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

Pancreatic duct cells secrete a HCO(3)(-)-rich (approximately 140 mM) fluid. Using a computer model of the pancreatic duct, Sohma, et al. have demonstrated that the activity of a Cl(-)/HCO(3)(-) exchanger with a 1: 1 stoichiometry at the apical membrane would have to be suppressed in order to achieve such a HCO(3)(-)-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(-)/2HCO(3)(-) exchange when expressed in Xenopus oocytes. To assess the implications of this 1: 2 stoichiometry for HCO(3)(-) 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 HCO(3)(-) secretion at higher concentrations.