ABSTRACT
Historically, USEP were introduced as a modality that can penetrate the cell plasma membrane, causing various intracellular e ects directly (Schoenbach et al. 2001, 2002, 2007, Beebe et al. 2003a,b). Apart from theoretical calculations, this mechanism of action was supported by the apparent lack of uptake of membrane-impermeable dyes (propidium and Trypan blue) by USEP-treated cells (Beebe et al. 2003a, Vernier et al. 2003, 2004, Pakhomov et al. 2004, 2007b). To test for propidium uptake, cells were bathed in a solution containing 3-30μM of propidium iodide and treated by USEP. Propidium is only weakly µuorescent in the extracellular solution, but will enhance emission 20-30-fold upon binding to the intracellular DNA and RNA. Propidium cations do not pass through undisturbed cell membranes, and therefore the lack of their uptake is commonly used as a convenient and sensitive marker of the membrane integrity. (Note: the terminology used in most publications refers to “propidium iodide (PI) uptake,” which is detected by “changes in PI µuorescence.” However, in water, PI dissociates into propidium cation and iodide anion; it is the propidium cation that penetrates the membrane and causes µuorescence changes, not the entire PI molecule. erefore, it is more accurate to use the term “propidium uptake,” especially when talking about the pore size.)
In fact, the lack of propidium uptake by USEP-treated cells did not necessarily mean that the cell membrane integrity was preserved. e USEP-opened membrane pores are too small to allow the passage of the propidium cation but allow passage of smaller ion species.
