Calcium Homeostasis Flashcards
How is vitamin D formed by the skin?
Sunlight converts cholesterol to vitamin D3
Describe stage 1 of vitamin D metabolism (in the liver)
Is it active yet?
Vitamin D from the skin and diet is 25-hydroxylated in the liver (inactive)
Describe stage 2 of vitamin D metabolism (in the kidneys)
Is it active yet?
25(OH)-D is 1-alpha-hydroxylated in the kidneys (active)
Vitamin D metabolism in the kidneys is stimulated by…
- PTH release from the parathyroids
- Low phosphate levels
How does parathyroid hormone (PTH) increase serum Ca2+? (3)
- Increases osteoclast activity (degrade bone)
- Increases Ca2+ absorption from the gut & kidneys
- Increases vitamin D activation by the kidneys, vitamin D also increases Ca2+ absorption in the gut
Hypercalcaemia is most often caused by…
Hyperparathyroidism
Describe -ve feedback of PTH release from the parathyroids due to raised Ca2+
- High gut uptake of vitamin D leads to raised serum Ca2+
- Ca2+ reaches calcium-sensing receptor (C ASR) in the parathyroid
- C ASR is a GPCR which acts to inhibit release of PTH
Hypocalcaemia can be caused by…
Hypoparathyroidism
List 4 factors that affect vitamin D supply to the skin/diet
- Age
- Immigration
- Food availability
- Sunlight availability
Name a factor which affects vitamin D metabolism by the liver
Anticonvulsants
List 4 factors that affect vitamin D activation by the kidneys
- Renal failure
- Osteomalacia/ Ricketts
- Nephrectomy
- Hyper/hypo-phosphataemia
How is serum Ca2+ affected by magnesium concentration?
Ca2+ release from cells is dependent on Mg2+
In magnesium deficiency, intracellular Ca2+ is high and serum Ca2+ is low
How is vitamin D formed by the skin?
Sunlight converts cholesterol to vitamin D3
