21 - Calcium Metabolism Flashcards
How do calcium and phosphate differ in their regulation?
- Calcium: tightly regulated between 1 and 1.3 mmol/L
- Phosphate: not so tightly regulated, fluctuate throughout the day
BOTH REGULATED BY THE SAME HORMONES IN OPPOSING WAYS
What hormones regulate calcium homeostasis and what organ systems to they act on?
Increase Ca: PTH and 1,25-dihydroxyvitamin D
Decrease Ca: calcitonin
Organ systems: kidney, gut, intestine
Draw a diagram of the calcium pool.
- 1kg in bone
- 1g elswhere
- 800 to 1200mg dietary intake a day with the gut absorbed and secreting at same time
How does calcium exist in the plasma?
- 2.2 to 2.7mmol/L normally
- 45% ionised, 45% protein bound, 10% complexed with small organic anions
- Only active one is the ionised form
How do blood tests measure calcium levels?
- Total calcium and then adjust this when albumin and other proteins are measure to determine free calcium range
What should you give a patient who has had multiple bags of blood transfusions and why?
- IV calcium
- Stored blood contains chelating agent citrate to stop blood clotting as calcium is factor IV, therefore all the chelating ions in the blood will cause hypocalcaemia
What are the symptoms of hypercalcaemia and hypocalcaemia?
- Hypercalcaemia: renal calculi, constipation, dehydration, kidney damage, tiredness and depression
- Hypocalcaemia: hyperexcitability of the nervous system, paraesthesia, tetany, paralysis, convulsions
What is the short and long term regulation of calcium levels?
Short: PTH
Long: calcitriol (active Vit D)
What does Vitamin D refer to and how is it activated?
- Collective term for vit D2 (ergocalciferol) and D3 (cholecalciferol)
- Undergoes two hydroxylation reactions in liver and kidneys to become Calcitriol (1,25-dihydroxycholecalciferol)
- PTH regulates renal C-1 hydroxylase
Why might someone with a malignancy have hypercalcaemia but a normal PTH level in the blood?
- Tumour cancer cells releases PTHrP leading to HHM (humeral hypercalcaemia of malignancy)
- Released by breast, prostate and myeloma
- PTHrp increases Ca release from bone, reduce Ca excretion and reduces renal phosphate absorption
Why might someone with malignant hypercalcaemia not have an elevated level of Vit D, even though PTH activated vit D?
- Not hyperparathyroidism, it is due to excretion of PTHrP
- PTHrP exerts no increase in activity of renal C-1 hydroxylase so does not increase the calcitriol concentration
What effect does calcitonin have on calcium levels, and where is it secreted from?
- In mammals it lowers Ca
What is the half life of 25-hydroxyvitamin and calcitriol?
- 15 days
- 5-8 hours
What part of the parathyroid produces the peptide hormone PTH?
Chief cells –> darker
Oxyphil –> light blurry staining
What is the mechanism of action to restore calcium homeostasis when it goes out of balance?
What is the best way to remember the signs of chronic hypercalcaemia?
- Stones
- Moans
- Groans
- Bones
Apart from the release of PTHrP, how can malignancy cause hypercalcaemia?
- Osteolytic bone metastases e.g breast, lung, renal, thyroid
- Not prostate as this is osteoblastic
What patients are most likely to get acute hypocalcaemia?
After 6 hours total-thyroidectomy if there is ischaemia or partial removal of the parathyroid. Will get tingling round mouth and in fingers and spasms
How does hypercalcaemia cause hypoexcitability of cells?
Calcium raises the threshold for nerve membrane depolarisation and the development of an action potential
What are the symptoms of severe hypercalcaemia?
Serum calcium >3mmol/L
- Lethargy, weakness, confusion, coma, renal failure
- Rehydration for treatment
What are some of the roles of calcium and phosphate?
How does the parathyroid gland sense plasma calcium concentration?
By G-protein coupled receptors than are activated by calcium
