Observations from animal models of HF, as well as from clinical studies, suggest that mitochondrial bioenergetic defects may underlie several aspects of the HF phenotypes. Besides their contribution to bioenergetic function, mitochondria are integrally involved in the regulation of intracellular Ca²⁺ flux, myocyte cell death, and remodeling events; in reactive oxygen species (ROS) generation and antioxidant response; and in furnishing cardioprotective responses to physiological insults.

The organelle manifold roles in bioenergetic production, sensor, and transport, as well as its effect on ROS generation and signaling, cell death, Ca²⁺ levels, and contractility, have often been overlooked (Table 18.1). In this section, we will focus on the mitochondrial changes that occur in HF, mainly on mitochondrial bioenergetics and biogenesis, ROS and oxidative stress (OS), and downstream effects on myocyte function (i.e., contractility and stimuli response) and cell death (i.e., apoptosis and necrosis). This discussion requires a larger view of mitochondria within the cellular context including interactions and cross talk with other organelles in the development and progression of HF. Current approaches to understand the role(s) that the organelle plays in the failing heart should include observations from experimental animal models, use of transgenic models, gene profiling analysis, in vitro studies with isolated cardiomyocytes (to identify pathway components in HF), and identification of potential targets for therapeutic interventions.

That cardiac metabolism plays a central role in HF has been confirmed by O’Brien et al. observations of significant decrease in ATP-utilizing enzymes including SR Ca²⁺-ATPase (SERCA) and myofibrillar Ca²⁺-ATPase activities in dogs paced to advanced HF (4 weeks of pacing), whereas in dogs with early HF (1 week of pacing), there was a significant decline in activities of the SR Ca²⁺ release channel, mitochondrial ATP synthesis, and Cr [32, 33].

Data obtained from biochemical analysis in paced dogs documented a significant decrease in cardiac respiratory complex III and V activities [34]. Included in this study was a limited temporal course analysis that showed significant reduction in specific ETC enzyme activities in early HF (between 1 and 2 weeks of pacing) [35]. Moreover, this early ETC dysfunction correlated with the appearance of markers of myocyte apoptosis and OS, whose presence has been previously described in paced failing hearts [36, 37]. Using electron spin resonance (ESR) spectroscopy to directly assess ROS levels in paced dog hearts, Ide et al. demonstrated a significant increase in superoxide anion levels in myocardial submitochondrial fractions; the elevated ROS production was attributed to the functional block of electron transport, resulting from a marked decrease in mitochondrial respiratory complex activities [38]. Furthermore, later studies by Ide et al. [39] using the paced dog HF model and modified ESR analysis showed a significant increase in hydroxyl radical levels and a significant positive correlation between myocardial ROS levels and increases in LV contractile dysfunction.

Several models of mechanically overloaded heart have presented further evidence of defective mitochondrial bioenergetics in HF. In both volume- and pressure-overloaded rat hearts [40], depletion of PCr occurs with LV hypertrophy. Patients with HF have shown marked reductions in levels of phosphocreatine (similarly decreased in HCM and DCM) [41], and reduced mCK activity and content were also demonstrated in a pressure-overloaded rat model [42]. In volume-overloaded hearts, alterations in myocardial high-energy phosphate (i.e., significantly lower myocardial PCr/ATP ratios) have been postulated to contribute to contractile/pump dysfunction occurring during exercise [43]. Interestingly, these mitochondrial abnormalities in function persisted despite adequate tissue oxygenation [44] and suggest that altered regulation of mitochondrial OXPHOS results in reduced PCr levels present in HF [45]. Modulation of OXPHOS activities could be achieved by differential substrate utilization, changes in ADP, inorganic phosphate, intramitochondrial NADH, and oxygen levels [46].

Data from several laboratories have shown that two major subpopulations of mitochondria are present in skeletal and cardiac muscles. One of these organelle subpopulations is associated with the sarcolemma (i.e., SSM), and the second population, the IMFM, is proximal to the myofilaments. A preferential decline in IMFM oxidative function in aged rats has been reported, with concomitant reduction of complex III and activities [47–49]. Presently, there is limited data ­concerning the different susceptibility of these mitochondrial subpopulations and its contribution to the overall mitochondrial dysfunction observed in HF. Analysis of skeletal muscle has shown a pacing-associated decline in complex I and III activities but limited to SSM mitochondria. On the other hand, a significant decline in complex V activity was detected in both IMFM and SSM (with a higher decline in the IMFM fraction). Taken together, these findings reveal that both discrete defects in bioenergetics and the onset of increased apoptosis are present in pacing-induced CM/HF and operative in the early stages of pacing.

The specificity and reproducibility of these changes suggest that these are not stochastic defects. Both changes were in parallel with elevated ROS/OS levels. The abnormal activity of several mitochondrial enzymes and the increased apoptogenic pathway appear to be underlain, at least in part, by an orchestrated shift in expression (both nuclear and mtDNA) of several respiratory chain subunits (e.g., cytb, ATP-β), mitochondrial bioenergetic enzymes (e.g., mtCK), global transcription factor (e.g., PGC-1), and apoptotic proteins (e.g., p53, p21).

Furthermore, these data also suggest that differences in specific mitochondrial subpopulations associated with distinct cardiac compartments may contribute to the observed enzymatic dysfunction observed in HF. Further studies appear to be necessary to unravel the genesis of the orchestration of the intercommunication involved and identify potential targets for new therapies.

Recently, Ashrafian et al. [50] have identified a mouse mutant, called Python, which develops DCM and HF secondary to a dominant fully penetrant mutation in the dynamin-1-like (Dnm1l) gene, which has been shown to be critical for mitochondrial fission. Heterozygous Python fibroblasts showed abnormal mitochondria and peroxisomes with reduced levels of mitochondrial enzyme complexes and cardiac ATP depletion. Of interest, the Python mutation affected mitochondrial function prior to major changes in heart structure and before there was any overt signs of HF. The homozygous Python mutation resulted in the death of embryos midway through gestation. As the authors indicated, this is the first time that a defect in a gene involved in mitochondrial remodeling can result in CM, implying that the function of this gene is necessary to maintain normal cellular function in a relatively tissue-specific manner.

In another model of HF, the murine model of myocardial infarct (MI) created by ligation of the left anterior descending coronary artery for 4 weeks, Ide et al. found that LV dilation and contractility dysfunction were accompanied by significant increases in levels of mitochondrial ROS (i.e., hydroxyl radicals) and lipid peroxides [51, 52]. Moreover, infarcted LV from MI mice exhibited diminished enzymatic activity of respiratory complexes I, III, and IV (enzymes each containing subunits encoded by mtDNA), while the mitochondrial enzymes encoded only by nuclear DNA (e.g., citrate synthase and complex II) were unaffected. These investigators also reported a parallel decrease in steady-state levels of several mtDNA-encoded transcripts, including subunits of complexes I, III, and IV as well as 12S and 16S rRNAs and a significant decline in mtDNA copy number. These data are consistent with the concept that membrane lipids, mtDNA, and mitochondrial respiratory enzymes are well-established targets of ROS oxidative damage (see Fig. 18.3), albeit the absence of effect on complex II, a particular common ROS target, was unexpected.

Another marker of enhanced OS in the mouse failing heart was increased level of mitochondrial 8-oxo-dGTPase, a DNA repair enzyme which prevents the incorporation of 8-oxo-dGTP into DNA, a significant type of ROS-induced DNA damage which can lead to defects in DNA replication. This HF model illustrates both the development of mitochondrial ROS and a number of its downstream deleterious effects (e.g., enzyme dysfunction, mtDNA damage) as well as an adaptive response (i.e., increased DNA repair activity) to remove that damage.

It is also important to note that while a number of animal studies have shown ROS activation in the failing heart, both arising from mitochondrial and extramitochondrial sources, some observations have suggested that endogenous antioxidants and ROS defense pathways can ameliorate ROS-mediated cardiac abnormalities, and the dynamic balance between these counteracting forces should be considered.

Since direct measurement of short-lived ROS is extremely difficult, several laboratories have reported increased levels of ROS-mediated damage (e.g., lipid peroxidation, DNA and protein oxidation) as an indirect index of ROS/OS in several animal models, including in the pacing-induced HF [34, 36, 53]. The LV from paced animals exhibited increased aldehyde levels and marked reductions in the activity of respiratory complexes III and V together with increased levels of large-scale mtDNA deletions [34].

While elevated ROS activation in the failing heart has been shown to arise from both mitochondrial and extramitochondrial sources, the role of endogenous antioxidants in ameliorating myocardial OS and the dynamic balance between these counteracting forces should be considered. It is well known that both cytosolic antioxidant enzymes (e.g., catalase, SOD1/CuSOD) and mitochondrial-localized antioxidants including SOD2/MnSOD, thioredoxin, and glutathione peroxidase can reduce ROS levels. Also, thioredoxin and thioredoxin reductase form an enzymatic antioxidant and redox regulatory system implicated in the regeneration of many antioxidant molecules, including ubiquinone, selenium-containing substances, lipoic acid, and ascorbic acid [54].

Both cytosolic antioxidant enzymes (e.g., catalase, SOD1, thioredoxin) and mitochondrial-localized antioxidants including SOD2, thioredoxin, and glutathione peroxidase can reduce ROS levels. Moreover, thioredoxin and thioredoxin reductase form an enzymatic antioxidant and redox regulatory system implicated in the regeneration of many antioxidant molecules, including ubiquinone (Q10), selenium-containing substances, lipoic acid, and ascorbic acid [54]. As we will see later, the relationship of antioxidants with HF has been underlined by several transgenic mouse studies.

While the mechanism by which the antioxidant response signaled in the presence of ROS remains unknown, recent observations suggest that mitochondria can act as an OS sensor and signal to the nucleus to upregulate antioxidant production. Storz et al. have shown that mitochondrial ROS release can activate a signal relay pathway in which the serine/threonine protein kinase D (PKD), acting as a mitochondrial sensor of OS, activates the NF-κB transcription factor leading to induction of SOD2 [55, 56]. PKD has been localized to the mitochondria in cells exposed to both exogenous and mitochondrial ROS, where it is phosphorylated and activated. Upon subsequent dissociation from mitochondria, activated PKD phosphorylate substrates participate in NF-κB activation, although how this is accomplished is not yet known. Experiments in which RNAi-mediated silencing of PKD blunted SOD2 induction confirm that SOD2 promoter activation is dependent on PKD. While the role of PKD in HF has not yet been established, the critical involvement of mitochondrial ROS in the signaling and generation of an antioxidant response suggests potential limitations with the use of antioxidants, which target mitochondrial ROS.

Although studies with animal models suggested that ROS and defective ETC play an integral role in the genesis and progression of HF, these findings were correlative and did not, however, establish a direct link between ROS, defective ETC, and either skeletal muscle or cardiac dysfunction. Notwithstanding, these observations did foreshadow recent findings using heart-/muscle-specific MnSOD-deficient mice, a transgenic mice in which OS, which selectively impaired mitochondrial respiration, can directly cause HF [57]. Furthermore, ROS may trigger a multiplicity of mechanisms that can lead to HF. For instance, complex IV (affected at both the transcriptional and activity levels in Ide’s studies) was not significantly decreased in the MnSOD-deficient mouse. On the other hand, this model exhibited a moderate decline in both complexes I and III and, particularly, a marked decline in the nuclear DNA-encoded complex II activity (shown to be mediated at the posttranscriptional level). Taken together, this latter study and the aforementioned pacing dog studies do not support the view of ROS-mediated decline in mtDNA copy number and in mitochondrial transcription as the definitive mechanism leading to reduced respiration and cardiac bioenergetic metabolism in HF.

Finally, studies using a porcine model of LV infarction and remodeling (induced by ligation of the left circumflex artery), reduced systolic performance accompanied by reductions in high-energy phosphate levels, myocardial OXPHOS, and concomitant reductions in ANT protein and in the β subunit of complex V were noted [58, 59]. These findings suggest that specific mitochondrial inner-membrane proteins may play a critical role during the remodeling that occurs in HF. Nevertheless, the order of the myocardial molecular and biochemical events leading to abnormal regulation of OXPHOS needs to be established.

The concept that the mitochondrial ETC is integrated by an orderly array of individual enzymatic complexes spreading freely in the inner membrane has been challenged by the discovery of a supramolecular organization of these complexes. Schagger and Pfeiffer [60] reported that all cytochrome c oxidase (complex IV) of Saccharomyces cerevisiae is bound to cytochrome c reductase (complex III), which exists in three forms: the free dimer and two supercomplexes comprising an additional one or two complex IV monomers. The distribution between these forms varies with the growth conditions. In mammalian mitochondria, almost all complex I was assembled into supercomplexes comprising complexes I and III and up to four copies of complex IV, a model for a network of respiratory chain complexes termed “respirasome.” Also, a fraction of total bovine ATP synthase (complex V) was isolated in a dimeric form, suggesting that a dimeric state is not limited to S. cerevisiae but also exists in mammalian mitochondria.

Observations by Shagger et al. [61] on the formation of respirasomes in human revealed that the mitochondrial ETC is organized in supramolecular assemblies; complexes I (NADH dehydrogenase) and III (cytochrome c reductase) formed a stable core respirasome to which complex IV (cytochrome c oxidase) may also bind. Analysis of the state of respirasomes in patients with an isolated deficiency of single complexes revealed that the formation of respirasomes is essential for the assembly/stability of complex I, the principal entry point of respiratory chain substrates. Moreover, genetic defects leading to a loss of complex III prevented respirasome formation and led to the secondary loss of complex I. Thus, primary complex III assembly deficiencies may manifest as a combined complex III/I defects.

The detection of supercomplexes and their constituent complexes became possible by using 2-D native electrophoresis, a technique pioneered by Schagger and von Jagow [62] employing the net negatively charged dye Coomassie Brilliant Blue G-250, which allows/improves the migration of proteins in a gel matrix driven by an electric field. Moreover, digitonin and blue native electrophoresis (BN-PAGE) have been applied for the initial separation of supercomplexes, followed by less mild native electrophoresis variants in the second dimension to release the individual complexes from the supercomplexes. By reducing the complexity of these supramolecular systems to the level of individual complexes, 2-D native systems may allow not only the identification of constituent proteins, but also their copy number [62]. In addition, the involvement of complex IV in the supramolecular organization of the ETC in photosynthetically active tissues of higher plant mitochondria has been possible in digitonin extracts of partially purified mitochondria of green plants using BN-PAGE and a gentler colorless native (CN)-PAGE; in this model a substantial proportion of the respiratory complex IV was assembled with complexes I and III into respirasome-like supercomplexes [63]. It is worth noting that in the case of supramolecular structure separation, in particular the ATP synthase, the Coomassie Brilliant Blue G-250 may need to be omitted. As pointed out by Seelert and Dencher [64], even for monomeric ATP synthase, this dye leads to partial dissociation into the subcomplexes F₁ and F_o, and only ATP synthases of some species are unaffected by this treatment. Thus, for applications with fragile complexes, clear or colorless native PAGE (CN-PAGE) and/or the variant high-resolution CN-PAGE are the methods of choice (as depicted in Fig. 18.4).

The first 3-D map of an ETC supercomplex has been recently reported by Schafer et al. [65] using random conical tilt electron microscopy analysis of a bovine supercomplex consisting of complex I, dimeric complex III, and complex IV (I₁III₂IV₁). In this 3-D map, the location and orientations of all the individual complexes in the supercomplex were distinctively achieved as seen in Fig. 18.5

Moreover, the ubiquinone and cytochrome c binding sites of each complex in the supercomplex were also located as depicted in Fig. 18.6.

The mobile electron carrier binding site of each complex was found to be in proximity to the binding site of the succeeding complex in the respiratory chain allowing structural evidence for direct substrate channeling in the supercomplex assembly with short diffusion distances for the mobile electron carriers. Also recently, Dudkina et al. [66] employing cryoelectron tomography and subvolume averaging reported the 3-D reconstruction of bovine heart respirasome (Fig. 18.7).

It was composed of dimeric complex III and single copies of complexes I and IV, at near 2.2-nm resolution. Unexpectedly, the distance between cytochrome c binding sites of complexes III₂ and IV was about 10 nm, and modeling indicated a loose interaction between the three complexes, which suggest that lipids are gluing them at the interfaces. These investigators have also observed a stable, dimeric ATP synthase supercomplex using mitochondria of the alga Polytomella. Structural analysis by electron microscopy and single-particle analysis revealed that dimer formation is based on specific interaction of the Fo parts, not the F1 headpieces which are not at all in close proximity. The angle between the two Fo parts was about 70°, which allowed marked local bending of the membrane. It seems that the function of ATP synthase dimerization is to control the particular architecture of the mitochondrial inner membrane [73].

There is evidence that the assembly of the functional respirasome is altered in HF. Rosca and Happle [74] have proposed a sequential mechanistic pathway in which the decrease in functional respirasome in HF is the primary event causing decreased OXPHOS and increased ROS production, leading to a progressive decrease in cardiac performance. Moreover, using native gel electrophoresis in the canine model of coronary microembolization HF, they found a dramatic decrease in ADP-stimulated respiration that was not relieved by an uncoupler, although the ADP/O ratio was normal, which indicates no defect in the phosphorylation apparatus. These findings inferred the presence of a defect in OXPHOS within the ETC, while the individual ETC enzyme activities were within normal limits. The respirasome-like supercomplex (complex I/complex III dimer and at least a significant proportion of complex IV) was decreased indicating that the mitochondrial defect lied in the supermolecular assembly rather than in the individual components of the ETC [75].

Furthermore, whether a decrease in the respirasome organization was due to changes in the phospholipid composition of the mitochondrial inner membrane or to modifications of the subunits of the ETC complexes has been analyzed by Rosca et al. [76]. The contents of the main phospholipid species, including cardiolipin, as well as the molecular species of cardiolipin were found unchanged in the failing heart mitochondria. However, employing phosphatase inhibitors, a specific decrease in complex I-supported respiration with glutamate was found. On the other hand, employing saponin-permeabilized cardiac fibers, preincubation with cAMP decreased OXPHOS, likely due to a localized defect at complex IV of the ETC. Taken together, these findings suggest that phosphorylation of specific complex IV subunits decreases OXPHOS either by limiting the incorporation of complex IV in supercomplexes or by decreasing supercomplex stability. Moreover, assembly of the ETC into respirasomes can provide structural support for mitochondrial OXPHOS, by facilitating electron channeling and perhaps by preventing electron leak and superoxide production. As Garcia-Palmer [77] has pointed out, these newly identified ETC assembly mechanisms open a new area of research into mitochondrial functional changes in general and in HF in particular. Further research on the structural conformation, presence, and assembling of mitochondrial supercomplexes will improve our understanding of their mechanistic implications and also increase the potential to develop new diagnostic methods and treatments for mitochondria-related diseases.

In mice transgenic models, disruption of mitochondrial bioenergy at specific loci or pathways can cause CM and HF. Gene ablation in mice (i.e., the generation of null mutations or gene knockouts) targeting a relatively wide spectrum of genes encoding specific mitochondrial proteins results in severe CM. Notably, mutational inactivation of the heart/muscle isoform of Ant1 gene in transgenic mice will result in the development of skeletal myopathy and CM leading to HF [78]. The Ant1 gene-deficient mice exhibit mitochondrial abnormalities including a partial deficit in ADP-stimulated respiration, consistent with impaired translocation of ADP into mitochondria in both skeletal muscle and heart. Ant1 ⁻ / ⁻ mice also exhibit a progressive cardiac hypertrophic phenotype coincident with the proliferation of mitochondria [79]. This mitochondrial biogenic response may be a compensatory mechanism to correct the energy deficit but could also be contributory to cardiac remodeling. Null mutations in either the mitochondrial or cytosolic CK gene in mice also lead to increased LV dilation and hypertrophy [42, 80]. Palmieri et al. [81] have reported the presence of a recessive mutation in the heart-/muscle-specific isoform of ANT1 in a patient with HCM and mild myopathy with exercise intolerance and lactic acidosis. This mutation resulted in complete loss of adenine nucleotide transport function and was also associated with increased levels of muscle mtDNA deletions.

Besides ANT [79], other targeted genes include MnSOD [82], glutathione peroxidase (GPx) [83], and factors involved in fatty acid metabolism, e.g., mitochondrial permeability transition pore (MPTP) subunits [84], mitochondrial transcription factor A (TFAM) [85], and frataxin [the protein responsible for Friedreich’s ataxia (FRDA)] [86]. Interestingly, ANT-deficient mice develop progressive HCM while MnSOD-deficient mice develop DCM, yet both types of null mutation cause severe cardiac ATP deficiency which is thought to underlie the resulting cardiac phenotype(s). Another major contribution of the transgenic mouse model has been to further our understanding of the family of transcriptional coactivators and factors including PPAR-α, PGC-1α, NRF-1, and MEF-2, which coordinately regulate myocardial energy metabolism, and their essential role in the developing embryonic heart, as well as to delineate the order of biochemical and molecular events in the metabolic and transcriptional cascade governing energy regulation in both the normal and diseased hearts [87].

In addition to examining the specific effects on cardiac phenotype by eliminating specific nuclear genes regulating mitochondrial function, tissue-specific knockout mice with mitochondrial CM have been used to identify modifying genes of potential therapeutic value [88]. At present, there is limited information about the impact on myocardium of knocking out nuclear genes involved directly in mitochondrial OXPHOS. A recent report described cardiac dysfunction in mice lacking cytochrome c oxidase subunit VIa-H, the heart isoform [89]. In addition, the “knockout” approach has not yet been accomplished with mtDNA genes due to the formidable technical difficulty involved in direct gene replacement or ablation of a multicopy gene in the setting of a nonnuclear multicopy organelle (i.e., the mitochondrion), although several promising approaches will be discussed in a later chapter.

The use of cardiac-specific overexpression of specific genes has also proved highly informative in furthering our understanding of the role of mitochondria in cardiac dysfunction. This technique involves fusing a regulatory region from a cardiac-specific gene with a candidate gene of interest and introduction into the transgenic mice, which will express the candidate gene specifically in cardiac muscle cells. Overexpression of genes which mediate the expression and control cardiac energy metabolism (e.g., PGC-1α, PPAR-α) has been shown to lead to severe cardiac dysfunction and marked changes in mitochondrial structure and function [90–92]. Similarly, transgenic mice containing cardiac-specific overexpression of calcineurin exhibited severe cardiac hypertrophy (which progresses to HF), marked mitochondrial respiratory dysfunction, and superoxide generation [93].

The development of animal models of mitochondrial-based cardiac dysfunction offers the possibility of direct testing for potential treatments. For example, the demonstration that MnSOD-deficient animals developed ROS toxicity and DCM prompted speculation that effective treatment with antioxidants could ameliorate the cardiac phenotype; indeed, peritoneal injection of MnSOD-deficient mice with the antioxidant MnTBAP [manganese (III) tetrakis (5,10,15,20 benzoic acid) porphyrin] eliminated the cardiac dysfunction and reversed the ROS accumulation [94]. Overexpression of the antioxidant GPx in transgenic mice inhibited the development of LV remodeling and failure after myocardial infarct (MI) and was associated with the attenuation of myocyte hypertrophy, apoptosis, and interstitial fibrosis [95]. Moreover, Schriner et al. have demonstrated that overexpression of catalase (primarily a peroxisomal-localized enzyme) targeted to the mitochondria increased overall mouse longevity, diminished OS- and ROS-mediated mitochondrial protein and mtDNA damage, and delayed the onset of aging-mediated cardiac pathology including subendocardial interstitial fibrosis, vacuolization of cytoplasm, variable myofiber size, hypercellularity, collapse of sarcomeres, mineralization, and arteriosclerosis, changes commonly observed in elderly human hearts and often found in association with congestive HF [96]. Another important finding was that it mattered in which subcellular compartment the overexpressed catalase was localized with little evidence of benefits from nuclear or peroxisomal-localized as compared to striking benefits of mitochondrial-localized catalase activity.

Among the most compelling evidences supporting a primary role for mitochondrial ROS and OS in CM and HF are findings with antioxidant genes in transgenic mice. Strains harboring null mutations in either MnSOD or TrxR2 encoding mitochondrial thioredoxin reductase exhibit DCM and HF. [57, 82, 97, 98] Li et al. [82] found that mice homozygous for MnSOD deficiency resulted in early neonatal death from severe DCM and metabolic acidosis. Moreover, these strains displayed severe reduction in myocardial succinate dehydrogenase (complex II) and aconitase (a TCA cycle enzyme) suggesting that MnSOD is required for maintaining the integrity of mitochondrial enzymes susceptible to direct inactivation by superoxide.

Mice, in which MnSOD deficiency was targeted to skeletal muscle and heart (i.e., H/M-Sod2 ^−/− strains), displayed progressive HF with depressed cardiac contractility by 8 weeks, cardiac enlargement by 16 weeks, and death from HF by 22 weeks [82]. Cardiac pathology was associated with specific defects in mitochondrial respiration (i.e., severely reduced respiratory complex II and moderately reduced complex I and III activities) and in myocardial mitochondrial ultrastructure. Immunoblot analyses showed significant expression of the SDHA and SDHB subunits from myocardial complex II of H/M-Sod2 ^−/− mice, with moderate suppression of complex Iα 9, Rieske iron–sulfur protein and core I subunit of complex III, and α and β subunits of complex V. Mitochondrial superoxide production was also significantly higher in these mice as was mitochondrial (but not cytosolic) lipid peroxidation, suggesting that oxidative damage was specifically localized in mitochondria. In addition, myocardial ATP production and content were significantly diminished, which may account for the absence of energy-dependent apoptosis in H/M-Sod2 ^−/− mice. This study also offered further evidence that ROS and OS are intimately linked to the progression of CM/HF since the administration at 8 weeks of age of the antioxidant MnSOD mimetic (MnTBAP) significantly improved cardiac contractility and ameliorated the overall phenotype.

It has been suggested that OS and alterations in protein metabolism may play an essential role in the pathogenesis of FRDA [99]. Schulz et al. [100] have assessed whether induction of mitochondrial metabolism may prevent the development of CM and HF. Transgenic mice were generated overexpressing the mitochondrial protein frataxin, which promotes mitochondrial energy conversion by ­controlling iron–sulfur cluster biogenesis and hereby mitochondrial electron flux. In this model, following doxorubicin-induced experimental CM, cardiac function and survival were significantly improved. Since the cardiac insulin/insulin-like growth factor 1 (IGF-1) cascade is markedly inhibited in wild-type mice following induction of CM, it seems that transgenic overexpression of frataxin may rescue the impaired insulin/IGF-1 signaling, providing enhanced cardiac stress resistance in the transgenic mice. These observations suggest that increased mitochondrial metabolism elicits an adaptive response due to mildly increased OS as a consequence of increased oxidative energy conversion; this in turn activates protective mechanisms which counteract cardiotoxic stress and promote survival in experimental CM.

The heart appears to function best when it simultaneously oxidizes both carbohydrates and fatty acids as bioenergetic substrates [109]. Under pathologic conditions, the heart relies more on glucose, as seen in cardiac hypertrophy, or may rely almost solely on fatty acids, as observed in cardiac tissue of animal models of diabetes [110]. Furthermore, the failing heart exhibits a decline in overall mitochondrial oxidative catabolism (including FAO), which is most evident in advanced HF while reliance on anaerobic glycolytic pathways tends to be increased [111–113]. Recent observations in an animal model of HF have downplayed the role of myocardial FAO downregulation in early, compensated HF [114]. Thus, initially this switch in metabolic substrate provides adequate energy to maintain normal cardiac function and may have an adaptive function by diminishing oxygen consumption. However, over time substrate switch in concert with declining OXPHOS becomes maladaptive, leading to a state of myocyte energy insufficiency (related to reduced capacity for myocardial mitochondrial ATP production and depletion in high-energy phosphates), resulting in diastolic HF. As previously noted, alterations in high-energy phosphates (PCr and ATP) have been identified by magnetic resonance spectroscopy both in animal models and human hearts with LV hypertrophy or HF [113, 115] and may be contributory to the pathological remodeling that occurs in end-stage HF.