On the dielectrically observable consequences of the diffusional motions of lipids and proteins in membranes. 2. Experiments with microbial cells, protoplasts and membrane vesicles

Role of Plasma Membrane at Dielectric Relaxations and Intermembrane Interaction in Human Erythrocytes

Dielectric relaxations at 1.4 MHz (β_sp) and 9 MHz (γ1_sp) on the erythrocyte spectrin network were studied by dielectric spectroscopy using dense suspensions of erythrocytes and erythrocyte ghost membranes, subjected to extraction with up to 0.2% volume Triton-X-100. The step-wise extraction of up to 60% of membrane lipids preserved γ1_sp and gradually removed β_sp-relaxation. On increasing the concentration up to 100 mM of NaCl at either side of erythrocyte plasma membranes, the β_sp-relaxation was linearly enhanced, while the strength of γ1_sp-relaxation remained unchanged. In media with NaCl between 100 and 150 mM β_sp-relaxation became slightly inhibited, while γ1_sp-relaxation almost disappeared, possibly due to the decreased electrostatic repulsion allowing erythrocytes to come into closer contact. When these media contained, at concentrations 10-30 mg/mL dextran (MW 7 kDa), polyethylene glycol or polyvinylpyrrolidone (40 kDa), or albumin or homologous plasma with equivalent concentration of albumin, the γ1_sp-relaxation was about tenfold enhanced, while β_sp-relaxation was strengthened or preserved. The results suggest the Maxwell-Vagner accumulation of ions on the lipid bilayer as an energy source for β_sp-relaxation. While β_sp-relaxation appears sensitive to erythrocyte membrane deformability, γ1_sp-relaxation could be a sensitive marker for the inter-membrane interactions between erythrocytes.

The promiscuous binding of pharmaceutical drugs and their transporter-mediated uptake into cells: what we (need to) know and how we can do so

A recent paper in this journal sought to counter evidence for the role of transport proteins in effecting drug uptake into cells, and questions that transporters can recognize drug molecules in addition to their endogenous substrates. However, there is abundant evidence that both drugs and proteins are highly promiscuous. Most proteins bind to many drugs and most drugs bind to multiple proteins (on average more than six), including transporters (mutations in these can determine resistance); most drugs are known to recognise at least one transporter. In this response, we alert readers to the relevant evidence that exists or is required. This needs to be acquired in cells that contain the relevant proteins, and we highlight an experimental system for simultaneous genome-wide assessment of carrier-mediated uptake in a eukaryotic cell (yeast).

The influence of the molecular structure of lipid membranes on the electric field distribution and energy absorption

We consider the influence of the molecular structure of phospholipid membranes on their dielectric properties in the radio frequency range. Membranes have a stratified dielectric structure on the submolecular level, with the lipid chains forming a central hydrophobic layer enclosed by the polar headgroups (HGs) and bound water layers. In our numerical model, isotropic permittivities of 2.2 and 48.8 were assigned to the lipid chain and bound water layers, respectively. The HG region was assumed to possess an anisotropic static permittivity with 142.2 and 30.2 in the tangential and normal directions, respectively. The permittivities of the HG and bound water regions have been assumed to disperse at frequencies around 51 and 345 MHz to become 2.2 and 1.8, respectively, in both the normal and tangential directions. Electric field distribution and absorption were calculated for phospholipid vesicles with 75 nm radius as an example. Significant absorption has been obtained in the HG and bound water regions. Averaging the membrane absorption over the layers resulted in a decreased absorption below 1 GHz but a more than 10-fold increase above 1 GHz, compared to a model with a homogeneous membrane of averaged properties. We propose single particle dielectric spectroscopy by AC electrokinetics at low-bulk medium conductivities for an experimental verification of our model.

Time-domain dielectric spectroscopy applied to cell suspensions

A precision difference time-domain reflectometry (TDR) technique is described for the investigation of the dielectric properties of cell suspensions. The dielectric spectra obtained for erythrocytes using TDR are comparable with those reported in previous dielectric studies employing frequency-domain technique.

Impedance spectroscopy of human erythrocytes: system calibration and nonlinear modeling

We present in detail our impedance measurement method, the cell embodding technique, for human erythrocytes, and an accurate calibration procedure for a true four-electrode impedance measurement system. This technique with the calibration procedure gives reliable impedance measurements over a wide frequency range--1 Hz to 10 MHz. To achieve high sensitivity in this frequency range, we embed the cells in the pores of a Nuclepore filter. The calibration procedure assumes that the measurement system is linear, and requires measurement of three reference impedances. The reliability of this procedure is demonstrated with various RC circuits, and application of it to the bio-impedancae measurement system eliminates a quasi-dispersion in the high-frequency range, and increases the bandwidth at both the low- and high-frequency ends of the range by about a decade. We model the impedance of the cells embedded within the filter with an equivalent circuit that is consistent with the geometry and interfaces present. The experimental data are fitted to this model by means of a complex nonlinear least squares (CNLS) fit, which simultaneously fits the real and imaginary parts of the impedance with the Levenberg-Marquardt algorithm. The impedance spectra of human erythrocytes are found to display constant-phase-angle (CPA) characteristics. A CPA element is an impedance of the form Z = A/(jw) alpha, where A is a constant, j = square root -1, omega is angular frequency, and 0 < alpha < 1, and has been used to describe the ac response of the interface between the cell surface and the external electrolyte solution. Such a CPA element may be related to fractal character of the interface.

Hypotonically induced changes in the plasma membrane of cultured mammalian cells

Cells from three cell lines were electrorotated in media of osmotic strengths from 330 mOsm to 60 mOsm. From the field-frequency dependence of the rotation speed, the passive electrical properties of the surfaces were deduced. In all cases, the area-specific membrane capacitance (Cm) decreased with osmolality. At 280 mOsm (iso-osmotic), SP2 (mouse myeloma) and G8 (hybridoma) cells had Cm values of 1.01 +/- 0.04 microF/cm2 and 1.09 +/- 0.03 microF/cm2, respectively, whereas dispase-treated L-cells (sarcoma fibroblasts) exhibited Cm = 2.18 +/- 0.10 microF/cm2. As the osmolality was reduced, the Cm reached a well-defined minimum at 150 mOsm (SP2) or 180 mOsm (G8). Further reduction in osmolality gave a 7% increase in Cm, after which a plateau close to 0.80 microF/cm2 was reached. However, the whole-cell capacities increased about twofold from 200 mOsm to 60 mOsm. L-cells showed very little change in Cm between 280 mOsm and 150 mOsm, but below 150 mOsm the Cm decreased rapidly. The changes in Cm correlate well with the swelling of the cells assessed by means of van't Hoff plots. The apparent membrane conductance (including the effect of surface conductance) decreased with Cm, but then increased again instead of exhibiting a plateau. The rotation speed of the cells increased as the osmolality was lowered, and eventually attained almost the theoretical value. All measurements indicate that hypo-osmotically stressed cells obtain the necessary membrane area by using material from microvilli. However, below about 200 mOsm the whole-cell capacities indicate the progressive incorporation of "extra" membrane into the cell surface.

Study of the electrical impedance of blood from house painters

Using a four-electrode technique the electrical impedance was measured at 150 kHz (100 microA) on whole blood samples from 13 male house painters (age 19-49 years; median 35 years) exposed occupationally to vapors from alkyd paints containing organic solvents and from a control group of 7 healthy unexposed men (age 27-53 years; median 37 years). The blood impedance of the two groups was monitored prior to and 9 days after an initial administration of acetylsalicylic (600 mg). The resistivity calculated from the impedance of whole blood from the painters remained significantly higher than the corresponding values for the controls. The hematological values for the two groups were within normal ranges, although the painters had significantly higher platelet counts than the controls. Effects of long-term exposure of the painters to volatile solvents are discussed.