Oxidative stress and myocardial injury in the diabetic heart

Abstract

Reactive oxygen or nitrogen species play an integral role in both myocardial injury and repair. This dichotomy is differentiated at the level of species type, amount, duration of free radical generated. Homeostatic mechanisms designed to prevent free radical generation in the first instance, scavenge, or enzymatically convert them to less toxic forms and water, play crucial roles in maintenance of cellular structure and function. The outcome between functional recovery and dysfunction is dependent upon the inherent ability of these homeostatic antioxidant defenses to withstand acute free radical generation, in the order of seconds to minutes. Alternatively, pre-existent antioxidant capacity (from intracellular and extracellular sources) may regulate the degree of free radical generation. This converts reactive oxygen and nitrogen species to the role of second messenger involved in cell signalling. The adaptive capacity of the cell is altered by the balance between death or survival signal converging at the level of the mitochondria, with distinct pathophysiologic consequences that extends the period of injury from hours to days and weeks. Hyperglycemia, hyperlipidemia, and insulin resistance enhance oxidative stress in diabetic myocardium that cannot adapt to ischemia reperfusion. Altered glucose flux, mitochondrial derangements and nitric oxide synthase uncoupling in the presence of decreased antioxidant defense and impaired prosurvival cell signalling may render the diabetic myocardium more vulnerable to injury, remodelling and heart failure.

Introduction

Coronary artery atherosclerosis is the leading cause of death in the western world [1] and its prevalence is increasing worldwide on an annual basis. It is estimated that more than one million people will experience or be at risk for life threatening myocardial infarction annually. Diabetes mellitus increases the risk of developing cardiovascular disease nearly five fold [2]. Nearly 25 million adult United States citizens have been diagnosed with concomitant diabetes and coronary artery atherosclerosis [3]. The risks of heart failure and death after myocardial infarction (MI) are increased 2 to 4 fold in patients with diabetes [4–6].

Effective re-establishment of coronary perfusion to preserve the myocardium is a mainstay of therapy for patients with coronary artery atherosclerosis. Unfortunately, diabetes is associated with increased cardiac morbidity and mortality following surgical or nonsurgical (angioplasty and vascular stenting) revascularization techniques [7–11]. Ischemia-reperfusion injury following myocardial revascularization is a major risk factor in the development of adverse outcomes in this patient population.

The principle mediator of myocardial injury secondary to ischemia and reperfusion in type 2 diabetes is oxidative stress [12]. Oxidative stress mediated myocardial injury is the consequence of an imbalance between the free radical generation and elimination due to increased reactive oxygen and nitrogen species (RONS) generation and/or inadequate antioxidant defense [13]. Oxidative stress arises directly or indirectly from hyperglycemia, hyperlipidemia, hyperinsulinemia, and insulin resistance that characterize type 2 diabetes. These perturbations, either alone or in combination, are thought to contribute to altered cellular structure and function of diabetic myocardium. The cellular and molecular mechanisms of diabetic cardiomyopathy in the absence of coronary artery atherosclerosis have been considered in detail elsewhere [14, 15]. This review will focus on the role of free radical biology in the pathogenesis of injury in diabetic heart with concomitant coronary artery atherosclerosis.

Oxidative stress and myocardial ischemia-reperfusion injury

The most common forms of free radicals identified in the human heart include the superoxide anion (O₂^•)⁻, the hydroxyl radical (OH^•), hydrogen peroxide (H₂O₂), singlet oxygen, carbon-centered radical, peroxynitrite (ONOO⁻), nitric oxide (NO^•), and nitrogen dioxide radical [16]. The O₂^•− serves as the precursor to the formation of most of the other free radicals [13].

The generation of free radicals in the heart is low under basal conditions. It is normally the results of electron leakage from the mitochondrial electron transport chains (ETC) under physiologic conditions. The cardiomyocyte response to low-level O₂^•− generation includes its conversion to less cytotoxic H₂O₂ by the action of enzyme superoxide dismutase (SOD) in cytoplasm or mitochondria. Hydrogen peroxide is subsequently converted to water by either catalase (CAT) or by the glutathione peroxidase (GPx) system [13].

In contrast, during ischemia and reperfusion, O₂^•− generation is markedly increased and originates from multiple cellular sources. These include the mitochondrial electron transport damage and uncoupling, nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, uncoupled nitric oxide synthase (NOS), xanthine oxidase, cytochrome P450 monooxygenase, and cyclooxygenase. In addition, the cellular antioxidant defense system is depleted during ischemia and reperfusion with lower activities of endogenous free radical scavenging enzymes such as SOD, GPx and CAT. The severity of cardiomyocyte damage during ischemia and reperfusion ranges from reversible to fatal, proportionally to the magnitude of RONS reactions within the cells. We have recently published a detailed review of the topic and this is not considered at length in this article [13].

The OH^•, which is thought to be the most damaging radical species in the cell, is extensively generated during ischemia and reperfusion [17]. Ischemia and early reperfusion result in low pH and hypoxic conditions that favor the release of iron cations (Fe²⁺) from metalloproteinases. In the presence of iron cations, H₂O₂ from SOD scavenge of O₂^•− will more likely be converted via the Fenton reaction to the OH^• (instead of being scavenged by the CAT or GPx) [18]. Nitric oxide, whose myocardial production is also greatly increased during ischemia-reperfusion, reacts with increased O₂^•− to yield ONOO⁻. With increased intracellular acidification, ONOO⁻ becomes more protonated to form peroxynitrous acid (ONOOH). ONOON rapidly forms nitrogen dioxide and OH^•. The ONOO⁻/ONOOH degradation contributes in parallel with the Fenton reaction to OH^• generation during ischemia and reperfusion [13].

The conditions of early reperfusion may be the primary determinant of tissue injury [19]. Reperfusion stimulates NADPH oxidase, cytochrome P450 and cyclooxygenase activity to increase and accelerate free radical production. Paradoxically, re-oxygenation of tissues with accumulated oxidative substrates uncouples the ETC, leading to a large-scale generation of RONS. Large-scale RONS generation [20, 21] and depleted energy stores [22, 23], induce a process called mitochondrial permeability transition (mPT) [24, 25]. mPT results in pore (mPTP) formation and opening on the inner mitochondrial membrane that permits the passage of molecules below 1.5 kDa in size [26, 27]. mPTP dissipates the mitochondrial membrane potential (ΔΨ_m) by providing protons an alternate route to the matrix. Cellular adenosine triphosphate (ATP) stores are further depleted. Molecules greater than 1.5 kDa exert a colloid osmotic pressure that causes the mitochondrial matrix to swell. The integrity of the outer membrane is compromised, cytochrome c is released to the cytoplasm to initiate pro-apoptotic signals [28]. The severity and the duration of mPT directly affect maintenance of ATP stores and cellular integrity. If mPT is transient, the cell can recover [29]. If mPT is prolonged and ATP depletion is severe, necrosis occurs [25]. To this end, the number of mitochondria undergoing mPT correlates with the likelihood of cardiomyocyte loss [30]. The severity of mPT is proportional to the functional recovery in an isolated heart model [31]. In general, acidic condition during ischemia inhibits mPT. As pH normalizes during early reperfusion, mPT is enhanced [32–34].

Myocardial injury during ischemia and reperfusion in diabetes

Although there is conflicting evidence from experimental studies on whether diabetic myocardium is more or less vulnerable to ischemia and reperfusion injury compared to normal heart, clinical data strongly supports an increased susceptibility to myocardial ischemia-reperfusion injury in patients with diabetes mellitus [35–38]. Clinically, diabetes is considered as an independent risk factor for myocardial injury during cardiac surgery [38]. The risk of post-myocardial infarction death is increased 2 to 4 fold in diabetic patients compared to those without diabetes [4, 6]. The pathophysiology of myocardial ischemia and reperfusion injury involves multifactorial mechanisms. The underlying mechanisms responsible for the increased injury in diabetic myocardium are not fully understood. The enhanced oxidative stress (excessive free radical generation and/or depleted endogenous antioxidant defense system) and impaired cellular cardioprotective prosurvival signal pathways have been implicated.

Enhanced oxidative stress in diabetic myocardium

It is well known that oxidative stress is involved in the development and progression of diabetes mellitus and diabetic complications [39, 40]. Yokota et al reported the increased O₂^•− production, lipid peroxidation, and NADPH oxidase activity in the plasma and tissue of type 2 diabetic mice [41]. The oxidative protein products and lipid peroxidation are elevated in patients with type 2 diabetes [42, 43]. Lack of functional GPx-1 was detected in diabetic apolipoprotein E-deficient mice [44]. In patients with type 2 diabetes, both the levels and the activities of enzymatic (GPx, SOD, and CAT) and non-enzymatic (β-carotene, retinol, vitamin C and E, and uric acid) antioxidants in erythrocytes were decreased compared to normal subjects [42]. Antioxidants therapy can normalize the increased ROS production in diabetic animals [39]. Although the beneficial effects of antioxidants on diabetes and its complications are still controversial, diabetic patients with antioxidants administration show a significant suppression of plasma markers for lipid oxidation [45]. These findings provide the confirmatory evidence of enhanced oxidative stress in diabetes.

In diabetic myocardium there are three major sources of free radicals: mitochondria, NADPH oxidase and/or NOS. Diabetes is associated with impaired mitochondrial morphology and function [46]. Myocardial mitochondrial perturbations were identified in both type 2 diabetic mice [47] and human diabetes [15]. Increased leakage of electron and the generation of O₂^•− from the ETC results from mitochondrial dysfunction. In contrast to normal conditions involving physiologic generation of O₂^•− by ETC Complex I and III, in diabetes, myocardial O₂^•− generation is enhanced by stimulation of ETC Complex II [48, 49]. Elevated NADPH oxidase activity was also observed in myocardium and vascular tissues from animal models with type 2 diabetes [50, 51]. In diabetic patients, both the activity of NADPH oxidase system and the levels of NADPH oxidase protein subunits were significantly increased [52, 53]. NADPH oxidase inhibitors were demonstrated to significantly inhibit the increased free radical formation in diabetic animals. In addition, reduced availability of the cofactor tetrahydrobiopterin (BH4) was identified in diabetic animal vessels and endothelial cells. The depletion of BH4 functionally “uncouples” endothelial NOS (eNOS) to generate more O₂^•− and less nitric oxide [54]. The accompanying increase in NADPH oxidase 2 and 4 in diabetes [55] further predisposes the heart to NOS uncoupling and ONOO⁻ generation. Moreover, inducible NOS (iNOS) is activated in diabetes by inflammatory mediators, which makes iNOS uncoupling a predominant contributor for oxidative/nitrosative stress in diabetic myocardium. Reversal of iNOS uncoupling by BH4 treatment is shown to increase myocardial tolerance to ischemia and reperfusion injury by increasing the bioavailability of iNOS-derived nitric oxide and eliminating oxidative stress in the diabetic rat heart [56].

The mechanisms underlying mitochondrial dysfunction, high NADPH oxidase activation and NOS uncoupling in diabetic myocardium are still unclear. However, various abnormalities in type 2 diabetes, including hyperglycemia, hyperlipidemia, hyperinsulinemia, and insulin resistance have all been implicated [57].

Hyperglycemia induces O₂^•− generation in the heart primarily by disruption of the ETC, activation of NADPH oxidase, and uncoupling of NOS [49, 58]. High glucose can impair the activities of ETC complex enzymes, and contribute to mitochondrial O₂^•− overproduction. Chronic elevated oxidative stress in type 2 diabetes may in turn impair mitochondrial energy metabolism, lead to further mitochondrial dysfunction [15] and form a vicious metabolic cycle of irreversible cell and tissue damage. High glucose has been demonstrated to activate NADPH oxidase and increased NADPH oxidase-derived O₂^•− formation in vitro. Hyperglycemia is also shown to increase NOS-dependent O₂^•− production in human endothelial cells. Alternative pathways of glucose metabolism and the formation and activation of advanced glycation endproducts (AGEs) may also play a major role in enhancing oxidative stress [59–61]. In addition, glycation can inactivate antioxidant enzymes like SOD to impair the myocardial antioxidant defense [62, 63]. Hyperglycemia has been shown to decrease total antioxidant capacity, erythrocyte glutathione (GSH) content and SOD activity in patients with impaired glucose regulation [64].

Free fatty acids (FFAs) levels are elevated in diabetes [47, 65]. FFAs stimulate NADPH oxidase and the ETC to generate ROS [66]. Elevated levels of FFAs also decrease intracellular GSH [67] to additionally impair the endogenous defense against free radical mediated injury.

Hyperinsulinemia and insulin resistance are usually linked to type 2 diabetes. Insulin resistance is related to oxidative stress in obese children [68]. Insulin administration increases NADPH oxidase activity to produce free radicals in cultured cells. By using a cardiomyocyte insulin receptor deletion mice model, Boudina et al reported impaired myocardial insulin signalling induced mitochondrial uncoupling, and promoted H₂O₂ production and oxidative stress [69].

Impaired endogenous cardioprotective signalling pathways in diabetic heart

Yue et al reported that cardiomyocyte necrosis is significantly increased in ventricular myocardial biopsies of diabetic patients, confirming a primary impairment in myocardial tolerance to ischemia and reperfusion injury [70]. It is well recognized that effective intraoperative myocardial protection is crucial to preserve high-risk myocardium and improve patient post-operative outcomes. To date, extensive research has focused on increasing the myocardial tolerance to ischemia using conditioning strategies.

Myocardial ischemia and/or pharmacological pre- and/or post-conditioning are known to reduce the severity of ischemia-reperfusion injury by activating cell survival pathways and altering free radical reactions in the myocardium. Unfortunately, the diabetic myocardium is resistant to physical or pharmacologic pre- and post-conditioning stimulus. This has been explained experimentally on the basis of prosurvival signal transduction impairment and enhanced mPT. This could explain enhanced susceptibility to injury in ischemia-reperfused diabetic myocardium.

Conditioning in general involves diverse cell signalling mechanisms. It appears to be predicated on the direct and/or indirect activation of several key cellular pro-survival pathways including the phosphoinositide-3 kinase (PI3K)-AKT and janus kinase 2 (JAK2)-signal transducer and activator of transcription 3 (STAT3) pathways, which in turn converge on key-end-effectors such as mPTP.

The PI3K/AKT prosurvival pathway is central to physical and pharmacologic pre- and postconditioning and salvaging the ischemia-reperfused myocardium. The activation of AKT induces a cytoprotective effect via the actions of its putative downstream effectors, such as eNOS and anti-apoptotic B-cell lymphoma-2 (BCL-2), to prevent mitochondrial-directed cell death [71, 72]. In diabetes, myocardium is resistant to preconditioning stimulus [73, 74]. The threshold stimulus for myocardial preconditioning is raised and subject to a critical level of AKT activation, to mediate a cardioprotective effect [73, 74]. The extent of AKT activation has been found to be significantly decreased in a Goto-Kakizaki rat type 2 diabetic model [74]. Impaired AKT activation has been characterized in patients with diabetes in clinical studies [73, 75]. It has been postulated that the principle negative regulator of the PI3K/AKT pathway is phosphatase and tensin homologue on chromosome 10 (PTEN) [76–78]. PTEN is constitutively expressed and subject to regulation by free radical biology. In part, hyperglycemia induced RONS generation affects PTEN antagonism of AKT activation [79, 80]. The level of PTEN has been found to be increased in Goto-Kakizaki Rat myocardium [77]. Our group recently found a positive correlation between blood glucose level, oxidative stress and PTEN level detected in human diabetic myocardium [43]. The extent of AKT phosphorylation negatively correlated with increased levels of myocardial PTEN in patients with diabetes. These findings suggest increased PTEN levels negatively regulate AKT prosurvival signalling in human diabetic myocardium [43]. Further study is required. In addition, high levels of circulating FFAs impair insulin-stimulated activation of PI3K, AKT, and eNOS [65, 81, 82]. FFAs are shown to up-regulate PTEN expression in vitro [81]. FFA treatment of cardiomyocytes has been shown to result in decreased insulin-stimulated eNOS activation [81]. These findings confirmed the impaired prosurvival AKT signalling in diabetic heart, which may render the heart resistant to cardioprotective preconditioning interventions.

The activation of transcription factor STAT3 is another cardioprotection obtained independently of AKT [83]. STAT3 is activated during ischemia and reperfusion by the JAKs [84]. STAT3 is phosphorylated by activated JAK at tyrosine 705. Activated STAT3 then translocates to the nucleus, regulating gene transcription [85]. In the heart, STAT3 positively regulates the expression of several anti-apoptotic genes such as B-cell lymphoma-extra large (BCL-XL) and BCL-2 [86]. In addition to its role as a transcriptional factor, STAT3 can also act as a signalling molecule by direct phosphorylation of various cytoplasmic components [85–87]. Mitochondrial energetics are preserved via direct action of STAT3 on the ETC [88] or prevention of mPT via the actions of AKT and BCL-2, two of STAT3’s potential downstream effectors [89, 90]. Experimentally, decreased STAT3 activity and phosphorylation, observed in aged mice [91] or with pharmacological inhibition, [92] is associated with a loss of the cardioprotection normally achieved by pre- and post-conditioning. STAT3-deficient mice are more susceptible to myocardial ischemia-reperfusion injury and MI, show increased cardiac apoptosis and infarct size, and have reduced cardiac function and survival [93]. In contrast, transgenic mice that over-express STAT3 have decreased infarct size subsequent to ischemia and reperfusion compared to wild type mice [94]. Although there is no direct evidence on STAT3 expression and activation in human diabetes, the rat modals of diabetes demonstrated significant decrease in myocardial STAT3 expression and activation, which was associated with a resistance to conditioning strategies [95].

Susceptibility to oxidant-mediated ischemia and reperfusion injury in the diabetic heart

The diabetic myocardium and vasculature appear more vulnerable to ischemia and reperfusion injury. This includes oxidant mediated structural alternations to cardiomyocyte sarcomeres, mitochondria and accompanying interstitial, and microvascular ultrastructure [96]. The underlying mechanisms responsible for exacerbation of oxidant-mediated injury in the ischemic reperfused diabetic heart are not clearly known. In addition to the general increased basal oxidative state and impaired cellular prosurvival signallings already described in previous sections, the primary etiology also relates to the impact of glucose flux, impaired cardiac stress or adaptive responses, and NOS uncoupling in the ischemic reperfused diabetic heart, to confer further injury (See Figure 1).

Figure 1.

Ischemia-reperfusion injury in the diabetic myocardium is multi-factorial and complex. a) Overexpression of phosphatase and tensin homologue on chromosome 10 (PTEN) in diabetes results in an inability to activate cellular protective pathways such as, PI3K/AKT and JAK2/STAT3 to prevent ischemia and reperfusion injury. b) Hexose biosynthase pathway (HBP) activation due to hyperglycemia results in glycosylation of proteins. Glycosylation of BAD, for example, leads to higher binding to and inactivation of anti-apoptotic BCL-2. This renders the cardiomyocyte more susceptible to mPTP. c) The multitude of effects of hyperglycemia and diabetes lead to uncoupling of the mitochondrial electron transport chains (ETC) and have a net effect of ROS overproduction and lower ATP production that renders the myocardium unable to ward off injury. d) Uncoupling of NOS enzymes and the resultant overproduction of peroxynitrite is a cyclical mode of cellular injury. This cycle is fed by overproduction of superoxide, which itself is formed from multiple sources during diabetes and ischemia and reperfusion. e) Release of divalent cations from metalloproteinases during ischemia and reperfusion promote production of OH^• radical from H₂O₂ through the fenton reaction. f) Activation of aldose reductase during ischemia and reperfusion depletes NADPH, the cofactor required for glutathione reductase (GR) activity. Decreased GR activity leads to lower levels of anti-oxidant glutathione (GSH), which also contributes to lower glutathione peroxidase (GPx) activity. Lower GPx activity decreases the amount of H₂O₂ neutralized and therefore contributes to higher oxidative stress and susceptibility to mPTP and resultant cardiac injury and remodeling.

In diabetic heart, the flux of glucose also activates aldose reductase, sorbital dehydrogenase, and the conversion of glucose to fructose to promote oxidant stress through the polyol pathway. Ischemia-reperfusion activates aldose reductase induced depletion of intracellular NADPH, the cofactor needed for the activity of glutathione reductase (GR) in the heart. The level of antioxidant GSH is depleted, rendering the heart vulnerable to oxidant mediated injury. It is reported that aldose reductase mediated oxidative stress enhances mPTP openings [97], which contributes to myocardial contractile dysfunction and tissue damage in ischemia-reperfused rat hearts [98, 99]. Indeed, the pharmacological inhibition of aldose reductase may represent a novel adjunctive approach for protecting the diabetic ischemic hearts.

In addition to high glucose, hyperlipidemia and RONS generation during ischemia and reperfusion create greater flux through the hexose biosynthase pathway (HBP). This may increase protein O-GlcNAc modification at regulatory serine/threonine phosphorylation sites. O-GlcNAc modification impairs prosurvival PI3K-AKT-eNOS signalling in diabetes [100, 101]. Increased O-GlcNAcylation of BCL-2-associated death promoter (BAD) will lead to an increase in BAD-BCL-2 (or BCL-XL) dimerization and the subsequent decrease in free BCL-2 (or BCL-XL) [102, 103]. The impact of blood glucose level on the downregulation of BCL-2 expression in skin biopsy from patients with diabetes has been previous described [104]. Similarly our group recently found an inverse correlation between blood glucose and BCL-2 level in human diabetic myocardium [43]. Decreased free BCL-2 (or BCL-XL) expression will predispose to mPT and increased cardiomyocyte death to extend injury. Targeting glycobiology of the diabetic heart may promote cell protection rather than corrupt important cellular prosurvival signalings responsible for metabolic homeostasis of the heart.

Compensatory mechanisms to protect the heart against oxidant-mediated injury include upregulation of metallothionein1 and 2 (MT1 and 2), redox regulators of free radicals scavenger GR [105]. MT expression increases in response to the oxidant stress in the diabetic heart. MT1 and 2 are also targets genes involved in STAT3-mediated cardioprotection [106]. However the response may be inadequate in diabetes. Decreased STAT3 expression in the diabetic heart may sensitize myocardium to the effects of RONS production during ischemia-reperfusion to exacerbate this form of injury [107, 108] via the insufficient regulation on MT1/2 expression.

Additionally, heme oxygensae (HO)-1 is the stress response protein responsible for oxidant heme degradation, or the generation of antioxidant bilirubin or carbon monoxide. Its absence has been associated with increased mortality following MI. Thus, the inability to elaborate HO-1 may also exacerbate ischemia-reperfusion injury in the diabetic heart [109].

Alternatively, thioredoxin (TRX)-1, a key intracellular antioxidant that regulates cell survival pathways [110], is also modified in diabetes. ONOO⁻ generation secondary to eNOS uncoupling may be responsible for nitration of TRX-1 [111]. Glycation of TRX-1 may also occur in hyperglycemia and diabetes [112]. The subsequent inactivation of TRX-1 due to nitration or glycation may represent a major mechanism responsible for ischemia and reperfusion in diabetes [110].

Progressive myocardial dysfunction and degeneration after MI

Oxidative stress mediated mechanisms of injury are also the leading cause of myocardial repair and remodeling after MI [113, 114]. This is especially the case with the diabetic heart [115], where oxidative stress is extensively enhanced. Smith et al demonstrated elevated oxidative stress in the diabetic MI animals as indicated by increased levels of myocardial 8-isoprostane (a marker of oxidative stress), oxidized glutathione (GSSG), SOD and CAT protein expression. This was associated with depressed indices of cardiac function at 4 weeks post-MI [116]. Aragno et al found that oxidative stress induced by experimental chronic hyperglycemia promoted profibrogenic gene expression and extracellular matrix deposition, which led to cardiac fibrosis and dysfunction [117]. These findings provide evidence to support oxidative stress as a key factor in the pathogeneses of myocardial dysfunction and degeneration after MI in diabetes.

Oxidative stress-induced cardiac remodeling involves multiple mechanisms [114, 118], including the effects of RONS on degeneration of lipid, protein and nuclei acid, RONS-promoted cardiomyocytes apoptosis, direct damage of RONS on cardiac contractile function, RONS-activated cardiac inflammatory response, regulation of RONS on extracellular matrix remodeling, and activation of a series of protein kinase which response for hypertrophic response by RONS stimulation. Study in diabetes demonstrated that the loss of myocardial insulin signalling led to accelerated post-MI left ventricular dysfunction [119], which may partially due to a reduction of substrate utilization and availability in the diabetic myocardium induced by oxidative stress related mitochondrial dysfunction (identified by a combination of declined mitochondrial fatty acid oxidative capacity and limited glucose transport capacity). In addition, mitochondria in human diabetic myocardium have decreased tolerance to calcium and increased propensity towards calcium induced mPTP opening, which is associated with increased levels of caspase-9, the mitochondrial cell death protease [120]. Oxidant induced alterations in key mPTP components including the adenine nucleotide transporter and glutathione depletion may play a role in mitochondrial calcium sensitivity of the mPTP. Finally, our group recently described the association between myocardial 15-F_2t-isoprostane (a marker for oxidative stress) generation, increased expression of PTEN, and decreased levels of key mitochondrial regulator BCL-2 in human diabetic myocardium [43], which may, in part, explain this effect in diabetic myocardium.

Taken together, the prolonged impact of metabolic perturbations, mitochondrial dysfunction in association with insulin resistance and oxidative stress in the diabetic state, has profound impact on the myocardium’s tolerance to ischemia reperfusion injury. Oxidative stress subsequently plays a role in the pathogenesis and increased frequency of heart failure following MI associated with diabetes.

Conclusions

Oxidative stress, an imbalance between the free radial generation and elimination or detoxification, is a principle mediator of myocardial injury during ischemia and reperfusion. The magnitude of oxidative stress is proportional to the functional recovery of the heart. Among the multiple cellular sources of free radicals during tissue ischemia and reperfusion, mitochondria-derived RONS is extremely important in the ischemic and reperfused myocardium.

In diabetes, enhanced oxidative stress arises directly or indirectly from hyperglycemia, hyperlipidemia, hyperinsulinemia, and insulin resistance. These perturbations, either alone or in combination, contribute to mitochondrial dysfunction, high NADPH oxidase activation and NOS uncoupling, which lead to excessive free radical generation in diabetic myocardium that cannot readily adapt to ischemia reperfusion. Altered glucose flux, mitochondrial derangements and NOS uncoupling in the presence of decreased antioxidant defense and impaired prosurvival cell signalling may render the diabetic myocardium more vulnerable to injury, remodelling and heart failure.

No therapeutic strategy has yet been demonstrated clinically effective against cardiac injury in diabetic population. The complex nature of the free radical biology in myocardial homeostasis, injury and repair strengthens the understanding of the underlying biologic mechanisms and the challenges to prevent cardiac injury in patients with diabetes. Simultaneously targeting perioperative glucose control, inhibition of oxidative stress in cardiac tissues, and activation of cellular prosurvival signalling may prove to be an effective treatment alternative for the cardioprotection of surgical patients with type 2 diabetes. This approach is the subject of ongoing research in laboratory models and clinical studies.