Download - Pharmacology of Nitric oxide
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.
• Endothelial cells release substances acting directly on vascular
smooth muscle cells, causing either contraction or relaxation.
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 O2-) protected EDRF.
ENDOTHELIUM DERIVED RELAXING FACTOR
•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 , Arginase , Asymmetric
Dimethylarginine (ADMA), Dimethylarginine Dimethylaminohydrolase (DDAH), etc.
acting in their corresponding receptors or cellular structures can affect production and
release of NO.
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.
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.
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.
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-
Increased 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
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
NOSConstitutive
Inducible
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
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Role of Nitric oxide
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.
Role in Neurodegenerative 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.
Play a role in long term memory
As a messenger that facilitates long term potentiation of neurons (memory)
Synthesis mechanism involve Ca2+/Calmodulin
activates NOS-I
activates Guanyl cyclase
cycle of nerve action potentials
catalyzes cGMP production
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 O2 binds to hemoglobin.
During O2 delivery, NO locally dilates blood vessels to aid in gas exchange
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
Role in the Immune system NOS II catalyzes synthesis of NO used in host defense reactions
Activation of NOS II is independent of Ca2+ 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.
Role In the Digestive systemNO 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.
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:
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.
Role in Inflammation
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.
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-
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
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)
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.
Organic nitritesSodium nitroprusside: Dilates 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.
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 strategiesAnother 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.