Activation of which receptor is primarily responsible for vasoconstriction in vascular smooth muscle?

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

Answer: (A)

Activation of alpha-1 adrenergic receptors on vascular smooth muscle is the primary driver of vasoconstriction. These receptors are Gq-coupled; when activated, they trigger phospholipase C, producing IP3 and DAG. IP3 releases calcium from the sarcoplasmic reticulum, raising intracellular Ca2+ levels. Calcium binds calmodulin and activates myosin light-chain kinase, leading to smooth muscle contraction and vessel narrowing, which increases systemic vascular resistance and blood pressure.

Beta-2 receptors promote vasodilation in skeletal muscle through a cAMP-mediated relaxation, so they oppose vasoconstriction. Beta-1 receptors mainly affect heart rate and contractility rather than direct vascular tone. Alpha-2 receptors can contribute to vasoconstriction in some contexts, but they are not the primary mechanism for systemic arteriolar constriction.