All are vasodilators except –
**Core Concept**
Nitric oxide (NO) and prostacyclin (PGI2) are potent vasodilators that act on the vascular smooth muscle to reduce blood pressure. These molecules trigger a cascade of intracellular signaling pathways that ultimately lead to the relaxation of smooth muscle cells and vasodilation.
**Why the Correct Answer is Right**
Prostacyclin (PGI2) and nitric oxide (NO) exert their vasodilatory effects through the activation of adenylate cyclase, leading to an increase in cyclic adenosine monophosphate (cAMP) levels. This results in the activation of protein kinase A (PKA), which in turn phosphorylates and relaxes the vascular smooth muscle. In contrast, the other options do not primarily act as vasodilators.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because, although it is a vasodilator, it is not the primary focus of the question. However, if we assume it is a distractor, it would be incorrect because it does not directly oppose the action of vasodilators.
**Option B:** This option is incorrect because it primarily acts as a bronchodilator and has some vasodilatory effects, but its primary mechanism is not vasodilation.
**Option C:** This option is incorrect because it primarily acts as a vasoconstrictor and is used to treat conditions like Raynaud's disease.
**Clinical Pearl / High-Yield Fact**
One key point to remember is that prostacyclin and nitric oxide are potent vasodilators, but they have different mechanisms of action. Prostacyclin primarily acts through the activation of adenylate cyclase, while nitric oxide activates guanylate cyclase.
**Correct Answer:** C. This option is a vasoconstrictor, making it the correct answer to the question.