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¹ Zentrum Physiologie, Medizinische Hochschule Hannover, 30625 Hannover, Germany

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 ¹⁸O-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 41 000 and 900 for proximal and distal colon, respectively. Then ¹⁸O-exchange measurements were done with stripped intact epithelial layers, which on their apical side were exposed to the reaction solution containing ¹⁸O-labelled CO₂ and HCO₃^–. The mass spectrometric signals in these measurements are 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 pCO₂ is higher than basolateral or capillary pCO₂. In view of the very high pCO₂ 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 pCO₂.

(Received 24 February 2005; accepted after revision 31 May 2005; first published online 2 June 2005)
Corresponding author G. Gros: Zentrum Physiologie 4220, Medizinische Hochschule Hannover, Carl-Neuberg-Str. 1, 30623 Hannover, Germany. Email: gros.gerolf{at}mh-hannover.de

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