Vitamin D effects on bone; Resorption or what? - USMLE Forums
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Old 07-13-2010
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Bones Vitamin D effects on bone; Resorption or what?

What is the mechanism of how Vitamin D stimulates bone resorption?
Is it the same as PTH stimulating osteoblast to secrete IL-1 which stimulates osteoclasts?

Also, in first aid it says vitamin D stimulates bone resorption, I thought it stimulates bone mineralization at low levels?

If so, why do all post-menopausal women take vitamin D supplements like crazy if it only worsens osteoporosis (+ their lack of estrogens inhibiting osteoclasts) by stimulating bone resorption?
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Old 07-13-2010
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Yeah, that is a trick that you should know.

Vitamin D at normal physiologic levels act on intestinal mucosa and the renal distal tubule to increase the absorption of Calcium. This Calcium will then be available for use in mineralizing new bone formation.

Therefore, when you have Vitamin D deficiency you will develop rickets (in children) or osteopenia (in adults).

The issue, is that if you have too much of Vitamin D (Vitamin D excess) then at that time it will work on the nuclear receptors in the osteoblasts and promote bone resorption.

Therefore, both deficiency and excess of Vitamin D can cause osteopenia and bone resorption.

Women with osteoporosis are advised to take Vitamin D in moderate amounts (the recommended daily allowance) but if they take too much that will for sure be counterproductive.

The effects of Vitamin D is nicely explained in Kaplan Physiology Notes page 437 (2007 version).
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Old 04-28-2011
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When excess vit d acts on OSTEOBLASTs receptor how come it can do bone resorption..?
bone resorption is job of osteoclats?? can u explain?
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Old 04-28-2011
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Quote:
Originally Posted by ikrimachoudry View Post
whn excess vit d acts on OSTEOBLASTs receptor how come it can do bone resorption..?
bone resorption is job of osteoclats?? can u explain?
It has been found that hormones affect osteoblasts which then relay the information to the neighboring osteoclasts. Receptors are on osteoblasts not on osteoclasts even though the job is done by osteoclasts.
This has been explained in detail in Kaplan videos and in fact it's a high yield info and they may trick you with it.

Here's quote for you from Wikipedia
Quote:
Stimulation is indirect since osteoclasts do not have a receptor for PTH; rather, PTH binds to osteoblasts, the cells responsible for creating bone. Binding stimulates osteoblasts to increase their expression of RANKL and inhibits their expression of Osteoprotegrin(OPG). OPG binds to RANKL and blocks it from interacting with RANK, a receptor for RANKL. The binding of RANKL to RANK (facilitated by the decreased amount of OPG) stimulates these osteoclast precursors to fuse, forming new osteoclasts which ultimately enhances bone resorption.
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Old 04-28-2011
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Quote:
Originally Posted by ikrimachoudry View Post
whn excess vit d acts on OSTEOBLASTs receptor how come it can do bone resorption..?
bone resorption is job of osteoclats?? can u explain?
i think that when you have vitamin D deficiency = low calcium = PTH will respond by increased resorption of Ca from bone = weak bones

so the reason you are seeing resorption of bone is due to PTH acting in compensation

correct me if im wrong
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Old 08-06-2012
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Quote:
Originally Posted by jattdoc View Post
i think that when you have vitamin D deficiency = low calcium = PTH will respond by increased resorption of Ca from bone = weak bones

so the reason you are seeing resorption of bone is due to PTH acting in compensation

correct me if im wrong

Vitamin D stimulate Osteoblast which produce cytokines, cytokines stimulates Osteoclast,Osteoclast produce acidic environment eventually lead to bone resorption ( old bones) which provide ca+ and Phosphate and these are again utilized for mineralization of new bone.
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