Which receptor mediates the bronchodilatory effect of beta-2 agonists?

• A. Alpha-1 adrenergic receptor on vascular smooth muscle
• B. Beta-2 adrenergic receptor on bronchial smooth muscle
• C. Beta-1 adrenergic receptor on cardiac tissue
• D. Dopamine receptor D2 on neurons

Answer: (B)

Bronchodilation from beta-2 agonists happens when the drug binds to beta-2 adrenergic receptors on bronchial smooth muscle. This receptor is linked to a Gs protein that activates adenylate cyclase, increasing cyclic AMP. The higher cAMP activates protein kinase A, which lowers intracellular calcium and promotes smooth muscle relaxation, widening the airways and improving airflow. That’s why these agents relieve bronchospasm in conditions like asthma. The other receptors listed don’t produce bronchodilation in the bronchi: alpha-1 on vascular smooth muscle causes vasoconstriction, beta-1 on cardiac tissue increases heart rate and contractility, and dopamine D2 receptors on neurons don’t mediate bronchodilation.