**Question:** Which of the following has cyclic GMP mediated action?

A. Nitric Oxide (NO)
B. Prostacyclin
C. Adenosine
D. Angiotensin II

**Correct Answer:** B. Prostacyclin

**Core Concept:**
Cyclic GMP (guanosine monophosphate) is an important second messenger molecule involved in various physiological processes, particularly in the cardiovascular system. It plays a role in regulating smooth muscle tone, vasodilation, and platelet aggregation.

**Why the Correct Answer is Right:**
Prostacyclin (PGI2) is a potent vasodilator synthesized from arachidonic acid by the enzyme cyclooxygenase-2 (COX-2). It acts via the activation of the G protein-coupled receptor IP (prostacyclin receptor) which stimulates the production of cGMP. This leads to vasodilation, inhibition of platelet aggregation, and reduction of vascular permeability.

**Why Each Wrong Option is Incorrect:**

A. Nitric Oxide (NO): NO is a potent vasodilator, but it acts via the stimulation of soluble guanylate cyclase (sGC) to produce cGMP, not directly through a specific receptor.
B. Adenosine: Adenosine is a vasodilator that acts via adenosine receptors (A1, A2A, A2B, and A3) and stimulates adenylyl cyclase, leading to cAMP production, not cGMP.
D. Angiotensin II: Angiotensin II is a potent vasoconstrictor acting via AT1 receptors and stimulating PLC (Phospholipase C) to produce IP3 and DAG (Diacylglycerol), which leads to calcium mobilization and vasoconstriction, not cGMP production.

**Clinical Pearl:**
Prostacyclin plays a significant role in cardiovascular homeostasis, particularly in the context of vasodilation, platelet aggregation control, and reducing vascular permeability. Understanding the mechanism of action of prostacyclin and cGMP is crucial for interpreting the effects observed in various clinical scenarios related to cardiovascular system.