## Mechanism of Action of Antihypertensives ### The EXCEPT Answer: Option C (Thiazide Diuretics) **High-Yield:** Thiazide diuretics do **NOT** cause vasodilation through direct relaxation of vascular smooth muscle as their primary antihypertensive mechanism. This statement is **FALSE**, making it the correct answer to this EXCEPT question. Their true antihypertensive mechanism operates in two phases: 1. **Acute phase (first 2–4 weeks):** Inhibition of Na⁺/Cl⁻ cotransporter in the distal convoluted tubule → natriuresis → reduced plasma volume → reduced cardiac output → lower BP 2. **Chronic phase (beyond 4 weeks):** Plasma volume normalizes, but BP remains reduced due to a fall in **peripheral vascular resistance** — the mechanism of which is incompletely understood. There is some evidence for opening of ATP-sensitive K⁺ channels in vascular smooth muscle, but this is **not** the same as the direct smooth muscle relaxation seen with true vasodilators like hydralazine or minoxidil. **Key Point:** The diuretic effect is integral to thiazide antihypertensive action. They are not classified as vasodilators. [KD Tripathi 8e, Ch 32] --- ### Why the Other Options Are CORRECT (and therefore not the answer) **Option A: Calcium Channel Blockers** CCBs inhibit **L-type (voltage-gated) calcium channels** in: - **Vascular smooth muscle** → vasodilation (dihydropyridines, e.g., amlodipine, are most selective here) - **Cardiac myocytes** → reduced contractility and heart rate (non-dihydropyridines, e.g., verapamil, diltiazem, are more prominent here) This statement is accurate. [KD Tripathi 8e, Ch 32] **Option B: Beta-Blockers** Beta-blockers reduce BP primarily by: - **Decreased heart rate** (negative chronotropy via β₁ blockade at the SA node) - **Reduced cardiac output** (negative inotropy via β₁ blockade at the myocardium) Non-selective agents (e.g., propranolol, nadolol) additionally block **β₂ receptors** on vascular smooth muscle. Since β₂ stimulation normally causes vasodilation, blocking these receptors leaves **α-adrenergic–mediated vasoconstriction unopposed**, resulting in peripheral vasoconstriction. While this is an undesirable side effect rather than a therapeutic goal, the statement that non-selective agents *cause* peripheral vasoconstriction is pharmacologically accurate. The SME note acknowledges this is slightly imprecise in wording, but the statement is not factually wrong — it correctly describes a known pharmacological consequence of non-selective β-blockade. [KD Tripathi 8e, Ch 19] **Option D: ACE Inhibitors** ACE inhibitors (e.g., enalapril, ramipril) block **angiotensin-converting enzyme**, preventing conversion of angiotensin I → angiotensin II. This reduces: - **Vasoconstriction** (loss of AII-mediated AT₁ receptor activation on vascular smooth muscle) - **Aldosterone secretion** (loss of AII-mediated stimulation of zona glomerulosa → less Na⁺/water retention) This statement is fully correct. [KD Tripathi 8e, Ch 32] --- ### Comparison Table | Drug Class | Primary Mechanism | Key Point | |---|---|---| | CCBs | Block L-type Ca²⁺ channels | Dihydropyridines: vasodilation; Non-DHP: ↓HR, ↓contractility | | Beta-blockers | ↓HR, ↓CO via β₁ blockade | Non-selective: peripheral vasoconstriction (unopposed α) | | ACE inhibitors | Block AII formation | ↓ Vasoconstriction, ↓ Aldosterone | | Thiazides | Diuresis → ↓ plasma volume → ↓ SVR (chronic) | **NOT** direct smooth muscle relaxation | **Clinical Pearl:** Direct vasodilators (hydralazine, minoxidil) act by directly relaxing arteriolar smooth muscle — this mechanism does NOT apply to thiazide diuretics, which is the basis of Option C being false. [cite: KD Tripathi Essentials of Medical Pharmacology, 8th ed., Ch 32]
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