Low carbon dioxide permeability of the apical epithelial membrane of guinea-pig colon

We have investigated the apical membrane permeability for CO2 of intact epithelia of proximal and distal colon of the guinea pig. The method used was the mass spectrometric O-18-exchange technique previously described. In a first step, we determined the intraepithelial carbonic anhydrase (CA) activity by studying vital isolated colonocytes before and after lysis with Triton X-100. Intraepithelial CA activity was found to be 41000 and 900 for proximal and distal colon, respectively. Then O-18-exchange measurements were done with stripped intact epithelial layers, which on their apical side were exposed to the reaction solution containing O-18-labelled CO2 and HCO3-. The mass spectrometric signals in these measurements arc determined by the intracellular epithelial CA activity, and by the apical membrane permeabilities for CO2 and HCO3-, P-CO2 and P-HCO3. From the signals, we calculated the two permeabilities while inserting the CA activities obtained from isolated colonocytes. From layers of intact colon epithelium, the apical P-CO2 was determined to be 1.5 x 10(-3) cm s(-1) for proximal and 0.77 x 10(-3) cm s(-1) for distal colon. These values are >= 200 times lower than the P-CO2 of the human red cell membrane as studied with the same technique (0.3 cm s-1). We conclude that the apical membrane offers a significant resistance towards CO2 diffusion, which implies that a major drop in CO2 partial pressure (PCO2) will occur across the apical membrane when luminal pCO(2) is higher than basolateral or capillary pCO(2). In view of the very high pCO(2) that can occur in the colonic lumen, this property of the apical membrane constitutes a significant protection of the cell against the high acid load associated with high pCO(2).