Myocardial and interstitial matrix metalloproteinase activity after acute myocardial infarction in pigs

A structural event during the evolution of a myocardial infarction (MI) is left ventricular (LV) remodeling. The mechanisms that contribute to early changes in LV myocardial remodeling in the post-MI period remain poorly understood. Matrix metalloproteinases (MMPs) contribute to tissue remodeling in several disease states. Whether and to what degree MMP activation occurs within the myocardial interstitium after acute MI remains to be determined. Adult pigs (n = 15) were instrumented to measure regional myocardial function and interstitial MMP levels within regions served by the circumflex and left anterior descending arteries. Regional function was measured by sonomicrometry, and interstitial MMP levels were determined by selective microdialysis and zymography as well as by MMP interstitial fluorogenic activity. Measurements were performed at baseline and sequentially for up to 3 h after ligation of the obtuse marginals of the circumflex artery. Regional fractional shortening fell by over 50% in the MI region but remained unchanged in the remote region after coronary occlusion. Release of soluble MMPs, as revealed by zymographic activity of myocardial interstitial samples, increased by 2 h post-MI. The increased zymographic activity after MI was consistent with MMP-9. Myocardial interstitial MMP fluorogenic activity became detectable within the ischemic region as early as 10 min after coronary occlusion and significantly increased after 1 h post-MI. MMP fluorogenic activity remained unchanged from baseline values in the remote region. The present study demonstrated that myocardial MMP activation can occur within the MI region in the absence of reperfusion. These unique results suggest that MMP release and activation occurs within the ischemic myocardial interstitium in the early post-MI period.     

Preconditioning decreases ischemia/reperfusion-induced release and activation of matrix metalloproteinase-2

Release and activation of matrix metalloproteinases (MMPs) significantly contribute to myocardial stunning injury immediately after ischemia and reperfusion, however, their role in preconditioning remains unknown. We therefore examined the effects of preconditioning and subsequent ischemia/reperfusion on MMP activity in isolated rat hearts. Hearts were subjected to a preconditioning protocol (three consecutive 5-min periods of global ischemia interspersed with 5 min of reperfusion) followed by 30 min ischemia and 5 min reperfusion. To measure MMP release, coronary effluent was collected: (a) during aerobic perfusion, (b) in reperfusion following each preconditioning ischemia, and (c) during the final reperfusion following test ischemia. MMP-2 activities could be detected by gelatin zymography in the ventricles and coronary effluent samples from the perfused hearts. The levels of MMP-2 activity in the effluent were markedly increased in effluent following test ischemia from control hearts without preconditioning. This was accompanied by a decrease in corresponding tissue MMP activities. Preconditioning significantly decreased the MMP-2 activity in the coronary effluent following test ischemia/reperfusion and preserved the MMP-2 protein content and activity in the myocardium. Our results demonstrate that classic preconditioning inhibits ischemia/reperfusion induced release and activation of MMP-2. These results suggest that preconditioning may exert part of its cardioprotective effects through the reduction of MMP-2 release.     

Erythropoietin protects post-ischemic hearts by preventing extracellular matrix degradation: role of Jak2-ERK pathway

Factors predisposing to extracellular matrix degradation associated with myocardial ischemia/reperfusion (IR) usually cause cell death. Recombinant human erythropoietin (EPO) protects the myocardium from IR, but whether it affects extracellular matrix (ECM) degradation is not known. This study examined the effect of the Jak2-ERK pathway, which is triggered by EPO, on the expression of matrix metalloproteinases (MMPs), tissue inhibitor of MMP 4 (TIMP-4), and collagen in post-ischemic hearts. Rat hearts were isolated and perfused in a Langendorff apparatus. IR was induced by 40 min of stopped flow and 120 min of aerobic reperfusion; EPO was added immediately before reperfusion. Compared to untreated controls, poor recovery of the left ventricular developed pressure (LVDP) was seen in IR hearts. IR resulted in myocyte injury measured by creatine kinase MB release and infarction. Western blot analysis showed increased levels of MMP-2 and MMP-9 and reduced levels of TIMP-4 and collagen III. IR rats given 5 IU/ml of EPO showed improved LVDP with reduced injury. EPO increased Jak2 and ERK activity, decreased MMP expression, increased TIMP-4 expression, and prevented collagen degradation in IR hearts. All these effects were blocked by the upstream ERK inhibitor, U0126 (5 microM). These observations show that EPO attenuates extracellular matrix degradation following IR and this may be the basis of the protection from cell death. Jak2-ERK phosphorylation may be an important signal in this process.     

Peroxynitrite decomposition catalyst prevents matrix metalloproteinase activation and neurovascular injury after prolonged cerebral ischemia in rats

Matrix metalloproteinases (MMPs) play an important role in reperfusion-induced brain injury following ischemia. To define the effects of peroxynitrite decomposition catalyst on MMP activation and neurovascular reperfusion injury, 5,10,15,20-tetrakis (2,4,6-trimethyl-3,5-disulfonatophenyl)-porphyrin iron (III) (FeTMPyP) was administered intravenously 30?min prior to reperfusion following a middle cerebral artery occlusion. Activation of MMP was assessed by in situ and gel zymography. Neurovascular injury was assessed using endothelial barrier antigen, collagen IV immunohistochemistry and Cresyl violet staining. Results were compared with sham and ischemia alone groups. We found that administration of FeTMPyP just before reperfusion after ischemia inhibited MMP-9 activation and total MMP-2 increases in the cortex and decreased active MMP-9 along with the total amounts of active MMP-9 and active MMP-2 in the striatum. Reperfusion-induced injury to the basal lamina of collagen IV-immunopositive microvasculature and neural cells in cortex and striatum was ameliorated by FeTMPyP. Losses of blood vessel endothelium produced by ischemia or reperfusion were also decreased in the cortex. These results suggest that administration of FeTMPy prior to reperfusion decreases MMP activation and neurovascular injury after prolonged cerebral ischemia. This strategy may be useful for future therapies targeted at preventing breakdown of the blood-brain barrier and hemorrhagic transformation.     

Myocardial ischemia and reperfusion injury in rats: lysosomal hydrolases and matrix metalloproteinases mediated cellular damage

The aim of this study was to evaluate the time course events of cellular damage during myocardial ischemia and reperfusion injury in rats and to find out a correlation between the structural alterations with respect to the biochemical changes. Cardiac biomarkers and lysosomal enzymes viz. cathepsin D, acid phosphatase and β-glucuronidase and matrix metalloproteinases (MMPs) were evaluated at different time points, in response to ischemia-reperfusion induced oxidative stress in an isolated rat heart model perfused in Langendorff mode. Microscopically, changes in myocardial architecture, myofibrillar degradation, and collagen (COL) integrity were studied using hematoxylin-eosin, Masson’s trichrome and toluidine blue staining techniques. A three-fold increase in the level of myoglobin was observed after 30 min of ischemia followed by 120 min of reperfusion as compared to 15 min ischemia, 120 min reperfusion. Similarly, a significant increase (P < 0.05) in the levels of lipid peroxides and superoxide anion coupled with a decrease in enzymatic and nonenzymatic antioxidant levels were observed. A concomitant increase in the activity of cathepsin D (24.07 ± 0.95) and a higher expression of MMPs after 120 min of reperfusion following 30 min ischemia were shown to correlate with the myocardial damage as shown by histopathology, suggesting that free radical induced activation of cathepsin D and MMPs could mediate early damage during myocardial ischemia and reperfusion.     

Toll-like Receptor 4 Mediates the Inflammatory Responses and Matrix Protein Remodeling in Remote Non-Ischemic Myocardium in a Mouse Model of Myocardial Ischemia and Reperfusion

The signaling mechanism that mediates inflammatory responses in remote non-ischemic myocardium following regional ischemia/reperfusion (I/R) remains incompletely understood. Myocardial Toll-like receptor 4 (TLR4) can be activated by multiple proteins released from injured cells and plays a role in myocardial inflammation and injury expansion. We tested the hypothesis that TLR4 occupies an important role in mediating the inflammatory responses and matrix protein remodeling in the remote non-ischemic myocardium following regional I/R injury. Methods and results: TLR4-defective (C3H/HeJ) and TLR4-competent (C3H/HeN) mice were subjected to coronary artery ligation (30 min) and reperfusion for 1, 3, 7 or 14 days. In TLR4-competent mice, levels of monocyte chemoattractant protein -1 (MCP-1), keratinocyte chemoattractant (KC), intercellular adhesion molecule 1 (ICAM-1) and vascular cell adhesion molecule 1 (VCAM-1) were elevated in the remote non-ischemic myocardium at day 1, 3, and 7 of reperfusion. Levels of collagen I, collagen IV, matrix metalloproteinase (MMP) 2 and MMP 9 were increased in the remote non-ischemic myocardium at day 7 and 14 of reperfusion. MMP 2 and MMP 9 activities were also increased. TLR4 deficiency resulted in a moderate reduction in myocardial infarct size. However, it markedly downgraded the changes in the levels of chemokines, adhesion molecules and matrix proteins in the remote non-ischemic myocardium. Further, left ventricular function at day 14 was significantly improved in TLR4-defective mice. In conclusion, TLR4 mediates the inflammatory responses and matrix protein remodeling in the remote non-ischemic myocardium following regional myocardial I/R injury and contributes to the mechanism of adverse cardiac remodeling.     

Regulation of cell invasion and morphogenesis in a three-dimensional type I collagen matrix by membrane-type matrix metalloproteinases 1, 2, and 3

During tissue-invasive events, migrating cells penetrate type I collagen-rich interstitial tissues by mobilizing undefined proteolytic enzymes. To screen for members of the matrix metalloproteinase (MMP) family that mediate collagen-invasive activity, an in vitro model system was developed wherein MDCK cells were stably transfected to overexpress each of ten different MMPs that have been linked to matrix remodeling states. MDCK cells were then stimulated with scatter factor/hepatocyte growth factor (SF/HGF) to initiate invasion and tubulogenesis atop either type I collagen or interstitial stroma to determine the ability of MMPs to accelerate, modify, or disrupt morphogenic responses. Neither secreted collagenases (MMP-1 and MMP-13), gelatinases (gelatinase A or B), stromelysins (MMP-3 and MMP-11), or matrilysin (MMP-7) affected SF/HGF-induced responses. By contrast, the membrane-anchored metalloproteinases, membrane-type 1 MMP, membrane-type 2 MMP, and membrane-type 3 MMP (MT1-, MT2-, and MT3-MMP) each modified the morphogenic program. Of the three MT-MMPs tested, only MT1-MMP and MT2-MMP were able to directly confer invasion-incompetent cells with the ability to penetrate type I collagen matrices. MT-MMP-dependent invasion proceeded independently of proMMP-2 activation, but required the enzymes to be membrane-anchored to the cell surface. These findings demonstrate that MT-MMP-expressing cells can penetrate and remodel type I collagen-rich tissues by using membrane-anchored metalloproteinases as pericellular collagenases.     

Glucose-insulin-potassium preserves systolic and diastolic function in ischemia and reperfusion in pigs

Clinical and experimental studies have suggested benefit of treatment with intravenous glucose-insulin-potassium (GIK) in acute myocardial infarction. However, patients hospitalized with acute coronary syndromes often experience recurrent myocardial ischemia without infarction that may cause progressive left ventricular (LV) dysfunction. This study tested the hypothesis that anticipatory treatment with GIK attenuates both systolic and diastolic LV dysfunction resulting from ischemia and reperfusion without infarction in vivo. Open-chest, anesthetized pigs underwent 90 min of moderate regional ischemia (mean subendocardial blood flow 0.3 ml x g(-1) x min(-1)) and 90 min reperfusion. Eight pigs were treated with GIK (300 g/l glucose, 50 U/l insulin, and 80 meq/l KCl; infused at 2 ml x kg(-1) x h(-1)) beginning 30 min before ischemia and continuing through reperfusion. Eight untreated pigs comprised the control group. Regional LV wall area was measured with orthogonal pairs of sonomicrometry crystals. GIK significantly increased myocardial glucose uptake and lactate release during ischemia. After reperfusion, indexes of regional systolic function (external work and fractional systolic wall area reduction), regional diastolic function (maximum rate of diastolic wall area expansion), and global LV function (LV positive and negative maximum rate of change in pressure with respect to time) recovered to a significantly greater extent in GIK-treated pigs than in control pigs (all P < 0.05). The findings suggest that the clinical utility of GIK may extend beyond treatment of acute myocardial infarction to anticipatory metabolic protection of myocardium in patients at risk for recurrent episodes of ischemia.     

Ischemia/reperfusion-induced myosin light chain 1 phosphorylation increases its degradation by matrix metalloproteinase 2

Degradation of myosin light chain 1 (MLC1) by matrix metalloproteinase 2 (MMP-2) during myocardial ischemia/reperfusion (I/R) has been demonstrated. However, the exact mechanisms controlling this process remain unknown. I/R increases the phosphorylation of MLC1, but the consequences of this modification are not known. We hypothesized that phosphorylation of MLC1 plays an important role in its degradation by MMP-2. To examine this, isolated perfused rat hearts were subjected to 20 min global ischemia followed by 30 min of aerobic reperfusion. I/R increased phosphorylation of MLC1 (as measured by mass spectrometry). When hearts were subjected to I/R in the presence of ML-7 (a myosin light-chain kinase inhibitor) or doxycycline (an MMP inhibitor), improved recovery of contractile function was observed compared to aerobic controls, and MLC1 was protected from degradation. Enzyme kinetic studies revealed an increased affinity of MMP-2 for the phosphorylated form of MLC1 compared to non-phosphorylated MLC1. We conclude that MLC1 phosphorylation is an important mechanism controlling the intracellular action of MMP-2 and promoting degradation of MLC1. These results further support previous findings implicating post-translational modifications of contractile proteins as a key factor in the pathology of cardiac dysfunction during and following ischemia.     

TGF-beta 1 attenuates myocardial ischemia-reperfusion injury via inhibition of upregulation of MMP-1

Ischemia-reperfusion (I/R) is thought to upregulate the expression and activity of matrix metalloproteinases (MMPs), which regulate myocardial and vascular remodeling. Previous studies have shown that transforming growth factor-beta(1) (TGF-beta(1)) can attenuate myocardial injury induced by I/R. TGF-beta(1) is also reported to suppress the release of MMPs. To study the modulation of MMP-1 by TGF-beta(1) in I/R myocardium, Sprague-Dawley rats were given saline and subjected to 1 h of myocardial ischemia [total left coronary artery (LCA) ligation] followed by 1 h of reperfusion (n = 9). Parallel groups of rats were pretreated with recombinant TGF-beta(1) (rTGF-beta(1), 1 mg/rat, n = 9) before reperfusion or exposure to sham I/R (control group). I/R caused myocardial necrosis and dysfunction, indicated by decreased first derivative of left ventricular pressure, mean arterial blood pressure, and heart rate (all P < 0.01 vs. sham-operated control group). Simultaneously, I/R upregulated MMP-1 (P < 0.01). Treatment of rats with rTGF-beta(1) reduced the extent of myocardial necrosis and dysfunction despite I/R (all P < 0.01). rTGF-beta(1) treatment also inhibited the upregulation of MMP-1 in the I/R myocardium (P < 0.05). To determine the direct effect of MMP-1 on the myocardium, isolated adult rat myocytes were treated with active MMP-1, which caused injury and death of cultured myocytes, measured as lactate dehydrogenase release and trypan blue staining, in a dose- and time-dependent manner (P < 0.05). Pretreatment with PD-166793, a specific MMP inhibitor, attenuated myocardial injury and death induced by active MMP-1. The present study for the first time shows that MMP-1 can directly cause myocyte injury or death and that attenuation of myocardial I/R injury by TGF-beta(1) may, at least partly, be mediated by the inhibition of upregulation of MMP-1.     

Lipoprotein receptor-mediated induction of matrix metalloproteinase by tissue plasminogen activator

Although thrombolysis with tissue plasminogen activator (tPA) is a stroke therapy approved by the US Food and Drug Administration, its efficacy may be limited by neurotoxic side effects. Recently, proteolytic damage involving matrix metalloproteinases (MMPs) have been implicated. In experimental embolic stroke models, MMP inhibitors decreased cerebral hemorrhage and injury after treatment with tPA. MMPs comprise a family of zinc endopeptidases that can modify several components of the extracellular matrix. In particular, the gelatinases MMP-2 and MMP-9 can degrade neurovascular matrix integrity. MMP-9 promotes neuronal death by disrupting cell-matrix interactions, and MMP-9 knockout mice have reduced blood-brain barrier leakage and infarction after cerebral ischemia. Hence it is possible that tPA upregulates MMPs in the brain, and that subsequent matrix degradation causes brain injury. Here we show that tPA upregulates MMP-9 in cell culture and in vivo. MMP-9 levels were lower in tPA knockouts compared with wild-type mice after focal cerebral ischemia. In human cerebral microvascular endothelial cells, MMP-9 was upregulated when recombinant tPA was added. RNA interference (RNAi) suggested that this response was mediated by the low-density lipoprotein receptor-related protein (LRP), which avidly binds tPA and possesses signaling properties. Targeting the tPA-LRP signaling pathway in brain may offer new approaches for decreasing neurotoxicity and improving stroke therapy.     

Inhibition of MMP-2 activation and release as a novel mechanism for HDL-induced cardioprotection

High density lipoproteins (HDL) protect the heart against ischemia/reperfusion (I/R) injury, and matrix metalloproteinase-2 (MMP-2) directly contributes to cardiac contractile dysfunction after I/R. To investigate the possible involvement of MMP-2 inhibition in HDL-mediated cardioprotection, isolated rat hearts underwent 20 min of low-flow ischemia and 30 min of reperfusion. Plasma-derived and synthetic HDL attenuated the I/R-induced cardiac MMP-2 activation and release in a dose-dependent way. The attenuation of I/R-induced MMP-2 activation by HDL correlated with the reduction of post-ischemic contractile dysfunction and cardiomyocyte necrosis. These results indicate prevention of MMP-2 activation as a novel mechanism for HDL-mediated cardioprotection.     

Hydrogen peroxide causes cardiac dysfunction independent from its effects on matrix metalloproteinase-2 activation

Hydrogen peroxide (H2O2) causes cardiac dysfunction through multiple mechanisms. As oxidative stress can activate matrix metalloproteinases (MMPs) and, in particular, MMP-2 activity is associated with oxidative stress injury in the heart, we hypothesized that MMP-2 activation by H2O2 in isolated rat hearts contributes to cardiac dysfunction in this model. Isolated working rat hearts were perfused at 37 degrees C with a recirculating Krebs-Henseleit buffer+/-5 mmol/L pyruvate, known to protect hearts from oxidative stress. H2O2 (300 micromol/L) was added as a single bolus after 20 min of equilibration, and cardiac function was monitored for 60 min. MMPs activities in both the heart and perfusate samples were assessed by gelatin zymography. Tissue high energy phosphates were analysed by HPLC. The actions of 2 MMP inhibitors, doxycycline (75 micromol/L) or Ro 31-9790 (3 micromol/L), were also assessed. H2O2 at 300 micromol/L produced a rapid decline in cardiac mechanical function, which was maximal at 5 min. A peak in perfusate MMP-2 activity was also observed at 5 min. The deleterious effect of H2O2 on cardiac function was abolished by pyruvate but not by the MMPs inhibitors. This study suggests that in intact hearts, H2O2 induces contractile dysfunction independent of MMPs activation.     

Left ventricular interstitial 8-hydroxy-deoxyguanosine concentration following myocardial ischemia and reperfusion in anesthetized rats

Summary

The effect of myocardial ischemia and reperfusion on left ventricular interstitial 8-hydroxydeoxyguanosine (8-OH-dG), a possible biomarker for in vivo oxidative deoxyribonucleic acid damage, in anesthetized rats was investigated. A microdialysis probe was implanted. Levels of 8-OH-dG in microdialysates were analyzed via an on-line high performance liquid chromatography system equipped with an electrochemical detector. Myocardial ischemia for 10 or 20 min, induced by clamping of the left anterior descending coronary artery, did not affect 8-OH-dG levels. However, reperfusion following either 10-min or 20-min ischemia significantly increased 8-OH-dG levels in collected microdialysates. Reperfusion-induced increases in 8-OH-dG levels were more prominent in the 20 min ischemia group (as high as 3.5 fold relative to basal levels) than in the 10 min ischemia group as high as 2.0 fold relative to basal levels). In conclusion, we observed that left ventricular interstitial 8-OH-dG concentration increased following myocardial ischemia and reperfusion in anesthetized rats. These results suggest that 8-OH-dG might be a useful biomarker for oxidative damage following myocardial ischemia and reperfusion.     

Selective spatiotemporal induction of matrix metalloproteinase-2 and matrix metalloproteinase-9 transcription after myocardial infarction

Myocardial remodeling after myocardial infarction (MI) is associated with increased levels of the matrix metalloproteinases (MMPs). Levels of two MMP species, MMP-2 and MMP-9, are increased after MI, and transgenic deletion of these MMPs attenuates post-MI left ventricular (LV) remodeling. This study characterized the spatiotemporal patterns of gene promoter induction for MMP-2 and MMP-9 after MI. MI was induced in transgenic mice in which the MMP-2 or MMP-9 promoter sequence was fused to the beta-galactosidase reporter, and reporter level was assayed up to 28 days after MI. Myocardial localization with respect to cellular sources of MMP-2 and MMP-9 promoter induction was examined. After MI, LV diameter increased by 70% (P < 0.05), consistent with LV remodeling. beta-Galactosidase staining in MMP-2 reporter mice was increased by 1 day after MI and increased further to 64 +/- 6% of LV epicardial area by 7 days after MI (P < 0.05). MMP-2 promoter activation occurred in fibroblasts and myofibroblasts in the MI region. In MMP-9 reporter mice, promoter induction was detected after 3 days and peaked at 7 days after MI (53 +/- 6%, P < 0.05) and was colocalized with inflammatory cells at the peri-infarct region. Although MMP-2 promoter activation was similarly distributed in the MI and border regions, activation of the MMP-9 promoter was highest at the border between the MI and remote regions. These unique findings visually demonstrated that activation of the MMP-2 and MMP-9 gene promoters occurs in a distinct spatial relation with reference to the MI region and changes in a characteristic time-dependent manner after MI.     

Transgenic MMP-2 expression induces latent cardiac mitochondrial dysfunction

Matrix metalloproteinases (MMPs) are central to the development and progression of dysfunctional ventricular remodeling after tissue injury. We studied 6 month old heterozygous mice with cardiac-specific transgenic expression of active MMP-2 (MMP-2 Tg). MMP-2 Tg hearts showed no substantial gross alteration of cardiac phenotype compared to age-matched wild-type littermates. However, buffer perfused MMP-2 Tg hearts subjected to 30 min of global ischemia followed by 30 min of reperfusion had a larger infarct size and greater depression in contractile performance compared to wild-type hearts. Importantly, cardioprotection mediated by ischemic preconditioning (IPC) was completely abolished in MMP-2 Tg hearts, as shown by abnormalities in mitochondrial ultrastructure and impaired respiration, increased lipid peroxidation, cell necrosis and persistently reduced recovery of contractile performance during post-ischemic reperfusion. We conclude that MMP-2 functions not only as a proteolytic enzyme but also as a previously unrecognized active negative regulator of mitochondrial function during superimposed oxidative stress.     

Matrix metalloproteinase interactions with collagen and elastin

Most abundant in the extracellular matrix are collagens, joined by elastin that confers elastic recoil to the lung, aorta, and skin. These fibrils are highly resistant to proteolysis but can succumb to a minority of the matrix metalloproteinases (MMPs). Considerable inroads to understanding how such MMPs move to the susceptible sites in collagen and then unwind the triple helix of collagen monomers have been gained. The essential role in unwinding of the hemopexin-like domain of interstitial collagenases or the collagen binding domain of gelatinases is highlighted. Elastolysis is also facilitated by the collagen binding domain in the cases of MMP-2 and MMP-9, and remote exosites of the catalytic domain in the case of MMP-12.     

Suppression of ischemic and reperfusion ventricular arrhythmias by inhalational anesthetic-induced preconditioning in the rat heart

Inhalational anesthetic-induced preconditioning (APC) has been shown to reduce infarct size and attenuate contractile dysfunction caused by myocardial ischemia. Only a few studies have reported the effects of APC on arrhythmias during myocardial ischemia-reperfusion injury, focusing exclusively on reperfusion. Accordingly, the aim of the present study was to examine the influence of APC on ventricular arrhythmias evoked by regional no-flow ischemia. APC was induced in adult male Wistar rats by 12-min exposures to two different concentrations (0.5 and 1.0 MAC) of isoflurane followed by 30-min wash-out periods. Ventricular arrhythmias were assessed in the isolated perfused hearts during a 45-min regional ischemia and a subsequent 15-min reperfusion. Myocardial infarct size was determined after an additional 45 min of reperfusion. The incidence, severity and duration of ventricular arrhythmias during ischemia were markedly reduced by APC. The higher concentration of isoflurane had a larger effect on the incidence of ventricular fibrillation than the lower concentration. The incidence of ventricular tachycardia and reversible ventricular fibrillation during reperfusion was also significantly reduced by APC; the same was true for myocardial infarct size. In conclusion, we have shown that preconditioning with isoflurane confers profound protection against myocardial ischemia- and reperfusion-induced arrhythmias and lethal myocardial injury.