Myocardial triggers involved in activation of remote ischaemic preconditioning

Is There a Mitochondrial Protection via Remote Ischemic Conditioning in Settings of Anticancer Therapy Cardiotoxicity?

PURPOSE OF REVIEW

To provide an overview of (a) protective effects on mitochondria induced by remote ischemic conditioning (RIC) and (b) mitochondrial damage caused by anticancer therapy. We then discuss the available results of studies on mitochondrial protection via RIC in anticancer therapy-induced cardiotoxicity.

RECENT FINDINGS

In three experimental studies in healthy mice and pigs, there was a RIC-mediated protection against anthracycline-induced cardiotoxicity and there was some evidence of improved mitochondrial function with RIC. The RIC-mediated protection was not confirmed in the two available studies in cancer patients. In adult cancer patients, RIC was associated with an adverse outcome. There are no data on mitochondrial function in cancer patients. Studies in tumor-bearing animals are needed to determine whether RIC does not interfere with the anticancer properties of the drugs and whether RIC actually improves mitochondrial function, ultimately resulting in improved cardiac function.

Is Intrinsic Cardioprotection a Laboratory Phenomenon or a Clinically Relevant Tool to Salvage the Failing Heart?

Cardiovascular diseases, especially ischemic heart disease, as a leading cause of heart failure (HF) and mortality, will not reduce over the coming decades despite the progress in pharmacotherapy, interventional cardiology, and surgery. Although patients surviving acute myocardial infarction live longer, alteration of heart function will later lead to HF. Its rising incidence represents a danger, especially among the elderly, with data showing more unfavorable results among females than among males. Experiments revealed an infarct-sparing effect of ischemic "preconditioning" (IPC) as the most robust form of innate cardioprotection based on the heart's adaptation to moderate stress, increasing its resistance to severe insults. However, translation to clinical practice is limited by technical requirements and limited time. Novel forms of adaptive interventions, such as "remote" IPC, have already been applied in patients, albeit with different effectiveness. Cardiac ischemic tolerance can also be increased by other noninvasive approaches, such as adaptation to hypoxia- or exercise-induced preconditioning. Although their molecular mechanisms are not yet fully understood, some noninvasive modalities appear to be promising novel strategies for fighting HF through targeting its numerous mechanisms. In this review, we will discuss the molecular mechanisms of heart injury and repair, as well as interventions that have potential to be used in the treatment of patients.

Remote ischemic preconditioning prevents sarcolemmal-associated proteolysis by MMP-2 inhibition

The death of myocytes occurs through different pathways, but the rupture of the plasma membrane is the key point in the transition from reversible to irreversible injury. In the myocytes, three major groups of structural proteins that link the extracellular and intracellular milieus and confer structural stability to the cell membrane: the dystrophin-associated protein complex, the vinculin-integrin link, and the spectrin-based submembranous cytoskeleton. The objective was to determine if remote ischemic preconditioning (rIPC) preserves membrane-associated cytoskeletal proteins (dystrophin and β-dystroglycan) through the inhibition of metalloproteinase type 2 (MMP-2) activity. A second objective was to describe some of the intracellular signals of the rIPC, that modify mitochondrial function at the early reperfusion. Isolated rat hearts were subjected to 30 min of global ischemia and 120 min of reperfusion (I/R). rIPC was performed by 3 cycles of ischemia/reperfusion in the lower limb (rIPC). rIPC significantly decreased the infarct size, induced Akt/GSK-3 β phosphorylation and inhibition of the MPTP opening. rIPC improved mitochondrial function, increasing membrane potential, ATP production and respiratory control. I/R increased ONOO- production, which activates MMP-2. This enzyme degrades β-dystroglycan and dystrophin and collaborates to sarcolemmal disruption. rIPC attenuates the breakdown of β-dystroglycan and dystrophin through the inhibition of MMP-2 activity. Furthermore, we confirm that rIPC activates different intracellular pathway that involves the an Akt/Gsk3β and MPTP pore with preservation of mitochondrial function.

Age-Dependent Effects of Remote Preconditioning in Hypertensive Rat Hearts are Associated With Activation of RISK Signaling

Remote ischemic preconditioning (RIPC) represents one of the forms of innate cardioprotection. While being effective in animal models, its application in humans has not been always beneficial, which might be attributed to the presence of various comorbidities, such as hypertension, or being related to the confounding factors, such as patients' sex and age. RIPC has been shown to mediate its cardioprotective effects through the activation of Reperfusion Injury Salvage Kinase (RISK) pathway in healthy animals, however, scarce evidence supports this effect of RIPC in the hearts of spontaneously hypertensive (SHR) rats, in particular, in relationship with aging. The study aimed to investigate the effectiveness of RIPC in male SHR rats of different age and to evaluate the role of RISK pathway in the effect of RIPC on cardiac ischemic tolerance. RIPC was performed using three cycles of inflation/deflation of the pressure cuff placed on the hind limb of anesthetized rats aged three, five and eight months. Subsequently, hearts were excised, Langendorff-perfused and exposed to 30-min global ischemia and 2-h reperfusion. Infarct-sparing and antiarrhythmic effects of RIPC were observed only in three and five months-old animals but not in eight months-old rats. Beneficial effects of RIPC were associated with increased activity of RISK and decreased apoptotic signaling only in three and five months-old animals. In conclusion, RIPC showed cardioprotective effects in SHR rats that were partially age-dependent and might be attributed to the differences in the activation of RISK pathway and various aspects of ischemia/reperfusion injury in aging animals.

Effects of Remote Ischaemic Preconditioning on the Internal Thoracic Artery Nitric Oxide Synthase Isoforms in Patients Undergoing Coronary Artery Bypass Grafting

Remote ischaemic preconditioning (RIPC) is a medical procedure that consists of repeated brief periods of transient ischaemia and reperfusion of distant organs (limbs) with the ability to provide internal organ protection from ischaemia. Even though RIPC has been successfully applied in patients with myocardial infarction during coronary revascularization (surgery/percutaneous angioplasty), the underlying molecular mechanisms are yet to be clarified. Thus, our study aimed to determine the role of nitric oxide synthase (NOS) isoforms in RIPC-induced protection (3 × 5 min of forearm ischaemia with 5 min of reperfusion) of arterial graft in patients undergoing urgent coronary artery bypass grafting (CABG). We examined RIPC effects on specific expression and immunolocalization of three NOS isoforms — endothelial (eNOS), inducible (iNOS) and neuronal (nNOS) in patients’ internal thoracic artery (ITA) used as a graft. We found that the application of RIPC protocol leads to an increased protein expression of eNOS, which was further confirmed with strong eNOS immunopositivity, especially in the endothelium and smooth muscle cells of ITA. The same analysis of two other NOS isoforms, iNOS and nNOS, showed no significant differences between patients undergoing CABG with or without RIPC. Our results demonstrate RIPC-induced upregulation of eNOS in human ITA, pointing to its significance in achieving protective phenotype on a systemic level with important implications for graft patency.

Impact of Maturation on Myocardial Response to Ischemia and the Effectiveness of Remote Preconditioning in Male Rats

Aging attenuates cardiac tolerance to ischemia/reperfusion (I/R) associated with defects in protective cell signaling, however, the onset of this phenotype has not been completely investigated. This study aimed to compare changes in response to I/R and the effects of remote ischemic preconditioning (RIPC) in the hearts of younger adult (3 months) and mature adult (6 months) male Wistar rats, with changes in selected proteins of protective signaling. Langendorff-perfused hearts were exposed to 30 min I/120 min R without or with prior three cycles of RIPC (pressure cuff inflation/deflation on the hind limb). Infarct size (IS), incidence of ventricular arrhythmias and recovery of contractile function (LVDP) served as the end points. In both age groups, left ventricular tissue samples were collected prior to ischemia (baseline) and after I/R, in non-RIPC controls and in RIPC groups to detect selected pro-survival proteins (Western blot). Maturation did not affect post-ischemic recovery of heart function (Left Ventricular Developed Pressure, LVDP), however, it increased IS and arrhythmogenesis accompanied by decreased levels and activity of several pro-survival proteins and by higher levels of pro-apoptotic proteins in the hearts of elder animals. RIPC reduced the occurrence of reperfusion-induced ventricular arrhythmias, IS and contractile dysfunction in younger animals, and this was preserved in the mature adults. RIPC did not increase phosphorylated protein kinase B (p-Akt)/total Akt ratio, endothelial nitric oxide synthase (eNOS) and protein kinase Cε (PKCε) prior to ischemia but only after I/R, while phosphorylated glycogen synthase kinase-3β (GSK3β) was increased (inactivated) before and after ischemia in both age groups coupled with decreased levels of pro-apoptotic markers. We assume that resistance of rat heart to I/R injury starts to already decline during maturation, and that RIPC may represent a clinically relevant cardioprotective intervention in the elder population.

Aging-Related Increase of cGMP Disrupts Mitochondrial Homeostasis in Leydig Cells

Since mitochondria play an essential role in the testosterone biosynthesis, serve as power centers and are a source of oxidative stress, a possible mitochondrial dysfunction could be connected with decreased activity of Leydig cells and lowered testosterone production during aging. Here we chronologically analyzed age-related alterations of mitochondrial function in Leydig cells correlated by the progressive rise of cGMP signaling and with respect to testosterone synthesis. To target cGMP signaling in Leydig cells, acute or long-term in vivo or ex vivo treatments with sildenafil (phosphodiesterase 5 [PDE5] inhibitor) were performed. Aging-related accumulation of cGMP in the Leydig cells is associated with mitochondrial dysfunction illustrated by reduced ATP and steroid production, lowered O2 consumption, increased mitochondrial abundance and mtDNA copies number, decreased expression of genes that regulate mitochondrial biogenesis (Ppargc1a/PGC1a-Tfam-Nrf1/NRF1), mitophagy (Pink1), fusion (Mfn1, Opa1), and increased Nrf2/NRF2. Acute in vivo PDE5 inhibition overaccumulated cGMP and stimulated testosterone but reduced ATP production in Leydig cells from adult, middle-aged, and old rats. The increased ATP/O ratio observed in cells from old compared to adult rats was diminished after stimulation of cGMP signaling. Opposite, long-term PDE5 inhibition decreased cGMP signaling and improved mitochondrial function/dynamics in Leydig cells from old rats. Mitochondrial abundance in Leydig cells decreased while ATP levels increased. Chronic treatment elevated Tfam, Nrf1, Nrf2, Opa1, Mfn1, Drp1, and normalized Pink1 expression. Altogether, long-term PDE5 inhibition prevented age-related NO and cGMP elevation, improved mitochondrial dynamics/function, and testosterone production. The results pointed on cGMP signaling in Leydig cells as a target for pharmacological manipulation of aging-associated changes in mitochondrial function and testosterone production.

Myocardial remote ischemic preconditioning: from cell biology to clinical application

Remote ischemic preconditioning (rIPC) is a cardioprotective phenomenon where brief periods of ischemia followed by reperfusion of one organ/tissue can confer subsequent protection against ischemia/reperfusion injury in other organs, such as the heart. It involves activation of humoral, neural or systemic communication pathways inducing different intracellular signals in the heart. The main purpose of this review is to summarize the possible mechanisms involved in the rIPC cardioprotection, and to describe recent clinical trials to establish the efficacy of these strategies in cardioprotection from lethal ischemia/reperfusion injury. In this sense, certain factors weaken the subcellular mechanisms of rIPC in patients, such as age, comorbidities, medication, and anesthetic protocol, which could explain the heterogeneity of results in some clinical trials. For these reasons, further studies, carefully designed, are necessary to develop a clearer understanding of the pathways and mechanism of early and late rIPC. An understanding of the pathways is important for translation to patients.

The disappearance of IPO in myocardium of diabetes mellitus rats is associated with the increase of succinate dehydrogenase-flavin protein

BACKGROUND

The aim of the present study was to investigate whether the disappearance of ischemic post-processing (IPO) in the myocardium of diabetes mellitus (DM) is associated with the increase of succinate dehydrogenase-flavin protein (SDHA).

METHODS

A total of 50 Sprague Dawley rats, weighing 300-400 g, were divided into 5 groups according to the random number table method, each with 10 rats. After DM rats were fed a high-fat and -sugar diet for 4 weeks, they were injected with Streptozotocin to establish the diabetic rat model. Normal rats were fed the same regular diet for the same number of weeks. Next, the above rats were taken to establish a cardiopulmonary bypass (CPB) model. Intraperitoneal glucose tolerance test (IPGTT) and oral glucose tolerance test (OGTT) were used to detect whether the DM rat model was established successfully. Taking blood from the femoral artery to collect the blood-gas analysis indicators, and judged whether the CPB model is established. After perfusion was performed according to the experimental strategy, the area of myocardial infarction (MI), and serum creatine kinase isoenzyme (CK-MB) and cardiac troponin (CTnI) levels were measured. Finally, the relative mRNA and protein expression of SDHA was detected.

RESULTS

The OGTT and IPGTT suggested that the DM rat model was successfully established. The arterial blood gas analysis indicated that the CPB model was successfully established. As compared with the N group, the heart function of the IR group was significantly reduced, the levels of myocardial enzyme markers, the area of MI, as well as the relative mRNA and protein expression of SDHA, were all increased. As compared with the IR group, the CK-MB and CTnI levels in the IPO group, the MI area, relative mRNA and protein expression of SDHA decreased. As compared with the IPO group, the myocardial enzyme content in the DM + IPO group, the MI area and the relative mRNA and protein expression of SDHA increased. As compared with the DM + IPO group, in the DM + IPO + dme group, the myocardial enzyme content, area of MI and relative mRNA and protein expression were all decreased.

CONCLUSION

IPO can inhibit the expression of SDHA, reduce MIRI and exert a cardioprotective effect in the normal rats. However, the protective effect of IPO disappears in the diabetic rats. The inhibitor dme combined with IPO can increase the expression of SDHA and restore the protective effect of IPO in DM myocardia.

Remote ischemic preconditioning does not induce activation of Akt and STAT5 in the rat heart

Remote ischemic preconditioning (RIPC) is hypothesized to be a promising cardioprotective strategy to protect hearts against ischemia and reperfusion (I/R) injury; however, the current understanding of the underlying signal transduction pathways involved remains unclear. It has been previously demonstrated that protein kinase B/AKT, which is a crucial protein of the reperfusion injury salvage kinases pathway, and STAT5, which is a member of the survivor activating factor enhancement pathway, serve a pivotal role in cardioprotection. However, whether and at what time-points (TPs) RIPC leads to the activation of AKT and STAT5 in a rat model of RIPC and I/R injury remains to be determined. The present study hypothesized that RIPC may induce the phosphorylation of AKT and/or STAT5 immediately following RIPC and/or at a later TP with or without subsequent I/R. In the first set of experiments (part A), male Wistar rats were randomized into 2 groups (n=6 per group): The first group underwent RIPC via a hind limb tourniquet (4x5 min I/R episodes), while the second group received the respective sham treatment. In the second set of experiments (part B), the rats were randomized into 4 groups (n=6 per group) that either underwent RIPC or sham treatment prior to 35 min of ischemia by occlusion of the left anterior descending coronary artery followed by 120 min reperfusion or a respective sham treatment. At the end of the experiments, the heart tissue was isolated in order to analyze the phosphorylation levels of AKT and STAT5. The results revealed that RIPC did not induce the immediate or late phosphorylation of AKT or STAT5. In addition, following I/R, the activation of AKT and STAT5 was not modulated by RIPC. In conclusion, the findings of the present study suggested that RIPC-induced cardioprotection may not be mediated by the activation of AKT or STAT5 at the investigated TPs.

Pre- and Post-Conditioning of the Heart: An Overview of Cardioprotective Signaling Pathways

Since the discovery of ischemic pre- and post-conditioning, more than 30 years ago, the knowledge about the mechanisms and signaling pathways involved in these processes has significantly increased. In clinical practice, on the other hand, such advancement has yet to be seen. This article provides an overview of ischemic pre-, post-, remote, and pharmacological conditioning related to the heart. In addition, we reviewed the cardioprotective signaling pathways and therapeutic agents involved in the above-mentioned processes, aiming to provide a comprehensive evaluation of the advancements in the field. The advancements made over the last decades cannot be ignored and with the exponential growth in techniques and applications. The future of pre- and post-conditioning is promising.

Remote ischemic preconditioning reduces myocardial ischemia-reperfusion injury through unacylated ghrelin-induced activation of the JAK/STAT pathway

Remote ischemic preconditioning (RIPC) offers cardioprotection against myocardial ischemia-reperfusion injury. The humoral factors involved in RIPC that are released from parasympathetically innervated organs have not been identified. Previous studies showed that ghrelin, a hormone released from the stomach, is associated with cardioprotection. However, it is unknown whether or not ghrelin is involved in the mechanism of RIPC. This study aimed to determine whether ghrelin serves as one of the humoral factors in RIPC. RIPC group rats were subjected to three cycles of ischemia and reperfusion for 5 min in two limbs before left anterior descending (LAD) coronary artery ligation. Unacylated ghrelin (UAG) group rats were given 0.5 mcg/kg UAG intravenously 30 min before LAD ligation. Plasma levels of UAG in all groups were measured before and after RIPC procedures and UAG administration. Additionally, JAK2/STAT3 pathway inhibitor (AG490) was injected in RIPC and UAG groups to investigate abolishment of the cardioprotection of RIPC and UAG. Plasma levels of UAG, infarct size and phosphorylation of STAT3 were compared in all groups. Infarct size was significantly reduced in RIPC and UAG groups, compared to the other groups. Plasma levels of UAG in RIPC and UAG groups were significantly increased after RIPC and UAG administration, respectively. The cardioprotective effects of RIPC and UAG were accompanied by an increase in phosphorylation of STAT3 and abolished by AG490. This study indicated that RIPC reduces myocardial ischemia and reperfusion injury through UAG-induced activation of JAK/STAT pathway. UAG may be one of the humoral factors involved in the cardioprotective effects of RIPC.

Neuro-autonomic changes induced by remote ischemic preconditioning (RIPC) in healthy young adults: Implications for stress

The mechanisms underlying the protective effects of remote ischemic preconditioning (RIPC) are not presently clear. Recent studies in experimental models suggest the involvement of the autonomic nervous system (ANS) in cardioprotection. The aim of this study was to investigate the changes in ANS in healthy young volunteers divided into RIPC (n = 22) or SHAM (n = 18) groups. RIPC was induced by 1 cycle of 4 min inflation/5 min deflation followed by 2 cycles of 5 min inflation/5 min deflation of a cuff placed on the upper left limb. The study included analysis of heart rate (HR), blood pressure (BP), heart rate variability (HRV), measurements of microcirculation and porphyrin fluorescence in the limb before and after the RIPC. RIPC caused reactive hyperemia in the limb and reduced blood porphyrin level. A mental load (serial sevens test) and mild motor stress (hyperventilation) were performed on all subjects before and after RIPC or corresponding rest in the SHAM group. Reduction of HR occurred during the experiments in both RIPC and SHAM groups reflecting RIPC-independent adaptation of the subjects to the experimental procedure. However, in contrast to the SHAM group, RIPC altered several of the spectral indices of HRV during the serial sevens test and hyperventilation. This was expressed predominantly as an increase in power of the very low-frequency band of the spectrum, increased values of detrended fluctuation analysis and weakening of correlation between the HRV parameters and HR. In conclusion, RIPC induces changes in the activity of ANS that are linked to stress resistance.

Perioperative Cardioprotection by Remote Ischemic Conditioning

Remote ischemic conditioning has been investigated for cardioprotection to attenuate myocardial ischemia/reperfusion injury. In this review, we provide a comprehensive overview of the current knowledge of the signal transduction pathways of remote ischemic conditioning according to three stages: Remote stimulus from source organ; protective signal transfer through neuronal and humoral factors; and target organ response, including myocardial response and coronary vascular response. The neuronal and humoral factors interact on three levels, including stimulus, systemic, and target levels. Subsequently, we reviewed the clinical studies evaluating the cardioprotective effect of remote ischemic conditioning. While clinical studies of percutaneous coronary intervention showed relatively consistent protective effects, the majority of multicenter studies of cardiac surgery reported neutral results although there have been several promising initial trials. Failure to translate the protective effects of remote ischemic conditioning into cardiac surgery may be due to the multifactorial etiology of myocardial injury, potential confounding factors of patient age, comorbidities including diabetes, concomitant medications, and the coadministered cardioprotective general anesthetic agents. Given the complexity of signal transfer pathways and confounding factors, further studies should evaluate the multitarget strategies with optimal measures of composite outcomes.

The Effect of Remote Ischaemic Preconditioning on Arterial Stiffness in Patients Undergoing Vascular Surgery: A Randomised Clinical Trial

OBJECTIVES

The main aim of this study was to evaluate the effect of remote ischaemic preconditioning (RIPC) on arterial stiffness in patients undergoing vascular surgery.

METHODS

This was a randomised, sham controlled, double blind, single centre study. Patients undergoing open abdominal aortic aneurysm repair, surgical lower limb revascularisation surgery or carotid endarterectomy were recruited. A RIPC or a sham procedure was performed, using a blood pressure cuff, along with preparation for anaesthesia. The RIPC protocol consisting of four cycles of 5 min of ischaemia, followed by 5 min of reperfusion was applied. Arterial stiffness and haemodynamic parameters were measured pre-operatively and 20-28 h after surgery. Two primary outcomes were selected: augmentation index and pulse wave velocity.

RESULTS

Ninety-eight patients were randomised. After dropouts 44 and 46 patients were included in the RIPC and sham groups, respectively. Both groups were comparable. There were no statistically significant differences in augmentation index (p = .8), augmentation index corrected for heart rate of 75 beats per minute (p = .8), pulse wave velocity (p = .7), large artery elasticity indices (p = .8), small artery elasticity indices (p = .6), or mean arterial pressure (p = .7) changes between the RIPC and sham groups. There occurred statistically significant (p ≤ .01) improvement in augmentation index (-5.8% vs. -5.5%), augmentation index corrected for a heart rate of 75 beats per minute (-2.5% vs. -2%), small artery elasticity indices (0.7 mL/mmHg × 100 vs. 0.9 mL/mmHg × 100), and mean arterial pressure post-operatively in both the RIPC and the sham groups (change median values in RIPC and sham groups, respectively).

CONCLUSIONS

RIPC had no significant effect on arterial stiffness, but there was significant improvement in arterial stiffness after surgery in both groups. Arterial stiffness and haemodynamics may be influenced by surgery or anaesthesia or oxidative stress or all factors combined. Further studies are needed to clarify these findings. CLINICALTRIALS.GOV: NCT02689414.

A Review of Humoral Factors in Remote Preconditioning of the Heart

A humoral mechanism of cardioprotection by remote ischemic preconditioning (RIP) has been clearly demonstrated in various models of ischemia-reperfusion including upper and lower extremities, liver, and the mesenteric and renal arteries. A wide range of humoral factors for RIP have been proposed including hydrophobic peptides, opioid peptides, adenosine, prostanoids, endovanilloids, endocannabinoids, calcitonin gene-related peptide, leukotrienes, noradrenaline, adrenomedullin, erythropoietin, apolipoprotein, A-I glucagon-like peptide-1, interleukin 10, stromal cell-derived factor 1, and microRNAs. Virtually, all of the components of ischemic preconditioning's signaling pathway such as nitric oxide synthase, protein kinase C, redox signaling, PI3-kinase/Akt, glycogen synthase kinase β, ERK1/2, mitoK_ATP channels, Connexin 43, and STAT were all found to play a role. The signaling pattern also depends on which remote vascular bed was subjected to ischemia and on the time between applying the rip and myocardial ischemia occurs. Because there is convincing evidence for many seemingly diverse humoral components in RIP, the most likely explanation is that the overall mechanism is complex like that seen in ischemic preconditioning where multiple components are both in series and in parallel and interact with each other. Inhibition of any single component in the right circumstance may block the resulting protective effect, and selectively activating that component may trigger the protection. Identifying the humoral factors responsible for RIP might be useful in developing drugs that confer RIP's protection in a more comfortable and reliable manner.

Adenosine A1 receptors and mitochondria: targets of remote ischemic preconditioning

Adenosine is involved in classic preconditioning in most species and acts especially through adenosine A₁ and A₃ receptors. The aim of the present study was to evaluate whether remote ischemic preconditioning (rIPC) activates adenosine A₁ receptors and improves mitochondrial function, thereby reducing myocardial infarct size. Isolated rat hearts were subjected to 30 min of global ischemia and 60 min of reperfusion [ischemia-reperfusion (I/R)]. In a second group, before isolation of the heart, a rIPC protocol (3 cycles of hindlimb I/R) was performed. Infarct size was measured with tetrazolium staining, and Akt/endothelial nitric oxide (NO) synthase (eNOS) expression/phosphorylation and mitochondrial function were evaluated after ischemia at 10 and 60 min of reperfusion. As expected, rIPC significantly decreased infarct size. This beneficial effect was abolished only when 8-cyclopentyl-1,3-dipropylxanthine (adenosine A₁ receptor blocker) and N^G-nitro-l-arginine methyl ester (NO synthesis inhibitor) were administered during the reperfusion phase. At the early reperfusion phase, rIPC induced significant Akt and eNOS phosphorylation, which was abolished by the perfusion with an adenosine A₁ receptor blocker. I/R led to impaired mitochondrial function, which was attenuated by rIPC and mediated by adenosine A₁ receptors. In conclusion, we demonstrated that rIPC limits myocardial infarct by activation of adenosine A₁ receptors at early reperfusion in the isolated rat heart. Interestingly, rIPC appears to reduce myocardial infarct size by the Akt/eNOS pathway and improves mitochondrial function during myocardial reperfusion. NEW & NOTEWORTHY Adenosine is involved in classic preconditioning and acts especially through adenosine A₁ and A₃ receptors. However, its role in the mechanism of remote ischemic preconditioning is controversial. In this study, we demonstrated that remote ischemic preconditioning activates adenosine A₁ receptors during early reperfusion, inducing Akt/endothelial nitric oxide synthase phosphorylation and improving mitochondrial function, thereby reducing myocardial infarct size.

Protecting the heart from ischemia/reperfusion injury: an update on remote ischemic preconditioning and postconditioning

PURPOSE OF REVIEW

The most effective strategy for reducing acute myocardial ischemic injury is timely and effective reperfusion. However, myocardial reperfusion can induce further cardiomyocyte death (reperfusion injury). Interventions that protect the heart from ischemia/reperfusion injury, reducing infarct size, can involve remote ischemic preconditioning and postconditioning. These interventions have a promising potential clinical application, and have been the focus of recent research. In this review, we provide an update of remote ischemic preconditioning and postconditioning mechanisms.

RECENT FINDINGS

Remote ischemic preconditioning cardioprotection can occur via a humoral pathway and/or a neural pathway. These two pathways have been described as mechanistically different, but it has been suggested that they could be interdependent. However, remote ischemic postconditioning mainly involves the humoral pathway. In this review, we will discuss the different pathways and mechanisms involved in remote ischemic preconditioning and postconditioning.

SUMMARY

Remote ischemic preconditioning and postconditioning is possible to perform in a clinical setting by intermittent ischemia of an upper or lower limb. Furthermore, clinical trials using this procedure in the context of predictable ischemia-reperfusion have produced promising results, and other studies to define the potential clinical use of these strategies are ongoing.

Connexins and Nitric Oxide Inside and Outside Mitochondria: Significance for Cardiac Protection and Adaptation

Irreversible myocardial damage happens in the presence of prolonged and severe ischemia. Several phenomena protect the heart against myocardial infarction and other adverse outcomes of ischemia and reperfusion (IR), namely: hibernation related to stunned myocardium, ischemic preconditioning (IPC), ischemic post-conditioning, and their pharmacological surrogates. Ischemic preconditioning consists in the induction of a brief IR to reduce damage of the tissue caused by prolonged and severe ischemia. Nitric oxide (NO) signaling plays an essential role in IPC. Nitric oxide-sensitive guanylate cyclase/cyclic guanosine-3′,5′-monophosphate (cGMP)-dependent protein kinase type I-signaling pathway protects against the IR injury during myocardial infarction. Mitochondrial ATP-sensitive and Ca²⁺-activated K⁺ channels are involved in NO-mediated signaling in IPC. Independently of the cGMP-mediated induction of NO production, S-nitrosation represents a regulatory molecular mechanism similar to phosphorylation and is essential for IPC. Unlike conditioning phenomena, the mechanistic basis of myocardial stunning and hibernation remains poorly understood. In this review article, we hypothesize that the disruption of electrical syncytium of the myocardium may underly myocardial stunning and hibernation. Considering that the connexins are the building blocks of gap junctions which represent primary structural basis of electrical syncytium, we discuss data on the involvement of connexins into myocardial conditioning, stunning, and hibernation. We also show how NO-mediated signaling is involved in myocardial stunning and hibernation. Connexins represent an essential element of adaptation phenomena of the heart at the level of both the cardio- myocytes and the mitochondria. Nitric oxide targets mitochondrial connexins which may affect electrical syncytium continuum in the heart. Mitochondrial connexins may play an essential role in NO-dependent mechanisms of myocardial adaptation to ischemia.

Noninvasive approach to mend the broken heart: Is "remote conditioning" a promising strategy for application in humans?

Currently, there are no satisfactory interventions to protect the heart against the detrimental effects of ischemia-reperfusion injury. Although ischemic preconditioning (PC) is the most powerful form of intrinsic cardioprotection, its application in humans is limited to planned interventions, due to its short duration and technical requirements. However, many organs/tissues are capable of producing "remote" PC (RPC) when subjected to brief bouts of ischemia-reperfusion. RPC was first described in the heart where brief ischemia in one territory led to protection in other area. Later on, RPC started to be used in patients with acute myocardial infarction, albeit with ambiguous results. It is hypothesized that the connection between the signal triggered in remote organ and protection induced in the heart can be mediated by humoral and neural pathways, as well as via systemic response to short sublethal ischemia. However, although RPC has a potentially important clinical role, our understanding of the mechanistic pathways linking the local stimulus to the remote organ remains incomplete. Nevertheless, RPC appears as a cost-effective and easily performed intervention. Elucidation of protective mechanisms activated in the remote organ may have therapeutic and diagnostic implications in the management of myocardial ischemia and lead to development of pharmacological RPC mimetics.

Co-dependence of the neural and humoral pathways in the mechanism of remote ischemic conditioning

The cardioprotection afforded by remote ischaemic conditioning (RIC) is mediated via a complex mechanism involving sensory afferent nerves, the vagus nerve, and release of a humoral blood-borne factor. However, it is unknown whether release of the protective factor depends on vagal activation or occurs independently. This study aimed to evaluate the co-dependence of the neural and humoral pathways of RIC, focussing on the vagus nerve and intrinsic cardiac ganglia. In the first study, anesthetised rats received bilateral cervical vagotomy or sham-surgery immediately prior to RIC (4 × 5 min limb ischemia–reperfusion) or sham-RIC. Venous blood plasma was dialysed across a 12–14 kDa membrane and dialysate perfused through a naïve-isolated rat heart prior to 35-min left anterior descending ischemia and 60-min reperfusion. In the second study, anesthetised rats received RIC (4 × 5-min limb ischemia–reperfusion) or control (sham-RIC). Dialysate was prepared and perfused through a naïve-isolated rat heart in the presence of the ganglionic blocker hexamethonium or muscarinic antagonist atropine, prior to ischemia–reperfusion as above. Dialysate collected from RIC-treated rats reduced infarct size in naïve rat hearts from 40.7 ± 6.3 to 23.7 ± 3.1 %, p < 0.05. Following bilateral cervical vagotomy, the protection of RIC dialysate was abrogated (42.2 ± 3.2 %, p < 0.05 vs RIC dialysate). In the second study, the administration of 50-μM hexamethonium (45.8 ± 2.5 %) or 100-nM atropine (36.5 ± 3.4 %) abrogated the dialysate-mediated protection. Release of a protective factor following RIC is dependent on prior activation of the vagus nerve. In addition, this factor appears to induce cardioprotection via recruitment of intrinsic cardiac ganglia.