12: Calcium & Phosphate Regulation Flashcards
What is FGF23
Fibroblast Growth factor 23
From osteocytes
Inhibits the sodium-phosphate transporter in PCT
Also inhibits calcitriol -> less phosphate reabsorption from gut
What is the role of PTH in phosphate regulation?
PTH inhibits sodium-phosphate transporters in the PCT
High PTH = LOW serum phosphate
Hence in primary hyperparathyroidism, you get LOW phosphate levels
What does PTH do?
Stimulates renal 1a-hydroxylase -> INCREASED CALCITRIOL
Also goes to bones and stimulates calcium/phosphate release
How do we normally make Vitamin D3?
7-dehydrocholesterol in skin is PRECURSOR
This gets converted to D3 via UVB light
What happens to Vitamin D3?
Goes to liver, converted to calcidiol (25 OH-D3)
Then goes to kidney, converted to calcitriol by renal 1a-hydroxylase
What is another source of Vitamin D?
Diet - Vitamin D2 (ergocalciferol)
What does calcitriol do?
- Ca absorption in gut
- Ca maintenance in bone
- Increased renal Ca absorption
- Negative feedback on PTH
State 5 causes of Vitamin D deficiency
- Dietary insufficiency
- Lack of sunlight (UVB)
- GI malabsorption (coeliac’s, IBD)
- Renal/Liver failure
- Vitamin D Receptor defects (autosomal recessive, rare)
How does extracellular Ca affect membrane excitability?
Hypercalcaemia -> HIGH ec Ca -> blocks Na influx -> LESS membrane excitability
Hypocalcaemia -> LOW ec Ca -> more Na influx -> MORE membrane excitability
Signs and symptoms of hypocalcaemia?
CATs go numb
Convulsions
Arrhythmias
Tetany
Parasthesia (hands, mouth, feet, lips)
Muscle cramps, tingling
Give 2 clinical tests for hypocalcaemia
CHVOSTEKS sign = tap facial nerve below zygomatic arch
Should cause twitching (neuromuscular irritability)
TROUSSEAU’S sign = Inflate BP cuff for few mins. Should cause CARPOPEDAL SPASM
