Name some functions of phosphate
- In DNA
- In cAMP
- In ATP
- In cell membrane
- In protein modification - kinases and phosphatases,
- Bone mineral - calcium hydroxyapatite
How is phosphate found in the body?
- In bone - calcium hydroxyapatite
- In serum, 50% as free ions, 35% complexed to Na, Ca and Mg
- 10% in protein bound
What occurs when there is too much/too little phosphate?
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
Name some dietary sources of phosphate
In protein - animal, dairy, soy, seeds and nuts
How is phosphate absorbed in the gut?
- In small intestine
- Passive at high concs
- Active transport (Na) - when low Ca concs
How is phosphate reabsorbed in the kidney?
- 80% is reabsorbed in proximal tubule (active - Na co transporter)
- 10% reabsorbed in distal tubule
- excess is excreted
What regulates phosphate metabolism? - what is the effect
- Parathyroid hormone
- increases active vit D
- decreases active gut phosphate absorption
- increases renal excretion due to reduced reabsorption - 1,25-dihydroxyvitamin D/ active vit D
- decreases active gut phosphate absorption
- increases renal excretion due to reduced reabsorption - 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
What occurs in rickets/osteomalacia?
- 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
What occurs due to low Vit D?
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