Which receptor stimulates contraction of smooth muscle and increases blood pressure via vasoconstriction?

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

Answer: (B)

Alpha-1 adrenergic receptors on vascular smooth muscle are the ones that drive contraction leading to vasoconstriction and a rise in blood pressure. When these receptors are activated, they signal through a Gq protein to kick off phospholipase C, producing IP3 and DAG. IP3 increases intracellular calcium, and the elevated calcium causes the smooth muscle to contract, narrowing the vessels and increasing systemic vascular resistance. That direct process is what raises blood pressure through vasoconstriction.

The other receptors don’t fit this exact mechanism. Alpha-2 receptors mainly reduce norepinephrine release, providing a modulatory effect rather than causing direct vasoconstriction. Beta-1 receptors are in the heart and raise blood pressure by increasing heart rate and contractility, not by constricting blood vessels. Dopamine can cause vasoconstriction at high doses via alpha receptors, but the direct receptor responsible for smooth muscle contraction and vasoconstriction is the alpha-1 receptor.