Calcium Homeostasis

Question 1

Give 5 roles of calcium in the body

Answer 1

1. Signalling - calcium is important in signalling molecules; exocytosis of synaptic vessels e.g. neurotransmitters/hormones etc, contraction of muscle fibres, alters enzyme function
2. Blood clotting - essential component of clotting cascade
3. Apoptosis - programmed cell death
4. Skeletal strength - 99% of calcium in the body is wrapped up in bone where it gives strength to the skeleton
5. Membrane Excitability - Ca2+ decreases Na+ permeability - most critical in short term homeostasis

Question 2

Describe hypercalcaemia

Answer 2

It decreases neuronal Na+ permeability which will reduce excitability and depress neuromuscular activity and in extreme cases trigger cardiac arrhythmias

Question 3

Describe hypocalcaemia

Answer 3

Increases neuronal Na+ permeability leading to hyperexcitation of neurons. In extreme cases causes tetany, and if spreads to the larynx and respiratory muscles causes asphyxiation

Question 4

How is the calcium on th body distributed?

Answer 4

Bone - 99%
Intracellular fluid - 0.9%
Extracellular fluid - 0.1%

Question 5

What is the normal value of calcium in the plasma?

Answer 5

2.2-2.6mM

Question 6

Where in bone in calcium stored?

Answer 6

In the calcified extracellular matrix

Question 7

Describe the affinity of calcium for proteins

Answer 7

It is very high

Question 8

In plasma, what percentage of calcium is bound to plasma proteins?

Answer 8

40%

Question 9

Which portion of calcium in the blood is physiologically active? What percentage of plasma protein does this make up?

Answer 9

Free (ionised) calcium

50% of plasma calcium

Question 10

What is the relationship between pH and free calciums plasma protein binding capacity?

Answer 10

The binding capacity is increased under alkalotic conditions and decreased under acidic conditions

Question 11

What is the effect of hyperventilation on calcium in the blood?

Answer 11

TH plasma pH rises and therefore plasma proteins bind to calcium more readily, causing calcium plasma concentration to fall and therefore may precipitate hypocalcaemic tetany

Question 12

What is hypocalcaemic tetany?

Answer 12

Where plasma pH rises and more calcium in the blood i sound to plasma proteins causing the calcium concentration in the blood to fall
NB the overall amount of calcium does not change

Question 13

What happens to calcium in the blood when there is acidosis?

Answer 13

Binding capacity of the calcium to the plasma proteins decreases and increases the amount of free calcium in the plasma but it is important to note that the overall amount of calcium does not change

Question 14

What is the relevance of the control of ECF calcium?

Answer 14

The distribution of calcium between bone and ECF is very important, although bone provides mechanical support for the body, its role in maintaining calcium balance takes precedence

Question 15

Outline Ca2+ storage in bone with reference to osteroblasts/clasts

Answer 15

There is a continuous turnover of bone which is forever being deposited and then reabsorbed

Osteoblasts are the bone-building cells which are highly active and lay down a collagen extracellular matrix which they then calcify. They differentiate to form osteocytes in established bone, they are much less active but appear to regulate the activity of the osteoblasts and osteoclasts

Osteoclasts are cells that are responsible for mobilising bone. They secrete H+ ions the dissolve the calcium salts and all provide proteolytic enzymes to digest the extracellular matrix

Question 16

What are the two key hormones which control calcium homeostasis?

Answer 16

1. Parathyroid hormone (PTH) - polypeptide hormone produced by the parathyroid glands
2. Calcitrol (aka Vit D3) - steroid hormone product from Vit D by the liver and kidneys

Question 17

Where are that parathyroid glands?

Answer 17

There are usually 4 lying on the poster surface of the thyroid gland

Question 18

What prompts release of PTH?

Answer 18

Decreased free calcium in the plasma

Question 19

How does PTH act to increase plasma calcium?

Answer 19

1. Stimulating osteoclasts to increase release of calcium and phosphate in the bone
2. Inhibiting osteoblasts to reduce calcium deposition in bone
3. Increasing reabsorption of calcium from the kidney tubules, therefore decreasing excretion in urine
4. Increasing renal excretion of phosphate thereby elevating free calcium by preventing it from being deposited back into bone, a process that requires phosphate
5. Stimulates the kidney to synthesise calcitrol from Vitamin D3 which promotes calcium adsorption at the gut and the kidney

Question 20

Hoe deos calcitrol act to complement the actions of PTH?

Answer 20

1. Increases adoption of calcium from the gut - calctirol controls the active transport system which dictates how much of the calcium is reabsorbed from the gut - this does not happen automatically like is the case with most nutrients like water, sodium and potassium
2. Facilitates renal absorption of calcium
3. Mobilises calcium stores in bone by stimulating osteoclast activity

Question 21

What is the level to which Vitamin D must fall to be considered deficient? What condition can it cause in adults and what other in children?

Answer 21

Less than 20g/ml

Adults - Osteomalacia
Children - Rickets

Question 22

Other than diseases, what does vitamin D3 deficiency lead to at first?

Answer 22

Calcium and phosphate deficiencies

Question 23

What is the only hormone which acts to decrease plasma calcium? Where is it produced? What stimulates its secretion?

Answer 23

Calcitonin, by the thyroid gland, increased plasma calcium

Question 24

What is the main action of calcitonin?

Answer 24

To bind and inhibit osteoclasts from bone resorption as well as increase renal secretion so as to further prevent any increases in plasma calcium

NB: there is little evidence that it is of significance in humans

Question 25

How does insulin affect calcium balance?

Answer 25

It increases bone formation and antagonises the action of cortisol. Diabetics have significant bone loss

Question 26

How does prolactin influence calcium balance?

Answer 26

Promotes calcium absorption from the gut by stimulating synthesis of calcitrol