Calcium Metabolism

Question 1

What are the functions of Ca2+ in the body?

Answer 1

• Formation of calcified tissues (bone, teeth)
• Necessary for action potentials in nerves and muscle
• Stimulates neurotransmitter release, and hormonal-glandular secretion
• Maintains integrity and permeability of cell membranes
• Cell adhesion
• Blood clotting

Question 2

How does Ca2+ act as a reservoir in bones?

Answer 2

Resorption can be accelerated to increase blood levels if low

Question 3

What states is calcium in the blood?

Answer 3

Bound with proteins e.g. albumin (45% of Ca2+ in blood)

Bound with organic ions (10%)

Free ionised Ca2+ (45%)

Question 4

Which calcium can be sensed?

Answer 4

Only free ionised Ca2+

Has a direct effect on hormone secretion

Question 5

Where is parathyroid hormone (PTH) secreted from?

Answer 5

Produced by chief cells of parathyroid gland

Question 6

Where are the parathyroid glands located?

Answer 6

Lateral to trachea in thyroid gland, embedded in thryoid

Question 7

When is PTH secreted?

Answer 7

Secreted in response to a low plasma Ca2+ level

Secretion inhibited when plasma Ca2+ level is high
- levels act directly on parathyroid

Question 8

How does PTH act to increase Ca2+ plasma levels?

Answer 8

1) Increases Ca2+ intestinal absorption via DHCCF (activated vit D)

2) Increased Va2+ resorption from bone
- stimulates osteolysis
- stimulates increased osteoclast activity

3) Decreased Ca2+ excretion
- Increases Ca2+ reabsorption from asc. loop of henle/DCT
- Reduced phosphate reabsorption from PCT/DCT

Question 9

What is osteolysis?

Answer 9

Transfer of Ca2+ from bone into canilicular fluid into osteocytes and from there into bone

Question 10

What is DHCCF?

Answer 10

1,25-Dihydroxycholecalciferol, also known as the active form of Vid-D

Question 11

How is DHCCF produced?

Answer 11

Vit-D metabolised in liver and kidney to form DHCCF
Vit-D3 is formed by action of UV light
Vit-D2 derived from plants

Production is accelerated by PTH - accelerates final hydroxylation step

Question 12

What are the actions of DHCCF?

Answer 12

On small intestine - increases Ca2+ absorption

On kidneys - increased Ca2+ reabsorption in DCT

On bone = increases bone resorption

Question 13

How does DHCCF act on small intestine?

Answer 13

Induced synthesis of Ca2+ channels and pumps

Induces expression of Ca2+ binding proteins on lumen wall

Question 14

How does DHCCF act on the kidneys?

Answer 14

Acts synergistically with PTH

Increases binding proteins and channels

Question 15

How does DHCCF act on bone?

Answer 15

Promotes development of osteoclasts from precursor cells

Question 16

What are the effects of DHCCF deficiency?

Answer 16

Leads to insufficient absorption of Ca2+

In children - Ricketts
In adults - Osteomalacia

Question 17

What is Ricketts?

Answer 17

Insufficient Ca2+ leading to insufficient mineralisation of bone
=> large amounts of unmineralised osteoid
=> bowing of growing weight-bearing long bones
- bones easily fractured due to softness

Question 18

What is osteomalacia?

Answer 18

Increased unmineralised osteoid and therefore decreased bone strength

• bowing doesn’t occur - bones not growing
• fractures develop

Question 19

What is calcitonin?

Answer 19

A small peptide hormone produced by parafolicular C cells of thyroid

Question 20

When is calcitonin secreted?

Answer 20

In response to increased Ca2+ plasma levels

Question 21

What action does calcitonin have?

Answer 21

Plays a little role day to day, but inhibits osteoclasts therefore reducing bone resorption

Question 22

What are the effects of hypercalcaemia on the patient?

Answer 22

Pain

Bone fractures and deformity

Question 23

What can cause hypercalcaemia?

Answer 23

Hyperparathyroidism as a result of a benign tumour (adenoma)

Question 24

What can cause chronic hypocalcaemia?

Answer 24

Loss of thyroid function, which may be a result of thyroidectomy

Question 25

What are the effects of hypocalcaemia?

Answer 25

Muscle spasms and seizures

Bone mass increase