Question

Mirabegron is used in the management of overactive bladder

• A. Beta 2 agonist
• B. Beta 3 agonist
• C. M3 antagonist
• D. Beta 1 agonist

Hint:

Beta-3 agonists like mirabegron are preferred in patients who cannot tolerate antimuscarinic side effects for overactive bladder.

Answer 1

The Correct Option is B

Mirabegron is a selective Beta-3 adrenergic receptor agonist primarily used in the treatment of overactive bladder (OAB). Activation of Beta-3 receptors in the detrusor muscle of the bladder leads to relaxation during the storage phase of the urinary bladder cycle, thereby increasing bladder capacity and reducing symptoms of urgency and frequency.
Unlike antimuscarinic agents (such as M3 antagonists), which inhibit bladder contractions by blocking muscarinic receptors, mirabegron works by a different mechanism and generally has a more favorable side effect profile, especially regarding dry mouth and constipation.

Answer 2

Mirabegron treats overactive bladder through a receptor mechanism distinct from the antimuscarinic drugs usually used for this condition. Let's check each receptor class listed against how mirabegron actually works.

1. Beta 2 agonist: Beta 2 receptors are mainly found in bronchial and vascular smooth muscle, and drugs like salbutamol act here for asthma. The detrusor muscle relaxation mirabegron produces is not mediated through this receptor subtype, so this option is incorrect.
2. Beta 3 agonist: Beta 3 adrenergic receptors are the dominant beta receptor subtype in the detrusor muscle of the bladder. Mirabegron selectively activates these receptors, relaxing the detrusor during the bladder's storage phase and increasing its capacity, which reduces urgency and frequency. This is exactly how mirabegron works, so this option is correct.
3. M3 antagonist: M3 muscarinic antagonists, such as oxybutynin and solifenacin, are the older class of overactive bladder drugs. They work by blocking acetylcholine-driven bladder contraction, a completely different receptor system from the one mirabegron targets, so this option is incorrect.
4. Beta 1 agonist: Beta 1 receptors are concentrated in cardiac tissue and control heart rate and contractility. They play no role in detrusor relaxation, so this option is incorrect.

The receptor mirabegron activates to relax the bladder and treat overactive bladder is the beta 3 subtype.

Therefore, the correct answer is Beta 3 agonist.