Paradoxical augmentation of bradykinin-induced vasodilatation by xanthine/xanthine oxidase-derived free radicals in isolated guinea pig heart.

Increased generation of reactive oxygen species contribute to endothelial dysfunction in atherosclerosis, hypertension and heart failure. Recently, it was suggested that bursts of superoxide anions may inactivate endothelial surface-bound enzymes such as angiotensin converting enzyme (ACE). Here, we tested effects of xanthine/xanthine oxidase-derived superoxide anions on vascular responses and ACE activity in the isolated guinea pig heart. We analysed effects of intracoronary infusion of low concentration of xanthine oxidase (10 mU/ml) in the presence of xanthine (0,5 mM) (X/XO) on bradykinin, other endothelium-dependent and independent vasodilators (acetylcholine, ADP, SNAP), as well as vasoconstrictor responses to angiotensin I and angiotensin II. Surprisingly, X/XO significantly augmented coronary response to bradykinin without an effect on responses to ADP, acetylcholine, SNAP, angiotensin I and angiotensin II. In contrast, inhibition of ACE by perindoprilate (100 nM) resulted in augmentation of bradykinin-induced vasodilatation as well as diminution of angiotensin I-evoked vasoconstriction without an influence on other responses. In summary, in the isolated guinea pig heart, X/XO-derived free radicals selectively augmented coronary vasodilator response to bradykinin, which cannot be explained by X/XO-induced derangement of ACE. The mechanism of this paradoxical phenomenon, which might represent a defensive response of the coronary circulation to oxidative stress requires further investigations.