PTH
synthesised as prohormone, then cleaved in Golgi apparatus and endoplasmic reticulum to hormone of 84 amino acids
9500 Da
fragments are metabolically active
Effect = ↑ plasma calcium, ↓ plasma phosphate
🏠 secreted by chief cells of parathyroid glands
regulation by disinhibition
adenylyl cyclase cAMP 2nd messenger system
Effect by system
| Site | Action |
|---|---|
| Bone | Increased osteocytic osteolysis Increased osteoclast activity via OPGL (RANK ligand) resulting in ↑ Ca2+, phosphate by bone breakdown (resorption) |
| Kidney | Increases Ca2+ reabsorption by ↑ activation of vitamin D in distal tubule Inhibits PO4 reabsorption in proximal tubule |
| Intestine | Indirectly enhances absorption of Ca2+, PO4 ions activates vit D = ↑ Ca reabsorption (converts 25-hydroxy vitamin D into 1,25-dihydroxy vitamin D (calcitriol)) |
Homeostasis
Trigger/stimulus
- ↑ serum PO4
- Adrenaline, histamine
- ↓ serum Ca