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Flashcards in Phosphate homeostasis Deck (9)
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1
Q

Name some functions of phosphate

A
  • In DNA
  • In cAMP
  • In ATP
  • In cell membrane
  • In protein modification - kinases and phosphatases,
  • Bone mineral - calcium hydroxyapatite
2
Q

How is phosphate found in the body?

A
  • In bone - calcium hydroxyapatite
  • In serum, 50% as free ions, 35% complexed to Na, Ca and Mg
  • 10% in protein bound
3
Q

What occurs when there is too much/too little phosphate?

A

High phosphate = excessive formation of calcium hydroxyapatite = deposition in tissues other than bone = calcification

Low phosphate = poor bone mineralisation = rickets or osteomalacia, pain and fractures

4
Q

Name some dietary sources of phosphate

A

In protein - animal, dairy, soy, seeds and nuts

5
Q

How is phosphate absorbed in the gut?

A
  • In small intestine
  • Passive at high concs
  • Active transport (Na) - when low Ca concs
6
Q

How is phosphate reabsorbed in the kidney?

A
  • 80% is reabsorbed in proximal tubule (active - Na co transporter)
  • 10% reabsorbed in distal tubule
  • excess is excreted
7
Q

What regulates phosphate metabolism? - what is the effect

A
  1. Parathyroid hormone
    - increases active vit D
    - decreases active gut phosphate absorption
    - increases renal excretion due to reduced reabsorption
  2. 1,25-dihydroxyvitamin D/ active vit D
    - decreases active gut phosphate absorption
    - increases renal excretion due to reduced reabsorption
  3. Fibroblast growth factor 23 - produced by osteocytes
    - main phosphate regulator
    - decreases the reabsorption and increases excretion of phosphate
    - may also suppress 1-alpha-hydroxylase, reducing its ability to activate vitamin D and subsequently impairing calcium absorption
8
Q

What occurs in rickets/osteomalacia?

A
  • Low bone density
  • Low serum phosphate
  • high urine phosphate

=bone pain, deformity, fracture

-rare, but can be tumour induced - high levels of FGF, this means high phosphate levels are excreted (due to low reabsorption) so serum levels are low = low bone density = high fracture risk

9
Q

What occurs due to low Vit D?

A

Low Vit D = Low Ca reabsorption
=low phosphate as active vit D is needed for gut absorption of phosphate and calcium
=low FBF-23 due to low phosphate levels