Adrenergic Antagonists
Introduction
- Adrenergic antagonists (Adrenergic blockers / Sympatholytics) are agents that bind to adrenoceptors & blocks the adrenergic receptors but do not trigger the usual receptor-mediated intracellular effects ( i.e. antagonize the receptor action of adrenaline and related drugs).
- These drugs act by either reversibly or irreversibly attaching to the adrenoceptors, thus preventing activation by endogenous catecholamines.
- Adrenergic antagonists have significant clinical use, primarily to treat diseases associated with the cardiovascular system.
Classification
On the basis of their relative affinities for α or β receptors in the sympathetic nervous system, adrenergic antagonists are classified as follows :-
1. α- Blockers
- α- blockers are drugs that inhibit adrenergic responses mediated through the α- adrenergic receptors without affecting those mediated through β- receptors.
- α- blockers profoundly affect blood pressure.
- E.g.
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2. β-Blockers
- β- blockers are drugs that inhibit adrenergic responses mediated through the β- adrenergic receptors without affecting those mediated through α- receptors.
- All of the clinically available β-blockers are competitive antagonists.
- Nonselective β-blockers act at both β1 and β2 receptors, whereas cardioselective β antagonists primarily block β1 receptors. [Note: There are no clinically useful β2 antagonists.]
- β-Blockers are effective in treating hypertension, angina, cardiac arrhythmias, myocardial infarction, heart failure, hyperthyroidism, glaucoma, and prophylaxis of migraine headaches.
- The names of all β-blockers end in “-olol” except for labetalol, carvedilol, sotalol etc.
- Although all β-blockers lower blood pressure, they do not induce postural hypotension, because the α- adrenoceptors remain functional.
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3. Drugs affecting Neuro transmitter uptake/release
- Some agents act on the adrenergic neuron, either to interfere with neurotransmitter release from storage vesicles or to alter the uptake of the neurotransmitter into the adrenergic neuron.
- However, due to the advent of newer and more effective agents with fewer side effects, these agents are seldom used therapeutically.
- E.g.
– Reserpine
Therapeutic uses
Adrenergic antagonists can be used in following way:
- Used to treat hypertension.
- Used to treat prostatic hypertrophy or benign prostatic hyperplasia (BPH).
- May be used to treat Raynauds disease and frostbite.
- Used in the treatment of pheochromocytoma.
- Useful in the chronic management of inoperable tumors.
- Occasionally used in secondary shock and peripheral vascular disease.
- Used to reverse or shorten the duration of soft-tissue anesthesia.
- Used to treat migraine (migraine headaches)
- May be advocated for treatment of dementia.
- Used as prophylactic agent for Myocardial infarction.
- May be useful in managing the Hyperthyroidism.
- Used for arrhythmias/tachycardias, premature ventricular contractions, digitalis-induced tachyarrhythmias, & essential tremor.
- Used for hypertension, coronary heart disease, congestive heart failure, angina pectoris, and acute MI.
- Used in the treatment of glaucoma & intraocular hypertension.
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