In exchange, protons are sent into the lumen where they combine with bicarbonate ions to form carbonic acid. In the DCT, a luminal carbonic acid anhydrase converts the carbonic acid into H 2 O and CO 2 which can enter DCT cells. Here they are back-converted by a second carbonic anhydrase. Protons are actively secreted in the DCT by a sodium-independent electrogenic ATPase, while the bicarbonate is recovered. CT The kidney can control pH to some extent by regulation of titratable acid.
Ammonia production can augment proton excretion on an as-needed basis.
The ammonium is exchanged with sodium in the lumen, and the ammonium remains charge-trapped in the lumen. The bicarbonate from the breakdown of glutamine is reabsorbed. Any ammonium that is not excreted is reabsorbed into circulation, and this is problematic because it is toxic. In the liver ammonium can be detoxified by conversion to urea, a process which consumes bicarbonate. Both hypercalcemia and hyperkalemia inhibit renal ammonium excretion.