Coronary artery disease (CAD) is a kind of atherosclerosis where the arteries that supply blood to heart muscle will become hardened and narrowed. This condition occurs due to the formation plaque inside the arteries, plaques usually contain cholesterol and other materials. This leads to the insufficient supply of blood and oxygen to the heart which will in turn leads to the angina, heart attack, heart failure, and arrhythmias. CAD will cause changes in the structure and function of the blood vessel. The endothelial cells of coronary artery produces important substances such as nitric oxide and prostacyclin which are necessary for the normal function of coronary arteries, if the endothelial cells become damaged it leads to the impaired relaxation and blood clot formation. These clots can partially or completely blocks the vessels.

Atherosclerosis:

Atherosclerosis literally means narrowing of vessels (‘Athero’ means fatty and ‘sclerosis’ means hardening). Blood is a fluid connective tissue which consists of several types of cells like red blood cells, white blood cells and platelets. Blood plasma carries variety of substances from one part to another part of the body including cholesterol on special particles called lipoproteins. The low-density lipoprotein (LDL) carries cholesterol to the walls of arteries, when LDL supplies excess of cholesterol to the artery than the required amount, another type of lipoprotein called high-density lipoprotein (HDL) will carry the cholesterol away from the tissues to liver. Hence there is a direct link between LDL and HDL in controlling the cholesterol deposition, if there is an impair in the LDL or HDL function or availability it will lead to the atherosclerosis.

Atherosclerosis Is An Inflammatory Disease:

Previously scientists thought that the atherosclerosis was result of cholesterol storage, but recent studies show that atherosclerosis is an inflammatory disease (Russell Ross, N Engl J Med 1999). Because high plasma concentrations of cholesterol, in particular those of low-density lipoprotein (LDL) cholesterol, are one of the principal risk factors for atherosclerosis,1. If the blood contains high concentration of LDL, a small amount of LDL which is in artery will be oxidised. The oxidisation process will take place with the help of chemical reactions in the endothelium, this oxidation triggers set of chain reactions. LDL which can be modified by oxidation and triggers immune response is the main cause for injury of endothelium underlying smooth muscle (Khoo JC et al., 1988, 1992; Navab M et al., 1996; Morel DW et al., 1983; Griendling KK et al., 1997). The oxidised LDL leads to the expression of a molecular glue called endothelial-leukocyte adhesion molecules. Production of this molecule attracts monocyte derived macrophages and T lymphocytes to endothelium. The lesions of atherosclerosis represent a series of highly specific cellular and molecular responses that can best be described, in aggregate, as an inflammatory disease (Ross R, Glomset JA, N Engl J Med 1973, 1976, 1986, 1993).

Endothelial Dysfunction:

The first stage of formation of atherosclerosis lesion takes place in endothelium with increased permeability to lipoproteins, which is mediated by nitric acid and other growth factors. The oxidised LDL leads to the migration of leucocytes into artery walls. This process is mediated by monocytes, Interlukin-8, Platelet derived growth factors, macrophage stimulating factors.

Endothelial Dysfunction in Atherosclerosis (Ross; NEJM 1999)

Formation of Fatty Streaks:

Fatty streaks basically consist of foam cells (Cells that are derived from both macrophages and smooth muscle cells which have accumulated LDL’s). In the later stages foam cells will join various smooth muscle cells. The foam-cell formation is mediated by oxidized low-density lipoprotein, macrophage colony-stimulating factor, tumour necrosis factor {alpha}, and interleukin-1 and platelet adherence and aggregation, which are stimulated by integrins, P-selectin, fibrin, thromboxane A2, tissue factor, and the factors are responsible for the adherence and migration of leukocytes (Russell Ross, N Engl J Med 1999).

Fatty-Streak and Lesion Formation in Atherosclerosis (Ross; NEJM 1999)

The fatty streaks lead to the intermediate and advanced lesions which in turn form a fibrous cap. This triggers a type of healing process. Number of lipids leucocytes and debris will form a necrotic core under the fibrous cap. The lesions will expand due to the continuous leukocyte adhesion and entry of other factors. The necrotic core leads to the apoptosis and necrosis and lipid accumulation.
Rupture of Fibrous Cap:

The thin sites of fibrous cap that covers the advanced lesion can lead to the rupture of fibrous cap. This rupture will rapidly cause thrombosis. The thrombosis will block the artery.
Rupture of the fibrous cap (Ross; NEJM 1999)

Role of NO:

Nitric oxide (NO) released by the coronary arterial endothelium will play an important role in basal myocardial flow regulation and contractility (Amrani et al. 1992; Ueeda, Silvia & Olsson, 1992; Brown, Thompson & Belloni, 1993).
The abnormal function of NO due to the various diseases like atherosclerosis and hypertension can affect the normal blood flow and other abnormalities of vascular functions.

Basic Properties Of Nitric Oxide:

Nitric Oxide is a highly reactive free radical in nature.
It is a highly diffusible water soluble gas.
Freely crosses cell membranes.
It can be synthesised easily when required from an precursor.

Biosynthesis of Nitric Oxide:

The amino acid L-arginine produces Nitric Oxide using an enzyme called nitric oxide synthase (NOS). The enzyme endothelial synthase exists in three endothelial forms namely constitutive soluble NOS (cNOS; type III), inducible soluble NOS (iNOS; type II) and constitutive particulate. NO can easily diffuse through membranes and provides communication endothelial and smooth muscle cells as well as between nerve endings and smooth muscle cells (Int Angiol. Et al., 1992). Decreased production of nitric oxide may lead to vasospasm, whereas its overproduction may cause pathological vasodilatation. Understanding of the role of nitric oxide signal transduction pathway in regulation of vascular tone will facilitate design of better strategies for the prevention and treatment of vascular diseases (Int Angiol. Et al., 1992). cNOS which is calcium and calmodulin dependent enzyme produces NO under normal conditions of blood vessels. cNOS are stimulated by two basic pathways, both the pathways requires release of calcium ions from subsarcolemmal storage sites.

Pathway 1 – Flow-Dependent NO Formation:
The blood flow on vascular endothelium triggers a shearing force on the cells which leads to the release of calcium and thereby activating the cNOS. Hence if the blood flow is increased it will stimulate the NO formation

Pathway 2 – Receptor-Stimulated NO Formation:
The endothelial cells contains receptors for acetylcholine, adenosine, bradykinin and others. A variety class of ligands will stimulate these receptors and leading the release of calcium and subsequent production of NO.

In year 1914 Dale found that acetylcholine is responsible for powerful vasodilatation.
In 1980, Furchgott and Zawadski discovered that intact endothelial cells produced a substance in response to acetylcholine stimulation known as termed endothelium-derived relaxing factor (EDRF) (Furchgott RF et al., 1980) The EDRF was produced only when endothelial cells acted on vascular smooth muscle cells to produce relaxation. Later studies showed that EDRF is NO. Palmer et al., in 1987 demonstrated NO release from endothelial cells using a chemilumin assay (Palmer, R.M.J et al., 1987).

The discovery of relationship between NO and cardiovascular system is an major milestone in the field of cardiovascular research. Endothelial dysfunction has now become synonymous with the reduced biologic activity of NO (Yetik-Anacak G et al., 2006).

Conversion Of NG-L arginine to NO:

Conversion of NG-L arginine to NO is carried out by family of nitric oxide synthase. During the process NADPH donates 5 electrons for the process which will be carried out by co-factors FAD, FMN and BH4. L-arginine and oxygen acts as co substrates to form NO.

Generation Of Nitric Oxide

Intracellular Mechanisms Of NO:
Superoxide anion have high affinity towards NO, these molecules have unpaired electron making them highly reactive. The presence of superoxide anion reduces NO bioavailability.

NO readily binds to the hemoglobin, the NO binds to a specific moiety of hemoglobin called heme moiety, which is present in vascular smooth muscle along with an enzyme called guanylyl.

The NO formed from vascular endothelium will diffuse into blood ultimately binds to the haemoglobin and breaks down. NO will also activates guanylyl cyclase.

This enzyme guanylyl catalyzes the dephosphorylation of GTP to cGMP, which is a second messenger and important cellular functions. cGMP plays an important role in the signalling smooth muscle relaxation.

Conclusion:

NO Has Many Beneficial Effects On The Vasculature:

NO helps in relaxation which leads to the vessel vasodilation (Moncada S, et al., 2002), it stimulates endothelial cell proliferation (Ziche M et al., 1994) and prevents endothelial cell apoptosis, (Tzeng E, Kim YM, Pitt BR, et al 1997) it inhibits Vascular Smooth Muscle Cell (VSMC) growth and migration (Garg UC et al., 1989; Mooradian DL et al., 1995) and stimulates VSMC apoptosis (Kibbe MR, Li JR, Nie SH, et al 2002).

NO leads to the inhibition of platelet aggregation and activation14, and also leukocyte adhesion to the cells and migration15. The NO shows thromboresistant properties, such as inhibition of platelet aggregation, adhesion, and activation; (Radomski MW et al., 1987). The damage or dysfunction of the endothelium leads to the abnormal production of NO by endothelial cells and this leads to the aggregation of platelets sequentially leading to inflammatory response. Impaired activity of NO can lead to the hyper – tension, atherosclerosis and excess of NO activity can lead to the inflammatory diseases.

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