Phosphate Flashcards
What is the weight and % of whole body phosphate
500-800g
1% total body weight
90% in bone mineral
What is the weight and % of serum phosphate
0.8-1.5 mmol/L
50% free ions
35% complexed with Na, Ca, Mg
10% protein-bound
What is the result of high phosphate
Excessive formation of hydroxyapatite
Deposition in tissues other than bone
What is the result of low phosphate
Poor bone mineralisation
- rickets or osteomalacia
- pain, fractures
What are the dietary source of phosphate
Protein
- animal
- dairy
- soy
- seeds and nuts
What is the daily recommended intake for phosphate
700mg
What regulates phosphate metabolism
Parathyroid hormone
1,25 dihydroxyvitamin D
FGF-23
What is the role of parathyroid hormone
Main function is regulation of calcium
Also affects phosphate
Increases 1,25 vitamin D - increases active gut absorption
Decreases tubular reabsorption of phosphate - increases renal excretion
What is the role of FGF-23 (fibroblast growth factor 23)
Now known to be a major regulator of phosphate metabolism
Produced by osteocytes
In response to
- rise in phosphate levels
- dietary phosphate loading
- PTH
- 1,25 vitamin D
What would genetic investigation be for rickets
Linkage analysis
- fixed genetic markers
- identify on which chromosome and region the abnormal gene is likely to be
Sequencing
- identify where DNA differs between affected and unaffected people
What are the actions of FGF-23
Decreases expression of Na transporter in renal tubule - increases renal excretion of phosphate
Decreases 1a-hydroxylation of vitamin D - decreases gut absorption of phosphate
Decreases whole body phosphate
How is phosphate secreted
Unbound phosphate (about 90%) is filtered
80% reabsorbed in proximal tubule by Na co-transporter
10% reabsorbed in distal tubule
Maximum rate of reabsorption is limited - excess is secreted
Describe inherited rickets
Presents in childhood or adulthood
Bone pain, deformity, fracture
Low bone density
Low serum phosphate
High urine phosphate
X-linked hypophosphatemic rickets
Autosomal dominant rickets
Describe tumour-induced osteomalacia
Rare form of osteomalacia with low phosphate
Seen in patients with small benign mesodermal tumours
Osteomalacia heals when tumour removed
- low serum phosphate
- high urine phosphate
What are the actions of FDF-23
Decreases expression of Na transporter in the renal tubule - increased renal excretion phosphate
Decrease 1a-hydroxylation of vitamin D - decreases gut absorption of phosphate
Decreases whole body phosphate
