**6. Mitochondria**

The mitochondrion plays a central role in ischemic injury. Not only is it the site of critical biochemical reactions in the cell, such as oxidative phosphorylation, beta-oxidation and the citric acid cycle, but it also occupies a unique position in the cellular balance between life and death. Inhibition of the mitochondrial respiratory chain as a result of oxygen depriva‐ tion is the cornerstone of metabolic disturbances.

Figure 3. This figure summarizes the ionic perturbations in an ischemic cell. **Figure 3.** This figure summarizes the ionic perturbations in an ischemic cell.

#### **6.1. Disturbance of ATP synthesis. 6.1. Disturbance of ATP synthesis.**

The main source of protons during ischemia comes from the production of lactate from pyr‐ uvate by lactate dehydrogenase. The accumulation of extracellular lactate greatly reduces the effectiveness of the lactate/proton cotransporter, preventing the removal of protons. Ad‐ ditionally, the residual metabolic activity also contributes to acidosis, as the hydrolysis of an

Ischemia induces a profound disturbance of the ionic homeostasis of a cell. The two major changes are the loss of ionic transmembrane gradients, which causes membrane depolariza‐

]i

Cellular depolarization occurs very rapidly after the onset of ischemia, and these mecha‐ nisms are not fully understood. However, it is recognized that both the inhibition of the Na

The accumulation of sodium in the cytosol is multifactorial. Acidosis stimulates Na+


cell at the expense of an intracellular accumulation of Ca2+. The massive entry of calcium in‐ to the cell disrupts the mechanisms that regulate its intracellular concentration and induces the release of calcium from the intracellular endoplasmic reticulum stores.[35] The lack of ATP prevents calcium excretion into the interstitium and its sequestration in the endoplas‐ mic reticulum. The accumulation of cytosolic calcium induces degradation of membrane phospholipids and cytoskeletal proteins, alters the both the calcium affinity and the efficien‐ cy of proteins involved in contractility, activates nitric oxide synthase (NOS) and proteases such as calpains and caspases, promotes the production of free radicals and alters the terti‐ ary structure of enzymes such as xanthine dehydrogenase, which is converted to xanthine

The mitochondrion plays a central role in ischemic injury. Not only is it the site of critical biochemical reactions in the cell, such as oxidative phosphorylation, beta-oxidation and the

) levels, leading to cellular edema.


), which is responsible for inducing a rise in

/H+ ex‐

. The

.[32]-[34] This net

and Ca2+, as well as an increase in the extracellular

affects the function of other membrane transporters,

. Progressive depolarization of the cell also promotes prolonged activa‐

, which results in increased intracellular Na+

is accompanied by osmotic water movement. Moreover, inhibition of the

/Ca2+ antiporter, an accelerator. This allows the extrusion of sodium from the

**5. Changes in the ionic cellular equilibrium (Figure 3)**

, Cl-

ATP molecule releases a proton.

the intracellular calcium ([Ca2+]i

lar concentrations of Na+

changers to purge cellular H+

high intracellular concentration of Na+

concentration of K+

movement of Na+

such as the Na+

oxidase. [36]-[38]

**6. Mitochondria**

+ /K+

8 Artery Bypass

Na+ /K+

tion, and increased intracellular sodium ([Na+

tion of voltage-dependent sodium channels. [29]

Without the respiratory chain oxidation-reduction reactions, proton accumulation in the mitochondrial intermembrane space is interrupted, disrupting the electrochemical gradient that allows ATP synthase to synthesize ATP. During ischemia, the protontranslocating F0F1-ATP synthase, which normally produces ATP, becomes an F0F1-ATPase and consumes ATP in order to pump protons from the matrix to the intermembrane space and maintain the mitochondrial membrane potential.[39],[40] The mitochondria therefore become a site of ATP consumption produced by anaerobic glycolysis. **6.2. An increase in free radical production**  Free radical oxygen species (ROS) are highly reactive chemical compounds because they have unpaired electrons in their electron cloud. ROS are capable of oxidizing cellular constituents such as proteins, deoxyribonucleic acid (DNA), membrane phospholipids Without the respiratory chain oxidation-reduction reactions, proton accumulation in the mi‐ tochondrial intermembrane space is interrupted, disrupting the electrochemical gradient that allows ATP synthase to synthesize ATP. During ischemia, the proton-translocating F0F1-ATP synthase, which normally produces ATP, becomes an F0F1-ATPase and con‐ sumes ATP in order to pump protons from the matrix to the intermembrane space and maintain the mitochondrial membrane potential.[39],[40] The mitochondria therefore be‐ come a site of ATP consumption produced by anaerobic glycolysis.

#### and other adjacent biological structures. In addition to their role in ischemia, ROS are constitutively generated during metabolic processes and have an important role in cell signaling. Mitochondrial respiration constitutively produces a small amount of ROS, **6.2. An increase in free radical production**

primarily the superoxide anion O2-● at complexes I and III of the electron transport chain. The anion is rapidly converted to hydrogen peroxide (H2O2) by metallo-enzymes and superoxide dismutase (SOD). [41]-[43] Cellular stress, particularly oxidative stress, dramatically increases mitochondrial ROS production by disrupting and later inhibiting oxidative phosphorylation. Moreover, the rise in mitochondrial calcium increases ROS production and greatly decreases the antioxidant capacity of mitochondria by decreasing the glutathione peroxidase concentration and SOD activity. **6.3. Intramitochondrial calcium overload**  Free radical oxygen species (ROS) are highly reactive chemical compounds because they have unpaired electrons in their electron cloud. ROS are capable of oxidizing cellular con‐ stituents such as proteins, deoxyribonucleic acid (DNA), membrane phospholipids and oth‐ er adjacent biological structures. In addition to their role in ischemia, ROS are constitutively generated during metabolic processes and have an important role in cell signaling. Mito‐ chondrial respiration constitutively produces a small amount of ROS, primarily the superox‐

mitochondrial swelling and the opening of the permeability transition pore.

**6.4. Opening of the mitochondrial permeability transition pore** 

The mitochondrial calcium concentration is in equilibrium between its cytosolic concentration and the proton gradient on either side of the inner membrane of mitochondria. The loss of this gradient due to the inhibition of the respiratory chain, as well as the elevated cytosolic calcium that results from ischemia, allows for the accumulation of calcium in the mitochondria and promotes

Ischemic disturbances within mitochondria, such as calcium overload, loss of membrane potential, oxidative stress, mass production of free radicals, low NADPH/NADP+ and reduced glutathione to oxidized glutathione ratios (GSH/GSSG), low intramitochondrial concentration of ATP or high inorganic phosphate, will promote opening of the permeability transition pore (mPTP) upon reperfusion, a major player in I/R injury-mediated cell lethality.[42],[44] mPTP is a nonspecific channel, and its opening ide anion O2 -● at complexes I and III of the electron transport chain. The anion is rapidly converted to hydrogen peroxide (H2O2) by metallo-enzymes and superoxide dismutase (SOD). [41]-[43] Cellular stress, particularly oxidative stress, dramatically increases mito‐ chondrial ROS production by disrupting and later inhibiting oxidative phosphorylation. Moreover, the rise in mitochondrial calcium increases ROS production and greatly decreases the antioxidant capacity of mitochondria by decreasing the glutathione peroxidase concen‐ tration and SOD activity.

communication and cell anchorage. Destruction of the internal architecture worsens I/R inju‐ ries and leads to apoptosis. [53],[56],[57] During ischemia, all elements of the cytoskeleton are affected, but with different kinetics.[54],[55] Moreover, the accumulation of osmotically active particles, including lactate, sodium, inorganic phosphate and creatine, induces cellu‐

Regulatory cellular mechanisms provide intracellular homeostasis that enables optimal en‐ zyme function in a relatively narrow range of environmental conditions. The conditions cre‐ ated by ischemia, such as acidosis and calcium overload, modify or inhibit the activity of many enzymes due to changes in the pH and tertiary structures, affecting cellular metabo‐ lism. For example, ischemia induces the conversion of xanthine dehydrogenase to xanthine oxidase.[36]-[38] These two enzymes catalyze the same reactions, converting hypoxanthine


**8. Protein synthesis and sarcoplasmic protein expression in an ischemic**

Protein synthesis is a complex process that requires continuous and adequate energy intake, strict control of ionic homeostasis of the cell and the smooth functioning of many other pro‐ teins. Ischemia disrupts these necessary conditions and therefore profoundly affects protein synthesis beyond acute injury. However, the transcription of several genes is initiated at the onset of ischemia, and the mechanisms underlying this phenomenon are not fully under‐ stood. Nevertheless, it appears that the mass production of free radicals, the high concentra‐ tion of calcium, acidosis and the activation of the family of mitogen-activated protein kinases (MAP kinases) play an important role. Nuclear factor heat shock transcription fac‐ tor-1 (HSF-1) activates the expression of heat shock proteins (HSPs), a family of chaperone proteins, and inhibits the expression of other proteins. HSPs are synthesized in different sit‐ uations of stress, including hyperthermia, ischemia, hypoxia and mechanical stress, and are intended to prevent the structural modifications of key metabolic and cytoskeletal enzymes

The low oxygen partial pressure during ischemia activates other nuclear factors, such as hy‐ poxia-inducible factor-1alpha (HIF-1α). HIF-1α stimulates the transcription of many genes involved in cellular defense, such as those encoding NOS and GLUT-1, and other enzymes

In addition, ischemia activates innate immunity by stimulating sarcoplasmic receptors, such as the Toll-like receptors (TLR) TLR-2 and TLR-6, the synthesis and sarcoplasmic expression of which are increased. Receptor stimulation supports the synthesis of chemokines and cyto‐

At the onset of ischemia, many substances are secreted by the cell. For example, ischemic cardiomyocytes secrete bradykinin, norepinephrine, angiotensin, adenosine, acetylcholine

as a cofactor, whereas the

Impact of Ischemia on Cellular Metabolism http://dx.doi.org/10.5772/54509 11

to xanthine and xanthine to uric acid. The first reaction uses NAD+

second uses oxygen and produces O2

and inhibit the activity of caspases. [58]-[60]

kines and contributes to I/R injury.[61]-[66]

involved in glucose metabolism.[61]

lar oedema.[38]

**cell**

#### **6.3. Intramitochondrial calcium overload**

The mitochondrial calcium concentration is in equilibrium between its cytosolic concentra‐ tion and the proton gradient on either side of the inner membrane of mitochondria. The loss of this gradient due to the inhibition of the respiratory chain, as well as the elevated cytosol‐ ic calcium that results from ischemia, allows for the accumulation of calcium in the mito‐ chondria and promotes mitochondrial swelling and the opening of the permeability transition pore.

#### **6.4. Opening of the mitochondrial permeability transition pore**

Ischemic disturbances within mitochondria, such as calcium overload, loss of membrane po‐ tential, oxidative stress, mass production of free radicals, low NADPH/NADP+ and reduced glutathione to oxidized glutathione ratios (GSH/GSSG), low intra-mitochondrial concentra‐ tion of ATP or high inorganic phosphate, will promote opening of the permeability transi‐ tion pore (mPTP) upon reperfusion, a major player in I/R injury-mediated cell lethality.[42], [44] mPTP is a nonspecific channel, and its opening suddenly increases the permeability of the inner mitochondrial membrane to both water and various molecules of high molecular weight (> 1,500 kDa). The opening of mPTPs abolishes the mitochondrial membrane poten‐ tial and uncouples oxidative phosphorylation, which empties the mitochondria of its matrix and induces apoptosis by releasing the intra-mitochondrial proteins cytochrome c, endonu‐ clease G, Smac/Diablo and apoptosis-inducing factor into the cytosol. [44]-[52]
