Which molecule is released during sympathetic activation that increases peripheral resistance?

• A. Adenosine.
• B. Atrial natriuretic peptide.
• C. Norepinephrine (catecholamines).
• D. Histamine.

Answer: (C)

During sympathetic activation, norepinephrine is released from sympathetic nerve endings and acts on alpha-1 adrenergic receptors on vascular smooth muscle. This triggers a signaling cascade that raises intracellular calcium, causing vasoconstriction of arterioles and increasing peripheral (systemic) resistance. The higher resistance contributes to higher arterial blood pressure in stress situations. The other molecules mentioned don’t primarily raise peripheral resistance: adenosine dilates vessels in many tissues, atrial natriuretic peptide promotes vasodilation and natriuresis, and histamine often causes vasodilation and increased permeability rather than constriction.