Loss of Ca 2+-mediated ion transport during colitis correlates with reduced ion transport responses to a Ca 2+-activated K + channel opener

Abstract

Background and purpose: Epithelial surface hydration is critical for proper gut function. However, colonic tissues from individuals with inflammatory bowel disease or animals with colitis are hyporesponsive to Cl - secretagogues. The Cl - secretory responses to the muscarinic receptor agonist bethanechol are virtually absent in colons of mice with dextran sodium sulphate (DSS)-induced colitis. Our aim was to define the mechanism underlying this cholinergic hyporesponsiveness. Experimental approach: Colitis was induced by 4% DSS water, given orally. Epithelial ion transport was measured in Ussing chambers. Colonic crypts were isolated and processed for mRNA expression via RT-PCR and protein expression via immunoblotting and immunolocalization. Key results: Expression of muscarinic M 3 receptors in colonic epithelium was not decreased during colitis. Short-circuit current (I SC) responses to other Ca 2+-dependent secretagogues (histamine, thapsigargin, cyclopiazonic acid and calcium ionophore) were either absent or severely attenuated in colonic tissue from DSS-treated mice. mRNA levels of several ion transport molecules (a Ca 2+-regulated Cl - channel, the intermediate-conductance Ca 2+-activated K + channel, the cystic fibrosis transmembrane conductance regulator, the Na +/K +-ATPase pump or the Na +/K +/2Cl - co-transporter) were not reduced in colonic crypts from DSS-treated mice. However, protein expression of Na +/K +-ATPase α1 subunits was decreased twofold during colitis. Activation of Ca 2+-activated K + channels increased I SC significantly less in DSS colons compared with control, as did the protein kinase C activator, phorbol 12-myristate 13-acetate. Conclusions and implications: Decreased Na +/K +-ATPase expression probably contributes to overall epithelial hyporesponsiveness during colitis, while dysfunctional K + channels may account, at least partially, for lack of epithelial secretory responses to Ca 2+-mediated secretagogues. © 2009 The British Pharmacological Society.