Phosphorus Homeostasis

Question 1

What is the difference between phosphorus and phosphate?

Answer 1

Phosphorus is the mineral element and phosphate is the compound PO4.

Question 2

When we measure blood are we measuring phosphorus or phosphate?

Answer 2

When we measure the serum and plasma we are measuring inorganic phosphorus

Question 3

What are the roles of phosphate?

Answer 3

Phosphate has a role in DNA/RNA and forms part of NADP. Phosphate is also included in esters such as glucose-6-phosphate and phospholipids and forms part of the compound hydroxyapatite.

Question 4

What does NADP stand for?

Answer 4

Nicotinamide-adenine dinucleotide phosphate

Question 5

Where is phosphate stored?

Answer 5

Phosphate is locked away in bones (85%) , some acts as all the molecules mentioned above and the rest is extracellular fluid (1%) where it acts as a buffe

Question 6

What are the acid and alkali forms that allow phosphate to act as a buffer?

Answer 6

HPO4(2-) weak alkali H2PO(4-) weak acid

Question 7

How is phosphate absorbed?

Answer 7

Phosphate is absorbed by the intestine promoted by 1,25 dihydroxyvitamin D (Calcitriol) and absorbed by the kidneys (around 80-90% by PCT and the rest is absorbed in the DCT)

Question 8

What hormone promotes phosphate loss?

Answer 8

PTH

Fibroblast growth factor 23 (FGF-23)

Question 9

How else is phosphate lost?

Answer 9

PTH promotes renal PO4 losses and phosphate is also lost (and recycled in cattle) by saliva.

Question 10

Fibroblast growth factor 23 (FGF-23) is a phosphatonin, what does this mean?

Answer 10

This means it is a hormone that acts to bring phosphate levels down, (a Hypophosphataemic peptide). It is secreted by the bone osteocytes to inhibit the alpha-1-hydroxylase in the kidney and to inhibit PTH to prevent any more phosphate absorption.

Question 11

What is the effect of PTH on phosphate?

Answer 11

PTH reduces the reabsorption of phosphate from the proximal tubule of the kidney, which means more phosphate is excreted through the urine. However, PTH enhances the uptake of phosphate from the intestine and bones into the blood.

Question 12

How can phosphate deficiency be caused?

Answer 12

Herbivores grazing phosphorus deficient pasture without grain
Bone mineralisation
Pica
All meat diets

Question 13

What is the ideal Ca:P ratio?

Answer 13

Close to or >1

Question 14

What can cause hyperphosphatemia?

Answer 14

Can be caused by reduced GFR – reduced clearance of phosphate
Calcitriol promotes intestinal absorption so a Vitamin D toxicity will lead to more absorption
Young and growing (growth hormone acts at renal tubules for more phosphate absorption for growth)

Question 15

What are the clinical symptoms of hyperphosphataemia?

Answer 15

Decreased calcitriol
Secondary renal hyperparathyroidism
Osetopenia, osteomalacia, rubber jaw
And if there is still enough phosphate when the calcium is drawn out then soft tissue mineralisation.

Question 16

What is secondary hyperparathyroidism?

Answer 16

Secondary hyperparathyroidism refers to the excessive secretion of parathyroid hormone (PTH) by the parathyroid glands in response to hypocalcemia (low blood calcium levels)

Question 17

How can high phosphate levels (from a reduced GFR in kidney disease for example) cause secondary hyperparathyroidism?

Answer 17

Reduced GFR leads to a reduced clearance of phosphate so FGF-23 is released in response.
This causes the complexed calcium to increase and the ionised calcium to decrease. This decrease in ionised Ca causes an increase in PTH which will lead to bone reabsorbtion.

Question 18

How does secondary hyperparathyroidism present in horses?

Answer 18

Loss of bone from the mandible and maxilla causes swelling and a big head

Question 19

What should be restricted in a renal diet?

Answer 19

Phosphate via oral antacids and lanthanum carbonate

Question 20

How do ruminants get urolithiasis?

Answer 20

Ruminants fed on high grain (high phosphorus) diets and have a reduced water intake produce two types of urolith:
Struvite (magnesium ammonium phosphate)
Apatite (calcium phosphate)

Question 21

What can cause hypophosphataemia?

Answer 21

Dietary deficiency 
Milk fever and eclampsia
Lack of calcitriol (as this stimulates intestinal uptake)
Insulin promotes uptake into cells 
Diuresis

Question 22

Downer cows usually have what two mineral deficiencies?

Answer 22

Hypocalcaemic milk fever cows are often also hypophosphataemic

Question 23

How do we take a blood sample to test for serum/plasma phosphorus levels?

Answer 23

Take from the coccygeal (tail vein) as the the sample from the neck will have phosphorus from the salivary gland blood flow
Haemolysis may cause a false increase