Dielectric behavior of the anion-exchange protein of human red blood cells

On the temperature dependence of the dielectric membrane properties of human red blood cells

Electrorotation (ER) spectra of human red blood cells (HRBCs) have been recorded in the frequency range from 10 kHz to 250 MHz in a 4-electrode microchip chamber. The cells were suspended at conductivities in the range from 0.02 to 3.00 S/m (corresponding to an ionic strength range from 1.6 to 343 mM) at temperatures between 10 degrees C and 35 degrees C. Generally, the characteristic frequencies as well as the rotation speeds of the first (membrane-dispersion) and second ER peaks increased with temperature. The rotation speed increase was largely correlated to the temperature dependence of the medium viscosity. Standard temperature dependencies were assumed for the conductivities and permittivities of cytoplasm, membrane, and external solution to explain the frequency shifts, starting from the cell parameters of Gimsa et al. [Gimsa et al., 1996, Biophys. J. 71: 495-506.]. The membrane capacitance was assumed to be temperature independent, based on the permittivity of alkyl-chains. Under these assumptions, the spectra could be well fitted only in a narrow temperature range around 20 degrees C. The temperature dependence of the first characteristic frequency was much stronger than predicted. In addition, around 15 degrees C, an anomalously high rotation speed was observed for the first peak at low external conductivities. Interestingly, this finding corresponds to the change in the chloride transport rate described by Brahm [Brahm, 1977, J. Gen. Physiol. 70: 283-306.].

A unified resistor-capacitor model for impedance, dielectrophoresis, electrorotation, and induced transmembrane potential

Dielectric properties of suspended cells are explored by analysis of the frequency-dependent response to electric fields. Impedance (IMP) registers the electric response, and kinetic phenomena like orientation, translation, deformation, or rotation can also be analyzed. All responses can generally be described by a unified theory. This is demonstrated by an RC model for the structural polarizations of biological cells, allowing intuitive comparison of the IMP, dielectrophoresis (DP), and electrorotation (ER) methods. For derivations, cells of prismatic geometry embedded in elementary cubes formed by the external solution were assumed. All geometrical constituents of the model were described by parallel circuits of a capacitor and a resistor. The IMP of the suspension is given by a meshwork of elementary cubes. Each elementary cube was modeled by two branches describing the current flow through and around the cell. To model DP and ER, the external branch was subdivided to obtain a reference potential. Real and imaginary parts of the potential difference of the cell surface and the reference reflect the frequency behavior of DP and ER. The scheme resembles an unbalanced Wheatstone bridge, in which IMP measures the current-voltage behavior of the feed signal and DP and ER are the measuring signal. Model predictions were consistent with IMP, DP, and ER experiments on human red cells, as well as with the frequency dependence of field-induced hemolysis. The influential radius concept is proposed, which allows easy derivation of simplified equations for the characteristic properties of a spherical single-shell model on the basis of the RC model.

Low frequency electrorotation of fixed red blood cells

Electrorotation of fixed red blood cells has been investigated in the frequency range between 16 Hz and 30 MHz. The rotation was studied as a function of electrolyte conductivity and surface charge density. Between 16 Hz and 1 kHz, fixed red blood cells undergo cofield rotation. The maximum of cofield rotation occurs between 30 and 70 Hz. The position of the maximum depends weakly on the bulk electrolyte conductivity and surface charge density. Below 3.5 mS/m, the cofield rotation peak is broadened and shifted to higher frequencies accompanied by a decrease of the rotation speed. Surface charge reduction leads to a decrease of the rotation speed in the low frequency range. These observations are consistent with the recently developed electroosmotic theory of low frequency electrorotation.