Veterinary Physiology-III

CALCITRIOL

• The third major hormone involved in the regulation of calci­um metabolism and skeletal remodelling is calcitriol or cholecalciferol (vitamin ‑D₃) or irradiated ergosterol (vitamin ‑D₂).
• Cholecalciferol is ingested in the diet and also synthesized from 7-dehydrocholesterol by ultraviolet irradiation.

Synthesis

• Epithelial cells of the skin synthesise the precursor of vitamin D, 7-dehydrocholesterol from acetate. Exposure to sun changes 7-dehydrocholesterol to inactive vitamin D.
• After absorption or synthesis, the cholecalciferol is bound to an alpha -2 globulin (vitamin D binding protein – DBP) in the blood, transported to the liver for its conversion into 25-hydroxycholecalciferol (calcidiol) by 25- hydroxylase.
• In the kidney 25‑hydroxy cholecalciferol is converted into 1, 25 dihydroxy cholecalciferol (DHCC)by a rate limiting enzyme 1-α hydroxylase. 1,25 DHCC exerts strong product feedback inhibition on 1 alpha hydroxylase in the kidney.
• When Ca is normal or in excess, kidneys produce 24,25 DHCC which is inactive.
• Activation of renal hydroxylase is stimulated by PTH, low serum Ca level, low serum PO₄ level, prolactin and estrogen and is inhibited by hypercalcemia, high serum PO₄ level and CT.