Identification of Acid-Base-Secreting Cells in Turtle Urinary Bladder by the Cellular Uptake of Br<sup>-</sup>

Abstract

In turtle urinary bladder H⁺- and HCO^-₃ secreting cells, thought to be equipped with either a serosal SITS-sensitive or a luminal anion exchanger, were identified by their Br^- uptake using electron microprobe analysis. Under all experimental conditions carbonic-anhydrase-rich (CA) cells exhibited a large variation in Cl^– or anion (sum of Cl^– and Br^-) concentrations, ranging between 2 and 55 mmol/kg wet weight (w.w.). From their location within the epithelium and their Br^– uptake characteristics, CA cells could be divided into three subpopulations: (1) Surface CA cells with a high anion concentration (Σ[C1^- + Br^-] ≧ 20 mmol/kg w.w.) were regarded as H⁺-secreting cells since the high serosal Br^– uptake within 7 min was similar to that expected from current models of H⁺ secretion, and could be reduced by serosal SITS or a luminal pH of 4.5. (2) Surface CA cells with a low anion concentration and low serosal, but substantial luminal, Br^– uptake were considered to be HCO^-₃-secreting cells. (3) CA cells without visible contact with the luminal surface and an intermediate Br^– uptake seem to represent surface CA cell precursors. The SITS inhibitable serosal Br^– uptake into granular and basal cells indicates the presence of a serosal anion exchanger, possibly involved in acid-base regulation of these cells.