goodies
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in FA on page 265, it says one of Vit D's functions is to increase bone resorption of calcium and phosphate. Is that true!?!?!? I thought Vitamin d just stimulated reabsorption of calcium and phosphate, not RESORPTION... which is what PTH does! Is FA wrong??
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| new_n_lost
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RESORPTION= The act or process of resorbing = To absorb again.
The major function of vitamin D is to increase the efficiency of calcium absorption from the small intestine. Heaney and colleagues demonstrated that maximum calcium absorption occurs at levels of 25-hydroxyvitamin D (25[OH]D) greater than 32 ng/mL. Vitamin D also enhances the absorption of phosphorus from the distal small bowel. Adequate calcium and phosphorus absorption from the intestine is important for proper mineralization of the bone. The second major function of vitamin D is for the maturation of osteoclasts to resorb calcium from the bones.
In bone, vitamin D may play a synergistic role with PTH in stimulating osteoclast proliferation and bone resorption.
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| new_n_lost
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The action of PTH is conversion of 25-hydroxyvitamin D to its most active metabolite, 1,25-dihydroxyvitamin D-3 [1,25-(OH)2 D3], by activation of enzyme 1-hydroxylase in the proximal tubules of the kidney.
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| Luckyall
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---->at normal levels, vit D promotes bone deposition by 1) increase reabsorb of Ca & Ph by intestin mucosa, the increased levels of Ca& Ph in ECF exceed the solubility product---> salt deposition in the bone matrix. 2) incr reabsorb Ca from Distal tubule
-----> at high vit D levels, vit D increases bone resorbtion and promotes release of Ca &Ph from the bone.Rec for vit D on the osteblast nuclues---> activates the osteoclasts--> bone rsorbing action.
Vit D resorbing action req. PTH presence !!!!!!!!!!!!!!!!!!!!
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| goodies
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I thought that vitamin D inhibited PTH release? so they have antagonistic and synergistic roles?
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| goodies
Forum Junior
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can someone please direct me to a resource that explains the receptor functions and interactions between osteoblasts and osteoclasts?
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| new_n_lost
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http://www.emedicine.com/ent/topic539.htm
PTH is a peptide containing 84 amino acids that is secreted by the parathyroid glands after cleavage from preproparathyroid hormone (115 amino acids) to proparathyroid hormone (90 amino acids) to the mature hormone. PTH has 3 major actions. The first action is to increase renal calcium resorption and phosphate excretion. In the kidney, PTH blocks reabsorption of phosphate in the proximal tubule while promoting calcium reabsorption in the ascending loop of Henle, distal tubule, and collecting tubule. Calcium also may exert a direct effect on renal resorption.
PTH promotes absorption of calcium from the bone in 2 ways. The rapid phase brings about a rise in serum calcium within minutes and appears to occur at the level of the osteoblasts and osteocytes. Although it may seem counterintuitive that the cells that promote deposition of bone are involved in resorption, these cells form an interconnected network known as the osteocytic membrane overlying the bone matrix, but with a small layer of interposed fluid termed bone fluid. When PTH binds to receptors on these cells, the osteocytic membrane pumps calcium ions from the bone fluid into the extracellular fluid.
The slow phase of bone resorption occurs over several days and has 2 components. First, osteoclasts are activated to digest formed bone, and second, proliferation of osteoclasts occurs. Interestingly, mature osteoclasts lack PTH membrane receptors; activation and proliferation appear to be stimulated by cytokines released by activated osteoblasts and osteocytes or by differentiation of immature osteoclast precursors that possess PTH and vitamin D receptors.
The third major action of PTH is conversion of 25-hydroxyvitamin D to its most active metabolite, 1,25-dihydroxyvitamin D-3 [1,25-(OH)2 D3], by activation of enzyme 1-hydroxylase in the proximal tubules of the kidney.
Negative inhibition of PTH release occurs primarily by direct effect of calcium at the level of the parathyroid gland. Although not well elucidated, 1,25-(OH)2 D3 appears to exert a mild inhibitory effect on the parathyroid gland as well.
Vitamin D-3 (cholecalciferol) is formed in the skin when a cholesterol precursor, 7-dehydroxycholesterol, is exposed to ultraviolet light. Activation occurs when the substance undergoes 25-hydroxylation in the liver and 1-hydroxylation in the kidney.
The primary action of 1,25-(OH)2 D3 is to promote gut absorption of calcium by stimulating formation of calcium-binding protein within the intestinal epithelial cells. Vitamin D also promotes intestinal absorption of phosphate ion, although the exact mechanism is unclear. Negatively charged phosphate ion may passively flow through the intestinal cell because of flux of the positively charged calcium ion. In bone, vitamin D may play a synergistic role with PTH in stimulating osteoclast proliferation and bone resorption.
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| Luckyall
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how come vit D would inhibit pth release ?
vit D is making sure Ca ECF levels are normal by resorbing Ca from the gut---> thas 1 statment
pth is released at low Ca levels, agree, that doesnt mean vit D inhibits pth!
in fact pth increase vit D formation by increasing the activity of 1 alpha hydroxylase enzyme.
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| new_n_lost
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Goodies have a look at this if still u have doubts then do let us know
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FORUM RULES-- Those who believe in telekinesis, raise my hand. I get enough exercise just by pushing my luck --P4U World.." The pure and simple truth is rarely pure and never simple."
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| Luckyall
Forum Guru
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as a very complete source i found kaplan more then enough
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| nandrolone
Forum Junior
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vit d in high as well low doses causes bone resorption
High doses=> more direct activation of osteoclasts thus resorption
low doses => decreased reabsorption of Ca and phosphates from intestines and kidney ( phosphate in PCT and Ca in DCT) => decreased solubility product hence bone resorption
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| nandrolone
Forum Junior
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also just to complete the picture
receptors
PTH => osteoblasts
vit d => osteoclasts
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