If norepinephrine binds to alpha1 receptors on vascular smooth muscle, what is the effect?

If norepinephrine binds to alpha1 receptors on vascular smooth muscle, what is the effect?

• A. Vasodilation
• B. No effect
• C. Vasoconstriction
• D. Relaxation

Answer: (C)

When norepinephrine binds to alpha1 receptors on vascular smooth muscle, it activates a Gq signaling pathway that opens phospholipase C. This leads to the production of IP3 and DAG, and IP3 increases intracellular calcium. The rise in calcium activates the contractile machinery (via calmodulin and myosin light-chain kinase), causing the smooth muscle to contract. Contraction of the vascular smooth muscle narrows the vessel lumen, producing vasoconstriction. This increase in vascular tone raises systemic vascular resistance and typically raises blood pressure. Epinephrine or beta2 activation can promote dilation in some beds, but norepinephrine mainly driving alpha1-mediated constriction explains why the effect here is vasoconstriction.

When norepinephrine binds to alpha1 receptors on vascular smooth muscle, it activates a Gq signaling pathway that opens phospholipase C. This leads to the production of IP3 and DAG, and IP3 increases intracellular calcium. The rise in calcium activates the contractile machinery (via calmodulin and myosin light-chain kinase), causing the smooth muscle to contract. Contraction of the vascular smooth muscle narrows the vessel lumen, producing vasoconstriction. This increase in vascular tone raises systemic vascular resistance and typically raises blood pressure. Epinephrine or beta2 activation can promote dilation in some beds, but norepinephrine mainly driving alpha1-mediated constriction explains why the effect here is vasoconstriction.