Axial heterogeneity of rabbit cortical collecting duct

The rabbit cortical collecting duct (CCD) consists of three major cell types: principal cells transport K⁺, β-intercalated cells absorb Cl^-, and α-intercalated cells secrete H⁺. We used functional and histological methods to assess axial distribution of these cell types along rabbit CCD. In perfused CCDs, lumen-to-bath Rb⁺ rate coefficient (an index of principal cell K⁺ transport) was not different in tubules from outer cortex (1 mm from renal surface) compared with those from inner cortex (2 mm from renal surface), suggesting that principal cell function is homogeneous along the CCD. In contrast, Cl^- rate coefficient (a measure of β-intercalated cell function) was twice as high in CCDs from outer compared with inner cortex, suggesting heterogeneity of β-intercalated cells along the CCD. To further investigate these regional differences, we fixed and embedded kidneys and identified three cell types in CCD cross sections using carbonic anhydrase staining and peanut lectin binding. Comparing tubule cross sections from outer with those from inner cortex, we found no axial difference in the fraction of cells that were either principal cells (64%) or total (lectin binding and nonlectin binding) intercalated cells (36%). However, the lectin-binding intercalated cell subset was significantly increased in outer compared with inner cortex. We conclude that there is not heterogeneity of principal cells along the rabbit CCD; however, β-cell number and function are increased in outer CCD. Collecting duct heterogeneity begins within the cortical segment.