Nitric oxide (NO) serves as a mediator in various biological systems, acting as the relaxing factor in endothelium, causing vasodilation by relaxing vascular smooth muscle. It also functions as a neurotransmitter, prevents platelet aggregation, and plays a vital role in macrophage function. Due to its reaction with O2, NO has a short half-life in tissues (3-10 seconds) and is converted into nitrates and nitrites, including the reactive nitrogen species peroxynitrite (O=NOO−). It should be noted that NO is a free radical gas often mistaken for nitrous oxide (N2O), the stable "laughing gas" used as an anesthetic.

1. The production of nitric oxide involves the use of cytosolic NO synthase (NOS), which requires substrates such as arginine, O2, and NADPH. Coenzymes including flavin mononucleotide (FMN), FAD, heme, and tetrahydrobiopterin participate in the reaction, resulting in the formation of NO and citrulline. There are three distinct NOS isozymes produced by different genes. Two isozymes are constitutive and dependent on calcium-calmodulin (Ca2+-CaM) as they are synthesized at a constant rate. These constitutive NOS isozymes, known as eNOS and nNOS, are primarily found in endothelium and neural tissue, respectively. They continuously produce low levels of NO, contributing to vasodilation and neurotransmission. On the other hand, the inducible iNOS enzyme can be expressed in various cell types, including macrophages and neutrophils, to combat pathogens. The specific inducers for iNOS differ depending on the cell type and may include proinflammatory cytokines like tumor necrosis factor-alpha (TNF-α), interferon-gamma (IFN-γ), and bacterial endotoxins such as lipopolysaccharide (LPS). When iNOS is synthesized, it can lead to the production of significant amounts of NO over extended periods, ranging from hours to days.

2. Nitric oxide plays a crucial role in the control of vascular smooth muscle tone. In endothelial cells, eNOS synthesizes NO, which then diffuses to vascular smooth muscle. Once there, NO activates the cytosolic form of guanylyl cyclase, resulting in the formation of cyclic guanosine monophosphate (cGMP). This reaction is similar to the formation of cyclic adenosine monophosphate (cAMP) by adenylyl cyclase. The increase in cGMP levels activates protein kinase G, which phosphorylates calcium channels. As a result, the entry of calcium into smooth muscle cells decreases. This reduction in calcium-CaM activation leads to a decrease in smooth muscle contraction and promotes relaxation. Nitroglycerin and other vasodilator nitrates are metabolized to NO, causing relaxation of vascular smooth muscle and subsequently lowering blood pressure. Therefore, NO can be considered as an endogenous nitrovasodilator. In hypoxic conditions, nitrite (NO2−) can be reduced to NO, which binds to deoxyhemoglobin. The bound NO is later released into the blood, leading to vasodilation and increased blood flow.

3. Macrophages exhibit low iNOS activity under normal circumstances. However, bacterial LPS and the release of IFN-γ in response to infection significantly stimulate the synthesis of iNOS enzyme in activated macrophages. These activated macrophages generate radicals that combine with NO, forming intermediates that decompose and produce highly bactericidal OH• radicals.

4. In addition to its other roles, NO acts as a potent inhibitor of platelet adhesion and aggregation by activating the cGMP pathway. It also functions as a neurotransmitter in both the central and peripheral nervous systems.
Nitric oxide (NO) is recognized as a mediator in a broad array of biologic systems. NO is the endothelium-derived relaxing factor that causes vasodilation by relaxing vascular smooth muscle. It also acts as a neurotransmitter, prevents platelet aggregation, and plays an essential role in macrophage function. It has a very short half-life in tissues (3–10 seconds) because it reacts with O2 and and is converted into nitrates and nitrites including peroxynitrite (O=NOO−), a reactive nitrogen species (RNS). [Note: NO is a free radical gas that is often confused with nitrous oxide (N2O), the “laughing gas” that is used as an anesthetic and is chemically stable.] 1. Nitric oxide synthase Arginine, O2, and NADPH are substrates for cytosolic NO synthase ([NOS], Fig. 13.9). Flavin mononucleotide (FMN), FAD, heme, and tetrahydrobiopterin (see p. 268) are coenzymes, and NO and citrulline are products of the reaction. Three NOS isozymes, each the product of a different gene, have been identified. Two are constitutive (synthesized at a constant rate), calcium (Ca2+)–calmodulin (CaM)-dependent enzymes (see p. 133). They are found primarily in endothelium (eNOS) and neural tissue (nNOS) and constantly produce very low levels of NO for vasodilation and neurotransmission. An inducible, Ca2+-independent enzyme (iNOS) can be expressed in many cells, including macrophages and neutrophils, as an early defense against pathogens. The specific inducers for iNOS vary with cell type and include proinflammatory cytokines, such as tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ), and bacterial endotoxins such as lipopolysaccharide (LPS). These compounds promote synthesis of iNOS, which can result in large amounts of NO being produced over hours or even days.
2.
Nitric oxide and vascular endothelium NO is an important mediator in the control of vascular smooth muscle tone. NO is synthesized by eNOS in endothelial cells and diffuses to vascular smooth muscle, where it activates the cytosolic form of guanylyl cyclase (or, guanylate cyclase) to form cyclic guanosine monophosphate (cGMP). [Note: This reaction is analogous to the formation of cyclic adenosine monophosphate (cAMP) by adenylyl cyclase (see p. 95).] The resultant rise in cGMP causes activation of protein kinase G, which phosphorylates Ca2+ channels, causing decreased entry of Ca2+ into smooth muscle cells. This decreases the Ca2+–CaM activation of myosin light-chain kinase, thereby decreasing smooth muscle contraction and favoring relaxation. Vasodilator nitrates, such as nitroglycerin, are metabolized to NO, which causes relaxation of vascular smooth muscle and, therefore, lowers blood pressure. Thus, NO can be envisioned as an endogenous nitrovasodilator. [Note: Under hypoxic conditions, nitrite (NO2−) can be reduced to NO, which binds to deoxyhemoglobin. The NO is released into the blood, causing vasodilation and increasing blood flow.] 3.
Nitric oxide and macrophage bactericidal activity In macrophages, iNOS activity is normally low, but synthesis of the enzyme is significantly stimulated by bacterial LPS and by release of IFN-γ response to infection. Activated macrophages form radicals that combine with NO to form intermediates that decompose, producing the highly bactericidal OH• radical.
4.
Additional functions NO is a potent inhibitor of platelet adhesion and aggregation (by activating the cGMP pathway). It is also characterized as a neurotransmitter in the central and peripheral nervous systems.