**1. Introduction**

Mitochondria are considered to be one of the most important organelles, not only in terms of their ability to control apoptosis [1] or necrosis [2], but also for their important participation in cardioprotection [3]. Mitochondria can cope with energy demanding situations due to their adaptability. The adaptation mechanisms of mitochondria are very important especially in the heart [4]. Cardiac mitochondria provide more than 90% of the total energy required for the cell [5]. Moreover, mitochondria are able to adapt to new conditions through signaling pathways affecting membrane remodeling, mitochondrial dynamics, or energy production [6,7].

Currently, many studies sugges<sup>t</sup> that regulation of mitochondrial permeability transition pore (mPTP) opening plays a key role in the induction of cardioprotection [8–11]. Modulation of mitochondrial membrane fluidity through its major component, cardiolipin, or signalization via mitochondrial connexin 43 (mtCx43) leads to myocardial energy maintenance under the conditions of reduced oxygen utilization.

The common denominator of cardioprotection induction seems to be the exposure of the organism to oxygen limiting conditions [12]. The partial or complete absence of oxygen (hypoxia, anoxia) or damage of the respiratory chain a ffect the changes of biochemical and metabolic processes and induce remodeling of membrane systems [13]. A limited supply or damage in oxygen processing activates signaling pathways that result in structural and functional changes involved in the adaptation of myocardium to pathological conditions.

The mPTP, cardiolipin, and mtCx43 signaling pathways are calcium associated. Calcium (Ca<sup>2</sup>+) ions as major inducers of mPTP opening show a high a ffinity to cardiolipin [14,15]. The process of hypoxia and subsequent reoxygenation also a ffect mPTP opening coupled with regulation of Ca2+ handling and cardiolipin oxidation [16]. Similarly, mtCx43 forms Ca2+ permeable hemichannels allowing Ca2+ entry and triggering a permeable transition leading to cell death [17]. In the following parts of this review, we discuss the signaling pathways through mPTP regulation in cooperation with cardiolipin and mtCx43 leading to myocardial adaptation in pseudohypoxia.

#### **2. Cardioprotection and Mitochondrial Energetics**

Myocardium is highly dependent on su fficient oxygen supply. For this reason, cardiac mitochondria must maintain adequate oxygen to continue oxidative phosphorylation [18–20]. Mitochondrial biogenesis is increased at the metabolically active site of the cell where the consumption of adenosine triphosphate (ATP) is increased [21,22]. Therefore, mitochondria occupy up to 35% of the cell volume of cardiomyocytes of heart ventricles [23–25]. The oxygen consumption varies depending on the physiological state of the organism [26]. Insu fficient oxygen supply, characteristic of pathological situations, is reflected in the reduction of energy production in cardiac mitochondria [22,27,28]. Although cardiac mitochondria are the main energy source of cells, their dysfunction contributes to the development of a wide range of diseases [29,30]. The most common diseases, such as ischemic heart disease [31] or diabetes mellitus [32,33], create conditions in which the organism is exposed to a significant lack of oxygen. Partial (hypoxia) or complete (anoxia) absence of oxygen or the inability to use available oxygen due to damage of the mitochondrial respiratory chain (pseudohypoxia) well characterizes the disease of diabetes mellitus [34] and changes of several biochemical and metabolic processes [32]. Therefore, attention is required to develop new therapeutic approaches directed to mitochondria as target organelles triggering cardioprotection.

The principle of the new cardioprotective models is based on controlled oxygen restriction [35]. One of the first well known phenomena of cardioprotection is ischemic preconditioning (IPC), consisting of several repetitions of short ischemic and subsequent reperfusion episodes that reduce myocardial sensitivity before the next prolonged ischemic episode of the heart [36]. The duration of ischemia is crucial for the rate of myocardial damage [37]. While the early phase of ischemia causes reversible changes of cardiomyocyte and decreases the contractility of myocardium, prolonged ischemia (more than 20 to 30 min) leads to irreversible changes in the metabolism, function, and ultrastructure of the heart [30].

Although many studies have confirmed the e fficacy of the classical form of IPC [38–40], attention is drawn to an alternative method of controlled induction of short term non-lethal series of ischemic and subsequent reperfusion impulses on specific organs or tissues remote from the heart, known as remote ischemic preconditioning (RPC) [41]. This phenomenon provides protection of myocardium against lethal ischemic damage [42].

An insu fficient oxygen supply and nutrients in cardiomyocytes is the main cause of heart ischemia/reperfusion (I/R) injury [43,44]. In a situation with a continuous lack of oxygen, anaerobic glycolysis is preferred [45]. A change in substrate preference used for energy production seems to be the key mechanism favorable for cells with a limited oxygen supply. This is one of the reasons why partial oxygen deprivation is the main factor used in experimental models for the induction of cardioprotection [46].

#### **3. Cardiac Mitochondrial Energetics in Partial Oxygen Deprivation**

Oxygen deprivation is reflected in specific metabolic changes that result in a balance disorder between fatty acids and glucose oxidation. The restriction of oxygen supply is reflected in changes in preferences for substrates used for energy production [47,48]. In comparison with fatty acid oxidation, a higher amount of ATP is produced by aerobic oxidation of glucose in relation to oxygen consumption [49]. Therefore, glucose is the preferred energy substrate. Despite the fact that fatty acids are less e fficient energy substrates compared to glucose, fatty acids are the preferred source of energy in situations associated with impaired mitochondrial function, reduced respiration, and decreased ATP production, such as ischemia of the heart or diabetes mellitus [50].

Increasing oxidation of fatty acids in the heart reduces oxidation of glucose and vice versa. The oxidation of fatty acids increases nicotinamide adenine dinucleotide (NADH) and acetyl-CoA levels, which inhibit pyruvate dehydrogenase (PDH) associated glucose metabolism reduction [51,52]. The process of mutual regulation of glucose and fatty acid metabolism is called the Randle cycle [53]. However, the predominance of fatty acid oxidation during reperfusion versus glucose oxidation negatively a ffects the activity of the heart [48,54]. Consequently, manipulating heart metabolism to redirect fatty acid oxidation during reperfusion to glucose utilization may constitute a proof-of-concept on how to preserve heart function after ischemia or hypoxia [55,56].

When a su fficient supply of oxygen is ensured, glucose is metabolized by aerobic oxidation [57]. The PDH complex metabolizes glucose to acetyl-CoA, which then enters into the Krebs cycle [58]. A limited supply of oxygen causes phosphorylation of PDH subunits, i.e., PDH inactivation, which is reflected in the inability to metabolize glucose to pyruvate and acetyl-CoA. Then, glucose is metabolized by anaerobic glycolysis to lactate [59,60]. This process is used mainly by cancer cells that are permanently in anaerobic conditions [61]. Oxygen deprivation stimulates the overexpression of hypoxia-inducible factor 1 α (HIF-1 α) and inactivation of PDH through pyruvate dehydrogenase kinase 1, resulting in the preference of anaerobic glucose oxidation [62–64]. This process in which the glucose metabolism is reprogrammed from aerobic to anaerobic is known as the Warburg e ffect [65]. Despite the fact that ATP production in anaerobic glycolysis is much lower, i.e., two molecules of ATP are produced by anaerobic glycolysis, but up to 36 molecules of ATP by the oxidative phosphorylation of one glucose molecule, anaerobic glycolysis is preferred due to low oxygen consumption [66]. Deletion of HIF-1 α affects heart function under normoxic conditions, despite the fact that the heart is protected by HIF-1 against hypoxia [32]. Since PDH and the electron transport chain of mitochondria are the major sources of reactive oxygen species (ROS), a preference for anaerobic glycolysis prevents the apoptosis of cancer cells [67,68]. Moreover, continuous production of ATP is ensured by constant glucose supply (malignancy or hyperglycemia) [69]. In addition, the process of anaerobic glycolysis is 100-times faster than oxidative phosphorylation [70]. A constant supply of small amounts of energy with a low oxygen consumption is advantageous for immediate energy supply [71,72]. Besides that, the function of mitochondria is considerably limited in a ffected cells; therefore, anaerobic glycolysis is the major mechanism of energy production. Since cancer cells are capable of increased proliferation even under these restricted conditions, we can consider that preference for anaerobic glycolysis is beneficial for cells exposed to hypoxia [66,73]. According to the above, we can suppose that cardiomyocytes from diabetic myocardium could be used to describe metabolic processes such as those of cancer cells [63]. Diabetic myocardium is characterized by a state of pseudohypoxia, as a result of electron transport chain damage associated with a limitation of oxidative phosphorylation and impairment of HIF-1 activation [32]. Pseudohypoxia is described as impaired cellular oxygen utilization capacity due to reduced levels of NAD, which may cause NADH accumulation with NADH/NAD redox imbalances [74,75]. Therefore, anaerobic glycolysis could also be advantageous for the cells of diabetic organisms [34,76]. Despite the known side e ffects of PDH inhibition, such as diabetes mellitus [58,77–79], metabolic syndrome [80], heart failure [81], and fatty liver [82], we can assume that cells of diabetic hearts use similar adaptation mechanisms to increase their survival. A su fficient supply of glucose ensures a prompt and continuous production of energy. These facts

explain the advantage of anaerobic glycolysis in a diabetic heart [52]. Another important factor is age, which leads to dysregulation of molecular pathways linked to mitochondria. Increased apoptosis, declined autophagy, increased disruption of mPTP, and worsened injury after hypoxic-ischemic insults are the results of aging. The age related decrease in NAD+ contributes to substrate starvation leading to a pseudohypoxic state [83].
