Title: Phosphorus accordion

Distribution

plasma distribution: (1.2 - 2 mmol/l)

- 55% ionic
- 10% protein bound
- 35% complexed

cellular distribution (100 mmol/l):

- free PO4: 100-200 nmol/l
- majority bound to
  - ribose, desoxyribose
  - proteins, phospholipids
  - Glucose-6-P
  - ATP, ADP, creatine-P, cAMP, IP3,…
- CaPO4 salts in mitochondria

Sources

Examples for uptake of 800 mg (daily need)

- 1 L milk
- 90 g Emmentaler cheese
- 500 g meat
- 400 g fish
- 600 g bread
- 200 peanuts

Intestinal Absorption

- small intestine (jejunum)
- paracellular: if luminal PO4- > 1mmol/l
- transcellular (active):
  - Na/PO4- cotransport (bbm)
  - carrier (blm)
  - stimulated by Vit D
- 60-80% of dietary intake (normal intake)
- proportional to dietary intake
- reduced in malabsorption syndromes

Renal Excretion

- ultrafiltration: 85% of serum PO4
- reabsorption (87.5 % of filtrated PO4):
  - 85% prox. tubulus (Na/PO4- cotransport)
  - 15% distal convoluted tubulus
- regulation:
  - dietary PO4- (↑→reabsorption ↓, ↓→ reabsorption↑ )
  - serum Ca (hypercalcaemia → reabsorption ↓, hypocalcaemia → reabsorption↑ )
  - ECF expansion → reabsorption ↓
  - PTH → reabsorption ↓ (Na/PO4- cotransport)
  - FGF-23 → reabsorption ↓ (Na/PO4- cotransport)

Homeostasis

- intestinal:
  - PO4 absorption proportional to dietary PO4 content
  - Vit D3 can stimulate active transport
- renal:
reabsorption is regulated by
  - dietary PO4
  - serum Ca
  - ECF volume
  - PTH
  - FGF-23

Hypophosphataemia

causes:

- malnutrition/malabsorption
- refeeding syndrome
- alcohol abuse (diuresis, malnutrition, respiratory alkalosis)
- hyperparathyroidism → hyperphosphaturia
- hungry bone syndrome

symptoms:

- muscle weakness
- neuronal disturbances: confusion, delirium, coma
- cell membrane instability → hemolysis, rhabdomyolysis

Hyperphosphataemia

causes:

- high intake and decreased renal excretion
- excessive bone resorption
- hyperparathyroidism and renal osteodystrophy

symptoms:

- hypocalcaemia
- tetany
- soft tissue calcification