21 - Calcium Metabolism Flashcards

1
Q

How do calcium and phosphate differ in their regulation?

A

- 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

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2
Q

What hormones regulate calcium homeostasis and what organ systems to they act on?

A

Increase Ca: PTH and 1,25-dihydroxyvitamin D

Decrease Ca: calcitonin

Organ systems: kidney, gut, intestine

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3
Q

Draw a diagram of the calcium pool.

A
  • 1kg in bone
  • 1g elswhere
  • 800 to 1200mg dietary intake a day with the gut absorbed and secreting at same time
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4
Q

How does calcium exist in the plasma?

A
  • 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
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5
Q

How do blood tests measure calcium levels?

A
  • Total calcium and then adjust this when albumin and other proteins are measure to determine free calcium range
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6
Q

What should you give a patient who has had multiple bags of blood transfusions and why?

A
  • 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
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7
Q

What are the symptoms of hypercalcaemia and hypocalcaemia?

A

- Hypercalcaemia: renal calculi, constipation, dehydration, kidney damage, tiredness and depression

- Hypocalcaemia: hyperexcitability of the nervous system, paraesthesia, tetany, paralysis, convulsions

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8
Q

What is the short and long term regulation of calcium levels?

A

Short: PTH

Long: calcitriol (active Vit D)

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9
Q

What does Vitamin D refer to and how is it activated?

A
  • 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
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10
Q

Why might someone with a malignancy have hypercalcaemia but a normal PTH level in the blood?

A
  • 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
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11
Q

Why might someone with malignant hypercalcaemia not have an elevated level of Vit D, even though PTH activated vit D?

A
  • 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
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12
Q

What effect does calcitonin have on calcium levels, and where is it secreted from?

A
  • In mammals it lowers Ca
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13
Q

What is the half life of 25-hydroxyvitamin and calcitriol?

A
  1. 15 days
  2. 5-8 hours
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14
Q

What part of the parathyroid produces the peptide hormone PTH?

A

Chief cells –> darker

Oxyphil –> light blurry staining

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15
Q
A
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16
Q

What is the mechanism of action to restore calcium homeostasis when it goes out of balance?

A
17
Q

What is the best way to remember the signs of chronic hypercalcaemia?

A
  • Stones
  • Moans
  • Groans
  • Bones
18
Q

Apart from the release of PTHrP, how can malignancy cause hypercalcaemia?

A
  • Osteolytic bone metastases e.g breast, lung, renal, thyroid
  • Not prostate as this is osteoblastic
19
Q

What patients are most likely to get acute hypocalcaemia?

A

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

20
Q

How does hypercalcaemia cause hypoexcitability of cells?

A

Calcium raises the threshold for nerve membrane depolarisation and the development of an action potential

21
Q

What are the symptoms of severe hypercalcaemia?

A

Serum calcium >3mmol/L

  • Lethargy, weakness, confusion, coma, renal failure
  • Rehydration for treatment
22
Q

What are some of the roles of calcium and phosphate?

A
23
Q

How does the parathyroid gland sense plasma calcium concentration?

A

By G-protein coupled receptors than are activated by calcium