Ischemic and pharmacological preconditioning in Girardi cells and C2C12 myotubes induce mitochondrial uncoupling

The Slo(w) path to identifying the mitochondrial channels responsible for ischemic protection

Mitochondria play an important role in tissue ischemia and reperfusion (IR) injury, with energetic failure and the opening of the mitochondrial permeability transition pore being the major causes of IR-induced cell death. Thus, mitochondria are an appropriate focus for strategies to protect against IR injury. Two widely studied paradigms of IR protection, particularly in the field of cardiac IR, are ischemic preconditioning (IPC) and volatile anesthetic preconditioning (APC). While the molecular mechanisms recruited by these protective paradigms are not fully elucidated, a commonality is the involvement of mitochondrial K⁺ channel opening. In the case of IPC, research has focused on a mitochondrial ATP-sensitive K⁺ channel (mitoK_ATP), but, despite recent progress, the molecular identity of this channel remains a subject of contention. In the case of APC, early research suggested the existence of a mitochondrial large-conductance K⁺ (BK, big conductance of potassium) channel encoded by the Kcnma1 gene, although more recent work has shown that the channel that underlies APC is in fact encoded by Kcnt2 In this review, we discuss both the pharmacologic and genetic evidence for the existence and identity of mitochondrial K⁺ channels, and the role of these channels both in IR protection and in regulating normal mitochondrial function.

The cardioprotective compound cloxyquin uncouples mitochondria and induces autophagy

Mitochondrial quality control mechanisms have been implicated in protection against cardiac ischemia-reperfusion (IR) injury. Previously, cloxyquin (5-chloroquinolin-8-ol) was identified via phenotypic screening as a cardioprotective compound. Herein, cloxyquin was identified as a mitochondrial uncoupler in both isolated heart mitochondria and adult cardiomyocytes. Additionally, cardiomyocytes isolated from transgenic mice expressing green fluorescent protein-tagged microtubule-associated protein light chain 3 showed increased autophagosome formation with cloxyquin treatment. The autophagy inhibitor chloroquine abolished cloxyquin-induced cardioprotection in both cellular and perfused heart (Langendorff) models of IR injury. Finally, in an in vivo murine left anterior descending coronary artery occlusion model of IR injury, cloxyquin significantly reduced infarct size from 31.4 ± 3.4% to 16.1 ± 2.2%. In conclusion, the cardioprotective compound cloxyquin simultaneously uncoupled mitochondria and induced autophagy. Importantly, autophagy appears to be required for cloxyquin-induced cardioprotection.

Exercise-induced ischemic preconditioning and the potential application to cardiac rehabilitation: a systematic review

Exercise-induced ischemic preconditioning (IPC) can be assessed by the results of the second of sequential exercise tests. Exercise-induced IPC is quantified by using the time to 1-mm ST-segment depression, the rate-pressure product at 1-mm ST-segment depression, the maximal ST-segment depression, and the rate-pressure product at the peak of exercise. Few studies reported whether exercise-induced IPC could be used in cardiovascular rehabilitation. A systematic review of the literature limited to human studies was performed using electronic databases, and the main key words were ischemic preconditioning, warm-up phenomenon, and exercise. After careful review, 38 articles were included in the systematic review. This review summarizes the molecular pathways of IPC and describes the first window of protection induced by sequential exercise tests, as well as the effect of medication on exercise-induced IPC. A section on the exercise protocol, mode of exercise, and intensity provides understanding as to what is needed for clinicians to induce IPC with sequential stress tests. The final section of the review is a discussion of the potential use of exercise-induced IPC in a cardiovascular rehabilitation setting. Even if exercise-induced IPC is a well-documented phenomenon, additional studies are needed in order to more fully understand its use in rehabilitation.

Physiology and metabolism of tissue-engineered skeletal muscle

Skeletal muscle is a major target for tissue engineering, given its relative size in the body, fraction of cardiac output that passes through muscle beds, as well as its key role in energy metabolism and diabetes, and the need for therapies for muscle diseases such as muscular dystrophy and sarcopenia. To date, most studies with tissue-engineered skeletal muscle have utilized murine and rat cell sources. On the other hand, successful engineering of functional human muscle would enable different applications including improved methods for preclinical testing of drugs and therapies. Some of the requirements for engineering functional skeletal muscle include expression of adult forms of muscle proteins, comparable contractile forces to those produced by native muscle, and physiological force-length and force-frequency relations. This review discusses the various strategies and challenges associated with these requirements, specific applications with cultured human myoblasts, and future directions.

Cardioprotection during diabetes: the role of mitochondrial DNA

BACKGROUND

Diabetes alters mitochondrial bioenergetics and consequently disrupts cardioprotective signaling. The authors investigated whether mitochondrial DNA (mtDNA) modulates anesthetic preconditioning (APC) and cardiac susceptibility to ischemia-reperfusion injury by using two strains of rats, both sharing nuclear genome of type 2 diabetes mellitus (T2DN) rats and having distinct mitochondrial genomes of Wistar and fawn-hooded hypertensive (FHH) rat strains (T2DN(mtWistar) and T2DN(mtFHH), respectively).

METHODS

Myocardial infarct size was measured in Wistar, T2DN(mtWistar), and T2DN(mtFHH) rats with or without APC (1.4% isoflurane) in the presence or absence of antioxidant N-acetylcysteine. Flavoprotein fluorescence intensity, a marker of mitochondrial redox state, 5-(and-6)-chloromethyl-2',7'-dichlorofluorescein fluorescence intensity, a marker of reactive oxygen species generation, and mitochondrial permeability transition pore opening were assessed in isolated rat ventricular cardiomyocytes with or without isoflurane (0.5 mmol/l).

RESULTS

Myocardial infarct size was decreased by APC in Wistar and T2DN(mtWistar) rats (to 42 ± 6%, n = 8; and 44 ± 7%, n = 8; of risk area, respectively) compared with their respective controls (60 ± 3%, n = 6; and 59 ± 9%, n = 7), but not in T2DN(mtFHH) rats (60 ± 2%, n = 8). N-acetylcysteine applied during isoflurane treatment restored APC in T2DN(mtFHH) (39 ± 6%, n = 7; and 38 ± 5%, n = 7; 150 and 75 mg/kg N-acetylcysteine, respectively), but abolished protection in control rats (54 ± 8%, n = 6). Similar to the data on infarct size, APC delayed mitochondrial permeability transition pore opening in T2DN(mtWistar) but not in T2DN(mtFHH) cardiomyocytes. Isoflurane increased flavoprotein and 5-(and-6)-chloromethyl-2',7'-dichlorofluorescein fluorescence intensity in all rat strains, with the greatest effect in T2DN(mtFHH) cardiomyocytes.

CONCLUSION

Differences in the mitochondrial genome modulate isoflurane-induced generation of reactive oxygen species which translates into differential susceptibility to APC and ischemia-reperfusion injury in diabetic rats.

Parvalbumin is overexpressed in the late phase of pharmacological preconditioning in skeletal muscle

Pharmacological preconditioning (PPC) with mitochondrial ATP-sensitive K(+) channel openers such as diazoxide, provides protection against ischemia in cardiac muscle, skeletal muscle, and other tissues. Effects on Ca(2+) homeostasis during the late phase of PPC have been described in cardiomyocytes, but no information is available regarding intracellular Ca(2+) changes in skeletal muscle fibers during late PPC. Intracellular Ca(2+) signals were measured in single fibers of adult mouse skeletal muscle, with fluorescent probes, 48 h after the administration of diazoxide. Parvalbumin levels in the myofibers were quantitated by Western blot. Diazoxide induction of late PPC was confirmed by partial protection of muscles from peroxide-induced damage. Late PPC was associated with a significant decrease in the duration of Ca(2+) signals during single twitches and tetanus with no changes in peak values. This effect was prevented by the reactive oxygen species (ROS) scavenger tiron. Late PPC was accompanied by a 30% increase in parvalbumin levels, and this effect was also blocked by tiron. Our data show, for the first time, a role of parvalbumin in late PPC in skeletal muscle.

Preconditioning by isoflurane elicits mitochondrial protective mechanisms independent of sarcolemmal KATP channel in mouse cardiomyocytes

Cardiac mitochondria and the sarcolemmal (sarc)KATP channels contribute to cardioprotective signaling of anesthetic-induced preconditioning. Changes in mitochondrial bioenergetics influence the sarcolemmal ATP-sensitive K (sarcKATP) channel function, but whether this channel has impacts on mitochondria is uncertain. We used the mouse model with deleted pore-forming Kir6.2 subunit of sarcKATP channel (Kir6.2 KO) to investigate whether the functional sarcKATP channels are necessary for isoflurane activation of mitochondrial protective mechanisms. Ventricular cardiomyocytes were isolated from C57Bl6 wild-type (WT) and Kir6.2 KO mouse hearts. Flavoprotein autofluorescence, mitochondrial reactive oxygen species production, and mitochondrial membrane potential were monitored by laser-scanning confocal microscopy in intact cardiomyocytes. Cell survival was assessed using H2O2-induced stress. Isoflurane (0.5 mM) increased flavoprotein fluorescence to 180% ± 14% and 190% ± 15% and reactive oxygen species production to 118% ± 2% and 124% ± 6% of baseline in WT and Kir6.2 KO myocytes, respectively. Tetramethylrhodamine ethyl ester fluorescence decreased to 84% ± 6% in WT and to 86% ± 4% in Kir6.2 KO myocytes. This effect was abolished by 5HD. Pretreatment with isoflurane decreased the stress-induced cell death from 31% ± 1% to 21% ± 1% in WT and from 44% ± 2% to 35% ± 2% in Kir6.2 KO myocytes. In conclusion, Kir6.2 deletion increases the sensitivity of intact cardiomyocytes to oxidative stress, but does not alter the isoflurane-elicited protective mitochondrial mechanisms, suggesting independent roles for cardiac mitochondria and sarcKATP channels in anesthetic-induced preconditioning by isoflurane.

Pretreatment with brain natriuretic peptide reduces skeletal muscle mitochondrial dysfunction and oxidative stress after ischemia-reperfusion

Brain natriuretic peptide (BNP) reduces the extent of myocardial infarction. We aimed to determine whether BNP may reduce skeletal muscle mitochondrial dysfunctions and oxidative stress through mitochondrial K(ATP) (mK(ATP)) channel opening after ischemia-reperfusion (IR). Wistar rats were assigned to four groups: sham, 3-h leg ischemia followed by 2-h reperfusion (IR), pretreatment with BNP, and pretreatment with 5-hydroxydecanoic acid, an mK(ATP) channel blocker, before BNP. Mitochondrial respiratory chain complex activities of gastrocnemius muscles were determined using glutamate-malate (V(max)), succinate (V(succ)), and N,N,N',N'-tetramethyl-p-phenylenediamine dihydrochloride ascorbate (V(TMPD/asc)). Apoptosis (Bax-to-Bcl2 mRNA ratio and caspase-3 activity) and oxidative stress (dihydroethidium staining) were also assessed. Compared with the sham group, IR significantly decreased V(max), reflecting complex I, II, and IV activities (-36%, 3.7 ± 0.3 vs. 5.8 ± 0.2 μmol O(2)·min(-1)·g dry wt(-1), P < 0.01), and V(TMPD/asc), reflecting complex IV activity (-37%, 8.6 ± 0.8 vs. 13.7 ± 0.9 μmol O(2)·min(-1)·g dry wt(-1), P < 0.01). IR increased Bax-to-Bcl2 ratio (+57%, 1.1 ± 0.1 vs. 0.7 ± 0.1, P < 0.05) and oxidative stress (+45%, 9,067 ± 935 vs. 6,249 ± 723 pixels, P > 0.05). BNP pretreatment reduced the above alterations, increasing V(max) (+38%, P < 0.05) and reducing Bax-to-Bcl2 ratio (-55%, P < 0.01) and oxidative stress (-58%, P < 0.01). BNP protection against deleterious IR effects on skeletal muscles was abolished by 5-hydroxydecanoic acid. Caspase-3 activities did not change significantly. Conversely, BNP injected during ischemia failed to protect against muscle injury. In addition to maintaining the activity of mitochondrial respiratory chain complexes and possibly decreasing apoptosis, pretreatment with BNP protects skeletal muscle against IR-induced lesions, most likely by decreasing excessive production of radical oxygen species and opening mK(ATP) channels.

Activation of IL-11/STAT3 pathway in preconditioned human skeletal myoblasts blocks apoptotic cascade under oxidant stress

AIM

To determine whether our novel approach of diazoxide-induced stem cell preconditioning might be extrapolated to human skeletal myoblasts to support their survival under lethal oxidant stress.

METHODS & RESULTS

Using an in vitro model of H(2)O(2) treatment of human skeletal myoblasts, we report the ability of diazoxide-preconditioned human skeletal myoblasts to express cytokines and growth factors, which act in an autocrine and paracrine fashion to promote their own survival. Preconditioning of skeletal myoblasts was cytoprotective and significantly reduced their apoptotic index (p < 0.05). IL-11 gene and protein expression was significantly increased in preconditioned skeletal myoblasts. Transfection of skeletal myoblasts with IL-11-specific siRNA incurred their death under oxidant stress. The cytoprotective effect of diazoxide preconditioning was blocked by Erk1/2 inhibitor PD98059 (20-100 µM), which abrogated STAT-3 phosphorylation, thus confirming a possible involvement of Erk1/2/STAT3 signaling downstream of IL-11 in cell survival. We also investigated the time course of subcellular changes and signaling pathway of skeletal myoblasts apoptosis under oxidant stress before and after preconditioning. Apoptosis was induced in skeletal myoblasts with 100-500 µM H(2)O(2) for time points ranging from 1 to 24 h. Release of lactate dehydrogenase, disruption of the mitochondrial membrane potential and cytochrome-c translocation into cytoplasm were the earliest signs of apoptosis. Total Akt protein remained unchanged whereas marked reduction in pAkt was observed in the native skeletal myoblasts. Terminal dUTP nick end-labeling and annexin-V positivity were significantly increased after 4 h. Ultra-structure studies showed condensed chromatin, shriveled nuclei and swollen mitochondria.

CONCLUSION

These data suggest that skeletal myoblasts undergo apoptosis under oxidant stress in a time-dependent manner and preconditioning of skeletal myoblasts significantly prevented their apoptosis via IL-11/STAT3 signaling.

Diazoxide pretreatment enhances L6 skeletal myoblast survival and inhibits apoptosis induced by hydrogen peroxide

Skeletal myoblast (SKM) transplantation is a promising approach to regenerate tissue and improve the function of the injured heart. However, the number of survival cells transplanted to host myocardium is quite poor due to high rate of apoptosis; diazoxide (DZ) is a highly selective mito-KATP channel opener that may reduce cell apoptosis by relieving reactive oxygen species (ROS) damage. The aim of this study is to explore the protective effects of DZ on L6 SKM damage induced by hydrogen peroxide (H(2)O(2) ) in vitro. Different dose and time of H(2)O(2) and DZ treatment were performed and only 24 hr of 1.00 mmol/L H(2) O(2) treatment and 200 μmol/L DZ pretreatment for 30 min were used for further experiment. L6 SKMs were cultured and divided into control group (no treatment), H(2)O(2) group (24 hr of 1.00 mmol/L H(2) O(2) treatment) and DZ + H(2)O(2) group (pretreated with 200 μmol/L DZ for 30 min before 24 hr of 1.00 mmol/L H(2) O(2) treatment). Compared with control group, H(2)O(2) treatment caused cell damage, increased lactate dehydrogenase release, cell apoptosis, and bax gene expression, while reduced cell proliferation and decreased bcl-2 expression. DZ pretreatment may protect cells from damage induced by H(2)O(2) and reduce cell apoptosis by increasing bcl-2 and decreasing bax expression. DZ pretreatment may also promote cell proliferation measured by both PCNA expression and flow cytometry method. These results suggest that DZ may protect L6 SKMs from damage induced by H(2)O(2) by maintaining integrity of cell membrane, reducing apoptosis and increasing proliferation in vitro.

PKCε promotes cardiac mitochondrial and metabolic adaptation to chronic hypobaric hypoxia by GSK3β inhibition

PKCε is central to cardioprotection. Sub-proteome analysis demonstrated co-localization of activated cardiac PKCε (aPKCε) with metabolic, mitochondrial, and cardioprotective modulators like hypoxia-inducible factor 1α (HIF-1α). aPKCε relocates to the mitochondrion, inactivating glycogen synthase kinase 3β (GSK3β) to modulate glycogen metabolism, hypertrophy and HIF-1α. However, there is no established mechanistic link between PKCε, p-GSK3β and HIF1-α. Here we hypothesized that cardiac-restricted aPKCε improves mitochondrial response to hypobaric hypoxia by altered substrate fuel selection via a GSK3β/HIF-1α-dependent mechanism. aPKCε and wild-type (WT) mice were exposed to 14 days of hypobaric hypoxia (45 kPa, 11% O(2)) and cardiac metabolism, functional parameters, p-GSK3β/HIF-1α expression, mitochondrial function and ultrastructure analyzed versus normoxic controls. Mitochondrial ADP-dependent respiration, ATP production and membrane potential were attenuated in hypoxic WT but maintained in hypoxic aPKCε mitochondria (P < 0.005, n = 8). Electron microscopy revealed a hypoxia-associated increase in mitochondrial number with ultrastructural disarray in WT versus aPKCε hearts. Concordantly, left ventricular work was diminished in hypoxic WT but not aPKCε mice (glucose only perfusions). However, addition of palmitate abrogated this (P < 0.05 vs. WT). aPKCε hearts displayed increased glucose utilization at baseline and with hypoxia. In parallel, p-GSK3β and HIF1-α peptide levels were increased in hypoxic aPKCε hearts versus WT. Our study demonstrates that modest, sustained PKCε activation blunts cardiac pathophysiologic responses usually observed in response to chronic hypoxia. Moreover, we propose that preferential glucose utilization by PKCε hearts is orchestrated by a p-GSK3β/HIF-1α-mediated mechanism, playing a crucial role to sustain contractile function in response to chronic hypobaric hypoxia.

Uncoupling and antioxidant effects of ursolic acid in isolated rat heart mitochondria

Ursolic acid (1), a pentacyclic triterpene acid, is one of the major components of certain traditional medicinal plants and possesses a wide range of biological effects, such as anti-inflammatory, antioxidative, and cytotoxic activities. Furthermore, 1, when present at 1.6-5 ng/mL concentrations in commercial herbal preparations used for patients with cardiac disorders, may also exert pro-cardiac activities. There are several indirect suggestions that the cardioprotective mechanism of ursolic acid could involve the mitochondria; however the mechanism of action is still not known. Therefore, the effects of 0.4-200 ng/mL ursolic acid (1) on the functions of isolated rat heart mitochondria oxidizing either pyruvate and malate, succinate, or palmitoyl-l-carnitine plus malate were investigated. It was found that 1 induced a statistically significant uncoupling of oxidative phosphorylation. A statistically significant decrease in H₂O₂ production in the mitochondria was observed after incubation with 5 ng/mL 1. This effect was comparable to the effectiveness of the classical uncoupler carbonyl cyanide 3-chlorophenylhydrazone. Since mild mitochondrial uncoupling has been proposed as one of the mechanisms of cardioprotection, the present results indicate that ursolic acid (1) has potential use as a cardioprotective compound.

Involvement of reperfusion injury salvage kinases in preconditioning depends critically on the preconditioning stimulus

Different preconditioning stimuli can activate divergent signaling pathways. In rats, adenosine-independent pathways (triple 3-min coronary artery occlusion [3CAO3]) and adenosine-dependent pathways (one 15-min coronary artery occlusion [ICAO15]) exist, both ultimately converging at the level of the mitochondrial respiratory chain. Furthermore, while 3CAO3, 1CAO15 and exogenous adenosine (ADO) are equally cardioprotective, only 1CAO15 increases interstitial myocardial adenosine levels. Reperfusion Injury Salvage Kinase (RISK) pathway kinases have been implicated in ischemic preconditioning, but not all preconditioning stimuli activate this pathway. Consequently, we evaluated in anesthetized rats the effects of three distinctly different preconditioning stimuli (3CAO3, 1CAO15 or ADO) on infarct size (IS), signaling pathways with a special emphasis on kinases belonging to the RISK pathway (phosphatidylinositol 3-kinase-Akt-nitric oxide synthase and extracellular signal-related kinase [ERK]) and mitochondrial respiration. All three stimuli increased state-2 respiration (using succinate as complex-II substrate), thereby decreasing the respiratory control index, which was accompanied by a limitation of IS produced by a 60-min coronary artery occlusion (CAO). Nitric oxide synthase inhibition abolished the mitochondrial effects and the cardioprotection by 3CAO3, 1CAO15 or ADO. In contrast, the PI3 kinase inhibitor, wortmannin, blocked protection by 1CAO15, but did not affect protection by 3CAO3 or ADO. Western blotting confirmed that phosphorylation of Akt and ERK were increased by 1CAO15 (which was inhibited by wortmannin), but not by 3CAO3 or ADO. In conclusion, while the three cardioprotective stimuli 3CAO3, 1CAO15 and ADO afford cardioprotection via nitric oxide-mediated modulation of mitochondrial respiration, only the 1CAO15 exerts its protection via activation of kinases belonging to the RISK pathway.

TNFα protects cardiac mitochondria independently of its cell surface receptors

Our novel proposal is that TNFα exerts a direct effect on mitochondrial respiratory function in the heart, independently of its cell surface receptors. TNFα-induced cardioprotection is known to involve reactive oxygen species (ROS) and sphingolipids. We therefore further propose that this direct mitochondrial effect is mediated via ROS and sphingolipids. The protective concentration of TNFα (0.5 ng/ml) was added to isolated heart mitochondria from black 6 × 129 mice (WT) and double TNF receptor knockout mice (TNFR1&2(-/-)). Respiratory parameters and inner mitochondrial membrane potential were analyzed in the presence/absence of two antioxidants, N-acetyl-L: -cysteine or N-tert-butyl-α-(2-sulfophenyl)nitrone or two antagonists of the sphingolipid pathway, N-oleoylethanolamine (NOE) or imipramine. In WT, TNFα reduced State 3 respiration from 279.3 ± 3 to 119.3 ± 2 (nmol O₂/mg protein/min), increased proton leak from 15.7 ± 0.6% (control) to 36.6 ± 4.4%, and decreased membrane potential by 20.5 ± 3.1% compared to control groups. In TNFR1&2(-/-) mice, TNFα reduced State 3 respiration from 205.2 ± 4 to 75.7 ± 1 (p < 0.05 vs. respective control). In WT mice, both antioxidants added with TNFα restored State 3 respiration to 269.2 ± 2 and 257.6 ± 2, respectively. Imipramine and NOE also restored State 3 respiration to 248.4 ± 2 and 249.0 ± 2, respectively (p < 0.01 vs. TNFα alone). Similarly, both antioxidant and inhibitors of the sphingolipid pathway restored the proton leak to pre-TNF values. TNFα-treated mitochondria or isolated cardiac muscle fibers showed an increase in respiration after anoxia-reoxygenation, but this effect was lost in the presence of an antioxidant or NOE. Similar data were obtained in TNFR1&2(-/-) mice. TNFα exerts a protective effect on respiratory function in isolated mitochondria subjected to an anoxia-reoxygenation insult. This effect appears to be independent of its cell surface receptors, but is likely to be mediated by ROS and sphingolipids.

Ethanolamine is a novel STAT-3 dependent cardioprotective agent

Ethanolamine is a biogenic amine found naturally in the body as part of membrane lipids and as a metabolite of the cardioprotective substances, sphingosine-1-phosphate (S1P) and anandamide. In the brain, ethanolamine, formed from the breakdown of anandamide protects against ischaemic apoptosis. However, the effects of ethanolamine in the heart are unknown. Signal transducer and activator of transcription 3 (STAT-3) is a critical prosurvival factor in ischaemia/reperfusion (I/R) injury. Therefore, we investigated whether ethanolamine protects the heart via activation of STAT-3. Isolated hearts from wildtype or cardiomyocyte specific STAT-3 knockout (K/O) mice were pre-treated with ethanolamine (Etn) (0.3 mmol/L) before I/R insult. In vivo rat hearts were subjected to 30 min ischaemia/2 h reperfusion in the presence or absence of 5 mg/kg S1P and/or the FAAH inhibitor, URB597. Infarct size was measured at the end of each protocol by triphenyltetrazolium chloride staining. Pre-treatment with ethanolamine decreased infarct size in isolated mouse or rat hearts subjected to I/R but this infarct sparing effect was lost in cardiomyocyte specific STAT-3 deficient mice. Pre-treatment with ethanolamine increased nuclear phosphorylated STAT-3 [control 0.75 ± 0.08 vs. Etn 1.50 ± 0.09 arbitrary units; P < 0.05]. Our findings suggest a novel cardioprotective role for ethanolamine against I/R injury via activation of STAT-3.

Ischaemic and morphine-induced post-conditioning: impact of mK(Ca) channels

BACKGROUND

Mitochondrial calcium-sensitive potassium (mK(Ca)) channels are involved in cardiac preconditioning. In the present study, we investigated whether also ischaemic-, morphine-induced post-conditioning, or both is mediated by the activation of mK(Ca) channels in the rat heart in vitro.

METHODS

Animals were treated in compliance with institutional and national guidelines. Male Wistar rats were randomly assigned to one of seven groups (each n = 7). Control animals were not further treated. Post-conditioning was induced either by 3 × 30 s of ischaemia/reperfusion (I-PostC) or by administration of morphine (M-PostC, 1 µM) for 15 min at the onset of reperfusion. The mK(Ca)-channel inhibitor paxilline (1 µM) was given with and without post-conditioning interventions (M-PostC+Pax, I-PostC+Pax, and Pax). As a positive control, we determined whether direct activation of mK(Ca) channels with NS1619 (10 µM) induced cardiac post-conditioning (NS1619). Isolated hearts underwent 35 min ischaemia followed by 120 min reperfusion. At the end of reperfusion, infarct sizes were measured by triphenyltetrazolium chloride staining.

RESULTS

In the control group, infarct size was 53 (5)% of the area at risk. Morphine- and ischaemic post-conditioning reduced infarct size in the same range [M-PostC: 37 (4)%, I-PostC: 35 (5)%; each P<0.05 vs control]. The mK(Ca)-channel inhibitor paxilline completely blocked post-conditioning [M-PostC+Pax: 47 (7)%, I-PostC+Pax: 51 (3)%; each P<0.05 vs M-PostC and I-PostC, respectively]. Paxilline itself had no effect on infarct size (NS vs control). NS1619 reduced infarct size to 33 (4)% (P < 0.05 vs control).

CONCLUSIONS

Ischaemic- and morphine-induced post-conditioning is mediated by the activation of mK(Ca) channels.

Mitochondrial reprogramming through cardiac oxygen sensors in ischaemic heart disease

Under hypoxic conditions, mitochondria can represent a threat to the cell because of their capacity to generate toxic reactive oxygen species (ROS). However, cardiomyocytes are equipped with an oxygen-sensing pathway that involves prolyl hydroxylase oxygen sensors and hypoxia-inducible factors (HIFs), which induces a tightly regulated programme to keep ischaemic mitochondrial activity under control. The aim of this review is to provide an update on the pathways leading to mitochondrial reprogramming, which occurs in the myocardium during ischaemia, with particular emphasis on those induced by HIF activation. We start by studying the mechanisms of mitochondrial damage during ischaemia and upon reperfusion, highlighting the importance of the formation of the mitochondrial permeability transition pore during reperfusion and its consequences for cardiomyocyte survival. Next, we analyse hypoxia-induced metabolic reprogramming through HIF and its important consequences for mitochondrial bioenergetics, as well as the phenomenon known as the hibernating myocardium. Subsequently, we examine the mechanisms underlying ischaemic preconditioning, focusing, in particular, on those that involve the HIF pathway, such as adenosine signalling, sub-lethal ROS generation, and nitric oxide production. Finally, the role of the mitochondrial uncoupling proteins in ischaemia tolerance is discussed.

Morphine induces preconditioning via activation of mitochondrial K(Ca) channels

Purpose

Mitochondrial calcium sensitive potassium (mK_Ca) channels are involved in cardioprotection induced by ischemic preconditioning. In the present study we investigated whether morphine-induced preconditioning also involves activation of mK_Ca channels.

Methods

Isolated rat hearts (six groups; each n = 8) underwent global ischemia for 30 min followed by a 60-min reperfusion. Control animals were not further treated. Morphine preconditioning (MPC) was initiated by two five-minute cycles of morphine 1 μM infusion with one five-minute washout and one final ten-minute washout period before ischemia. The mK_Ca blocker, paxilline 1 μM, was administered, with and without morphine administration (MPC + Pax and Pax). As a positive control, we added an ischemic preconditioning group (IPC) alone and combined with paxilline (IPC + Pax). At the end of reperfusion, infarct sizes were determined by triphenyltetrazoliumchloride staining.

Results

Infarct size was (mean ± SD) 45 ± 9% of the area at risk in the Control group. The infarct size was less in the morphine or ischemic preconditioning groups (MPC: 23 ± 8%, IPC: 20 ± 5%; each P < 0.05 vs Control). Infarct size reduction was abolished by paxilline (MPC + Pax: 37 ± 7%, P < 0.05 vs MPC and IPC + Pax: 36 ± 6%, P < 0.05 vs IPC), whereas paxilline alone had no effect (Pax: 46 ± 7%, not significantly different from Control).

Conclusion

Cardioprotection by morphine-induced preconditioning is mediated by activation of mK_Ca channels.

Postconditioning inhibits mPTP opening independent of oxidative phosphorylation and membrane potential

Mitochondrial permeability transition pore (mPTP) inhibition plays a relevant role in postconditioning (PostC). Ischemia damages the electron transport chain, and the potential contribution of additional modifications in mitochondrial function caused by PostC remains unknown. We sought to determine which mitochondrial functions are involved in the inhibition of mPTP opening during the first minutes of reperfusion. Anesthetized New Zealand White rabbits underwent 30-min ischemia followed by 10-min reperfusion. At reperfusion, they received either no intervention (Control, C), PostC with 4 cycles of 1-min ischemia followed by 1-min reperfusion, or an IV injection of 5 mg/kg cyclosporine A (CsA: a powerful inhibitor of mPTP opening). Sham rabbits underwent no ischemia throughout the 40-min experiment. At the end of the 10-min reperfusion, mitochondria were isolated from the area at risk by differential centrifugations. Calcium retention capacity (CRC) and mitochondrial membrane potential (DeltaPsi(m)) were assessed by fluorimetry in subsarcolemmal (SSM) and interfibrillar (IFM) mitochondria. Oxidative phosphorylation was assessed using a Clark-type electrode, and oxidative stress via protein carbonylation by Western blotting. PostC and CsA treatments improved CRC when compared to the C group. Control, PostC and CsA mitochondria exhibited a comparable significant dissipation of DeltaPsi(m), together with a comparable significant decrease in state 3 and an increase in state 4 respiration, in both SSM and IFM. However, PostC but not CsA treatment reduced total heart oxidative stress. These data suggest that during the early minutes of reperfusion, PostC reduces oxidative stress and inhibits mPTP opening, independent of alteration of oxidative phosphorylation or of DeltaPsi(m).

Myasthenia gravis sera have no effect on cardiomyocytes in vitro

Myasthenia gravis (MG) is an autoimmune disorder primarily caused by circulating autoantibodies targeting the nicotinic acetylcholine receptor. Several studies have suggested a link between MG and heart disease. Girardi heart cells were treated with MG sera, measuring cytotoxic effects using flow cytometry, adenylate kinase (AK) release and evaluating morphology. MG sera did not induce morphological changes in the cells. AK release from cells treated with MG sera did not exceed controls and flow cytometric examination did not reveal any increase in dead or apoptotic cells. We conclude that MG sera have no cytotoxic effect in our heart cell culture system.

A novel potassium channel in skeletal muscle mitochondria

In this work we provide evidence for the potential presence of a potassium channel in skeletal muscle mitochondria. In isolated rat skeletal muscle mitochondria, Ca(2+) was able to depolarize the mitochondrial inner membrane and stimulate respiration in a strictly potassium-dependent manner. These potassium-specific effects of Ca(2+) were completely abolished by 200 nM charybdotoxin or 50 nM iberiotoxin, which are well-known inhibitors of large conductance, calcium-activated potassium channels (BK(Ca) channel). Furthermore, NS1619, a BK(Ca)-channel opener, mimicked the potassium-specific effects of calcium on respiration and mitochondrial membrane potential. In agreement with these functional data, light and electron microscopy, planar lipid bilayer reconstruction and immunological studies identified the BK(Ca) channel to be preferentially located in the inner mitochondrial membrane of rat skeletal muscle fibers. We propose that activation of mitochondrial K(+) transport by opening of the BK(Ca) channel may be important for myoprotection since the channel opener NS1619 protected the myoblast cell line C2C12 against oxidative injury.

Sustained endothelial progenitor cell dysfunction after chronic hypoxia-induced pulmonary hypertension

Circulating endothelial progenitor cells (EPCs) contribute to neovascularization of ischemic tissues and repair of injured endothelium. The role of bone marrow-derived progenitor cells in hypoxia-induced pulmonary vascular remodeling and their tissue-engineering potential in pulmonary hypertension (PH) remain largely unknown. We studied endogenous mobilization and homing of EPCs in green fluorescent protein bone marrow chimeric mice exposed to chronic hypoxia, a common hallmark of PH. Despite increased peripheral mobilization, as shown by flow cytometry and EPC culture, bone marrow-derived endothelial cell recruitment in remodeling lung vessels was limited. Moreover, transfer of vascular endothelial growth factor receptor-2+/Sca-1+/CXCR-4+-cultured early-outgrowth EPCs failed to reverse PH, suggesting hypoxia-induced functional impairment of transferred EPCs. Chronic hypoxia decreased migration to stromal cell-derived factor-1alpha, adhesion to fibronectin, incorporation into a vascular network, and nitric oxide production (-41%, -29%, -30%, and -32%, respectively, vs. normoxic EPCs; p < .05 for all). The dysfunctional phenotype of hypoxic EPCs significantly impaired their neovascularization capacity in chronic hind limb ischemia, contrary to normoxic EPCs cultured in identical conditions. Mechanisms contributing to EPC dysfunction include reduced integrin alphav and beta1 expression, decreased mitochondrial membrane potential, and enhanced senescence. Novel insights from chronic hypoxia-induced EPC dysfunction may provide important cues for improved future cell repair strategies.

Ischemic preconditioning modulates mitochondrial respiration, irrespective of the employed signal transduction pathway

We tested in the in vivo rat heart the hypothesis that although ischemic preconditioning can employ different signal transduction pathways, these pathways converge ultimately at the level of the mitochondrial respiratory chain. Infarct size produced by a 60-min coronary artery occlusion (69%+/-2% of the area at risk) was limited by a preceding 15-min coronary occlusion (48%+/-4%). Cardioprotection by this stimulus was triggered by adenosine receptor stimulation, which was followed by protein kinase C and tyrosine kinase activation and then mitochondrial K(+)(ATP)-channel opening. In contrast, cardioprotection by 3 cycles of 3-min coronary occlusions (infarct size 27%+/-5% of the area at risk) involved the release of reactive oxygen species, which was followed by protein kinase C and tyrosine kinase activation, but was independent of adenosine receptor stimulation and K(+)(ATP)-channel activation. However, both pathways decreased respiratory control index (RCI; state-3/state-2, using succinate as complex-II substrate) from 3.1+/-0.2 in mitochondria from sham-treated hearts to 2.4+/-0.2 and 2.5+/-0.1 in hearts subjected to a single 15-min and triple 3-min coronary occlusions, respectively (both P<0.05). The decreases in RCI were due to an increase in state-2 respiration, whereas state-3 respiration was unchanged. Abolition of cardioprotection by blockade of either signal transduction pathway was paralleled by a concomitant abolition of mitochondrial uncoupling. These observations are consistent with the concept that mild mitochondrial uncoupling contributes to infarct size limitation by various ischemic preconditioning stimuli, despite using different signal transduction pathways. In conclusion, in the in vivo rat heart, different ischemic preconditioning (IPC) stimuli can activate highly different signal transduction pathways, which seem to converge at the level of the mitochondria where they increase state-2 respiration.

The regulation of mitochondrial respiration by opening of mKCa channels is age-dependent

The protective potency of ischemic preconditioning decreases with increasing age. A key step in ischemic preconditioning is the opening of mitochondrial Ca(2+) sensitive K(+) (mK(Ca)) channels, which causes mild uncoupling of mitochondrial respiration. We hypothesized that aging reduces the effects of mK(Ca) channel opening on mitochondrial respiration. We measured the effects of mK(Ca) channel opener NS1619 (30 microM) on mitochondrial respiration in isolated heart mitochondria from young (2-3 months) and old (22-26 months) Wistar rats. Oxygen consumption was monitored online after addition of 250 microM ADP (state 3 respiration), and after complete phosphorylation of ADP to ATP (state 4 respiration) in the presence or absence of the mK(Ca) channel blocker paxilline (5 microM). The respiratory control index (RCI) was calculated as state 3/state 4. In mitochondria from young rats, NS1619 increased state 4 respiration by 11.9+/-4.1% (mean+/-S.E.M.), decreased state 3 respiration by 7.6+/-2.5%, and reduced the RCI from 2.6+/-0.03 (control) to 2.1+/-0.06 (all P<0.05, n=12 for all groups). Paxilline blocked the effect of NS1619 on state 4 respiration (0.7+/-2.8%), but did not affect the decrease in state 3 respiration; paxilline blunted the decrease of RCI. In mitochondria from old rats, NS1619 had neither effect on state 4 (0.4+/-1.6%), and state 3 respiration (-7.4+/-1.5%), nor on RCI (3.0+/-0.13 vs. 3.2+/-0.11, n=12). Increasing age reduced the effects of mK(Ca) opening on mitochondrial respiration. This might be one underlying reason of the decreased protective potency of ischemic preconditioning in the aged myocardium.

Increased mitochondrial mass in cells with functionally compromised mitochondria after exposure to both direct gamma radiation and bystander factors

The bystander effect describes radiation-like damage in unirradiated cells either in the vicinity of irradiated cells or exposed to medium from irradiated cells. This study aimed to further characterize the poorly understood mitochondrial response to both direct irradiation and bystander factor(s) in human keratinocytes (HPV-G) and Chinese hamster ovarian cells (CHO-K1). Oxygen consumption rates were determined during periods of state 4, state 3 and uncoupled respiration. Mitochondrial mass was determined using MitoTracker FM. CHO-K1 cells showed significantly reduced oxygen consumption rates 4 h after exposure to 5 Gy direct radiation and irradiated cell conditioned medium (ICCM) and an apparent recovery 12-24 h later. The apparent recovery was likely due to the substantial increase in mitochondrial mass observed in these cells as soon as 4 h after exposure. HPV-G cells, on the other hand, showed a sustained increase in oxygen consumption rates after ICCM exposure and a transient increase 4 h after exposure to 5 Gy direct radiation. A significant increase in mitochondrial mass per HPV-G cell was observed after exposure to both direct radiation and ICCM. These findings are indicative of a stress response to mitochondrial dysfunction that increases the number of mitochondria per cell.

Ischemic preconditioning does not protect via blockade of electron transport

Ischemic preconditioning (IPC) before sustained ischemia decreases myocardial infarct size mediated in part via protection of cardiac mitochondria. Reversible blockade of electron transport at complex I immediately before sustained ischemia also preserves mitochondrial respiration and decreases infarct size. We proposed that IPC would attenuate electron transport from complex I as a potential effector mechanism of cardioprotection. Isolated, Langendorff-perfused rat hearts underwent IPC (3 cycles of 5-min 37°C global ischemia and 5-min reperfusion) or were perfused for 40 min without ischemia as controls. Subsarcolemmal (SSM) and interfibrillar (IFM) populations of mitochondria were isolated. IPC did not decrease ADP-stimulated respiration measured in intact mitochondria using substrates that donate reducing equivalents to complex I. Maximally expressed complex I activity measured as rotenone-sensitive NADH:ubiquinone oxidoreductase in detergent-solubilized mitochondria was also unaffected by IPC. Thus the protection of IPC does not occur as a consequence of a partial decrease in complex I activity leading to a decrease in integrated respiration through complex I. IPC and blockade of electron transport both converge on mitochondria as effectors of cardioprotection; however, each modulates mitochondrial metabolism during ischemia by different mechanisms to achieve cardioprotection.

Downmodulation of mitochondrial F0F1 ATP synthase by diazoxide in cardiac myoblasts: a dual effect of the drug

Similar to ischemic preconditioning, diazoxide was documented to elicit beneficial bioenergetic consequences linked to cardioprotection. Inhibition of ATPase activity of mitochondrial F(0)F(1) ATP synthase may have a role in such effect and may involve the natural inhibitor protein IF(1). We recently documented, using purified enzyme and isolated mitochondrial membranes from beef heart, that diazoxide interacts with the F(1) sector of F(0)F(1) ATP synthase by promoting IF(1) binding and reversibly inhibiting ATP hydrolysis. Here we investigated the effects of diazoxide on the enzyme in cultured myoblasts. Specifically, embryonic heart-derived H9c2 cells were exposed to diazoxide and mitochondrial ATPase was assayed in conditions maintaining steady-state IF(1) binding (basal ATPase activity) or detaching bound IF(1) at alkaline pH. Mitochondrial transmembrane potential and uncoupling were also investigated, as well as ATP synthesis flux and ATP content. Diazoxide at a cardioprotective concentration (40 muM cell-associated concentration) transiently downmodulated basal ATPase activity, concomitant with mild mitochondria uncoupling and depolarization, without affecting ATP synthesis and ATP content. Alkaline stripping of IF(1) from F(0)F(1) ATP synthase was less in diazoxide-treated than in untreated cells. Pretreatment with glibenclamide prevented, together with mitochondria depolarization, inhibition of ATPase activity under basal but not under IF(1)-stripping conditions, indicating that diazoxide alters alkaline IF(1) release. Diazoxide inhibition of ATPase activity in IF(1)-stripping conditions was observed even when mitochondrial transmembrane potential was reduced by FCCP. The results suggest that diazoxide in a model of normoxic intact cells directly promotes binding of inhibitor protein IF(1) to F(0)F(1) ATP synthase and enhances IF(1) binding indirectly by mildly uncoupling and depolarizing mitochondria.

AMP-activated protein kinase mediates preconditioning in cardiomyocytes by regulating activity and trafficking of sarcolemmal ATP-sensitive K(+) channels

Brief periods of ischemia and reperfusion that precede sustained ischemia lead to a reduction in myocardial infarct size. This phenomenon, known as ischemic preconditioning, is mediated by signaling pathway(s) that is complex and yet to be fully defined. AMP-activated kinase (AMPK) is activated in cells under conditions associated with ATP depletion and increased AMP/ATP ratio. In the present study, we have taken advantage of a cardiac phenotype overexpressing a dominant negative form of the alpha2 subunit of AMPK to analyze the role, if any, that AMPK plays in preconditioning the heart. We have found that myocardial preconditioning activates AMPK in wild type, but not transgenic mice. Cardiac cells from transgenic mice could not be preconditioned, as opposed to cells from the wild type. The cytoprotective effect of AMPK was not related to the effect that preconditioning has on mitochondrial membrane potential as revealed by JC-1, a mitochondrial membrane potential-sensitive dye, and laser confocal microscopy. In contrast, experiments with di-8-ANEPPS, a sarcolemmal-potential sensitive dye, has demonstrated that intact AMPK activity is required for preconditioning-induced shortening of the action membrane potential. The preconditioning-induced activation of sarcolemmal K(ATP) channels was observed in wild type, but not in transgenic mice. HMR 1098, a selective inhibitor of sarcolemmal K(ATP) channels opening, inhibited preconditioning-induced shortening of action membrane potential as well as cardioprotection afforded by AMPK. Immunoprecipitation followed by Western blotting has shown that AMPK is essential for preconditioning-induced recruitment of sarcolemmal K(ATP) channels. Based on the obtained results, we conclude that AMPK mediates preconditioning in cardiac cells by regulating the activity and recruitment of sarcolemmal K(ATP) channels without being a part of signaling pathway that regulates mitochondrial membrane potential.

Diazoxide-induced respiratory inhibition - a putative mitochondrial K(ATP) channel independent mechanism of pharmacological preconditioning

The ischemic preconditioning biological phenomenon has been explored to identify putative pharmacologic agents to mimic this cytoprotective program against cellular ischemic injury. Diazoxide administration confers this cytoprotection, however, whether this is via direct activation of the putative mitochondrial K(ATP) (mK(ATP)) channel which was originally proposed has been questioned. Here, we present data supporting an alternate hypothesis evoking mitochondrial respiratory inhibition rather than mK(ATP) channel activation, as a mediating event in the diazoxide-activated cytoprotective program. Mitochondrial respiration and reactive oxygen species (ROS) production was measured in digitonin-permeabilized C2C12 myotubes, allowing for the modulation of mK(ATP) conductance by changing the potassium concentration of the medium (0-130 mM). Diazoxide dose-dependently attenuated succinate-supported respiration, an effect that was independent of mK(ATP) channel conductance. Similarly, 5-hydroxydecanoate (5-HD), a putative mK(ATP) channel blocker, released diazoxide-induced respiratory inhibition independently of potassium concentration. Since diazoxide-induced cytoprotection and respiratory inhibition are both integrally linked to ROS generation we repeated above experiments following ROS generation using DCF fluorescence. Cytoprotective doses of diazoxide increased ROS generation independently of potassium concentration and 5-HD inhibited ROS production under the same conditions. Collectively these data support the hypothesis that diazoxide-mediated cytoprotection is independent of the conductance of the mK(ATP) channel and rather implicate mitochondrial respiratory inhibition-triggered ROS signaling.

TNFα-induced cytoprotection requires the production of free radicals within mitochondria in C2C12 myotubes

We previously reported that tumour necrosis factor alpha (TNFalpha) can mimic classic ischemic preconditioning (IPC) in both cells and heart. However, the signalling pathways involved remain incompletely understood. One potential protective pathway could be TNFalpha-induced reactive oxygen species (ROS). We hypothesized that TNFalpha cytoprotection occurs through the generation of ROS which originate within the mitochondria. C(2)C(12) myotubes were preconditioned with either a short period of hypoxia (IPC) or a low concentration of TNFalpha (0.5 ng/ml) prior to a simulated ischemic insult. ROS generation was evaluated on cells stained with dichlorofluorescin diacetate (DCFH-DA) by flow cytometry. The source of TNFalpha-induced ROS was examined with Mitotracker Red CM-H(2)XRos. The bioenergetics of the mitochondria were evaluated by investigation of the respiratory parameters and the inner mitochondrial membrane potential. Pretreatment with TNFalpha improved cell viability compared with the simulated ischemic control (TNFalpha: 75 +/- 1% versus 34 +/- 1% for the control: p<0.001). The ROS scavenger, N-2-mercaptopropionyl-glycine (MPG), reduced the viability of TNFalpha-stimulated cells to 15 +/- 1% (p<0.001 versus TNFalpha). Similar results were obtained with IPC. TNFalpha stimulation increased ROS production mainly in the mitochondria, and this increase was abolished in the presence of MPG. Addition of TNFalpha to the cells increased State 2 respiration and modestly depolarised the membrane potential prior to the ischemic insult. In conclusion, TNFalpha-induced ROS generation can occur within the mitochondria, resulting in temporal mitochondrial perturbations which may initiate the cytoprotective effect of TNFalpha.

3'Phosphoinositide-dependent kinase-1 is essential for ischemic preconditioning of the myocardium

Brief periods of ischemia and reperfusion that precede sustained ischemia lead to a reduction in myocardial infarct size. This phenomenon, known as ischemic preconditioning, is mediated by signaling pathway(s) that are yet to be fully defined. 3'-Phosphoinositide-dependent kinase-1 (PDK1) has been implicated in numerous cellular processes. However, the involvement of PDK1 in preconditioning has yet to be elucidated. Studying PDK1 is not as straightforward as it is for the majority of kinases, due to the lack of a specific inhibitor of PDK1. Therefore, we have taken advantage of PDK1 hypomorphic mutant mice with reduced expression of PDK1 to study the role of PDK1 in preconditioning. Whole heart and single cell models of preconditioning demonstrated that the hearts and cardiac cells from PDK1 hypomorphic mice could not be preconditioned. The cardioprotective effect of PDK1 was not related to the effect that preconditioning has on sarcolemmal membrane action potential as revealed by di-8-ANEPPS, a sarcolemmal-potential sensitive dye, and laser confocal microscopy. In contrast, experiments with JC-1, a mitochondrial membrane potential-sensitive dye, has demonstrated that intact PDK1 levels were required for preconditioning-mediated regulation of mitochondrial membrane potential. Western blotting combined with functional experiments have shown that intact PDK1 levels were required for preconditioning-induced phosphorylation of protein kinase B (PKB), glycogen synthase kinase-3beta (GSK-3beta), and cardioprotection. We conclude that PDK1 mediates preconditioning in the heart by regulating activating PKB-GSK-3beta to regulate mitochondrial but not sarcolemmal membrane potential. 3'Phosphoinositide-dependent kinase-1 (PDK1) is essential for ischemic preconditioning of the myocardium.

Diazoxide-induced cardioprotection via ΔΨm loss depending on timing of application

Although the role of mitochondrial ATP-sensitive potassium (mitoKATP) channels in cardioprotection is widely accepted, it remains unclear when their opening is critical for protection. We tested the hypothesis that the mitoKATP channel acts as a trigger or mediator of protection against apoptosis through loss of mitochondrial inner membrane potential (DeltaPsim). Exposure of neonatal rat cardiomyocytes to H2O2 (0.5 mmol/L) resulted in apoptosis associated with severe DeltaPsim loss. Pretreatment with diazoxide (20 to 100 micromol/L) prevented H2O2-induced apoptosis and DeltaPsim loss at 2 but not 18 h after exposure, while the latter was prevented by cotreatment with diazoxide. Lack of protection by pretreatment with diazoxide was observed in cardiomyocytes cultured in a medium containing H2O2 for 2 h and then not containing for 16 h. The slopes of the regression lines of the relationship between the proportion of apoptotic cells and DeltaPsim loss (y = -0.89 vs. -0.42) and the proportion of cells with high side scatter signal differed between cardiomyocytes exposed H2O2 for 2 and 18 h. Diazoxide per se caused a transient DeltaPsim loss (within 30 min) with a recovery followed by persistent DeltaPsim loss (after 6 h). Inhibition of the former by 5-hydroxydecanoate (5-HD, 0.5 mmol/L) abolished protection of pretreatment with diazoxide (trigger phase), while that of the latter prevented the protection of cotreatment with diazoxide (mediator phase). Our results suggest that mitoKATP channels act as a trigger and mediator of cardioprotection through a transient or persistent DeltaPsim loss depending on phenotypic consequence in response to oxidants.

Mitochondrial Ca2+-induced K+ influx increases respiration and enhances ROS production while maintaining membrane potential

We recently demonstrated a role for altered mitochondrial bioenergetics and reactive oxygen species (ROS) production in mitochondrial Ca(2+)-sensitive K(+) (mtK(Ca)) channel opening-induced preconditioning in isolated hearts. However, the underlying mitochondrial mechanism by which mtK(Ca) channel opening causes ROS production to trigger preconditioning is unknown. We hypothesized that submaximal mitochondrial K(+) influx causes ROS production as a result of enhanced electron flow at a fully charged membrane potential (DeltaPsi(m)). To test this hypothesis, we measured effects of NS-1619, a putative mtK(Ca) channel opener, and valinomycin, a K(+) ionophore, on mitochondrial respiration, DeltaPsi(m), and ROS generation in guinea pig heart mitochondria. NS-1619 (30 microM) increased state 2 and 4 respiration by 5.2 +/- 0.9 and 7.3 +/- 0.9 nmol O(2).min(-1).mg protein(-1), respectively, with the NADH-linked substrate pyruvate and by 7.5 +/- 1.4 and 11.6 +/- 2.9 nmol O(2).min(-1).mg protein(-1), respectively, with the FADH(2)-linked substrate succinate (+ rotenone); these effects were abolished by the mtK(Ca) channel blocker paxilline. DeltaPsi(m) was not decreased by 10-30 microM NS-1619 with either substrate, but H(2)O(2) release was increased by 44.8% (65.9 +/- 2.7% by 30 muM NS-1619 vs. 21.1 +/- 3.8% for time controls) with succinate + rotenone. In contrast, NS-1619 did not increase H(2)O(2) release with pyruvate. Similar results were found for lower concentrations of valinomycin. The increase in ROS production in succinate + rotenone-supported mitochondria resulted from a fully maintained DeltaPsi(m), despite increased respiration, a condition that is capable of allowing increased electron leak. We propose that mild matrix K(+) influx during states 2 and 4 increases mitochondrial respiration while maintaining DeltaPsi(m); this allows singlet electron uptake by O(2) and ROS generation.

Ceramide attenuates hypoxic cell death via reactive oxygen species signaling

We have previously demonstrated that tumor necrosis factor alpha (TNFalpha), a cytokine known to be induced by ischemia, independently promotes preconditioning in part via ceramide generation. As reactive oxygen species (ROS) signaling is evoked by ischemic preconditioning, by TNFalpha and by ceramide we reasoned that ceramide-induced preconditioning is ROS-mediated. Fibroblastic L-cells were subjected to 8 hours simulated ischemia and were preconditioned by pretreatment with cell permeable c2 ceramide (1 microM) with or without the antioxidant N-mercaptopropionyl glycine (MPG; 1 mM). Pretreatment with ceramide reduced lactate dehydrogenase release at the end of the simulated ischemia but this cytoprotective effect was lost in the presence of MPG. Concurrent temporal ROS generation was measured using confocal microscopy on cells stained with dichlorofluorescein diacetate (DCF-DA). Ceramide increased ROS production after 30 minutes and this induction was decreased by MPG. Incubation of ceramide with cyclooxygenase-2 inhibitor, NS 398 (10 microM), or with a mitochondrial respiratory chain inhibitor, rotenone (10 microM) reduced the cytoprotective effect of ceramide in parallel with a partial diminution in ROS generation. In contrast, inhibition of other ROS-producing systems including nitric oxide synthase, xanthine oxidase, or NADPH oxidase failed to modulate ceramide-induced cytoprotection. Collectively, these data demonstrate that ceramide induces a cell survival program through ROS signaling activated, in part, via cyclooxygenase and the mitochondrial respiratory chain.

Different mechanisms of mitochondrial proton leak in ischaemia/reperfusion injury and preconditioning: implications for pathology and cardioprotection

The mechanisms of mitochondrial proton (H+) leak under various pathophysiological conditions are poorly understood. In the present study it was hypothesized that different mechanisms underlie H+ leak in cardiac IR (ischaemia/reperfusion) injury and IPC (ischaemic preconditioning). Potential H(+) leak mechanisms examined were UCPs (uncoupling proteins), allosteric activation of the ANT (adenine nucleotide translocase) by AMP, or the PT (permeability transition) pore. Mitochondria isolated from perfused rat hearts that were subjected to IPC exhibited a greater H+ leak than did controls (202+/-27%, P<0.005), and this increased leakage was completely abolished by the UCP inhibitor, GDP, or the ANT inhibitor, CAT (carboxyattractyloside). Mitochondria from hearts subjected to IR injury exhibited a much greater amount of H+ leak than did controls (411+/-28%, P<0.001). The increased leakage after IR was weakly inhibited by GDP, but was inhibited, >50%, by carboxyattractyloside. In addition, it was inhibited by cardioprotective treatment strategies including pre-IR perfusion with the PT pore inhibitors cyclosporin A or sanglifehrin A, the adenylate kinase inhibitor, AP5A (diadenosine pentaphosphate), or IPC. Together these data suggest that the small increase in H+ leak in IPC is mediated by UCPs, while the large increase in H+ leak in IR is mediated by the ANT. Furthermore, under all conditions studied, in situ myocardial O2 efficiency was correlated with isolated mitochondrial H+ leak (r2=0.71). In conclusion, these data suggest that the modulation of H+ leak may have important implications for the outcome of IR injury.

Bcl-2 regulation of mitochondrial energetics

Recent data suggest that in addition to regulating apoptosis, Bcl-2 (an anti-apoptotic protein overexpressed in B-cell lymphoma) and Bcl-2 family members also regulate mitochondrial and cell physiology. t-Bid, a Bcl-2 family member, has been shown to modulate reorganization of mitochondrial cristae. Bcl-2 appears to regulate voltage-dependent anion channel permeability, which has important consequences for mitochondrial transport of adenine nucleotides, Ca(2+), and other metabolites. BAD, a pro-apoptotic Bcl-2 family member, is required for the binding of glucokinase to a mitochondrial complex, and BAD null mice have altered glucose homeostasis. It has been suggested that Bcl-2 family members may regulate important mitochondrial/cell functions and serve as sentinels to detect abnormalities in these pathways and, when the abnormalities are severe enough, to initiate or facilitate cell death. Understanding the physiologic processes controlled by Bcl-2 will be important in understanding cell regulation, and it may also provide new insights into the regulation of apoptosis.

Mitochondrial dysfunction in cardiac ischemia–reperfusion injury: ROS from complex I, without inhibition

A key pathologic event in cardiac ischemia reperfusion (I-R) injury is mitochondrial energetic dysfunction, and several studies have attributed this to complex I (CxI) inhibition. In isolated perfused rat hearts, following I-R, we found that CxI-linked respiration was inhibited, but isolated CxI enzymatic activity was not. Using the mitochondrial thiol probe iodobutyl-triphenylphosphonium in conjunction with proteomic tools, thiol modifications were identified in several subunits of the matrix-facing 1alpha sub-complex of CxI. These thiol modifications were accompanied by enhanced ROS generation from CxI, but not complex III. Implications for the pathology of cardiac I-R injury are discussed.

Biochemical dysfunction in heart mitochondria exposed to ischaemia and reperfusion

Heart tissue is remarkably sensitive to oxygen deprivation. Although heart cells, like those of most tissues, rapidly adapt to anoxic conditions, relatively short periods of ischaemia and subsequent reperfusion lead to extensive tissue death during cardiac infarction. Heart tissue is not readily regenerated, and permanent heart damage is the result. Although mitochondria maintain normal heart function by providing virtually all of the heart's ATP, they are also implicated in the development of ischaemic damage. While mitochondria do provide some mechanisms that protect against ischaemic damage (such as an endogenous inhibitor of the F1Fo-ATPase and antioxidant enzymes), they also possess a range of elements that exacerbate it, including ROS (reactive oxygen species) generators, the mitochondrial permeability transition pore, and their ability to release apoptotic factors. This review considers the process of ischaemic damage from a mitochondrial viewpoint. It considers ischaemic changes in the inner membrane complexes I-V, and how this might affect formation of ROS and high-energy phosphate production/degradation. We discuss the contribution of various mitochondrial cation channels to ionic imbalances which seem to be a major cause of reperfusion injury. The different roles of the H+, Ca2+ and the various K+ channel transporters are considered, particularly the K+(ATP) (ATP-dependent K+) channels. A possible role for the mitochondrial permeability transition pore in ischaemic damage is assessed. Finally, we summarize the metabolic and pharmacological interventions that have been used to alleviate the effects of ischaemic injury, highlighting the value of these or related interventions in possible therapeutics.

Uncoupling proteins 2 and 3 function in concert to augment tolerance to cardiac ischemia

Transient cardiac ischemia activates cell survival signaling, conferring subsequent ischemia tolerance to the heart. This biological phenomenon, termed ischemic preconditioning, results in improved clinical outcome and attenuated infarct size following myocardial infarction. To explore genomic modifications underpinning this ischemia tolerance, we delineated the regulation and function of the cardiac enriched mitochondrial uncoupling proteins 2 and 3 during delayed ischemic preconditioning in the rat. Cardiac transcripts of genes encoding uncoupling proteins 2 and 3 are up-regulated in parallel with infarct size reduction in preconditioned hearts. Mitochondria isolated from preconditioned hearts exhibit an augmented inducible proton leak. In parallel, following anoxia-reoxygenation these mitochondria generate less hydrogen peroxide compared with non-preconditioned mitochondria. Preconditioning in rat cardiac derived myoblasts is abolished following uncoupling protein-2 depletion by RNA-interference. RNAi of uncoupling protein-3 partially attenuates the capacity to precondition these cells. Functional characterization of anoxia and reoxygenation tolerance following uncoupling protein 2 or 3 and combined 2 and 3 RNAi shows the largest reduction in viability follows depletion of both homologues. Uncoupling protein-2 depletion alone significantly attenuates anoxia-reoxygenation tolerance but uncoupling protein-3 depletion does not reduce anoxia tolerance. In parallel combined uncoupling protein depletion and isolated uncoupling protein-2 depletion augments ROS production in viable cardiomyocytes following anoxia-reoxygenation. Concurrent anti-oxidant administration ameliorates the uncoupling protein-depleted anoxia-susceptible phenotype. In conclusion, mitochondrial uncoupling proteins are necessary components of ischemia tolerance and function as components of the cellular antioxidant defense program. In the cytoprotective hierarchy, uncoupling protein-2 appears to play a greater role than uncoupling protein-3 in modulating ischemia/anoxia tolerance in heart-derived cells.

Preconditioning improves postischemic mitochondrial function and diminishes oxidation of mitochondrial proteins

This study examines the hypothesis that ischemic or pharmacologic preconditioning improves postischemic mitochondrial function by attenuating oxidation of mitochondrial proteins. Isolated rat hearts were perfused for 38 min preischemia, followed by 25 min global ischemia and then 60 min reperfusion. Hearts were preconditioned by two episodes of 3 min global ischemia, followed by 2 min of reflow (IP), or by perfusion with 50 micromol/l nicorandil (Nic) for 10 min, followed by 10 min washout. IP and Nic significantly (p <.05) improved postischemic function, which was abolished by bracketing the protocols with 200 micromol/l 5-hydroxydecoanate (5HD) or 300 micromol/l alpha-mercaptopropionylglycine (MPG). After isolation of cardiac mitochondria, the respiratory control index (RCI) was calculated from State 3 and State 4 respiration. Both IP and Nic significantly (p <.05) improved postischemic RCI, which was depressed 71% from preischemic values in control hearts. The protective effects of IP and Nic were partially abolished by bracketing with 5HD or MPG. Furthermore, mitochondria from ischemic hearts had significantly (p <.05) less ability to resist swelling on Ca2+ loading, which was improved by both IP and Nic. By use of an immunoblot technique, carbonyl content of multiple bands of mitochondrial proteins was observed to be elevated after 25 min ischemia, and still elevated by the end of 60 min reperfusion. Both IP and Nic attenuated the increased protein oxidation observed at the end of ischemia. The protective effect of IP was almost completely abolished by MPG and partially by 5HD, which also partially abolished the protective effect of Nic. These studies support the conclusion that one mechanism for enhanced postischemic function in the preconditioned heart is improved mitochondrial function as a result of decreased oxidation of mitochondrial proteins.

p70s6 kinase is a functional target of insulin activated Akt cell-survival signaling

Insulin administration attenuates cardiac ischemia-reperfusion apoptosis via activation of Akt-mediated cell-survival signaling. As p70s6 kinase is a cognate Akt-mediated phosphorylation target we evaluated whether p70s6 kinase activation is a functional requirement in insulin-mediated cell survival program during post-ischemic reoxygenation. Human cardiac-derived girardi cells were subjected to 6h of simulated ischemia and 2h of reoxygenation+/-insulin treatment [0.3mU/ml]. Concurrently, cells were pre-treated with anti-sense oligodeoxynucleotides (ODNs) corresponding to the initiation start-site of human p70s6 kinase mRNA. Sense ODN and scrambled ODN were used as controls. Cell viability was measured using lactate dehydrogenase (LDH) release and propidium iodide (PI) exclusion. Insulin at reoxygenation enhanced cell viability with attenuated LDH release (>or=50% , p<0.001 vs. ischemic controls) and reduced PI uptake by >or=30% vs. ischemic controls. The protection afforded by insulin was abolished by anti-sense ODN targeting p70s6 kinase, but not by the sense or scrambled ODNs. In parallel, insulin administration at reoxygenation significantly increased p70s6 kinase levels and activity compared with controls. P70s6 kinase activity was abolished by pre-treatment with anti-sense ODNs. Collectively, these data demonstrate that p70s6 kinase activation is a functional target of Akt following insulin-activated cytoprotection during ischemia-reoxygenation-induced injury.

Diazoxide triggers cardioprotection against apoptosis induced by oxidative stress

Although mitochondrial ATP-sensitive potassium (mitoK(ATP)) channels have been reported to reduce the extent of apoptosis, the critical timing of mitoK(ATP) channel opening required to protect myocytes against apoptosis remains unclear. In the present study, we examined whether the mitoK(ATP) channel serves as a trigger of cardioprotection against apoptosis induced by oxidative stress. Apoptosis of cultured neonatal rat cardiomyocytes was determined by flow cytometry (light scatter and propidium iodide/annexin V-FITC fluorescence) and by nuclear staining with Hoechst 33342. Mitochondrial membrane potential (DeltaPsi) was measured by flow cytometry of cells stained with rhodamine-123 (Rh-123). Exposure to H(2)O(2) (500 microM) induced apoptosis, and the percentage of apoptotic cells increased progressively and peaked at 2 h. This H(2)O(2)-induced apoptosis was associated with the loss of DeltaPsi, and the time course of decrease in Rh-123 fluorescence paralleled that of apoptosis. Pretreatment of cardiomyocytes with diazoxide (100 microM), a putative mitoK(ATP) channel opener, for 30 min before exposure to H(2)O(2) elicited transient and mild depolarization of DeltaPsi and consequently suppressed both apoptosis and DeltaPsi loss after 2-h exposure to H(2)O(2). These protective effects of diazoxide were abrogated by the mitoK(ATP) channel blocker 5-hydroxydecanoate (500 microM) but not by the sarcolemmal K(ATP) channel blocker HMR-1098 (30 microM). Our results suggest for the first time that diazoxide-induced opening of mitoK(ATP) channels triggers cardioprotection against apoptosis induced by oxidative stress in rat cardiomyocytes.

Adenine nucleotide translocase mediates the K(ATP)-channel-openers-induced proton and potassium flux to the mitochondrial matrix

KATP channel openers have been shown to protect ischemic-reperfused myocardium by mimicking ischemic preconditioning, although their mechanisms of action have not been fully clarified. In this study we investigated the influence of the adenine nucleotide translocase (ANT) inhibitors--carboxyatractyloside (CAT) and bongkrekic acid (BA)--on the diazoxide- and pinacidil-induced uncoupling of isolated rat heart mitochondria respiring on pyruvate and malate (6 + 6 mM). We found that both CAT (1.3 microM) and BA (20 microM) markedly reduced the uncoupling of mitochondrial oxidative phosphorylation induced by the K(ATP) channel openers. Thus, the uncoupling effect of diazoxide and pinacidil is evident only when ANT is not fixed by inhibitors in neither the C- nor the M-conformation. Moreover, the uncoupling effect of diazoxide and pinacidil was diminished in the presence of ADP or ATP, indicating a competition of K(ATP) channel openers with adenine nucleotides. CAT also abolished K+-dependent mitochondrial respiratory changes. Thus ANT could also be involved in the regulation of K(ATP)-channel-openers-induced K+ flux through the inner mitochondrial membrane.

K(ATP) channel opener protects neonatal rabbit heart better than St. Thomas' solution

OBJECTIVES

Myocardial protection with ATP-sensitive potassium channel (K(ATP) channel) openers is as effective as St. Thomas' cardioplegia (StTCP) in adult rabbit hearts. This study compares the effectiveness of the K(ATP) channel opener pinacidil to StTCP in protecting neonatal rabbit hearts exposed to global ischemia.

METHODS

Seventeen neonatal rabbit hearts (7-9 days old) perfused with Krebs-Henseleit buffer (KHB) on a Langendorff apparatus underwent 90 min of normothermic ischemia. Six (ischemia control) received no pretreatment before or during ischemia. Six others (pinacidil) received a 3-min infusion of 50 microM pinacidil in KHB without StTCP at the onset of ischemia. Five others (StTCP) received a 3-min infusion of StTCP at the onset of ischemia. After 60 min of KHB reperfusion, recovery of left ventricular (LV) performance and coronary flow (CF) were measured and compared to preischemia. A paired t test was used for comparison between drug-treated and untreated groups.

RESULTS

Pinacidil-treated hearts had significantly better recovery of left ventricular developed pressure (47 +/- 3.8 mmHg vs 32 +/- 2.5 mmHg, P < 0.05), contractility (+dP/dt(max); 885.4 +/- 74 mmHg vs 643.7 +/- 65 mmHg, P < 0.05), left ventricular end diastolic pressure (10.5 +/- 0.9 mmHg vs 17.4 +/- 1.2 mmHg P < 0.05), compliance (-dP/dt(max); 994.2 +/- 86 mmHg vs 673.6 +/- 69 mmHg, P < 0.05), and CF (5.9 +/- 0.4 ml/min vs 4.2 +/- 0.2 ml/min, P < 0.05) compared to ischemic control. StTCP only improved the recovery of -dP/dt(max) (877.4 +/- 73 mmHg/s vs 673.6 +/- 69 mmHg/s, P < 0.05) and CF (5.7 +/- 0.3 ml/min vs 4.2 +/- 0.2 ml/min, P < 0.05) compared to control.

CONCLUSIONS

Pinacidil pretreatment provided superior recovery of systolic performance compared to St. Thomas' cardioplegia solution in neonatal hearts. Myocardial protection by pretreatment with the K(ATP) channel opener pinacidil may be a new strategy for myocardial protection during pediatric cardiac surgery.

Opening of potassium channels modulates mitochondrial function in rat skeletal muscle

We have investigated the presence of diazoxide- and nicorandil-activated K+ channels in rat skeletal muscle. Activation of potassium transport in the rat skeletal muscle myoblast cell line L6 caused a stimulation of cellular oxygen consumption, implying a mitochondrial effect. Working with isolated rat skeletal muscle mitochondria, both potassium channel openers (KCOs) stimulate respiration, depolarize the mitochondrial inner membrane and lead to oxidation of the mitochondrial NAD-system in a strict potassium-dependent manner. This is a strong indication for KCO-mediated stimulation of potassium transport at the mitochondrial inner membrane. Moreover, the potassium-specific effects of both diazoxide and nicorandil on oxidative phosphorylation in skeletal muscle mitochondria were completely abolished by the antidiabetic sulfonylurea derivative glibenclamide, a well-known inhibitor of ATP-regulated potassium channels (K(ATP) channels). Since both diazoxide and nicorandil facilitated swelling of de-energised mitochondria in KSCN buffer at the same concentrations, our results implicate the presence of a mitochondrial ATP-regulated potassium channel (mitoK(ATP) channel) in rat skeletal muscle which can modulate mitochondrial oxidative phosphorylation.

Metabolic plasticity and the promotion of cardiac protection in ischemia and ischemic preconditioning

The concept of metabolic protection of the ischemic myocardium is in constant evolution and has recently been supported by clinical studies. Historically, enhanced glucose metabolism and glycolysis were proposed as anti-ischemic cardioprotection. This hypothesis is supported by the sub-cellular linkage between key glycolytic enzymes and the activity of two survival-promoting membrane-bound pumps, namely the sodium-potassium ATPase, and the calcium uptake pump of the sarcoplasmic reticulum. Moreover, improved resistance against ischemia follows the administration of glucose-insulin-potassium in a variety of animal models and in patients following acute myocardial infarction. The metabolic plasticity paradigm has now been expanded to include (1) the benefit of improved coupling of glycolysis to glucose oxidation, which explains the action of anti-ischemic fatty acid inhibitors such as trimetazidine and ranolazine; (2) the role of malonyl CoA in the glucose-fatty acid interaction; and (3) the anti-apoptotic role of insulin. Furthermore, we argue for a protective role of increased glucose uptake in the preconditioning paradigm. Additionally, we postulate an adaptive role of mitochondrial respiration in the promotion of cardioprotection in the context of ischemic preconditioning. The mechanisms driving these mitochondrial perturbations are still unknown, but are hypothesized to involve an initial modest uncoupling of respiration from the production of mitochondrial ATP. These perturbations are in turn thought to prime the mitochondria to augment mitochondrial respiration during a subsequent ischemic insult to the heart. In this review we discuss studies that demonstrate how metabolic plasticity can promote cardioprotection against ischemia and reperfusion injury and highlight areas that require further characterization.