Transport mechanisms involved in hydrogen ion (H+) secretion and bicarbonate (HCO3–) and potassium (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. Carbon dioxide and water combine to form carbonic acid, which then dissociates into H+ and HCO3–; this reaction is catalyzed by carbonic anhydrase. The H+ ions 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+. Driven by their electrochemical concentration gradients, cellular HCO3– ions enter 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.
