Acid-secreting type A intercalated cells

Transport mechanisms involved in hydrogen secretion and HCO₃^– and K⁺ reabsorption in type A intercalated cells, which are present from the late distal convoluted tubule to the initial portion of the inner medullary collecting duct. Water within the cell dissociates into H⁺ and OH^– ions. The former are secreted into the lumen by H-ATPase pumps in the luminal membrane, where they combine with urinary buffers to generate titratable acid (eg, convert HPO₄^–2 to H₂PO₄^–) and convert NH₃ to NH₄⁺. The OH^– ions in the cell combine with CO₂ to form HCO₃^– in a reaction catalyzed by carbonic anhydrase (CA). Driven by their electrochemical concentration gradients, cellular bicarbonate enters the peritubular capillaries in exchange for extracellular chloride via Cl-HCO₃ exchangers on the basolateral membrane. H-K-ATPase pumps, which secrete H⁺ and reabsorb K⁺, are also present in the luminal membrane of the type A intercalated cells. The number and activity of these pumps are increased by K⁺ depletion, suggesting that they may be important for K⁺ conservation.