Endocrine 9: Vitamin D Flashcards
1
Q
Formation of Vitamin D
A
- sunlight convertes 7-dehydroxycholesterol into cholecalcifirol
- Cholecalcifirol is converted in the liver to 25-hydroxy-cholecalciferol (25-HCC)
- 25-HCC is converted in PCT of kindey into 1,25DHCC (activated Vitamin D) under the effect of alpha-hydroxylase enzyme
2
Q
Nature of Vitamin D
A
Steroid
3
Q
all vitamin D moves in the blood by:
A
Being bound to globulin —> Vitamin D binding protein (DBP)
4
Q
Mechanism of action of Vitamin D
A
- Binds to cytoplasmic receptor
- the hormone receptor complex enters the nucleus
- it’s facilitate transcription of mRNA of calcium binding protein called Calbindin D
5
Q
Most common sites of Vitamin D receptors
A
- Intestines
- Bone
- Kidney
6
Q
action of vitamin D on the intestines
A
- stimulates formation of Calbindin D
- Calbindin D transports calcium from brush border to the basolateral membrane
- also, it stimulates calcium-ATP pump at the basolateral border, and the alkaline phosphatase enzyme
7
Q
Action of Vitamin D on the kidney
A
- Increases calcium reabsorption by Calbindin D
- increases for Phosphate reabsorption
8
Q
Action of Vitamin D in the bone is variable, and it depends on:
A
Concentration of Ca2+ & Phosphate:
* If high —> Osteoblastic activity
* If low —> (along with PTH) osteolysis osteocytic membrane System + Osteoblast release of RANKL & IL-6 for Osteoclast proliferation
9
Q
relationship between Vitamin D & Ca2+ concentration in the blood
A
Inversely proportional
10
Q
explain the regulation of Vitamin D in relation to plasma Ca2+
A
- Decreased Ca2+ increases PTH
—> PTH stimulates, alpha-1 hydroxylase enzyme –> increased vitamin D synthesis –> increased calcium absorption - vitamin D itself has a negative feedback on alpha-1 hydroxylase enzyme therefore limiting more formation of vitamin D
- vitamin D stimulates convertase enzyme, which inactivates vitamin D into 24،25DHCC
