Levamisole inhibits intestinal Cl- secretion via basolateral K+ channel blockade.

Phenylimidazothiazoles have recently been shown to activate wild-type and mutant cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels in transfected cells and were proposed as therapy for cystic fibrosis. The aim of this study was to investigate the effects of phenylimidazothiazoles on regulated transepithelial Cl- transport in intact epithelia. T84 intestinal epithelial cells grown on permeable supports and stripped human colonic mucosal sheets were studied by conventional current-voltage clamping. Selective permeabilization of apical or basolateral membranes with the monovalent ionophore nystatin was used to isolate basolateral K+ and apical Cl- channel activity, respectively. 86Rb+ uptake was assessed for Na/K/2Cl cotransporter and Na+,K(+)-adenosine triphosphatase activity. In T84 monolayers and human colon, levamisole and its brominated derivative bromotetramisole failed to activate transepithelial secretion. In fact, these compounds dose-dependently inhibited secretory responses to the cyclic adenosine monophosphate agonist forskolin and the Ca2+ agonist carbachol. In permeabilized T84 monolayers, phenylimidazothiazoles weakly activated apical Cl- currents (consistent with their reported action on CFTR) and did not affect bumetanide-sensitive or bumetanide-insensitive 86+Rb+ uptake. Instead, they profoundly inhibited the basolateral Ba(2+)-sensitive and Ba(2+)-insensitive K+ currents. Phenylimidazothiazoles block K+ channels required for Cl(-)-secretory responses elicited by diverse pathways in model epithelia and native colon, an effect that outweighs their ability to activate apical Cl- channels.