Preconditioning protects by inhibiting the mitochondrial permeability transition

Mitochondrial ion channels: gatekeepers of life and death

Continuous generation of ATP by mitochondrial oxidative phosphorylation is essential to maintain function in mechanically active cells such as cardiomyocytes. Emerging evidence indicates that mitochondrial ion channels activated by reactive oxygen species can induce a mitochondrial "critical" state, which can scale to cause electrical and contractile dysfunction of the cardiac cell and, ultimately, the whole heart. Here we focus on how mitochondrial ion channels participate in life-and-death decisions of the cell and discuss the challenges ahead for translating recent findings into novel therapeutic applications.

Cardioprotection afforded by chronic exercise is mediated by the sarcolemmal, and not the mitochondrial, isoform of the KATP channel in the rat

This study was conducted to examine the role of myocardial ATP-sensitive potassium (K(ATP)) channels in exercise-induced protection from ischaemia-reperfusion (I-R) injury. Female rats were either sedentary (Sed) or exercised for 12 weeks (Tr). Hearts were excised and underwent a 1-2 h regional I-R protocol. Prior to ischaemia, hearts were subjected to pharmacological blockade of the sarcolemmal K(ATP) channel with HMR 1098 (SedHMR and TrHMR), mitochondrial blockade with 5-hydroxydecanoic acid (5HD; Sed5HD and Tr5HD), or perfused with buffer containing no drug (Sed and Tr). Infarct size was significantly smaller in hearts from Tr animals (35.4 +/- 2.3 versus 44.7 +/- 3.0% of the zone at risk for Tr and Sed, respectively). Mitochondrial K(ATP) blockade did not abolish the training-induced infarct size reduction (30.0 +/- 3.4 versus 38.0 +/- 2.6 in Tr5HD and Sed5HD, respectively); however, sarcolemmal K(ATP) blockade completely eradicated the training-induced cardioprotection. Infarct size was 71.2 +/- 3.3 and 64.0 +/- 2.4% of the zone at risk for TrHMR and Sed HMR. The role of sarcolemmal K(ATP) channels in Tr-induced protection was also supported by significant increases in both subunits of the sarcolemmal K(ATP) channel following training. LV developed pressure was better preserved in hearts from Tr animals, and was not influenced by addition of HMR 1098. 5HD decreased pressure development regardless of training status, from 15 min of ischaemia through the duration of the protocol. This mechanical dysfunction was likely to be due to a 5HD-induced increase in myocardial Ca2+ content following I-R. The major findings of the present study are: (1) unlike all other known forms of delayed cardioprotection, infarct sparing following chronic exercise was not abolished by 5HD; (2) pharmacological blockade of the sarcolemmal K(ATP) channel nullified the cardioprotective benefits of exercise training; and (3) increased expression of sarcolemmal K(ATP) channels was observed following chronic training.

Selective role for superoxide in InsP3 receptor-mediated mitochondrial dysfunction and endothelial apoptosis

Reactive oxygen species (ROS) play a divergent role in both cell survival and cell death during ischemia/reperfusion (I/R) injury and associated inflammation. In this study, ROS generation by activated macrophages evoked an intracellular Ca2+ ([Ca2+]i) transient in endothelial cells that was ablated by a combination of superoxide dismutase and an anion channel blocker. [Ca2+]i store depletion, but not extracellular Ca2+ chelation, prevented [Ca2+]i elevation in response to O2*- that was inositol 1,4,5-trisphosphate (InsP3) dependent, and cells lacking the three InsP3 receptor (InsP3R) isoforms failed to display the [Ca2+]i transient. Importantly, the O2*--triggered Ca2+ mobilization preceded a loss in mitochondrial membrane potential that was independent of other oxidants and mitochondrially derived ROS. Activation of apoptosis occurred selectively in response to O2*- and could be prevented by [Ca2+]i buffering. This study provides evidence that O2*- facilitates an InsP3R-linked apoptotic cascade and may serve a critical function in I/R injury and inflammation.

Mitochondrial permeability transition pore as a target for cardioprotection in the human heart

After an episode of myocardial ischemia, opening of the mitochondrial permeability transition pore (mPTP), at the onset of reperfusion, is a critical determinant of myocyte death. We investigated the role of the mPTP as a target for cardioprotection in the human heart. We subjected human atrial tissue, harvested from patients undergoing cardiac surgery, to a period of lethal hypoxia and investigated the effect of suppressing mPTP opening at the onset of reoxygenation. We found that suppressing mPTP opening at the onset of reoxygenation with known mPTP inhibitors cyclosporin A (CsA, 0.2 micromol/l) and sanglifehrin A (SfA, 1.0 micromol/l) 1) improved recovery of baseline contractile function from 29.4 +/- 2.0% under control conditions to 48.7 +/- 2.2% with CsA and 46.1 +/- 2.3% with SfA (P < 0.01) and 2) improved cell survival from 62.8 +/- 5.3% under hypoxic control conditions to 91.4 +/- 4.1% with CsA and 87.2 +/- 6.2% with SfA (P < 0.001). Furthermore, with a cell model in which oxidative stress was used to induce mPTP opening in human atrial myocytes, we demonstrated directly that CsA and SfA mediated their cardioprotective effects by inhibiting mPTP opening, as evidenced by an extension in the time required to induce mPTP opening from 116 +/- 8 s under control conditions to 189 +/- 10 s with CsA and 183 +/- 12 s with SfA (P < 0.01). We report that suppressing mPTP opening at the onset of reoxygenation protects human myocardium against lethal hypoxia-reoxygenation injury. This suggests that, in the human heart, the mPTP is a viable target for cardioprotection.

Mitochondrial ATP-Sensitive K+ Channels Prevent Oxidative Stress, Permeability Transition and Cell Death

Ischemia followed by reperfusion results in impairment of cellular and mitochondrial functionality due to opening of mitochondrial permeability transition pores. On the other hand, activation of mitochondrial ATP-sensitive K(+) channels (mitoK(ATP)) protects the heart against ischemic damage. This study examined the effects of mitoK(ATP) and mitochondrial permeability transition on isolated rat heart mitochondria and cardiac cells submitted to simulated ischemia and reperfusion (cyanide/aglycemia). Both mitoK(ATP) opening, using diazoxide, and the prevention of mitochondrial permeability transition, using cyclosporin A, protected against cellular damage, without additive effects. MitoK(ATP) opening in isolated rat heart mitochondria slightly decreased Ca(2+) uptake and prevented mitochondrial reactive oxygen species production, most notably in the presence of added Ca(2+). In ischemic cells, diazoxide decreased ROS generation during cyanide/aglycemia while cyclosporin A prevented oxidative stress only during simulated reperfusion. Collectively, these studies indicate that opening mitoK(ATP) prevents cellular death under conditions of ischemia/reperfusion by decreasing mitochondrial reactive oxygen species release secondary to Ca(2+) uptake, inhibiting mitochondrial permeability transition.

Mitochondrial K(ATP) channels in cell survival and death

Since the discovery of the mitochondrial ATP-sensitive potassium channel (mitoK(ATP)) more than 13 years ago, it has been implicated in the processes of ischemic preconditioning (IPC), apoptosis and mitochondrial matrix swelling. Different approaches have been employed to characterize the pharmacological profile of the channel, and these studies strongly suggest that cellular protection well correlates with the opening of mitoK(ATP). However, there are many questions regarding mitoK(ATP) that remain to be answered. These include the very existence of mitoK(ATP) itself, its degree of importance in the process of IPC, its response to different pharmacological agents, and how its activation leads to the process of IPC and protection against cell death. Recent findings suggest that mitoK(ATP) may be a complex of multiple mitochondrial proteins, including some which have been suggested to be components of the mitochondrial permeability transition pore. However, the identity of the pore-forming unit of the channel and the details of the interactions between these proteins remain unclear. In this review, we attempt to highlight the recent advances in the physiological role of mitoK(ATP) and discuss the controversies and unanswered questions.

Nicorandil: a review of its use in the management of stable angina pectoris, including high-risk patients

Nicorandil (Adancor, Angicor, Dancor, Nikoril [Europe], Ikorel [Europe and Oceania], Sigmart [Japan, Korea and Taiwan]) is an adenosine triphosphate (ATP)-sensitive potassium (KATP) channel agonist with nitrate-like properties used in the management of stable angina pectoris. With well established monotherapeutic antianginal activity and a beneficial effect (when added to optimal antianginal therapy) on clinical outcomes in high-risk patients with stable angina, twice-daily oral nicorandil is a useful alternative or addition to other antianginal therapy.

Nitric oxide donors protect murine myocardium against infarction via modulation of mitochondrial permeability transition

Mitochondrial permeability transition (MPT) pores have recently been implicated as a potential mediator of myocardial ischemic injury. Nitric oxide (NO) donors induce a powerful late phase of cardioprotection against ischemia-reperfusion injury; however, the cellular mechanisms involved are poorly understood. The role of MPT pores as a target of cardioprotective signaling pathways activated by NO has never been explored in detail. Thus mice were administered the NO donor diethylenetriamine (DETA)/NO (4 doses of 0.1 mg/kg i.v. each) 24 h before 30 min of coronary artery occlusion followed by 24 h of reperfusion. Infarct size was significantly reduced in DETA/NO-treated mice (30 +/- 2% of risk region in treated mice vs. 50 +/- 2% in control mice; P < 0.05), which demonstrates powerful cardioprotection. To examine the role of MPT pores, mice were administered atractyloside (Atr; 25 mg/kg i.v.), which induces adenine nucleotide translocase-dependent MPT, 20 min before ischemia. Atr blocked the infarct-sparing effects of DETA/NO (infarct size, 58 +/- 1 vs. 30 +/- 2% of risk region in DETA/NO; P < 0.05), whereas Atr alone had no effect. Mitochondria isolated from DETA/NO-treated mice exhibited increased resistance to Ca(2+)-induced swelling by 20 micromol/l CaCl(2) or by the higher concentration of 200 micromol/l, which suggests that cardioprotection involves decreased propensity for MPT. Preincubation of mitochondria from control hearts with 30 nmol/l of the pore inhibitor cyclosporin A prevented swelling by 200 micromol/l CaCl(2), thereby confirming that Ca(2+) induces mitochondrial swelling via MPT. In accordance with the effects on infarct size, administration of Atr to the mice significantly abrogated DETA/NO-induced protection against Ca(2+)-induced mitochondrial swelling. These phenotypic alterations were associated with an increase in the antiapoptotic protein Bcl-2, which suggests that the underlying mechanisms may involve inhibition of cell death by Bcl-2. These data suggest that a critical process during NO donor-induced cardioprotection is to prevent MPT pore opening potentially via targeting of the adenine nucleotide translocator.

New Insights into the Cellular and Molecular Mechanisms of Cold Storage Injury

Solid organ grafts, but also other biologic materials requiring storage for a few hours to a few days, are usually stored under hypothermic conditions. To decrease graft injury during cold storage, organ preservation solutions were developed many years ago. However, since then, modern biochemical and cell biologic methods have allowed further insights into the molecular and cellular mechanisms of cold storage injury, including further insights into alterations of the cellular ion homeostasis, the occurrence of a mitochondrial permeability transition, and the occurrence of free–radical-mediated hypothermic injury and cold-induced apoptosis. These new aspects of cold storage injury, which are not covered by preservation solutions in current clinical use and offer the potential for improvement of organ and tissue preservation, are presented here.

Inhibit GSK-3beta or there's heartbreak dead ahead

Numerous signaling pathways have been shown to mediate cardioprotection, but the end effectors that mediate protection are only beginning to be elucidated. Numerous cardioprotective drugs are shown to converge on glycogen synthase kinase-3beta (GSK-3beta). The phosphorylation and inhibition of GSK-3beta lead to inhibition or delayed activation of the mitochondrial permeability transition, a key regulator of apoptosis.