Calcium Homeostasis

Question 1

What body functions is calcium important for?

Answer 1

o Growth and maintenance of skeletal system 
        - Bone and tooth formation
o Neurotransmitter release
o Muscle contraction
        - Smooth muscle activity 
o Hormone secretion 
        - Secretory activity of glands 
o Blood clotting 
o Intracellular Ca2+ signalling 
o Apoptosis

Question 2

What can block calcium absorption?

Answer 2

Alcohol and cigarettes

Question 3

What 2 hormones does the regulation of calcium homeostasis involve?

Answer 3

Calcitonin and parathyroid hormone

Question 4

What can abnormal Ca2+ regulation cause?

Answer 4

Disease

Question 5

How is calcium involved in bone turnover?

Answer 5

During resorption, there is a transfer of calcium from bone fluid to the blood

Question 6

What are the diseases of calcium homeostasis?

Answer 6

• Primary hyperparathyroidism
• Secondary hyperparathyroidism (renal failure)
• Osteoporosis
• Rickets (vitamin D deficiency or mutation affecting vitamin D synthesis)
• Calcium stones
• Receptor mutations (PTH receptor, extracellular Ca2+ sensing receptor)

Question 7

What produces calcitonin?

Answer 7

Parafollicular C cells that are found between follicle cells in the thyroid gland

Question 8

What does the thyroid gland secrete?

Answer 8

o T4 – Levothyroxine (Thyroxine) 80%

o T3 - Tri-iodothyronine (Liothyronine) 20%

Question 9

How much T3 and T4 are protein bound in the blood?

Answer 9

99%

Question 10

Are thyroid hormones available as pharmaceuticals?

Answer 10

Yes:
o Levothyroxine = oral (most common)
o Liothyronine = oral or parenteral

Question 11

What is the half life of calcitonin?

Answer 11

5 mins

Question 12

What is the main function of calcitonin (thyrocalcitonin)?

Answer 12

To reduce blood calcium [Ca2+] ion concentration.

*It also opposes the effect of parathyroid glands

Question 13

Where are the parathyroid gland(s) located?

Answer 13

FOUR parathyroid glands located on the posterior surface of thyroid gland (below)

Question 14

What are the 2 cells of the parathyroid gland?

Answer 14

• Chief cells (few in number)

- Oxyphil cells (unknown function)

Question 15

What do Chief cells produce?

Answer 15

Parathyroid hormone (PTH), a small helical protein hormone of 84aa (half life <20 mins)

Question 16

What is the function of PTH?

Answer 16

To INCREASE blood [Ca2+] when it gets too low

Question 17

How is low [Ca2+] detected?

Answer 17

Detected by the G-protein-coupled Ca2+ sensing receptors (caSR)

Question 18

How does PTH raise [Ca2+] ?

Answer 18

o Indirectly stimulating osteoclasts to release more Ca2+ from bone (bone resorption)
o Increasing renal Ca2+ reabsorption (i.e. reducing Ca2+ excretion via filtration)
o Increasing production of vitamin D, which stimulates the uptake of Ca2+ from the intestine, as well as increasing bone resorption and renal Ca2+ reabsorption

(OPPOSITE effect to calcitonin)

Question 19

How is calcium ion increase regulated?

Answer 19

By a negative feedback:

• Increase in blood [Ca2+] decreases PTH secretion
• Increase in [Ca2+] also increases bone formation

Question 20

How does calcitonin decrease blood [Ca2+]?

Answer 20

Reduces osteoclast activity to reduce bone resorption and allows rapid bone deposition by osteoblasts

Question 21

How is decreasing blood [Ca2+] regulated?

Answer 21

o Decrease in blood [Ca2+] decreases calcitonin secretion

o Decrease in blood [Ca2+] reduces bone formation

Question 22

Do high/low levels of calcitonin cause disease?

Answer 22

NO - minor effects