Ca2+ Entry into an Electroporated Sea Urchin Egg




Hosoi, M. & Kinosita, K. Jr., unpublished.



When a cell is exposed to an external electric field, ions accumulate on both sides of the cell membrane which is basically impermeable to ions (see Figure). The accumulated ions create a large transmembrane potential, which is outside positive on the positive-electrode side and outside negative on the negative-electrode side. When the absolute magnitude of the transmembrane voltage reaches about 1 V, the membrane somehow becomes permeable to small molecules and ions (the phenomenon called 'electroporation.') In the movie on the right, a sea urchin egg (diameter 100 μm) is electroporated by applying an electric field at 400 V/cm (maximal transmembrane voltage about 3 V) for 400 μs. This results in the influx of Ca2+ ions in the surrounding sea water into the egg interior (Ca2+ level shown in false colors), initially more extensive on the positive-electrode side and later on the negative side. The cause of this poration asymmetry is not yet clear. The porated membrane is eventually resealed spontaneously. On the whole, more Ca2+ ions enter from the negative side, leading to the formation of a larger fertilization envelope on the negative side.
             Hibino, M. et al., Biophys. J. 64, 1789-1800 (1993).
             Kinosita, K., Jr. et al., J. Cell Biol. 115, 67-73 (1991).

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