Calcium and Phosphorus

Question 1

Functions of calcium

Answer 1

Bone mineralization (99%) and ionized as Ca 2+ messaging (1%)

Question 2

Describe calcium as a second messenger

Answer 2

1. Messenger opens calcium channel
2. Calcium enters cell through channel
3. Calcium acts as a second messenger, activating calmodulin
4. Activated calmodulin (Ca-calmodulin) activates enzymes: protein kinase
5. Enzyme then performs function

Question 3

How does Ca2+ enter the cell?

Answer 3

Diffusion or channels

Question 4

How is calcium removed from the cell

Answer 4

ATP dependent pumps that use Mg2+ and Na+ and ATPase

Question 5

Efficiency of calcium absorption

Answer 5

Low (20-30% effective).

Question 6

Methods of calcium absorption

Answer 6

• Transcellular (major route - occurs in duodenum): saturable, requires energy + channel + binding protein (calbindin)
• Paracellular (occurs in ileum/jejunum): non-saturable, energy independent, concentration dependent
• Colonic fermentation (large intestine - 4-10%): Fibre fermentation can release Ca2+

Question 7

Efficiency of phosphate absorption

Answer 7

60-70% effective (twice as much as calcium)

Question 8

What compounds increase calcium absorption?

Answer 8

1,25(OH)2D (active vitD), Estrogen, PTH

Question 9

What is required for absorption of phosphorus

Answer 9

Absorbed as phosphate
- requires magnesium
- can enter through diffusion
- can enter through carrier mediated active transport

Question 10

Function of PTH on Vit D

Answer 10

Regulates conversion of vit D to active form in the kidney

Question 11

Calcium regulation by PTH

Answer 11

1. Low blood Ca2+ signals PTH release
2. PTH binds to bone and triggers resorption/breakdown of bone mineral to release Ca2+
3. PTH acts on kidneys to synthesize active vit D and calcitriol
4. PTH and calcitriol act to reabsorb calcium from kidney into blood
5. Calcitriol goes to intestine and promotes calbindin synthesis to absorb more calcium across brush border from intestinal cells into basal
6. Calcium enters blood from (1) release from bone (2) release from kidneys (3) asorption from intestinal cells

Question 12

What kind of hormone is calcitonin

Answer 12

Peptide hormone from the thyroid gland; released when Ca2+ is high. Opposite effects of PTH.

Question 13

How much does bone remodelling occur

Answer 13

10% of skeletal bone mass is replaces each year in adults - remodelling is constant

Question 14

What is the purpose of bone remodelling?

Answer 14

To repair bone defects and maintain blood Ca2+ levels.

Question 15

What are osteoclasts vs. osteoblasts

Answer 15

Osteoclasts: destroy and resorb old bone
Osteoblasts: deposit new bone into place
(They are coupled)

Question 16

Modes of calcium absorption:

Answer 16

Transcellular - saturable, energy requiring, stimulated by low Ca2+ doets and calcitriol (genetic), mostly duodenum
Paracellular - nonsaturable, passive, mostly ileum/jejunum
Colonic fermentation - 4-10% Ca2+

Not highly effective in general (20-30%)

Question 17

Phosphate absorption

Answer 17

Absorbed linear to intake (mostly HPO4 2-), similar mechanisms as Ca2+ (can be transported actively when complexed with other minerals or passively as organic phosphate/free)
- Highly effective (60-70%); responds to calcitriol but lesser extent than Ca2+

Question 18

Renal reabsorption of Ca2+

Answer 18

Mostly paracellular, moves in parallel with water

Question 19

Phosphorus urinary excretion

Answer 19

Linear relationship with urinary excretion.