Sympathomimetics Or Adrenergic agonist Drugs

Adrenergic agonist Drugs

Those drugs that stimulates the adrenergic receptors are known as adrenergic agonists or adrenergic drugs or sympathomimetic Drugs
There are three types of classification of these drugs
1)Classification Based On Direct Acting And Indirect Acting sympathomimetic or adrenergic agonist

A)Direct Acting
Salbutamol or albuterol
Isoproterenol or isoprenaline
Adrenaline (Epinephrine)
Noradrenaline (Norepinephrine)

B)Indirect acting drugs:
Amphetamine
Tyramine
Ephedrine

2)Classification based on chemical structure
On the basis of chemical structure sympathomimetic drugs or adrenergic agonist are divided into two groups:
Catecholamines
Non-Catecholamine

A) Catecholamines
Those drugs which contains catechol ring in their chemical structure.Catechole ring or nucleus consist of (OH) groups at position three and four of benzene ring.Following are catecholamines drugs or agents
Adrenaline (epinephrine)
Noradrenaline (Norepinephrine)
Dobutamine
Dopamine
Isoprenaline (isoproterenol)

B) Non-Catecholamine
Those sympathomimetics drugs which don’t contain catechol ring or nucleus in their chemical structure.Following are the agents in this group
Salbutamol
Terbutaline
Phenylephrine
Amphetamine
Ephedrine
Fenoterol

3)Classification based on effects of drugs on receptor types

A)Both alpha and Beta agonist
Adrenaline
Noradrenaline
Ephedrine
Amphetamine

B)Mainly Alpha Agonists
These are further subdivided into two types

Mainly alpha 1 agonists
Methoxamine
Phenylephrine

Mainly alpha 2 agonist
Clonidine
Methyldopa
Gianabenz
Guanfacine

C)Mainly Beta Agonists
They are further divided into the following types:
1)Mainly Beta 1 + Beta 2 agonists
Isoproterenol (isoprenaline)
2)Mainly Beta 1 agonists
Dobutamine
Prenalterol
3)Mainly Beta 2 agonists
Salbutamol
Terbutaline
Ritodrine
Fenoterol

D)Dopamine Agonists
Dopamine
Ibopamine
Bromocriptine
Fenoldopam
Important Drugs

# (1)Salbutamol:
Salbutamol is a direct acting selective B2 agonist and is mainly used as a bronchodiator.
Clinical Uses:
A)Salbutamol is used for the treatment of asthma in the form of inhalational dosage form.when use by inhalation ,a smaller dose of drug is required to produce the maximum beneficial effects with minimum systemic adverse reactions as compare to systemic administration.Dose of 0.25-0.5 mg can be given by Slow Intravenous,Subcuteneous or intramuscular .Each metered dose inhalar delover 100micrograms of salbutamol by one compression of its piston.1-2 inhalation can be used after every four to six hoursduring an attack of bronchial asthma.

B)Salbutamol can also be used in patient suffering from emphysema and chronic bronhitis.

Pharmacokinetics:
It can be give Orally,Intramuscularly,intraveneously and by inhalation.Best absorb from GiT and it is metabolised mainly in liver by monoamine oxidase enzyme.Cause bronchodilation in few minutes and last for 3-4 hours when given by inhalational route.When given by subcutaneous route its effects occur in about five minutes and persist for about 4 hours.When given orally ,bronchodilation occurs after an hour and last for 5-6 hours.
# (2)Isoproterenol (Isoprenaline)
It is a direct acting synthetic agent which stimulate both B1 and B2 Receptors bit has no effect on alpha Receptors.

Clinical Uses:
(1)Bronchial Ashtma
Isprenaline is used through inhalational route or in the form of nebuliser.
(2)Heart Block & Cardiac Arrest:
Isoprenaline can be used by parenteral injection

Adverse Reactions:
Headache,Tachycardia,Cardiac Arrhythmias,and Flushing of skin can be occur.

# (3)Adrenaline Or Epinephrine
Its a direct acting Endogenous catecholamine and is administered by Intramiscular and subcuteneous and sometime rarely by INtraveneous Route.Epinephrine is mostly metabolised in the body and metabolites are excreted in urine.Adrenaline is metabolised in the body by two enzymes ,monoamine oxidase (MAO) And Catechol-O-methyl transferase (COMT).

Uses:
Following are some of the very important Uses of adrenaline or norepinephrine.

Bronchial asthma:
Epinephrine is useful in bronchial asthma because it causes dilation of bronchioles by stimulation of beta2 receptors on their smooth muscle.it can be given in a dose of 0.3-0.5mg by subcutaneous injection.in children 0.01mg/kg can be given .it can be repeated after every 15-20minutes .
Anaphylactic shock
Anaphylactic shock is a serious condition which occurs due to due to introduction of foreign substances or drugs in a person,who is allergic to these agents.these agents acting as antigens cause the formation of antibodies which become attached to the surface of mast cells and basophils.Re-Exposure of a person to antigens leads to interaction of antibodies with antigen on the surface of mast cell and basophils.Re.exposure of a person to antigens leads to interaction of antibodies with antigens on the surface of mast cell and basophils leading to their rupture and release of histamine and other chemicals.
These chemical agents will cause dilation of blood vessels in the body which will cause severe falls in blood pressure and ultimately less blood supply to tissues.Also Histamine and other chemicals thus released from mast cells and basophils will cause bronchoconstriction ,causing difficulty in breathing.Epinephrine is a drug of choice in anaphylactic shock .it reverse the vasodilation by causing stimulation of alpha receptors on the smooth muscles of blood vessels.in this way it increase the blood pressure and restores it to normal level.it causes bronchodilation by stimulation of Beta2 receptors on the smooth muscle of bronchioles.Thus breathing become easy.Adrenaline or epinephrine can be given in a dose of 0.3-0.5mg By intramuscular injection.It can be repeated after every 15-20 minutes if required.

Urticaria and angioedema
Urtecaria are multiple small swelling of the skin they are allergic in nature and are associated with pruritis.Angioedema is also allergic in nature and it is manifested in the form of swelling of face ,lips ,eyelids,tongue and glottis.Swelling of glottis can be life threatening.Both Urticaria and angioedma can be treated with adrenaline ,which reduces the swelling by causin vasoconstriction.

Cardiac arrest
In cardiac arrest the heart stop beating .it is best treated with electric ,cardioversion but if it is not available then 0.5mg of adrenaline or epinephrine can be injected directly into the heart which can be repeated every five minutes if required.

Adverse Effects of Epinephrine or adrenaline
Following are some Adverse events which occur when we used epinephrine ,
Hypertension
Cardiac arrhythmias
Nervousness,Restlessness,headache,tremor,and anxiety etc
Subarachoid and cereberal haemorrhage.

# Contraindications

Hypertension
It may caused subarachoid and cerebral haemorrhage due to increase in blood pressure.

Cardiac arrhythmias
Epinephrine or adrenaline can make the arrhythmias more severe and in severe cases ventricular fibrillation can occur.

Hyperthyroidism
It can prescipitate various types of cardiac arrhythmias in these patients.

Ischemic Heart disease
Epinephrine or adrenaline can precipitate angina pectoris or Myocardial infarction (MI)in patient with ishemic heart disease.there fore it is avoided in patient with ischemic heart disease and in old people.

# 4)Norepinephrine or Noradrenaline
It is an important endogenous neurotranmitter.it stimulates alpha1, alpha 2 and Beta 1 adrenoceptors but has no significant effect on beta2 adrenoceptors.

Uses
It is sometimes used for the treatment of hypotension by intravenous infusion.

# 5) Dopamine
It is an endogenous catecholamine with important neurotransmitter activity.

Clinical uses:
In cardiogenic and septicemic shock dopamine increase the blood flow to the kidney and other vital organs by causing dilation of their arteries.in these patients it is usually given in doses of 2-5microgram/kg/minute by intravenous infusion.Dopamine is not effective orally as it is destroyed during absorption from GIT.

Adverse effects:
Cardiac arrhythmias,Palpitations,Tachucardia,headache,Hypertension,Nausea and vomiting.

# 6) Dobutamine :
It is a synthetic catecholamine which mainly stimulates the beta1 adrenoceptor of heart,causing increase in the heart rate and force of its contraction,leading to increase in the cardiac output.

Clinical Uses:
A)Cardiogenic Shock
It is the drug of choice in the treatment of shock due to heart diseases,such as shock due to acute myocardial infarction.it is given in a dose of 2.5-15microgram/kg/minute by IV infusion.

B)Acute Congestive Cardiac failure
Here it increase the cardiac output and improve the blood supply to the tissues.it can be given at dose of 2.5-15microgram/kg/minute by IV infusion.

Adverse Reactions
Cardiac arrhythmias,Tachycardia,Palpitation,and nausea and vomiting .

# 7) Ibupamine
It releases dopamine in the body and is use orally .its clinical use not known.

# 8) Fenoldopam
It is a D1 agonist that causes dilation of some blood vessels causing fall in blood pressure .it is used by IV infusion for severe hypertension.

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