Calcitriol

Calcitriol is the final active form of vitamin D₃. Calcitriol binds to the vitamin D receptor (VDR) in the cell nucleus and regulates gene transcription and protein synthesis. In the intestine, activated vitamin D receptors increase the expression of calbindin-D9k and the calcium channel TRPV6, enhancing calcium ion and phosphate absorption by small intestinal epithelial cells and contributing to the regulation of blood calcium and phosphate homeostasis. In bone, calcitriol binds to vitamin D receptors in osteoblasts and increases the expression of receptor activator of NF-κB ligand (RANKL), thereby regulating the expression of bone-related genes and influencing bone formation and bone resorption. In addition, calcitriol indirectly promotes the differentiation and activation of osteoclasts.