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Old 07-20-2013
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Question Vitamin D and PTH Confusion Here!

I find this so confusing sometimes, because they just seem so contradictory to each other!

PTH

1) inc alpha-1 hydroxylase synthesis ---> Inc active Vit D

2) inc Ca uptake by early distal tubules

3) dec phosphate uptake -----> Phosphaturia

So based on the above, it seems obvious that PTH is concerned about raising ionized calcium (pee out the phosphate, keep the calcium), which makes sense But then PTH also increases Vit D activation, which has the following effects:

Vit D:

-- increased Calcium and phosphate reabsorption by kidney ---> inc serum calcium, and inc serum phosphate

works together with PTH to increase bone resorption (bone remodelling)

-- So, on one hand, PTH increases ionized calcium, and you'd think that's the end of the story. But on the other hand, PTH also stimulates Vitamin D synthesis...which has a direct effect of elevating both Calcium AND phosphate, so that bone can be remodeled.

So you could say that PTH promotes the release of Calcium. But you could also say, that PTH (via Vit D activation), also promotes the formation of bone, using both Calcium AND phosphate, whereas without the Vit D, it has a phosphaturic effect.

Can you see why this sounds contradictory to me? Is there a single source out there, which could make sense out of this madness? Or is there something simple that I'm not understanding? Is there a bigger picture here somewhere? Thanks, this has been bugging me for some time now.
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Old 07-20-2013
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Default confusion

PTH activates Vitd but vitd does not cause calcium and phosphate reabsorption from the kidney...

VIT d causes phosphate and Ca reabsoprtion from the small intestine.. more so Ca by activating Calbindin protein.. this calbindin takes Ca from intestine and into blood..

lets say you have hypo calcemia

so PTH increases..
activates Vit D
vitamin D causes Ca and Phosphate reaborption from small intestine
Pth activates osteoblasts which through cytokines activate osteoclasts that cause bone breakdown and release phosphate and calcium
Pth then causes increased reabsorption of ca in distal tuble and inhibits the phosphate pump in proximal tubule so that phosphate is not reabsorbed and excreted to prevent high phosphate concentration in blood.


hope this helped
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Old 07-20-2013
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Quote:
Originally Posted by takeonusmle View Post
PTH activates Vitd but vitd does not cause calcium and phosphate reabsorption from the kidney...

VIT d causes phosphate and Ca reabsoprtion from the small intestine.. more so Ca by activating Calbindin protein.. this calbindin takes Ca from intestine and into blood..

lets say you have hypo calcemia

so PTH increases..
activates Vit D
vitamin D causes Ca and Phosphate reaborption from small intestine
Pth activates osteoblasts which through cytokines activate osteoclasts that cause bone breakdown and release phosphate and calcium
Pth then causes increased reabsorption of ca in distal tuble and inhibits the phosphate pump in proximal tubule so that phosphate is not reabsorbed and excreted to prevent high phosphate concentration in blood.


hope this helped

I think i'm trying to figure out the connection between PTH, Vit D, and Calcium/Phosphate regulation, in terms of BOTH 1) Calcium homeostasis, and 2) Bone remodeling.

I still find it very confusing.

I understand that PTH is all about raising calcium levels when it's too low. And to do this, it must decrease phosphate reabsorption (so that calcium in the serum will not complex with phosphate for bone deposition).

That makes sense. But Why is Vitamin D also controlled by PTH? Because Vitamin D increases BOTH Calcium AND Phosphate, this means you'll get bone deposition...and PTH is not able to do what it needs to do (which is to increase calcium reabsorption).

Does that make sense?
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Old 07-20-2013
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If vit. D doesnt increase phosphate then it will go dangerously low as PTH is trashing phosphate continuosuly..
So it also maintains mineralization-demineralion balance... otherwise after some attacks of hypocalcemia you will end up with brittle bones
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