Claudia Penna

Post–conditioning induced cardioprotection requires signaling through a redox–sensitive mechanism, mitochondrial ATP–sensitive K+ channel and protein kinase C activation

Post–conditioning (Post–C) induced cardioprotection involves activation of guanylyl–cyclase. In the ischemic preconditioning scenario, the downstream targets of cGMP include mitochondrial ATP–sensitive K+ (mKATP) channels and protein kinase C (PKC), which involve reactive oxygen species (ROS) production. This study tests the hypothesis that mKATP, PKC and ROS are also involved in the Post–C protection. Isolated rat hearts underwent 30 min global ischemia (I) and 120 min reperfusion (R) with or without Post–C (i.e., 5 cycles of 10 s R/I immediately after the 30 min ischemia). In 6 groups (3 with and 3 without Post–C) either mKATP channel blocker, 5– hydroxydecanoate (5–HD), or PKC inhibitor, chelerythrine (CHE) or ROS scavenger, N–acetyl–cysteine (NAC), were given during the entire reperfusion (120 min). In other 6 groups (3 with and 3 without Post–C), 5–HD, CHE or NAC were infused for 117 min only starting after 3 min of reperfusion not to interfere with the early effects of Post–C and/or reperfusion. In an additional group NAC was given during Post–C maneuvers (i.e., 3 min only). Myocardial damage was evaluated using nitro–blue tetrazolium staining and lactate dehydrogenase (LDH) release. Post–C attenuated myocardial infarct size (21 ± 3% vs. 64 ± 5% in control; p < 0.01). Such an effect was abolished by 5–HD or CHE given during either the 120 or 117 min of reperfusion as well as by NAC given during the 120 min or the initial 3 min of reperfusion. However, delayed NAC (i.e., 117 min infusion) did not alter the protective effect of Post– C (infarct size 32 ± 5%; p < 0.01 vs. control, NS vs. Post–C). CHE, 5–HD or NAC given in the absence of Post–C did not alter the effects of I/R. Similar results were obtained in terms of LDH release. Our data show that Post–C induced protection involves an early redox–sensitive mechanism as well as a persistent activation of mKATP and PKC, suggesting that the mKATP/ROS/PKC pathway is involved in post–conditioning.

Nitroxyl affords thiol-sensitive myocardial protective effects akin to early preconditioning.

Nitric oxide (NO) donors mimic the early phase of ischemic preconditioning (IPC). The effects of nitroxyl (HNO/NO(-)), the one-electron reduction product of NO, on ischemia/reperfusion (I/R) injury are unknown. Here we investigated whether HNO/NO(-), produced by decomposition of Angelis salt (AS; Na(2)N(2)O(3)), has a cardioprotective effect in isolated perfused rat hearts. Effects were examined after intracoronary perfusion (19 min) of either AS (1 microM), the NO donor diethylamine/NO (DEA/NO, 0.5 microM), vehicle (100 nM NaOH) or buffer, followed by global ischemia (30 min) and reperfusion (30 min or 120 min in a subset of hearts). IPC was induced by three cycles of 3 min ischemia followed by 10 min reperfusion prior to I/R. The extent of I/R injury under each intervention was assessed by changes in myocardial contractility as well as lactate dehydrogenase (LDH) release and infarct size. Postischemic contractility, as indexed by developed pressure and dP/dt(max), was similarly improved with IPC and pre-exposure to AS, as opposed to control or DEA/NO-treated hearts. Infarct size and LDH release were also significantly reduced in IPC and AS groups, whereas DEA/NO was less effective in limiting necrosis. Co-infusion in the triggering phase of AS and the nitroxyl scavenger, N-acetyl-L-cysteine (4 mM) completely reversed the beneficial effects of AS, both at 30 and 120 min reperfusion. Our data show that HNO/NO(-) affords myocardial protection to a degree similar to IPC and greater than NO, suggesting that reactive nitrogen oxide species are not only necessary but also sufficient to trigger myocardial protection against reperfusion through species-dependent, pro-oxidative, and/or nitrosative stress-related mechanisms.

Ischemia/reperfusion injury and cardioprotective mechanisms: Role of mitochondria and reactive oxygen species

Reperfusion therapy must be applied as soon as possible to attenuate the ischemic insult of acute myocardial infarction (AMI). However reperfusion is responsible for additional myocardial damage, which likely involves opening of the mitochondrial permeability transition pore (mPTP). In reperfusion injury, mitochondrial damage is a determining factor in causing loss of cardiomyocyte function and viability. Major mechanisms of mitochondrial dysfunction include the long lasting opening of mPTPs and the oxidative stress resulting from formation of reactive oxygen species (ROS). Several signaling cardioprotective pathways are activated by stimuli such as preconditioning and postconditioning, obtained with brief intermittent ischemia or with pharmacological agents. These pathways converge on a common target, the mitochondria, to preserve their function after ischemia/reperfusion. The present review discusses the role of mitochondria in cardioprotection, especially the involvement of adenosine triphosphate-dependent potassium channels, ROS signaling, and the mPTP. Ischemic postconditioning has emerged as a new way to target the mitochondria, and to drastically reduce lethal reperfusion injury. Several clinical studies using ischemic postconditioning during angioplasty now support its protective effects, and an interesting alternative is pharmacological postconditioning. In fact ischemic postconditioning and the mPTP desensitizer, cyclosporine A, have been shown to induce comparable protection in AMI patients.

Post-conditioning reduces infarct size in the isolated rat heart: role of coronary flow and pressure and the nitric oxide/cGMP pathway.

AbstractWe aimed to assess the role of the nitric oxide (NO)–cGMP pathway in cardioprotection by brief intermittent ischemias at the onset of reperfusion (i.e., post–conditioning (Post–con)). We also evaluated the role of coronary flow and pressure in Post–con. Rat isolated hearts perfused at constant– flow or –pressure underwent 30 min global ischemia and 120 min reperfusion. Post–con obtained with brief ischemias of different duration (modified, MPost–con) was compared with Post–con obtained with ischemias of identical duration (classical, C–Post–con) and with ischemic preconditioning (IP). Infarct size was evaluated using nitro–blue tetrazolium staining and lactate dehydrogenase (LDH) release. In the groups, NO synthase (NOS) or guanylyl–cyclase (GC) was inhibited with LNAME and ODQ, respectively. In the subgroups, the enzyme immunoassay technique was used to quantify cGMP release. In the constant–flow model, M–Post–con and C–Post–con were equally effective, but more effective than IP in reducing infarct size. The cardioprotection by M–Post–con was only blunted by the NOS–inhibitor, but was abolished by the GC–antagonist. Post–ischemic cGMP release was enhanced by MPost–con. In the constant–pressure model IP, M–Post–con and C–Post–con were equally effective in reducing infarct size. Post–con protocols were more effective in the constant–flow than in the constant–pressure model. In all groups, LDH release during reperfusion was proportional to infarct size. In conclusion, Post–con depends upon GC activation, which can be achieved by NOS–dependent and NOS–independent pathways. The benefits of M– and CPost–con are similar. However, protection by Post–con is greater in the constant–flow than in the constant–pressure model.

The paradigm of postconditioning to protect the heart

•  Introduction •  Reperfusion injury ‐  Causes of reperfusion injury ‐  Effects of reperfusion injury •  Definition of postconditioning ‐  Postconditioning algorithm •  Protective effects of postconditioning ‐  Infarct size reduction ‐  Reduction of apoptosis ‐  Reduction in endothelial dysfunction ‐  Reduction in endothelial activation, and neutrophil adherence ‐  Reduction of stunning ‐  Anti‐arrhythmic effects •  Potentiality of postconditioning ‐  Remote postconditioning ‐  Pharmacological postconditioning ‐  Postconditioning the human heart ‐  Postconditioning in diseased hearts •  Mechanisms involved in postconditioning ‐  Passive mechanisms ‐  Mechanical mechanisms ‐  Cellular mechanisms ‐  Active mechanisms (intramyocardiocyte mechanisms) ‐  Triggers ‐  Mediators ‐  End effectors ‐  Cardioprotection by pre‐ and post‐conditioning is redox‐sensitive •  Conclusions

Mitochondrial Pathways, Permeability Transition Pore, and Redox Signaling in Cardioprotection: Therapeutic Implications

Reperfusion therapy is the indispensable treatment of acute myocardial infarction (AMI) and must be applied as soon as possible to attenuate the ischemic insult. However, reperfusion is responsible for additional myocardial damage likely involving opening of the mitochondrial permeability transition pore (mPTP). A great part of reperfusion injury occurs during the first minute of reperfusion. The prolonged opening of mPTP is considered one of the endpoints of the cascade to myocardial damage, causing loss of cardiomyocyte function and viability. Opening of mPTP and the consequent oxidative stress due to reactive oxygen and nitrogen species (ROS/RNS) are considered among the major mechanisms of mitochondrial and myocardial dysfunction. Kinases and mitochondrial components constitute an intricate network of signaling molecules and mitochondrial proteins, which interact in response to stressors. Cardioprotective pathways are activated by stimuli such as preconditioning and postconditioning (PostC), obtained with brief intermittent ischemia or with pharmacological agents, which drastically reduce the lethal ischemia/reperfusion injury. The protective pathways converging on mitochondria may preserve their function. Protection involves kinases, adenosine triphosphate-dependent potassium channels, ROS signaling, and the mPTP modulation. Some clinical studies using ischemic PostC during angioplasty support its protective effects, and an interesting alternative is pharmacological PostC. In fact, the mPTP desensitizer, cyclosporine A, has been shown to induce appreciable protections in AMI patients. Several factors and comorbidities that might interfere with cardioprotective signaling are considered. Hence, treatments adapted to the characteristics of the patient (i.e., phenotype oriented) might be feasible in the future.

Guanylate-cyclase-mediated inhibition of cardiac ICa by carbachol and sodium nitroprusside.

We studied the role of cyclic guanosine monophosphate (cGMP) as a mediator of the reduction of L-type calcium current (ICa) induced by muscarinic receptor stimulation and by nitric oxide in isolated guinea-pig ventricular cells using the whole-cell patchclamp technique. Our results show that when the level of cyclic adenosine monophosphate was increased by the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX), stimulation of a pertussis-toxin (PTX)-sensitive muscarinic receptor by carbachol (1 μM) reduced the calcium current increase from 80.6±23.5% to 19.8±9.6% over the control and this effect was prevented by methylene blue (10 μM), an inhibitor of the soluble guanylate cyclase. Pipette solution containing 10 μM cGMP reduced the enhancement of ICa by IBMX from 121.9±11.6% to 14.2±5.4% above the control. Sodium nitroprusside (10 μM), a spontaneous donor of nitric oxide, and consequently a stimulator of soluble guanylate cyclase, also reduced IBMX-stimulated ICa from 115.2±13.2% to 32.2±6.9% above control and the sodium nitroprusside effect was also suppressed by methylene blue. The latter two reagents were ineffective on basal ICa.

Cardioprotective pathways during reperfusion: focus on redox signaling and other modalities of cell signaling.

Post-ischemic reperfusion may result in reactive oxygen species (ROS) generation, reduced availability of nitric oxide (NO•), Ca(2+)overload, prolonged opening of mitochondrial permeability transition pore, and other processes contributing to cell death, myocardial infarction, stunning, and arrhythmias. With the discovery of the preconditioning and postconditioning phenomena, reperfusion injury has been appreciated as a reality from which protection is feasible, especially with postconditioning, which is under the control of physicians. Potentially cooperative protective signaling cascades are recruited by both pre- and postconditioning. In these pathways, phosphorylative/dephosphorylative processes are widely represented. However, cardioprotective modalities of signal transduction also include redox signaling by ROS, S-nitrosylation by NO• and derivative, S-sulfhydration by hydrogen sulfide, and O-linked glycosylation with beta-N-acetylglucosamine. All these modalities can interact and regulate an entire pathway, thus influencing each other. For instance, enzymes can be phosphorylated and/or nitrosylated in specific and/or different site(s) with consequent increase or decrease of their specific activity. The cardioprotective signaling pathways are thought to converge on mitochondria, and various mitochondrial proteins have been identified as targets of these post-transitional modifications in both pre- and postconditioning.

Postconditioning and intermittent bradykinin induced cardioprotection require cyclooxygenase activation and prostacyclin release during reperfusion

Postconditioning (PostC), obtained with brief intermittent cycles of ischemia alternating with reperfusion applied after the ischemic event, has been shown to reduce infarct size. Recently, we have shown that PostC triggering includes B2 receptor activation and its downstream pathway. Moreover, we showed that BK intermittent infusion induces a cardioprotection similar to PostC. The aim of this study was to investigate the involvement of cyclooxygenase-(COX)-derivated prostaglandins, such as prostacyclin (PGI2) pathway in the cardioprotective action mediated by intermittent BK infusion.Isolated rat hearts underwent 30 min ischemia and 120 min reperfusion. Myocardial damage was evaluated using nitro-blue-tetrazolium staining. The production of metabolite of PGI2, 6-keto-PGF1α, was evaluated with EIA assay on the samples collected during reperfusion. The perfusion pressure and the left ventricular pressure were monitored. In Control hearts, the infarct size was 64% ± 4% of risk area. PostC reduced significantly the infarct size (28% ± 4% P < 0.001 Vs. Control). BK intermittent protocol to mimic PostC, attenuated infarct size (40% ± 2% P < 0.01 Vs. Control). The BK-intermittent and PostC protections were abolished with COX-inhibition. Intermittent BK and PostC enhanced the release of prostacyclin metabolite, 6-keto-PGF1α, in the late phase of reperfusion (i.e., 6-keto-PGF1α peaked 30 min after protective maneuvers). Also the stable PGI2 analogue, Iloprost, given in the early reperfusion reduced infarct size and improved post-ischemic heart function. In conclusion, protection by PostC and intermittent BK requires COX activation and PGI2 release during late reperfusion. These data suggest that COX must not be inhibited to have PostC protection. This finding should be kept present by future clinical studies on PostC.

Redox balance and cardioprotection

Coronary artery disease is a major cause of morbidity and mortality in the Western countries. Acute myocardial infarction is a serious and often lethal consequence of coronary artery disease, resulting in contractile dysfunction and cell death. It is well known that unbalanced and high steady state levels of reactive oxygen and nitrogen species (ROS/RNS) are responsible for cytotoxicity, which in heart leads to contractile dysfunction and cell death. Pre- and post-conditioning of the myocardium are two treatment strategies that reduce contractile dysfunction and the amount of cell death considerably. Paradoxically, ROS and RNS have been identified as a part of cardioprotective signaling molecules, which are essential in pre- and post-conditioning processes. S-nitrosylation of proteins is a specific posttranslational modification that plays an important role in cardioprotection, especially within mitochondria. In fact, mitochondria are of paramount importance in either promoting or limiting ROS/RNS generation and reperfusion injury, and in triggering kinase activation by ROS/RNS signaling in cardioprotection. These organelles are also the targets of acidosis, which prevents mitochondrial transition pore opening, thus avoiding ROS-induced ROS release. Therefore, we will consider mitochondria as either targets of damage or protection from it. The origin of ROS/RNS and the cardioprotective signaling pathways involved in ROS/RNS-based pre- and post-conditioning will be explored in this article. A particular emphasis will be given to new aspects concerning the processes of S-nitrosylation in the cardioprotective scenario.