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 patter ns or organization. For this review we focus on the organization of in tra- and submembrane particules at the scale of the apical cell membra ne of the H+ secreting, alpha intercalated cells. Particles were exami ned quantitatively by thin section and freeze-fracture (FF) electron m icroscopy. Ongoing studies in turtle bladder indicate that the-density of submembrane particles (studs) per mu m(2) is approximately the sam e as that of spherical units (SPUs) forming linear (rod-shaped) arrays on FF. This one-to-one relationship is observed in the presence or ab sence of CO2 and suggests that CO2-induced changes in H+ secretion do not involve dissociation of the intramembrane (channel) and cytoplasmi c (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 fixati on permit functional correlation across scales of study.