Phosphate

Phosphate is abundant in the body and is an important intracellular and extracellular anion. Much of the phosphate inside cells is covalently attached to lipids and proteins. Phosphorylation and dephosphorylation of enzymes are important mechanisms in the regulation of metabolic activity. Most of the body’s phosphate is in bone (Fig 37.1). Phosphate changes accompany calcium deposition or resorption of bone. Control of ECF phosphate concentration is achieved by the kidney, where tubular reabsorption is reduced by PTH. The phosphate that is not reabsorbed in the renal tubule acts as an important urinary buffer.

Plasma inorganic phosphate

At physiological hydrogen ion concentrations, phosphate exists in the ECF both as monohydrogen phosphate and as dihydrogen phosphate. Both forms are together termed ‘phosphate’, and the total is normally maintained within the limits 0.80–1.40 mmol/L. Sometimes the term ‘inorganic phosphate’ is used to distinguish these forms from organically bound phosphate such as in ATP. Approximately 20% of plasma phosphate is attached to protein, although in contrast to the binding of calcium, this is of little significance.

In plasma, calcium and phosphate often have a reciprocal relationship. In particular, if phosphate rises, calcium falls.

Hyperphosphataemia

Persistent hyperphosphataemia may result in calcium phosphate deposition in soft tissues. Causes of a high serum phosphate concentration include:

Hypophosphataemia

Severe hypophosphataemia (<0.3 mmol/L) is rare and causes muscle weakness, which may lead to respiratory impairment. The symptomatic disorder requires immediate intravenous infusion of phosphate. Modest hypophosphataemia is much more common. Alcoholic patients are especially prone to hypophosphataemia.

Causes of a low serum phosphate include:

 Hyperparathyroidism. The effect of a high PTH is to increase phosphate excretion by the kidneys and this contributes to a low serum concentration.

 Treatment of diabetic ketoacidosis. The effect of insulin in causing the shift of glucose into cells may cause similar shifts of phosphate, which may result in hypophosphataemia.

 Alkalosis. Especially respiratory, due to movement of phosphate into cells.

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