Burkhard Hornig

ROLE OF OXIDATIVE STRESS IN ATHEROSCLEROSIS

The common risk factors for atherosclerosis increase production of reactive oxygen species (ROS) by endothelial, vascular smooth muscle, and adventitial cells. These ROS initiate processes involved in atherogenesis through several important enzyme systems, including xanthine oxidase, nicotinamide adenine dinucleotide phosphate (NADPH) oxidases, and nitric oxide synthase. Physical forces also regulate vascular production of ROS. Oscillatory shear, which is present at sites where atherosclerosis develops, seems a particularly potent stimulus of superoxide production. The signaling cascade for activation of the NAD(P)H oxidase by angiotensin II has recently been elucidated and seems to involve a feed-forward mechanism that permits ongoing production of ROS for prolonged periods. Oxidative stress in humans with coronary artery disease is also exacerbated by a reduction of vascular extracellular superoxide dismutase, normally an important protective enzyme against the superoxide anion.

Vascular Oxidative Stress and Endothelial Dysfunction in Patients With Chronic Heart Failure Role of Xanthine-Oxidase and Extracellular Superoxide Dismutase

Background—Impaired flow-dependent, endothelium-mediated vasodilation (FDD) in patients with chronic heart failure (CHF) results, at least in part, from accelerated degradation of nitric oxide by oxygen radicals. The mechanisms leading to increased vascular radical formation, however, remain unclear. Therefore, we determined endothelium-bound activities of extracellular superoxide dismutase (ecSOD), a major vascular antioxidant enzyme, and xanthine-oxidase, a potent radical producing enzyme, and their relation to FDD in patients with CHF. Methods and Results—ecSOD and xanthine-oxidase activities, released from endothelium into plasma by heparin bolus injection, were determined in 14 patients with CHF and 10 control subjects. FDD of the radial artery was measured using high-resolution ultrasound and was assessed before and after administration of the antioxidant vitamin C (25 mg/min; IA). In patients with CHF, endothelium-bound ecSOD activity was substantially reduced (5.0±0.7 versus 14.4±2.6 U · mL−1 · min−1;P <0.01) and closely related to FDD (r =0.61). Endothelium-bound xanthine-oxidase activity was increased by >200% (38±10 versus 12±4 nmol O2·− · &mgr;L−1;P <0.05) and inversely related to FDD (r =−0.35) in patients with CHF. In patients with low ecSOD and high xanthine-oxidase activity, a greater benefit of vitamin C on FDD was observed, ie, the portion of FDD inhibited by radicals correlated negatively with ecSOD (r =−0.71) but positively with xanthine-oxidase (r =0.75). Conclusions—These results demonstrate that both increased xanthine-oxidase and reduced ecSOD activity are closely associated with increased vascular oxidative stress in patients with CHF. This loss of vascular oxidative balance likely represents a novel mechanism contributing to endothelial dysfunction in CHF.

Electron Spin Resonance Characterization of Vascular Xanthine and NAD(P)H Oxidase Activity in Patients With Coronary Artery Disease Relation to Endothelium-Dependent Vasodilation

Background—Increased inactivation of nitric oxide by superoxide (O2·−) contributes to endothelial dysfunction in patients with coronary disease (CAD). We therefore characterized the vascular activities of xanthine oxidase and NAD(P)H oxidase, 2 major O2·−-producing enzyme systems, and their relationship with flow-dependent, endothelium-mediated vasodilation (FDD) in patients with CAD. Methods and Results—Xanthine- and NAD(P)H-mediated O2·− formation was determined in coronary arteries from 10 patients with CAD and 10 controls by using electron spin resonance spectroscopy. Furthermore, activity of endothelium-bound xanthine oxidase in vivo and FDD of the radial artery were determined in 21 patients with CAD and 10 controls. FDD was measured before and after infusion of the antioxidant vitamin C (25 mg/min i.a.) to determine the portion of FDD inhibited by radicals. In coronary arteries from patients with CAD, xanthine- and NAD(P)H-mediated O2·− formation was increased compared with controls (xanthine: 12±2 versus 7±1 nmol O2·−/&mgr;g protein; NADH: 11±1 versus 7±1 nmol O2·−/&mgr;g protein; and NADPH: 12±2 versus 9±1 nmol O2·−/&mgr;g protein; each P <0.05). Endothelium-bound xanthine oxidase activity was increased by >200% in patients with CAD (25±4 versus 9±1 nmol O2·−/&mgr;L plasma per min;P <0.05) and correlated inversely with FDD (r =−0.55;P <0.05) and positively with the effect of vitamin C on FDD (r =0.54;P <0.05). Conclusions—The present study represents the first electron spin resonance measurements of xanthine and NAD(P)H oxidase activity in human coronary arteries and supports the concept that increased activities of both enzymes contribute to increased vascular oxidant stress in patients with CAD. Furthermore, the present study suggests that increased xanthine oxidase activity contributes to endothelial dysfunction in patients with CAD and may thereby promote the atherosclerotic process.

The relevance of tissue angiotensin-converting enzyme: manifestations in mechanistic and endpoint data

Angiotensin-converting enzyme (ACE) is primarily localized (>90%) in various tissues and organs, most notably on the endothelium but also within parenchyma and inflammatory cells. Tissue ACE is now recognized as a key factor in cardiovascular and renal diseases. Endothelial dysfunction, in response to a number of risk factors or injury such as hypertension, diabetes mellitus, hypercholesteremia, and cigarette smoking, disrupts the balance of vasodilation and vasoconstriction, vascular smooth muscle cell growth, the inflammatory and oxidative state of the vessel wall, and is associated with activation of tissue ACE. Pathologic activation of local ACE can have deleterious effects on the heart, vasculature, and the kidneys. The imbalance resulting from increased local formation of angiotensin II and increased bradykinin degradation favors cardiovascular disease. Indeed, ACE inhibitors effectively reduce high blood pressure and exert cardio- and renoprotective actions. Recent evidence suggests that a principal target of ACE inhibitor action is at the tissue sites. Pharmacokinetic properties of various ACE inhibitors indicate that there are differences in their binding characteristics for tissue ACE. Clinical studies comparing the effects of antihypertensives (especially ACE inhibitors) on endothelial function suggest differences. More comparative experimental and clinical studies should address the significance of these drug differences and their impact on clinical events.

Endothelial Function and Bradykinin in Humans

SummaryThe endothelium controls vascular smooth muscle tone by secreting relaxing and contracting factors. There is a constant release of endothelium-derived relaxing factors (EDRFs) under basal conditions. In addition, the endothelium can increase the release of EDRFs in response to humoral stimulation by vasoactive substances such as acetylcholine or bradykinin. Under physiological conditions, the most important stimulus to the release of EDRFs is an increase in blood flow leading to increased shear stress on endothelial cells. Recent experimental studies raised the possibility that bradykinin plays an important role in the regulation of vascular tone at rest and during flow-stimulated conditions. Bradykinin is a very potent vasodilator that exerts its vasodilatory actions by causing endothelial release of nitric oxide, prostacyclin and/or a hyperpolarising factor [endothelium-derived hyperpolarising factor (EDHF)]. This concept is also supported by recent studies in humans demonstrating that bradykinin contributes to the regulation of coronary vascular tone under resting and flow-stimulated conditions. This mechanism has now been shown to be important in both human peripheral and coronary arteries. Angiotensin converting enzyme (ACE) inhibitors not only reduce angiotensin II, but also increase bradykinin levels, since the angiotensin converting enzyme is identical to kininase II, an enzyme that degrades bradykinin. This raises the possibility that beneficial vascular effects of ACE inhibitors may be related to increased availability of bradykinin. Indeed, we have recently shown that ACE inhibition improves flow-dependent, endothelium-mediated vasodilation and that this beneficial effect of ACE inhibition is bradykinin dependent. These findings raise the possibility that the beneficial effects of ACE inhibition in heart failure and coronary artery disease might be partly due to improved endothelial function.

Importance of endothelial function in chronic heart failure

Chronic heart failure (CHF) is associated with neurohumoral activation and alterations of the peripheral circulation. Several mechanisms are involved in the impaired peripheral perfusion, including increased sympathetic tone. Recent data suggest an important role of the endothelium for peripheral perfusion in CHF. Endothelium-dependent dilation of resistance vessels is blunted in patients with severe CHF and may be involved in the impaired reactive hyperemia in these patients. In conductance vessels, flow-dependent dilation is reduced in CHF, reflecting endothelial dysfunction of large conduit vessels. To investigate endothelial function in humans in vivo, agents such as acetylcholine are used to stimulate the release of endothelium-derived nitric oxide (NO). Conversely, N-mono-methyl-L-arginine (L-NMMA), a specific inhibitor of NO synthesis from L-arginine, decreases forearm blood flow by inhibiting the basal release of NO. Conversely to the impaired stimulated release of NO by acetylcholine, the decrease in blood flow induced by L-NMMA appears to be exaggerated in CHF. The blood flow response to nitroglycerin or sodium nitroprusside, both endothelium-independent vasodilators, is usually preserved in patients with nonedematous CHF, indicating a normal response of the vascular smooth muscle of resistance vessels to exogenous NO. Therefore, impaired endothelium-dependent dilation of peripheral resistance vessels emerges in CHF, suggesting a reduced release of NO on stimulation. Endothelial dysfunction may therefore be involved in the impaired vasodilator capacity in the peripheral circulation, e.g., during exercise. In contrast, the basal release of NO from endothelium of resistance vessels appears to be enhanced and may play an important compensatory role in CHF.

Reversal of endothelial dysfunction in humans.

Correspondence and requests for reprints to Dr Burkhard Hornig, Abteilung Kardiologie und Angiologie, Medizinische Hochschule Hannover, Hannover, Germany. Tel: +49 511 532 6628; fax: +49 511 532 3357; e-mail: hornig.burkhard@mh-hannover.de Introduction The endothelium plays a key role for the maintenance of a normal vascular function and structure 1 . Vice versa, an abnormal endothelial function impairs vascular tone, favors platelet aggregation and monocyte adhesion and is associated with an increased risk of future cardiovascular events 2 4 . Thus, restoration of normal endothelial function becomes an attractive therapeutic Ž . goal. Pharmacological interventions Table 1 that improve endothelial function may have the potential to preserve vasomotor tone, and prevent vascular events such as unstable angina or myocardial infarction including cardiovascular mortality.

Steps ahead in the non-invasive testing of vascular function.

Atherosclerosis is the major cause of death in all Western countries. Accordingly, it is of general interest to characterize individuals with increased risk for cardiovascular events [such as stroke, myocardial infarction or need for percutaneous transluminal coronary angioplasty (PTCA)/coronary artery bypass graft (CAGB)] as early as possible using easy to use, noninvasive methods. In this respect, the present study of Vuilleumier et al. [1] represents a step in the right direction.