Changes

In the liver, dietary vitamin D is hydroxylated to 25-hydroxyvitamin D by the 25-hydroxylase enzyme. This is the first step in vitamin D activation and 25-hydroxyvitamin D is then bound to vitamin D binding protein and released into circulation. Protein-bound 25-hydroxyvitamin D is transported to the kidney for the second and final step in activation. Renal 1α-25 hydroxylase is located on the proximal tubule of the nephron and converts 25-hydroxyvitamin D to 1,25-hydroxyvitamin D (i.e., calcitriol). This renal enzyme is under control of PTH. During periods of low circulating ionized calcium concentrations the parathyroid gland releases PTH, which in turn stimulates 1α-25 hydroxylase activity to produce more calcitriol. Low circulating PTH concentration result in low 1α-25 hydroxylase activity. There are '''two primary target tissues for calcitriol: the intestinal epithelium and bone'''. In the enterocyte calcitriol stimulates production of a number of proteins, including the calcium binding protein calbindin, which facilitates uptake of dietary calcium. Receptors to calcitriol are also found on osteoblasts within bone. Binding of calcitriol to osteoblastic receptors stimulates production of cytokines that regulate mineral deposition and osteoclastic activity.

In the liver, dietary vitamin D is hydroxylated to 25-hydroxyvitamin D by the 25-hydroxylase enzyme. This is the first step in vitamin D activation and 25-hydroxyvitamin D is then bound to vitamin D binding protein and released into circulation. Protein-bound 25-hydroxyvitamin D is transported to the kidney for the second and final step in activation. Renal 1α-25 hydroxylase is located on the proximal tubule of the nephron and converts 25-hydroxyvitamin D to 1,25-hydroxyvitamin D (i.e., calcitriol). This renal enzyme is under control of PTH. During periods of low circulating ionized calcium concentrations the parathyroid gland releases PTH, which in turn stimulates 1α-25 hydroxylase activity to produce more calcitriol. Low circulating PTH concentration result in low 1α-25 hydroxylase activity. There are '''two primary target tissues for calcitriol: the intestinal epithelium and bone'''. In the enterocyte calcitriol stimulates production of a number of proteins, including the calcium binding protein calbindin, which facilitates uptake of dietary calcium. Receptors to calcitriol are also found on osteoblasts within bone. Binding of calcitriol to osteoblastic receptors stimulates production of cytokines that regulate mineral deposition and osteoclastic activity.

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[[File:Canine vitamineD.jpg|500px|center]]

[[File:Canine vitamineD.jpg|500px|center|thumb|© Diffomédia/Masure]]

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