Calcium homeostasis 2

Question 1

How much calcium does a young adult body contain?

Answer 1

1100g.

Question 2

What is extracellular calcium necessary for?

Answer 2

Nerve function, muscle contraction, coagulation, skeletal mineralization and activation of most cell types (signalling pathways).

Question 3

What is the physiologically active calcium?

Answer 3

It is unbound.

Question 4

What forms does calcium take in the body?

Answer 4

Complexes with anions, lactate, phosphate, urate, sulfate, bicarbonate, bound to albumin and unbound.

Question 5

What percentage of calcium in the body is unbound?

Answer 5

45%.

Question 6

When are calcium requirements higher?

Answer 6

During pregnancy and nursing.

Question 7

How is calcium excreted?

Answer 7

Through urine and feces.

Question 8

If 20mmol of calcium is taken in per day, how much is excreted via the feces?

Answer 8

18mmol.

Question 9

What does calcitonin do and what is it?

Answer 9

It’s a hormone secreted by the thyroid that lowers the blood calcium.

Question 10

What does parathyroid hormone do?

Answer 10

It increases calcium levels when they are too low.

Question 11

What is calcitriol?

Answer 11

An active form of vitamin D that is essential for calcium absorption and bone mineralization.

Question 12

What is the structure of parathyroid hormone?

Answer 12

It is a single chain polypeptide made from 84 amino acids that has a molecular weight of 9500. It is derived from the larger precursor peptides pre-proPTH and proPTH.

Question 13

Where is parathyroid hormone produced?

Answer 13

Chief cells of the parathyroid gland.

Question 14

What are the normal plasma levels of PTH?

Answer 14

10-55 pg/ml.

Question 15

How is PTH secretion regulated on a short-term scale?

Answer 15

Calcium ions act on the G protein coupled calcium sensing receptor. If there is decreased free plasma calcium there is an increase in PTH secretion.

Question 16

How is PTH secretion regulated on a long term scale?

Answer 16

1,25(OH)2D3 acts directly on the PTH to decrease preproPTH mRNA.

Question 17

What are the organs in which PTH acts to increase plasma calcium levels?

Answer 17

The kidney, the bone and the GI tract.

Question 18

What are the ways in which PTH acts on the kidneys to increase plasma calcium levels?

Answer 18

It stimulates calcium reabsorption in the distal tube, it inhibits PO43- reabsorption in the proximal tubule and increases the activity of 1-alpha-hydroxylase and decreases 2,4-hydroxylase and the net effect is a gradual increase in renal production of 1,25(OH)2D3.

Question 19

What are the ways in which PTH acts on the bone to increase plasma calcium levels?

Answer 19

It stimulates the rapid efflux of calcium from freely exchangeable calcium pool (an effect on osteocytes and bone-lining cells) and increases the number and activity of osteoclasts via action on osteoblasts which causes a gradual increase in bone resorption (calcium and PO43- release).

Question 20

What are the ways i which PTH acts on the GI tract to increase plasma calcium levels?

Answer 20

It stimulates the absorption of calcium and phosphorus ions. The effect is delayed and indirect.

Question 21

What is 1,25(OH)2D3?

Answer 21

The active metabolite of vitamin D3 it is a secosteroid (open B ring).

Question 22

Where is 1,25(OH)2D3 produced?

Answer 22

In the kidney by 1-alpha hydroxylation of 25(OH)D3.

Question 23

What is transcalciferin?

Answer 23

A vitamin D-binding protein that 1,25(OH)2D3 is bound to.

Question 24

What happens to vitamin D3 in the body?

Answer 24

It is rapidly converted in the liver to 25(OH)D3.

Question 25

What determines plasma levels of 1,25(OH)2D3?

Answer 25

The rate of conversion of 25(OH)D3 to 1,25(OH)2D3 and the rate of conversion of this to 24,25(OH)2D3. The first step is the activation step and the second step is the inactivation step.

Question 26

What is the effect of 1,25(OH)2D3 on the GI tract?

Answer 26

It stimulates the absorption of Ca2+ in the ileum and the absorption of PO43- in the jejunum and ileum.

Question 27

What is the effect of 1,25(OH)2D3 on the bone?

Answer 27

It increases the number and activity of osteoclasts which leads to an increase in bone resorption, resulting in Ca2+ and PO43- release.

Question 28

What is the effect of 1,25(OH)2D3 on the kidney?

Answer 28

It facilitates calcium reabsorption.

Question 29

What is the structure of calcitonin?

Answer 29

A single chain polypeptide consisting of 32 amino acids and a molecular weight of 3500.

Question 30

Where is calcitonin secreted?

Answer 30

Parafollicular ‘C’ cells of the thyroid gland.

Question 31

What is the secretion of calcitonin regulated by?

Answer 31

Calcium and gastrin - if levels of these increase the secretion increases.

Question 32

What is the effect of calcitonin secretion?

Answer 32

There is a fall in plasma calcium levels.

Question 33

What effect does calcitonin have on the bone?

Answer 33

It decreases the release of calcium and phosphate ions and decreases the rapid efflux across the bone membrane and acts directly on osteoclasts to inhibit bone resorption.

Question 34

What effect does calcitonin have on the kidneys?

Answer 34

It decreases tubular reabsorption of calcium and phosphate ions.

Question 35

What is the physiological roles of calcitonin?

Answer 35

It may protect against postprandial hypercalcaemia, protect the female skeleton during pregnancy and lactation and may act to prevent excessive bone destruction.

Question 36

What are the features of hypercalcaemia?

Answer 36

It is associated with XS parathyroid hormone and affects bone, kidneys, GI tract as well as neurological symptoms.

Question 37

What are the features of hypocalcaemia?

Answer 37

It is due to lack of PTH effect, lack of vitamin D intake (diet or drug interaction), the symptoms are related to neuromuscular excitability. Long term lack of vitamin D affects bone growth.