EFFECTS OF MEMBRANE-POTENTIAL AND MUSCARINE ON POTASSIUM M-CHANNEL KINETICS IN RAT SYMPATHETIC NEURONS

1. Using cell-attached patch pipettes, sustained activity of single po tassium M-channels was recorded from dissociated rat superior cervical ganglion neurones. Previous results indicated that this activity, con sisting of three main levels of open-channel conductance (congruent-to 7, congruent-to 12 and congruent-to 19 pS) was activated by membrane depolarization and inhibited by muscarine added outside the patch. Con sequently, a kinetic analysis was undertaken in order to identify M-ch annel states sensitive to muscarine and membrane potential. 2. Channel activity recorded at 30 mV positive to the resting membrane potential level (congruent-to - 60 mV) showed three shut and two open times. Me an shut times were: tau(s1) 8.0 +/- 2.2 ms; tau(s2) = 71.3 +/- 8.6 ms and tau(s3) = 740 +/- 220 ms. Mean open times were: tau(o1) 10.6 +/- 1 .9 ms and tau(o2) = 59.3 +/- 8.7 ms. When bursts of channel openings w ere determined as those including tau(s1), two exponential components were evident in burst duration distributions (tau(b1) = 11.0 +/- 0.9 m s and tau(b1) = 80.4 +/- 11.0 ms). 3. Membrane hyperpolarization signi ficantly lengthened all three shut times and shortened both open times . It also slightly enhanced the relative contribution of high-conducta nce channels and decreased the relative contribution of low-conductanc e channels to overall activity. 4. All three shut times of the M-chann els were lengthened by 10 muM muscarine without significantly affectin g their open times. 5. It is concluded that both open and shut states of the M-channel are voltage sensitive while only shut states are sens itive to muscarine.