Basolateral ClC-2 chloride channels in surface colon epithelium: regulation by a direct effect of intracellular chloride

Abstract

Background & aims: The principal function of the colon in fluid homeostasis is the absorption of NaCl and water. Apical membrane Na(+) channels, Na(+)/H(+), and Cl(-)/HCO(3)(-) exchangers have been postulated to mediate NaCl entry into colonocytes. The basolateral exit pathway for Cl(-) has recently been proposed to be via ClC-2 channels present in that membrane domain in surface epithelium. The aim of this report is to obtain functional data for a basolateral localization of ClC-2 and explore a possible direct regulation by intracellular Cl(-).

Methods: Guinea pig colon epithelium with the apical membrane perforated with nystatin in Ussing chambers is used to show a basolateral Cl(-) conductance. Gramicidin D perforated-patch configuration of the patch-clamp technique is used on isolated surface colonocytes. Heterologous expression of the recombinant channel and the whole-cell configuration are used to investigate a direct regulation by intracellular Cl(-).

Results: A basolateral membrane conductance with the characteristics of ClC-2 channels, including Cd(2+) sensitivity, selectivity, and inhibition by extracellular alkalinization, is present in distal colon epithelium. The effect of intracellular Cl(-) on this conductance suggests activation by the permeant anion. Using the recombinant ClC-2 channel, a strong dependence of its activity on intracellular Cl(-) is shown, with a shift of activation to more positive voltages as [Cl(-)](i) is increased.

Conclusions: It is suggested that ClC-2 serves as an exit pathway for Cl(-) in the basolateral membranes of the distal colon and that its dependence on [Cl(-)](i) might provide a cross-talk mechanism to match fluxes at the apical and basolateral domains of these epithelial cells.