Calcium homeostasis

Question 1

Describe the process through which the body obtains vitamin D from the sun.

• where is it made?
• what are the intermediate molecules called?
• which organs are part of the process?
• which enzymes are involved?

Answer 1

UVB radiation from the sun triggers the conversion of cholesterol to vitamin D3 in keratinocytes.
Vitamin D3 goes to the liver via bloodstream.
In the liver Vitamin D3 is converted to calcidiol.
In the kidneys calcidiol is converted into calcitriol using the enzyme 1-α-hydroxylase.

Question 2

Hormone secreted by parathyroid gland relevant in calcium homeostasis. Function of the hormone.

Answer 2

Parathyroid hormone - secreted in response to LOW serum Ca levels functions to increase plasma Ca levels.
- Increases levels of the enzyme 1-α-hydroxylase which converts calcidiol to calcitriol in the kidneys. Calcitriol in turn increases blood Ca levels by increasing Ca reabsorption from the kidney tubules.
- increase osteoclast activity to release Ca from bone fluid and decreases osteoblast activity.
“Vitamin D helps body absorb calcium”.

Question 3

What is the function of the active form of vitamin D in calcium homeostasis? Name site and action.

Answer 3

Calcitriol (active form of vitamin D) acts on the intestine to increase Ca absorption, thereby increasing plasma Ca levels.

Question 4

What is Paget’s disease? How can it be treated?

Answer 4

When the cycle of bone renewal is disrupted causing weak and deformed bones.
Treated using salmon calcitonin.

Question 5

Describe PTH signalling in calcium homeostasis.

Answer 5

• PTH1R is expressed on osteoblasts and tubular cells of kidneys.
• when PTH binds, osteoblasts will produce the RANK-L.
  RANK-L binds to its receptor on osteoclasts.
• osteoclasts activity increase and demineralise the bones to release Ca and phosphate ions (bone resorption).

Question 6

What is the function of calcitonin? What triggers its release? Where is it released from?

Answer 6

Triggered due to hypercalcaemia so acts to reduce blood Ca levels.
Released by C cells of the thyroid gland.
- Increases osteoblast activity and decreases osteoclast activity.
- Also decreases Ca reabsorption from the kidney tubules by inhibiting reabsorption.