Direct observation in the millisecond time range of fluorescent molecule asymmetrical interaction with the electropermeabilized cell membrane.

Interaction of two stains (propidium iodide and ethidium bromide) with electropermeabilized living Chinese hamster ovary cells is observed using an ultrafast fluorescence image acquisition system. The computing process is linked to an ultra-low-light intensifying camera working with a very short time resolution (3.33 ms per image). Altered parts of the cell membrane were identified via the enhancement in fluorescence intensity of the dyes. They reflect the electropermeabilized part of the membrane in which free flow of dye occurred. Images of the fluorescence interaction patterns of the two dyes, in a maximum 20-ms time lag after pulsation, reveal asymmetrical permeabilization of the cell membrane. For electric field intensities higher than a first threshold value, permeabilization is always observed on the anode-facing side of the cell. For electric field intensities over a second higher threshold value, the two electrode-facing hemispheres of the cell are permeabilized, the hemisphere facing the anode being most permeable. These data support the conclusion that electropermeabilization of living cell membrane is affected by its resting potential. The asymmetrical pattern of the dye interaction is not dependent on the nature or concentration of the dye, the ionic strength of the pulsing buffer, or the duration of the pulse. The field intensity determines the fraction of the membrane in which molecular alterations can occur. The extent of alteration in this localized region is determined by the duration of the pulse when a single pulse in the millisecond time range is applied.