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AT CHEMPATH > ChemPath: Calcium Metabolism > Flashcards

ChemPath: Calcium Metabolism Flashcards

Review calcium metabolism and homeostasis, recognising the importance of a fixed calcium level on nerve and muscle function. Common calcium disorders Hypercalcaemia Hypocalcaemia Common metabolic bone disorders Osteporosis Osteomalacia / Rickets Pagets To understand the effects of vitamin D and PTH Renal stones

1
Q

What percent of calcium in the body is stored in bones?

A

99%

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2
Q

What percent of calcium is stored in serum?

A

1%

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3
Q

What are the three forms of serum Calcium?

A
  • Free (“ionised”) ~50% - biologically active
  • Protein-bound ~40% - albumin
  • Complexed ~10% - citrate / phosphate
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4
Q

What is the normal range of total calcium serum?

A

2.2 - 2.6 mmol/L

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5
Q

How is corrected Calcium calculated?

A

serum calcium + 0.02 * (40 – serum albumin in g/L)

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6
Q

Why is control of serum calcium levels so important?

A

Calcium levels important in muscle depolarisation and thus in the control of nerve and muscle

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7
Q

If you have a low albumin, the bound calcium will be ____, but the free calcium will be _____. Thus the corrected calcium tells you that the problem is with the ______ and that the ionised calcium will also be ______.

A

If you have a low albumin, the bound calcium will be low, but the free calcium will be normal. Thus the corrected calcium tells you that the problem is with the albumin and that the ionised calcium will also be normal.

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8
Q

How are circulating calcium levels maintained? & where is ca obtained from in LT calcium deficiency?

A

Plasma concentration of calcium must be maintained despite calcium and vitamin D deficiency.

Chronic calcium deficiency thus results in loss of calcium from bone in order to maintain circulating calcium.

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9
Q

Describe briefly calcium homeostasis in response to decreased Calcium?

A

Hypocalcaemia is detected by parathyroid gland.

Parathyroid gland releases PTH.

PTH “obtains” Calcium from 3 sources: Bone, Gut (absorption), Kidney (resorption and renal 1 alpha hydroxylase activation)

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10
Q

What are the main roles of PTH?

A
  • Stimulate osteoclasts to release Ca from bone
  • Stimulate renal Ca resorption
  • Stimulates 1,25 (OH)2 Vit D synthesis (1alpha-hydroxylation)
  • Stimulates renal phosphate wasting (Phosphate trashing hormone)
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11
Q

What enzyme does PTH activate in the kidney?

A

1alpha-hydroxylase

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12
Q

Describe Vitamin D synthesis.

A
  1. 7-dehydrocholesterol is converted into cholecalciferol (D3) by UV light.
  2. Cholecalciferol (D3) is converted into 25-hydroxycholecalciferol (25-OH D3) by 25-hydroxylase in the liver.
  3. 25-hydoxycholecalciferol (25-OH D3) is converted by 1,25-dihydroxycholecalciferol (1,25-(OH)2 D3) by 1alpha-hydroxylase in the kidney.
  4. 1,25(OH)2 D3 / calcitriol is the physiologically active form of Vitamin D
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13
Q

What inactive form is Vit D stored in the body as?

A

25-hydroxycholecalciferol

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14
Q

What is the physiologically active form of Vit D?

A

1,25-dihydroxycholecalciferol
/calcitriol

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15
Q

Which of these is a plant product?
A. Ergocalciferol (D2)
B. Cholecalciferol (D3)

A

A. Ergocalciferol (D2)

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16
Q

What percent of any absorbed Vit D is hydroxylated at the 25 position in the liver?

A

100%

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17
Q

What enzyme in the liver hydroxylases Vit D at the 25 position?

A

25-hydroxylase

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18
Q

What is the rate limiting step in Vit D activation?

A

1-alpha hydroxylase in the kidney. This enzyme is activated by PTH only when calcium is needed.

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19
Q

Where may 1-alpha hydroxylase sometimes be expressed pathologically?

A

Rarely, it can be expressed in lung cells of sarcoid tissue.

With sarcoidosis, there are macrophages in the lung that may express this enzyme in a non-regulated fashion. This may cause hypercalcaemia but only during the summer time when Vit D levels are increased -> more calcium

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20
Q

What are the main roles of 1,25 (OH)2 D3?

A
  • Intestinal Ca absoprtion
  • Intestinal Pi absorption (but increase Pi excretion in kidneys)
  • Critical for bone formation

Other:

  • Vit D receptor controls many genes eg for cell proliferation, immune system etc
  • Vit D deficiency associated with cancer, autoimmune disease, metabolic syndrome
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21
Q

What other conditions are Vit D deficiency associated with?

A

Poverty, Cancer, TB, Infectious diseases

22
Q

What effect does PTH have on the bone?

A

PTH activates osteoclasts to release calcium from the bone. It causes resorption of calcium and phosphate stimulated by PTH.

In response to osteoclast activation, osteoblasts try and build up the bone and pull calcium and phosphate back. Osteoblast activation causes release of ALP.

23
Q

In the presence of sky high ALP but normal ALT and AST, what pathology does this suggest?

A

Bone disease rather than liver disease

24
Q

What is the role of the skeleton from an orthopaedic viewpoint?

A 
Structural framework (strong, relatively lightweight, mobile) 
Protects vital organs 
Capable of orderly growth and remodelling
25
Q

What is the role of the skeleton from a metabolic view point?

A

Metabolic role in calcium homeostasis

Main reservoir of calcium, phosphate and magnesium

26
Q

What does Vit D deficiency cause in adults and children?

A

Osteomalacia and Rickets

27
Q

What are the risk factors for vitamin D deficiency?

A

Lack of sunlight exposure
Dark skin
Dietary
Malabsorption

28
Q

Why does chappati consumption increase vit D deficiency?

A

Phytic acid

(This chelates vit D in gut and prevents absorption, contributing to the vit D deficiency)

29
Q

What are lifestyle treatments for osteoporosis?

A
  • Weight-bearing exercise
  • Stop smoking
  • Reduce EtOH
30
Q

Is the bone structure in osteomalacia normal or abnormal?

A

Abnormal. Vit D deficiency causes defective bone mineralisation.

31
Q

What are the clinical features of Osteomalacia?

A
  • Bone and muscle pain
  • Increased fracture risk
  • Looser’s Zones (pseudofractures)
32
Q

What are the clinical features of Rickets?

A

BMWC
- Bowed legs
- Widened epiphyses at the wrists
- Myopathy
- Costochondral swelling

33
Q

What is the biochemistry of Osteomalacia?

A

Low vit D
Low Calcium
Low Phosphate
Raised ALP (due to osteoblasts trying to build up the bone)
Raised PTH

34
Q

What type of drug may induce breakdown of Vit D?

A

Anticonvulsants (phenytoin, phenobarbital)

35
Q

What hormone does the placenta make that plays an important role in calcium metabolism in babies?

A

PTHrp

36
Q

Is calcium high, normal or low in secondary hyperparathyroidism?

A

Calcium must be low to stimulate PTH

37
Q

Is bone structure normal or abnormal in osteoporosis?

A

Normal

38
Q

Osteoporosis has _____ loss but with _____ calcium. It is due to a reduction in bone ____ with ____ mineralisation.

A

Osteoporosis has bone loss but with normal calcium. It is due to a reduction in bone density with normal mineralisation.

39
Q

What are major causes of Osteoporosis?

A
  • Old age
  • Childhood illness (failure to attain peak bone mass meaning you start off with a lower bone density)
  • Lack of oestrogen (menopause or POI)
  • Hyperprolactinaemia (switch off HPG so no oestrogen)
  • Cushing’s and hyperthyroidism (inc cortisol/t4 increases bone turnover, accelearing normal ageing of bone)
  • Lifestyle: Immobilisation, smoking, etoh
40
Q

What is the biochemistry of osteoporosis?

A

Normal

41
Q

What T score is used to define osteoporosis and osteopenia?

A

Osteoporosis: T-score < -2.5
Osteopaenia: T-score -1 to -2.5

42
Q

What are the typical fractures seen in osteoporosis?

A

Neck of femur
Vertebral (kyphosis)
Wrist - Colle’s fracture

43
Q

How is osteoporosis diagnosed?

A

DEXA scan (dual energy X-ray absorptiometry)

  • hip (femoral neck etc) & lumbar spine
  • T-score – sd from mean of young healthy population (useful to determine  risk)
  • Z-score – sd from mean of age and gender-matched control (useful to identify accelerated bone loss in younger patients)
44
Q

What are lifestyle, endocrine, drugs and other causes of Osteoporosis?

A
  • Lifestyle: sedentary, EtOH, smoking, low BMI/nutritional
  • Endocrine: hyperprolactinaemia, thyrotoxicosis, Cushings
  • Drugs: steroids
  • Others eg genetic, prolonged intercurrent illness
45
Q

What are drug treatments for osteoporosis?

A
  • Vitamin D/Ca
  • Bisphosphonates (eg alendronate) –↓ bone resorption
  • Teriparatide (PTH derivative) – anabolic
  • Strontium – anabolic + anti-resorptive
  • (Oestrogens – HRT)
  • SERMs eg raloxifene
46
Q

What is the calcium level in Paget’s disease?

A

Normal because even though turnover is high the balance of calcium is normal

ALP will be high

47
Q

Recall the symptoms of Paget’s disease

A

PAIN, warmth, deformity, fracture, increased risk of high output cardiac failure

48
Q

Which bones are most commonly affected by Paget’s?

A

Pelvis, femur, skull and tibia

49
Q

What is the gold standard investigation for diagnosing Paget’s disease?

A

IV radiolabelled bisphosphonates

50
Q

How is pain treated in Paget’s disease?

A

Bisphosphonates

51
Q

In pri hyperparathyroidism, are PTH levels high/normal/low?

A

High
May also be inappropriately normal (since if calcium is high pth should be low/zero to suppress calcium)

AT CHEMPATH (24 decks)

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