Effects of diabetes on myocardial infarct size and cardioprotection by preconditioning and postconditioning

Cardiovascular diseases are the leading cause of death, accounting for approximately 30% of all deaths worldwide [1]. Among the various cardiovascular diseases, acute myocardial infarction (AMI) has a high rate of mortality, and infarct size is a primary determinant of prognosis in these patients. The only established and clinically approved method to limit infarct size is restoration of coronary blood flow by percutaneous coronary interventions (PCI), thrombolytic agents or coronary bypass surgery. However, our recent review of clinical infarct size data indicated that infarct size after current reperfusion therapy was larger than 75% of the area at risk and larger than 20% of the left ventricle in one fourth of AMI patients [2]. Lack of substantial myocardial salvage in those patients cannot be simply attributable to long symptom-to-reperfusion time, and involvement of reperfusion injury [3] is also possible. Nevertheless, significant percentages of AMI patients suffer from insufficient cardiac function and have a poor prognosis even after successful restoration of coronary flow [2]. Therefore, the establishment of a novel strategy to limit the extent of infarction during ischemia/reperfusion is of great clinical importance.

As a cardioprotective strategy, ischemic preconditioning (PC) has received much attention for its potent infarct size-limiting effect since its first report by Murry et al. in 1986 [4]. Zhao et al. found that “conditioning” with repetitive ischemia/reperfusion at the time of reperfusion also affords protection and named the phenomenon ischemic postconditioning (PostC) [5]. Laboratory investigations to date have clarified outlines of the intracellular mechanisms of PC and PostC [6‐9]. In addition, several agents targeting signaling relevant to PC and PostC have been shown to act as PostC mimetics [6‐8]. However, clinical benefits of PC, PostC and their mimetics were not always demonstrated in clinical trials [8, 10]. The discrepancy between results of animal studies and clinical trials may be explained by notable differences between animal models of AMI and AMI in humans [2, 10]. First, there are species differences in regulatory mechanisms of cardiomyocytes such as contribution of sarcolemmal Ca²⁺ ATPase to overall Ca²⁺ handling. Second, the timing of administration of the test agent was not exactly the same in the positive animal studies and negative clinical trials. Third, most of the patients with AMI were pretreated with pharmacological agents for their co-morbidities at the onset of AMI, whereas animals were untreated, except for anesthetic agents, before AMI in most of the previous studies. Pharmacological agents used for treatment of hypertension, dyslipidemia and/or diabetes mellitus (DM) potentially modify intracellular signaling relevant to cytoprotection and thus myocardial responses to PC, Post and PC mimetics. Fourth, AMI patients recruited to clinical studies were generally old and had concomitant diseases, whereas relatively young and healthy animals were used in most of the previous animal studies. Aging and co-morbidities have been shown to modify or even abrogate the infarct size-limiting effects of interventions in animal experiments [6, 11] and thus possibly underlie the negative results in clinical trials.

Of morbidities that potentially compromise the protective mechanisms of the heart, DM appears primarily important to study today. The number of patients with DM has been increasing worldwide in the past two decades, and these patients are predisposed to serious cardiovascular morbidity and mortality [12]. Despite recent progress in coronary intervention strategies, DM is associated with higher mortality after AMI due to more extensive atherosclerotic lesions and also hypertrophied and dysfunctional left ventricle [13‐15]. Interestingly, Lamblin et al. recently showed that rates of cardiovascular death and heart failure after the first AMI were higher in DM patients than in non-DM patients, though ventricular function and remodeling were comparable in the two groups [16]. The increased mortality after AMI in DM patients is unlikely to be prevented simply by improvement of glycemic control, since intensive glycemic controls failed to reduce cardiovascular events as well as mortality rate in DM patients in recent large-scale clinical trials [17‐19]. Hence, we need novel strategies for protecting DM patients with coronary artery diseases from excessive myocardial ischemia-reperfusion injury. To gain an insight into a novel strategy, we thoroughly review reported effects of DM on myocardial infarct size and on protective mechanisms against infarction and discuss future perspectives in this article. Our discussion on pathophysiology and clinical approach to diabetic cardiomyopathy was recently published elsewhere [20].

Clinical studies showed that DM increased the susceptibility of the myocardium to ischemia/reperfusion injury [21, 22]. However, results of infarct size studies in diabetic animal models were contradictory (Tables 1 and 2), and the reason for the inconsistency remains unclear.

Multiple mechanisms are involved in cardiomyocyte necrosis induced by ischemia/reperfusion. Depletion of intracellular ATP, intracellular overloading with Na⁺ and Ca²⁺, and increased fragility of the cell membrane are major changes during ischemia relevant to cell necrosis. Accumulating evidence supports the notion that the mitochondrial permeability transition pore (mPTP) is primed during ischemia and is opened by Ca²⁺ overload and burst of reactive oxygen species at the time of reperfusion, leading to irreversible loss of mitochondrial functions and cell necrosis [23]. Theoretically, modification of ischemia-induced changes or reperfusion-induced opening of the mPTP should underlie the change in myocardial susceptibility to infarction in DM hearts. However, such modification has not been understood well.

DM is one of the co-morbidities that potentially modify myocardial response to protective interventions, including PC and PostC. A number of studies have been carried out to determine the efficacy of ischemic PC and PostC for protecting the myocardium in animal models of DM (Tables 3 and 4) and diabetic patients (Table 5). The majority of those studies showed that DM interferes with protective mechanisms of cardioprotective interventions. Myocardial protection by PC and PostC is achieved by activation of multiple protective signaling pathways that appear to converge to inhibit mPTP opening upon reperfusion via phosphorylation of glycogen synthase kinase-3β (GSK-3β) at Ser9 [6, 8, 9], and DM-induced defects in the protective signaling may be different depending on the model and/or phase of DM.

Possible adverse effects of sulfonylureas on cardiac events in DM patients have been debated for decades, and recent studies have indicated that cardiovascular risk was significantly higher in DM patients treated with sulfonylureas than in those who received metformin monotherapy [93, 94]. Sulphonylureas have been shown to inhibit protection of PC and PostC by blocking the ATP-sensitive K⁺ channel (K_ATP channel) in cardiomyocytes, raising concern that such actions of sulphonylureas may be detrimental at the time of AMI in DM patients. However, the effects on PC and PostC are actually not uniform across sulfonylureas. Glibenclamide, a non-specific blocker of the K_ATP channel, abrogated the infarct size-limiting effect of PC and PostC in non-diabetic animals [6, 7, 95]. In contrast, glimepiride, a second-generation sulfonylurea, did not block the protective effect of PC in the animal myocardium or human myocardium [96, 97], presumably because this agent has few cardiac actions [98]. In addition, glimepiride was shown to have a PostC-mimetic action by activation of PI3K/Akt in the rabbit myocardium [99]. However, the effects of sulfonylureas on PC in diabetes have been tested in only a few studies [51, 96], since PC and PostC failed to limit infarction in most of the DM models. Interestingly, Nieszner et al. [51] reported that infarct-sparing effect of PC in alloxan-treated diabetic rabbits was restored by pretreatment with a low dose, but not a high dose, of glibenclamide. The mechanism by which glibenclamide restored myocardial response to PC in the DM rabbit is unclear. However, beneficial effects of glycemic control, possibly on PI3K/Akt signaling, might have overwhelmed the effect of partial inhibition of cardiac K_ATP channels when an appropriate dose of glibenclamide was selected.

Metformin is an insulin-sensitizing agent that reduces hepatic glucose output and increases the uptake of glucose in peripheral tissues, including the skeletal muscle. In non-diabetic animals, metformin reduced infarct size by activation of AMP-activated protein kinase and/or Akt [95]. However, whether metformin is additive to ischemic PC or PostC has not been examined. Infarct size-limiting effect of metformin in diabetic animals was controversial: there are two positive reports [40, 100] and one negative report [101].

Thiazolidinediones (pioglitazone, rosiglitazone) administered before ischemia reduced infarct size in both normal and diabetic animals [39, 41, 95]. However, the impact of the cardioprotective effect of thiazolidinediones on clinical outcome is unclear, since the effects of pioglitazone and rosiglitazone on risk of cardiovascular events appear discrepant [102].

In the past few years, growing evidence has demonstrated cardiovascular effects of incretin-based therapy using glucagon-like peptide-1 (GLP-1) receptor agonists and inhibitors of dipeptidyl peptidase-4 (DPP-4) [95, 103‐105]. However, the effects of GLP-1 receptor agonists or DPP-4 inhibitors on ischemia/reperfusion injury have not been examined in diabetic animals except for a study by Huisamen et al. [106] They reported that enlargement of infarct size in obese pre-diabetic rats was reduced by treatment with a DPP-4 inhibitor for 4 weeks [106]. Further investigation is necessary for clarifying the effects of GLP-1 analogues and DPP-4 inhibitors on intracellular protective signaling and its modification by DM, though there are a number of on-going large-scale clinical trials to test the effects of incretin-based therapy on cardiovascular outcomes [107].

DM potentially increases myocardial susceptibility to ischemia/reperfusion injury, though it cannot be explained by chronic hyperglycemia alone. DM also modifies myocardial responses to ischemic and pharmacological PC and PostC by disruption of intracellular signaling responsible for enhancement of resistance to cell death. These alterations in the diabetic heart appear to underlie the poor prognosis of DM patients after AMI. In addition, a defect of the protective mechanisms caused by aging, DM and possibly other co-morbidities is one of possible reasons why infarct size-limiting effects of agents in animal studies could not have been translated into AMI patients.

Analyses of modifications of cytoprotective signaling by DM, heart failure, dyslipidemia and hypertension generally showed that defects in the signaling by the pathological factors are upstream of a step of GSK-3β phosphorylation and mPTP regulation [6, 9]. Hence, a reasonable approach to protect the heart in DM patients is manipulation of protective mechanisms at the level of GSK-3β or mPTP. Blockade of mPTP opening by cyclosporine A at the time of reperfusion is one such approach, and its effect was first examined in 58 AMI patients [108]. Infarct size determined by MRI at 5 days after AMI was significantly limited by cyclosporine A and the beneficial effects persisted until 6 month after AMI [109]. The results are very promising and support the feasibility of a strategy for cardioprotection targeting the mPTP. However, how DM modifies the mPTP regulation and how we can suppress opening of the mPTP at the time of reperfusion need to be further investigated for devising a novel therapy for cardioprotection in DM patients.