Pulsed electric fields (PEFs) above a certain voltage threshold cause electroporation for microsecond pulses and intracellular effects for submicrosecond pulses. Models describing these effects often depend on the electrical properties of the cell, which are altered by the PEF. We used time domain dielectric spectroscopy to provide data for these models and to measure effects on cell suspension conductivity. We applied single 50-/spl mu/s and 10-ns pulses to HL-60 cells, with the voltages chosen so the pulses have approximately the same energy. For 1.1-kV/cm, 50-/spl mu/s pulses, the conductivity rose within a minute after the pulse and dropped dramatically approximately 40 min after the pulse. For 78-kV/cm, 10-ns pulses, we observed a brief delay prior to the conductivity rise and noted the same drop in conductivity after approximately 40 min. For both pulse durations, higher voltages frequently led to membrane poration followed by a gradual recovery approximately 30-40 min after the pulse. For 2-kV/cm, 50-/spl mu/s pulses, we observed significantly more scatter in Trypan Blue uptake measurements due to stronger effects on the cell membrane. By using a cell model, we showed that a 50-/spl mu/s pulse caused a much larger rise in membrane conductivity than a 10-ns pulse of the same energy.

Despite a basic knowledge of cells' biochemical processes, their electrical properties, particularly the changes in membrane properties upon the application of pulsed electric fields (PEF's), have not yet been fully characterized. Microsecond pulses above a certain threshold cause electroporation of the cell membrane while nanosecond pulses of higher voltage additionally porate the inner organelles. We used Time Domain Dielectric Spectroscopy to measure the conductivity of HL-60 (human leukemia) cell suspensions as a function of time after 10 ns, 78.5 kV/cm pulses and 50 mus, 1.1 kV/cm pulses, which have the same energy. The conductivity increased immediately after the 50 mus pulse, indicating that ion channels in the HL-60 membranes initially opened. However, the conductivity decreased immediately after the ultrashort pulse, indicating that ion channels initially closed. The conductivity decreases significantly approximately 40 minutes after both pulses. This suggests that not only do the pores or channels opened close, but pores or channels open in the membrane prior to the pulse may close as well. These measurements were an intermediate step in determining the electrical properties of HL-60 cells using a two-shell model. Once determined, these electrical parameters will be used in electroporation models developed at Old Dominion University.