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

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-/HCO3- exchangers have been postulated to mediate NaCl entry into colonocytes. The basolateral exit pathway for Cl- has recently been proposed to be via CIC-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 CIC-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 CIC-2 channels, including Cd2+ 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 CIC-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 CIC-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.