Voltage-clamp experiments on oxidized cholesterol membranes modified with excitability-inducing material and comparison with a model.

The conductance of oxidized cholesterol membranes modified with excitability-inducing material was observed in membranes containing either single conductance channels or 100-1000 channels. Membranes containing single channels have several conductance states for each voltage polarity, and the current through membranes containing manychannels decays with at least two, and probably three, time constants following a step change in voltage (voltage-clamp). The time constants differ by about an order of magnitude. The multi-state behavior seems more pronounced in membranes made from highly oxidized cholesterol. Although a given conductance state could occur at either positive or negative voltages, each state was much more frequent at one polarity or the other. The most frequently observed single-channel conductance states in 0.1 M NaCl are about 0.3, 0.1, 0.03, and 0.0 n-1 for negative voltages and 0.25, 0.05, 0.03, and 0.0 n-1 for positive voltages. The current following a voltage clamp decays to a quasi-steady state within 1 min for positive voltages and 1-15 min for negative voltages. When the holding voltage is --20 mV, the decay constants and quasi-steady state conductances as functions of clamping voltage are reasonably well described by either a three-state model of the conductance or a two-state model applied independently at negative and positive voltages. However, for high voltages, the quasi-steady state does not appear to approach a state in which all the channels are in a low conductance state.