Fig. 5. Electrochemical potential for the apical anion exchange (AE) process calculated assuming intracellular Cl^- concentration and apical membrane potential of 30 mmol l^-1 and -100 mV, respectively, and using luminal Cl^- and HCO₃^- concentrations from unfed gulf toadfish (Taylor and Grosell, 2006a) and different intracellular HCO₃^- concentrations ranging from 1.5 to 40 mmol l^-1 (indicated by different symbols). Absorption of Cl^- and HCO₃^- secretion via 1:1 anion exchange can only occur when the AE electrochemical potential is >0, which requires intracellular HCO₃^- concentrations of >10 mmol l^-1 in the anterior intestine and close to 40 mmol l^-1 in the mid intestine. See text for further details.