Reperfusion-induced coronary endothelial injury: A new target for ischemic preconditioning

Although cardiac ischemia-reperfusion is well known as a disease of the myocytes, it is now clear that the consequences of this disease also extend to the vascular wall and especially to the endothelium. A rat model of ischemia-reperfusion in vivo was used to detect severe endothelial dysfunction characterized by a decreased nitric oxide (NO)-dependent relaxation to acetylcholine in isolated coronary arteries. Given the essential role of the endothelium and NO in the regulation of vascular tone, protection of the coronary endothelial cells is an important therapeutic target. For this purpose, a focus on the concept of endogenous protection against ischemia, ie, preconditioning, showed that endothelial dysfunction could be reversed by both the early and the delayed phase of preconditioning. With regard to the mechanisms of the coronaroprotective effects of preconditioning, it was shown that both free radicals and NO seem to have an important triggering role, leading to a delayed increase in NO production and decreased adhesion of neutrophils to endothelial cells. Identification of the precise triggers and mediators of this protection will allow the development of new therapeutic agents targeting both the myocardium and the coronary vasculature.     

Coronary endothelial cells: a target of ischemia reperfusion and its treatment?

Ischemia/reperfusion injury of the heart is not limited to cardiomyocytes but also extends to coronary vascular cells, and especially coronary endothelium. Indeed, in different animal models, ischemia followed by reperfusion (but not ischemia alone) markedly decreases endothelium-dependent relaxations of coronary arteries, and especially those induced by nitric oxide (NO), while endothelium-independent responses and smooth muscle responsiveness are usually maintained. Such injury to the endothelium appears to depend on the increased production of oxygen-derived free radicals upon reperfusion, leading to an increased degradation of NO and an acute inflammatory response characterized by an increased adhesion of neutrophils to endothelial cells. Indeed, reperfusion injury to the endothelium may be prevented by free radical scavengers, by prevention of adhesion and/or activation of neutrophils, by exogenous NO supply or increased endogenous production of NO, as well as by ischemic preconditioning. Given the essential role of the endothelium and of NO in the regulation of vasomotor tone, as well as platelet and leukocyte function, it is likely that such changes in coronary endothelial function have important adverse consequences in terms of altered perfusion, and increased risk of vasospasm, but also on the long-term risk of thrombosis and atherosclerosis. Although these coronary endothelial alterations have been essentially evaluated in experimental models and are indeed difficult to assess in the human coronary circulation in the context of myocardial infarction, data obtained in healthy volunteers demonstrate that such post-ischemic alterations of endothelial function may be detected in the peripheral circulation, with underlying molecular mechanisms similar to those demonstrated experimentally. A better understanding of the mechanisms responsible for such endothelial injury may lead to the development of new treatments that protect the endothelium during ischemia and reperfusion, but also possibly in other diseases associated with endothelial dysfunction.     

The role of neutrophils in myocardial ischemia-reperfusion injury

Reperfusion of ischemic myocardium is necessary to salvage tissue from eventual death. However, reperfusion after even brief periods of ischemia is associated with pathologic changes that represent either an acceleration of processes initiated during ischemia per se, or new pathophysiological changes that were initiated after reperfusion. This 'reperfusion injury' shares many characteristics with inflammatory responses in the myocardium. Neutrophils feature prominently in this inflammatory component of postischemic injury. Ischemia-reperfusion prompts a release of oxygen free radicals, cytokines and other proinflammatory mediators that activate both the neutrophils and the coronary vascular endothelium. Activation of these cell types promotes the expression of adhesion molecules on both the neutrophils and endothelium, which recruits neutrophils to the surface of the endothelium and initiate a specific cascade of cell-cell interactions, leading first to adherence of neutrophils to the vascular endothelium, followed later by transendothelial migration and direct interaction with myocytes. This specific series of events is a prerequisite to the phenotypic expression of reperfusion injury, including endothelial dysfunction, microvascular collapse and blood flow defects, myocardial infarction and apoptosis. Pharmacologic therapy can target the various components in this critical series of events. Effective targets for these pharmacologic agents include: (a) inhibiting the release or accumulation of proinflammatory mediators, (b) altering neutrophil or endothelial cell activation and (c) attenuating adhesion molecule expression on endothelium, neutrophils and myocytes. Monoclonal antibodies to adhesion molecules (P-selectin, L-selectin, CD11, CD18), complement fragments and receptors attenuate neutrophil-mediated injury (vascular injury, infarction), but clinical application may encounter limitations due to antigen-antibody reactions with the peptides. Humanized antibodies and non-peptide agents, such as oligosaccharide analogs to sialyl Lewis, may prove effective in this regard. Both nitric oxide and adenosine exhibit broad spectrum effects against neutrophil-mediated events and, therefore, can intervene at several critical points in the ischemic-reperfusion response, and may offer greater benefit than agents that interdict at a single point in the cascade. The understanding of the molecular processes regulating actions of neutrophils in ischemic-reperfusion injury may be applicable to other clinical situations, such as trauma, shock and organ or tissue (i.e. vascular conduits) transplantation.     

Clinical importance of coronary endothelial vasodilator dysfunction and therapeutic options

The vascular endothelium plays a key role in the control of vasomotor tone, local haemostasis and vascular wall proliferation processes. These responses are mediated by a variety of substances released from the endothelium in response to physiological stimuli, including prostacyclin, endothelin, and most importantly nitric oxide (NO). NO mediates vasodilation and furthermore inhibits platelet aggregation, expression of adhesion molecules for monocytes and adhesion of neutrophils, and it impairs growth of vascular smooth muscle cells. Risk factors for coronary atherosclerosis, such as hypercholesterolaemia, impair NO bioactivity, mainly due to an oxidative stress by superoxide radicals (O2-), which are able of rapidly inactivating endothelium-derived NO. Impaired NO bioactivity leads to unopposed paradoxical vasoconstriction of epicardial conductance vessels in response to physiological stimuli such as sympathetic activation as well as impaired vasodilator function of coronary resistance vessels. Therefore, endothelial dysfunction contributes to ischaemic manifestation of coronary artery disease. In addition, enhanced paradoxical vasoconstriction and a loss of endothelial antithrombotic activities might unfavourably modulate the course of acute coronary syndromes. Thus, the aim of therapeutic interventions is to increase NO bioavailability by either increasing NO production or decreasing O2- production in the endothelium. This goal can be reached, for example by ACE inhibitors, lipid-lowering drugs, increased shear-stress by physical exercise, oestrogens, and L-arginine, which have already been shown to improve endothelial vasodilator function. Nevertheless, it has to be determined whether ameliorated endothelial function will contribute to improved patients prognosis.     

Cardioprotective actions of transforming growth factor-β

Transforming growth factor-beta (TGF-β) has been shown to have protective effects in experimental models of myocardial, mesenteric, and cerebral ischemia-reperfusion injury. These effects are mediated by its ability to block adhesion of neutrophils to endothelium and to preserve endothelial function in terms of physiologic release of nitric oxide (NO). TGF-β also maintains the rhythmicity of cultured cardiac myocytes and blocks the suppressive effects of interleukin-1 on their beating rate by antagonizing the pathologic induction of NO synthase. These data suggest that TGF-β will be useful clinically in treatment of both reperfusion injury and inflammatory diseases of the heart.     

Ischemic preconditioning: from theory to practice

Brief periods of ischemia and reperfusion are able to protect the heart from irreversible injury induced by consequent prolonged ischemia and reperfusion stress. This phenomenon called ischemic preconditioning (IP) may limit infarct size, enhance postischemic recovery of cardiac function, reduce reperfusion arrhythmias and vascular dysfunction. Mechanisms of IP are tightly related to alterations of efficiency of metabolic pathways and maintenance of ion homeostasis in ischemic cardiac myocytes. They may be initiated by formation of various triggers (adenosine, bradykinin, NO, free oxygen radicals etc.) that interact with receptors of cardiomyocytes and vascular endothelium or directly alter activity of enzymes. These interactions lead to activation of different pathways of intracellular signal conduction involving contribution of mediators and complex of the secondary messengers of IP. The most typical of them are e-isoform of protein kinase C and the ATP-dependent potassium channels. Biochemical pathways of molecular signaling in the preconditioned myocardium may be different, but always have same final effectors -- intracellular metabolism and ion homeostasis. As a rule, successfully preconditioned myocardium exhibits improved energy state of ischemic cardiomyocytes, reduced Ca(+) overload and attenuated damage of the sarcolemma and mitochondrial membranes. These beneficial changes provide myocardial salvage under conditions of deficient supply of cardiomyocytes with energy substrates and oxygen. Stimulation of adaptative mechanisms of IP is possible with specific receptor agonists or activation of secondary messenger pathways without causing ischemia. At present study of such pharmacological approaches to treating ischemia is a high priority task. In clinical practice selective openers of ATP-dependent potassium channels, A(1) and A(2) adenosine receptor agonists and Na(+)/H(+) exchange inhibitors are used to affect the final effectors of signaling pathways. These pharmacological agents are explored in transluminal coronary angioplasty, cardiac surgery and organ transplantation.     

再灌注心脏保护的新策略:后处理和预处理的比较（英文）

心肌缺血/再灌注引起的损伤（RMMI）是由多种触发物、媒介物和效应器参与的复杂生物反应过程,导致炎症反应、内皮细胞损伤、血流障碍、心功能异常、心肌细胞坏死和凋亡。过去二十多年来,尽管人们开发了多种心脏保护措施（药物性干预）以减少RMMI,但结果并不令人满意。因此,在临床上寻求可行和有效的治疗措施以减轻RMMI有着极大的价值。我们实验室近来报道了在再灌注或恢复供氧早期,快速反复中断冠脉血流或氧供（缺血或缺氧后处理）,可减少心肌组织或细胞内自由基的生成,减轻钙超载,减轻内皮功能的损伤,降低黏附蛋白的表达,减少坏死和凋亡。后处理的这些保护作用可能和内源性生成物如腺苷和一氧化氮增多、蛋白酶（包括PI3KAkt和ERK1/2）的激活、线粒体的ATP依赖性K+通道开放和线粒体通透性转换孔关闭有关。与预处理比较,后处理具有同样的保护效应。在长时间再灌注后仍有减少梗死范围的作用。目前的实验结果和临床观察证实,在缺血后恢复血供时,后处理的应用在治疗缺血/再灌注损伤方面开启了一个新的治疗窗口。     

The role of nitric oxide and NO-donor agents in myocardial protection from surgical ischemic-reperfusion injury

The coronary vascular endothelium is injured by ischemia-reperfusion, which may facilitate the pathophysiological role played by neutrophils. Hearts undergoing coronary artery bypass surgery or other surgical procedures requiring cardiopulmonary bypass and elective cardioplegia undergo repetitive episodes of ischemia and reperfusion, which leads to endothelial injury as well as contractile dysfunction and morphological injury, despite the use of cardioprotective cardioplegic solutions and other strategies of myocardial protection. In cardiac surgery, as in coronary occlusion, endothelial injury seems to occur upon reperfusion with unmodified blood. Blood cardioplegia does not prevent this surgical ‘reperfusion injury’, but does prevent extension of endothelial injury during the period of hypothermic cardioplegic arrest (‘protected ischemia’). It is not known whether global cardioplegic ischemia in preoperatively injured hearts impairs the basal release of nitric oxide (NO) and hence obtunds this endogenous protective mechanism. However, enhancement of blood cardioplegia with the NO precursor, -arginine, reduces postsurgical myocardial injury, suggesting that endogenous or basal release of NO participates in the modulation of ischemic-reperfusion injury. In addition, an NO-donor agent also protects the myocardium from surgical ischemic-reperfusion injury. Both cardioprotective strategies involve inhibition of neutrophil accumulation, consistent with the known inhibitory effects of NO on neutrophil adherence and neutrophil-mediated damage to the coronary endothelium. Therefore, NO-related therapy offers a new strategy to protect the myocardium, including the coronary endothelium, from surgically imposed ischemic-reperfusion injury.     

Nitric Oxide Modulation of Neutrophil-Endothelium Interaction: Difference Between Arterial and Venous Coronary Bypass Grafts

Objectives. This study sought to evaluate the relation between the pattern of neutrophil-endothelial adhesion in saphenous vein (SV) and internal mammary artery (IMA) grafts and the endothelial production of nitric oxide (NO).Background. Autologous IMA and SV grafts (SVGs) are increasingly used as conduits for coronary bypass grafting. Previous studies have demonstrated a greater production of endothelial-derived relaxing factor (NO) from IMA than from SVGs. Because of the well known role of NO in modulating the adhesion of polymorphonuclear leukocytes to the endothelium, we studied the pattern of neutrophil adhesion to the endothelium of IMA and SVs under basal conditions and after inhibition of NO synthesis.Methods. Segments of IMA and SVs were obtained from 20 patients undergoing coronary artery bypass graft surgery. We evaluated the adhesion of both unstimulated and activated neutrophils to the endothelial surface of IMA and SVs in both basal conditions and after inhibition of NO synthesis with N^ω-nitro-l-arginine methyl ester.Results. Under basal conditions, no difference in unstimulated neutrophil adhesion to endothelium was observed between the two vessel conduits. After neutrophil activation, a significantly (p < 0.05) greater adhesion of neutrophils was observed in the SV than in the IMA. After inhibition of NO release, the adhesion of activated neutrophils increased in both vessels, and no significant difference between them was observed. The increased adhesion was attenuated by both l-arginine and sodium nitroprusside.Conclusions. The lesser neutrophil adhesion to the endothelium of the IMA is a consequence of enhanced release of NO at this level; this effect could be responsible for the better early and long-term patency of this conduit over the SVG in coronary bypass grafting.     

Nitric oxide and cardioprotection during ischemia-reperfusion

Coronary artery reperfusion is widely used to restore blood flow in acute myocardial infarction and limit its progression. However, reperfusion of ischemic myocardium results in reperfusion injury and persistent ventricular dysfunction even when achieved after brief periods of ischemia. Normally, small amounts of nitric oxide (NO) generated by endothelial NO synthase (eNOS) regulates vascular tone. Ischemia-reperfusion triggers the release of oxygen free radicals (OFRs) and a cascade involving endothelial dysfunction, decreased eNOS and NO, neutrophil activation, increased cytokines and more OFRs, increased inducible NO synthase (iNOS) and marked increase in NO, excess peroxynitrite formation, and myocardial injury. Low doses of NO appear to be beneficial and high doses harmful in ischemia-reperfusion. eNOS knock-out mice confirm that eNOS-derived NO is cardioprotective in ischemia-reperfusion. iNOS overexpression increases peroxynitrite but did not cause severe dysfunction. Increased angiotensin II (AngII) after ischemia-reperfusion inactivates NO, forms peroxynitrite and produces cardiotoxic effects. Beneficial effects of angiotensin-converting-enzyme inhibition and AngII type 1 (AT₁) receptor blockade after ischemia-reperfusion are partly mediated through AngII type 2 (AT₂) receptor stimulation, increased bradykinin and NO. Interventions that enhance NO availability by increasing eNOS might be beneficial after ischemia-reperfusion.     

Involvement of neutrophils in the pathogenesis of lethal myocardial reperfusion injury

Neutrophils respond to myocardial ischemia-reperfusion in a manner similar to the bacterial invasion of a host. The inflammatory-like response that follows the onset of reperfusion involves intense interactions with the coronary vascular endothelium, arterial wall, and cardiomyocytes in a very well-choreographed manner. Neutrophils have been implicated as primary and secondary mediators of lethal injury after reperfusion to coronary vascular endothelium and cardiomyocytes. The involvement of neutrophils in the pathogenesis of lethal myocardial injury has been inferred from (1) their presence and accumulation in reperfused myocardium in temporal agreement with injury induced, (2) the armamentarium of toxic agents such as oxidants and proteases that are released by neutrophils in reperfused myocardium, (3) responsivity to (recruitment by and/or activation by) inflammatory factors released by reperfused myocardium, and (4) inhibition of lethal post-ischemic myocyte or endothelial cell injury by strategies that interdict neutrophil interactions at any number of stages. However, whether neutrophils are directly involved in the pathogenesis of lethal reperfusion injury in the myocardium, are just pedestrian (first) responders to inflammatory signals released after the onset of reperfusion, or are important to an early but not clinically important phase of pathology are still points of controversy. As with the general area of myocardial protection itself, the failure to reproduce the salubrious effects of anti-neutrophil therapeutic strategies and to successfully translate these strategies into clinical practice has not only fueled the debate, but has jeopardized the further pursuit of myocardial protection therapeutics to improve post-ischemic outcomes. This review will describe the molecular responses of neutrophils to ischemia-reperfusion, discuss the cellular and tissue damage inflicted either directly or indirectly by these white cells, and discuss the physiological impact of interdiction of neutrophil-mediated interactions with myocardial cells at various levels on lethal post-ischemic injury. In addition, it will discuss the arguments for and against the involvement of neutrophils in responses to ischemia-reperfusion in experimental models, and the failure to translate experimentally successful therapy into clinical practice.     

The role of oxidants and free radicals in reperfusion injury

While timely reperfusion of acute ischemic myocardium is essential for myocardial salvage, reperfusion results in a unique form of myocardial damage. Functional alterations occur, including depressed contractile function and decreased coronary flow as well as altered vascular reactivity. Both myocardial stunning and infarction are seen. Over the last two decades, it has become increasingly clear that oxidant and oxygen radical formation is greatly increased in the post-ischemic heart and serves as a critical central mechanism of post-ischemic injury. This oxidant formation is generated through a series of interacting pathways in cardiac myocytes and endothelial cells and triggers subsequent leukocyte chemotaxis and inflammation. Nitric oxide (NO) production and NO levels are also greatly increased in ischemic and post-ischemic myocardium, and this occurs through NO synthase (NOS)-dependent NO formation and NOS-independent nitrite reduction. Recently, it has been shown that the pathways of oxygen radical and NO generation interact and can modulate each other. Under conditions of oxidant stress, NOS can switch from NO to oxygen radical generation. Under ischemic conditions, xanthine oxidase can reduce nitrite to generate NO. NO and peroxynitrite can inhibit pathways of oxygen radical generation, and, in turn, oxidants can inhibit NO synthesis from NOS. Ischemic preconditioning markedly decreases NO and oxidant generation, and this appears to be an important mechanism contributing to preconditioning-induced myocardial protection.     

Endotheliale Dysfunktion — eine Bestandsaufnahme und Ansätze zur Therapie

The vascular endothelium is the inner lining of all blood vessels and serves as an important autocrine and paracrine organ, that regulates vascular wall functions. Because of its strategic location between the circulating blood and the vascular wall, the endothelium interacts with cellular and neurohumoral mediators, thus controlling vascular contractile state and cellular composition. The vascular endothelium maintains vascular homeostasis by modulating blood vessel tone, by regulating local cellular growth and extracellular matrix deposition and by controlling hemostatic as well as inflammatory responses. One of the best characterized and most important substances released from the endothelium is nitric oxide (NO). NO is a soluble gas which is continuously synthesized from the amino acid L-arginine in endothelial cells by the constitutively expressed nitric oxide synthase. The most important stimuli represent physical factors such as shear stress and pulsatile stretching of the vessel wall as well as circulating and locally released vasoactive substances. The endothelium can be seen as a biosensor, reacting to a large variety of stimuli and therefore maintaining adequate NO release. A large number of risk factors for atherosclerosis including hypercholesterolemia, systemic hypertension, smoking and diabetes have been associated with impaired endothelial NO-mediated vasodilation. "Endothelial dysfunction" is an early marker of atherosclerosis and may be closely related to the disease process. In acute coronary syndromes dysfunctional endothelium may trigger the devastating event of plaque rupture by promoting adhesion of leukocytes, vasoconstriction, activation of platelets and thrombos formation. Atherosclerotic blood vessels are further characterized by activation through zytokines and expression of cellular adhesion molecules such as vascular cell adhesion molecule-1 (VCAM-1), intercellular adhesion molecule-1 (ICAM-1) and endothelial-leukocyte adhesion molecule-1 (E-Selectin). After adhesion to the endothelium mononuclear cells migrate to the subendothelial space to take up oxidized LDL, thus transforming into foam cells, a hall mark of early atherosclerotic lesions. A number of conditions including infection with Chlamydia pneumoniae may cause continuous activation of the endothelium and inflammation of the vessel wall. Continuous endothelial dysfunction and activation, caused by risk factors and infection, represent the basis for atherogenesis and acute coronary syndromes.     

Endothelial dysfunction in uraemia

Cardiovascular disease is a major cause of morbidity and mortality in chronic renal failure (CRF). The endothelium plays a central role in the control of many aspects of vascular function, and abnormalities may contribute to the generation of atherosclerosis. The endothelium produces a wide range of regulatory molecules which, in health, function in concert to provide a carefully balanced anti-atherogenic environment. Endothelial dysfunction has been repeatedly demonstrated in renal failure, is present in the absence of anatomically obvious disease and appears to be useful in the prediction of morbidity and mortality in other cardiovascular risk groups. One of the most intensively studied and important mediators of endothelial function is nitric oxide (NO), whose production is reduced in CRF. A number of possible mechanisms for reduced NO bioavailability have been investigated including substrate limitation, competitive inhibition of NO synthase by endogenous NO synthase inhibitors known to accumulate in renal failure, and premature quenching of NO by free radicals present in high concentrations in this group. A clearer understanding of the pathogenesis of endothelial dysfunction in CRF has potential clinical implications. It may provide avenues for therapeutic interventions before the onset of clinically obvious cardiovascular disease in this high-risk patient group.     

Clinical importance of endothelial function in arteriosclerosis and ischemic heart disease.

The vascular endothelium is a dynamic endocrine organ that regulates vascular tone, local homeostasis, and the fibro-inflammatory-proliferative process. These responses are mediated by various substances released from the endothelium in response to physiologic stimuli, including prostacyclin, endothelin and, most importantly, nitric oxide (NO). NO mediates vasodilation and inhibits platelet aggregation, thrombus formation, expression of adhesion molecules and chemokines for leukocytes, and oxidative stress. It also attenuates growth and proliferation of vascular smooth muscle cells. Risk factors for atherosclerosis, such as hypercholesterolemia, hypertension, diabetes and cigarette smoking, impair endothelial function, which leads to atherosclerosis and results in ischemic manifestations such as acute coronary syndrome and stroke. Thus, therapeutic intervention aimed at increasing NO bioavailability by statins or angiotensin-converting enzyme inhibitors might improve patient prognosis. Vascular endothelial function is an important and clinically relevant therapeutic target for cardiovascular disease.     

Effect of classic preconditioning and diazoxide on endothelial function and O2- and NO generation in the post-ischemic guinea-pig heart

OBJECTIVES: A hypothesis was tested that a reaction product between superoxide (O2-) and nitric oxide (NO) mediates post-ischemic coronary endothelial dysfunction that ischemic preconditioning (IPC) protects the endothelium by preventing post-ischemic cardiac O2- and/or NO formation, and that the opening of the mitochondrial ATP-dependent potassium channel (mKATP) plays a role in the mechanism of IPC. METHODS: Langendorff-perfused guinea-pig hearts were subjected either to 30 min global ischemia/30 min reperfusion (IR) or were preconditioned prior to IR with three cycles of either 5 min ischemia/5 min reperfusion or 5 min infusion/5 min wash-out of mKATP opener, diazoxide (0.5 microM). Coronary flow responses to acetylcholine (ACh) and nitroprusside were used as measures of endothelium-dependent and -independent vascular function, respectively. Myocardial outflow of O2- and NO, and functional recoveries were followed during reperfusion. RESULTS: IR impaired the ACh response by approximately 60% and augmented cardiac O2- and NO outflow. Superoxide dismutase (150 U/ml) and NO synthase inhibitor, l-NMMA (100 microM) inhibited the burst of O2- and NO, respectively, and afforded partial preservation of the ACh response in IR hearts. NO scavenger, oxyhemoglobin (25 microM), afforded similar endothelial protection. IPC and diazoxide preconditioning attenuated post-ischemic burst of O2-, but not of NO, and afforded a complete endothelial protection. Diazoxide given after 30-min ischemia increased the O2- burst and was not protective. The effects of IPC and diazoxide preconditioning were not affected by HMR-1098 (25 microM), a selective blocker of plasmalemmal KATP, and were abolished by glibenclamide (0.6 microM) and 5-hydroxydecanoate (100 microM), a nonselective and selective mK(ATP) blocker, respectively. 5-Hydroxydecanoate produced similar effects, whether it was given as a continuous treatment or was washed out prior to IR. CONCLUSION: The results suggest that in guinea-pig heart: (i) a reaction product between O2- and NO mediates the post-ischemic endothelial dysfunction; (ii) the mK(ATP) opening serves as a trigger of the IPC and diazoxide protection; and (iii) the mK(ATP) opening protects the endothelium in the mechanism that involves the attenuation of the O2- burst at reperfusion.     

Ischemia and reperfusion: the endothelial perspective. A radical view

Ischemia and reperfusion (IR) injury causes a variety of changes in tissue homeostasis that lead to necrosis and/or programmed cell death. Due to its strategic location at the luminal surface of vessels, the vascular endothelium is particularly sensitive to IR. In particular, endothelial biosynthetic activities (and their protective effects) appear to be impaired by the oxidative burst induced by a sudden increase in oxygen free radical species upon reperfusion. Importantly, this endothelial damage can be easily assessed in vivo in humans by measuring endothelium-dependent vasorelaxation. Paradoxically, recent studies have emphasized the central role of free radicals (including oxygen free radicals and nitric oxide) also in a protective process, denominated ischemic preconditioning, i.e. a condition whereby a given stimulus can increase the tolerance of a tissue to IR damage. We discuss the role of the endothelium in determining the mechanism of IR injury, and on the other side, the effect of IR injury on endothelial function. In particular, we focus on the role of reactive free radicals in endothelial IR injury and in the development of ischemic preconditioning.     

Delayed endothelial protective effects of monophosphoryl lipid A after myocardial ischemia and reperfusion in rats.

Monophosphoryl lipid A (MLA) induces delayed (24 h) myocardial protection in various animal models of ischemia/reperfusion injury, and thus mimics the second window of preconditioning against cardiac injury. However, the potential endothelial protective effects of this drug have not been evaluated. The present study was designed to assess whether MLA exerts delayed protective effects against reperfusion-induced coronary endothelial dysfunction in rats, as well as the protective role of iNOS in this protection. Wistar rats received a single i.v. injection of MLA (450 microg/kg) or solvent. Twenty-four hours later, they were anesthetized and subjected to 20 min ischemia with 60 min reperfusion, in the absence or the presence of the iNOS inhibitor aminoguanidine (300 mg/kg i.p.). At the end of reperfusion, 1.5-2 mm coronary segments (average diameter 250 microm) were removed distal to the site of occlusion and mounted in wire myographs. Endothelium-dependent relaxations to acetylcholine were determined in arteries pre-contracted by serotonin. Ischemia/reperfusion induced a marked decrease in the coronary responses to acetylcholine (maximal relaxations: sham 64+/-8%, n=8; ischemia/reperfusion: 41+/-9%, n=8 P<0.05). This impaired response was partially restored by MLA (55+/-4%, n=10 P<0.05 vs ischemia/reperfusion). The effect of MLA was not affected by aminoguanidine (57+/-5%, n=6). Thus, in addition to protecting myocytes, MLA induces a delayed protection against coronary endothelial dysfunction. However, in contrast to its effects on myocytes, the endothelial protective effects do not appear to involve iNOS.     

The endothelium and lipoproteins: insights from recent cell biology and animal studies

Both lipoproteins and the endothelium play critical roles in the initiation and progression of atherosclerosis. An understanding of the interactions between lipoproteins and the endothelium facilitates our understanding of atherogenesis and could suggest new therapeutic targets. Lipoproteins have important effects on endothelial cells. Atherogenic lipoproteins such as remnants, low-density lipoprotein (LDL), and oxidized LDL act on endothelial cells to cause upregulation of endothelial adhesion molecules and selectins, promotion of oxygen radicals, increased apoptosis, and reduced endothelium-dependent relaxation. Antiatherogenic lipoproteins such as HDL protect endothelial cells from oxidative stress and apoptosis and reduce adhesion molecule expression. Conversely, the endothelium has major effects on lipoprotein metabolism and function. Several lipases, including lipoprotein lipase, hepatic lipase, endothelial lipase, and secretory phospholipase A2, are bound to the endothelial cell matrix and have the ability to hydrolyze lipoprotein triglycerides and phospholipids. Furthermore, endothelial cells express a variety of lipoprotein receptors including the VLDL receptor, scavenger receptor A, SR-BI, CD36, and LOX-1, although little is known about their function on endothelial cells. Although a great deal is known about endothelial-lipoprotein interactions, more research is needed in this important area.