Which of the following relaxes mesangial cells in glomerulus?
**Core Concept**
The relaxation of mesangial cells in the glomerulus is crucial for increasing glomerular filtration rate (GFR) and blood flow to the kidneys. This process involves the contraction or relaxation of mesangial cells, which surround the capillaries of the glomerulus and regulate the surface area available for filtration.
**Why the Correct Answer is Right**
Prostaglandin E2 (PGE2) is a potent vasodilator that relaxes mesangial cells in the glomerulus, thereby increasing the surface area available for filtration and enhancing GFR. PGE2 acts through its receptors, specifically EP1, EP2, EP3, and EP4, to activate adenylate cyclase, increasing cyclic AMP (cAMP) levels. Elevated cAMP levels lead to the activation of protein kinase A (PKA), which phosphorylates and relaxes the mesangial cells. This results in the dilation of the afferent arteriole and the relaxation of mesangial cells, ultimately increasing blood flow and GFR to the glomerulus.
**Why Each Wrong Option is Incorrect**
**Option A:** While nitric oxide (NO) is a potent vasodilator, it primarily acts on vascular smooth muscle cells to relax the blood vessels and increase blood flow. It does not specifically target mesangial cells in the glomerulus.
**Option B:** Adenosine is a vasodilator that acts on the renal vasculature to increase blood flow. However, it does not have a direct effect on mesangial cells in the glomerulus.
**Option C:** Angiotensin II is a potent vasoconstrictor that acts on the renal vasculature to decrease blood flow and increase blood pressure. It does not relax mesangial cells in the glomerulus.
**Clinical Pearl / High-Yield Fact**
The relaxation of mesangial cells in the glomerulus is an important mechanism for regulating GFR and blood flow to the kidneys. Understanding the role of PGE2 in this process is crucial for managing conditions such as hypertension and kidney disease.
**Correct Answer:** C. Prostaglandin E2 (PGE2)