Metabolism S11 - Calcium Metabolism

Question 1

What organs/systems regulate calcium balance in the body?

Answer 1

• Gut
• Kidneys
• Bone

Question 2

List the uses of calcium in the body

Answer 2

• Maintaining bone and teeth
• Regulates heart rhythm
• Nerve transmission at neuromuscular junction
• Reduces blood cholesterol
• Assists blood clotting
• Normal kidney function
• Lowers blood pressure
• Maintains nerve and muscle function

Question 3

Where is calcium stored in the body?

Answer 3

99% stored as hydroxyapatite crystals in the skeleton 1Kg of calcium found in humans

Question 4

In what forms are calcium found in the plasma?

Answer 4

• Biologically active free ionised Ca+
• Bound to plasma protein Eg Albumin
• Bound to citrate (low organic anions)

Question 5

Describe the structure and half-life of PTH

Answer 5

• Polypeptide hormone
• Synthesised as pre pro hormone and cleaved to 84 aa
• Half-life of 4 minutes

Question 6

What is the role of parathyroid hormone?

Answer 6

Increases serum calcium concentration

• Stimulates bone resorption
• Stimulates calcium reabsorption and excretion of phosphate
• Stimulates hydroxylation of vitamin D to active calcitrol

Question 7

How is synthesis of parathyroid hormone regulated?

Answer 7

• Low serum calcium up regulates transcription of PTH in chief cells
• High serum calcium down regulates transcription of PTH Negative feedback mechanism

Question 8

How is high calcium levels detected by chief cells?

Answer 8

• Calcium ions bind to G-protein coupled calcium receptors
• G protein disassociates and stimulates phospholipase C
• This inhibits adenylyl cyclase leading to reduced cAMP and reduced PTH

Question 9

How does parathyroid hormone impact vitamin D?

Answer 9

• Stimulates conversion of vitamin D to its active form which increases calcium uptake in the gut

Question 10

What are the two forms of vitamin D?

Answer 10

D3 - cholecalciferol - made form the skin and from dairy

D2 - ergocalciferol - from yeast and fungi

Question 11

How is active calcitriol produced?

Answer 11

• Vitamin D3 or D2 undergo two hydroxylation reactions
• One in liver to 25-hydroxyvitamin D - Second in kidney to produce active calcitriol

Question 12

What is the cause of hypercalcaemia?

Answer 12

• Over secretion of PTH due to primary hyperparathyroidism
• Usually caused by parathyroid adenoma or PTH related peptide (PTHrP) from various tumours

Question 13

What are the symptoms of hypercalcaemia?

Answer 13

Renal calculi, kidney damage, constipation, dehydration, tiredness and depression

Question 14

How is hypercalcaemia treated?

Answer 14

• Fluid replacement
• Usually removal of tumour (which could be secreting parathyroid hormone related peptide (PTHrP)

Question 15

What is hypocalcaemia?

Answer 15

Low serum calcium due to PTH deficiency typically caused by surgical removal of parathyroid glands

Question 16

What are the symptoms of hypocalcaemia?

Answer 16

Hyper-excitability of the neuromuscular junction, pins and needles, tetany, paralysis, convulsions…..death

Question 17

How are phosphate levels regulated in the body?

Answer 17

• PTH stimulates phosphate excretion in kidney so reduces likelihood of calcium-phosphate stone formation
• Vitamin D stimulates uptake of both calcium and phosphate in the gut