30 Questions
What represents the main form of vitamin D in the body?
25-OH D
Where is vitamin D stored when there is supraphysiological intake?
Adipose tissue
Which hormone regulates the renal synthesis of calcitriol?
Parathyroid hormone (PTH)
In response to increased PTH concentrations, where is 25-OH D taken up?
Kidneys
Which tissue contains primarily nonhydroxylated vitamin D?
Adipose tissue
What is the half-life of 25-OH D in the body?
15 days to 3 weeks or more
What is the precursor to vitamin D3 formed in the skin upon exposure to UVB photons?
Previtamin D3
Which inactive metabolite of vitamin D3 is generated from 7-dehydrocholesterol in the presence of ultraviolet light?
Lumisterol
How is dietary vitamin D (D3 and D2) absorbed within the body?
Passive diffusion into the intestinal cell
Where is vitamin D primarily incorporated after absorption into the body?
Chylomicrons in the lymphatic system
Which protein serves as the main blood transport protein for vitamin D?
Vitamin D–binding protein (DBP)
What percentage of dietary vitamin D is typically absorbed within the distal small intestine?
50%
Where are the receptors for calcitriol found, besides the intestine, bone, and kidneys?
All of the above
Which protein is associated with the non-genomic actions of vitamin D?
MARRS protein
How does calcitriol primarily affect body processes?
By interacting with VDRE on genes
What is one of the actions initiated by intracellular signaling pathways activated by calcitriol?
Increased intracellular calcium concentration
How does calcitriol regulate gene expression?
By binding to nuclear vitamin receptors in the cytosol
What stimulates the secretion of PTH from the parathyroid gland?
Low serum calcium concentrations
What is the similarity between calcitriol's genomic mechanism of action and that of retinoic acid?
Both bind to nuclear vitamin receptors in the cytosol
What is the normal range for serum calcium concentrations?
8.5 to 10.5 mg/dL
Which tissues do NOT have receptors for calcitriol based on the text?
Muscle and bone
Which enzyme is stimulated by PTH in the kidneys to convert 25-OH D to calcitriol?
1-hydroxylase
How does vitamin D influence PTH?
By interacting with VDRE in the PTH gene promoter region
What is the role of calcitriol released into the blood bound to DBP?
Raises serum calcium concentrations
How does calcitriol affect phosphate absorption in the intestine?
Increases the number of carriers for phosphate absorption
What is the effect of calcitriol on serum phosphorus concentrations?
Increases serum phosphorus concentrations
How does calcitriol influence muscle function?
Enhances calcium uptake in muscle cells
What role does calcitriol play in non-calcium regulating tissues regarding cell growth?
Stimulates cell growth and proliferation
Which condition is commonly associated with vitamin D deficiency in relation to muscle health?
Muscle weakness and pain
What effect does calcitriol have on phosphorus resorption in the kidneys?
Enhances phosphorus resorption in the distal tubules
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.
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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