Which adrenergic receptor subtype mediates vasodilation and bronchodilation in smooth muscle?

• A. Beta 2 receptors
• B. Beta 1 receptors
• C. Alpha 1 receptors
• D. Alpha 2 receptors

Answer: (A)

Beta-2 adrenergic receptors on smooth muscle produce relaxation, leading to both bronchodilation in the airways and vasodilation in certain vascular beds. When epinephrine or other beta-2 agonists bind these receptors, they activate Gs proteins, increasing cAMP, which activates protein kinase A and lowers intracellular calcium. That shift reduces myosin light-chain phosphorylation, causing the smooth muscle to relax. This is why beta-2 agonists like albuterol are effective for treating asthma and why these receptors mediate dilation in the bronchial smooth muscle; they also mediate dilation in some skeletal muscle vessels, though vascular tone depends on multiple factors with alpha-1–mediated constriction often predominating in other beds.