  There is mounting evidence that lipid bilayers display conductive properties.
However, when interpreting the electrical response of biological membranes to
voltage changes, they are commonly considered as inert insulators. However,
lipid bilayers under voltage-clamp conditions do not only display current
traces with discrete conduction-steps indistinguishable from those attributed
to the presence of protein channels. In current-voltage (I-V) plots they may
also display outward rectification, i.e., voltage-gating. Surprisingly, this
has even been observed in chemically symmetric lipid bilayers. Here, we
investigate this phenomenon using a theoretical framework that models the
electrostrictive effect of voltage on lipid membranes in the presence of a
spontaneous polarization, which can be recognized by a voltage offset in
electrical measurements. It can arise from an asymmetry of the membrane, for
example from a nonzero spontaneous curvature of the membrane. This curvature
can be caused by voltage via the flexoelectric effect, or by hydrostatic
pressure differences across the membrane. Here, we describe I-V relations for
lipid membranes formed at the tip of patch pipettes situated close to an
aqueous surface. We measured at different depths relative to air/water surface,
resulting in different pressure gradients across the membrane. Both linear and
nonlinear I-V profiles were observed. Nonlinear conduction consistently takes
the form of outward rectified currents. We explain the conductance properties
by two mechanisms: One leak current with constant conductance without pores,
and a second process that is due to voltage-gated pore opening correlating with
the appearance of channel-like conduction steps. In some instances, these
nonlinear I-V relations display a voltage regime in which dI/dV is negative.
This has also been previously observed in the presence of sodium channels. ...
