Vitamin D Synthesis Process
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Questions and Answers

What represents the main form of vitamin D in the body?

  • 25-OH D (correct)
  • Calcitriol
  • Vitamin D stored in muscle
  • DBP-bound vitamin D

    • Where is vitamin D stored when there is supraphysiological intake?

  • Muscle tissue
  • Liver
  • Kidneys
  • Adipose tissue (correct)

    • Which hormone regulates the renal synthesis of calcitriol?

  • Fibroblast-like growth factor (FGF) 23
  • Insulin
  • Parathyroid hormone (PTH) (correct)
  • Glucagon

    • In response to increased PTH concentrations, where is 25-OH D taken up?

    <p>Kidneys</p> Signup and view all the answers

    Which tissue contains primarily nonhydroxylated vitamin D?

    <p>Adipose tissue</p> Signup and view all the answers

    What is the half-life of 25-OH D in the body?

    <p>15 days to 3 weeks or more</p> Signup and view all the answers

    What is the precursor to vitamin D3 formed in the skin upon exposure to UVB photons?

    <p>Previtamin D3</p> Signup and view all the answers

    Which inactive metabolite of vitamin D3 is generated from 7-dehydrocholesterol in the presence of ultraviolet light?

    <p>Lumisterol</p> Signup and view all the answers

    How is dietary vitamin D (D3 and D2) absorbed within the body?

    <p>Passive diffusion into the intestinal cell</p> Signup and view all the answers

    Where is vitamin D primarily incorporated after absorption into the body?

    <p>Chylomicrons in the lymphatic system</p> Signup and view all the answers

    Which protein serves as the main blood transport protein for vitamin D?

    <p>Vitamin D–binding protein (DBP)</p> Signup and view all the answers

    What percentage of dietary vitamin D is typically absorbed within the distal small intestine?

    <p>50%</p> Signup and view all the answers

    Where are the receptors for calcitriol found, besides the intestine, bone, and kidneys?

    <p>All of the above</p> Signup and view all the answers

    Which protein is associated with the non-genomic actions of vitamin D?

    <p>MARRS protein</p> Signup and view all the answers

    How does calcitriol primarily affect body processes?

    <p>By interacting with VDRE on genes</p> Signup and view all the answers

    What is one of the actions initiated by intracellular signaling pathways activated by calcitriol?

    <p>Increased intracellular calcium concentration</p> Signup and view all the answers

    How does calcitriol regulate gene expression?

    <p>By binding to nuclear vitamin receptors in the cytosol</p> Signup and view all the answers

    What stimulates the secretion of PTH from the parathyroid gland?

    <p>Low serum calcium concentrations</p> Signup and view all the answers

    What is the similarity between calcitriol's genomic mechanism of action and that of retinoic acid?

    <p>Both bind to nuclear vitamin receptors in the cytosol</p> Signup and view all the answers

    What is the normal range for serum calcium concentrations?

    <p>8.5 to 10.5 mg/dL</p> Signup and view all the answers

    Which tissues do NOT have receptors for calcitriol based on the text?

    <p>Muscle and bone</p> Signup and view all the answers

    Which enzyme is stimulated by PTH in the kidneys to convert 25-OH D to calcitriol?

    <p>1-hydroxylase</p> Signup and view all the answers

    How does vitamin D influence PTH?

    <p>By interacting with VDRE in the PTH gene promoter region</p> Signup and view all the answers

    What is the role of calcitriol released into the blood bound to DBP?

    <p>Raises serum calcium concentrations</p> Signup and view all the answers

    How does calcitriol affect phosphate absorption in the intestine?

    <p>Increases the number of carriers for phosphate absorption</p> Signup and view all the answers

    What is the effect of calcitriol on serum phosphorus concentrations?

    <p>Increases serum phosphorus concentrations</p> Signup and view all the answers

    How does calcitriol influence muscle function?

    <p>Enhances calcium uptake in muscle cells</p> Signup and view all the answers

    What role does calcitriol play in non-calcium regulating tissues regarding cell growth?

    <p>Stimulates cell growth and proliferation</p> Signup and view all the answers

    Which condition is commonly associated with vitamin D deficiency in relation to muscle health?

    <p>Muscle weakness and pain</p> Signup and view all the answers

    What effect does calcitriol have on phosphorus resorption in the kidneys?

    <p>Enhances phosphorus resorption in the distal tubules</p> Signup and view all the answers

    Study Notes

    Vitamin D Synthesis

    • Ultraviolet B (UVB) photons from sunlight penetrate the epidermis and dermis, triggering the conversion of 7-dehydrocholesterol in skin cells to previtamin D3.
    • Previtamin D3 undergoes thermal isomerization to form vitamin D3/cholecalciferol over a period of several hours to a few days.
    • The skin prevents excess vitamin D3 production by generating inactive metabolites like lumisterol and tachysterol, which are lost as the skin sloughs off.

    Absorption

    • Dietary vitamin D (D3 and D2) is absorbed from the distal small intestine, requiring no digestion and aided by bile salts.
    • Vitamin D is incorporated into chylomicrons, which enter the lymphatic system and eventually the blood, with about 50% of dietary vitamin D being absorbed.

    Transport, Metabolism, and Storage

    • Chylomicrons contain 40% of vitamin D in the blood, with some being transferred to vitamin D-binding protein (DBP).
    • Vitamin D is stored in the body's adipose tissue, muscle, and blood, with the blood being the largest single pool and major storage site.
    • The half-life of 25-OH D in the blood is around 15 days to 3 weeks or more.

    Functions and Mechanisms of Action

    • Calcitriol exerts its functions through both genomic and non-genomic mechanisms.
    • Non-genomic actions of calcitriol include activating signal transduction pathways, increasing calcium uptake, and regulating intracellular calcium concentration.
    • Genomic mechanisms of calcitriol involve binding to nuclear vitamin receptors, regulating gene expression, and influencing cell growth, differentiation, and proliferation.

    Cell Differentiation, Proliferation, and Growth

    • Calcitriol helps maintain normal cell growth, differentiation, and proliferation in various tissues, including skin, bone, and kidneys.
    • Vitamin D's ability to stimulate skin epidermal cell differentiation while inhibiting proliferation has been applied in the treatment of psoriasis.

    Calcitriol and Muscle

    • Vitamin D deficiency can lead to muscle dysfunction, including muscle weakness, pain, and difficulty walking.
    • Calcitriol functions in muscle to enhance calcium uptake and intracellular calcium concentrations, which are important for muscle contraction and relaxation.

    Serum Calcium Homeostasis

    • Calcitriol plays a crucial role in regulating serum calcium concentrations, which are normally maintained between 8.5 to 10.5 mg/dL.
    • Calcitriol synthesis is stimulated in response to changes in serum calcium concentrations and the release of PTH.
    • PTH stimulates the conversion of 25-OH D to calcitriol, which acts on its target tissues (intestine, bone, and kidneys) to increase serum calcium concentrations.

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    Description

    Learn about the synthesis of vitamin D in the skin upon exposure to ultraviolet B (UVB) radiation. Understand how previtamin D3 is formed from 7-dehydrocholesterol and eventually converted to cholecalciferol (vitamin D3) through thermal isomerization.

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