A 52-year-old man with hypertension is prescribed two antihypertensive agents: one acts as a β₁-selective antagonist and the other as a non-selective β-blocker. Which feature best distinguishes the β₁-selective agent from the non-selective β-blocker?

Explanation

## Distinguishing β₁-Selective from Non-Selective β-Blockers ### Receptor Selectivity and Organ Distribution **Key Point:** β₁-adrenergic receptors are predominantly located in the heart (sinoatrial node, ventricular myocardium), while β₂-adrenergic receptors are abundant in bronchial smooth muscle, vascular smooth muscle, and skeletal muscle. **High-Yield:** β₁-selective agents (e.g., metoprolol, atenolol, bisoprolol) preferentially block cardiac β₁-receptors at therapeutic doses, leaving β₂-receptors in the lungs relatively unaffected. Non-selective β-blockers (e.g., propranolol, nadolol) block both β₁ and β₂-receptors. ### Clinical Consequences | Feature | β₁-Selective Agent | Non-Selective β-Blocker | |---------|-------------------|------------------------| | Airway resistance | Minimal increase | Significant increase (contraindicated in asthma/COPD) | | Heart rate reduction | Moderate | Moderate (similar) | | Cardiac contractility | Reduced | Reduced (similar) | | Bronchodilation (β₂) | Preserved | Blocked | | Metabolic effects | Minimal | Pronounced (↓ glucose awareness, ↑ lipids) | | Peripheral vasoconstriction | Absent | Present (β₂ blockade) | **Clinical Pearl:** The preservation of β₂-mediated bronchodilation in β₁-selective agents makes them safer in patients with reactive airway disease, asthma, or COPD. This is the single most important discriminating feature between the two classes. **Warning:** Even β₁-selective agents lose selectivity at higher doses and can block β₂-receptors; they should still be used cautiously in asthmatic patients.