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Preconditioning, postconditioning, and remote conditioning of the myocardium enhance the ability of the heart to withstand a prolonged ischemia/reperfusion insult and the potential to provide novel therapeutic paradigms for cardioprotection. While many signaling pathways leading to endogenous cardioprotection have been elucidated in experimental studies over the past 30 years, no cardioprotective drug is on the market yet for that indication. One likely major reason for this failure to translate cardioprotection into patient benefit is the lack of rigorous and systematic preclinical evaluation of promising cardioprotective therapies prior to their clinical evaluation, since ischemic heart disease in humans is a complex disorder caused by or associated with cardiovascular risk factors and comorbidities. These risk factors and comorbidities induce fundamental alterations in cellular signaling cascades that affect the development of ischemia/reperfusion injury and responses to cardioprotective interventions. Moreover, some of the medications used to treat these comorbidities may impact on cardioprotection by again modifying cellular signaling pathways. The aim of this article is to review the recent evidence that cardiovascular risk factors as well as comorbidities and their medications may modify the response to cardioprotective interventions. We emphasize the critical need for taking into account the presence of cardiovascular risk factors as well as comorbidities and their concomitant medications when designing preclinical studies for the identification and validation of cardioprotective drug targets and clinical studies. This will hopefully maximize the success rate of developing rational approaches to effective cardioprotective therapies for the majority of patients with multiple comorbidities. SIGNIFICANCE STATEMENT: Ischemic heart disease is a major cause of mortality; however, there are still no cardioprotective drugs on the market. Most studies on cardioprotection have been undertaken in animal models of ischemia/reperfusion in the absence of comorbidities; however, ischemic heart disease develops with other systemic disorders (e.g., hypertension, hyperlipidemia, diabetes, atherosclerosis). Here we focus on the preclinical and clinical evidence showing how these comorbidities and their routine medications affect ischemia/reperfusion injury and interfere with cardioprotective strategies., (Copyright © 2023 by The Author(s).)

Connexin 43 (Cx43) forms gap junction channels that are essential for the propagation of electrical depolarization in cardiomyocytes, but also with important roles in the pathophysiology of reperfusion injury. However, more recent studies have shown that Cx43 has also important functions independent from intercellular communication between adjacent cardiomyocytes. Some of these actions have been related to the presence of Cx43 in the mitochondria of these cells (mitoCx43). The functions of mitoCx43 have not been completely elucidated, but there is strong evidence indicating that mitoCx43 modulates mitochondrial respiration at respiratory complex I, production of radical oxygen species and ATP synthesis. These functions of mitoCx43 modulate mitochondrial and cellular tolerance to reperfusion after prolonged ischemia and are necessary for the cardioprotective effect of ischemic preconditioning. In the present review article we discuss available knowledge on these functions of mitoCx43 in relation to reperfusion injury, the molecular mechanisms involved and explore the possibility that mitoCx43 may constitute a new pharmacological target in patients with ST-segment elevation myocardial infarction (STEMI). This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve., (Copyright © 2017 Elsevier B.V. All rights reserved.)

To commemorate the auspicious occasion of the 30th anniversary of IPC, leading pioneers in the field of cardioprotection gathered in Barcelona in May 2016 to review and discuss the history of IPC, its evolution to IPost and RIC, myocardial reperfusion injury as a therapeutic target, and future targets and strategies for cardioprotection. This article provides an overview of the major topics discussed at this special meeting and underscores the huge importance and impact, the discovery of IPC has made in the field of cardiovascular research., Competing Interests: HEB is shareholder of CellAegis Inc. PF is a founder and CEO of Pharmahungary, a group of R&D companies. GH served as a consultant to Servier. MO was a consultant for Neurovive Pharmaceuticals. DGD served as consultant to Neurovive Pharmaceuticals. All other authors have no relevant disclosures.

Connexin 43 (Cx43) deficiency increases myocardial tolerance to ischemia-reperfusion injury and abolishes preconditioning protection. It is not known whether modifications in baseline signaling through protective RISK or SAFE pathways or in response to preconditioning may contribute to these effects. To answer this question we used Cx43(Cre-ER(T)/fl) mice, in which Cx43 expression is abolished after 4-hydroxytamoxifen (4-OHT) administration. Isolated hearts from Cx43(Cre-ER(T)/fl) mice, or from Cx43(fl/fl) controls, treated with vehicle or 4-OHT, were submitted to global ischemia (40 min) and reperfusion. Cx43 deficiency was associated with reduced infarct size after ischemia-reperfusion (11.17 ± 3.25 % vs. 65.04 ± 3.79, 59.31 ± 5.36 and 65.40 ± 4.91, in Cx43(fl/fl) animals treated with vehicle, Cx43(fl/fl) mice treated with 4-OHT, and Cx43(Cre-ER(T)/fl) mice treated with vehicle, respectively, n = 8-9, p < 0.001). However, the ratio phosphorylated/total protein expression for Akt, ERK-1/2, GSK3β and STAT3 was not increased in normoxic samples from animals lacking Cx43. Instead, a reduction in the phosphorylation state of GSK3β was observed in Cx43-deficient mice (ratio: 0.15 ± 0.02 vs. 0.56 ± 0.11, 0.77 ± 0.15, and 0.46 ± 0.14, respectively, n = 5-6, p < 0.01). Furthermore, ischemic preconditioning (IPC, 4 cycles of 3.5 min of ischemia and 5 min of reperfusion) increased phosphorylation of ERK-1/2, GSK3β, and STAT3 in all hearts without differences between groups (n = 5-6, p < 0.05), although Cx43 deficient mice were not protected by either IPC or pharmacological preconditioning with diazoxide. Our data demonstrate that modification of RISK and SAFE signaling does not contribute to the role of Cx43 in the increased tolerance to myocardial ischemia-reperfusion injury and in preconditioning protection.

Connexin 43 (Cx43) plays an important role in cardioprotective signalling by mechanisms at least in part independent of gap junctional communication. To investigate whether this role is related to specific properties of this connexin isoform, we used a knock-in mouse model in which the coding region of Cx43 is replaced by that of Cx32. Homozygous Cx43KI32 mice showed reduced cell-to-cell Lucifer Yellow transfer (P < 0.01), but QRS duration and left ventricular fractional shortening (echocardiography) were similar to those in wild-type animals. NMR spectroscopy detected reduced ATP and increased lactate content in myocardium from homozygous Cx43KI32 animals (P < 0.05). Despite this, isolated homozygous Cx43KI32 hearts showed smaller infarcts after ischaemia-reperfusion (40 min/60 min) as compared to hearts from heterozygous and wild-type animals (13 and 31% reduction, respectively, P < 0.05). Cardiac myocytes isolated from Cx43KI32 mouse hearts also showed a reduced rate of cell death after simulated ischaemia-reperfusion. In a separate series of experiments, both ischaemic (4 cycles of 3.5 min of ischaemia and 5 min of reperfusion) and pharmacological (50 micromol l(-1) diazoxide, 10 min) preconditioning reduced infarct size in hearts from wild-type mice (by 24.84 and 26.63%, respectively, P < 0.05), but only ischaemic preconditioning was effective in hearts from heterozygous animals and both preconditioning strategies failed to protect Cx43KI32 homozygous hearts. These results demonstrate that Cx43 has an important and previously unknown modulatory effect in myocardial energy metabolism and tolerance to ischaemia, and plays a critical role in preconditioning protection, by mechanisms that are specific for this connexin isoform.

Preserved mitochondrial function (respiration, calcium handling) and integrity (cytochrome c release) is central for cell survival following ischemia/reperfusion. Mitochondrial function also requires import of proteins from the cytosol via the translocase of the outer and inner membrane (TOM and TIM complexes). Since mitochondrial function following ischemia/reperfusion is better preserved by ischemic preconditioning (IP), we now investigated whether expression of parts of the import machinery is affected by ischemia/reperfusion without or with IP in vivo. We analyzed the mitochondrial content of the presequence receptor Tom20, the pore forming unit Tom40 and Tim23. Goettinger minipigs were subjected to 90 min of low-flow ischemia without or with preconditioning by 10 min ischemia and 15 min reperfusion. Mitochondria were isolated from the ischemic or preconditioned anterior wall of the left ventricle and from the control posterior wall. Infarct size was significantly reduced by IP (20.1 +/- 1.6% of area at risk (non-preconditioned) vs. 6.5 +/- 2.5% of area at risk (IP)). Using Western blot analysis, the ratio of Tom20 (normalized to Ponceau S) between mitochondria isolated from the anterior ischemic and posterior control wall was reduced (0.72 +/- 0.11, a.u., n = 8), whereas the mitochondrial Tom20 content was preserved by IP (1.17 +/- 0.16 a.u., n = 7, P < 0.05). The mitochondrial Tom40, Tim23 and adenine nucleotide transporter (ANT) contents were not significantly different between non-preconditioned and preconditioned myocardium. The preservation of the mitochondrial Tom20 protein level may contribute to the improved mitochondrial function after IP.

Our understanding of the end-effectors involved in preconditioning protection is still very limited. This is partially due to an incomplete knowledge of the mechanisms responsible for acute sarcolemmal rupture and cell death during the first minutes of reperfusion, including the relative roles of hypercontracture-mediated sarcolemmal rupture and mitochondrial permeability transition pore (MPTP) opening-mediated cell death. In the present article, the role of proposed end-effectors of preconditioning protection, defined as molecules directly involved in cell death that are modified by ischemic preconditioning (IP), is examined. IP attenuates hypercontracture-mediated cell death, probably through several mechanisms, including attenuated calpain activation during reperfusion leading to preserved cytoskeletal integrity and accelerated recovery of Na+/K+-ATPase function, but probably also protein kinase G (PKG)-mediated improved calcium handling. The potential role of gap junctions in preconditioning protection is controversial, but the recently discovered mitochondrial localisation of connexin43 seems to play an important role in protection that has not yet been completely defined. Several recent studies suggest that IP can reduce MPTP opening during reperfusion and limit infarct size through this mechanism, although the contribution of this widely accepted mechanism to the infarct size reduction induced by IP in the intact heart needs to be established.

Objective: Connexin 43 (Cx43) is involved in infarct size reduction by ischemic preconditioning (IP); the underlying mechanism of protection, however, is unknown. Since mitochondria have been proposed to be involved in IP's protection, the present study analyzed whether Cx43 is localized at mitochondria of cardiomyocytes and whether such localization is affected by IP., Methods and Results: Western blot analysis on mitochondrial preparations isolated from rat, mouse, pig, and human hearts showed the presence of Cx43. The preparations were not contaminated with markers for other cell compartments. The localization of Cx43 to mitochondria was also confirmed by FACS sorting (double staining with MitoTracker Red and Cx43) and immuno-electron and confocal microscopy. To study the role of Cx43 in IP, mitochondria were isolated from the ischemic anterior wall (AW) and the control posterior wall (PW) of pig myocardium at the end of 90 min low-flow ischemia without (n=13) or with (n=13) a preceding preconditioning cycle of 10 min ischemia and 15 min reperfusion. With IP, the mitochondrial Cx43/adenine nucleotide transporter ratio was 3.4+/-0.7 fold greater in AW than in PW, whereas the ratio remained unchanged in non-preconditioned myocardium (1.1+/-0.2, p<0.05). The enhancement of the mitochondrial Cx43 protein level occurred rapidly, since an increase of mitochondrial Cx43 was already detected with two cycles of 5 min ischemia/reperfusion in isolated rat hearts to 262+/-63% of baseline., Conclusion: These data demonstrate that Cx43 is localized at cardiomyocyte mitochondria and that IP enhances such mitochondrial localization.

Objectives: It has been shown that sarcolemmal rupture can occur during reenergization in cardiomyocytes in which previous ischemia has induced sarcolemmal fragility by calpain-dependent hydrolysis of structural proteins. We tested the hypothesis that attenuated calpain activation contributes to the protection against reperfusion-induced cell death afforded by ischemic preconditioning (IPC), and investigated the involvement of protein kinase A (PKA) in this effect., Methods: Calpain activity and degradation of different structural proteins were studied along with the extent of necrosis in isolated rat hearts submitted to 60 min of ischemia and 30 min of reperfusion with or without previous IPC (two cycles of 5 min ischemia-5 min reperfusion), and the ability of different treatments to mimic or blunt the effects of IPC were analyzed., Results: IPC accelerated ATP depletion and rigor onset during ischemia but reduced LDH release during reperfusion by 69% (P<0.001). At the end off reperfusion, calpain activity was reduced by 66% (P<0.001) in IPC, and calpain-dependent degradation of sarcolemmal proteins was attenuated. Addition of the calpain inhibitor MDL-28170 mimicked the effects of IPC on protein degradation and reduced LDH release by 48% (P<0.001). The effects of IPC on calpain, alpha-fodrin, and LDH release were blunted by the application of the PKA inhibitor H89 or alprenolol during IPC, while transient stimulation of PKA with CPT-cAMP or isoproterenol before ischemia attenuated calpain activation, alpha-fodrin degradation, and markedly reduced LDH release (P<0.001). In hearts exposed to Na(+)-free perfusion, IPC attenuated calpain activation by 67% (P<0.001) and reduced by 56% (P<0.001) LDH release associated to massive edema occurring during Na(+) readmission without modifying its magnitude., Conclusion: These results are consistent with PKA-dependent attenuation of calpain-mediated degradation of structural proteins being an end-effector mechanism of the protection afforded by IPC.

The end-effectors of ischemic preconditioning (IPC) are not well known. It has been recently shown that transgenic mice underexpressing the gap junction protein connexin43 (Cx43) cannot be preconditioned. Because gap junctions allow spreading of cell death during ischemia-reperfusion in different tissues, including myocardium, we hypothesized that the protection afforded by IPC is mediated by effects on gap junction-mediated intercellular communication. To test this hypothesis, we analyzed the effect of IPC (5 min ischemia-5 min reperfusion x 2) on the changes in electrical impedance (four electrode probe) and impulse propagation velocity (transmembrane action potential) induced by ischemia (60 min) and reperfusion (60 min) in isolated rat hearts. IPC (n = 8) reduced reperfusion-induced lactate dehydrogenase release by 65.8% with respect to control hearts (n = 9) (P = 0.04) but had no effect on the time of onset of rigor contracture (increase in diastolic tension), electrical uncoupling (sharp changes in tissue resistivity and phase angle in impedance recordings), or block of impulse propagation during ischemia. Normalization of electrical impedance during reperfusion was also unaffected by IPC. The lack of effect of IPC on ischemic rigor contracture and on changes in tissue impedance during ischemia-reperfusion were validated under in vivo conditions in pigs submitted to 48 min of coronary occlusion and 120 min of reperfusion. IPC (n = 12) reduced infarct size (triphenyltetrazolium) by 64.9% (P = 0.01) with respect to controls (n = 17). We conclude that the protection afforded by IPC is not mediated by effects on electrical coupling. This result is consistent with recent findings suggesting that Cx43 could have effects on cell survival independent on changes in cell-to-cell communication.

Gap junction-mediated communication can modulate cell death in different tissues. In myocardium, gap junction communication is altered during ischemia, which contributes to the development of arrhythmias, but still allows synchronization of the onset of rigor contracture in the progression of injury. During reperfusion, gap junction communication allows cell-to-cell spread of hypercontracture and cell death. Since the intracellular signal transduction systems involved in modulation of gap junction-mediated communication are activated during ischemic preconditioning, the hypothesis can be raised that gap junctions are end-effectors of preconditioning contributing to its protective effect on cell death. This paper reviews the available information supporting this hypothesis. It has been shown that ischemic preconditioning may influence gap junction-mediated intercellular communication by activation of different kinases, including PKC and MAPK cascades, and by preservation of cGMP among other mechanisms. Connexin phosphorylation by PKC, p38/MAPK, and PKG, tends to reduce intercellular communication. This effect of ischemic preconditioning seems to have no relevant consequences during prolonged ischemia, and does not significantly modify the time course of either electrical uncoupling or the frequency or temporal distribution of ventricular arrhythmias during this period. However, any modification of gap junction communication during initial reperfusion could contribute to the reduced extent of hypercontracture and cell death observed in preconditioned hearts. The potential role of gap junctions as effectors of ischemic preconditioning against lethal injury secondary to ischemia-reperfusion deserves to be investigated in depth.