Biochem: Calcium Regulation

Question 1

Top two causes of hyperphosphatemia.

Answer 1

Renal Failure

Rhabdomyolysis

Question 2

What hormones cause the rapid and delayed response for calcium regulation?

Answer 2

Rapid: PTH
Delayed: Vit D

Question 3

Name the 3 major actions of PTH.

Answer 3

Main Target is kidney:

1. Increased Tubular Calcium Reabsorption
2. Increased 1 alpha-hydroxylase expression to activate Vit. D

Also acts on the bone
3. Increases osteoclast activity

Question 4

What is the importance of the carboxyl-fragment sequence of the parathyroid hormone?

Answer 4

It is the portion of the PTH protein that has the longest half-life. In chronic renal failure it will accumulate in the blood. This is because the kidneys can’t reabsorb calcium and the body will secrete a lot of PTH to try and raise plasma calcium levels, resulting in high levels of the C-fragment sequence.

Question 5

What are the genomic and non-genomic effects of calcium binding the calcium receptor (CaR) on the chief cells of the parathyroid gland?

Answer 5

Genomic: when calcium binds the CaR, it halts transcription of the PTH gene preventing PTH formation.

Non-genomic: calcium binding prevents vesicles containing PTH from fusing and exocytosing contents into the blood.

Question 6

When calcium binds CaR, what signaling mechanism is used to stop PTH formation?

Answer 6

Both Gq and Gi,

The result is a decrease in cAMP

Question 7

What effect does epinephrine have on PTH secretion from chief cells and by what mechanism?

Answer 7

Increases PTH secretion by Gs pathway. The rise in cAMP allows vesicle fusion and exocytosis.

Question 8

Signaling pathway that PTH induces when it binds to target cells.

Answer 8

Both Gs and Gq

Question 9

Name the 3 major functions of Vit. D

Answer 9

1. Increases duodenal absorption of calcium
2. Increases osteoclast activity
3. Negative feedback on parathyroid chief cells by decreasing PTH gene transcription

Question 10

Describe the formation of the active form of Vit. D.

Answer 10

1. 7-dehydrocholesterol stored in skin
2. UV-B light converts it to Cholecalciferol and it is released into the blood
3. 25-hydroxylase converts cholecalciferol into 25-hydroxycholecalciferol in the liver
4. 1 alpha-hydroxylase converts 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol in the kidney

Question 11

How does the body prevent excessive serum Vit. D levels even with continued UV exposure?

Answer 11

There are two storage forms of Vit. D

1. Lumisterol
2. Tachysterol

Question 12

Source of calcitonin

Answer 12

Thyroid C cells

Question 13

What are the two clinical uses for Calcitonin?

Answer 13

1. Tumor marker for Medullary Carcinoma of the Thyroid

2. Inhibitor of bone resorption to prevent osteoporosis

Question 14

What effect does Vit. D have on serum phosphate concentration?

Answer 14

Increase

Question 15

What effect does PTH have on serum phosphate concentration?

Answer 15

Decrease