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Beta-Blockers: Classification and Propranolol

Classification of Beta-Blockers

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1. By Receptor Selectivity

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2. By Intrinsic Sympathomimetic Activity (ISA / Partial Agonist Activity)

• With ISA (partial agonists): Pindolol, Acebutolol, Celiprolol, Oxprenolol
  • Cause less resting bradycardia; may preserve resting cardiac output
  • Less likely to cause cold extremities
• Without ISA: Propranolol, Metoprolol, Atenolol, Bisoprolol, Carvedilol

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3. By Lipophilicity (determines CNS penetration)

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4. By Generation (Clinical Classification)

• First-generation: Non-selective - Propranolol, Timolol, Nadolol
• Second-generation: Cardioselective (β1) - Metoprolol, Atenolol, Bisoprolol, Esmolol
• Third-generation: Vasodilating - Carvedilol (α1 block), Nebivolol (NO release), Labetalol (α1 + β block)

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5. By Route of Elimination

• Hepatic metabolism: Propranolol, Metoprolol, Carvedilol
• Renal excretion: Atenolol, Nadolol
• Both: Bisoprolol, Acebutolol

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PROPRANOLOL - Detailed Pharmacology

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Mechanism of Action (MoA)

At β1 Receptors (cardiac):

• Blocks catecholamine (adrenaline/noradrenaline) binding to β1 receptors on sinoatrial node, AV node, and myocardium
• Reduces adenylyl cyclase activation → decreased cAMP → decreased PKA activity
• Results in: decreased heart rate (negative chronotropy), decreased force of contraction (negative inotropy), slowed AV nodal conduction (negative dromotropy), and decreased automaticity

At β2 Receptors (peripheral/smooth muscle):

• Blocks bronchial β2 receptors → bronchoconstriction (clinically significant in asthmatics)
• Blocks vascular smooth muscle β2 receptors → unopposed α vasoconstriction → initial rise in peripheral resistance
• Blocks β2 in skeletal muscle vasculature

Antihypertensive Mechanism (Multi-factorial):

1. Decreased cardiac output - primary mechanism; propranolol decreases heart rate and contractility, reducing CO
2. Inhibition of renin secretion - blocks β1-mediated renin release from juxtaglomerular cells → reduced angiotensin II and aldosterone → less sodium/water retention
3. Central action - reduces central sympathetic outflow
4. Peripheral presynaptic β-blockade - reduces sympathetic vasoconstrictor nerve activity
5. Resetting of baroreceptors over time

Membrane Stabilizing Activity (MSA / Quinidine-like Action):

• At high concentrations, propranolol also blocks fast sodium channels in the myocardium (a local anesthetic/quinidine-like effect). This contributes to its antiarrhythmic activity but is not the primary mechanism at therapeutic doses.

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Pharmacological Actions

1. Cardiovascular Effects

2. Respiratory Effects

• Bronchoconstriction via β2 blockade - significant and potentially dangerous in asthma/COPD
• Increases airway resistance

3. Metabolic Effects

• Inhibits glycogenolysis (β2 receptor-mediated glycogenolysis in liver/muscle is blocked) → prolonged and masked hypoglycemia in diabetics
• Masks tachycardia (the warning sign of hypoglycemia), but sweating is preserved (cholinergic)
• Inhibits lipolysis and may raise triglycerides, lower HDL cholesterol
• Blocks β2-mediated potassium uptake into cells → may cause mild hyperkalemia

4. Renal Effects

• Decreases renin secretion (β1 block on JG cells) → antihypertensive mechanism
• No direct renal vasodilatory effect

5. Ocular Effects

• Reduces aqueous humor production → lowers intraocular pressure (basis for topical timolol in glaucoma; propranolol used systemically)

6. CNS Effects

• Being highly lipophilic, propranolol crosses the BBB readily
• Causes fatigue, sleep disturbances, nightmares, depression, and (rarely) hallucinations
• Reduces anxiety and tremor (β-mediated peripheral mechanism)

7. Uterine Effects

• May prolong labor by blocking catecholamine-mediated β2 relaxation of the uterus

8. Thyroid Effects

• Inhibits peripheral conversion of T4 to T3 (independent of β-blockade, likely MSA)
• Controls symptoms of thyrotoxicosis (tachycardia, tremor, anxiety, sweating)

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Pharmacokinetics

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Therapeutic Uses

1. Cardiovascular

• Effective in mild to moderate hypertension; particularly useful in patients with coexisting angina or post-MI status
• Mechanism: decreased CO + decreased renin

• Reduces myocardial O2 demand by decreasing HR, contractility, and wall stress
• Prolongs diastole (better subendocardial perfusion)
• Drug of choice for chronic stable (effort) angina

• Controls arrhythmias arising from increased sympathetic activity (exercise-induced, thyrotoxicosis, phaeochromocytoma)
• Slows ventricular rate in atrial fibrillation and flutter (AV nodal blocking effect)
• Suppresses ventricular premature contractions and tachyarrhythmias
• Effective in AVNRT and other SVTs (IV use for acute termination)
• Used in long QT syndrome (congenital LQTS) to prevent VF

• Reduces re-infarction, sudden cardiac death, and overall mortality post-MI
• Initiated within hours of STEMI; continued long-term

• Carvedilol and metoprolol succinate are preferred over propranolol for CHF
• Propranolol is used carefully; improves survival in stable CHF (systolic) when initiated at low doses and titrated up slowly

• Reduces outflow tract obstruction by decreasing HR and contractility
• Reduces symptoms of dyspnea, angina, and syncope

• Slows heart rate and reduces aortic wall stress; used with sodium nitroprusside

2. Thyroid Disease

• Rapidly controls sympathomimetic symptoms: tachycardia, tremor, anxiety, heat intolerance
• Inhibits peripheral T4 → T3 conversion
• Used as adjunct while awaiting effect of antithyroid drugs

3. Neurological / Psychiatric Uses

• Drug of choice for essential tremor; reduces peripheral (β2-mediated) muscle tremor

• One of the most effective preventive agents for migraine; mechanism unclear (possibly central serotonergic effects)

• Blocks peripheral sympathetic manifestations: palpitations, tremor, sweating

• Reduces splanchnic blood flow and portal pressure
• Used to prevent first variceal bleed and rebleeding in cirrhosis

4. Endocrine Uses

• Used only after adequate α-blockade (phenoxybenzamine given first) to prevent hypertensive crisis
• Controls tachycardia and arrhythmias perioperatively

• NOT recommended; masks tachycardia, prolonging hypoglycemia

5. Other Uses

• Fallot's spells (hypercyanotic spells in Tetralogy of Fallot) - relaxes right ventricular infundibular spasm
• Glaucoma - reduces aqueous production (less common; topical timolol preferred)
• Akathisia (antipsychotic-induced) - reduces motor restlessness

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Adverse Effects

1. Cardiovascular

• Bradycardia (most common) - excessive slowing of HR; dangerous in pre-existing sick sinus syndrome
• AV block - may progress to complete heart block in susceptible individuals
• Worsening heart failure - if used in decompensated HF; may precipitate acute decompensation
• Cold extremities / Raynaud's phenomenon - due to β2 blockade → unopposed α-mediated vasoconstriction in peripheral vessels
• Rebound phenomenon on abrupt withdrawal - nervousness, tachycardia, worsening angina, risk of myocardial infarction; due to upregulation/supersensitivity of β-receptors from chronic blockade. Must taper gradually over 1-2 weeks.

2. Respiratory

• Bronchospasm - life-threatening in asthmatics and COPD patients due to β2 blockade
• Increased airway resistance

3. Metabolic

• Hypoglycemia unawareness - masks tachycardia (the warning sign of hypoglycemia); sweating is preserved but may be blunted
• Prolonged hypoglycemia - by blocking glycogenolysis and gluconeogenesis
• Dyslipidaemia - raises serum triglycerides, reduces HDL cholesterol (less effect with cardioselective agents)
• Hyperkalemia - blocks β2-mediated cellular potassium uptake

4. CNS (due to high lipophilicity)

• Fatigue and lethargy (most common CNS effect)
• Sleep disturbances - insomnia, vivid dreams, nightmares
• Depression
• Cognitive impairment (especially in elderly)
• Hallucinations (rare)

5. Other

• Sexual dysfunction - impotence and decreased libido (mechanism uncertain; possibly reduced penile blood flow)
• Masking of hyperthyroid symptoms - may delay diagnosis
• GI disturbances - nausea, diarrhea (uncommon)

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Contraindications

Absolute Contraindications

Relative Contraindications

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Key Points Summary

Sources: Katzung's Basic and Clinical Pharmacology, 16th Ed. | Goodman & Gilman's The Pharmacological Basis of Therapeutics