Calcium, magnesium and phosphate homeostasis

Question 1

Physiological importance of calcium

Answer 1

muscle contraction, neuronal excitation, enzyme activity (Na/K ATPase, hexokinase, etc), blood clotting

Question 2

Total body calcium

Answer 2

• Around 1kg
• 99% bon
• 1% intracellular, 0.1% extracellular

Question 3

Total Ca =

Answer 3

ionised Ca + bound Ca + complexed Ca

Question 4

If low albumin, what happpns t calcium levels?

Answer 4

reduction in plasma protein to bind = lower bound calcium and total calcium; same ionised calcium

Question 5

Adjusted Ca

Answer 5

• adjusted for if patient had normal albumin

- Total Ca + [ (40 – Alb) x 0.025]

Question 6

Phosphate

Answer 6

• Mainly intracellular
• P in ATP – body fuel, intracellular signalling, cellular metabolic processes
• Backbone of DNA + in lipids

Question 7

Distribution of phosphate

Answer 7

• Around 700g
• 85% in bone
• 14 % extra, 1% intra
• 70% organic form – covalently bound e.g. phospholipids
• 30% inorganic as phosphate e.g. HPO42-

Question 8

Homeostasis of calcium: Two key controlling factors

Answer 8

• PTH

- Vitamin D and metabolites

Question 9

PTH

Answer 9

• PTH secreted in response to low calcium/high phosphate
• Act on bone (resorption) and act on kidney (increase calcium reuptake from filtrate + increase excretion of phosphate)
• Increase serum calcium
• Decrease serum phosphate

Question 10

Vitamin D

Answer 10

• 5-Dihydroxyvitamin D which is active form
• increase the activity of 1-alpha hydroxylase enzyme to increase the activation of Vit D
• Active Vit D acts on intestine to increase absorption of calcium and phosphate
• Precursor of Vit D has higher conc in blood

Question 11

Hypocalcaemia

Answer 11

• Symptoms include tetany and paraesthesia in extremities
• Most common cause is Vit D deficiency
• Low levels of calcium resorption due to insufficient PTH and thyroid issue

Question 12

Hypercalcaemia

Answer 12

• Stones, bones, moans. groans
• Caused by hyper parathyroid - too much PTH
• Most common cause = Malignant tumours that secrete chemicals increasing resorption rate

Question 13

How is ionised calcium measured

Answer 13

blood gas analyser

Question 14

Phosphate deficiency symptoms

Answer 14

• Haematological issues - haemolysis
• Severe muscle weakness
• Degradation of muscle cells
• Rickets/osteomalacia
• Convulsions, coma, death

Question 15

Refeeding syndrome

Answer 15

• Cause of phosphate deficiency
• upon refeeding patient calories kickstarts insulin secretion starting glycolysis, protein synthesis, glycogenesis - all require phosphate = severe depletion of circulation concentration of Phosphate

Question 16

Hyperphosphatemia

Answer 16

• Not as serious as hypo
• Symptoms usually due to hypocalcaemia
• Most common cause is renal failure - AKI or CKD
• Spurious (miscalculation) due to haemolysis or assay interference

Question 17

Magnesium

Answer 17

• Cofactor for ATP, Enzymatic function – cofactor, Regulates ion channels
• Around 1.1 mol
• 54 % bone
• 45 % intracellular, 0.1 % extracellular
• 60% is ionised MG, 25% bound to albumin, 15% complexed to anions

Question 18

Homeostasis of magnesium

Question 19

How does PTH interact with magnesium

Answer 19

• PTH release is stimulated by decreased magnesium and inhibited by increased magnesium
• PTH release is magnesium-dependent so severe hypomagnesaemia will inhibit PTH release and cause hypocalcaemia which won’t resolve with calcium supplementation – need Mg

Question 20

Hypomagnesaemia signs and symptoms

Answer 20

• CNS – neuromuscular hyperexcitability (tremor, tetany, convulsion),
• Cardiovascular – ECG changes, reduced contractility, arrhythmia
• GI – nausea and anorexia
• hypokalaemia, hypocalcaemia, with associated symptoms

Question 21

Hypomagnesaemia causes

Answer 21

• Inadequate intake (diet)
• Renal loss due to drugs, Hypercalcaemic states
• Redistribution into cells (refeeding syndrome)

Question 22

Hypermagnesemia

Answer 22

• Very rare as kidneys have large capacity to excrete excess
• there is a high level of magnesium in the blood. - Symptoms include weakness, confusion, decreased breathing rate, and decreased reflexes.

Question 23

Hypermagnesemia causes

Answer 23

• Renal Osteodystrophy
• Post-op recent blood transfusion
• Likely vitamin D deficiency
• Primary hypoparathyroidism