Fig. (2).

Schematic model of the α-intercalated cell and the H⁺ secretion in cortical collecting tubule. The α-intercalated cell is responsible for H⁺ secretion by a vacuolar H⁺-ATPase (main pump) and also by a H⁺-K⁺-ATPase. The luminal ammonia (NH₃) buffers H⁺ to form nondiffusible ammonium (NH₄⁺) and divalent basic phosphate (HPO₄^-) is converted to the monovalent acid form (H₂PO₄^-) in H⁺ presence. Intracellularly formed HCO₃^- leaves the cell via Cl^- - HCO^{3 -} exchange, facilitated by an anion exchanger (AE1). Cytoplasmic carbonic anhydrase II (CA II) is necessary to secret H⁺.