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Endocrinology > 12 - Endocrine control of calcium metabolism > Flashcards

12 - Endocrine control of calcium metabolism Flashcards

1
Q

What are the roles of calcium in the body?

A 
Neuromuscular excitability
Muscle contraction 
Bone strength
Intracellular second messenger
Intracellular co-enzyme
Hormone/neurotransmitter stimulus-secretion coupling
Blood coagulation (factor IV)
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2
Q

How is calcium found in the body?

A

As calcium salts.
99% in the bones as complex calcium salts.

Some is present in the blood as Ca2+ (ionised calcium)
Some is bound to protein and others are soluble salts.

Only the free unbound Ca2+ is bioactive.

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3
Q

What percentage of calcium stored in the blood is bioactive?

A

50% is unbound and bioactive
45% is bound to plasma proteins
5% is diffusible salts

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4
Q

Where does calcium enter the body and where in the body is it needed and excreted from?

A 
Enters in gastro-intestinal tract.
Goes into blood.
Blood takes it to bones and kidneys.
Some lost through kidneys in urine
Most lost in GI tract in faeces.
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5
Q

What increases [Ca2+]?

A

Parathyroid hormone (PTH)

1,25 (OH2) Vitamin D3 (Calcitriol)

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6
Q

What decreases [Ca2+]?

A

Calcitonin

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7
Q

Where is parathyroid hormone (PTH) secreted from?

A

Parathyroid glands

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8
Q

Where is calcitonin secreted from?

A

Parafollicular cells (outside a thyroid follicle, outside the follicular cells)

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9
Q

How are calcium levels sensed?

What happens if calcium is detected?

A

G-protein coupled cell surface receptors.

When calcium is sensed, the receptors recognise this and suppress PTH release.

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10
Q

What is an antagonist and what happens if this binds to the calcium receptor?

A

It stops calcium from binding to the receptor.

This makes the body think there is less calcium so more PTH is released.

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11
Q

Where may calcium receptors be found?

A

Parathyroid gland
GI tract
Kidney

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12
Q

What is the precursor to parathyroid hormone?

How long is the final PTH?

A

Pre-proPTH.

Final PTH is 84 amino acids long.

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13
Q

How does PTH interact with receptors?

A

PTH binds to transmembrane G-protein linked receptors.

This activates adenyl cyclase & PLC (phospholipase C) as a second messenger.

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14
Q

What are the actions of PTH on the kidneys?

A
  • Increases Ca2+ reabsorption
  • Increases (PO4)3- excretion. (Phosphate)
  • Stimulates 1a hydroxyls enzyme which is used to make 1,25 (OH)2 Vitamin D3.
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15
Q

What is 1,25 (OH)2 V3 used for?

A

It stimulates the small intestine to:

  • Absorb more Ca2+
  • Absorb more (PO4)3- (phosphate)
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16
Q

What are the actions of PTH on the bones?

A
  • Stimulates osteoclasts
  • Inhibits osteoblasts
    = More bone broken down
    = Increased bone reabsorption
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17
Q

What is the overall action of PTH?

A

INCREASES blood Ca2+

Via -
Increasing Ca2+ reabsorption in the kidneys
Increasing Ca2+ adsorption in the small intestine
Increasing Ca2+ mobilisation in the bones.

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18
Q

How does PTH work in bones?

A

PTH binds to PTH receptor on osteoblasts.
This stimulates osteoclast activating factors (OAFs), e.g. RANKL
They cause osteoclasts to reabsorb bone and extracting calcium from it.

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19
Q

How is PTH regulated? (Think feedback loops)

A

Decreased plasma [Ca2+] causes PTH release.

PTH causes an increase in [Ca2+].

Negative feedback on parathyroid gland so less PTH released.

ALSO:

PTH increases synthesis of Vit D3. This also has -ve feedback on pituitary.

Catecholamines stimulate parathyroids to make more PTH.

20
Q

Explain how dihydroxy-cholecalciferol is synthesised.

1,25 (OH)2 D3

A

Cholecalciferol (Vitamin D3) is made from:

  • Diet
  • 7-dehydrocholesterol in skin with UV light

In the LIVER:
Cholecalciferol is made into 25-hydroxy-cholecalciferol (25 (OH) D3)

PTH stimulates 1a hydroxylase enzyme…

In the KIDNEYS:

25(OH)D3 is made into 1,25 Di-hydroxy-cholecalciferol
(1,25 (OH)2 D3) = main bioactive form.

21
Q

What is 1,25 Di-hydroxy-cholecalciferol also called?

A

Calcitriol

22
Q

What are the actions of 1,25 (OH)2 D3 in the bones, kidneys and small intestine?

A

SMALL INTESTINE:
Increased Ca2+ and phosphate absorption

KIDNEYS:
Increased Ca2+ and phosphate reabsorption

BONE:
Takes in Ca2+ and stimulates osteoblasts

23
Q

What does Fibroblast Growth Factor 23 do?

A

Regulates phosphate metabolism. Protects you from having high phosphate levels.
High phosphate and high Vit D3 triggers FGF23 to decrease phosphate level.

24
Q

How does FGF23 and PTH affect phosphate reabsorption in the kidneys?

A

FGF23 and PTH inhibit the sodium phosphate transporter.
This means that LESS sodium and phosphate are reabsorbed into the blood, so more in excreted in the urine.
Calcitriol (Vit D3) stimulates FGF23.

25
Q

What is the precursor for calcitonin?

How long is the final calcitonin?

A

Pre-procalcitonin.

Final calcitonin is 32 amino acids long.

26
Q

What does calcitonin bind to?

A

Transmembrane G-protein linked receptors.

Which activate adenyl cyclase or other second messengers.

27
Q

What is the main function of calcitonin?

A

To lower [Ca2+]

28
Q

How does increased plasma [Ca2+] lead to calcitonin production and regulation of [Ca2+] in the bone and kidneys?

A

Increased plasma [Ca2+] leads to:

Parafollicular cells secrete Calcitonin…

BONE:
Inhibits osteoclasts

KIDNEY:
Increased urinary excretion of Ca2+

Which all leads to DECREASED [Ca2+]

29
Q

What may also stimulate the parafollicular cells to secrete calcitonin?

A

Gastrin

30
Q

What is hypocalcaemia?

A

Low blood calcium levels

31
Q

What are 3 causes of hypocalcaemia?

A

Hypoparathyroidism
Pseudohypoparathyroidism
Vitamin D deficiency

32
Q

What are 2 clinical signs of hypocalcaemia?

A

Tetany -
Trousseau’s sign - hand spasm
Chaostek’s sign - Twitch in nose when ear is flicked

Easier for sodium to enter and depolarise cells so more action potentials fired = spasms

33
Q

What are the causes of hypoparathyroidism?

A
  • Idiopathic (
  • Hypomagnesaemia
  • Negative feedback - suppression by raised calcium conc.
34
Q

What is pseudohypoparathyroidism also known as?

A

Allbright hereditary osteodystrophy.

35
Q

What is pseudohypoparathyroidism?

A

Target organ resistance to PTH.

Due to ineffective G proteins.

36
Q

What are the features of someone with pseudohypoparathyroidism?

A 
Short stature
Round face
Low IQ
Subcutaneous calcification and bone abnormalities (shortening of metacarpals) 
Other associated endocrine disorders.
37
Q

What is vitamin D deficiency called in children and adults?

A

Children - Rickets

Adults - Osteomalacia

38
Q

What are the clinical features of Vitamin D deficiency?

A

Decreased calcification of bone matrix = softer bone
Fracture in adults
Bowing of bones in children.

39
Q

How would you diagnose between hypoparathyroidism,

pseudohypoparathyroidism and vitamin D deficiency?

A

Hypoparathyroidism:
Calcium - Low
Phosphate - High
PTH - Low

Pseudohypoparathryoidism:
Calcium - Low
Phosphate - High
PTH - High

Vitamin D deficiency:
Calcium: Low
Phosphate: Low
PTH: High

40
Q

What are the endocrine causes of hypercalcaemia?

A

Primary hyperparathyroidism
Tertiary hyperparathyroidism
Vitamin D toxicosis

41
Q

What is primary hyperparathyroidism?

A

Adenoma in parathyroid.
PTH increases
[Ca] increases
Negative feedback doesn’t work so too much PTH is released.

42
Q

What is secondary hyperparathyroidism?

A

Renal failure.
Kidneys can’t reabsorb Ca so [Ca] falls.
Causes more PTH to be released.

43
Q

What is tertiary hyperparathyroidism?

A

Low [Ca] = parathyroid releases PTH.
Causes [Ca] to increase.
Parathyroid is autonomous so does not respond to negative feedback so keeps pumping out PTH.

44
Q

What are the effects of excess parathyroid hormone in the kidneys?

A 
KIDNEY:
More Ca reabsorbed
More phosphate excreted.
Polyuria
Renal stones
Vitamin D synthesis
45
Q

What are the effects of excess parathyroid hormone in the GI tract?

A

GI tract:
Gastric acid
Duodenal ulcers

46
Q

What are the effects of excess parathyroid hormone in the bones?

A

Bones:
Bone lesions
Bone rarefaction
Fractures

Endocrinology (18 decks)

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