Monochloramine directly modulates Ca 2+-activated K + channels in rabbit colonic muscularis mucosae

Abstract

Background & Aims: Mesenteric ischemia, infection, and inflammatory bowel disease may eventuate in severe colitis, complicated by toxic megacolon with impending intestinal perforation. Monochloramine (NH 2Cl) is a membrane-permeant oxidant generated during colitis by the large amount of ambient luminal NH 3 in the colon. Reactive oxygen metabolites can modulate smooth muscle ion channels and thereby affect colonic motility, which is markedly impaired in colitis. Methods: Effects of NH 2Cl on ionic currents in the innermost smooth muscle layer of the colon, the tunica muscularis mucosae, were examined using the patch clamp technique. Membrane potential in whole tissue strips was measured using high-resistance microelectrodes. Results: Whole cell voltage clamp experiments showed that NH 2Cl (3-30 μmol/L) enhanced outward currents in a dose-dependent manner, increasing currents more than 8-fold at a test potential of +30 mV. Tail current analysis showed that the currents enhanced by NH 2Cl were K + currents. Inhibition by tetraethylammonium and iberiotoxin suggested that these currents represented activation of large-conductance, Ca 2+-activated K + channels. The membrane-impermeant oxidant taurine monochloramine, however, had no effect on whole cell currents. Single-channel studies in inside-out patches showed that NH 2Cl increased open probability of a 257-pS channel in symmetrical (140 mmol/L) K +. In the presence of NH 2Cl, the steady-state voltage dependence of activation was shifted by −22 mV to the left with no change in the single-channel amplitude. The sulfhydryl alkylating agent N-ethylmaleimide prevented NH 2Cl-induced channel activation. NH 2Cl also hyperpolarized intact muscle strips, an effect blocked by iberiotoxin. Conclusions: NH 2Cl, at concentrations expected to be found during colitis, may contribute to smooth muscle dysfunction by a direct oxidant effect on maxi K + channels. GASTROENTEROLOGY 1999;117:906-917