Which receptor subtype commonly mediates vasoconstriction in vascular smooth muscle?

• A. Alpha-1
• B. Beta-1
• C. Beta-2
• D. Muscarinic

Answer: (A)

The action of many vessels is controlled by sympathetic signals that tell the smooth muscle around arteries to tighten or relax. The receptor subtype that commonly drives vasoconstriction is the alpha-1 adrenergic receptor. When norepinephrine (and to some extent epinephrine) binds this receptor on vascular smooth muscle, it activates Gq signaling, which increases IP3 and DAG. This raises intracellular calcium, and the calcium allows the smooth muscle fibers to contract, narrowing the vessel and increasing resistance.

That’s why alpha-1 is the best answer here. Beta-2 receptors, in contrast, trigger relaxation and vasodilation via cAMP. Beta-1 receptors are mainly in the heart, increasing rate and force of contraction rather than constricting blood vessels. Muscarinic receptors in vessels generally promote vasodilation through endothelium-derived NO rather than constriction.