxide to th e vasodilator effect of BK in the rat heart. Using rat renal microsome s as a source of P450, we verified that P450-dependent metabolism of A A generated superoxide, as detected by chemiluminescence with lucigeni n. The signal was almost abolished by inhibition of P450 with clotrima zole and the superoxide scavenger 4,5-dihydroxy-1,3-benzene sulfonic a cid. However, base-line superoxide formation, detected by chemilumines cence, in cardiac slices and perfused hearts was unchanged in response to BK or AA. Furthermore, in perfused hearts treated with nitroargini ne and indomethacin to eliminate NO and prostaglandins and elevate per fusion pressure, dose-dependent vasodilator responses to BK were unaff ected by superoxide dismutase plus catalase, a combination that abolis hed dilator responses to hydrogen peroxide. Similarly, the superoxide scavengers 4,5-dihydroxy-1,3-benzene sulfonic acid and 4-hydroxy-2,2,6 ,6-tetramethylpiperidinenoxyl were without effect on vasodilator respo nses to BK. Thus, the coronary vasodilator action of BK is independent of superoxide or its derivatives, which can be excluded as hyperpolar izing factors mediating NO-independent vasodilation in the rat.