Electrogenic proton transport by intercalated cells of tight urinary epithelia

Structure-function studies of the turtle bladder indicate that electrogenic proton secretion into the urinary compartment is accomplished by alpha-type intercalated cells which are rich in carbonic anhydrase. In the absence of electrochemical potential gradients (delta mu H = 0), the rate of H+ secretion (JH) is a function of the number of H+ pumps in position at the apical cell membrane, as judged from morphometric and freeze-fracture studies of apical membrane area characterized by a cytoplasmic coating with studs and by rod-shaped intramembrane particles (RSP). At a given pump population, JH is a sigmoid function of delta mu H, with delta pH and delta psi having equivalent effects on JH. The JH versus delta mu H relation reflects the intrinsic properties of the H+ pump and suggests a H+ pump model consisting of two components, a channel through the apical membrane across which delta mu H falls, and a catalytic unit located within the cytoplasm (outside of delta mu H). Each intramembrane RSP is associated with several cytoplasmic studs, but the precise relations between the two remain to be clarified.