The effects of sulfhydryl reagents on M-type voltage-dependent potassium currents (I(K(M))) were examined in NG108-15 cells transformed to express ml muscarinic acetylcholine receptors (mAChRs), a NGPM1-27 clone. Focal application of glutathione at millimolar concentrations dissolved in acidic solutions caused a transient inward current in NGPM1-27 cells at holding potentials of -30 mV, associated with an inhibition of I(K(M)). The glutathione-induced response was mimicked by cysteine. These effects were also reproduced by superfusion with micromolar concentrations of HgCl2, AgNO3, N-methylmaleimide and p-chloromercuribenzoic acid (pCMB), agents which target protein thiols. Glutathione, HgCl2, AgNO3 and pCMB inhibited the peak conductance of I(K(M)) without shifting the half activating voltage (V(1/2)), which was comparable to the acetylcholine (ACh)-induced response. The voltage dependence of time constants for I(K(M)) deactivation in sulfhydryl reagent-, ACh- and non-treated cells resembled, but differed from that in Ba2+-treated cells. These results reveal that there is an accessible cysteine moiety, but not a disulfide bond, either on the M channel protein itself or on a protein directly involved in agonist-M channel coupling.
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