Which catecholamine primarily acts on beta-2 receptors?

• A. Norepinephrine
• B. Epinephrine
• C. Dopamine
• D. Serotonin

Answer: (B)

Beta-2 receptor activation is driven mainly by epinephrine because this catecholamine has a strong affinity for beta receptors, especially beta-2. When beta-2 receptors are stimulated, they couple to Gs proteins and raise cAMP in smooth muscle cells, leading to relaxation—think bronchodilation and vasodilation in certain vascular beds. That pronounced beta-2 effect contrast with norepinephrine, which mostly targets alpha-1 and beta-1 receptors and has only minimal action on beta-2 at typical levels. Dopamine acts primarily on dopamine receptors and, at higher doses, can influence dopaminergic and some adrenergic receptors, not primarily beta-2. Serotonin isn’t a catecholamine at all and works on serotonin receptors. So the catecholamine most associated with beta-2–mediated responses is epinephrine.