Vit D doesnot resorb bone!!!!
This is the classic confusion between PTH and Vit D who does what and how!!! So lets get it sorted once and for all...
Before that lets talk about some basic things:
1. Bone Formation: Bone formation occurs when there is either rise in Ca2+ or Phosphorus ion and the solubility product of both is greater than what requires to form bone(layman = when conc of either one of them rises bone is formed) and Osteoblasts are responsible for this! They form osteoid which is nothing but bone!
2. Bone destruction: Bone is usually destroyed by the osteoclasts when osteoid is broken down to release Ca2+ and Phosphorus.
Now who does what:
PTH:
- Increases Ca2+ reabosorbtion from Kidney
- Decreases Phosphorus reabsorbtion from Kidney
- Activates Vit D via 1-alpha hydroxylation in the kidney
- PTH binds to osteoblast which releases IL-1, IL-1 activates osteoclast which breaks down bone
VIT D:
- Increases Ca2+ reabosorbtion from Kidney
- Increases Phosphorus reabsorbtion from Kidney
- Increases Ca2+ reabsorbtion from the gut
- Vit D binds to osteoblast and stimulates bone formation
Look that both hormones bind to Osteoblasts but one increases bone formation other decreases it!
Only Calcitonin hormone binds to osteoclasts directly and inhibits osteoclasts!
LOGIC:
Vit D forms bone...Vit D raises Ca2+ level to form bone...
PTH job is to maintain Ca2+ level and it doesnot care about bone and breaks it!
Credits : Sir
Edward Goljan