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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.
