1, 25 dihydrocholecalciferol acts on ?
**Core Concept:** 1,25 dihydroxyvitamin D is the biologically active form of vitamin D, primarily responsible for calcium and phosphorus homeostasis. It acts on the parathyroid glands, bones, and intestines to regulate calcium and phosphate levels in the body.
**Why the Correct Answer is Right:** 1,25 dihydroxyvitamin D is produced by the action of two enzymes: vitamin D 1Ξ±-hydroxylase (CYP27B1) and vitamin D 24-hydroxylase (CYP24A1). It binds to the vitamin D receptor (VDR), which is expressed in various tissues, including the parathyroid glands, bones, and intestines. In the parathyroid glands, 1,25 dihydroxyvitamin D inhibits parathyroid hormone (PTH) secretion, reducing the release of calcium from bones and promoting calcium absorption in the intestines. In bones, it promotes calcium release and inhibits bone resorption, while in the intestines, it enhances calcium absorption.
**Why Each Wrong Option is Incorrect:**
A. Vitamin D receptors are not expressed in the liver, so option A is incorrect.
B. Vitamin D receptors are not expressed in the kidneys, so option B is incorrect.
C. Vitamin D receptors are expressed in the liver, where 25-hydroxyvitamin D (calcidiol) is converted to 1,25 dihydroxyvitamin D (calcitriol), making option C incorrect.
D. Vitamin D receptors are expressed in the kidneys where calcitriol is converted to 1,25 dihydroxyvitamin D (calcitriol), making option D correct.
**Clinical Pearl:** Understanding the role of vitamin D and its active form, calcitriol (1,25 dihydroxyvitamin D), in calcium and phosphate homeostasis is crucial for preventing conditions like rickets in children and osteomalacia/osteoporosis in adults, which result from impaired vitamin D synthesis or action.