Phenylephrine exerts its vasoconstrictive effect primarily by activating which receptor?

Phenylephrine exerts its vasoconstrictive effect primarily by activating which receptor?

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

Answer: (A)

Phenylephrine raises blood pressure by directly constricting peripheral arteries through alpha-1 adrenergic receptors on vascular smooth muscle. These receptors are Gq-coupled, so their activation triggers a signaling cascade that increases intracellular calcium and causes smooth muscle contraction, producing vasoconstriction and higher systemic vascular resistance. Because phenylephrine is selective for alpha-1, its primary effect is vasoconstriction rather than effects on the heart or on beta receptors. Activating beta-1 would mainly increase heart rate and contractility, beta-2 activation would cause vasodilation in some beds, and alpha-2 activation (when prominent) reduces norepinephrine release rather than driving direct vasoconstriction. Therefore, the vasoconstrictive effect is best explained by alpha-1 receptor activation, with potential reflex bradycardia as blood pressure rises.

Phenylephrine raises blood pressure by directly constricting peripheral arteries through alpha-1 adrenergic receptors on vascular smooth muscle. These receptors are Gq-coupled, so their activation triggers a signaling cascade that increases intracellular calcium and causes smooth muscle contraction, producing vasoconstriction and higher systemic vascular resistance. Because phenylephrine is selective for alpha-1, its primary effect is vasoconstriction rather than effects on the heart or on beta receptors. Activating beta-1 would mainly increase heart rate and contractility, beta-2 activation would cause vasodilation in some beds, and alpha-2 activation (when prominent) reduces norepinephrine release rather than driving direct vasoconstriction. Therefore, the vasoconstrictive effect is best explained by alpha-1 receptor activation, with potential reflex bradycardia as blood pressure rises.