Phosphorus homeostasis

Question 1

Where in the body do you find phosphorus?

Answer 1

• Teeth/ bone
• Intracellular organic molecules
• ECF (buffers pH)

Question 2

Why is it important to keep the Ca:P ratio low?

Answer 2

-Because high concentrations of either or both in solution causes insoluble precipitates to form

Question 3

Describe the absorption of phosphorus

Answer 3

• Intestinal absorption regulated by Calcitriol

- Renal resorption occurs in the PCT and some in the DCT

Question 4

Describe the excretion of phosphate

Answer 4

• PTH PROMOTES RENAL PHOSPHATE LOSS
• Some lost in saliva and recycling (cattle)
• FGF-23

Question 5

What secretes FGF-23?

Answer 5

Osteocytes

Question 6

What are the effects of a deficiency in dietary phosphorus?

Answer 6

Rickets

Question 7

What are the effects of excess dietary phosphorus?

Answer 7

Excess phosphorus leads to calcium deficiency

Question 8

Describe the causes of hyperphosphatemia

Answer 8

• Reduced GFR leads to reduced renal clearance
• Vit D toxicity= too much calcitriol hence excess intestinal absorption
• Hypoparathyroidism- not enough PTH
• Increased bone turnover e.g. hyperthyroidism/ HAC

Question 9

What does FGF23 do?

Answer 9

Decreases calcitriol

Causes secondary renal hyperparathyroidism

Question 10

Discuss Bran disease in horses

Answer 10

• Diet of low calcium grasses and high phosphorus grains leads to low dietary Ca:P ratio
• FGF-23 causes decreased calcitriol
• Ionised calcium levels decrease and PTH increases resulting in bone reabsorption
• Bone loss from the skull causes swelling resulting in the ‘big head’ symptom

Question 11

Which uroliths contain phosphate?

Answer 11

• Struvite= magnesium ammonium phosphate

- Apatite= calcium phosphate

Question 12

Discuss ruminant urolithiasis

Answer 12

• High grain diets
• Caused by hyperphosphataemia
• Causes uroliths=struvite or apatite
• Results in alkaline urine
• Reduced water intake makes it worse= more common in winter
• May/may not have an obstruction in sigmoid flexure or veriform appendage

Question 13

How does hypophosphataemia occur?

Answer 13

• Increased PTH results in high clearance levels
• Dietary deficiency of phosphorus
• Milk fever and eclampsia
• Lack of calcitriol
• High levels of insulin promote uptake of phosphorus into cells
• Fanconi syndrome- defect of the PCT

Question 14

Describe the clinical presentation of hypophosphataemia

Answer 14

• Muscle weakness, pain
• Haemolytic anaemia
• In dairy animals= poor milk yield, poor growth, low fertility

Question 15

What are the diagnostic tests for phosphorus disorders?

Answer 15

• Serum/ plasma phosphorus
• Urea, creatinine- to look at renal dysfunction
• Levels of FGF-23
• Fractional excretion of phosphorus