Mechanism of action of NO is –
**Core Concept**
Nitric oxide (NO) is a crucial signaling molecule involved in various physiological processes, including vasodilation, platelet aggregation, and neurotransmission. Its mechanism of action is centered around the activation of guanylyl cyclase, leading to the production of cyclic guanosine monophosphate (cGMP).
**Why the Correct Answer is Right**
NO acts by diffusing into vascular smooth muscle cells, where it binds to soluble guanylyl cyclase (sGC). This binding causes a conformational change in the enzyme, leading to an increase in guanylyl cyclase activity. As a result, cGMP levels rise, causing the relaxation of vascular smooth muscle cells and subsequent vasodilation. This effect is mediated through the activation of protein kinase G (PKG), which phosphorylates and inhibits myosin light chain kinase (MLCK), ultimately reducing intracellular calcium levels and leading to muscle relaxation.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the mechanism of action of NO.
**Option B:** While NO is involved in neurotransmission, its primary mechanism of action is not through the activation of adenylate cyclase.
**Option C:** This option is incorrect as NO does not act through the inhibition of protein kinase A (PKA).
**Clinical Pearl / High-Yield Fact**
NO is synthesized from L-arginine by the enzyme nitric oxide synthase (NOS), which is present in various cell types, including endothelial cells, neurons, and macrophages. The production of NO is tightly regulated by various factors, including calcium levels, pH, and the presence of cofactors.
**Correct Answer:** C.