Which among the following is a vasoconstrictor?
**Core Concept**
Angiotensin II is a potent vasoconstrictor that plays a crucial role in the renin-angiotensin system, which regulates blood pressure and fluid balance. It acts on the vascular smooth muscle to cause vasoconstriction, thereby increasing blood pressure.
**Why the Correct Answer is Right**
Angiotensin II exerts its vasoconstrictive effects by binding to the angiotensin type 1 (AT1) receptors on vascular smooth muscle cells. This binding triggers a cascade of intracellular signaling events that ultimately lead to the contraction of vascular smooth muscle cells, causing blood vessels to constrict. The resulting increase in peripheral resistance increases blood pressure.
**Why Each Wrong Option is Incorrect**
**Option A:** Vasopressin, also known as antidiuretic hormone (ADH), is primarily involved in regulating water reabsorption in the kidneys and has minimal vasoconstrictive effects.
**Option B:** Acetylcholine is a neurotransmitter that typically causes vasodilation in blood vessels, particularly in the endothelium-dependent relaxation pathway.
**Option C:** Nitric oxide (NO) is a potent vasodilator that is produced by the endothelium and plays a crucial role in maintaining vascular tone and preventing thrombosis.
**Clinical Pearl / High-Yield Fact**
The renin-angiotensin system is a critical regulator of blood pressure, and angiotensin II is a key player in this system. Inhibitors of the renin-angiotensin system, such as ACE inhibitors and angiotensin receptor blockers (ARBs), are commonly used to treat hypertension and heart failure.
**Correct Answer:** C. Nitric oxide