Which statement best describes the beta-2 receptor involvement differentiating epinephrine from norepinephrine?

Which statement best describes the beta-2 receptor involvement differentiating epinephrine from norepinephrine?

• A. Beta-2 stimulation is inhibited by norepinephrine
• B. Both stimulate beta-2 equally
• C. Neither stimulates beta-2 receptors
• D. Epinephrine stimulates beta-2 receptors; norepinephrine does not

Answer: (D)

Beta-2 receptor stimulation produces smooth muscle relaxation, including bronchodilation and some vascular effects in skeletal muscle. Epinephrine activates beta-2 receptors strongly, giving bronchodilation and certain vasodilatory effects, while norepinephrine has little affinity for beta-2 receptors and thus does not produce those beta-2–mediated effects at typical concentrations. That makes the statement that epinephrine stimulates beta-2 receptors while norepinephrine does not the best description of their differences. The other ideas aren’t correct because norepinephrine mainly targets alpha-1 (and beta-1) with negligible beta-2 activity, so it doesn’t produce the beta-2–driven responses seen with epinephrine.

Beta-2 receptor stimulation produces smooth muscle relaxation, including bronchodilation and some vascular effects in skeletal muscle. Epinephrine activates beta-2 receptors strongly, giving bronchodilation and certain vasodilatory effects, while norepinephrine has little affinity for beta-2 receptors and thus does not produce those beta-2–mediated effects at typical concentrations. That makes the statement that epinephrine stimulates beta-2 receptors while norepinephrine does not the best description of their differences. The other ideas aren’t correct because norepinephrine mainly targets alpha-1 (and beta-1) with negligible beta-2 activity, so it doesn’t produce the beta-2–driven responses seen with epinephrine.