Experimental studies

Infarct size in animal models of DM was larger or smaller than or similar to, depending on reports, that in non-diabetic controls (Table 1). As shown in Table 1, there were numerous differences in experimental preparations and protocols, and a single factor cannot entirely explain the discrepancy in the effects of DM on infarct size. Duration of the diabetic state and plasma level of insulin (i.e., type 1 DM vs. type 2 DM) appear to influence myocardial tolerance against infarction. In a study by Ravingerová et al. [24], infarct size after 30-min ischemia was smaller in diabetic rat hearts at 1 week after streptozotocin (STZ) injection than in controls, but this infarct size limitation was not detected 8 weeks later. Two other studies have also shown that resistance of diabetic hearts to ischemia/reperfusion injury was detectable at the early phase of DM and disappeared later [25, 26]. However, enlargement of infarct size was observed as early as 8 days after STZ injection [27–29], indicating that duration of DM is not the only factor responsible for inconsistent effects of DM on infarct size.

Difference in metabolic profiles associated with DM (i.e., presence or absence of hyperinsulinemia and types of dyslipidemia) may also be responsible for different changes in infarct size in DM animals. With a few exceptions [30, 31], DM models with obesity and hyperinsulinemia showed increased myocardial susceptibility to ischemia/reperfusion-induced necrosis [32–41]. In contrast, high glucose level induced simply by glucose or dextrose infusion did not affect infarct size in the majority of studies, including two recent studies [42, 43] (Table 2).

Clinical studies

Increased myocardial susceptibility to infarction in DM patients was indicated by two clinical studies: infarct size determined by SPECT imaging after reperfusion therapy was larger by 30 ~ 70% in DM patients than in non-DM patients [21, 22]. The change in myocardial tolerance to infarction is consistent with worse short- and long-term prognosis after AMI in DM patients [44, 45]. Of particular relevance, Haffner et al. [46] showed that cardiovascular event risk in DM patients without prior myocardial infarction was comparable to the risk in non-diabetic patients with prior myocardial infarction. However, the untoward effects of DM on prognosis after AMI cannot be explained solely by increased myocardial vulnerability to ischemia/reperfusion injury, since the risk of heart failure and death was significantly higher in DM patients than in non-DM patients with comparable infarct sizes and left ventricular ejection fractions [47]. Myopathic changes (“diabetic cardiomyopathy”), impaired collateral recruitment, microcirculatory abnormalities, and underuse of evidence-based therapies are possible explanations, in addition to increased susceptibility to infarction, for detrimental prognosis after AMI in DM patients.

Experimental studies

With a few exceptions, previous studies demonstrated that cardioprotection achieved by ischemic PC or PostC is impaired in DM: the infarct size-limiting effect was lost or required extra cycles of ischemia/reperfusion in experimental diabetes (Table 3). Tsang et al. [48] reported that one, two and three cycles of ischemic PC significantly reduced infarct size in normal rats; however, three cycles of ischemic PC were required to limit infarct size in diabetic Goto-Kakizaki rats. Their findings suggested that the threshold for PC protection is increased in the diabetic myocardium. Liu et al. [49] were the first to examine the effects of DM on PC and they reported preservation of PC protection in STZ-induced DM. However, since they used a PC protocol with multiple cycles of ischemia/reperfusion only, the change in the threshold for inducing PC protection might have been missed in their study. In fact, impairment of PC by DM has been observed in multiple species (rats, rabbits and dogs) and different models of DM (STZ-induced type 1 DM and genetic models of type 2 DM) [30, 37, 50–54]. Mimetics of ischemic PC and PostC (diazoxide, erythropoietin, [D-Ala², D-Leu⁵-enkephalin acetate [DADLE] and isoflurane) were also ineffective for limitation of infarct size in DM hearts [33, 34, 37, 55–59], confirming that DM impairs intracellular signaling mechanisms relevant to myocardial protection.

Several steps in cytoprotective signaling have been found to be disrupted by DM in the heart. Blocked or attenuated signaling via phosphoinsitide-3 kinase (PI3K)-Akt, extracellular signal-regulated kinase (ERK) and/or signal transducer and activator of transcription-3 (STAT3) pathways was detected in association with loss or reduced efficiency of PC in protection against infarction [31, 33, 34, 36, 48, 57–59]. Our recent studies have shown that phosphorylation of Jak2, being upstream of PI3K-Akt signaling, is inhibited by enhanced calcineurin activity and that phosphorylation of GSK-3β by ERK is lost by an endoplasmic reticulum stress-dependent mechanism in OLETF, a rat model of obese type 2 DM [33, 34]. Furthermore, protein level of active GSK-3β, a pro-necrotic and pro-apoptotic kinase, was increased in mitochondria, leading to reduction in the threshold for mPTP opening in response to Ca²⁺ overload [34]. On the other hand, a protective mechanism downstream of GSK-3β phosphorylation appears to be intact in diabetic hearts, since direct pharmacological inhibitors of GSK-3β limited infarct size in diabetic hearts as in non-diabetic animals [33, 34, 54, 57, 58].

An important and clinically relevant issue is whether hyperglycemia per se is responsible for disruption of protective signaling in PC and PostC. Acute hyperglycemia induced by dextrose infusion attenuated infarct size limitation by ischemic PC, a mitochondrial ATP-sensitive K⁺ channel (K_ATP channel) opener and anesthetic agents (Table 4). These results indicate a primary role of hyperglycemia in impairment of protective signaling. Interestingly, Przyklenk et al. [31] reported that the cardioprotective effect of ischemic PostC was re-established in STZ-induced diabetic mice by pancreas islet cell transplantation. Transplantation of islet cells in diabetic mice normalized both blood glucose level and free fatty acid level and cellular signaling, such as ERK phosphorylation, activated by ischemic PostC. Since dyslipidemia was also shown to attenuate the infarct size-limiting effect of ischemic PC [60, 61], restoration of myocardial response to PostC in the diabetic heart by islet cell transplantation might be achieved by normalization of both plasma glucose level and lipid profile. Nevertheless, circumstantial evidence in animal experiments supports the notion that normalization of the metabolic profile restores protective signaling mechanisms in the DM heart.

Clinical studies on PC

PC protection in human hearts has been demonstrated by results of in vitro experiments using human ventricular myocytes [62] and atrial trabeculae [63] and by detailed analyses of myocardial responses in patients with naturally occurring ischemic syndromes [64, 65]. Since ischemic PC and PC-like phenomena in human hearts have been recently reviewed elsewhere [7, 65], we briefly summarize the results that are relevant to the effects of DM on ischemic PC. As for PC-like effects of preinfarct angina, different groups reported that pre-infarct angina was associated with smaller infarct size determined by creatine kinase (CK) release, improved left ventricular function and reduced mortality [64, 66]. Furthermore, in overall analysis of AMI patients, angina before AMI was associated with better long-term prognosis [67, 68]. However, in patients with DM, beneficial effects of pre-infarct angina were not detected [69]. Ishihara et al. [69] showed that CK release and recovery of cardiac function and in-hospital mortality after AMI were similar in DM patients with and without pre-infarct angina. DM-induced impairment of ischemic PC protection in human hearts has also been indicated by studies in which myocardial injury was assessed during angioplasty [70] and during a treadmill exercise test [71] by use of electrocardiography.

Direct evidence for diabetes-induced loss of ischemic PC protection in the human myocardium was provided by an in vitro experiment using atrial trabeculae obtained at open heart surgery. CK release and contractile dysfunction after hypoxia/reoxygenation in vitro was significantly suppressed by PC in trabeculae from non-diabetic patients but not in trabeculae from diabetic patients [63, 72].

Clinical studies on PostC

In contrast to PC, PostC or administration of its mimetic at the time of reperfusion is clinically feasible. In 2005, Staat et al. [73] translated for the first time the concept of ischemic PostC into the clinical setting. PostC with four cycles of 1-min inflation and 1-min deflation before full reperfusion successfully reduced CK release in a small and selected cohort of patients with AMI. Efficacy of ischemic PostC in patients with AMI was subsequently confirmed in a number of small-scale studies, using as endpoints cardiac biomarkers, ECG ST-segment resolution, and anatomic infarct size (Table 5). However, three recent studies failed to show protection by ischemic PostC in terms of CK and troponin release, LV function or infarct size that was determined by delayed enhancement magnetic resonance imaging (MRI) [74–76]. The reasons for the discrepant results remain uncertain, but there are some possibilities: differences in the protocols of ischemic PostC, presence of co-morbidities and use of different pharmacological agents for the co-morbidities.

In the PostC protocol, duration from reperfusion to the first re-occlusion and number of re-occlusions have been shown to influence the infarct size-limiting effects of PostC in animal models of AMI [77–79]. Retrospective analyses by Darling et al. [80] and Wang et al. [81] suggest that larger numbers of balloon inflations–deflations were associated with greater protection in patients with AMI. However, an appropriate algorithm of PostC stimulus to maximally protect the human myocardium has not been established. In addition, protection by PostC in animal models of AMI is attenuated with aging and presence of co-morbidities, such as DM, hypertension and hypercholesterolemia [6, 11]. Although characteristics of patients, such as age and prevalence of DM, seem to be similar in the human studies (Table 5), difference in durations of the co-morbidities might have been involved in the different outcomes. Nevertheless, the effect of DM on protection afforded by PostC has not been specifically examined in clinical studies.

The effects of pharmacological PostC (i.e., administration of pharmacological agents at reperfusion) on myocardial injury have been examined in a number of clinical trials. Results of the trials are a mixture of positive and negative results, and the reason for the discrepancy remains unknown [82–92]. Adenosine infusion limited infarct size, but neither the death rate during 6-month follow-up nor the rate of re-hospitalization for heart failure were reduced by the adenosine treatment [82, 84]. A post hoc analysis suggested that only patients with early reperfusion therapy received clinical benefits of adenosine, such as a reduction in death or heart failure [85]. In the J-WIND trial [86], the effects of nicorandil and ANP on infarct size in different study arms were examined. Administration of nicorandil failed to significantly limit infarct size, though recombinant ANP infusion was shown to induce a small but significant reduction in infarct size. The results of studies on the effect of erythropoietin administered at reperfusion were discrepant [87–92]. Taken together, there are a few promising agent for infarct size limitation in AMI patients, but none of their effects has been unequivocally demonstrated to be sufficient for improving prognosis.