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====Actions====
====Actions====
There are four biological actions of Calcitriol (active Vitamin D3):
There are four biological actions of Calcitriol (active Vitamin D3):
#'''Increase calcium absorption from the intestine''' - via active transport mechanisms. It increases the synthesis of '''Calbindin''' (Calcium binding protein) which transports calcium from the lumen to the Vitamin D activated Calcium ATPase pumps on the basolateral membrane of the enterocytes (secondary active transport). This process takes approximatley 48 hours.
#'''Increase calcium absorption from the intestine''' via active transport mechanisms. Calcitriol increases the synthesis of '''Calbindin''' (Calcium binding protein) which transports calcium from the intestinal lumen to the Vitamin D activated Calcium ATPase pumps on the basolateral membrane of the enterocytes (via secondary active transport). This process takes approximatley 48 hours.
#'''Increase phosphorous absorption from the intestine''' - Phosphorous is found in grains, and is absorbed in the small intestine via active transport which responds to calcitriol.
#'''Increase phosphorous absorption from the intestine''' - Phosphorous is found in grains, and is absorbed in the small intestine via active transport mechanisms which are respondsive to calcitriol.
#'''Decrease Calcium and Phosphorous excretion via the kidney''' - Calcitriol acts on the renal tubular epithelial cells to increase calcium and phosphorous reabsorption from the nephron. This action is WEAK compared to the PTH action (which acts to reabsorb calcium but lose phosphorous from the nephron).
#'''Decrease Calcium and Phosphorous excretion via the kidney''' - Calcitriol acts on the renal tubular epithelial cells to increase calcium and phosphorous reabsorption from the nephron. This action is WEAK compared to the action of PTH, which acts to reabsorb calcium but lose phosphorous from the nephron.
#'''Normal bone functioning (osteoclast and osteoblast functions)''' - Calcitriol is needed for normal bone absorption and deposition. Without Vitamin D3, bone is not resorbed in response to PTH.
#'''Normal bone functioning (osteoclast and osteoblast functions)''' - Calcitriol is needed for normal bone absorption and deposition. Without Vitamin D3, bone is not resorbed in response to PTH.