Scales of urine acidification: apical membrane-associated particles in turtle bladder

Since the time of Smith, studies of urinary acidification have shifted their focus to ever smaller scales and have revealed iterative patterns or organization. For this review we focus on the organization of intra- and submembrane particles at the scale of the apical cell membrane of the H+ secreting, alpha intercalated cells. Particles were examined quantitatively by thin section and freeze-fracture (FF) electron microscopy. Ongoing studies in turtle bladder indicate that the density of submembrane particles (studs) per micron 2 is approximately the same as that of spherical units (SPUs) forming linear (rod-shaped) arrays on FF. This one-to-one relationship is observed in the presence or absence of CO2 and suggests that CO2-induced changes in H+ secretion do not involve dissociation of the intramembrane (channel) and cytoplasmic (catalytic) parts of the H-ATPase. Structure-function studies based on density estimates of the particles, morphometry of the H+ secreting cell population, and measurement of H+ transport rate prior to fixation permit functional correlation across scales of study.