Localization of transport compartments in turtle urinary bladder

To characterize different transport compartments in the urinary bladder epithelium of postabsorptive turtles, the electrolyte composition of individual cells was determined using electron microprobe analysis. After blocking the transepithelial Na transport, the short-circuit current decreased from positive to negative values (from 26.5±17.7 to −3.9±2.9 after ouabain and from 25.4±17.2 to −8.0±5.1 μA/cm2 after amiloride). Whereas under control conditions the Na and K concentrations were similar in all cell types and the same was true for Cl in most of the cells, some cells exhibited very low Cl concentrations. The epithelial cells were subdivided according to their electrolyte composition into ouabain-sensitive and ouabain-insensitive ones. In the ouabain-sensitive cells, which made up the majority of epithelial cells and showed a relatively high Cl concentration (about 36 mmol/kg wet weight), the Na concentration increased after ouabain by about 90 mmol/kg wet weight and the K concentration decreased by a similar amount. Since these alterations could largely be prevented when amiloride was applied before ouabain, it is suggested that the granular and basal cells form a syncytial Na transport compartment similar to that in other multilayered epithelia. The ouabain-insensitive cells, in which almost no alteration in Na and K concentrations was observed after ouabain, were subdivided into a Cl-rich (34.6±7.6 mmol/kg wet weight) and a Cl-poor (12.0±5.6 mmol/kg wet weight) population. Since in these cells no large mucin granules were detectable, they are regarded as carbonic anhydrase-rich cells involved in H and HCO3 transport.