REVIEW paper: pathophysiology of myocardial reperfusion injury: the role of genetically engineered mouse models

Coronary heart disease is the leading cause of death worldwide, affecting millions of men and women each year. Following an acute myocardial infarction, early and successful reperfusion therapy with thrombolytic therapy or primary percutaneous coronary intervention plays an important role in minimizing tissue injury associated with cessation of blood flow. The process of restoring blood flow to the ischemic myocardium, however, can induce additional injury. This phenomenon, termed myocardial ischemia-reperfusion (MI-R) injury, can paradoxically reduce the beneficial effects of myocardial reperfusion. MI-R injury is characterized by the formation of oxygen radicals upon reintroduction of molecular oxygen to the ischemic tissue, resulting in widespread lipid and protein oxidative modifications, mitochondrial injury, and cell death. In addition, studies have shown that MI-R is characterized by an inappropriate immune response in the microcirculation, resulting in leukocyte-endothelial cell interactions mediated by the upregulation of both leukocyte and endothelial cell adhesion molecules. Furthermore, MI-R ameliorates the production of certain cardioprotective factors such as nitric oxide. Advances in the generation of genetically modified mouse models enable researchers to identify the functional importance of genes involved in these processes.

Statin therapy and myocardial no-reflow

HMG-CoA reductase inhibitors (statins) have now become one of the most powerful pharmacological strategies in the treatment of cardiovascular diseases. Originally, the cardioprotective effects of statins were thought to be mediated through lipid lowering actions. However, it has now become increasingly clear that the beneficial effects of statins are not related to the lipid lowering effects, but rather to a number of pleiotropic actions. Of particular interest, statins have been shown to increase bioavailability of nitric oxide and protect against vascular inflammation and cardiac cell death in a number of cardiovascular disease states. In this present issue of the British Journal of Pharmacology, Zhao and colleagues provide a novel mechanism of action for statins with the observation that simvastatin reduces myocardial 'no-reflow' after ischemia and reperfusion by activating the mitochondrial K(ATP) channel. The findings of the present study have very profound implications for the treatment of cardiovascular disease. This commentary discusses the implications of these findings and how they relate to the established cardioprotective actions of statins.

Control of myocardial oxygen consumption in transgenic mice overexpressing vascular eNOS

Our objective was to investigate the potential role of selective endothelial nitric oxide (NO) synthase (eNOS) overexpression in coronary blood vessels in the control of myocardial oxygen consumption (MVo2). Transgenic (Tg) eNOS-overexpressing mice (eNOS Tg) ( n = 22) and wild-type (WT) mice ( n = 24) were studied. Western blot analysis indicated greater than sixfold increase of eNOS in cardiac tissue. Echocardiography in awake mice indicated no difference in cardiac function between WT and eNOS Tg; however, systolic pressure in eNOS Tg mice decreased significantly (126 ± 2.3 to 109 ± 2.3 mmHg; P < 0.05), whereas heart rate (HR) was not different. Total peripheral resistance (TPR) was also decreased (9.8 ± 0.8 to 7.6 ± 0.4 4 mmHg·ml−1·min; P < 0.05) in eNOS Tg. Furthermore, female eNOS Tg mice showed even lower TPR (7.2 ± 0.4 mmHg·ml−1·min) compared with male eNOS mice (8.6 ± 0.5, mmHg·ml·min−1; P < 0.05). Left ventricular slices were isolated from WT and eNOS Tg mice. With the use of a Clark-type oxygen electrode in an airtight bath, MVo2was determined as the percent decrease during increasing doses (10−10to 10−4mol/l) of bradykinin (BK), carbachol (CCh), forskolin (10−12to 10−6mol/l), or S-nitroso- N-acetyl penicillamine (SNAP; 10−7to 10−4mol/l). Baseline MVo2was not different between WT (181 ± 13 nmol·g−1·min−1) and eNOS Tg (188 ± 14 nmol·g−1·min−1). BK decreased MVo2(10−4mol/l) in WT by 17% ± 1.1 and 33% ± 2.7 in eNOS Tg ( P < 0.05). CCh also decreased MVo2, 10−4mol/l, in WT by 20% ± 1.7 and 31% ± 2.0 in eNOS Tg ( P < 0.05). Forskolin (10−6mol/l) or SNAP (10−4mol/l) also decreased MVo2in WT by 24% ± 2.8 and 36% ± 1.8 versus eNOS 31% ± 1.8 and 37% ± 3.5, respectively. N-nitro-l-arginine methyl ester (10−3mol/l) inhibited the MVo2reduction to BK, CCh, and forskolin by a similar degree ( P < 0.05), but not to SNAP. Thus selective overexpression of eNOS in cardiac blood vessels in mice enhances the control of MVo2by eNOS-derived NO.

Increased disease activity in eNOS-deficient mice in experimental colitis

Oral dextran sodium sulfate (DSS, 3%) produces experimental colitis with many features of human inflammatory bowel disease (IBD), (leukocyte extravasation, cachexia, and histopathology). Previous studies suggest that the inducible nitric oxide synthase (iNOS) in blood cells or in the endothelium contribute to this injury. However, until now no study has been performed to directly evaluate the role of endothelial nitric oxide synthase (eNOS) in IBD. We compared disease activity in wild-type (eNOS+/+) and eNOS-deficient (eNOS-/-) mice in the DSS model of colitis. Administration of DSS induced weight loss, stool blood, and overt histopathology in both mouse strains. Disease activity was dramatically increased in eNOS-/- mice compared to wild types. Histologically, eNOS-deficient mice had greater leukocyte infiltration, gut injury, and expressed higher levels of the mucosal addressin, MAdCAM-1. These results demonstrate that eNOS plays an important role in limiting injury to the intestine during experimental colitis and altered eNOS content and/or activity may contribute to human IBD.

Pretreatment with simvastatin attenuates myocardial dysfunction after ischemia and chronic reperfusion

We have previously demonstrated that simvastatin attenuates myocardial cell necrosis after acute myocardial ischemia and reperfusion via induction of endothelial cell NO synthase. However, it remains unknown whether the cardioprotective effects of statins can persist after extended periods of reperfusion. Furthermore, it is unknown whether simvastatin therapy can attenuate postischemic cardiac dysfunction. Pretreatment with simvastatin attenuated myocardial injury after 30 minutes of myocardial ischemia and 24 hours of reperfusion. However, the protective effects are not recognized unless simvastatin is given at least 3 hours before myocardial ischemia. Subsequently, we pretreated mice with vehicle or simvastatin and subjected the mice to 30 minutes of myocardial ischemia and 6 months of reperfusion. Myocardial infarct size (percentage of left ventricle) was significantly reduced by 51% in the simvastatin-treated group compared with the vehicle-treated group. Left ventricular diastolic and systolic dilatation was significantly (P<0.05) reduced in simvastatin-treated mice compared with vehicle-treated mice. Additionally, the decrement in fractional shortening after 6 months of reperfusion was minimized in simvastatin-treated mice (P=NS versus baseline) compared with vehicle-treated mice (P<0.05 versus baseline). Left ventricular end-diastolic pressure was significantly (P<0.01) elevated in vehicle-treated mice (21+/-4 mm Hg) but not simvastatin-treated mice (5+/-2 mm Hg) compared with baseline values. These data demonstrate that simvastatin treatment before myocardial ischemia attenuates infarct size and preserves myocardial function after chronic reperfusion in mice.

CD8(+)-T-cell depletion ameliorates circulatory shock in Plasmodium berghei-infected mice

The Plasmodium berghei-infected mouse model is a well-recognized model for human cerebral malaria. Mice infected with P. berghei exhibit (i) metabolic acidosis (pH < 7.3) associated with elevated plasma lactate concentrations, (ii) significant (P < 0.05) vascular leakage in their lungs, hearts, kidneys, and brains, (ii) significantly (P < 0.05) higher cell and serum glutamate concentrations, and (iv) significantly (P < 0.05) lower mean arterial blood pressures. Because these complications are similar to those of septic shock, the simplest interpretation of these findings is that the mice develop shock brought on by the P. berghei infection. To determine whether the immune system and specifically CD8(+) T cells mediate the key features of shock during P. berghei malaria, we depleted CD8(+) T cells by monoclonal antibody (mAb) treatment and assessed the complications of malarial shock. P. berghei-infected mice depleted of CD8(+) T cells by mAb treatment had significantly reduced vascular leakage in their hearts, brains, lungs, and kidneys compared with infected controls treated with rat immunoglobulin G. CD8-depleted mice were significantly (P < 0.05) protected from lactic acidosis, glutamate buildup, and diminished HCO(3)(-) levels. Although the blood pressure decreased in anti-CD8 mAb-treated mice infected with P. berghei, the cardiac output, as assessed by echocardiography, was similar to that of uninfected control mice. Collectively, our results indicate that (i) pathogenesis similar to septic shock occurs during experimental P. berghei malaria, (ii) respiratory distress with lactic acidosis occurs during P. berghei malaria, and (iii) most components of circulatory shock are ameliorated by depletion of CD8(+) T cells.

Cardioprotective actions of acute HMG-CoA reductase inhibition in the setting of myocardial infarction

A known risk factor for the development of coronary artery disease and subsequent myocardial infarction is hypercholesterolaemia. The widespread nature of this phenomenon in the western world has led to the development of agents which reduce serum cholesterol levels. One such class of agents, HMG-CoA reductase inhibitors (statins) are very effective in cholesterol reduction. Recently, clinical and experimental evidence has amassed suggesting that patients taking statins receive cardiovascular benefits that occur independent of cholesterol reduction. Experimental data suggest that statins may increase levels of nitric oxide (NO) in vivo. This review will address the 'cholesterol-independent' vasculoprotective and cardioprotective effects of statins in animal models. Upon completion, the reader will be familiar with the proposed cholesterol-independent pathways of statins and understand that the cholesterol-independent benefits may arise from enhanced production of NO.

Heart-targeted overexpression of caspase3 in mice increases infarct size and depresses cardiac function

Up-regulation of proapoptotic genes has been reported in heart failure and myocardial infarction. To determine whether caspase genes can affect cardiac function, a transgenic mouse was generated. Cardiac tissue-specific overexpression of the proapoptotic gene Caspase3 was induced by using the rat promoter of alpha-myosin heavy chain, a model that may represent a unique tool for investigating new molecules and antiapoptotic therapeutic strategies. Cardiac-specific Caspase3 expression induced transient depression of cardiac function and abnormal nuclear and myofibrillar ultrastructural damage. When subjected to myocardial ischemia-reperfusion injury, Caspase3 transgenic mice showed increased infarct size and a pronounced susceptibility to die. In this report, we document an unexpected property of the proapoptotic gene caspase3 on cardiac contractility. Despite inducing ultrastructural damage, Caspase3 does not trigger a full apoptotic response in the cardiomyocyte. We also implicate Caspase3 in determining myocardial infarct size after ischemia-reperfusion injury, because its cardiomyocyte-specific overexpression increases infarct size.

Cardioprotective actions of endogenous IL-10 are independent of iNOS

Myocardial ischemia-reperfusion (I/R) is a well-known stimulus for acute inflammatory responses that promote cell death and impair pump function. Interleukin-10 (IL-10) is an endogenous, potent anti-inflammatory cytokine. Recently, it has been proposed that IL-10 inhibits inducible nitric oxide synthase (iNOS) activity after myocardial I/R and consequently exerts cardioprotective effects. However, whether this actually occurs remains unclear. To test this hypothesis, we utilized iNOS-deficient (-/-), IL-10 -/-, and IL-10/iNOS -/- mice to examine the potential mechanism of IL-10-mediated cardioprotection after myocardial I/R. Wild-type, iNOS -/-, IL-10 -/-, and IL-10/iNOS -/- mice were subjected to in vivo myocardial ischemia (30 min) and reperfusion (24 h). Deficiency of iNOS alone did not significantly alter the extent of myocardial necrosis compared with wild-type mice. We found that deficiency of IL-10 resulted in a significantly (P < 0.05) larger infarct size than that in wild-type hearts. Interestingly, deficiency of both IL-10 and iNOS yielded significantly (P < 0.01) larger myocardial infarct sizes compared with wild-type animals. Histological examination of myocardial tissue samples revealed augmented neutrophil infiltration into the I/R myocardium of IL-10 -/- and IL-10/iNOS -/- mice compared with hearts of wild-type mice. These results demonstrate that 1) deficiency of endogenous IL-10 exacerbates myocardial injury after I/R; 2) the cardioprotective effects of IL-10 are not dependent on the presence or absence of iNOS; and 3) deficiency of IL-10 enhances the infiltration of neutrophils into the myocardium after I/R.

Simvastatin exerts both anti-inflammatory and cardioprotective effects in apolipoprotein E-deficient mice

BACKGROUND

Simvastatin attenuates ischemia and reperfusion in normocholesterolemic animals by stabilizing endothelial nitric oxide synthase activity and inhibiting neutrophil-mediated injury. Because endothelial dysfunction is a detrimental effect of hypercholesterolemia, we examined whether short-term treatment with simvastatin could inhibit leukocyte-endothelium interaction and attenuate myocardial ischemia-reperfusion injury in apoE-deficient (apoE(-/-)) mice fed a high-cholesterol diet.

METHODS AND RESULTS

We studied leukocyte-endothelium interactions in apoE(-/-) mice fed a normal or a high-cholesterol diet after short-term (ie, 18 hours) simvastatin treatment. We also studied simvastatin treatment in myocardial ischemia-reperfusion injury by subjecting apoE(-/-) mice to 30 minutes of ischemia and 24 hours of reperfusion. ApoE(-/-) mice fed a high-cholesterol diet exhibited higher blood cholesterol levels, which were not affected by short-term simvastatin treatment. However, the increased leukocyte rolling and adherence that occurred in cholesterol-fed apoE(-/-) mice (P<0.001 versus control diet) were significantly attenuated by simvastatin treatment (P<0.01 versus vehicle). Cholesterol-fed apoE(-/-) mice subjected to myocardial ischemia-reperfusion also experienced increased myocardial necrosis (P<0.01 versus control diet), which was significantly attenuated by simvastatin (P<0.01 versus vehicle). Simvastatin therapy also significantly increased vascular nitric oxide production in apoE(-/-) mice.

CONCLUSIONS

Simvastatin attenuates leukocyte-endothelial cell interactions and ameliorates ischemic injury in hypercholesterolemic mice independently of lipid-lowering actions.

Acute exposure to a high cholesterol diet attenuates myocardial ischemia-reperfusion injury in cholesteryl ester transfer protein mice

BACKGROUND

Previous experiments have demonstrated that acute exposure to a high-cholesterol diet (HCD) increases the severity of myocardial infarction in animals. Recent results suggest that the process is modulated by multiple genes and their interactions with circulating cholesterol.

DESIGN

In the present study cholesteryl-ester-transfer-protein (CETP) transgenic mice were generated and fed a normal rodent-chow diet, HCD for 1 week, or a HCD for 6 weeks in order to define the role of CETP in myocardial infarction after acute exposure to a HCD.

METHODS

Cholesterol levels in mice of all groups were measured. Separate groups of mice were exposed to 30 min of in-vivo occlusion of coronary artery and 2 h of reperfusion. We assessed the sizes of the ischemic zone and infarct using Evans blue and 2,3,5-triphenyltetrazolium chloride.

RESULTS

The extent of infarction (percentage infarct/area at risk) was significantly less (P < 0.05) after 1 week of a HCD (18.7 +/- 7.0%) than those for the normal diet group (51.4 +/- 5.5%) and the group fed a HCD for 6 weeks (44.4 +/- 5.2%). Additionally, there was significantly less infiltration of neutrophils into the ischemic-reperfused mouse hearts for mice fed a HCD for 1 week. Levels of reduced and oxidized glutathione in the hearts of CETP mice were measured for separate groups of animals. The reduced:oxidized-glutathione ratio was significantly (P < 0.01) lower for mice fed a HCD for 1 week (1.5 +/- 0.1) than it was for mice fed a normal diet (3.6 +/- 0.3) and a HCD for 6 weeks (3.3 +/- 0.2).

CONCLUSIONS

These data suggest that activity of CETP in hypercholesterolemic mice has an acute effect on size of infarct after 1 week of a HCD. This suggests that CETP induces tolerance of ischemia in the mice fed a HCD via mild oxidative stress.

Myocardial reperfusion injury in neuronal nitric oxide synthase deficient mice

BACKGROUND

A substantial amount of data suggesting that endothelial cell nitric oxide synthase (eNOS) plays a cardioprotective role in animal models of ischemia-reperfusion injury has amassed. We have previously demonstrated that eNOS-deficient (-/-) mice exhibit significantly larger myocardial infarcts than do wild-type mice. Few investigations have examined the neuronal form of nitric oxide synthase in the heart. The two constitutive isoforms have been demonstrated to play differing roles in studies of cerebral ischemia-reperfusion.

OBJECTIVE

To characterize the role of neuronal nitric oxide synthase (nNOS) in myocardial ischemia-reperfusion injury.

METHODS

Wild-type and nNOS -/- mice were subjected to 20 min of coronary artery occlusion and 120 min of reflow.

RESULTS

We found no significant difference between the two groups in terms of infarct size. Microscopic cross-sections from both groups were examined for infiltration of polymorphonuclear leukocyte. Hearts of nNOS -/- mice exhibited significantly (P < 0.05) more polymorphonuclear leukocytes than did hearts of wild-type mice.

CONCLUSION

Despite the fact that eNOS plays a cardioprotective role in the ischemic-reperfused myocardium, we observed no change in size of myocardial infarcts when nNOS was genetically disrupted.

PR-39, a potent neutrophil inhibitor, attenuates myocardial ischemia-reperfusion injury in mice

We investigated the effects of PR-39, a recently discovered neutrophil inhibitor, in a murine model of myocardial ischemia-reperfusion injury. Mice were given an intravenous injection of vehicle (n = 12) or PR-39 (n = 9) and subjected to 30 min of coronary artery occlusion followed by 24 h of reperfusion. In addition, the effects of PR-39 on leukocyte rolling and adhesion were studied utilizing intravital microscopy of the rat mesentery. The area-at-risk per left ventricle was similar in vehicle- and PR-39-treated mice. However, myocardial infarct per risk area was significantly (P < 0.01) reduced in PR-39 treated hearts (21.0 +/- 3.8%) compared with vehicle (47.1 +/- 4.8%). Histological analysis of ischemic reperfused myocardium demonstrated a significant (P < 0.01) reduction in polymorphonuclear neutrophil (PMN) accumulation in PR-39-treated hearts (n = 6, 34.3 +/- 1.7 PMN/mm(2)) compared with vehicle-treated myocardium (n = 6, 59.7 +/- 3.1 PMN/mm(2)). In addition, PR-39 significantly (P < 0.05) attenuated leukocyte rolling and adherence in rat inflamed mesentery. These results indicate that PR-39 inhibits leukocyte recruitment into inflamed tissue and attenuated myocardial reperfusion injury in a murine model of myocardial ischemia-reperfusion.

The MEK1-ERK1/2 signaling pathway promotes compensated cardiac hypertrophy in transgenic mice

Members of the mitogen-activated protein kinase (MAPK) cascade such as extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK) and p38 are implicated as important regulators of cardiomyocyte hypertrophic growth in culture. However, the role that individual MAPK pathways play in vivo has not been extensively evaluated. Here we generated nine transgenic mouse lines with cardiac-restricted expression of an activated MEK1 cDNA in the heart. MEK1 transgenic mice demonstrated concentric hypertrophy without signs of cardiomyopathy or lethality up to 12 months of age. MEK1 transgenic mice showed a dramatic increase in cardiac function, as measured by echocardiography and isolated working heart preparation, without signs of decompensation over time. MEK1 transgenic mice and MEK1 adenovirus-infected neonatal cardiomyocytes each demonstrated ERK1/2, but not p38 or JNK, activation. MEK1 transgenic mice and MEK1 adenovirus-infected cultured cardiomyocytes were also partially resistant to apoptotic stimuli. The results of the present study indicate that the MEK1-ERK1/2 signaling pathway stimulates a physiologic hypertrophy response associated with augmented cardiac function and partial resistance to apoptotsis.

Leukocyte and endothelial cell adhesion molecules in a chronic murine model of myocardial reperfusion injury

Expression of endothelial and leukocyte cell adhesion molecules is a principal determinant of polymorphonuclear neutrophil (PMN) recruitment during inflammation. It has been demonstrated that pharmacological inhibition of these molecules can attenuate PMN influx and subsequent tissue injury. We determined the temporal expression of alpha-granule membrane protein-40 (P-selectin), endothelial leukocyte adhesion molecule 1 (E-selectin), and intercellular cell adhesion molecule 1 (ICAM-1) after coronary artery occlusion and up to 3 days of reperfusion. The expression of all of these cell adhesion molecules peaked around 24 h of reperfusion. We determined the extent to which these molecules contribute to PMN infiltration by utilizing mice deficient (-/-) in P-selectin, E-selectin, ICAM-1, and CD18. Each group underwent 30 min of in vivo, regional, left anterior descending (LAD) coronary artery ischemia and 24 h of reperfusion. PMN accumulation in the ischemic-reperfused (I/R) zone was assessed using histological techniques. Deficiencies of P-selectin, E-selectin, ICAM-1, or CD18 resulted in significant (P < 0.05) attenuation of PMN infiltration into the I/R myocardium (MI/R). In addition, P-selectin, E-selectin, ICAM-1, and CD18 -/- mice exhibited significantly (P < 0.05) smaller areas of necrosis after MI/R compared with wild-type mice. These data demonstrate that MI/R induces coronary vascular expression of P-selectin, E-selectin, and ICAM-1 in mice. Furthermore, genetic deficiency of P-selectin, E-selectin, ICAM-1, or CD18 attenuates PMN sequestration and myocardial injury after in vivo MI/R. We conclude that P-selectin, E-selectin, ICAM-1, and CD18 are involved in the pathogenesis of MI/R injury in mice.

Myocardial ischemia/reperfusion injury in NADPH oxidase-deficient mice

Previous studies have suggested that oxygen-derived free radicals are involved in the pathophysiology of myocardial ischemia/reperfusion (MI/R) injury. Specifically, neutrophils have been shown to mediate postischemic ventricular arrhythmias and myocardial necrosis. We hypothesized that MI/R injury would be reduced in the absence (-/-) of NADPH oxidase. Heterozygous control mice (n=23) and NADPH oxidase(-/-) mice (n=24) were subjected to 30 minutes of coronary artery occlusion and 24 hours of reperfusion. Myocardial area at risk per left ventricle was similar in heterozygous control hearts (55+/-3%) and NADPH oxidase(-/-) hearts (61+/-4%). Contrary to our hypothesis, the size of infarct area at risk was similar in the heterozygous control mice (42+/-4%) and NADPH oxidase(-/-) mice (34+/-5%) (P=not significant). In addition, echocardiographic examination of both groups revealed that left ventricle fractional shortening was similar in NADPH oxidase(-/-) mice (n=8; 27+/-2.5%) and heterozygous control mice (n=10; 23.3+/-3. 3%) after MI/R. Superoxide production, as detected by cytochrome c reduction, was significantly impaired (P<0.01) in NADPH oxidase(-/-) mice (n=6) compared with heterozygous mice (n=7) (0.04+/-0.03 versus 2.2+/-0.08 nmol O(2).min(-1).10(6) cells(-1)). Intravital microscopy of the inflamed mesenteric microcirculation demonstrated that leukocyte rolling and adhesion were unaffected by the absence of NADPH oxidase. Oyster glycogen-stimulated neutrophil transmigration into the peritoneum was also similar in both the heterozygous control mice and NADPH oxidase(-/-) mice (P:=not significant). These findings suggest that NADPH oxidase does not contribute to the development of myocardial injury and dysfunction after MI/R.

The HMG-CoA reductase inhibitor simvastatin activates the protein kinase Akt and promotes angiogenesis in normocholesterolemic animals

Recent studies suggest that statins can function to protect the vasculature in a manner that is independent of their lipid-lowering activity. We show here that statins rapidly activate the protein kinase Akt/PKB in endothelial cells. Accordingly, simvastatin enhanced phosphorylation of the endogenous Akt substrate endothelial nitric oxide synthase (eNOS), inhibited apoptosis and accelerated vascular structure formation in vitro in an Akt-dependent manner. Similar to vascular endothelial growth factor (VEGF) treatment, both simvastatin administration and enhanced Akt signaling in the endothelium promoted angiogenesis in ischemic limbs of normocholesterolemic rabbits. Therefore, activation of Akt represents a mechanism that can account for some of the beneficial side effects of statins, including the promotion of new blood vessel growth.

Oxidative stress and cardiac disease

Reactive oxygen species (ROS) are formed at an accelerated rate in postischemic myocardium. Cardiac myocytes, endothelial cells, and infiltrating neutrophils contribute to this ROS production. Exposure of these cellular components of the myocardium to exogenous ROS can lead to cellular dysfunction and necrosis. While it remains uncertain whether ROS contribute to the pathogenesis of myocardial infarction, there is strong support for ROS as mediators of the reversible ventricular dysfunction (stunning) that often accompanies reperfusion of the ischemic myocardium. The therapeutic potential of free radical-directed drugs in cardiac disease has not been fully realized.

Pharmacology of selectin inhibitors in ischemia/reperfusion states

Recently, the selectin family of glycoprotein adhesion molecules (P-selectin, E-selectin, and L-selectin) has been implicated in the pathogenesis of a number of inflammatory disease states. The selectins modulate the early adhesive interactions between circulating neutrophils and the endothelium. Both P-selectin and E-selectin can be expressed on the surface of endothelial cells following stimulation by a number of inflammatory mediators. In contrast, L-selectin is constitutively expressed on the surface of neutrophils at very high levels. In addition, neutrophils also express ligands for the endothelial selectins, including the carbohydrate sialyl Lewis(x) and the high-affinity ligand P-selectin glycoprotein ligand 1, which facilitate neutrophil-endothelial interactions. Selectins have been extensively investigated in ischemia/reperfusion injury states. The study of selectin involvement in ischemia/reperfusion injury has been facilitated by the development of highly specific selectin antagonists, including monoclonal antibodies, carbohydrates, small molecule inhibitors, and soluble forms of P-selectin glycoprotein ligand 1. This article reviews the results of current studies of selectin antagonists in experimental models of ischemia/reperfusion injury.

Role of nitric oxide in the regulation of acute and chronic inflammation

Recent studies by a number of different laboratories have implicated nitric oxide (NO) as an important modulator of a variety of acute and chronic inflammatory disorders. A hallmark of inflammation is the adhesion of leukocytes to post-capillary venular endothelium and the infiltration of leukocytes into the tissue interstitium. Leukocyte adhesion and infiltration is known to be dependent on interaction of the leukocytes with the endothelial cell surface via a class of glycoproteins collectively known as endothelial cell adhesion molecules (ECAMs). Several recent studies suggest that NO may modulate cytokine-induced ECAM expression in cultured endothelial cells in vitro by regulating the activation of nuclear transcription factor kappa B (NF-kappaB). This discussion reviews some of the more recent studies that assess the role of the different NOS isoforms on the inflammatory response in vivo.

Effects of hypercholesterolemia on myocardial ischemia-reperfusion injury in LDL receptor-deficient mice

Hypercholesterolemia is a primary risk factor for atherosclerosis, coronary artery disease, and myocardial infarction. We subjected low density lipoprotein receptor-deficient (LDLr -/-) and control (wild-type) mice to 30 minutes of myocardial ischemia and 120 minutes of reperfusion. Myocardial infarction per area at risk (AAR) was noted under baseline conditions to be significantly (P<0.05) smaller in the LDLr -/- mice compared with wild-type mice (24.7+/-3. 2% and 38.8+/-4.3% of AAR, respectively). Subsequently, mice were fed a high-cholesterol diet (HCD) for 2 or 12 weeks, which resulted in significant increases in serum cholesterol levels in both LDLr -/- and wild-type groups. After 2 weeks of the HCD, the LDLr -/- mice demonstrated a significant elevation (P<0.01) in myocardial necrosis per AAR (50.2+/-5.36% of AAR) compared with the normal-diet LDLr -/- group, whereas the short-term HCD-fed wild-type mice demonstrated no significant difference from baseline. In contrast, wild-type mice fed the HCD for 12 weeks revealed a significant (P<0. 05) decrease in necrosis per AAR, which was 22.5+/-3.2% of the AAR in comparison with that in the normal-diet wild-type mice (38.8+/-4. 3% of AAR). LDLr -/- mice on the same long-term HCD showed a similar significantly (P<0.05) decreased infarct size, which was 13.2+/-4.0% of the AAR. In additional experiments, we determined that myocardial tissue total glutathione (GSH) levels were reduced after 2 weeks of the HCD and were significantly increased after 12 weeks of the HCD in the LDLr -/- mouse heart. These data suggest that short-term cholesterol feeding renders the myocardium of LDLr -/- mice more susceptible to ischemia-reperfusion injury, whereas more long-term hypercholesterolemia confers cardioprotection in the LDLr -/- mouse heart.

Cardioprotective effects of abciximab (ReoPro) in an isolated perfused rat heart model of ischemia and reperfusion

Ischemia followed by reperfusion in the presence of polymorphonuclear leukocytes (PMNs) results in cardiac contractile dysfunction as well as myocardial injury, due in large part to endothelial dysfunction, upregulation of cell adhesion molecules and subsequent neutrophil induced cardiac injury. We studied the effects of abciximab (ReoPro), an anti-IIb/IIIa antibody, which has been shown to attenuate platelet interactions, in a neutrophil-platelet mediated isolated perfused rat heart model of ischemia (I) (20 min) and reperfusion (R) (45 min). Administration of abciximab (6.5 micrograms/kg) 10 min prior to the perfusion of the PMN + platelet perfused I/R heart improved post-reperfusion coronary flow and preserved post-reperfusion left ventricular developed pressure (LVDP) and +dP/dt max as indices of cardiac contractile function. Abciximab-treated hearts reperfused in the presence of PMNs and platelets preserved all indices of cardiac contractile function. I/R heart perfused with PMNs and platelets produced a profound injury to the hearts which was attenuated with the treatment of abciximab. In addition, abciximab significantly reduced PMN accumulation in the ischemic myocardium from 38 +/- 1 PMNs/mm2 in untreated hearts to 7 +/- 1 in rats given abciximab. Similar results were obtained with PMN perfused I/R rat hearts without platelets. These results provide evidence that abciximab is a potent and effective cardioprotective agent that inhibits leukocyte-endothelial cell interactions as well as platelet-endothelial cell interaction and preserves cardiac contractile function and coronary perfusion following myocardial ischemia and reperfusion. Therefore, IIb/IIIa may be important in attenuating both platelet and neutrophil-mediated myocardial dysfunction.

Reperfusion injury is not affected by blockade of P-selectin in the diabetic mouse heart

We examined the mechanisms responsible for myocardial ischemia-reperfusion (MI-R) injury in a well-characterized animal model of type II diabetes mellitus. Diabetic (db/db) mice and their littermate nondiabetic controls were subjected to 30 min of left anterior descending coronary artery occlusion and 2 h of reperfusion. Diabetic and nondiabetic mice experienced similar-sized areas at risk per left ventricle: 50.4 +/- 2.0 and 53.4 +/- 4.1%, respectively. However, myocardial necrosis (percentage of area at risk) was significantly greater (P < 0.001) in diabetic than in nondiabetic animals: 56.3 +/- 2.8 and 27.2 +/- 3.1%, respectively. Histological examination revealed significantly (P < 0.05) more neutrophils (PMNs) in the diabetic than in the nondiabetic hearts. Coronary endothelial expression of P-selectin was determined using radiolabeled monoclonal antibodies (MAbs). MI-R elicited a more intense (P < 0.05) upregulation of P-selectin in the ischemic zone of diabetic than of nondiabetic myocardium: 0.310 +/- 0.034 and 0. 161 +/- 0.042 microgram MAb/g tissue. Immunoneutralization of P-selectin (RB40.34) reduced PMN accumulation in the diabetic myocardium but failed to reduce the extent of myocardial necrosis. Conversely, administration of an MAb directed against CD18 (GAME46) reduced PMN infiltration and attenuated the infarct size in the diabetic hearts. These results suggest that the diabetic heart is more susceptible to ischemia-reperfusion injury than normal myocardium. Furthermore, the mechanism of this injury may not be critically dependent on P-selectin in diabetic hearts.

Simvastatin preserves the ischemic-reperfused myocardium in normocholesterolemic rat hearts

BACKGROUND

Ischemia followed by reperfusion in the presence of polymorphonuclear leukocytes (PMNs) results in cardiac contractile dysfunction as well as cardiomyocyte injury. These deleterious effects are due in large part to endothelial dysfunction leading to the upregulation of cell adhesion molecules and subsequent neutrophil-endothelium interaction. At clinically relevant doses, simvastatin, an HMG-CoA reductase inhibitor, has been shown to lower serum cholesterol levels and normalize endothelial cell function. We wanted to test the effects of simvastatin on neutrophil-mediated cardiac dysfunction in a controlled model of myocardial ischemia-reperfusion.

METHODS AND RESULTS

This study examines the effects of simvastatin in a neutrophil-dependent isolated perfused rat heart model of ischemia (I) (20 minutes) and reperfusion (R) (45 minutes) injury. Administration of simvastatin 25 micrograms/rat improved coronary flow and preserved left ventricular developed pressure (LVDP) and dP/dtmax, indexes of cardiac contractile function. Final LVDP was 95+/-5 mm Hg in I/R hearts perfused with PMNs and simvastatin, compared with 49+/-4 mm Hg in PMN-perfused I/R hearts receiving only vehicle (P<0.001). In addition, simvastatin significantly reduced PMN accumulation in the ischemic myocardium (P<0.01). In PMN-perfused rat hearts after I/R, simvastatin also significantly attenuated P-selectin expression, CD18 upregulation in rat PMNs, and PMN adherence to rat vascular endothelium. Significant, although less potent, effects were obtained with pravastatin.

CONCLUSIONS

These results provide evidence that HMG-CoA reductase inhibitors are potent and effective cardioprotective agents that inhibit leukocyte-endothelial cell interactions and preserve cardiac contractile function and coronary perfusion after myocardial ischemia and reperfusion. Moreover, these effects are unrelated to the cholesterol-lowering action of this agent and appear to be mediated by enhanced endothelial release of NO.

Vascular endothelial growth factor attenuates leukocyte-endothelium interaction during acute endothelial dysfunction: essential role of endothelium-derived nitric oxide

Vascular endothelial growth factor (VEGF) is an endothelium-specific secreted protein that induces vasodilation and increases endothelial release of nitric oxide (NO). NO is also reported to modulate leukocyte-endothelium interaction. Therefore, we hypothesized that VEGF might inhibit leukocyte-endothelium interaction via increased release of NO from the vascular endothelium. We used intravital microscopy of the rat mesenteric microcirculation to measure leukocyte-endothelium interactions 2, 4, and 24 h after systemic administration of VEGF to the rat (120 microg/kg, i.v., bolus). Superfusion of the rat mesentery with either 0.5 U/ml thrombin or 50 microM L-NAME consistently increased the number of rolling, adhering, and transmigrated leukocytes (P<0.01 vs. control mesenteries superfused with Krebs-Henseleit buffer). At 4 and 24 h posttreatment, VEGF significantly attenuated thrombin-induced and L-NAME-induced leukocyte rolling, adherence, and transmigration in rat mesenteric venules. In addition, adherence of isolated rat PMNs to thrombin-stimulated mesenteric artery segments in vitro was significantly reduced in mesenteric arteries isolated from VEGF-treated rats (P<0.001 vs. control rats). Direct measurement of NO demonstrated a threefold increase in basal NO release from aortic tissue of rats injected with VEGF, at 4 and 24 h posttreatment (P<0. 01 vs. aortic tissue from control rats). Finally, systemic administration of VEGF to ecNOS-deficient mice failed to inhibit leukocyte-endothelium interactions observed in peri-intestinal venules. We concluded that VEGF is a potent inhibitor of leukocyte-endothelium interaction, and this effect is specifically correlated to augmentation of NO release from the vascular endothelium.--Scalia, R., Booth, G., Lefer, D. J. Vascular endothelial growth factor attenuates leukocyte-endothelium interaction during acute endothelial dysfunction: essential role of endothelium-derived nitric oxide.

Leukocyte-endothelial cell interactions in nitric oxide synthase-deficient mice

Nitric oxide (NO) is known to be an important endogenous modulator of leukocyte-endothelial cell interactions within the microcirculation. We examined leukocyte rolling and adhesion under baseline conditions and following thrombin (0.25 U/ml) superfusion in the mesentery of wild-type, inducible NOS (iNOS)-deficient (-/-), neuronal NOS (nNOS) -/-, and endothelial cell NOS (ecNOS) -/- mice to further our understanding of NO and leukocyte function. Baseline leukocyte rolling (cells/min) was significantly elevated in both the nNOS -/- (30.0 +/- 4.0) and ecNOS -/- mice (67.0 +/- 12.0) compared with wild-type mice (11.0 +/- 1.4). In addition, baseline leukocyte adherence (cells/100 micrometers of vessel) was also significantly elevated in the nNOS -/- (5.2 +/- 1.0) and ecNOS -/- (13.0 +/- 1.3) compared with wild-type animals (1.3 +/- 0.5). Deficiency of iNOS had no effect on baseline leukocyte rolling or adhesion in the mesentery. Baseline surface expression of P-selectin was observed in 68.0 +/- 9.0% of intestinal venules in ecNOS -/- mice compared with 10.0 +/- 2.0% in wild-type mice. Additional studies demonstrated that administration of an anti-P-selectin monoclonal antibody (RB40. 34) or the soluble P-selectin ligand, PSGL-1, completely inhibited the increased rolling and firm adhesion response in nNOS -/- and ecNOS -/- mice. Transmigration of neutrophils into the peritoneum following thioglycollate injection was also significantly augmented in nNOS -/- and ecNOS -/- mice. These studies clearly indicate the NO derived from both nNOS and ecNOS is critical in the regulation of leukocyte-endothelial cell interactions.

Myocardial ischemia-reperfusion injury is exacerbated in absence of endothelial cell nitric oxide synthase

Myocardial ischemia and reperfusion (MI/R) initiates a cascade of polymorphonuclear neutrophil (PMN)-mediated injury, the magnitude of which may be influenced by the bioavailability of nitric oxide (NO). We investigated the role of endothelial cell nitric oxide synthase (ecNOS) in MI/R injury by subjecting wild-type and ecNOS-deficient (-/-) mice to 20 min of coronary artery occlusion and 120 min of reperfusion. Myocardial infarct size represented 20.9 +/- 2.9% of the ischemic zone in wild-type mice, whereas the ecNOS -/- mice had significantly (P < 0.01) larger infarcts measuring 46.0 +/- 3.8% of the ischemic zone. Because P-selectin is thought to be involved with the pathogenesis of neutrophil-mediated I/R injury, we assessed the effects of MI/R on P-selectin expression in the myocardium of wild-type and ecNOS -/- mice. P-selectin expression measured with a radiolabeled monoclonal antibody (MAb) technique after MI/R in wild-type mice was 0.037 +/- 0.009 microgram MAb/g tissue, whereas ecNOS -/- coronary vasculature was characterized by significantly (P < 0.05) higher P-selectin expression (0.080 +/- 0.013 microgram MAb/g tissue). Histological examination of the postischemic myocardium revealed significantly (P < 0.01) more neutrophils in the ecNOS -/- (29.5 +/- 2.5 PMN/field) compared with wild-type (5.0 +/- 0.9 PMN/field) mice. A similar trend in infarct size and neutrophil accumulation was observed when wild-type and ecNOS -/- mice were subjected to 30 min of ischemia and 120 min of reperfusion. These novel in vivo findings demonstrate a cardioprotective role for ecNOS-derived NO in the ischemic-reperfused mouse heart.

Nitric oxide. II. Nitric oxide protects in intestinal inflammation

This article examines the evidence for nitric oxide (NO) as a protective agent in splanchnic ischemia-reperfusion and other forms of acute intestinal inflammation. Four major points emerge from this body of data. First, acute intestinal inflammation results in an early (i.e., <5 min) and severe decrease in endothelium-derived NO. Thus the early trigger event in this condition is a functional loss of NO. Second, administration of exogenous NO, NO donors, or NO precursors ameliorate splanchnic ischemia-reperfusion and other forms of acute intestinal inflammation (i.e., splanchnic trauma). These beneficial effects occur at physiological levels of NO when given early in the course of the inflammatory state. Third, blockade of nitric oxide synthase (NOS) or gene deletion of NOS exacerbates intestinal inflammation. Fourth, there are a variety of signaling mechanisms that may mediate the protective effect of NO.

Myocardial ischemia-reperfusion injury in CD18- and ICAM-1-deficient mice

Previous studies have demonstrated that circulating neutrophils (PMNs) contribute to the pathophysiology of myocardial ischemia-reperfusion (MI/R) injury. PMN-endothelial cell interactions are highly regulated by adhesive interactions between PMN CD11/CD18 and coronary endothelial cell intercellular adhesion molecule-1 (ICAM-1). We investigated the effects of MI/R in wild-type, CD18-, and ICAM-1-deficient (-/-) mice. Wild-type (n = 6), CD18 -/- (n = 6), and ICAM-1 -/- (n = 6) mice were subjected to 30 min of myocardial ischemia and 120 min of reperfusion to determine the extent of PMN infiltration and myocardial cell necrosis. Myocardial infarction (% of the area at risk) was 45.1 +/- 5.9 in wild-type mouse hearts. In contrast, the extent of myocardial infarction was significantly (P < 0.05) reduced in the CD18 (19.3 +/- 5.1%)- and ICAM-1 (17.9 +/- 3.2%)-deficient mice. Similarly, PMN infiltration into the ischemic-reperfused myocardium was attenuated by 54% in the CD18 -/- mice and by 32% in ICAM-1 -/- mice compared with wild-type hearts. Deficiency in either CD18 or ICAM-1 expression results in a marked reduction in PMN accumulation and myocardial necrosis after acute MI/R.

Coronary endothelial P-selectin in pathogenesis of myocardial ischemia-reperfusion injury

We investigated in vivo coronary P-selectin expression and its pathophysiological consequences in a murine model of myocardial ischemia-reperfusion (MI/R) using wild-type and P-selectin deficient (-/-) mice. Coronary P-selectin expression [microgram monoclonal antibody (MAb)/g tissue] was measured using a radiolabeled MAb method after 30 min of myocardial ischemia and 20 min of reperfusion. P-selectin expression in wild-type mice was significantly (P < 0. 01) elevated in the ischemic zone (0.070 +/- 0.010) compared with the nonischemic zone (0.037 +/- 0.008). Myocardial P-selectin expression was nearly undetectable in P-selectin -/- mice after MI/R. Furthermore, myocardial infarct size (% of area at risk) after 30 min of myocardial ischemia and 120 min of reperfusion was 42.5 +/- 4. 4 in wild-type mice and 24.4 +/- 4.0 in P-selectin -/- mice (P < 0. 05). In additional experiments of prolonged myocardial ischemia (60 min) and reperfusion (120 min), myocardial infarct size was similar in P-selectin -/- mice and wild-type mice. Our results clearly demonstrate the involvement of coronary P-selectin in the development of myocardial infarction after MI/R.

Synergism between platelets and neutrophils in provoking cardiac dysfunction after ischemia and reperfusion: role of selectins

BACKGROUND

Neutrophils (PMNs) are known to contribute to both cardiac dysfunction and myocardial necrosis after reperfusion of an ischemic heart. Moreover, platelets are also important blood cells that can aggravate myocardial ischemic injury. This study was designed to test the effects of PMNs and platelets separately and together in provoking cardiac dysfunction in isolated perfused rat hearts after ischemia and reperfusion.

METHODS AND RESULTS

Control rat hearts not subjected to ischemia were perfused without blood cells for 80 minutes. Additional control rat hearts were perfused with 75x106 PMNs, with 100x106 platelets, or with 75x106 PMNs+100x106 platelets over a 5-minute perfusion followed by a 75-minute observation period. No significant reduction in coronary flow, left ventricular developed pressure (LVDP), or the first derivative of LVDP (dP/dtmax) was observed at the end of the observation period in any nonischemic group. Similarly, global ischemia (I) for 20 minutes followed by 45 minutes of reperfusion (R) produced no sustained effects on the final recovery of any of these parameters in any group of hearts perfused in the absence of blood cells. However, I/R hearts perfused with either PMNs or platelets alone exhibited decreases in these variables of 10% to 12% (P<0.05 from control). Furthermore, I/R hearts perfused with both PMNs and platelets exhibited decreases of 50% to 60% in all measurements of cardiac function (P<0.001). These dual-cell-perfused I/R hearts also exhibited marked increases in cardiac myeloperoxidase (MPO) activity, indicating a significant PMN infiltration, and enhanced P-selectin expression on the coronary microvascular endothelium. All cardiodynamic effects as well as MPO accumulation and PMN infiltration were markedly attenuated by a sialyl LewisX-oligosaccharide or a recombinant soluble P-selectin ligand, which inhibits selectin-mediated cell adhesion.

CONCLUSIONS

These results provide evidence that platelets and neutrophils act synergistically in provoking postreperfusion cardiac dysfunction and that this may be largely due to cell-to-cell interactions mediated by P-selectin. These findings may help explain the reperfusion injury phenomenon.

Modulation of leukocyte-endothelial interactions by reactive metabolites of oxygen and nitrogen: relevance to ischemic heart disease

Ischemia and reperfusion (I/R) are thought to play an important role in the pathophysiology of ischemic diseases of the heart. It is now well appreciated that leukocyte-endothelial cell interactions are important determinants for I/R-induced microvascular injury and dysfunction. There is a growing body of experimental data to suggest that reactive metabolites of oxygen and nitrogen are important physiological modulators of leukocyte-endothelial cell interactions. A number of investigators have demonstrated that I/R enhances oxidant production within the microcirculation resulting in increases in leukocyte adhesion and transendothelial cell migration. Several other studies have shown that exogenous nitric oxide (NO) donors may attenuate leukocyte and platelet adhesion and/or aggregation in a number of different inflammatory conditions including I/R. The objective of this review is to discuss the physiological chemistry of reactive metabolites of oxygen and nitrogen with special attention given to those interactions that may modulate leukocyte-endothelial cell interactions, provide an overview of the evidence implicating reactive metabolites of oxygen and nitrogen as modulators of leukocyte-endothelial cell interactions in vivo, and discuss how these mechanisms may be involved in the pathophysiology of ischemic heart disease.

Lymph isolated from a regional scald injury produces a negative inotropic effect in dogs

Large surface-area burns in patients have been associated with a severe impairment in cardiac performance, as evidenced by a decline in cardiac output. The mechanisms responsible for this profound myocardial dysfunction are largely unknown. We investigated the effects of lymph isolated from the scalded hind limb of dogs on regional myocardial blood flow, coronary vascular reactivity, and contractile performance. Dogs were instrumented with ultrasonic dimension crystals in the myocardium supplied by the left anterior descending (LAD) and by the left circumflex (LCx) coronary arteries. After cannulating a hind limb lymphatic, lymph was infused directly into the LAD before and after a 10-second 100 degrees C hind limb scald. Scalding alone did not alter myocardial contractile performance in the LAD or LCx regions, coronary artery blood flow, or systemic hemodynamics. Interestingly, postburn lymph infused into the LAD resulted in a 38% decline in LAD zone segment shortening (p < 0.01 vs baseline) that lasted throughout the 5-hour observation period. In contrast, segment shortening in the (control) LCx region was unaffected by postburn lymph injections into the LAD. Regional myocardial blood flow (radiolabeled microspheres) in the LAD and LCx regions was unchanged after scald injury or intracoronary injection of postburn lymph. In addition, LAD coronary artery vascular reactivity to acetylcholine and nitroglycerin was also unaffected by the regional thermal injury or by injection of lymph into the LAD. These data suggest that a regional scald injury results in the production and release of a potent myocardial depressant factor(s) that produces a direct negative inotropic effect on the canine myocardium.

Age-related differences in response to neutrophil-mediated reperfusion injury in the neonatal piglet heart

BACKGROUND

Neonatal hearts have altered adhesion molecule interactions in response to ischemia-reperfusion. How this affects myocardial function is unknown.

METHODS

Isolated, buffer perfused 0- to 2-day (newborn) and 2-week piglet hearts were first subjected to 20-minute global, normothermic ischemia, followed by 45 minutes of reperfusion during which 150 x 10(6) newborn or 2-week neutrophils were infused. In some hearts, an antibody to SLe(x) (CSLEX-1) was infused with neutrophils during reperfusion. Hemodynamic variables, including left ventricular developed pressure (LVDP), were recorded at timed intervals. Neutrophil CD-18, L-selectin, and SLe(x) contents were measured by flow cytometry.

RESULTS

Full recovery of LVDP was observed in newborn hearts receiving newborn or 2-week-old neutrophils. Recovery of LVDP was depressed (p < 0.01, ANOVA) in 2-week-old hearts receiving 2-week old, not newborn, neutrophils. Infusion of CSLEX-1 in 2-week-old hearts restored LVDP to baseline. Whereas flow cytometry showed higher (p < 0.01, Student's t test) CD-18 and L-selectin expression on newborn versus 2-week-old neutrophils, newborn neutrophils expressed lower (p < 0.01) SLe(x) levels.

CONCLUSIONS

Initial "loose" neutrophil-endothelial selectin interactions are a necessary prelude to "firm" adhesion and reperfusion injury. Operations performed soon after birth may be better tolerated than when surgery is delayed; anti-SLe(x) preparations may prove beneficial when performing cardiac procedures on older infants.

Postischemic endothelium-dependent vascular reactivity is preserved in adhesion molecule-deficient mice

Previous studies utilizing monoclonal antibodies directed against specific leukocyte-endothelial cell adhesion proteins have suggested that neutrophils mediate endothelial cell injury in a number of vascular beds after ischemia-reperfusion (I/R). In the present study, we investigated superior mesenteric artery (SMA) vascular reactivity to acetylcholine (ACh) and sodium nitroprusside (SNP) after occlusion and reperfusion in wild-type (C57BL/6) mice and in gene-targeted mice that are deficient in either CD11/CD18, intercellular adhesion molecule 1 (ICAM-1), or P-selectin. All mice were 4 wk of age, and the SMA was occluded for 45 min and then reperfused for 45 min. Segments of SMA were isolated and suspended in organ chambers and contracted with phenylephrine (10(-5) M), and the maximal vasorelaxation to ACh (10(-6) M) and SNP (10(-6) M) was measured. SMA from sham-operated C57BL/6 mice relaxed 83.5 +/- 3.3% to ACh and 91.7 +/- 3.4% to SNP. In contrast, segments of SMA from C57BL/6 mice subjected to I/R demonstrated a marked impairment in vasorelaxation to ACh (51.3 +/- 4.7%, P < 0.01 vs. sham) without any impairment in the vasoreactivity to SNP (86.1 +/- 5.5%). In CD11/CD18-deficient mice, SMA reactivity to ACh (84.7 +/- 2.3%) and SNP (91.2 +/- 2.8%) was unaffected by I/R. Similarly, SMA rings from ICAM-1-deficient mice exhibited normal vasorelaxation to ACh and SNP with maximal vasorelaxation of 83.1 +/- 2.9 and 87.4 +/- 3.0%, respectively. We also observed profound preservation of endothelium-dependent vasorelaxation after I/R in P-selectin-deficient mice. These findings indicate that leukocyte-endothelial cell adhesion molecule deficiency is associated with the preservation of endothelium-dependent vascular responses after I/R.

Mild preischemia hypothermia adversely affects postischemic myocardial function in the neonatal piglet heart

BACKGROUND

During cardiac surgery, operative hypothermia has been shown to be beneficial in certain situations, although in children perioperative hypothermia has been associated with several physiologic alterations that have proven detrimental to their postoperative function. Little attention has been given to the effects of mild (34.5 degrees C) perioperative hypothermia on postischemic myocardial function in the pediatric population. It was hypothesized that mild hypothermia would be detrimental to postischemic ventricular function in the neonatal heart.

METHODS

Neonatal (0-2 days old) piglets were subjected to mild perioperative hypothermia without rewarming (HT-only, n = 6), hypothermia followed by rewarming (HT-RW, n = 6), or continuous normothermia (NT, n = 8). The hearts were rapidly excised, suspended on an isolated perfusion apparatus, and allowed to spontaneously beat while being perfused with an asanguinous solution. All hearts were subjected to 20 min global, normothermic, zero-flow ischemia followed by 45 min oxygenated crystallite buffer reperfusion (I-R).

RESULTS

Compared to that of NT piglets, there were significant (P < 0.05) reductions in recovery of left ventricular (LV) diastolic and systolic function following ischemia and reperfusion in HT-RW animals. When the hearts were rendered ischemic without first rewarming, the degree of myocardial dysfunction was not as severe. In contrast to the NT piglets, HT-RW animals demonstrated significant (P < 0.05) reductions in the final recovery of LV developed pressure (71 +/- 6 vs 105 +/- 6 in NT), LV rate pressure product (52 +/- 4 vs 102 +/- 9 NT), and LV end diastolic pressure (32 +/- 7 vs 3 +/- 1 in NT) following I-R. When compared to the HT-RW group, HT-only piglets did not exhibit significant differences in systolic function, although diastolic function was minimally altered initially as evidenced by the slight elevation of LV end diastolic pressure at 5 min, with reperfusion in the HT-only group (P < 0.05).

CONCLUSIONS

In this newborn piglet model, mild hypothermia significantly reduces recovery of systolic and diastolic left ventricular function when followed by an episode of global myocardial ischemia-reperfusion only when the animals are returned to normothermia prior to the ischemic insult. When hypothermia is immediately followed by the ischemic event, left ventricular function is unaffected.

Effect of short-term treatment with a monoclonal antibody to P-selectin on balloon catheter-induced: intimal hyperplasia, re-endothelialization, and attenuation of endothelial-dependent relaxation

The effects of an anti-P-selectin monoclonal antibody (MAb, PB1.3; Cytel Corporation) on neoendothelialization; neoendothelial function, as evidenced by acetylcholine-induced relaxation (nitric oxide formation); and intimal hyperplasia following embolectomy catheter-induced injury to the rabbit thoracic aorta were investigated. Catheter injury was induced in two groups of New Zealand White rabbits. One group received no treatment, while the second group received short-term treatment with the MAb (i.p., immediately before and 12 h after induction of catheter injury). A third group underwent a sham operation and served as uninjured controls. Following sacrifice at 2 weeks after injury, aortic rings were assessed for degree of intimal hyperplasia, neoendothelial morphology (scanning electron microscopy), and acetylcholine-induced relaxation. Aortic tissue from catheter-injured animals that received treatment exhibited improved neoendothelial morphology, as compared with tissue from untreated but catheterized animals; however, no statistically significant attenuation of the hyperplastic response or improvement in the attenuated neoendothelial-dependent acetylcholine-induced relaxant response that is characteristic of neoendothelium that forms after catheter denudation was observed. These data suggest that short-term attenuation of P-selectin-mediated polymorphonuclear leukocyte (PMN)/endothelium, PMN/platelet interactions, and/or thrombin formation beneficially affects neoendothelialization of the vascular wall following balloon catheter-induced injury.

Monoclonal antibody to ICAM-1 preserves postischemic blood flow and reduces infarct size after ischemia-reperfusion in rabbit

Neutrophils are pivotal in the pathogenesis of reperfusion injury leading to myocardial infarction. Firm adhesion of PMN to endothelium may be initiated by the interaction between constitutively expressed intercellular adhesion molecule-1 (ICAM-1) on endothelium and beta2 integrin (CD11b/CD18) on neutrophils. We tested the hypothesis that a monoclonal antibody (mAb RR1/1) against ICAM-1 would preserve postischemic myocardial blood flow and attenuate myocardial injury in an anesthetized rabbit model of coronary occlusion and reperfusion. Either mAb RR1/1 or isotypematched control mAb (R3.1) was injected 10 min before reperfusion. Postischemic myocardial blood flow in the area at risk (Ar) and necrotic area was significantly improved with mAb RR1/1 treatment compared with vehicle and mAb R3.1 during the reperfusion period. RR1/1 had no effect on nonischemic zone blood flow. The Ar as a percent of left ventricle was comparable between groups. Infarct size (TTC) as a percent of Ar was significantly reduced by mAb RR1/1 compared with saline vehicle and mAb R3.1. Plasma creatine kinase activity confirmed the reduction of infarct size in mAb RR1/1 group. In in vitro studies, 40 microg/mL mAb RR1/l, which approximates the plasma concentration of 2 mg/kg mAb RR1/1, markedly inhibited platelet-activating factor-stimulated neutrophil adherence to rabbit aortic endothelium. We conclude that blockade of ICAM-1 during reperfusion reduces postischemic perfusion defects and attenuates the progression of myocardial injury leading to necrosis. This cardioprotection by mAb RR1/1 may be due to inhibition of neutrophil adhesion to the coronary endothelium.

Levels of expression of P-selectin, E-selectin, and intercellular adhesion molecule-1 in coronary atherectomy specimens from patients with stable and unstable angina pectoris

Unstable angina occurs when atherosclerotic plaque ruptures. Recent evidence suggests a role for inflammation in this process. Leukocyte-endothelial cell interactions are important in inflammation and are regulated by cell adhesion molecules. This study was designed to examine the vascular expression of cell adhesion molecules and cytokines in patients with unstable angina. Directional coronary atherectomy was performed in patients with unstable and stable angina. Expression of the cell adhesion molecules P-selectin, E-selectin, and intercellular adhesion molecule-1 in the tissue obtained was examined using immunohistochemistry. In addition, expression of the cytokines tumor necrosis factor-alpha and interleukin-1beta, which participate in the regulation of cell adhesion molecule expression, was also examined. Atherectomy specimens had significantly greater P-selectin expression from patients with unstable angina than from patients with stable angina. P-selectin expression was observed primarily on endothelial cells. There were no differences in any of the other factors between patients with unstable and stable angina. In addition, other clinical and angiographic variables were not associated with differential expression of any of the cell adhesion molecules or cytokines. These results indicate a possible role for P-selectin in the process of unstable angina.

Peroxynitrite inhibits leukocyte-endothelial cell interactions and protects against ischemia-reperfusion injury in rats

Peroxynitrite (ONOO-) anion, formed by the interaction of superoxide with nitric oxide (NO), has previously been implicated as a cytotoxic agent. However, the effects of this free radical species on neutrophil (PMN)-endothelial cell interactions is largely unknown. We investigated the direct actions of ONOO- on PMN adhesion to endothelial cells in vitro and in vivo, as well as the effects of ONOO- on PMN-mediated myocardial ischemia-reperfusion injury. In vitro, peroxynitrite (100-1,000 nM) inhibited the adhesion of rat PMNs to the endothelium of isolated thrombin- or H2O2-stimulated rat mesenteric artery (P < 0.01 vs. thrombin or H2O2 alone). In vivo, in the rat mesentery, thrombin (0.5 U/ml) or N(G)-nitro-L-arginine-methyl ester (50 microM) significantly increased venular leukocyte rolling and adherence, which were also significantly (P < 0.01) attenuated by ONOO (800 nM) accompanied by reduced P-selectin expression on the endothelial cell surface. Isolated perfused rat hearts were subjected to global ischemia and reperfusion with rat PMNs (10(8) cells), which resulted in profound cardiac depression (i.e., a marked reduction in left ventricular developed pressure and maximal rate of development of left ventricular pressure). Infusion of ONOO- reversed the myocardial contractile dysfunction of ischemic-reperfused rat hearts to near baseline levels, and markedly attenuated the accumulation of PMNs in the postischemic heart. The present study provides strong evidence that nanomolar concentrations of ONOO- both inhibit leukocyte-endothelial cell interactions and exert cytoprotective effects in myocardial ischemia-reperfusion injury. Furthermore, our results suggest that the inhibition of P-selectin expression by peroxynitrite is a key mechanism of the modulatory actions of ONOO- on leukocyte-endothelial cell interactions.

Combined inhibition of P-selectin and ICAM-1 reduces myocardial injury following ischemia and reperfusion

Neutrophil-endothelial cell interactions are mediated by a number of cell adhesion proteins. We investigated the effects of inhibition of P-selectin and intercellular adhesion molecule-1 (ICAM-1), individually or in combination, in the ischemic-reperfused canine myocardium. Monoclonal antibodies PB1.3 (anti-P-selectin) and CL 18/6 (anti-ICAM-1) were administered to dogs subjected to coronary artery occlusion and reperfusion. After reperfusion, untreated dogs experienced a 61% decline (P < 0.01 vs. baseline) in myocardial blood flow and a ninefold increase in ischemic zone neutrophil accumulation (4.7 +/- 0.9 U/100 mg tissue myeloperoxidase activity). In contrast, PB1.3 and CL 18/6 administered individually preserved myocardial blood flow (11 and 24% decrease from baseline, respectively, both P < 0.01 vs. saline), and significantly attenuated myeloperoxidase activity (1.4 +/- 0.3 and 1.5 +/- 0.26 U/100 mg tissue, respectively, both P < 0.01 vs. saline). PB1.3 and CL 18/6 in combination resulted in significant coronary vascular and myocardial protection that was not superior to treatment with either antibody alone. Thus the coadministration of anti-P-selectin and anti-ICAM-1 monoclonal antibodies does not enhance the degree of myocardial protection in this model of reperfusion injury.

A sialyl Lewis(x)-containing carbohydrate reduces infarct size: role of selectins in myocardial reperfusion injury

Previous studies have implicated the selectins (P- and L-selectin) in the acute phase of myocardial reperfusion injury. However, it is unclear whether these adhesion molecules are involved in the pathogenesis of myocardial reperfusion associated with longer periods of reperfusion. Dogs (n = 8/group) were subjected to 90 min of coronary ischemia and 48 h of reperfusion. Animals were initially treated with a 35 mg/kg intravenous bolus of a sialyl Lewis(x) oligosaccharide (SLe(x)-OS) 10 min before reperfusion, followed by a 1.75 mg.kg-1.h-1 infusion for the first 24 h of reperfusion. A control group of dogs received a normal saline bolus followed by saline infusion for the first 24 h of reperfusion. In a subsequent group of dogs treatment consisted of only the 35 mg/kg bolus of SLe(x)-OS to help elucidate the time course of selectin involvement. The saline control group exhibited marked decreases in blood flow in the ischemic-reperfused myocardium, sustained depression of left ventricular function, an average infarct size of 29 +/- 5% of the myocardial area at risk, and excessive polymorphonuclear leukocyte accumulation in the infarcted myocardium after 48 h of reperfusion. Dogs that received a bolus followed by an infusion of SLe(x)-OS exhibited significant preservation of myocardial blood flow and left ventricular function at 4.5 and 48 h of reperfusion, dramatic attenuation (56%) of infarct size (P < 0.05), and a 55% reduction (P < 0.05) in polymorphonuclear leukocyte accumulation compared with the saline group. Interestingly, SLe(x)-OS bolus treatment alone exerted early (i.e., at 4.5 h) cardioprotective effects that waned by 48 h of reperfusion. These results demonstrate that the selectin family of adhesion molecules plays an extended role in myocardial reperfusion injury and is not only involved in the acute phase of this disease process.

The role of nitric oxide and cell adhesion molecules on the microcirculation in ischaemia-reperfusion

The microcirculation undergoes a profound degree of endothelial dysfunction within minutes (i.e., 2.5 to 5 min) following reperfusion of ischaemic vasculature. This has been documented in the coronary and mesenteric microcirculation. The endothelial dysfunction is characterized by a loss in basal and agonist-mediated nitric oxide (NO) produced by the vascular endothelium. The loss of NO results in upregulation of cell adhesion molecules (CAMs) particularly P-selectin 10-20 min following reperfusion. Thus, CAM upregulation renders the endothelium sticky, and a marked degree of leukocyte adherence (particularly neutrophils) occurs 20 min following reperfusion. This enhanced involvement of neutrophils leads to neutrophil infiltration into the underlying tissue (e.g., myocardium) within 2-3 h of reperfusion. The infiltration of neutrophils leads to reperfusion injury (i.e., necrosis) which is significant at 3 h but becomes profound at 4.5 h following reperfusion. Cardiac necrosis can be significantly attenuated by treatment with NO, an organic NO donor, L-arginine, or specific blockers of CAMs given just prior to reperfusion. This approach is a promising one for a variety of types of reperfusion injury.

An anti-CD18 antibody limits infarct size and preserves left ventricular function in dogs with ischemia and 48-hour reperfusion

OBJECTIVES

This study investigated whether an antibody against neutrophil adhesion protein CD18 could limit myocardial infarct size and preserve left ventricular function after prolonged reperfusion in a canine model.

BACKGROUND

Myocardial reperfusion injury is mediated in part by accumulation of activated neutrophils. Although antibodies against CD18 have been shown to reduce neutrophil influx and infarct size after ischemia and 3 to 4 h of reperfusion, it is unknown whether protection is sustained beyond this time or whether there is meaningful preservation of ventricular function.

METHODS

Dogs undergoing 90-min circumflex coronary artery occlusion and 48-h reperfusion were randomized to receive 1 mg/kg bodyweight of R15.7 (an anti-CD18 antibody, n = 12) or saline (control, n =12) 10 min before reperfusion. Contrast left ventriculography was used to measure left ventricular ejection fraction and regional chord shortening at baseline, during occlusion and at 48 h. Microspheres injected during occlusion were used to measure collateral flow and risk region size. Postmortem infarct size was measured with triphenyltetrazolium chloride.

RESULTS

In the dose administered, R15.7 bound to neutrophils in vivo, with >85% saturation of CD18 for >24 h, with sustained antibody excess in the plasma. R15.7 significantly reduced infarct size after adjusting for the effect of collateral flow (p = 0.0002, analysis of covariance). In a subgroup of dogs with collateral flow <30% of nonischemic flow, infarct size was reduced from 34.6 +/- 3.9% (mean +/- SE) of the region at risk in the control group to 19.5 +/- 3.3% in the antibody group (p = 0.008). Ejection fraction and regional chord shortening did not differ between the two groups at baseline or during occlusion, but after 48-h reperfusion, ejection fraction and inferior wall regional cord shortening (representing the infarct zone) were both higher in the R15.7 group than the control group (43.6 +/- 2.9% vs. 28.5 +/- 1.8%, p < 0.01; 2.55 +/- 0.29% vs. 1.06 +/- 0.18%, p < 0.05).

CONCLUSIONS

A single injection of an anti-CD18 antibody given before reperfusion can limit myocardial infarct size by nearly 50% and preserve global and regional left ventricular function after 48 h of reperfusion.