Differences between K channels in motor and sensory nerve fibres of the frog as revealed by fluctuation analysis

Differences between K channels in the nodal membrane of sensory and motor myelinated nerve fibres of the frog were investigated by fluctuation analysis. Spectral densities, S(f), between 3 Hz and 5 kHz were determined from K-current fluctuations measured between 145 and 460 ms after the onset of depolarizations V between 16 and 80 mV. Fits by the sum of a 1/f component and Lorentzian spectra corresponding to Hodgkin-Huxley n4-kinetics gave significant deviations from the measured spectra. The best fit was obtained by: S(f) = S1/[1+(f/fc)1.5]+S2. The first term can be interpreted as a diffusion spectrum which would originate from gating of K channels governed by an electrodiffusion process. To describe the spectral density at frequencies above 1 kHz it was necessary to add the plateau S2. Time constants taun* = 1/(2pifc) are roughly equal to the conventional Hodgkin-Huxley time constant taun only for pulses V < 40mV. At higher depolarizations taun increases with increasing depolarization in contrast to taun. The variance, var, of conductance fluctuations was determined by integration of the first component of S(f). From var, the probability of the open channel state, and the steady-state K current the single-channel conductance gamma and the number N of K channels per node were calculated; all parameters were corrected for K accumulation during depolarizing pulses. gamma and N were found to be only weakly voltage-dependent. The mean values over all voltages are for motor fibres: gamma=2.7 pS, N = 5.7 x 10(4), and for sensory fibres: gamma = 4.6 pS, N = 5.2 x 10(4). The results suggest two different kinds of K channels in motor and sensory nerve fibres.