In previous studies we have shown that colonic epithelium possesses a significant barrier towards diffusion of CO2.We have now exposed isolated colonic mucosa from guinea pig to different CO2 partial pressures in the solutions on the apical and mucosal side, respectively. A double-barrelled CO2/pH microelectrode was employed to determine the pCO2 gradient across the tissue. We found a major step in pCO2 associated with penetration of the apical membrane by the electrode. This indicates that the apical membrane constitutes the main barrier to CO2 diffusion in the mucosa. This view is confirmed by the observation that this barrier was lost when Triton X-100 was added on the apical side. We show in addition that opening the tight junctions of distal colon epithelium by applying carbonic anhydrase inhibitors results also in a loss of the apical CO2 barrier. By using mucolytic drugs (glycosidases, dithiothreitol), we demonstrated that the apical mucus is not responsible for the observed barrier. We conclude that high resistance to CO2 diffusion of apical membranes as well as tight junctions is responsible for the apical gas barrier.

Supported by DFG, Gr 489/19.