Efficient cell electrotransfection can be achieved using combinations of high-voltage (HV; 800 V/cm, 100 micros) and low-voltage (LV; 80 V/cm, 100 ms) pulses. We have developed equipment allowing the generation of various HV and LV combinations with precise control of the lag between the HV and LV pulses. We injected luciferase-encoding DNA in skeletal muscle, before or after pulse delivery, and measured luciferase expression after various pulse combinations. In parallel, we determined permeabilization levels using uptake of (51)Cr-labeled EDTA. High voltage alone resulted in a high level of muscle permeabilization for 300 seconds, but very low DNA transfer. Combinations of one HV pulse followed by one or four LV pulses did not prolong the high permeabilization level, but resulted in a large increase in DNA transfer for lags up to 100 seconds in the case of one HV + one LV and up to 3000 seconds in the case of one HV + four LV. DNA expression also reached similar levels when we injected the DNA between the HV and LV pulses. We conclude that the role of the HV pulse is limited to muscle cell permeabilization and that the LV pulses have a direct effect on DNA. In vivo DNA electrotransfer is thus a multistep process that includes DNA distribution, muscle permeabilization, and DNA electrophoresis.

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Electroporation is a transient phenomenon that increases permeability of the cell plasma membrane. In the state of high permeability, the plasma membrane allows ions and small and large molecules to be introduced into the cytoplasm, although the cell plasma membrane in its normal state represents a considerable barrier for them. Besides introduction of different substances to the cytoplasm, the permeabilized cell membrane allows cell fusion or insertion of proteins into cell membrane. Efficacy of electroporation and its applications strongly depends on parameters that can be divided into parameters of the electric field (i.e., pulse amplitude, pulse duration, pulse repetition frequency, number of pulses, and pulse shape), and parameters that define the state of cells, their surroundings, and cell geometry (i.e., temperature, osmotic pressure, cell size and shape, etc.). With properly chosen values of the electric field parameters, the process of electroporation is reversible and cells return into their normal physiological state. If these parameters exceed certain values (e.g., amplitude of pulses is too high or duration of pulses is too long), cells are irreversibly permeabilized and lose their viability.