## Alpha-1 Adrenergic Blockade: Physiological Consequences ### Mechanism of Alpha-1 Receptors **Key Point:** Alpha-1 adrenergic receptors are found on **vascular smooth muscle (arterioles and veins), cardiac muscle, and the internal urethral sphincter**. They are NOT present on salivary glands — salivary secretion is controlled by **parasympathetic muscarinic receptors** (M3 subtype). **High-Yield:** Alpha-1 blockade causes **vasodilation → ↓ BP → baroreceptor reflex → ↑ HR and contractility**. This is why alpha-1 antagonists often cause reflex tachycardia. ### Correct Effects of Alpha-1 Blockade | Effect | Mechanism | Clinical Significance | | --- | --- | --- | | **Vasodilation** | Loss of α1-mediated vasoconstriction on arterioles/veins | ↓ SVR, ↓ BP | | **Reflex tachycardia** | Baroreceptor unloading from BP drop | Compensatory ↑ HR | | **Urethral sphincter relaxation** | Loss of α1-tone on internal sphincter smooth muscle | Urinary hesitancy, retrograde ejaculation | | **Salivary secretion** | Unchanged or ↑ (parasympathetic M3 intact) | NOT affected by α1 blockade | ### Why Option 2 Is Wrong **Salivary glands are innervated exclusively by parasympathetic fibres** (via CN VII and IX) acting on **M3 muscarinic receptors**. Alpha-1 adrenergic receptors are not functionally present on salivary acini. Therefore, alpha-1 blockade does **NOT decrease** salivary secretion; if anything, loss of any residual sympathetic inhibition might slightly increase it. **Clinical Pearl:** Anticholinergic drugs (which block M3 receptors) cause dry mouth; alpha-1 antagonists do not. ### Verification of Other Options **Option 0 (Vasodilation):** ✓ Correct. Alpha-1 blockade removes sympathetic vasoconstriction, causing arteriolar and venous dilation. **Option 1 (Reflex tachycardia):** ✓ Correct. The drop in blood pressure from vasodilation triggers baroreceptor-mediated sympathetic reflex, increasing heart rate. **Option 3 (Urethral sphincter relaxation):** ✓ Correct. The internal urethral sphincter is rich in α1 receptors; blockade causes relaxation and can lead to urinary hesitancy or incontinence. [cite:KD Tripathi Pharmacology 8e Ch 12]
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