Putative M-type potassium channels in neuroblastoma-glioma hybrid cells: inhibition by muscarine and bradykinin

Putative M-type K(+)-channels ('M-channels') were recorded in differentiated NG108-15 neuroblastoma x glioma hybrid cells transformed to express m1 muscarinic acetylcholine receptors using cell-attached patch-electrodes. Channels showed multiple conductances, with peaks at 6-9 and 12-15 pS. Averaged currents showed time-dependent activation during 1 s depolarization steps to around -30 mV. Steady-state Po increased in a voltage-dependent manner when the membrane was depolarized between 10 and 60 mV, with a limiting slope of 5.5 mV/e-fold change in Po. Steady-state kinetics were fit by two open and three shut times: depolarization shortened shut times and lengthened open times. Application of muscarine (10 microM) or bradykinin (10 microM) to the membrane outside the patch reversibly reduced steady-state in-patch channel activity to 38.4 +/- 11.7 and 28.8 +/- 6.1% of control values, respectively. Inhibition was accompanied by a lengthening of channel shut times without significant change in open times or distribution of conductance levels. No effect of muscarine or bradykinin on whole-cell or membrane patch delayed rectifier currents was detected. It is concluded that M-channels in NG108-15 cells are qualitatively similar to, but sparser than, those previously reported in rat sympathetic neurones. Their inhibition by extra-patch acetylcholine and bradykinin suggests that a mobile messenger is involved in the transduction process leading from receptor activation to channel closure.