Regulation of calcium metabolism

Calcium ion regulation:

[Ca²⁺] is increased by:

• Parathyroid hormone (Parathormone, PTH)
• 1,25 (OH)₂ vitamin D₃ (Dihydrocholecalciferol)

[Ca²⁺] is decreased by:

• Calcitonin

Actions of PTH:

On the kidneys:

• Increases Ca²⁺ reabsorption
• Increases PO^4- excretion
• Stimulates 1-α-hydroxylase activity (which increases 1,25 (OH)₂ vitamin D₃ synthesis)

On bone:

• Stimulates osteoclasts
• Inhibits osteoblasts

On the small intestine:

• Indirect action (by increasing the synthesis of 1,25 (OH)₂ vitamin D₃)
• Increases Ca²⁺ absorption
• Increases PO^4- absorption

Synthesis of 1,25 (OH)₂ vitamin D₃:

Cholecalciferol à 25-hydroxyxholcalciferol à 1,25-dihydroxycholecalciferol

The last step is catalysed by Stimulates 1-α-hydroxylase activity (in the kidneys)

Actions of 1,25 (OH)₂ vitamin D₃:

On the kidneys:

• Increases Ca²⁺ reabsorption
• Increases PO^4- reabsorption

On bone:

• Increases osteoblast activity

On the small intestine:

• Increases Ca²⁺ absorption
• Increases PO^4- absorption

Actions of calcitonin:

On the kidneys:

• Increases excretion of Ca²⁺, Na⁺ and PO₄^3-

On bone:

• Inhibits osteoclasts