Intra- and submembrane particle densities during CO2 stimulation of H+ secretion in turtle bladder

The apical cell membranes of the H+ secreting, alpha-intercalated cells of turtle urinary bladder (TB) are characterized by studs (cytoplasmic domains of V-adenosinetriphosphatase) on thin-section transmission electron microscopy and by intramembrane particles (spherical units, SPUs) occurring as rod-shaped particles on freeze-fracture electron microscopy. To examine the relationship between studs and SPUs, morphometric studies were carried out on bladders maintained in 5% CO2 and in the absence of exogenous CO2. The stud density per square micrometer of apical membrane was 3,909 +/- 352 (+/-SE) in four TBs (29 alpha-cells) at 5% CO2 and 3,667 +/- 448 (+/-SE) in the paired halves of the same bladders without CO2 (25 alpha-cells). Corresponding densities of SPUs counted on apical membranes of the same bladders (n = 4) were 3,941 +/- 545 in 5% CO2 and 3,599 +/- 511 without CO2. The similarity of the densities of studs and SPUs under both conditions indicates that each SPU within the membrane is matched by one stud projecting into the cytoplasm. The one-for-one relationship between studs and SPUs was preserved over a wide range of transport rates. Addition of CO2 caused only inconsistent increments in the densities of studs and SPUs despite substantial increases in H+ transport rate. Slight variations in spacing of studs were consistent with patterns of distribution of SPUs on fracture surfaces.