A novel role for carbonic anhydrase: cytoplasmic pH gradient dissipation in mouse small intestinal enterocytes

1. The spatial and temporal distribution of intracellular H+ ions in respon se to activation of a proton-coupled dipeptide transporter localized at the apical pole of mouse small intestinal isolated enterocytes was investigate d using intracellular carboxy-SNARF-1. fluorescence in combination with who le-cell microspectrofluorimetry or confocal microscopy. 2. In Hepes-buffered Tyrode solution, application of the dipeptide Phe-Ala (10 mM) to a single enterocyte reduced pH(i) locally in the apical submembr anous space. After a short delay (8 s), a fall of pH(i) occurred more slowl y at the basal pole. 3. In the presence of CO2/HCO3--buffered Tyrode solution, the apical and ba sal rates of acidification were not significantly different and the time de lay was reduced to 1 s or less. 4. Following application of the carbonic anhydrase inhibitor acetazolamide (100 mu M) in the presence of CO2/HCO3- buffer, addition of Phe-Ala once ag ain produced a localized apical acidification that took 5 s to reach the ba sal pole. Basal acidification was slower than at the apical pole. 5. We conclude that acid influx due to proton-coupled dipeptide transport c an lead to intracellular pH gradients and that intracellular carbonic anhyd rase activity, by facilitating cytoplasmic H+ mobility, limits their magnit ude and duration.