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

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

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

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 alpha1 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.