Mechanisms of Na and Cl absorption across the distal colon epithelium of the pig

Porcine distal colon epithelium was mounted in Ussing chambers and bathed in plasma-like Ringer solution. Tissue conductances ranged from 10 to 15 mS and the short-circuit current (I_sc) ranged from-15 to 220 μA·cm^-2. Variations in basal I_sc resulted from differences in the amount of amiloride (10μM mucosal addition)-sensitive Na⁺ absorption. Ion substitution and transepithelial flux experiments showed that 10 μM amiloride produced a decrease in the mucosal-to-serosal (M-S) and net Na flux, and that this effect on I_sc was independent of Cl^- and HCO₃^-replacement. When the concentration of mucosal amiloride was increased from 10 to 100 μM, little change in I_sc was observed. However, increasing the concentration to 1 m M produced a further inhibition, which often reversed the polarity of the I_sc. The decrease in I_sc due to 1 m M amiloride was dependent on both Cl^- and HCO₃^-, and was attributed to reductions in the M-S and net Na⁺ fluxes as well as the M-S unidirectional Cl^- flux. Ion replacement experiments demonstrated that Cl^- substitution reduced the M-S and net Na fluxes, while replacement of HCO₃^-with HEPES abolished net Cl^- absorption by reducing the M-S unidirectional Cl^- flux. From these data it can be concluded that: (1) Na⁺ absorption is mediated by two distinct amiloride-sensitive transport pathways, and (2) Cl^- absorption is completely HCO₃^--dependent (presumably mediated by Cl^-/HCO₃^-exchange) and occurs independently of Na⁺ absorption.