Biology of Acid-Base Transport in Distal Urinary Epithelia

Summary

The biology of acid-base transport in distal urinary epithelia is reviewed under four headings: 1) The major function of urinary acidification is based in a intercalated cells or carbonic anhydrase cells. The a cells secrete H⁺ into the urinary compartment by means of a vacuolar H⁺-ATPase located at the apical cell membrane. Structure-function studies reveal that the number of pumps in an apical membrane position varies with acid-base conditions via a mechanism of exocytosis and endocytosis. The basolateral membrane of the a cell contains Cl-HCO₃ exchangers, which closely resemble the red cell band 3 protein, and parallel conductive Cl channels. The high transport rates achieved by α cells appear to be a function of the efficient packing of H⁺-ATPase molecules in the apical membrane. 2) The function of bicarbonate secretion is confined to a small population of β intercalated cells and is less well understood. β cell function is observed in turtle urinary bladder and in the cortical segments of the mammalian collecting duct. The apical membrane of the β cell contains a Cl-HCO₃ exchanger that differs from the basolateral one of the α cell. Transport is driven by a basolateral H⁺-ATPase. 3) The sodium transporting cells of the distal nephron usually contain basolateral Na/H antiporters involved in the regulation of intracellular pH. In some segments an apical Na/H antiporter may contribute to vectorial H⁺ secretion. 4) Overall acid-base transport in the distal nephron is a complex function of the geometry of the transporting systems and the availability of proton acceptors.