Apical Cl^-/HCO₃^- exchanger stoichiometry in the modeling of HCO₃^- transport by pancreatic duct epithelium

Pancreatic duct cells secrete a HCO₃^--rich (∼140 mM) fluid. Using a computer model of the pancreatic duct, Sohma, et al. have demonstrated that the activity of a Cl^-/HCO₃^- exchanger with a 1:1 stoichiometry at the apical membrane would have to be suppressed in order to achieve such a HCO₃^--rich secretion. Recently the apical exchanger in pancreatic ducts has been identified as SLC26A6 and this probably mediates most of Cl^--dependent HCO ₃^- secretion across the apical membrane. SLC26A6 is reported to mediate electrogenic Cl^-/2HCO₃^- exchange when expressed in Xenopus oocytes. To assess the implications of this 1:2 stoichiometry for HCO₃^- secretion, we have reconstructed the Sohma model using MATLAB/Simulink. To do this we have formulated an expression for the turnover rate of Cl^-/2HCO ₃^- exchange using network thermodynamics and we have estimated the constants from published experimental data. Preliminary data suggest that the 1:2 stoichiometry of SLC26A6 would favor HCO₃ ^- secretion at higher concentrations.