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Functions of Vitamin D (Calcitriol )
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- Vitamin D acts on target organs like bones, kidneys & intestinal mucosa to regulate calcium & phosphate metabolism.
- Calcitriol is the physiological active form of Vitamin D.
- It increases the blood calcium level.
1. Intestinal Absorption of Calcium & Phosphate
- Calcitriol (active form of Vitamin D) increases the absorption of Calcium from the brush border surface of the intestine (passive).
- From the intestinal cell to plasma it is transported by using energy “active transport”.
- It is either by the Sodium-Calcium exchange mechanism or by pumping out the Calcium-Calbindin Complex.
- The action of Calcitriol is like a steroid hormone.
- It enters the cell and binds to a specific nuclear receptor called VDR (Vitamin D Receptor) which binds to VDRE to form a dimer to activate Transcription of specific genes that code for Calbindin.
- Due to the increased availability of calcium binding protein, the absorption of calcium is increased.
2. Mineralization of Bones
- Calcitriol increases the activity of osteoblasts, releasing phosphatase leading to mineralization of bones.
- It has its action on all 3 types of cells namely the osteoblasts, osteoclasts and osteocytes.
- It is responsible for increased production of bone matrix proteins such as collagen, osteocalcein and osteopontin.
3. Other Effects
- Vitamin D helps in the renal re-absorption of calcium & phosphorus.
- It lowers the pH in certain parts of gut and increases urinary pH.
- It also helps in the Signal transduction to brain and the immune system.
- Recent research has proved that most tissues possess Vitamin D receptor and several tissues have the enzymes to generate calcitriol.
- 1, 25-DHCC has been found to have a modulatory effect on immune-hematopoietic system.
- Therapeutic doses given to children with rickets have been found to correct the anemia, hypocellularity of the bone marrow & increased susceptibility to infection.
- It has also been found to reduce the risk of cancer & coronary vascular disease.
- Beneficial effects have been observed in patients with AIDS.
- Calcitriol exerts its pleiotropic actions via the specific receptor VDR, an important transcription factor.
- Vitamin D has been found to be a modulator of immune response.