Calcium homeostasis

Question 1

Where is the majority of calcium stored?

Answer 1

In the bone (extracellular matrix)

Question 2

What form is calcium in the blood?

Answer 2

50% ionised
40% bound
10% in complexes

Question 3

What is the normal total calcium?

Answer 3

2.2-2.6mmol/L

Question 4

What re the 2 lines of defence against fluctuations?

Answer 4

Buffering

Hormonal control

Question 5

Which 3 hormones are involved in the regulation of ionised calcium?

Answer 5

PTH
Calcitonin
Calcitriol

Question 6

What are the 4 actions of PTH?

Answer 6

1= bone (fast) phase from bone fluid
2= bone (slow) phase from bone
3=kidney, reabsorption within tubules
4= intestine, indirect effect through the activation of vit D

Question 7

Describe the fast phase

Answer 7

• Begins in minutes and progressively increases for hours
• Acts on OSTEOCYTES and OSTEOBLASTS
• Increased uptake from bone fluid
• The PTH interacts with receptors on the osteocytes and osteoblasts, this increases permeability to calcum
• Increased calcium uptake from bone fliud
• Calcium in the bone fluid is replaced by calcium phosphate crystals.

Question 8

Describe the slow phase

Answer 8

• Activation of OSTEOCLASTS
• Osteoblasts have no receptors for PTH, signal comes from activated osteocytes and oesteoblasts
• Involves activation of existing osteoclasts and formation of new ones.

Question 9

Describe the kidney phase of action for PTH

Answer 9

• Increases Ca reabsorption in late DCT and CD
• Results in retention of Ca and Mg
• Decreases phosphorus reabsorption in PCT
• All results in rapid loss of phosphorus

Question 10

Which enzyme activates vit D in the kidney?

Answer 10

-1 alpha hydoxylase

Question 11

What activates 1-alpha hydroxylase?

Answer 11

PTH

Question 12

What effect does calcitriol have?

Answer 12

• Increases uptake of calcium from the intestinal epithelial cells
• Increases synthesis of calcium binding protein (increases conc of calcbindin)
• This is a SLOW process- take a couple days
• Activates calcium ATPase pumps on basolateral membrane

Question 13

Discuss the important of vitamin D on normal bone health

Answer 13

• Very important

- Without vit D bone cannot reabsorbed in response to PTH

Question 14

Name the 2 things that activate vit D

Answer 14

• PTH

- Low blood phosphorus

Question 15

How is calcitonin secretion regulated?

Answer 15

Mainly by the plasma concentration of ionised calcium- stimulated by hypercalcaemia

Question 16

What are the effects of calcitonin?

Answer 16

• Fast phase bone resorption= inhibits osteoclast activity

- Slow phase bone response= reduces formation of new osteoclasts

Question 17

Is magnesium hormonally controlled?

Answer 17

NO