Surface pH of the distal colonic epithelium of guinea-pigs

A method for the continuous measurement of proton activity at the luminal surface of the gastro intestinal mucosa was developed. The pH-sensitive fluorescent dye 5-N-hexadecanoyl-amino fluorescein (HAF) was used to monitor the surface pH (pHs). HAF integrates in the apical membrane of the colonic cells because of its amphophilic character. By excitation at two different wavelengths, the ratio of the two fluorescence intensities corresponds to the pHs. pHs in the presence of bicarbonate is relatively independent of the luminal pH. In bicarbonate-free, HEPES-buffered solutions, such a constant "microclimate" was not present. Addition of 113 mmol.l-1 butyrate to the luminal solution caused an increase of the surface pH by 0.14 +/- 0.07 (n = 18) pH units in bicarbonate-containing medium. This increase was not observed in bicarbonate-free medium, indicating that the increase in pHs is due to a gain of bicarbonate during butyrate absorption.