Proximal tubule: phosphate reabsorption and PTH
Proximal tubule · detail · NaPi-IIa PTH FGF23
About 80-85% of filtered phosphate is reclaimed by apical sodium-phosphate cotransporters in the proximal tubule. PTH and FGF23 retrieve these transporters from the membrane, making the proximal tubule the site of the kidney's phosphate regulation.
Traced from the start
- Glomerular filtration barrier
- Proximal tubule: phosphate reabsorption and PTH
Detail
- Reabsorbs
- 80-85% of filtered phosphate (transport-maximum limited); also ~65% of filtered Ca2+ passively (paracellular, solvent drag) and ~15-20% of Mg2+
- Transporter
- Apical NaPi-IIa and NaPi-IIc (Na+-phosphate cotransport)
- Hormone
- PTH (PTH1R, cAMP/PKA) and FGF23 (FGFR-Klotho) internalise NaPi-IIa causing phosphaturia; PTH also stimulates 1-alpha-hydroxylase here to make calcitriol
- Diuretic
- None specific; acetazolamide and SGLT2 inhibitors modestly increase phosphate excretion
When it goes wrong
Primary hyperparathyroidism
Hypercalcaemia with hypophosphataemia and raised urinary phosphate; hypercalciuria despite PTH-driven distal calcium reabsorption because the filtered calcium load rises
X-linked hypophosphataemic rickets (PHEX mutation, excess FGF23) or Fanconi syndrome
Renal phosphate wasting with low TmP/GFR, rickets in children, osteomalacia in adults
Falling nephron mass in CKD
Phosphate retention despite rising FGF23 and PTH: secondary hyperparathyroidism and mineral bone disease
Practise this structure
1 question in the bank tagged Parathyroid.