Alpha & Beta Adrenergic Blockers

Learning Objectives

At the end of this chapter, the Phase II MBBS student will be able to:

1. Classify the alpha adrenergic blockers into irreversible, competitive, selective α1 and α1A-selective groups, with examples. (PH1.13 — Knows)
2. Describe the first-dose phenomenon of prazosin and its prevention. (PH1.13 — Knows)
3. Explain the use of alpha blockers in BPH and the alpha-before-beta rule in phaeochromocytoma. (PH1.13 — Knows-how)
4. Classify the beta blockers by cardioselectivity, intrinsic sympathomimetic activity, membrane-stabilising activity and combined α+β action. (PH1.13 — Knows)
5. Describe the therapeutic uses of beta blockers and their receptor basis. (PH1.13 — Knows)
6. Explain why beta blockers are hazardous in asthma and can mask hypoglycaemia in diabetes. (PH1.13 — Knows)
7. Describe the adverse effects, withdrawal syndrome and overdose management (glucagon) of beta blockers. (PH1.13 — Knows)
8. Describe the drug classes used in glaucoma and their mechanisms. (PH1.13 — Knows)
9. Select an appropriate beta blocker for a given patient profile (asthmatic, diabetic, heart failure). (PH1.13 — Shows-how)
10. Differentiate propranolol from atenolol by lipophilicity and its consequences. (PH1.13 — Knows)

Must-Know Summary

The adrenergic antagonists — alpha and beta blockers — are the mirror image of CH12's agonists, and their clinical value is enormous: alpha blockers for hypertension and BPH, beta blockers for hypertension, angina, arrhythmias, heart failure, migraine and glaucoma. The pharmacology turns on a few distinctions: cardioselective (β1) versus non-selective (β1+β2) blockers, the presence of intrinsic sympathomimetic activity (ISA) or membrane-stabilising activity (MSA), and lipophilicity. Two rules dominate the examinations: block alpha before beta in phaeochromocytoma, and never give a non-selective beta blocker to an asthmatic.

In one line each:

• Phenoxybenzamine — irreversible, non-selective alpha blocker
• Prazosin — selective α1, first-dose syncope
• Tamsulosin — α1A-selective for BPH
• Cardioselective beta blockers — atenolol, metoprolol, bisoprolol, esmolol
• Propranolol — non-selective, lipophilic, with membrane-stabilising activity
• Pindolol — has intrinsic sympathomimetic activity
• Esmolol — ultra-short acting, esterase-metabolised
• Alpha before beta — the phaeochromocytoma rule
• Non-selective beta blocker in asthma — precipitates bronchospasm
• Timolol — beta blocker that reduces aqueous production in glaucoma

Classification

Box 1 — Alpha adrenergic blockers

• Non-selective, irreversible — phenoxybenzamine (pre-operative phaeochromocytoma)
• Non-selective, competitive — phentolamine (acute hypertensive crisis)
• Selective α1 — prazosin, terazosin, doxazosin (hypertension, BPH; first-dose syncope)
• α1A-selective — tamsulosin, silodosin (BPH, less hypotension)

Box 2 — Beta adrenergic blockers

• Cardioselective (β1) — atenolol, metoprolol, bisoprolol, esmolol
• Non-selective (β1+β2) — propranolol, nadolol, timolol, sotalol
• With ISA (partial agonist) — pindolol, acebutolol
• With MSA — propranolol (high dose)
• Combined α+β — labetalol, carvedilol
• Ultra-short acting — esmolol

Core Concepts

1. Alpha adrenergic blockers — classification and pharmacology

Alpha blockers antagonise the α1 (and sometimes α2) adrenergic receptors, opposing the vasoconstriction and smooth-muscle contraction of α1 stimulation. They are classified by selectivity and reversibility:

• Non-selective, irreversible — phenoxybenzamine. It binds covalently to both α1 and α2 receptors, producing a long-lasting (irreversible) blockade that persists until new receptors are synthesised. It is used pre-operatively in phaeochromocytoma to control blood pressure before surgery. Its α2 blockade increases noradrenaline release (loss of presynaptic inhibition), causing reflex tachycardia.
• Non-selective, competitive — phentolamine. A short-acting competitive α1/α2 antagonist used in the acute hypertensive crisis of phaeochromocytoma and in local extravasation of noradrenaline.
• Selective α1 — prazosin, terazosin, doxazosin. These block α1 (vasodilation, relaxation of the bladder neck and prostate) without the α2-mediated tachycardia. Prazosin is the prototype and is known for the first-dose phenomenon — a marked postural hypotension and syncope after the first dose, prevented by starting with a low dose at bedtime.
• α1A-selective — tamsulosin, silodosin. These preferentially block the α1A receptor of the prostate and bladder neck, so they treat BPH with less hypotension than the non-selective α1 blockers.

2. Uses of alpha blockers — BPH and phaeochromocytoma

Benign prostatic hyperplasia (BPH). α1 blockade relaxes the smooth muscle of the bladder neck and prostate, relieving the obstructive symptoms of BPH. Tamsulosin (α1A-selective) is preferred for its uroselectivity and lesser blood-pressure effect; prazosin, terazosin and doxazosin are alternatives (also useful when hypertension coexists).

Phaeochromocytoma. This catecholamine-secreting tumour produces dangerous hypertension. The cardinal rule is "alpha before beta": an alpha blocker (phenoxybenzamine pre-operatively, phentolamine acutely) must be given before any beta blocker. The reason is that beta-blockade alone removes β2-mediated vasodilation while leaving the tumour's α-mediated vasoconstriction unopposed, precipitating a hypertensive crisis. Once alpha blockade is established, a beta blocker is added to control the reflex tachycardia.

3. Beta blockers — classification and pharmacokinetics

Beta blockers are classified by several overlapping properties:

• Cardioselective (β1-selective) — atenolol, metoprolol, bisoprolol, esmolol: at usual doses they block cardiac β1 with relative sparing of bronchial/vascular β2, making them safer in asthma and diabetes.
• Non-selective (β1+β2) — propranolol, nadolol, timolol, sotalol: block both subtypes.
• With intrinsic sympathomimetic activity (ISA — partial agonist) — pindolol, acebutolol: they partially stimulate the receptor while blocking full agonists, so they cause less bradycardia.
• With membrane-stabilising activity (MSA — a quinidine-like, local-anaesthetic effect) — propranolol (at high doses); this is not the therapeutic antiarrhythmic mechanism.
• Combined α+β blockade — labetalol, carvedilol: useful in hypertensive emergencies (labetalol) and heart failure (carvedilol).
• Ultra-short acting — esmolol: metabolised by red-cell esterases (half-life ~9 minutes), given intravenously for perioperative and supraventricular tachyarrhythmias.

Pharmacokinetic distinction: propranolol is lipophilic — well absorbed, high first-pass metabolism, crosses the blood-brain barrier (CNS effects, nightmares), and hepatic clearance; atenolol is hydrophilic — renally excreted, less CNS penetration. This lipophilicity difference is a recurring discriminator.

4. Beta-blocker pharmacological properties and the asthma/diabetes problem

Beta blockade has three principal cardiovascular effects: β1 blockade reduces heart rate, contractility and conduction, and lowers renin release; this reduces myocardial oxygen demand (the basis of the anti-anginal and antihypertensive effect). The β2 blockade of non-selective agents underlies their two most important hazards:

• Asthma/COPD — blocking bronchial β2 removes the dilator pathway, so non-selective beta blockers can precipitate bronchospasm in asthmatics; cardioselective agents are preferred (and even they are used cautiously).
• Diabetes — β-blockers mask the warning signs of hypoglycaemia (tremor, palpitations, anxiety, sweating are adrenergic), so a diabetic on insulin may not recognise an impending hypoglycaemic episode; non-selective agents also impair glycogenolysis. Cardioselective agents are preferred.

5. Therapeutic uses of beta blockers

• Hypertension — reduce cardiac output and renin (first-line in younger patients, CH26).
• Angina — reduce heart rate and contractility, lowering myocardial oxygen demand (CH27).
• Arrhythmias — slow conduction (rate control in atrial fibrillation, supraventricular tachycardia).
• Post-myocardial infarction — reduce mortality by reducing reinfarction and arrhythmic risk.
• Heart failure — carvedilol, bisoprolol, metoprolol (the mortality-reducing agents) given in stable, compensated heart failure (CH28).
• Migraine prophylaxis — propranolol.
• Glaucoma — timolol reduces aqueous humour production (see Section 7).
• Hyperthyroidism — propranolol controls the sympathetic symptoms and blocks the peripheral conversion of T4 to T3.
• Essential tremor and performance anxiety — propranolol.
• Portal hypertension — propranolol reduces portal pressure in variceal prophylaxis.

6. Adverse effects, overdose and withdrawal

Adverse effects reflect the receptor blockade: bradycardia and heart block (β1), bronchospasm (β2, in asthmatics), cold extremities (peripheral β2 blockade), fatigue, depression and sexual dysfunction, masked hypoglycaemia (diabetics), and — with non-selective agents — an unfavourable lipid profile (raised triglycerides). Propranolol's CNS penetration adds nightmares and insomnia.

Withdrawal. Abrupt cessation of chronic beta-blockade causes a rebound tachycardia, hypertension and angina from up-regulation of β receptors during therapy. Beta blockers are therefore tapered over 1–2 weeks, never stopped suddenly.

Overdose. Severe beta-blocker overdose produces bradycardia, hypotension and heart block. Management includes atropine (for bradycardia), glucagon (which stimulates the heart by activating adenylate cyclase independently of the β receptor, the specific antidote), and inotropic/pacing support.

7. Drugs used in glaucoma

Glaucoma is managed by reducing intraocular pressure, and the drug classes act by either reducing aqueous production or increasing its outflow:

• Beta blockers — timolol (and betaxolol): reduce aqueous production by blocking ciliary-body β receptors; a mainstay, given as eye drops.
• Prostaglandin analogues — latanoprost, bimatoprost, travoprost: increase uveoscleral outflow; now the first-line agents for open-angle glaucoma.
• Alpha agonists — brimonidine, apraclonidine: reduce aqueous production and increase uveoscleral outflow.
• Carbonic anhydrase inhibitors — dorzolamide, brinzolamide (topical), acetazolamide (systemic): reduce aqueous production by inhibiting carbonic anhydrase in the ciliary body.
• Cholinergic — pilocarpine (CH10): increases trabecular outflow by ciliary-muscle contraction.
• Osmotic — mannitol: used in acute angle-closure glaucoma to rapidly lower pressure.

Tables

Table 1 — Alpha blockers

Table 2 — Beta-blocker classification

Table 3 — Cardioselective versus non-selective

Table 4 — Beta-blocker uses

Table 5 — Beta-blocker adverse effects

Table 6 — Glaucoma drug classes

Table 7 — Phaeochromocytoma management

Figures

Figure 1 — Alpha and beta adrenergic blockade. Diagram contrasting alpha-1 blockade, which causes vasodilation and lowers blood pressure, with beta-1 blockade, which slows the heart, and beta-2 blockade, which can cause bronchospasm.

Figure 2 — Beta-blocker classification. Classification map of beta blockers into cardioselective, non-selective, intrinsic sympathomimetic activity, combined alpha plus beta, and ultra-short acting groups, with example drugs in each.

Figure 3 — The alpha-before-beta rule. Sequence diagram showing that in phaeochromocytoma the alpha blocker must be given before the beta blocker, because beta blockade alone would leave alpha-mediated vasoconstriction unopposed and precipitate a hypertensive crisis.

Figure 4 — Glaucoma drug classes. Diagram of the eye showing the glaucoma drug classes and their sites of action — beta blockers and carbonic anhydrase inhibitors reducing aqueous production, pilocarpine increasing trabecular outflow, and prostaglandin analogues increasing uveoscleral outflow.

Clinical Correlation

Vignette 1 — Phaeochromocytoma before surgery

A 45-year-old woman with a phaeochromocytoma is being prepared for surgery. The anaesthetist insists on starting phenoxybenzamine before adding any beta blocker.

Reasoning: This is the "alpha before beta" rule. The tumour releases large amounts of catecholamines; if a beta blocker is given first, it removes the β2-mediated vasodilation while leaving the tumour's α-mediated vasoconstriction unopposed, precipitating a hypertensive crisis. Therefore an alpha blocker (phenoxybenzamine) is given first to control the blood pressure, and only then is a beta blocker added for the reflex tachycardia. This sequence is the single most examined point in phaeochromocytoma pharmacology.

Vignette 2 — Propranolol in an asthmatic

A patient with asthma is inadvertently given propranolol for migraine prophylaxis and develops acute wheezing and breathlessness.

Reasoning: Propranolol is a non-selective beta blocker; its β2 blockade removes the bronchodilator pathway, precipitating bronchospasm in the asthmatic. The correct approach is to avoid non-selective beta blockers entirely in asthma and, if a beta blocker is truly needed (e.g. post-MI), to use a cardioselective (β1) agent such as metoprolol or bisoprolol with caution. This vignette is the clinical face of the β1-versus-β2 selectivity distinction.

Vignette 3 — Abrupt beta-blocker cessation

A patient on long-term atenolol stops it abruptly before a minor procedure and develops severe chest pain, tachycardia and a blood pressure of 190/110 mmHg.

Reasoning: Chronic beta-blockade causes up-regulation of β receptors; when the drug is withdrawn abruptly, the now-supersensitive receptors respond to circulating catecholamines, producing a rebound tachycardia, hypertension and angina — potentially precipitating myocardial infarction. Beta blockers must be tapered over 1–2 weeks, and the vignette illustrates the hazard of sudden discontinuation.

Vignette 4 — First-dose prazosin syncope

A patient started on prazosin for hypertension takes the first dose in the morning and collapses on standing, with a transient loss of consciousness.

Reasoning: This is the first-dose phenomenon of prazosin — a marked postural hypotension and syncope after the first dose of an α1 blocker, due to pronounced vasodilation before tolerance develops. It is prevented by starting with a low dose at bedtime and rising slowly. The phenomenon is specific to the α1 blockers and is a recurring examination point.

Practical Linkage

Selecting the beta blocker

Exercise (PH1.13 — select the beta blocker)

For each patient, choose the appropriate beta blocker (or alternative) and justify.

Discussion point

Why is glucagon used in severe beta-blocker overdose?

Expected: glucagon activates adenylate cyclase through its own receptor, bypassing the blocked β receptor, and restores cardiac contractility and rate.

Rapid Revision

• Phenoxybenzamine — irreversible non-selective alpha blocker
• Phentolamine — short-acting competitive alpha blocker
• Prazosin — selective alpha-1, first-dose syncope
• Tamsulosin — alpha-1A selective, for BPH
• Alpha before beta — the phaeochromocytoma rule
• Cardioselective beta blockers — atenolol, metoprolol, bisoprolol, esmolol
• Non-selective beta blockers — propranolol, nadolol, timolol, sotalol
• ISA — pindolol and acebutolol, less bradycardia
• MSA — propranolol at high doses
• Alpha plus beta — labetalol and carvedilol
• Esmolol — ultra-short, esterase-metabolised
• Propranolol — lipophilic, crosses the blood-brain barrier
• Atenolol — hydrophilic, renally excreted
• Non-selective beta blocker in asthma — precipitates bronchospasm
• Beta blockers in diabetes — mask the warning signs of hypoglycaemia
• Mortality-reducing beta blockers in heart failure — carvedilol, bisoprolol, metoprolol
• Timolol — reduces aqueous production in glaucoma
• Latanoprost — increases uveoscleral outflow, first-line glaucoma drug
• Dorzolamide — carbonic anhydrase inhibitor for glaucoma
• Mannitol — osmotic, for acute angle-closure
• Beta-blocker withdrawal — rebound from receptor up-regulation
• Glucagon — the antidote for beta-blocker overdose
• Propranolol in hyperthyroidism — blocks T4 to T3 conversion
• Sotalol — non-selective beta blocker with Class III action
• Beta blockers reduce myocardial oxygen demand — by lowering heart rate and contractility

Viva Questions

• Classify the alpha blockers — Irreversible (phenoxybenzamine), competitive (phentolamine), selective alpha-1 (prazosin), alpha-1A (tamsulosin).
• What is the first-dose phenomenon — Marked postural hypotension and syncope after the first dose of prazosin.
• What is the alpha-before-beta rule — In phaeochromocytoma, give the alpha blocker before the beta blocker to avoid unopposed alpha vasoconstriction.
• Name the cardioselective beta blockers — Atenolol, metoprolol, bisoprolol and esmolol.
• What is intrinsic sympathomimetic activity — Partial agonist activity (pindolol), producing less bradycardia.
• What is membrane-stabilising activity — A quinidine-like local anaesthetic effect of propranolol at high doses.
• Why are non-selective beta blockers dangerous in asthma — They block beta-2 bronchodilation, precipitating bronchospasm.
• How do beta blockers mask hypoglycaemia — They blunt the adrenergic warning signs such as tremor and palpitations.
• Which beta blockers reduce mortality in heart failure — Carvedilol, bisoprolol and metoprolol.
• What is timolol's mechanism in glaucoma — Reduction of aqueous humour production.
• What is latanoprost's mechanism — Increase in uveoscleral outflow.
• Why is mannitol used in acute angle-closure — Its osmotic effect rapidly lowers intraocular pressure.
• What causes beta-blocker withdrawal rebound — Up-regulation of beta receptors during therapy.
• What is the antidote for beta-blocker overdose — Glucagon, which bypasses the beta receptor.
• Differentiate propranolol from atenolol — Propranolol is lipophilic and crosses the blood-brain barrier; atenolol is hydrophilic and renally excreted.

References

1. Tripathi KD. Essentials of Medical Pharmacology. 9th ed. New Delhi: Jaypee Brothers Medical Publishers; Chapter 12 (Adrenergic Antagonists).
2. Katzung BG, Vanderah TW (eds). Basic & Clinical Pharmacology. 16th ed. New York: McGraw Hill; Chapter 10 (Adrenoceptor Antagonist Drugs).
3. Brunton LL, Knollmann BC (eds). Goodman & Gilman's The Pharmacological Basis of Therapeutics. 14th ed. New York: McGraw Hill; Chapters 12 and 13 (Adrenergic Agonists and Antagonists).
4. Ritter JM, Flower RJ, Henderson G, et al. Rang & Dale's Pharmacology. 10th ed. Edinburgh: Elsevier; Chapter 14 (Noradrenergic Transmission).
5. National Medical Commission. Competency Based Undergraduate Curriculum for the Indian Medical Graduate, Volume II — Pharmacology (Code: PH), competency PH1.13.
6. Frishman WH. Beta-adrenergic blockers: a 50-year historical perspective. American Journal of Therapeutics. 2008;15(6):565–576.
7. European Glaucoma Society. Terminology and Guidelines for Glaucoma. 4th ed. Savona: PubliComm; 2014.
8. Prichard BNC, Owens CWI. Beta-adrenoceptor blocking drugs. In: Handbook of Hypertension. Amsterdam: Elsevier.