The conductance of cellular membranes at supra-physiological voltages

• Membrane permeabilization by pulsed electric fields is best detected by monitoring conductance.
• Conductance can either mean ‘slope conductance’ or ‘chord conductance’.
• Gchord increases steeply in tobacco cell membranes charged beyond threshold voltage.
• Gchord saturates at extreme voltages, indicating that optimum pore density exists.
• Data are described quantitatively with a Boltzmann equation for a two-state model.

Membrane permeabilization by pulsed electric fields (electroporation), that is supposed to be caused by the formation of aqueous pores, is widely used in biomedicine and biotechnology. It is detected most precisely by measuring membrane conductance. When whole-cell patch-clamp experiments are used to screen a wide voltage range, poration becomes manifest by large currents elicited at extreme hyper-/depolarization. The slope conductance, Gslope, can be obtained from non-linear current–voltage relations by differentiation of the current–voltage curve. Alternatively, the chord conductance, Gchord, is defined as the slope of straight lines connecting each point on the current–voltage curve with the zero-current (reversal) potential on the voltage axis. Here, Boltzmann functions were fitted to plots of Gchord versus voltage recorded on protoplasts from bright-yellow-2 tobacco cells. These plots are supposed to reflect transition from a non-porated to a porated membrane state. Consistently, Gchord saturated at extremely negative and positive voltages at values well below those expected for a complete demolition of the membrane (half-maximum voltages: ~− 332 mV and + 294 mV, respectively). The slope factor allowed inferring the change in dipole moment associated with water intrusion into the bilayer. It was − 6.19 10−4 and  3.35 10− 4C∗ m ∗ mol− 1, respectively. Outside-out patches rendered similar results, but half-maximum voltages were shifted to more extreme voltages with respect to whole-cell experiments.