Calcium Phosphate Pyhsiology Dr. Pond

Question 1

Function of Parathyroid hormone (PTH)

Answer 1

-increases serum Ca2+
-decreases serum PO3- (phosphorus)
-increases active Vitamin D

main target: kidney

Question 2

PTH Physiology

Answer 2

in the kidney

-increase Ca reabsorption (decreases excretion) -> more serum Ca
-increases PO3- excretion -> less serum PO3-

-increases production of active Vitamin D

Question 3

How is PTH regulated?

Answer 3

-high level of Ca:
Calcium-sensitive protease cleaving PTH into fragments
calcium-sensing receptor (CaR): PTH will be turned OFF when Ca binds CaR

-Phosphate (indirect): if PO3- is high ->
PO3- binds Ca (Ca3PO4) -> so CaR will be free, and PTH stays ON -> PTH will decrease PO3-

-active Vitamin D inhibits PTH

Question 4

Function of Vitamin D

Answer 4

-increases serum Ca2+
-increases serum PO3- (phosphorus)

main target: Gut

Question 5

Vitamin D Physiology

Answer 5

mainly in the gut:
-increases Ca and PO3- absorption

in the kidney: weak
-increase Ca and PO3- reabsorption (decrease in excretion)

Question 6

What are the different Vitamin D forms?

Answer 6

-Vitamin D3 (Cholecalciferol): diet and skin (7-dehydrocholesterol to cholecalciferol via UV and heat)

-Vitamin D2 (Ergocalciferol): plants, yeast, fortified milk

Question 7

How is Vitamin D activated?

Answer 7

works with Vitamin D2 or D3
1. in the liver: add OH on the 25 carbon (via 25-hydroxylase)

2. in the kidney: add OH to the 1st carbon (1-hydroxylase)

the product: Calcitriol (1,25 (OH)2 D3) Rx

Question 8

How does PTH stimulate Vitamin D production?

Answer 8

by stimulating the 1-alpha-hydroxylase in the kidney

Question 9

What would happen to active Vitamin D production if Ca and PO3- are high?

Answer 9

the production would shift to the side product of 1-alpha-hydroxylase -> secalciferol (24,25 (OH)2 D3)

Question 10

How is Vitamin D production downregulated?

Answer 10

by FGF-23 -> inhibiting 1-alpha-hydroxylase in the kidney

Question 11

Interaction of Vitamin D

Answer 11

-inhibits PTH
-increases FGF-23

Question 12

Where is FGF-23 produced?

Answer 12

-in the bones by osteoblasts and osteocytes
-binds to FGF-I and IIIc receptor in the presence of Klotho receptors in the kidney

Question 13

Function of FGF-23

Answer 13

-decreases serum Ca2+
-decreases serum PO3- (phosphorus)

main target: kidney

Question 14

FGF-23 Physiology

Answer 14

in the kidney:
-increases Ca and PO3- excretion

-also inhibits 1-alpha-hydroxylase to lower active Vitamin D production (since active Vit D increases Ca and PO3-)

Question 15

How is FGF-23 regulated?

Answer 15

think of FGF-23 as down regulator: it lowers Ca and PO3-

-high active Vitamin D stimulates FGF-23
-high PO3- stimulates FGF-23
-> to bring them down

Question 16

How does FGF-23 affect active Vitamin D production?

Answer 16

FGF-23 is a down-regulator (decreases Ca and PO3-)

-decreases active vitamin D by inhibiting 1-alpha-hydroxylase

Question 17

Function of Calcitonin

Answer 17

-decreases serum Ca2+
-decreases serum PO3- (phosphorus)

main target: bones

Question 18

Calcitonin Physiology

Answer 18

mainly in the bones:
-keeps Ca and PO3- in the bones

in the kidney:
-increases Ca and PO3- excretion