Changes

==Roles in the Body==

==Roles in the Body==

#'''[[Calcium#Calcium Homeostasis|Calcium Homeostasis]]<ref>Holick MF. Vitamin D. In Biochemical and physiological aspects of human nutrition. 2000 Philadelphia, PA: WB Saunders Company p.625-639.</ref><ref name="NRC">National Research Council (NRC). Vitamins. In Nutrient Requirements for Dogs and Cats. 2006 Washington, DC: National Academies Press p.194-200.</ref>:''' 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 [[Nephron Microscopic Anatomy#Proximal Tubule|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 [[Calcium#Parathyroid Hormone (PTH)|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 [[Calcium#Calcitriol (Activated Vitamin D3)|calcitriol]]. Low circulating PTH concentration results in low 1α-25 hydroxylase activity. There are '''two primary target tissues for calcitriol: the intestinal epithelium and [[Bones - Anatomy & Physiology|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.

#'''[[Calcium#Calcium Homeostasis|Calcium Homeostasis]]<ref>Holick MF. Vitamin D. In Biochemical and physiological aspects of human nutrition. 2000 Philadelphia, PA: WB Saunders Company p.625-639.</ref><ref name="NRC">National Research Council (NRC). Vitamins. In Nutrient Requirements for Dogs and Cats. 2006 Washington, DC: National Academies Press p.194-200.</ref>:''' 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 [[Nephron Microscopic Anatomy#Proximal Tubule|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 [[Calcium#Parathyroid Hormone (PTH)|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 [[Calcium#Calcitriol (Activated Vitamin D3)|calcitriol]]. Low circulating PTH concentration results in low 1α-25 hydroxylase activity. There are '''two primary target tissues for calcitriol: the intestinal epithelium and [[Bones - Anatomy & Physiology|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|thumb|© Diffomédia/Masure]]

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