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 (Isc) ranged from -15 to 220 microA.cm-2. Variations in basal Isc resulted from differences in the amount of amiloride (10 microM mucosal addition)-sensitive Na+ absorption. Ion substitution and transepithelial flux experiments showed that 10 microM amiloride produced a decrease in the mucosal-to-serosal (M-S) and net Na flux, and that this effect on Isc was independent of Cl- and HCO3- replacement. When the concentration of mucosal amiloride was increased from 10 to 100 microM, little change in Isc was observed. However, increasing the concentration to 1 mM produced a further inhibition, which often reversed the polarity of the Isc. The decrease in Isc due to 1 mM 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.