Expansion
Mostly passive in the proximal tubule and loop, actively regulated in the distal tubule
Expansion
- Proximal tubule: about 65 per cent, passively with sodium and water
- Thick ascending limb: about 25 per cent, passively, driven by the lumen-positive potential created by NKCC2
- Distal convoluted tubule: about 8 per cent, actively and under hormonal control
- Excreted: 1 to 2 per cent
Parathyroid hormone increases distal reabsorption; it also increases phosphate excretion and stimulates 1-alpha hydroxylation of vitamin D.
Drug effects follow the mechanism:
- Loop diuretics abolish the lumen-positive potential in the thick ascending limb, so calcium reabsorption falls and excretion rises. Useful in hypercalcaemia
- Thiazides act distally, and by depleting cell sodium they enhance basolateral sodium-calcium exchange, so excretion falls. Useful in recurrent calcium stones, and a recognised cause of hypercalcaemia
The calcium-sensing receptor on the thick ascending limb reduces reabsorption when plasma calcium is high; activating mutations cause hypocalcaemia with hypercalciuria, and inactivating mutations cause familial hypocalciuric hypercalcaemia.