Calcium and phosphate homeostasis

Question 1

What is the function of intracellular calcium?

Answer 1

Second messenger
Signalling pathways of many cells
(needs controlling +-2%)

Question 2

What 3 factors regulate plasma calcium concentration?

Answer 2

1. PTH - parathyroid hormone released in response to dec plasma [Ca2+] conc increasing it AND activated Vit D 3
2. Vitamin D: (Calcitrol)
   Increases Ca2+ from SI, inc renal absorption and metabolism of Ca from bones
3. Calcitronin- from Thyroid gland responds to inc Ca blood plasma by decreasing it

Also in renal disease, inc P forms complexes with Ca = decreasing [Ca in blood] and inc P inc PHT

Question 3

What is the major function of Vitamin D3? What is D3 also known as?

Answer 3

Regulates calcium absorption from the gut

Cholecaliferol, converted to calcitriol (bioactive D3) in the kidneys

Question 4

Urinary excretion of calcium normally remains constant. When might it increase and decrease?

Answer 4

If increased Ca in blood = increased excretion and vice versa

Question 5

How is urinary excretion of calcium reduced?

Answer 5

Calcitriol (D3) and PTH

Cause increased calcium absorption in renal tubulese

Question 6

What are some functions of phosphate?

Answer 6

Part of boy
ATP, ADP etc
Act as buffers in cells and in H+ excretion by the kidney

Question 7

What regulates phosphate absorption? Is this active or passive?
How is the regulation of phosphate different to reg of Ca?

Answer 7

Calcitriol (D3)
Active - requires energy

1. The regulation of phosphate largely depends on urinary excretion
2. The regulation of calcium largely depends on differential absorption of calcium from the small intestine
3. Parathyroid hormone (PTH) decreases urinary excretion of calcium and increases urinary excretion of phosphate

Question 8

What does phosphate deposition in the bone depend on?

Answer 8

Plasma conc of Para Thyroid Hormone and calcitriol - mobilises calcium and phosphate from the bone matrix

Question 9

Where is phosphate excreted? What is the effect of PTH on phosphate?

Answer 9

Kidneys

Inhibits reabsorption, increases excretion

Question 10

Where is PTH synthesised? How is it stored?

Answer 10

Parathyroid glands

As vesicles

Question 11

What is the main factor that controls PTH secretion?

Answer 11

Amount of ionised calcium in the blood
Low Ca2+ = stimulate parathyroid gland to inc secretion of PTH
High Ca2+ = decreases secretion

Question 12

How does hyperphosphataemia alter PTH secretion?

Answer 12

Increases PTH secretion, as PTH decreases phosphate

Question 13

How does PTH increase calcium concentration? (3 ways)

Answer 13

Mobilises calcium from the bone
Increases reabsorption by the kidneys
Stimulates synthesis of D3, which increases calcium absorption from gut

Question 14

What are the main effects of PTH on calcium and phosphate in the blood?

Answer 14

Increases calcium

Decreases phosphate

Question 15

What is vitamin D/calciferol? What is their main action?

Answer 15

Lipophilic precursors

Calcium homeostasis - increases calcium absorption in gut, decreases secretion from kidney, mobilises calcium from bone

Question 16

How can vitamin D3 be made? (2 ways). What happens to it after?

Answer 16

Cholecalciferol - made in the skin following UVB light exposure or by cholesterol in intestinal wall
Converted to calcitriol by the kidneys

Question 17

What is vitamin D2?

Answer 17

Dietary ergocalciferol

Question 18

What regulates the conversion of calcefidiol to calcitriol? What is this also influenced by?

Answer 18

PTH

Also influenced by P concentration and calcitriol itself (negative feedback)

Question 19

What are the main functions of calcitriol?

Answer 19

Increase calcium reabsorption in kidneys
Increase absorption of Ca and P from gut
Mobilise Ca from bone

Question 20

Calcitonin is a peptide hormone. Where is it secreted from? What causes secretion?

Answer 20

C cells of thyroid follicles

Hypercalcaemia

Question 21

What is the main action of calcitonin? What else does it do?

Answer 21

Decrease bone reabsorption of calcium - mainly acts on bone
Decreases calcium reabsorption by the kidneys

Question 22

How do glucocorticoids affect bone formation?

Answer 22

Inhibit osteoblast differentiation

Stimulate osteoclast activity

Question 23

What is an important adverse effect of glucocorticoid administration?

Answer 23

Osteoporosis

Question 24

Apart from the expected hormones (calcitonin, PTH, D3), what else can have an impact on calcium homeostasis?

Answer 24

Growth hormone

Sex steroids

Question 25

What are calcium salts given for? When might they be given?

Answer 25

Replace calcium

Dietary deficiency, post parturient hypocalcaemia, hyperphosphataemia

Question 26

How is calcium carbonate used to treat hyperphosphataemia?

Answer 26

Acts as an antacid

Poorly absorbed in gut, binds to phopshate

Question 27

How can calcium be administered? What should be given with it if administering orally?

Answer 27

Orally or parenterally

Oral - give with vitamin D as poorly absorbed from diet

Question 28

What are the adverse affects of calcium salts when given orally or via IV?

Answer 28

Orally - GI upset

IV - cardiac arrhythmias and arrest

Question 29

How are vitamin D preparations given? What do they do?

Answer 29

Orally

Mange chronic hypocalcaemia

Question 30

When may chronic hypocalcaemia occur?

Answer 30

After thyroidectomy
Immune mediated hypoparathyroidism
Osteodystrophy

Question 31

What are the adverse effects of vitamin D preparations?

Answer 31

Small therapeutic index (effective dose vs toxic dose)
Hypercalcaemia
Hyperphosphataemia
Nephrocalcinosis