Mechanism of action of Calcitriol is:
**Question:** Mechanism of action of Calcitriol is:
A. Activation of vitamin D receptor
B. Increased calcium absorption
C. Inhibition of parathyroid hormone secretion
D. Enhanced osteoclast activity
**Core Concept:** Calcitriol is the active form of Vitamin D, which plays a crucial role in calcium homeostasis and bone health. It achieves this by binding to the Vitamin D Receptor (VDR), a nuclear hormone receptor that regulates gene expression in target cells.
**Why the Correct Answer is Right:** Calcitriol primarily acts through its interaction with the Vitamin D Receptor (VDR). When calcitriol binds to the VDR, it enters the nucleus and binds to specific DNA sequences called vitamin D response elements (VDREs), which leads to the activation of target genes. These target genes are involved in calcium and phosphate homeostasis, bone remodeling, and immune system modulation.
**Why Each Wrong Option is Incorrect:**
A. Activation of vitamin D receptor: While calcitriol does activate the VDR, option A is not entirely correct because it does not specify the nuclear action of the VDR.
B. Increased calcium absorption: While calcitriol does increase calcium absorption in the intestine, the correct answer should focus on the VDR activation and subsequent gene regulation, rather than just calcium absorption.
C. Inhibition of parathyroid hormone secretion: Although calcitriol does suppress parathyroid hormone (PTH) secretion, the mechanism should focus on the VDR activation and gene regulation, not just PTH suppression.
D. Enhanced osteoclast activity: Enhanced osteoclast activity is a side effect of calcitriol, but the correct answer should emphasize the role of VDR activation and gene regulation.
**Clinical Pearl:** The correct mechanism of action of calcitriol highlights its role in gene regulation via the Vitamin D Receptor (VDR) in maintaining calcium homeostasis, bone health, and immune system modulation. Understanding the nuclear action of the VDR is crucial for clinical scenarios involving vitamin D deficiency, hypercalcemia, or hypocalcemia.