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BMS15 > Calcium Homeostasis & Vitamin D > Flashcards

Calcium Homeostasis & Vitamin D Flashcards

1
Q

Where is calcium found in the diet?

A

Cereals

Milk

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2
Q

How much calcium do we need per day?

A

807mg/day

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3
Q

Where is phosphorus found in the diet?

A

Cereals

Milk

Meat

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4
Q

What food/ingredient is fortified with calcium by law?

A

White flour

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5
Q

How much calcium is absorbed in the diet and from supplements?

A

Diet - 25-35%

Supplements - 35-40%

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6
Q

What can increase calcium bioavailability?

A

Casein and lactose in dairy products

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7
Q

What substances inhibit calcium absorption in the gut?

A

Oxalate

Phytate

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8
Q

What is the most important inhibitor of calcium absorption in the gut?

A

Phytate

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9
Q

What is the most potent inhibitor of calcium absorption in the gut?

A

Oxalate

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10
Q

When do calcium requirements increase?

A

During growth spurts

Lactation (+550mg/day)

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11
Q

What are normal plasma calcium levels?

A

2.25-2.6mM (~2mM)

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12
Q

Where in the GI tract is calcium absorbed?

A

Duodenum

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13
Q

How is calcium absorbed in the GI tract? (3)

A

Taken up in ion form through ECaC/CaT1 down concentration gradient

Binds to calcium-binding protein/calbindin which translocates calcium to the BL membrane

Ca-ATPase or Na/Ca exchanger used to export calcium

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14
Q

What is the ratio of ions transported by the Na/Ca exchanger?

A

3Na to 1Ca

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15
Q

What is upregulated in the duodenum by vitamin D?

A

ECaC and CaBP expression

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16
Q

Which calcium-binding protein is larger: duodenal or renal?

A

Renal

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17
Q

What percentage of circulating calcium is free/ionised?

A

~50%

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18
Q

What form of calcium is freely filtered at the renal corpuscle?

A

Ionised calcium

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19
Q

Where is calcium freely and passively reabsorbed from the nephron?

A

PCT and thick ascending limb ~95%

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20
Q

Where does controlled reabsorption of calcium occur in the nephron?

A

DCT and upper part of collecting duct

21
Q

What is NCX1?

A

Na/Ca exchanger on BL membrane of duodenal enterocytes and cells of DCT

22
Q

What does PMCA1b stand for?

A

Plasma membrane Ca-ATPase 1b

23
Q

What stimulates reabsorption of calcium from the nephron?

A

Vitamin D and parathyroid hormone

24
Q

What inhibits reabsorption of calcium from the nephron?

A

Calcitonin

25
Q

What type of hormone is calcitonin?

A

Peptide

26
Q

Where are calcium-sensing receptors found? (4)

A

Parathyroid gland

Thyroid

Bone

Renal tubules

27
Q

What is stimulated by low plasma calcium (hormonal)?

A

Increased PTH and inhibition of calcitonin secretion

28
Q

What is stimulated by high plasma calcium (hormonal)?

A

Increased calcitonin and inhibition of PTH secretion

29
Q

Describe what happens when plasma calcium decreases below normal.

A

Detection by calcium-sensing receptors

Increased PTH and inhibition of calcitonin secretion

PTH activates vitamin D

Increased:

  • Renal calcium reabsorption
  • Intestinal calcium absorption
  • Bone resorption
30
Q

Describe what happens when plasma calcium increases above normal.

A

Detection by calcium-sensing receptors

Increased calcitonin and inhibition of PTH secretion

Decreased renal reabsorption/increased renal excretion

31
Q

What are the two precursors of vitamin D?

A

Cholecalciferol (D3)

Ergocalciferol (D2)

32
Q

Where does the majority of vitamin D come from in humans?

A

Action of UV B-radiation on skin to convert 7-dehydrocholesterol into cholecalciferol (photoconversion)

33
Q

What takes up pre-vitamin D for storage?

A

Liver

34
Q

What is the first metabolite of vitamin D and how does it circulate?

A

25 hydroxy vitamin D (25-OH-D)

Bound to vitamin D-binding protein

35
Q

Where is 25-OH-D activated?

A

Kidneys

36
Q

What causes activation of 25-OH-D and how?

A

Decreased plasma levels of vitamin D, calcium or phosphate; parathyroid hormone

Stimulates an α-hydroxylase in kidney to cause a second hydroxylation to form 1, 25 dihydroxy vitamin D

37
Q

What is the active form of vitamin D?

A

1, 25 dihydroxy vitamin D

38
Q

What is the “off switch” in vitamin D activation?

A

If vitamin D, calcium or phosphate levels rise too quickly then a second hydroxylation forms 24, 25 dihydroxy vitamin D (inactive) instead

39
Q

In the UK, when is vitamin D formed by photoconversion?

A

April to September

40
Q

How much vitamin D is recommended per day?

A

10ug/day (above 4yo)

41
Q

What foods contain more vitamin D than others? (4)

A

Egg yolk (5ug/100g)

Margarines

Cod liver oil (210ug/100g)

Dairy in USA

42
Q

What food is fortified with vitamin D by law in the UK?

A

Margarines

43
Q

How does phytate affect vitamin D stores?

A

Decreases stores

Blocks calcium absorption so less plasma calcium which stimulates activation of vitamin D to maintain homeostasis

44
Q

How do anticonvulsants affect vitamin D activation?

A

Inhibit activation

Inhibit the cytochrome-based enzymes involved in activation

45
Q

What are some renal risk factors for vitamin D deficiency? (6)

A

Low PTH

High phosphate

Rickets

Some tumours

Renal failure

Nephrectomy

46
Q

What role does vitamin D have on growing bones?

A

Controls growth and turnover

Acts on growth plates in proliferative zone - main trigger in differentiation of chondrocytes from proliferative to hypertrophic cells

47
Q

How does a vitamin D deficiency cause rickets?

A

Cells in growth plates remain proliferative

More matrix and plate expands and becomes too flimsy to support weight

48
Q

What are the clinical features of rickets? (3)

A

Knock-knees/bow-legs (femur)

Enlarged wrists

Rachitic rosary (ribcage)

49
Q

What effect does vitamin D deficiency have on adults? (3)

A

Osteomalacia

Increased PTH levels so more bone resorption but insufficient mineral to complete remodelling

Unmineralised, collagen-rich matrix appears as looser zones/pseudofractures on x-rays

BMS15 (4 decks)

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