Regulation of Calcium and Phosphate

Question 1

Where is most of our calcium located?

Answer 1

Skeleton and teeth- extracellular ionised calcium is a biologically active component

Question 2

2 sources of Vitamin D?

Answer 2

Diet and skin synthesis from sunshine- slight structural differences, D2 and 3

Question 3

3 steps of Vitamin D metabolism?

Answer 3

1. UVB light catalyses 7-dehydrocholesterol to pre-vitamin D3 and then vitamin D3. Transported to liver.
2. 25-hydroxylase converts Vitamin D3 to 25(OH) cholecalciferol. Transported to kidneys
3. 1-alpha-hydroxylase acts to produce 1,25(OH)2 cholecalciferol (calcitriol)

Question 4

What substrate is used an as indicator of body Vitamin D status?

Answer 4

25(OH) cholecaliciferol

Question 5

How is calcitriol regulated?

Answer 5

It regulates itself by decreasing transcription of 1-alpha hydroxylase

Question 6

How does calcitriol increase calcium and phosphate?

Answer 6

Increases calcium absorption from bones
Increases Ca2+ from kidney
Increases Ca2+ from gut
Increases phosphate absorption from gut

Question 7

Calcitriol action in the bones?

Answer 7

Low serum calcium- activates osteoclasts to increase Ca2+ absorption from bones
Normal serum calcium- activates osteoblasts to increase bone formation

Question 8

PTH secreted by?

Answer 8

Chief cells in parathyroid glands as a large precursor molecule and then cleaved

Question 9

Chief cells receptors?

Answer 9

G-coupled protein receptors

High extracellular Ca2+ binds to receptors which inhibits PTH secretion

Question 10

PTH effects?

Answer 10

Ca2+ absorption from skeleton
Ca2+ reabsorption and phosphate excretion in kidneys- stimulates 1-alpha-hydroxylase activity
Ca2+ and phosphate absorption from gut

Question 11

How is PTH regulated?

Answer 11

Negative feedback from increased serum calcium and 1,25(OH)2D3 synthesis

Question 12

Calcitonin secreted by and 2 effects?

Answer 12

Parafollicular thyroid cells
Decreases osteoclast activity
Increases Ca2+ in kidney

Question 13

Fibroblast Growth Factor 23?

Answer 13

FGF23 reduces phosphate levels by:
inhibiting Na+/PO43- co-transporter in proximal tube cells of kidney
Inhibits calcitriol to reduce phosphate reabsorption from gut

Question 14

Hypocalcaemia causes?

Answer 14

Low PTH levels- hypoparathyroidism, surgery, auto-immune disorder, magnesium deficiency
Vitamin D deficiency- inadequate sun exposure, malabsorption or dietary insufficiency, liver disease, renal disease

Question 15

Hypocalcaemia symptoms?

Answer 15

Paraesthesia
Convulsions
Arrhythmias
Tetany- contract but can’t relax muscles

Question 16

Chvostek’s sign?

Answer 16

Tap facial nerve below zygomatic arch

Positive response= twitching of facial muscles

Question 17

Trousseau’s sign?

Answer 17

Inflation of BP cuffs for several minutes to induce carpopedal spasm

Question 18

Hypercalcaemia symptoms?

Answer 18

Stones-renal effects
Abdominal moans- GI effects, anorexia, nausea, constipation, dyspepsia
Psychic groans- fatigue, depression, impaired concentration, altered mentation

Question 19

Hypercalcaemia causes?

Answer 19

Primary hyperparathyroidism- parathyroid adenoma
Malignancy- bony metastases produce local factors to activate osteoclasts
Vitamin D excess (rare)