Summary of the Lecture on Adrenergic Agonists and Antagonists
This lecture discusses adrenergic drugs, which affect the autonomic nervous system
by stimulating or blocking alpha (α) and beta (β) receptors. These drugs influence
heart rate, blood pressure, respiratory function, metabolism, and more.
Understanding their mechanisms, uses, and side effects is essential for treating
conditions like hypertension, asthma, cardiac arrest, and shock.
1. Adrenergic Agonists (Sympathomimetics)
These drugs stimulate adrenergic receptors to produce effects similar to the body's
natural neurotransmitters (epinephrine and norepinephrine).
A. Epinephrine (Adrenaline)
A naturally occurring catecholamine that stimulates both α and β receptors.
Dosage-dependent effects:
o Low doses: β effects dominate → causes vasodilation (widens blood vessels, lowers
blood pressure).
o High doses: α effects dominate → causes vasoconstriction (narrows blood vessels,
increases blood pressure).
Effects on the body:
o Cardiovascular system:
Strengthens heart contractions (β1 effect) → increases cardiac output.
Can cause increased oxygen demand on the heart.
Stimulates renin release from kidneys → leads to more vasoconstriction and higher
blood pressure.
o Respiratory system:
Powerful bronchodilator (β2 effect) → used in asthma attacks.
Prevents the release of histamine, which causes allergic reactions.
o Metabolic system:
Raises blood sugar by promoting glycogen breakdown in the liver (β2 effect).
Inhibits insulin release, making it harder for the body to lower blood sugar (α2 effect).
Therapeutic Uses:
o Asthma attacks (emergency bronchodilator).
o Anaphylactic shock (severe allergic reactions).
o Cardiac arrest (restores heart rhythm).
o Used with local anesthetics to reduce bleeding by constricting blood vessels.
B. Norepinephrine (Noradrenaline)
A neurotransmitter and adrenergic agonist that primarily affects α receptors.
Effects:
o Powerful vasoconstriction → raises blood pressure significantly.
o Can cause reflex bradycardia (a slowing of the heart rate due to high blood pressure).
Main use: Treatment of shock (when blood pressure drops dangerously low).
C. Isoproterenol (β1 and β2 agonist)
Stimulates β1 and β2 receptors, but not α receptors.
Effects:
o Increases heart rate and contractility (β1 effect).
o Strong bronchodilation (β2 effect).
o No vasoconstriction because it doesn’t activate α receptors.
Not commonly used today because more selective drugs are available.
D. Dopamine
Precursor to norepinephrine, naturally present in the body.
Dose-dependent effects:
o Low doses → Improves kidney function (D1 receptor activation → increases blood
flow to kidneys).
o Moderate doses → Increases heart rate and cardiac output (β1 receptor activation).
o High doses → Causes vasoconstriction (α1 receptor activation).
Used for:
o Shock treatment (maintains blood pressure and kidney function).
o Heart failure (increases cardiac output).
E. Other Adrenergic Agonists
Dobutamine: Selective β1 agonist used in acute heart failure to increase cardiac
output.
Albuterol, Terbutaline: Selective β2 agonists used for asthma (bronchodilators).
Phenylephrine: Selective α1 agonist, used as a nasal decongestant and to increase
blood pressure.
Clonidine: Selective α2 agonist, reduces blood pressure and helps with opioid
withdrawal.
2. Adrenergic Antagonists (Adrenergic Blockers)
These drugs block adrenergic receptors, preventing the effects of epinephrine and
norepinephrine.
A. Alpha (α) Blockers
Non-selective α blockers (e.g., Phentolamine, Phenoxybenzamine):
o Used for pheochromocytoma (a tumor that causes excess adrenaline production).
o Can cause reflex tachycardia (increased heart rate).
Selective α1 blockers (e.g., Prazosin, Doxazosin, Tamsulosin):
o Used for hypertension and benign prostatic hyperplasia (BPH) (improves urine flow).
o Fewer side effects compared to non-selective α blockers.
B. Beta (β) Blockers
Non-selective β blockers (e.g., Propranolol, Timolol):
o Reduce heart rate and blood pressure.
o Used for hypertension, angina, arrhythmias, and migraine prevention.
o Can cause bronchoconstriction, so they should be avoided in asthma patients.
Selective β1 blockers (e.g., Metoprolol, Atenolol):
o Preferred for heart disease patients because they don’t affect the lungs.
o Used in hypertension, heart failure, and post-heart attack management.
Mixed α and β blockers (e.g., Labetalol, Carvedilol):
o Used for hypertension and heart failure.
Key Differences Between Agonists and Antagonists
Feature Adrenergic Agonists (Stimulators) Adrenergic Antagonists (Blockers)
Function Stimulate α and β receptors Block α and β receptors
Effect on Blood Pressure Can increase or decrease BP Usually reduce BP
Effect on Heart Rate Increase HR (β1 activation) Reduce HR (β1 blocking)
Effect on Lungs Bronchodilation (β2 activation) Bronchoconstriction (β blocking)
Main Uses Asthma, shock, cardiac arrest Hypertension, arrhythmias, heart disease
Clinical Applications
Epinephrine: Used for anaphylaxis, cardiac arrest, asthma.
Norepinephrine: Used for shock, low blood pressure.
Beta-blockers (e.g., Propranolol, Metoprolol): Used for hypertension, heart disease,
arrhythmias.
Alpha-blockers (e.g., Prazosin, Tamsulosin): Used for hypertension, prostate
enlargement (BPH).
5. Conclusion
Adrenergic drugs play a crucial role in managing conditions related to the heart, blood vessels,
and respiratory system. Understanding their mechanisms, effects, and uses is essential for
effective treatment.
