On the audio- and radio-frequency dielectric behaviour of anchorage-independent, mouse L929-derived LS fibroblasts

Abstract

The utility of dielectric methods as a means for estimating the biomass of animal cells in suspension culture was assessed, using mouse L929-derived LS fibroblasts. The dielectric increment of the β-dispersion was found to be a linear function of both cell number (30–70 permittivity units per 10 6 cells/ml, depending on the batch of cells) and volume fraction in the range measured (up to 1.28 × 10 g cells/ml, volume fraction = 0.14). The notional distribution of relaxation times as encompassed in the Cole/Cole α (0.13 ± 0.03 SD) was rather modest. If the cells were treated as spherical shell capacitors of their observable diameter and number, the apparent capacitance of the plasma membrane was some 1.9–4.0 μF/cm 2. This value significantly exceeded those (0.5–1 μF/cm 2) usually encountered or claimed, due predominantly to the possession by these cells of numerous plasma membrane protrusions. As the osmolarity of the suspending medium was increased using the non-permeant solute sorbitol, the apparent specific capacitance of the plasma membrane and the Cole/Cole α were increased, whilst the dielectric increment per 10 6 cells/ml was unchanged. In addition, a secondary β-dispersion, with a characteristic frequency greater than that of the main β-dispersion, became increasingly prominent as the medium osmolarity was increased. It is proposed that this β 2-dispersion is dominated by a Maxwell—Wagner mechanism taking place in the region of the plasma membrane protrusions of these cells.