INTRAMEMBRANE AND SUBMEMBRANE PARTICLE DENSITIES DURING CO2 STIMULATION OF H+ SECRETION IN TURTLE BLADDER

The apical cell membranes of the H+ secreting, alpha-intercalated cell s of turtle urinary bladder (TB) are characterized by studs (cytoplasm ic domains of V-adenosinetriphosphatase) on thin-section transmission electron microscopy and by intramembrane particles (spherical units, S PUs) occurring as rod-shaped particles on freeze-fracture electron mic roscopy. To examine the relationship between studs and SPUs, morphomet ric studies were carried out on bladders maintained in 5% CO2 and in t he absence of exogenous CO2. The stud density per square micrometer of apical membrane was 3,909 +/- 352 (+/- SE) in four TBs (29 alpha-cell s) at 5% CO2 and 3,667 +/- 448 (+/- SE) in the paired halves of the sa me bladders without CO2 (25 alpha-cells). Corresponding densities of S PUs counted on apical membranes of the same bladders (n = 4) were 3,94 1 +/- 545 in 5% CO2 and 3,599 +/- 511 without CO2. The similarity of t he densities of studs and SPUs under both conditions indicates that ea ch SPU within the membrane is matched by one stud projecting into the cytoplasm. The one-for-one relationship between studs and SPUs was pre served over a wide range of transport rates. Addition of CO2 caused on ly inconsistent increments in the densities of studs and SPUs despite substantial increases in H+ transport rate. Slight variations in spaci ng of studs were consistent with patterns of distribution of SPUs on f racture surfaces.