Monochloramine directly modulates Ca2+-activated K+ channels in rabbit colonic muscularis mucosae

Background & Aims: Mesenteric ischemia, infection, and inflammatory bowel d isease may eventuate in severe colitis, complicated by toxic megacolon with impending intestinal perforation. Monochloramine (NH2Cl) is a membrane-per meant oxidant generated during colitis by the large amount of ambient lumin al NH3 in the colon. Reactive oxygen metabolites can modulate smooth muscle ion channels and thereby affect colonic motility, which is markedly impair ed in colitis. Methods: Effects of NH2Cl on ionic currents in the innermost smooth muscle layer of the colon, the tunica muscularis mucosae, were exam ined using the patch clamp technique. Membrane potential in whole tissue st rips was measured using high-resistance microelectrodes. Results: Whole cel l voltage clamp experiments showed that NH2Cl (3-30 mmol/L) enhanced outwar d 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 curre nts enhanced by NH2Cl were K+ currents. Inhibition by tetraethylammonium an d iberiotoxin suggested that these currents represented activation of large -conductance, Ca2+-activated K+ channels. The membrane-impermeant oxidant t aurine monochloramine, however, had no effect on whole cell currents. Singl e-channel studies in inside-out patches showed that NH2Cl increased open pr obability of a 257-pS channel ih symmetrical (140 mmol/L) K+. In the presen ce of NH2Cl the steady-state voltage dependence of activation was shifted b y -22 mV to the left with no change in the single-channel amplitude. The su lfhydryl alkylating agent N-ethylmaleimide prevented NH2Cl-induced channel activation. NH2Cl also hyperpolarized intact muscle strips, an effect block ed by iberiotoxin. Conclusions: NH2Cl at concentrations expected to be foun d during colitis, may contribute to smooth muscle dysfunction by a direct o xidant effect on maxi K+ channels.