A 62-year-old man with benign prostatic hyperplasia is prescribed doxazosin for lower urinary tract symptoms. Regarding β-adrenergic receptor subtypes and their tissue distribution, all of the following are correct EXCEPT:

Explanation

## Analysis of β-Adrenergic Receptor Subtypes ### Correct Statements **Option 1 (β1 receptors):** β1-adrenergic receptors are the predominant β-receptor subtype in the heart. Their stimulation increases heart rate (positive chronotropic effect) and contractility (positive inotropic effect). β1-selective antagonists like metoprolol block these effects and are used in hypertension and angina. [cite:KD Tripathi 8e Ch 10] **Option 2 (β2 receptors in airways):** β2-adrenergic receptors in bronchial smooth muscle mediate smooth muscle relaxation and bronchodilation. Salbutamol (albuterol) is a selective β2-agonist used as a bronchodilator in asthma and COPD. This is correct. **Option 3 (β3 receptors):** β3-adrenergic receptors are found in adipose tissue and mediate lipolysis and thermogenesis. Mirabegron (a β3-agonist) is FDA-approved for overactive bladder, and newer β3-agonists are under investigation for metabolic disorders including type 2 diabetes. This statement is correct. ### The Incorrect Statement (Option 4) **Key Point:** β2-adrenergic receptors in vascular smooth muscle mediate **vasodilation**, NOT vasoconstriction. This is a critical pharmacological principle. **High-Yield:** The vascular effects of catecholamines depend on the balance between α1 (vasoconstriction) and β2 (vasodilation) receptor activation: - **Epinephrine** at low doses → predominant β2 activation → vasodilation - **Epinephrine** at high doses → α1 activation dominates → vasoconstriction - **Norepinephrine** → predominantly α1 activation → vasoconstriction **Warning:** Stating that "β2-agonists cause vasoconstriction" is fundamentally incorrect and contradicts basic autonomic pharmacology. β2-agonists cause vasodilation and would WORSEN cardiogenic shock by reducing systemic vascular resistance. ### β-Receptor Subtype Distribution and Effects Table | Receptor Subtype | Primary Location | Effect of Agonist | Effect of Antagonist | | --- | --- | --- | --- | | β1 | Heart, kidney, adipose tissue | ↑ HR, ↑ contractility, ↑ renin | ↓ HR, ↓ contractility, ↓ renin | | β2 | Bronchi, blood vessels, uterus, GI smooth muscle | Bronchodilation, vasodilation, uterine relaxation | Bronchoconstriction, vasoconstriction | | β3 | Adipose tissue, GI tract, bladder | Lipolysis, thermogenesis, bladder relaxation | Decreased lipolysis | **Clinical Pearl:** In cardiogenic shock, the goal is to INCREASE systemic vascular resistance and cardiac output. β1-agonists (dobutamine) or α1-agonists (norepinephrine, phenylephrine) are used — NOT β2-agonists, which would worsen the shock state.