**2.3. Cardiac mitochondria during aging**

and other interacting mitochondrial molecules such as the phosphate carrier, BH3 proteins, and p53 [29]. However, genetic ablation of the proposed components revealed that only the deletion of the CyPD gene resulted in impaired opening of the mPTP, suggesting that the

Recent studies indicated that the ATP synthase is a major component of the mPTP [32]. There are two working proposals about the mechanism for ATP synthase in the mPTP formation. The first one suggests that the pore forms at the interface of two dimers of ATP synthase [33]. It has been showed that the current that was observed from reconstituted lipid bilayers with purified dimers of the ATP synthase was electrophysiologically equivalent to that of the mPTP. Additionally, genetic ablation of two specific subunits of the F0 subcomplex that are necessary for dimerization did not result in opening of the pore, which further underscores the importance of dimerization for the formation of the mPTP [34]. The second hypothesis focuses on the c-subunit ring of the F0 subcomplex [35]. In purified ATP synthase extracts in yeast, the ring structure produced by the c-subunits exhibited a current that was equivalent to that of the mPTP [34], and the currents were inhibited by regulators of the mPTP, suggesting that the ring and the mPTP are the same. While debate continues about the precise components and mechanism of the mPTP, its importance in physiology and pathology is clear and its regulation is paramount to cell survival. While a short-term opening of the mPTP appears to act as a normal calcium-release mechanism that is required for proper metabolic regulation [29, 36–38], irreversible formation and consequent opening of the mPTP are key factors in mitochondrial dysfunction and mitochondria-driven cell death [32, 39, 40]. When the mitochondria are exposed to high concentrations of calcium, they undergo a massive and permanent swelling that leads to an abrupt increase in permeability to small solutes of the IMM, abolishing the chemiosmotic gradient across the IMM [29], which subsequently uncouples OXPHOS, leading to a decrease in ATP production and an increase in ROS formation [25]. Further rupture of the outer mitochondrial membrane results in the extrusion of cytochrome c, a key step in the initiation of apoptosis [41]. The mPTP may also play a role in the regulation of energy production due to the dual role of the

other proposed components are not a necessary part of the pore [30, 31].

36 Mitochondrial Diseases

ATP synthase in both ATP production and mPTP formation [25].

cardiomyocyte through an iNOS-dependent mechanism [9].

development through the fetus, neonatal, and adult heart.

**2.2. Cardiac mitochondrial changes during cardiac development**

Interestingly, small increases in *O*-GlcNAcylation were correlated with improved ability of cardiac mitochondria to sequester Ca2+ as well as resistance to mPTP opening. Key regulatory proteins in the mPTP, the VDAC and ANT, were also found to be able to be *O*-GlcNAcylated. The ATP synthase, the key molecule that forms the mPTP, has also been shown to be able to be *O*-GlcNAcylated [6]. Since mPTP opening is influenced by the loss of the mitochondrial potential as well as calcium overload, any change in mitochondrial potential or calcium dynamics may have adverse effects in the mitochondria. Key calcium signaling participants of mPTP regulation include the pore of the outer membrane, VDAC, the pore of the IMM calcium uniporter, and a key regulator of the mPTP, cyclophilin D [42]. Our most recent study also showed that overexpression of VCP protects against stress-induced mPTP opening in

There are significant differences in mitochondrial metabolism and function during the cardiac

Aging is a major risk factor for cardiovascular diseases. During aging, mitochondrial oxidative stress responses, mitochondrial damage, and biogenesis as well as the cross-talk between mitochondria and cellular signaling are changed.

Aging may induce changes to the shape and size of mitochondria in the heart [48]. In aged mice, mitochondria appeared more rounded and less spherical [49]. It was further noted that aged mitochondria exhibit a lower total area of inner membrane per mitochondria, suggesting a reduced capacity for OXPHOS [50]. Reciprocally, increased levels of large-scale deletions and point mutations in cardiac mtDNA, as well as reduced levels of mitochondrial enzymatic activities, may occur with aging.

Additionally, the multiple metabolic changes that occur in cardiac muscle with advancing age include increasing levels of saturated fatty acids and reduced levels of polyunsaturated fatty acids and cardiolipin [51]. Cardiolipin is a key cellular phospholipid and an important constituent of the mitochondrial inner membrane. Reduced cardiolipin influences cardiac mitochondrial membrane transport function, fluidity, and stability of the membrane and facilitates optimal energy generation [51]. Significant reduction in carnitine and acetyl carnitine levels has also been reported in older subjects, suggesting lowered ability to transfer fatty acids into the mitochondria to be metabolized [52]. In addition, the effect of aging on cardiac OXPHOS enzymatic function has been reported. Within cardiomyocytes, the interfibrillar mitochondria consume less oxygen and show a decrease in the ETC enzyme activity, particularly complexes III and IV during aging [53, 54]. This decrease in enzyme activity may lead to the lowered ability to meet the energy demands of the heart as aging ensues.

the imbalance between the requirement and supplement of oxygen in the cardiac muscle during the contraction and relaxation cycle. Consequently, the bioenergetic requirements of the heart are beyond what the mitochondria can cope with, and the heart begins to progress to HF. Thus, energy deficiency can be a cause and effect of HF. There are considerable evidences of links between HF and impairment of the energetics of myocardial mitochondria, such as declined mitochondrial synthesis/resynthesis of ATP, shifted fuel selection, impaired mito-

Mitochondria and Heart Disease

39

http://dx.doi.org/10.5772/intechopen.72611

Like all the other cells, there are three energy systems that contribute to the production of ATP in cardiac muscles: phosphagen system (ATP-creatine phosphate cycling; high power, short duration), glycolysis (moderate power/short duration), and FAO (low power/long duration). Three energy systems can be selectively recruited, depending on the amount of oxygen available, as part of the cellular respiration process to generate the ATP for the cardiac muscles. Since the heart has a limited capacity for substrate storage, energy is required to rebuild or resynthesize it. The energy released from any of these three series of reactions is coupled with

ATP-CP system is the quickest way to resynthesize ATP. CP, like ATP, is stored in cardiac muscle cells and serves as the main energy store in myocardium. If oxygen is unavailable, the ATP-CP system does not use oxygen and does not produce lactic acid. This is the primary system behind the very short, powerful movements of the cardiac contraction and relaxation cycle. When CP is broken down, a large amount of energy is released. This energy released is coupled to the energy requirement necessary for the resynthesis of ATP. CP can easily diffuse through the inner mitochondrial membrane to the cytosol to generate ATP from ADP catalyzed by the cytosolic CK (cytoCK). Normal beating cardiomyocytes, even under variations of workload, maintain a constant level of ATP and CP in the cytosolic and mitochondrial compartments [58]. The CP/ATP ratio of 1.7–2.1 reflects normal mitochondrial ATP production and CK efficiency. This ratio has become a powerful index of the bioenergetics of the heart and its decrease has been reported in both the human and animal models of HF [61–65]. In HF patients and animal models, the total CK, as well as both the cytoCK and mitoCK, positively correlates with ejection fraction and can decrease as much as 50% [66–68]. It is observed that the decrease in CK activity, rather than the level of hypertrophy itself, is a hallmark of the transition from severe hypertrophy to HF [62, 69]. Interestingly, healthy myocardial cell size, myofibrillar and cytoskeletal organization, and positioning of the mitochondria near the SR allow for the ATP production in both mitochondrial and cytosolic regions and work concurrently to meet the energy demand [69]. However, in the failing hearts, the increase in myocardial cell size, the shrinkage of mitochondrial content, the alterations in microtubules, and the disorganization of cytoskeletal protein and their reduced expression contribute to decrease the efficiency of mitoCK and cytoCK for the energy transfer between the mitochondria and the cytosol [70–73]. The glycolysis system is the second-fastest way to resynthesize ATP. In the normal heart, pyruvate is converted into a metabolic intermediary molecule called acetyl coenzyme A (acetyl-CoA), which enters the mitochondria for oxidation and the production of more ATP. In the failing heart, the conversion to lactate occurs due to the greater demand for oxygen than the available supply. Although the catabolism of sugar supplies the necessary energy from which

chondrial biogenesis, and abnormal calcium transport.

the energy requirements of the reaction that resynthesizes ATP.

*3.1.1. Reduction of ATP synthesis*

Furthermore, mitochondrial abnormalities have been proposed due to the increased mitochondrial production of ROS during aging. The rate of oxidative phosphorylation decreases with aging, allowing for increased leakage of electrons [48], these electrons are then able to interact with oxygen, generating superoxide anions and other forms of ROS. Excessive ROS formation has harmful consequences, including cellular dysfunction and cell death [48]. This high level of ROS is also able to oxidize mtDNA. Moreover, opening of the mPTP has been found to be changed in the heart during aging [55]. Increased opening of the mPTP may be linked to higher ROS levels and thus may be facilitating the aging process.
