Parathyroid hormone physiology and calcium-phosphate regulation
Core concept
- CaSR on the parathyroid chief cell senses ionised Ca -> a fall releases stored PTH within seconds
- PTH raises calcium and lowers phosphate - three actions, one receptor (PTH1R)
- Bone - PTH1R is on the osteoblast, not the osteoclast
- inc RANKL, dec OPG -> osteoclast recruitment -> Ca + PO4 released
- Kidney
- inc Ca reabsorption at DCT (TRPV5, cAMP-mediated)
- inhibits NaPi-IIa in PCT -> phosphaturia
- stimulates 1-alpha-hydroxylase -> calcitriol
- Gut - indirect only, via calcitriol -> TRPV6 -> inc Ca and PO4 absorption
- Bone - PTH1R is on the osteoblast, not the osteoclast
- Net phosphate falls because the renal leak outweighs the bone and gut gain
- PTH and FGF-23 are the two phosphaturic hormones
- Discriminator: PTH stimulates 1-alpha-hydroxylase, FGF-23 suppresses it
3 more sections, plus exam facts
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