A 58-year-old man with benign prostatic hyperplasia and hypertension is being started on tamsulosin. Unlike prazosin, tamsulosin does not cause first-dose orthostatic hypotension to the same degree. Which best explains the pharmacological basis for this difference?

• A Tamsulosin has a higher affinity for alpha-1B receptors in blood vessel walls
• B Tamsulosin has partial agonist activity at alpha-1 receptors
• C Tamsulosin is a non-competitive antagonist and therefore acts more slowly
• D Tamsulosin is a selective alpha-1A/alpha-1D antagonist with lower affinity for alpha-1B vascular receptors ✓

Explanation

Alpha-1 adrenoreceptors have three subtypes: alpha-1A (predominant in prostate/bladder neck), alpha-1B (blood vessel smooth muscle), and alpha-1D (also in bladder). Tamsulosin shows greater selectivity for alpha-1A and alpha-1D subtypes, relaxing the prostate with relatively less blockade of vascular alpha-1B receptors, resulting in less orthostatic hypotension compared to non-selective prazosin. It has no partial agonist or non-competitive properties.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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