Endocrine Control of Calcium Balance

Question 1

What is the role of calcium in the body?

5

Answer 1

Cell signalling - Increase in intracellular Ca triggers exocytosis of vesicles
Blood Clotting - Essential component of clotting cascade
Apoptosis - Programmed cells death
Skeletal strength - 99% of Ca stored in bones and gives strength to skeleton
Membrane Excitability - Ca decreases Na permeability

Question 2

What is the relationship between Ca changes and Membrane excitability?

Answer 2

Hypocalcemia = Increases Na permeability and then hyperactivity of neurones which can lead to tetany

Hypercalcemia = Decrease in Na permeability which reduces excitability and depresses neuromuscular activity leading to cardiac arrhythmias

Question 3

What percentage of Ca is physiologically active?

Answer 3

0.05% - Free in solution

Question 4

What form is Ca stored in the Bones?

Answer 4

Hydroxyapatite (Ca10(PO4)6(OH)2)

As it has phosphate in it this is also important in determining Ca balance

Question 5

Where is Ca thats not stored in the bones found?

Answer 5

Intracellular:

• Mitochondria
• Sarcoplasmic reticulum

Extracelular:

• 50% is bound to proteins or plasma anions
• 50% is free in solution

Question 6

How does pH affect plasma Ca binding?

Answer 6

The affinity of plasma proteins for Ca increases with a drop in pH

Lower H2 Ions = More Alkaline
More plasma proteins bind with Ca
Less free Ca = Hypocalcemia

This is because the H2 Ions compete with Ca for the binding points on plasma proteins

Question 7

How do Osteocytes breakdown bone?

2

Answer 7

Release of proteolytic enzymes

Secrete H2 ions (create acidic environment)

Question 8

Which hormones play a role in Ca regulation?

3

Answer 8

Increasing Ca:
Parathyroid hormone (Polypeptide hormone / Parathyroid glands)
Calcitriol (Steroid hormone / Vit D in Liver & Kidneys)

Decreasing Ca:
Calcitonin (Peptide hormone / Parafollicular cells of the thyroid)

Question 9

How many parathyroid glands does the average person have?

Answer 9

4

Posterior side of the thyroid gland

Question 10

What are the affects of parathyroid hormone?

Answer 10

Stimulates Osteoclast
Increases bone resorption which releases Ca
Effects seen in 12-24hours

Inhibits Osteoblasts

Increases renal reabsorption of Ca

Increases renal exertion of Phosphate (prevents Ca from being reabsorbed into bone)

Question 11

What are the affects of Calcitrol hormone?

Answer 11

Vitamin D precursor is in the skin
Converts inactive vitamin D into Active vitamin D in the liver and then kidneys
Also needs Parathyroid hormone to covert in the kidney
Increases Ca permeability though the GI tract

Question 12

What are the affects of Calcitonin hormone?

Answer 12

Stops the Osteoclasts from releasing Ca
Promotes renal excretion of Ca

any excess Calcitonin is overridden by PTH

Question 13

What are the other hormones alter Ca Balance?

5

Answer 13

Cortisol:

• Inhibits osteoblasts
• Increases renal excretion of Ca & Phosphate
• Decreases plasma Ca
• Increase in PTH = Bone resorption = Bone loss

Insulin:

• Increases bone formation and antagonises cortisol
• May lead to bone loss
• Decrease in plasma Ca

Oestrogen:

• Promotes bone formation via oestrogen receptors on osteoblasts
• Decrease in plasma Ca

Growth hormone:

• Constant stimulus for bone formation
• Decrease in plasma Ca

Prolactin:

• Stimulates synthesis of calcitrol
• Promotes calcium absorption from Gut
• Increase in plasma Ca