Nitric Oxide PDF

28/12/2023 Nitric oxide Osaze Edosuyi. PharmD. Ph.D. 1 2 1  28/12/2023 Robert Furchgott Ph.D. (1916-2009) Furchgott, R., Zawadz...

28/12/2023 Nitric oxide Osaze Edosuyi. PharmD. Ph.D. 1 2 1  28/12/2023 Robert Furchgott Ph.D. (1916-2009) Furchgott, R., Zawadzki, J. The obligatory role of endothelial cells in the relaxation of arterial smooth muscle by acetylcholine. Nature 288, 373–376 (1980). https://doi.org/10.1038/288373a0. 3 Introduction The innermost layer of the blood vessel (endothelium) releases a short-lived vasodilator initially called endothelium-derived relaxing factor (EDRF), but now known as Nitric oxide (NO). NO is an endogenous signaling molecule of physiological and pathophysiological relevance in major organs or systems. NO has been found to act as a significant biological mediator in the cardiovascular, neurological, and immunological systems 4 2  28/12/2023 Synthesis NO synthesis is principally regulated by the NO synthase isoforms. These enzymes include endothelial dependent nitric oxide synthase (eNOS), neuronal dependent nitric oxide synthase (nNOS) and inducible nitric oxide synthase (iNOS). eNOS and nNOS are physiological components of the system and are termed constitutive while iNOS is activated or induced by cytokines in situations involving microbial attack. The actions of eNOS and nNOS are calcium dependent. These enzymes act of the substrate, L-arginine, metabolizing it to NO and citrulline. This reaction occurs in the presence of co-factors such as tetrahydrobiopterin (BH4) and oxygen. Also, NO could also be generated via the nitrate-nitrite pathway under hypoxic conditions. 5 6 3  28/12/2023 Pharmacokinetics NO has a short half-life and is usually metabolized via different pathways. It could be exhaled via the lungs; it could also react with haemoglobin, and other radicals to form nitrosylated haemoglobin which is still oxygen sensitive and peroxynitrite, respectively. It could also dissolve in systemic circulation as nitrogen (ii) tetraoxide and excreted in the urine. 7 Mechanism of action Nitric oxide synthase (NOS) is activated by calcium in the cytosol. Endogenous ligands such as acetylcholine and shear stress increase intracellular calcium levels. The activated NOS metabolizes L-arginine to NO and citrulline. NO activates guanylyl cyclase (GC), which breaks down guanosine triphosphate (GTP) to cyclic guanosine monophosphate (cGMP), leading to smooth muscle relaxation. This results in the regulation of ion channels and phosphodiesterases, subsequently resulting in smooth muscle relaxation. 8 4  28/12/2023 9 Pharmacological Effects of NO The effects of NO are widespread but it most prominent in the cardiovascular, nervous and immune systems. NO could produce dual effects depending on its concentration, the physiology or pathology of the system. At low concentrations NO exerts beneficial effects but at higher concentrations NO could become toxic. For instance, NO could combine with the reactive specie, superoxide anion to form peroxynitrite which is a very toxic specie. Most of the pathological effects of NO arise because of the inducible nitric oxide synthase (iNOS) enzyme. 10 5  28/12/2023 Cardiovascular system NO is a principal vasodilator in the vascular system. NO constantly regulates vascular tone and by extension, blood pressure. NO also stimulates angiogenesis and acts as an anti-platelet drug. As a result of its effect of platelet activity and endothelial function, NO deficiency can lead to the generation of atherosclerotic plaques. 11 b) Immune system Nitric oxide acts as an antimicrobial, antitumour, and cytotoxic agent. It achieves this by binding modifying proteins in these organisms, leading to inactivation and death. Its cytotoxic effects are due to the free radical action of both NO and its toxic derivatives superoxide and peroxynitrite. Immune processes such as apoptosis, proliferation, cytokine generation and activation of adhesion molecules are NO mediated. 12 6  28/12/2023 c) Nervous system NO is a neurotransmitter present in the central and peripheral nervous system. It acts as a neuromodulator, regulates the pain perception and nerve transmission. 13 14 7  28/12/2023 Nitric oxide in therapy The activity of NO can be potentiated or inhibited in therapy. Therapeutic use of NO involves either the supplementation with exogenous nitric oxide donors or the pharmacological alteration of steps involved in NO synthesis with agents. a) Inhaled NO Nitric oxide has been inhaled to manage pulmonary hypertension and respiratory distress syndrome. 15 Drugs that act on nitric oxide synthesis i) Endogenous NOS inhibitors include L-arginine may undergo proteolytic transformation to methylarginines. These methylarginines (asymmetric demethylated arginine (ADMA) and L-N- methylarginine (L-NMA) are naturally occurring and compete with arginine at the active site of NOS. These compounds are degraded by dimethylarginine dimethylamino hydroxylase (DDAH). Inducers of DDAH could thus increase NO bioavailability. ADMA is a non-competitive inhibitor while L-NMA is a competitive inhibitor. 16 8  28/12/2023 ii) Synthetic NOS inhibitors N-nitro-L-arginine methyl ester (L-NAME) and Ng-nitro-L-arginine (L-NArg) are non-selective NOS inhibitors that also inhibit NOS activity and NO production. iv) Downstream inhibitors Methylene blue dye is an inhibitor of guanylyl cyclase and had been used to manage septic shock. H-[1,2,4] oxadiazoloquinoxalin-1-one (ODQ) is also an inhibitor of guanylyl cyclase used in improving motor deficits Parkinson’s disease. 17 Potentiate Nitric oxide donors (nitrovasodilators) Nitrovasodilators induced vasodilatation by increasing NO generation. They include organic nitrates (amyl nitrite, nitroglycerin, isosorbide mono- and dinitrate) and the inorganic nitrates (sodium nitroprusside, SNP). The organic nitrates are principally used in managing angina pectoris while SNP is employed in hypertensive emergencies. S-nitroacetylpenicillamine (SNAP) and s-nitrosoglutathione (SNOG) are also nitric oxide donors. 18 9  28/12/2023 19 10