**1. Introduction**

Cardiovascular diseases (CVDs), i.e., ischemic heart disease and stroke, remain the leading cause of death in the past decades around the world, especially in the developed countries [1]. CVDs can start from risk factors that may cause local vascular lesion and end up with systematic complications, which lead to organ failure and death. Thus, understanding the biochemistry of events involved in the whole process of CVD progression is crucial to prevent and treat the disease.

Epidemiological data show that various factors are associated with the increase of cardiovascular morbidity and mortality, including hypertension, smoking,

hypercholesterolemia, diabetes mellitus, obesity, stress, low fruit and vegetable dietary, lack of regular exercise, and abnormal sleep [2]. Current therapeutic strategies mainly focus on reducing patients' blood pressure, restoring redox balance, controlling cholesterol, and implementing physical activity programs [3]. In this chapter, we explore the physiological and pathological events in the cardiovascular system from the molecular biology's perspectives. Molecular mechanism of muscle contraction and relaxation in the cardiovascular system will be discussed first. Then, we will delve into the biological effects of Nobel Prize molecule nitric oxide (NO), the most important vasodilator in the body. In addition, due to the inspiring clinical outcomes of using isosorbide dinitrate (an NO stimulus) and hydralazine (an antioxidant) to treat patients with symptomatic congestive heart failure [4], we will also discuss how nitroso-redox balance mediates cardiovascular functions.

## **2. Muscle contraction and relaxation**

#### **2.1 Sliding filament theory**

Skeletal, cardiac, and smooth muscles have different structures and regulatory mechanisms, but they share the same molecular mechanism of contraction and relaxation, i.e., the relative sliding between myofilaments [5]. To understand how the heart beats and how blood vessels regulate their tones, it is important to look into the subcellular structure of these tissues (**Figure 1**).

Heartbeat relies on myofibrils, a fiber bundle structure that abounds in cardiomyocyte (**Figure 1c**). When a number of myofibrils are highly aligned, sarcomere, a repeat unit in the myofibril, can be observed under the microscope. Sarcomere is the basic unit for motion. Two most important proteins in the sarcomere are: myosin and actin. A myosin contains the N-terminal globular head domain, the short neck domain, and the long C-terminal coiled-coil tail domain. The globular head works as a specialized adenosine triphosphatase (ATPase), responsible for adenosine triphosphate (ATP) binding, actin binding, and generating force from ATP hydrolysis. The neck domain transduces force generated by the head. And the fibrous tails are bundled together to form the thick filament (**Figure 1g**). Actin, together with troponin and tropomyosin, forms the thin filament. When the two fibers slide toward each other, the overlapped region increases, which is the mechanism of muscle contraction. Similarly, when the fibers slide away from each other, muscles relax (**Figure 1e**).

#### **2.2 Cross-bridge cycling**

The filament sliding depends on cross-bridge cycling [6]. A cross-bridge refers to the two globular heads of myosin, which take turns to bind, pull, and detach from the actin fiber to achieve relative movement between the filaments. An analogy is that a person alternately uses two hands to pull a rope. One alteration of the hand is considered to be one cycle. There are four basic states [7] (sometimes detailed to six states [8]) in cross-bridge cycling. Each state corresponds to one behavior of ATP and one response of myosin. State 1: activation of myosin head, when ATP binds to myosin, it is hydrolyzed to ADP and Pi (inorganic phosphate); myosin becomes the "cocked position." State 2: cross-bridge formation, the activated myosin binds to actin; Pi is released to stabilize the binding. State 3: power stroke, ADP is released; myosin generates force to pull actin filament. State 4: detachment of cross bridge, another ATP binds to myosin; myosin disengages from actin; then State 1 is repeated. The continuous cross-bridge cycling allows myosin to pull actin to its tail side, resulting in

**5**

**Figure 1.**

*cross-bridge cycling (g).*

*Nitric Oxide and Oxidative Stress-Mediated Cardiovascular Functionality: From Molecular…*

filament sliding and muscle contraction. At the resting state, actin's myosin-binding site is blocked by troponin and tropomyosin [9] (**Figure 1g**). A switch mechanism is needed to expose and mask the myosin-binding site to regulate muscle contraction.

*Muscle contraction illustrated on different structural levels in the cardiovascular system: tissue level—heart (a) and blood vessel (b); cell level—cardiomyocyte (c) and vascular smooth muscle cells (SMCs) (d); subcellular level—filament sliding in cardiomyocyte (e) and SMCs (f); and molecular level—thin and thick filament for* 

*DOI: http://dx.doi.org/10.5772/intechopen.82556*

*Nitric Oxide and Oxidative Stress-Mediated Cardiovascular Functionality: From Molecular… DOI: http://dx.doi.org/10.5772/intechopen.82556*

#### **Figure 1.**

*Vascular Biology - Selection of Mechanisms and Clinical Applications*

**2. Muscle contraction and relaxation**

into the subcellular structure of these tissues (**Figure 1**).

**2.1 Sliding filament theory**

muscles relax (**Figure 1e**).

**2.2 Cross-bridge cycling**

hypercholesterolemia, diabetes mellitus, obesity, stress, low fruit and vegetable dietary, lack of regular exercise, and abnormal sleep [2]. Current therapeutic strategies mainly focus on reducing patients' blood pressure, restoring redox balance, controlling cholesterol, and implementing physical activity programs [3]. In this chapter, we explore the physiological and pathological events in the cardiovascular system from the molecular biology's perspectives. Molecular mechanism of muscle contraction and relaxation in the cardiovascular system will be discussed first. Then, we will delve into the biological effects of Nobel Prize molecule nitric oxide (NO), the most important vasodilator in the body. In addition, due to the inspiring clinical outcomes of using isosorbide dinitrate (an NO stimulus) and hydralazine (an antioxidant) to treat patients with symptomatic congestive heart failure [4], we will also discuss how nitroso-redox balance mediates cardiovascular functions.

Skeletal, cardiac, and smooth muscles have different structures and regulatory mechanisms, but they share the same molecular mechanism of contraction and relaxation, i.e., the relative sliding between myofilaments [5]. To understand how the heart beats and how blood vessels regulate their tones, it is important to look

Heartbeat relies on myofibrils, a fiber bundle structure that abounds in cardiomyocyte (**Figure 1c**). When a number of myofibrils are highly aligned, sarcomere, a repeat unit in the myofibril, can be observed under the microscope. Sarcomere is the basic unit for motion. Two most important proteins in the sarcomere are: myosin and actin. A myosin contains the N-terminal globular head domain, the short neck domain, and the long C-terminal coiled-coil tail domain. The globular head works as a specialized adenosine triphosphatase (ATPase), responsible for adenosine triphosphate (ATP) binding, actin binding, and generating force from ATP hydrolysis. The neck domain transduces force generated by the head. And the fibrous tails are bundled together to form the thick filament (**Figure 1g**). Actin, together with troponin and tropomyosin, forms the thin filament. When the two fibers slide toward each other, the overlapped region increases, which is the mechanism of muscle contraction. Similarly, when the fibers slide away from each other,

The filament sliding depends on cross-bridge cycling [6]. A cross-bridge refers to the two globular heads of myosin, which take turns to bind, pull, and detach from the actin fiber to achieve relative movement between the filaments. An analogy is that a person alternately uses two hands to pull a rope. One alteration of the hand is considered to be one cycle. There are four basic states [7] (sometimes detailed to six states [8]) in cross-bridge cycling. Each state corresponds to one behavior of ATP and one response of myosin. State 1: activation of myosin head, when ATP binds to myosin, it is hydrolyzed to ADP and Pi (inorganic phosphate); myosin becomes the "cocked position." State 2: cross-bridge formation, the activated myosin binds to actin; Pi is released to stabilize the binding. State 3: power stroke, ADP is released; myosin generates force to pull actin filament. State 4: detachment of cross bridge, another ATP binds to myosin; myosin disengages from actin; then State 1 is repeated. The continuous cross-bridge cycling allows myosin to pull actin to its tail side, resulting in

**4**

*Muscle contraction illustrated on different structural levels in the cardiovascular system: tissue level—heart (a) and blood vessel (b); cell level—cardiomyocyte (c) and vascular smooth muscle cells (SMCs) (d); subcellular level—filament sliding in cardiomyocyte (e) and SMCs (f); and molecular level—thin and thick filament for cross-bridge cycling (g).*

filament sliding and muscle contraction. At the resting state, actin's myosin-binding site is blocked by troponin and tropomyosin [9] (**Figure 1g**). A switch mechanism is needed to expose and mask the myosin-binding site to regulate muscle contraction.

Intracellular Ca2+ works as a secondary messenger that quickly bonds to troponin, causing a quick conformational change of troponin and tropomyosin [10]. Thus, the myosin-binding site on actin filaments is exposed, and cross-bridge cycling proceeds. Intracellular Ca2+ concentration, or [Ca2+]i, can return to a very low level to cease the contraction and cause relaxation by different mechanisms, such as extruding Ca2+ out of cells or storing cytosolic Ca2+ into sarcoplasmic reticulum (SR) which functions as the Ca2+ reservoir in the cardiomyocyte. Similar mechanisms exist in the vascular tissue. SMC layer lies in between the endothelium layer and adventitia. There is no organized contractile protein fibril or sarcomere structure in SMCs [11]. Instead, the contractile fibrous proteins along with other intermediate filaments form bundles that are immobilized by anchoring proteins onto cell cytoskeletons. These filaments distribute all over the cytoplasm and connect each other through anchoring proteins (dense bodies) to form a threedimensional network (**Figure 1d** and **f**). Unlike in cardiac muscles, actin filament in smooth muscles is associated with caldesmon, tropomyosin, and calmodulin (CaM) [12]. CaM is an important Ca2+ sensing protein, which binds and mediates many enzymes' activities upon Ca2+ signaling. Caldesmon binds to actin, which inhibits the activity and motility of actin-myosin ATPase, and this binding is greatly strengthened by tropomyosin [13]. Ca2+ binds and activates CaM to uncouple the interaction between caldesmon and actin. Thus, actin's myosin-binding sites are exposed to myosins. Different from skeleton or cardiac tissues, the contraction in smooth muscles also depends on phosphorylation level of myosin light chain, which is adjusted by the enzyme activity of CaM-dependent myosin light chain kinase (CaM-dependent MLCK) and myosin light chain phosphatase (MLCP) [14]. MLCK adds the phosphoryl group to the myosin light chain, while MLCP removes it. Thus, increase of [Ca2+]i also facilitates muscle contraction through enhancing myosin phosphorylation [15].

Cardiovascular contractility is crucial for blood pressure homeostasis, thermal exchanging, mass transfer, immune responses, and organ functions [16]. Impaired coronary artery contractility (caused by the block of blood vessel or poor dilation) incurs ischemia heart disease [17]. To maintain cardiomyocyte viability and vascular tones, enhancing vasodilation and restricting oxidative stress are critical.
