MULTIPLE KINETIC STATES UNDERLYING MACROSCOPIC M-CURRENTS IN BULLFROG SYMPATHETIC NEURONS

M-current is a time- and voltage-dependent potassium current which is suppressible by muscarinic receptor activation. We have used curve fitting and noise analysis to determine if macroscopic M-currents deviate from a previously predicted simple two-state kinetic scheme. The M-current was best described by three kinetically distinct components: 'fast' (tau-0), 'intermediate' (tau-1) and 'slow' (tau-2) time constants. The 'fast' (tau-0) and 'intermediate' (tau-1) components were identified from the spectral of M-current noise at potentials positive to the cells' resting membrane potential. The 'intermediate' (tau-1) and 'slow' (tau-2) components were seen by curve fitting M-current deactivation currents. The 'intermediate' (tau-1) time constant was voltage dependent (decreasing e-fold in 23 mV), but voltage dependence of the, 'fast' (tau-0) and 'slow' (tau-2) components was not obvious. All kinetic components were sensitive to muscarine, with the 'intermediate' (tau-1) and 'slow' (tau-2) being equally so. These data suggest that all components may derive from the same channel population, and that the M-channel may have at least four kinetic states.