Figure 1. Schematic representation of reactions and transport processes associated with ¹⁸O exchange of colon epithelial cells, either in the intact epithelial sheet or as isolated colonocytes in suspension.

The partial pressure of C¹⁸O¹⁶O in the fluid sample is followed continuously by mass spectrometry. The fractions 2/3 and 1/3 indicate that in each dehydration reaction step there is a 2/3 chance that the ¹⁸O of HC¹⁸O¹⁶O₂⁻ ends up in CO₂, and a 1/3 chance that it is transferred into water. Because the water pool is very much greater than the pool of CO₂/HCO₃⁻, eventually almost all of the ¹⁸O will disappear into the water pool. The crucial parameters describing the overall process in the presence of cells are: (1) the intracellular activity of carbonic anhydrase (CA), which is determined in an independent experiment from colonocytes in suspension; (2) the membrane CO₂ permeability (P_CO2) and; (3) the membrane HCO₃⁻ permeability (P_HCO3); the latter two are determined from the recorded time course of C¹⁸O¹⁶O from experiments such as in Figs 4B and 5. The water permeability P_H2O of red cells is so high that it does not become rate limiting in these measurements.