Pharmacology of Nitric oxide

Pharmacology of Nitric oxide

1. Nitric oxide Pharmacology, Donors & Inhibitors RVS Chaitanya koppala

2. ENDOTHELIUM

3. ENDOTHELIUM The endothelium is the thin layer of cells that lines the interior surface of Blood vessels and lymphatic vessels forming an interface between circulating blood and lymph in the lumen and the rest of the vessel wall. The cells that form the endothelium are called endothelial cells . Endothelial cells in direct contact with blood are called vascular endothelial cells where as those in direct contact with lymph are known as lymphatic endothelial cells.

4. Endothelial cells release substances acting directly on vascular smooth muscle cells, causing either contraction or relaxation .

5. In 1980 Furchgott & Zawadzki first described endothelium- dependent relaxation of the blood vessels  by acetylcholine . Further studies in 1984 revealed that other factors such as bradykinin, histamine and 5-hydroxytryptamine release endothelium derived relaxing factor (EDRF), which can modulate vessel tone. In 1987 Furchgott proposed that EDRF might be nitric oxide (NO) based on a study of the transient relaxations of endothelium-denuded rings of rabbit aorta to ‘acidified’ inorganic nitrite (NO - ) solutions and the observations that superoxide dismutase (SOD, which removes O 2 - ) protected EDRF. ENDOTHELIUM DERIVED RELAXING FACTOR

6. The following year in 1989 the enzyme responsible for NO production, NO synthase , was discovered and NO pathway was proposed. Neuronal and humoral mediators, e.g. Ach, adrenaline , noradrenaline , histamine , 5-HT, ATP, Adenosine , Substance P , Arginine Vasopressin (AT/VP), Bradykinin , Thrombin and Ca ++ , Insulin , Angiotensin , TNF- α , IL-6 , A rginase , A symmetric D imethylarginine (ADMA), Dimethylarginine Dimethylaminohydrolase (DDAH ), etc. acting in their corresponding receptors or cellular structures can affect production and release of NO.

7. NITRIC OXIDE A chemical compound with formula NO is a free radical gas. It is first identified as endothelial derived releasing factor(E D R F ). At high concentration , fight against infectious organism and cancer cell. At lower concentration helps in regulating the circulatory and central nervous system. Nitric oxide differs from other neurotransmitter and hormones in a way that it is not regulated by storage, release , or targeted degradation. NO does not require receptor for its action when synthesized immediately utilized. Ca++ clamudulin complex is necessary for nitric oxide synthesis.

8. Synthesis of Nitric Oxide Nitric oxide is synthesized from L-arginine. This reaction is catalyzed by nitric oxide synthase, a 1,2,9,4 amino acid enzyme.

9. Release & MOA of NO-

10. INTRACELLULAR MECHANISM: Nitric oxide also binds to the heme moiety of hemoglobin and heme moiety of enzyme gunayl cyclase , which is found in smooth muscle cell and most other cells of body. When NO formed by vascular endothelium it rapidly diffuses into the blood where it binds to hemoglobin & subsequently broken down .

11. which serve as a second messenger for many important cellular function , particular for signaling smooth muscle contraction. cGMP induces smooth muscle relaxation by multiple mechanism including- I ncreased intracellular cGMP which inhibit ca++ entry into the cell and decrease intracellular ca++ concentration. Activates k+ channel which leads to hyper polarization & relaxation . Stimulates a cGMP dependent protein kinase that activates myosin light chain phosphate (MLCK) the enzyme that dephosphorylate myosin light chain leads to smooth muscle relaxation

12. Types of NOS NOS I or n NOS Central and peripheral neuronal cells, brain, spinal cord, platelets. Ca ++ dependent, used for neuronal communication Constitutive NOS II or iNOS Most nucleated cells, particularly macrophages Independent of intracellular Ca ++. Inducible in presence of inflammatory cytokines, bacterial liposaccharides. NOS III or e NOS Present on Vascular endothelial cells and neuronal cells Ca+2 dependent Vascular regulation NOS Constitutive Inducible

13. Nitric Oxide in the human body has many uses which are best summarized under five categories. NO in the nervous system NO in the circulatory system NO in the muscular system NO in the immune system NO in the digestive system No in the reproductive system NO in the gene toxicity No in the apoptosis 14 Role of Nitric oxide

14. NO is a signaling molecule, but not necessarily a neurotransmitter. NO signals inhibition of smooth muscle contraction, adaptive relaxation, and localized vasodilation. NO diffuses out of the cells making it vesicular storage in vesicles and release by exocytosis NO does not bind to surface receptors, but instead exits cytoplasm, enters the target cell, and binds with intracellular guanylyl cyclase Present in presynaptic terminal Natural removal from synaptic junction 15 Nitric oxide in the Nervous system NO serves in the body as a neurotransmitter, but there are definite differences.

15. Role in N eurodegenerative disease Implicated in :- Alzheimer disease Parkinson disease All are related to the excessive release of NO & glutamate both. But in Parkinson's disease Glial cells produce excessive levels of nitric oxide, which may be neurotoxic for a sub population of dopaminergic neurons. The presence of glial cells expressing nitric oxide synthase in the substantia nigra of patients with Parkinson's disease represents a consequence of dopaminergic neuronal loss.

16. Play a role in long term memory As a messenger that facilitates long term potentiation of neurons (memory) Synthesis mechanism involve Ca 2+ / Calmodulin activates NOS-I activates Guanyl cyclase cycle of nerve action potentials catalyzes cGMP production

17. Nitric oxide in the circulatory system NO serves as a vasodilator Released in response to high blood flow rate and signaling molecules (Ach and bradykinin) Highly localized and effects are brief If NO synthesis is inhibited, blood pressure increases NO aids in gas exchange between hemoglobin and cells Hemoglobin is a vasoconstrictor, Fe scavenges NO NO is protected by cysteine group when O 2 binds to hemoglobin. During O 2 delivery, NO locally dilates blood vessels to aid in gas exchange

18. Nitric oxide in the Muscular system NO was originally called EDRF (endothelium derived relaxation factor ) NO signals inhibition of smooth muscle contraction Ca 2+ is released from the vascular lumen activating NOS NO is synthesized from NOS III in vascular endothelial cells This causes guanyl cyclase to produce cGMP A rise in cGMP causes Ca2+ pumps to be activated, thus reducing Ca2+ concentration in the cell This causes muscle relaxation

19. Role in Blood vessels

20. Role in the Immune system NOS II catalyzes synthesis of NO used in host defense reactions Activation of NOS II is independent of Ca 2+ in the cell Synthesis of NO happens in most nucleated cells, particularly macrophages. NO is a potent inhibitor of viral replication. NO is a bactericidal agent NO is created from the nitrates extracted from food near the gums. This kills bacteria in the mouth that may be harmful to the body.

21. Role In the Digestive system NO is used in adaptive relaxation NO promotes the stretching of the stomach in response to filling. When the stomach gets full, stretch receptors trigger smooth muscle relaxation through NO releasing neurons . Role In the Reproductive system: NOS localized in pelvic nerve neuron innervating the corpora cavrinosa and the neuronal plexuses of the adventitial layer of the penile arteries – proven most effective for erectile dysfunction.

22. NO and its derivatives produced in inflamed tissue contribute to the carcinogenesis process due to direct or indirect DNA damage. Direct DNA damage: DNA deamination , peroxynitrite induced adult formation single strand break in the DNA Indirect DNA damage: interaction of NO reactive species with other molecule like amines , thioles , lipids. -NO after reaction with O2/superoxide forms genotoxicity . Role In Genotoxicity :

23. Role in wound healing & tissue repair No is powerful stimulator of cell division maturation and differentiation . Necessary mediator of neuro vascularization i.e. angiogenesis and lymph ducts to nourish the healing of tissue. Role in Apoptosis Nitric oxide and its reaction products either promotes or prevent apoptosis . Pro-apoptosis effect of NO- induction of apoptosis by NO resulting in the accumulation of tumor suppressor protein p-53. Anti-apoptotic effect of NO- some studies suggest that endogenous iNOS expression or exposure to low dose of NO donors inhibits apoptosis.

24. Role in I nflammation NO has shown to act as a mediator of inflammatory processes. This process has enhanced the effect of cyclooxygenases and stimulates the production of pro-inflammatory eicosanoids .

25. Mechanism of action of NO

26. Oxides of nitrogen Name Known function Nitric oxide (NO) Vasodilator, Platelet inhibitor, immune regulator, neurotransmitter. Peroxynitrite (NO3-) Oxidant and nitrating agent Nitroxyl anion (NO-) Exhibit NO like action by oxidising like NO Nitrous oxide (N2O) Anesthetic Dinitrogen trioxide (N2O3) Auto oxidation product of NO that can nitrosylate protein binds Nitrite (NO2-) Decomposes to NO at acidic Ph Nitrate (NO3-) Stable oxidation of NO-

27. Nitric oxide synthesis inhibitors Primary strategy to reduce NO generation is to use NO synthesis inhibitors. Majority of inhibitors are L-arginine analogs which bind to the NOS arginine binding site. No selectivity for specific NOS and hence acts generally on all the NOS NOS Approach for inhibition n NOS Neurodegenrative diseases e NOS Heamostatic signalling i NOS Inflammatory disorders

28. Nitric oxide Donors NO donors which release NO or related NO species , are used to elicit Smooth muscle relaxation. Different classes of NO donors have different biologic response. Organic nitrates Organic nitrites Sodium nitroprusside NO gas inhalation Alternate strategies (type 5 phosphodiesterase )

29. Organic nitrates Nitroglycerin dilates veins and coronary arteries is metabolized to NO by aldehyde reductase. Venous dilation --- decrease cardiac preload Arterial dilation --- antianginal effects. Isosorbide dinitrate metabolized to NO releasing species through a poorly understood enzymatic path. Less significant effect on aggregation of platelets. Organic nitrates exhibits rapid tolerance during continuous administration.

30. Organic nitrites Sodium nitroprusside : D ilates arterioles and venules . Used for rapid pressure reduction in arterial hypertension In response to light/chemical/enzymatic mechanism in cell membranes SNP breaks down to generate five cyanide molecules and a single NO.

31. NO gas inhalation NO itself can be used therapeutically, Results in reduced pulmonary artery pressure Improved perfusion of ventilated areas of the lung. Inhaled NO- pulmonary hypertension/ acute hypoxemia and cardiopulmonary resuscitation. Alternate strategies Another mechanism to potentiate the action of NO is to inhibit the Phosphodiesterase enzymes that degrade cGMP Inhibitors of type 5 phosphodiesterase such as Sildenafil result in prolongation of the duration of NO- induced cGMP elevations in various tissues.

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