calcium homeostasis

Question 1

why is calcium homeostasis important?

Answer 1

Ca2+ ion plays an important role in many biological processes.

Question 2

what do Ca2+ ions play an important role in?

Answer 2

• Excitation-contraction coupling
• Excitation-secretion coupling
• Excitability of nerves and muscle
• Clotting of blood

Question 3

what do muscle contractions require in terms of Ca2+ concentration?

Answer 3

an increase in intracellular Ca2+ ion concentration

Question 4

what happens if there is an increase in Ca2+ ion concentration?

Answer 4

an increase in extracellular Ca2+ ion concentration decreases neuromuscular contractility and hence reduces likelihood of muscle contraction occurring.

Question 5

what is normal plasma calcium level?

Answer 5

9-10 mg/dl (2.4±0.2 mmol/L)

Question 6

what makes up total body calcium?

Answer 6

• Bone - 99%
• Intracellular in soft tissues - 0.9%
• Extracellular fluid - 0.1%

Question 7

which hormones control calcium homeostasis directly?

Answer 7

Parathyroid hormone or Parathormone (PTH)

Vitamin D metabolite (1,25-dihydroxyvitamin D3 or Calcitriol

Calcitonin

Question 8

where is parathyroid hormone secreted from?

Answer 8

Chief cells of parathyroid glands (4 glands embedded in the back surface of thyroid gland)

Question 9

where is PTH metabolised?

Answer 9

by the liver and kidney

Question 10

what is the half-life of PTH?

Answer 10

2-5 minutes

Question 11

what controls PTH secretion?

Answer 11

The concentration of Ca2+ in plasma regulates parathyroid gland secretion

Question 12

what happens to PTH if there is low plasma Ca2+?

Answer 12

Low plasma Ca2+ cause increased secretion and high Ca2+ has an opposite effect

Question 13

what does sustained hypocalcaemia result in?

Answer 13

hyperplasia and hypertrophy of the gland.

Question 14

what is the primary function of PTH?

Answer 14

Maintain a normal level of Ca2+ in ECF

Question 15

what organs does PTH target?

Answer 15

• bone
• kidney
• intestine

Question 16

what are the effects of PTH on the kidney?

Answer 16

• Decrease Ca2+ excretion

- Increase PO43- excretion

Question 17

what are the effects of PTH on the intestine?

Answer 17

• Increases absorption of both Ca2+ & PO43- by activation of Vitamin D

Question 18

what are the effects of PTH on the bone?

Answer 18

• Fast Ca2+ efflux into the plasma from the bone fluid [Normal action]
• Slow transfer of Ca2+ and PO43- by dissolution of bone itself [during chronic hypocalcaemia]

Question 19

what is the cause of PTH deficiency?

Answer 19

Most common cause: operative procedure on either thyroid or parathyroid gland

Question 20

what are the effects of PTH deficiency?

Answer 20

Hypocalcaemia

Question 21

what are the signs and symptoms of PTH deficiency?

Answer 21

• Tetany of hands and feet with mechanical stimulation of peripheral nerves
• With complete removal of the glands death occur due to asphyxiation due to tetany of respiratory muscle
• Occasionally seizers can occur due to increased excitability in the brain

Question 22

what is the treatment for PTH deficiency?

Answer 22

Vitamin D and Calcium supplementation

Question 23

what causes excess PTH?

Answer 23

A tumour in one of the parathyroid glands

Question 24

what are the effects of excess PTH?

Answer 24

Hypercalcaemia & hypophosphataemia

Question 25

what are the effects of excess PTH on the bone?

Answer 25

Mild: no effect Severe: extensive decalcification with large punched out areas of bone filled with osteoclasts

Question 26

what are the neuromuscular effects of excess PTH?

Answer 26

``` muscle weakness decreased mentation poor memory constipation, abdominal pain (groan) peptic ulcer depressed relaxation of heart during diastole ```

Question 27

what are the effects excess PTH on the kidney?

Answer 27

Increased tendency for formation of kidney stones

Question 28

what is the treatment for excess PTH?

Answer 28

surgical removal of tumour if possible

Question 29

what doe metabolite vitamin D enhance?

Answer 29

intestinal absorption of calcium and phosphate the responsiveness of bone to PTH

Question 30

what does reduced calcium absorption lead to?

Answer 30

Reduced Calcium absorption leads to demineralisation of bone resulting from PTH action to maintain normal plasma Ca2+ level.

Question 31

what is osteoporosis?

Answer 31

a condition characterised by reduction in bone mass that result from bone resorption exceeding bone formation.

Question 32

what is calcitonin?

Answer 32

hypocalcaemic hormone, the effects generally opposite to those of PTH

Question 33

where is calcitonin produced?

Answer 33

by parafollicular cells or C cells of thyroid gland

Question 34

what stimulates the secretion of calcitonin?

Answer 34

stimulated by hypercalcaemia and inhibited by hypocalcaemia

Question 35

what is a therapeutic use of calcitonin?

Answer 35

To reduce hypercalcaemia, useful in disorders of increased skeletal remodeling such as Paget’s disease