Voltage-induced nonconductive pre-pores and metastable single pores in unmodified planar lipid bilayer

Electric fields promote pore formation in both biological and model membranes. We clamped unmodified planar bilayers at 150-550 mV to monitor transient single pores for a long period of time. We observed fast transitions between different conductance levels reflecting opening and closing of metastable lipid pores. Although mean lifetime of the pores was 3 +/- 0.8 ms (250 mV), some pores remained open for up to approximately 1 s. The mean amplitude of conductance fluctuations (approximately 500 pS) was independent of voltage and close for bilayers of different area (40,000 and 10 microm(2)), indicating the local nature of the conductive defects. The distribution of pore conductance was rather broad (dispersion of approximately 250 pS). Based on the conductance value and its dependence of the ion size, the radius of the average pore was estimated as approximately 1 nm. Short bursts of conductance spikes (opening and closing of pores) were often separated by periods of background conductance. Within the same burst the conductance between spikes was indistinguishable from the background. The mean time interval between spikes in the burst was much smaller than that between adjacent bursts. These data indicate that opening and closing of lipidic pores proceed through some electrically invisible (silent) pre-pores. Similar pre-pore defects and metastable conductive pores might be involved in remodeling of cell membranes in different biologically relevant processes.