Exposure to CO2 acidifies the cytosol of mitochondria-rich cells in turtle bladder epithelium. The result of the decrease in pH in these, the acid-secreting cells of the epithelium, is a transient increase in cell calcium, which causes exocytosis of vesicles containing proton-translocating ATPase. Because mitochondria-rich cells have rapid luminal membrane turnover, we were able to identify single mitochondria-rich cells by their endocytosis of rhodamine-tagged albumin. Using fluorescence emission of 5,6-carboxyfluorescein at two excitation wavelengths, we measured cell pH in these identified mitochondria-rich cells and found that although the cell pH fell, it recovered within 5 min despite continuous exposure to CO2. This pH recovery also occurred at the same rate in Na+-free media. However, pH recovery did not occur when luminal pH was 5.5, a condition under which the H+-pump does not function, suggesting that recovery of cell pH is due to the luminally located H+ ATPase. Chelation of extracellular calcium by EGTA prevented the CO2-induced rise in cell calcium measured with the intracellular fluorescent dyes Quin 2 or Fura 2 and also prevented recovery of cell pH. When the change in cell calcium was buffered by loading the cells with high concentrations of Quin 2, the CO2-induced decrease in pH did not return back to basal levels. We had found previously that buffering intracellular calcium transients prevented CO2-stimulated exocytosis. Further, we show here that the increased H+ current in voltage-clamped turtle bladders, which is directly proportional to the number of H+-pump-containing vesicles that fuse with the luminal membrane, was significantly reduced in calcium-depleted bladders. These results suggest that pH regulation in these acid-secreting cells occurs by calcium-dependent exocytosis of vesicles containing proton pumps, whose subsequent turnover restores the cell pH to its initial levels.

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