Pharmacological Account of Propranolol

1. Mechanism of Action

• β₁ blockade: Heart, juxtaglomerular cells → ↓ heart rate, contractility, AV conduction, renin secretion
• β₂ blockade: Bronchi, peripheral vasculature, liver → bronchoconstriction, vasoconstriction, ↓ glycogenolysis

2. Pharmacological Actions by System

A. Cardiovascular System (CVS)

• ↓ Cardiac output: Negative inotropic (↓ contractility) + negative chronotropic (↓ heart rate) effects via β₁ blockade
• ↓ SA and AV node activity: Slows heart rate; prolongs AV conduction (useful in arrhythmias)
• ↓ Myocardial oxygen consumption: Basis for use in angina
• Blood pressure: Acutely, CO falls with a reflex rise in peripheral resistance; chronically, peripheral resistance normalizes or falls → net antihypertensive effect
• ↓ Renin secretion: Via β₁ blockade at juxtaglomerular cells → contributes to BP reduction
• Peripheral vasoconstriction: β₂ blockade prevents catecholamine-mediated vasodilation → ↑ peripheral resistance (a potential adverse effect)

B. Respiratory

• Bronchoconstriction via β₂ receptor blockade in bronchial smooth muscle
• Not clinically significant in normal individuals, but potentially life-threatening in asthma and significant in COPD
• β₁-selective agents (atenolol, metoprolol) are preferred when a beta-blocker is unavoidable in airway disease; COPD patients tolerate them better than asthma patients

C. Eye

• ↓ Intraocular pressure (IOP) by reducing aqueous humor production
• Used topically (e.g., timolol) in glaucoma

D. Metabolic and Endocrine Effects

• ↓ Glycogenolysis (β₂-mediated): Inhibition of hepatic and skeletal muscle glycogenolysis
• Masks hypoglycemia warning signs: Tachycardia is suppressed; sweating persists
• ↓ Glucagon-mediated glucose recovery: Delays recovery from hypoglycemia in insulin-dependent diabetics — use with caution, especially in those with low glucagon reserve or post-pancreatectomy
• Lipid effects: ↑ VLDL, ↑ triglycerides, ↓ HDL cholesterol → potentially unfavorable cardiovascular risk profile
• ↓ Lipolysis: Inhibits sympathetic stimulation of fat breakdown
• β₁-selective agents are less prone to these metabolic effects

E. CNS

• No major CNS effects at therapeutic doses
• May cause behavioral changes, forgetfulness, nightmares (CNS penetration due to high lipophilicity)
• Suppresses anxiety: Reduces somatic manifestations (palpitations, tremor); used in performance anxiety ("stage fright")
• Reduces skeletal muscle tremor (essential tremor)

F. Membrane-Stabilizing (Local Anesthetic) Effect

• Blocks sodium channels in nerve/cardiac membranes — a class I antiarrhythmic property
• Clinically insignificant at usual therapeutic plasma concentrations

3. Pharmacokinetics

4. Therapeutic Uses (Clinical Indications)

5. Adverse Drug Reactions (ADRs)

1. Bradycardia / Heart block — excessive β₁ blockade
2. Precipitation of heart failure — loss of sympathetic support, especially in compromised hearts; β₁-selective agents preferred, add cautiously
3. Bronchoconstriction / Asthma exacerbation — β₂ blockade; life-threatening in asthma
4. Peripheral vasoconstriction — cold extremities, worsening of peripheral arterial disease, Raynaud's phenomenon
5. Hypoglycemia (masked) — delayed recovery + suppressed warning signs in diabetics
6. Dyslipidemia — ↑ triglycerides, ↓ HDL → increased cardiovascular risk
7. CNS effects — fatigue, depression, nightmares, sexual dysfunction
8. Sodium/fluid retention — ↓ renal perfusion → compensatory renin-angiotensin activation; usually combined with a diuretic
9. Variant (Prinzmetal) angina exacerbation — unopposed α-mediated coronary vasospasm
10. Withdrawal syndrome — abrupt discontinuation causes rebound tachycardia, angina, hypertension (up-regulation of β receptors); must taper gradually

6. Contraindications

7. Drug Interactions

8. Selectivity Comparison

Sources: Katzung's Basic and Clinical Pharmacology, 16th Ed. | Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th Ed.