A 65-year-old man with acute urinary retention due to postoperative atonic bladder is given bethanechol. Which receptor subtype and tissue action combination best explains its therapeutic effect?

• A M1 receptor stimulation increasing detrusor muscle tone
• B M2 receptor stimulation relaxing the bladder trigone
• C Nn receptor stimulation at the pelvic ganglion enhancing parasympathetic outflow
• D M3 receptor stimulation causing detrusor contraction and bladder emptying ✓

Explanation

Bethanechol is a direct-acting muscarinic agonist that selectively stimulates M3 receptors on the detrusor muscle of the bladder, causing contraction and promoting urine voiding in atonic bladder and postoperative urinary retention. M1 receptors are mainly neuronal and gastric; M2 receptors dominate in the heart; Nn receptors are nicotinic ganglionic receptors not primarily responsible for detrusor contraction.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.