**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]


3.1.1. Weld Appearance

The resistance spot weld appearance depends on the coupling effect of heat, electric, and load, which will influence the welded joint quality, corrosion resistance, and look. Figure 3 shows the appearances of typical resistance spot welded joints, which were gained with different welding currents. During the resistance spot welding process, while the welding current was 8.5 kA~10.5 kA, there was no welding spatter on the spot weld surface. Until the welding current was 11 kA, the welding spatters were generated, and the welding spatters obviously increased. The welding spatter is caused by the large welding heat input, which results in the faster speed of melting metal than the expansion speed of the plastic ring, and the melted liquid metals fly out of the plastic ring. Because the welding current is the main parameter to determine the heat input (*Q* = *i* 2*Rt*, *Q* is the welding heat input, *i* is the welding current, and *t* the is welding time), the welding heat input increases sharply when the welding current is increased, then the high heat input brings about more welding spatters.

**Figure 3.** Resistance spot weld appearance gained with different welding currents: (**a**) 8.5 kA; (**b**) 10.0 kA; (**c**) 11.0 kA; (**d**) 12.0 kA.

Figure 4 indicates the three feature zones on the resistance spot weld surface, and it comprises of three circle zones (circle zone I, circle zone II, and circle zone III) from the center to the outside. The circle zone I was the area where the electrodes contacted with the base metal during the resistance spot welding process, and is located in the centre of the spot weld surface. It was produced by various physical factors, such as the heat, electricity, and load, of the electrodes and the binding force of the base metal. Because of the highest temperature in the circle zone I and the low melting point of Zn (692 K), the welding heat caused the Zn layer to melt and be squeezed away by the electrodes, meanwhile the base metal substrate was exposed and oxidized. The circle zone II is located outside the adjacent region of the circle zone I. The profile of the circle zone II depended on the working surface shape of the electrodes and the indentation depth of the spot welded joint. On the circle zone II, the Zn extruded from circle zone I along the edge of the electrodes, and molten Zn on circle zone II was aggregated by the action of gravity and surface tension, then solidified to form the Zn island as shown in Figure 4. The SEM image and element (white spots in Figure 5b–d) map distribution of the Zn island (on the circle zone II in Figure 4 as an example) are shown in Figure 5, and the main components of the Zn island comprised of O (8.57%), Fe (15.44%), and Zn (65.24%). The circle zone III was the heat affected zone of spot welded joints. The base metal in this zone was heated and the Zn layer was oxidized to form ZnO. The Zn island also could generate on the circle zone III.

**Figure 4.** Three feature zones on the resistance spot weld surface.

**Figure 5.** SEM image and element map distribution of Zn island: (**a**) SEM image; (**b**)OKα 1; (**c**) Fe Kα 1; (**d**) Zn Kα 1.

From Figure 3a, due to the low welding current, the Zn on the surface of circle zone I was partially melted and the color was not much different from that on the base metal; the profile of circle zone II was small with a smooth transition, and the Zn island was generated in the circle zone III. While the welding current was 10.0 kA, the Zn layer on the surface of the circle zone was not seriously damaged, and the inner and outer colors of circle zone II were quite different. The Zn island was formed on the adjacent part of circle zone I, because the Zn layer on the circle zone II was melted and extruded, and then cooled and crystallized on the outside. There was a Zn island on the circle zone III. When the welding current continued to increase to 11.0 kA, the Zn layer on the circle zone I was seriously damaged. There was a Cu-Zn alloy formed by the reaction of the molten Zn layer and copper on the edge of circle zone II, and the electrodes' adhesion occurred. When the welding current reached 12.0 kA, the appearance quality of the spot welded joint decreased obviously and many spatters were generated because of the uneven heat distribution of the electrodes. Figure 6 indicates the relation between the welding current and the diameters of the three feature zones on the weld appearance. When the welding current was 8.5~9.5 kA, the diameters of circle zone I were almost unchanged. With the continued increase of the welding current, the diameters of circle zone I increased to the maximum at first and then decreased gradually. The diameter of the circle zone II changed little with a low welding current, which was the largest with a 10.5 kA current. The diameter of the circle zone III increased with the increase of the current from 8.5 kA to 10.5 kA, but when the welding current was higher than 10.5 kA, the diameter of circle zone III did not change obviously. The results displayed that the welding current had an important influence on the weld appearance, and a low welding current was used on the basis of meeting the strength requirements of the welded joint.

**Figure 6.** Effect of the welding current on sizes of three feature zones on the weld appearance.

### 3.1.2. Main Dimensions of Welded Joint Cross-Section

The main dimensions of the resistance spot welded joint cross-section include the indentation (usually expressed by indentation rate, D/δ) and weld nugget width at the overlap surface (W), as shown in Figure 7. The indentation influences the weld appearance smoothness, reduces the welded joint cross-section size, and causes stress concentration, which results in reducing the strength of the welded joint. The tensile strength of the resistance spot welded joint is mainly controlled by the W.

**Figure 7.** Schematic diagram of the main dimensions of the welded joint cross-section.

The effect of the welding current on the indentation rate is indicated in Figure 8a. The results indicated that the indentation rate was small and increased less with a low welding current because the welding heat input was small, which caused a small amount of base metal melting. When the welding current was between 9.5 kA and 11.0 kA, the welding heat input increased rapidly, so more base metal was melted and the indentation rate increased. If the welding current was greater than 11.0 kA, the indentation rate increased gradually due to welding spatters and other defects, and the indentation was too serious to satisfy the welding quality requirements. Figure 8b displays the relationship between the welding current and the W. The weld nugget width increased rapidly from 7.36 mm to 8.64 mm with an increase of the current from 8.5 kA to 10.0 kA, and the maximum value was 8.75 mm at the 10.5 kA welding current. While the current changed from 10.0 kA to 10.5 kA, the welding heat input reached a quasi-steady state and the change of the weld nugget width was small. When the welding current was greater than 10.5 kA, the current density was higher, and a large number of welding spatters were generated, which reduced the amount of melted base metal in the weld nugget and thus decreased the weld nugget width. If the welding current continued to increase to 12.0 kA, the weld nugget width increased because of the larger welding heat input, but there were many welding spatters, and also some shrinkage and crack defects in the weld nugget.

**Figure 8.** Relationships between the welding current and (**a**) indentation rate (D/δ); (**b**) weld nugget width at overlap surface (W).


**Author details**

Joaquim Miranda Maloa Department of Geosciences at Rovuma University - Niassa Extension, Lichinga, Mozambique

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