Phosphorus homeostasis

Question 1

where is phosphorus found?

Answer 1

Bone - 85%
Intracellular organic molecules - 14%
Extracellular fluid - 1%

Question 2

Describe the relationship between Ca and phosphorus

Answer 2

Law of mass action:
- Ca and PO4 levels kept at levels suitable for bone mineralisation but not soft tissue mineralisation
Food high in phosphorous is usually low in Ca
Connected control mechanisms - Calcitriol, PTH

Question 3

Describe the absorption of phosphate

Answer 3

Intestinal phosphate absorption promoted by calcitriol
Renal resorption mainly in PCT

Question 4

Describe the protective losses caused by PTH

Answer 4

PTH => Ca and phosphorous increase from bone
PTH => increased phosphorous excretion from renal tubules
This prevents phosphorous from taking iCa and making it unavailable

Question 4

Describe the control of phosphate excretion

Answer 4

PTH promotes renal PO4 losses
FGF-23:
- secreted by bone in response to PO4
- anti PTH - inhibits PTH stimulating calcitriol formation => prevents phosphorous being resorbed though gut
- promotes phosphate loss through urine

Question 5

What factors control PO4

Answer 5

Dietary intake and absorption
Calcitriol
PTH
renal tubular resorption
Phosphatonins/FGF-23

Question 6

What causes hyperphosphataemia?

Answer 6

Reduced GFR => reduced clearance
Vit D toxicity => increased intestinal absorption
Hypoparathyroidism - no PTH so no encouragement of renal PO4 loss
Young and growing
Increased bone turnover

Question 7

Describe the action of acute hyperphosphataemia

Answer 7

=> hypocalcaemia => tetany

Question 8

Describe the effects of secondary renal hyperparathyroidism

Answer 8

Renal disease:
- reduced GFR
- reduced clearance of PO4
- increased serum PO4 => FGF-23
- Decreased iCa in blood
- Increased PTH => bone resorption
- polyuria due to Ca losses
- tubular damage and FGF-23 => decreased calcitriol
- poor appetite and decreased calcitriol => poor Ca uptake

Question 9

What is the outcome of secondary hyperparathyroidism?

Answer 9

Bone demineralisation
Rubber jaw - soft mandible

Question 10

Give an example of the effects of secondary hyperparathyroidism in horses

Answer 10

Bran-disease/big head
Increased bone resorption => bone loss from skull => swelling

Question 11

How is PO4 restricted in renal failure management?

Answer 11

PO4 restricted diets
PO4 binders - prevent resorption:
- oral antacids (CaCO3)

Question 12

Describe the effect of hyperphosphataemia in ruminants

Answer 12

Urolithiasis:
- high grain diets, dietary phosphorous => phosphate containing uroliths
- alkaline urine
- reduced water intake
- may get obstruction (bladder stones)
- surgical therapy needed

Question 13

what causes hypophosphataemia?

Answer 13

Increased PTH promotes clearance
Dietary deficiency
Milk fever
Lack of calcitriol - not enough Vit D from food and sunlight
Insulin promotes phosphorous uptake into cells
Diuresis
Fanconi syndrome

Question 14

Describe the clinical signs of hypophosphataemia

Answer 14

Large skeletal sores
Long term => osteomalacia, deformity, pain
Muscle:
- weakness, pain
- glycolysis
RBCs:
- haemolytic anaemia
- Increased oxygen binding => hypoxia
Dairy:
- poor growth
- poor milk yields
- low fertility

Question 15

Describe diagnostic tests of phosphorous disorders

Answer 15

Serum/plasma phosphorous (beware false increase in haemolysis due to PO4 in RBCs)
Urea, creatinine - evidence of renal dysfunction
Total Ca, iCa, albumin
Fractional excretion of phosphorous
FGF-23 - early marker of renal dysfunction