M06.05.010 β Receptor Antagonists

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Learning Objective

Explain the pharmacologic actions, classifications, clinical uses, and unique properties of β-adrenergic receptor antagonists (beta blockers), including cardioselectivity, intrinsic sympathomimetic activity, lipid effects, CNS penetration, and mixed α/β blockade.

β Receptor Antagonism

β₁ Blockade Effects

  • ↓ Heart rate
  • ↓ Stroke volume
  • ↓ Cardiac output
  • ↓ Renin release

β₂ Blockade Effects

  • May precipitate bronchospasm (asthma)
  • May worsen vasospasm (vasospastic disorders)
  • ↓ Aqueous humor production
  • Metabolic effects:
    • ↓ glycogenolysis
    • ↓ gluconeogenesis
    • ↑ LDL, ↑ triglycerides

Features of Common Beta Blockers

Drug β₁-Selective ISA Sedation Lipids
Acebutolol + + + + −
Atenolol + − − ↑↑
Metoprolol + − + ↑↑
Pindolol − + + + −
Propranolol − − +++ ↑↑
Timolol − − ++ ↑↑

Key Concepts

Cardioselectivity (β₁-selective blockers)

Safer in:

  • Asthma
  • Diabetes
  • Peripheral vascular disease

Examples: atenolol, metoprolol, acebutolol

Intrinsic Sympathomimetic Activity (ISA)

Partial agonists:

  • Cause less bradycardia
  • Produce mild vasodilation/bronchodilation
  • Less effect on plasma lipids

Examples: pindolol, acebutolol

CNS Penetration

  • High CNS entry → propranolol, timolol
  • Low CNS entry → atenolol

Major Clinical Uses

  • Angina
  • Hypertension
  • Post-MI mortality benefit
  • Arrhythmias (Class II): propranolol, acebutolol, esmolol
  • Glaucoma: timolol
  • Migraine, essential tremor, thyrotoxicosis, anxiety: propranolol

Mixed α₁/β Blockers

  • Labetalol → hypertensive emergencies
  • Carvedilol → CHF

Special Agents

  • Sotalol → K⁺ channel blocker + β-blocker (Class III antiarrhythmic)
  • Nebivolol → β₁ + β₂ blocker + β₃ agonist → ↑ NO → vasodilation
  • Used in hypertension

Activity

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