Which receptor mediates relaxation of the detrusor muscle to allow bladder filling in overactive bladder?

• A. Beta-1 adrenergic receptor
• B. Beta-3 adrenergic receptor
• C. Alpha-1 adrenergic receptor
• D. Muscarinic M3 receptor

Answer: (B)

Relaxation of the detrusor during bladder filling is driven by sympathetic signaling through beta-3 adrenergic receptors on the detrusor smooth muscle. When these receptors are activated, they stimulate Gs proteins and raise cAMP, which promotes smooth muscle relaxation and increases bladder capacity. In overactive bladder, beta-3 agonists like mirabegron are used to enhance this relaxation and reduce urgency and frequency. By contrast, muscarinic M3 receptors mediate detrusor contraction when acetylcholine is released, so activating M3 would promote voiding rather than filling. Alpha-1 receptors, mainly on the bladder neck and internal sphincter, cause contraction there and do not relax the detrusor; beta-1 receptors are not central to detrusor tone.