Progression of change in membrane capacitance and cytoplasm conductivity of cells during controlled starvation using dual-frequency DEP cytometry.

The dielectric properties of cells are directly related to their morphological and physiological properties and can be used to monitor their status when exposed to stress conditions. In this work, dual-frequency dielectrophoresis (DEP) cytometry was employed to measure changes in the membrane capacitance and cytoplasm conductivity of single Chinese hamster ovary (CHO) cells during the progression of starvation-induced apoptosis. Our dual-frequency DEP cytometer enables simultaneous measurement of multiple dielectric properties of single cells and identification of their state (viable or apoptotic) within a heterogeneous sample. We employed one frequency to determine each cell's viability state and the other frequency to characterize the change in membrane capacitance or cytoplasm conductivity. Cells were starved by incubation in a medium lacking glucose and glutamine and monitored every 12 h over a 64 h period. Our results showed a subpopulation of early apoptotic cells emerged after 40 h in the starvation medium, which rapidly increased during the next 12 h. After 52 h, a complete transition from viable to apoptotic state was observed. Analyzing the subpopulation of viable cells over the first 52 h showed that the membrane capacitance gradually declined from an initial value of 2.0 to 1.2 μF/cm2, and was 0.9 μF/cm2 for apoptotic cells. The cytoplasm conductivity of viable cells initially remained constant and then declined from 0.40 to 0.27 S/m after 40 h, coinciding with onset of apoptotic processes. A dramatic decrease in cytoplasm conductivity from 0.27 to 0.07 S/m was observed after 52 h, corresponding to apoptotic cells. As membrane capacitance is related to membrane morphology and cytoplasm conductivity is related to intracellular ion concentrations, the results indicate that during controlled starvation the cell membrane smooths gradually whereas intracellular ion concentrations are initially maintained near homeostatic levels until a later dramatic decline occurs.