Ba2+ was used to introduce K+ current fluctuations in addition to those arising from the intrinsic 'gating' mechanism studied in the preceding paper. In the presence of 0.5 to 2.0 mM-Ba2+, Lorentzian spectra were observed at potentials between -10 and -63 mV. The amplitude and corner frequency of these Lorentzian spectra varied with voltage in the general manner to be expected from the voltage- and time-dependent blocking action of Ba2+, but the microscopically determined time constant was consistently faster than that determined from the intensification of block upon hyperpolarization. The variance of the Ba2+-induced fluctuations could be used to calculate the limiting single-channel conductance, gamma, without the need to assume a specific model for inward rectification. The value of gamma obtained for preparations in symmetrical 120 mM-K+ was about 8 pS (20-24 degrees), in good agreement with that found in the preceding paper. Some fibres underwent a spontaneous, large increase in membrane conductance. This conductance showed inward rectification in the positive quadrant. At negative potentials this high conductance was blocked by 1 mM-Ba2+ in a voltage- and time-dependent manner. The block became faster and more complete at more negative potentials. Analysis of current noise in the high conductance state in the presence of Ba2+ gave Lorentzian spectra with corner frequencies in general agreement with the time constant of macroscopic current relaxations. The unitary conductance in that state was estimated to be at least 70 pS.
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