What is the mechanism of action of mirabegron?

• A. It inhibits acetylcholine release at the neuromuscular junction.
• B. It is a beta-3 adrenergic receptor agonist that relaxes the detrusor muscle to increase bladder capacity.
• C. It blocks beta-2 receptors to cause urinary retention.
• D. It stimulates muscarinic receptors to increase detrusor contraction.

Answer: (B)

Mirabegron works by activating beta-3 adrenergic receptors in the detrusor smooth muscle. When these receptors are stimulated, they signal via Gs proteins to raise cyclic AMP, which causes the detrusor to relax during the storage phase. This relaxation increases bladder capacity and reduces urge incontinence associated with overactive bladder. It’s not about inhibiting acetylcholine release at the neuromuscular junction, nor about blocking beta-2 receptors to cause retention, nor about stimulating muscarinic receptors to boost detrusor contraction. The key idea is beta-3 receptor activation leading to detrusor relaxation and greater storage capacity.