Network connectivity between psychological resilience and depression in prostate cancer patients

OBJECTIVES

Prostate cancer (PCa) patients often experience depression. One possible buffer against stress-related depression is psychological resilience (PR), which has been described as heterogeneous in structure, like major depressive disorder (MDD). Although both of these constructs are central to understanding and assisting distressed PCa patients, no data have been reported on how they connect via network arrays at a component and symptom level. Such information has the potential to inform clinical practice with depressed PCa patients.

METHODS

Using a cross-sectional design, 555 PCa patients completed the Patient Health Questionnaire-9 (PHQ-9) and the Connor-Davison Resilience Scale (CDRISC). Data were analysed via network analysis.

RESULTS

Network analysis indicated that various CDRISC factors interacted with different PHQ-9 symptoms. For example, trust in one's instincts, tolerance of negative affect, and strengthening effects of stress (CDRISC) was associated with concentration problems and suicidal ideation (PHQ-9); positive acceptance of change, and secure relationships (CDRISC) was linked to low self-worth, anhedonia, fatigue/lethargy, motor problems, depressed mood, and concentration and appetite problems (PHQ-9). Similarly heterogeneous associations were found between individual CDRISC items and PHQ-9 symptoms. Network analytic figures depict both these sets of associations.

CONCLUSIONS

As well as confirming the heterogeneous nature of PR and MDD in PCa patients, these findings argue for the further development of 'individualised' medicine approaches when working with PCa patients and their experiences of depression.

Frontal Alpha Asymmetry Argues for the Heterogeneity of Psychological Resilience

Depression is associated with frontal alpha asymmetry (FAA) and Psychological Resilience (PR), although in different ways. Only cursory attention has been given to how these three constructs interact despite the possible clinical and research implications of those associations. One limitation of recent research into these associations has been conceptualising PR as a unitary construct, whereas it has been shown to be multi-component. This study investigated the underlying components of PR, their correlations with FAA, and the effect that participants' depressive status had upon those correlations in a community sample of 54 males and 46 females aged between 18 yr and 75 years. Results confirmed the overall inverse association between total PR and depression for four of the original five PR components and for one of the two components found in this sample. Similarly, there were differences between the ways that FAA and PR components were associated, depending upon the depressive status of participants. Source localisation data indicated that the PR components were not uniformly correlated with alpha activity in the same brain regions. These findings of content, efficacy, and neurophysiological differences between the five components of PR and their associations with FAA argue against consideration of PR as a unitary construct.

Network analysis of frontal lobe alpha asymmetry confirms the neurophysiological basis of four subtypes of depressive behavior

Introduction

Although depression is widespread carries a major disease burden, current treatments remain non-universally effective, arguably due to the heterogeneity of depression, and leading to the consideration of depressive "subtypes" or "depressive behavior subtypes." One such model of depressive behavior (DB) subtypes was investigated for its associations with frontal lobe asymmetry (FLA), using a different data analytic procedure than in previous research in this field.

Methods

100 community volunteers (54 males, 46 females) aged between 18 yr. and 75 years (M = 32.53 yr., SD = 14.13 yr) completed the Zung Self-rating Depression Scale (SDS) and underwent 15 min of eyes closed EEG resting data collection across 10 frontal lobe sites. DB subtypes were defined on the basis of previous research using the SDS, and alpha-wave (8-13 Hz) data produced an index of FLA. Data were examined via network analysis.

Results

Several network analyses were conducted, producing two models of the association between DB subtypes and FLA, confirming unique neurophysiological profiles for each of the four DB subtypes.

Discussion

As well as providing a firm basis for using these DB subtypes in clinical settings, these findings provide a reasonable explanation for the inconsistency in previous FLA-depression research.

Network analysis of depression in prostate cancer patients: Implications for assessment and treatment

OBJECTIVES

Many prostate cancer patients also suffer from depression, which can decrease their life satisfaction and also impede recovery from their cancer. This study described the network structure of depressive symptomatology in prostate cancer patients, with a view to providing suggestions for clinical interventions for depressed patients.

METHODS

Using a cross-sectional design, 555 prostate cancer patients completed the Patient Health Questionnaire-9 (PHQ-9).

RESULTS

Network analysis and multidimensional scaling indicated that anhedonia was the most central symptom for these men, and that several sets of depression symptoms were closely associated with each other. These included anhedonia-depressed mood; sleeping problems-fatigue/lethargy; and suicidal ideation-low self-worth-depressed mood. Other depression symptoms such as appetite problems, concentration problems, and motor problems, were less well-related with the remainder of the network. Patients receiving treatment for reocurring prostate cancer (PCa) had significantly higher PHQ9 scores than patients undergoing their initial treatment, but no major differences in their network structures. Implications for clinical practice were derived from the relationships between individual depression symptoms and the overall depression network by examining node predictability.

CONCLUSIONS

The use of total depression scores on an inventory does not reflect the underlying network structure of depression in PCa patients. Identification and treatment of the central symptom of anhedonia in PCa patients suggests the need to adopt specific therapies that are focussed upon this symptom.

Cluster analysis of autism spectrum disorder symptomatology: Qualitatively distinct subtypes or quantitative degrees of severity of a single disorder?

The decision to collapse several related disorders into a single diagnosis of Autism Spectrum Disorder (ASD) generated significant controversy and debate. There has been mixed evidence as to whether various ASD subtypes are qualitatively distinct or if they exist on a spectrum of symptom severity. The present study conducted a two-step cluster analysis of major ASD symptoms in a sample of 147 young males with ASD aged between 6yr and 18yr with IQ > 70. Results indicated that a two-cluster solution (high and low severity of ASD symptomatology) was reliable and valid. Further, the construct of challenging behaviour was not a necessary component of the two-cluster solution, verifying the new conceptualisation of ASD. Further replication of these findings with other subsets of individuals with ASD is needed.

A comparison of age, cognitive, hormonal, symptomatic and mood correlates of Aggression towards Others in boys with ASD

BACKGROUND

Aggression is a major problem in children with Autism Spectrum Disorder (ASD) but little is known about the possible contributors to this behaviour.

AIMS

To determine the relative strength of the relationships between developmental, cognitive, symptomatic, hormonal and mood factors and 'Aggression towards Others' in boys with ASD.

METHOD

Predictors of Aggression towards Others were investigated in a sample of 136 boys with Autism Spectrum Disorder (M age=11.3yr, SD=3.2yr, range=6yr to 17yr). Data were collected from the boys themselves and their parents (14 fathers, 122 mothers).

RESULTS

Results indicated that age and Low Registration on the Sensory Profile were the only significant correlates of this form of aggression. Importantly, testosterone levels did not account for level of social aggression.

CONCLUSIONS

These data suggest that these boys may have learnt more effective methods of dealing with their frustration as they grew older or benefitted from cognitive maturation, and that having a high neurological threshold may be a source of frustration for these boys. The relationship between Aggression towards Others and Low Registration is discussed and clinical implications of the findings explicated.

Electropermeabilization and fluorescent tracer exchange: the role of whole-cell capacitance

Transmembrane crossing of charged fluorescent tracers such as propidium iodide (PI) and carboxyfluorescein+ (CF) can be used to quantitate membrane permeabilization. Murine myeloma Sp2/0-Ag14 cells were loaded with CF (0.1 fmol/cell) before electropulsation (0.5-3.0 kV/cm, 40 microseconds) in medium containing 25-50 micrograms/ml PI at 21-23 degrees C. Cytograms of PI vs. CF fluorescence showed three readily distinguishable subpopulations: 1) intact living cells with CF but without PI (these form > 95% of the prepulsed population), 2) transiently electropermeabilized but resealed cells showing both CF and low-level PI fluorescence, and 3) permanently permeabilized cells without CF but with very high PI fluorescence. Despite the ready influx of PI, the efflux of CF from transiently permeabilized cells was negligible and was insensitive to pulse parameters; however, electrically killed cells (subpopulation 3) lost all CF fluorescence and probably lost their cytoplasm. This difference in transmembrane passage of the dyes is best explained by binding of intracellular CF to macromolecules (and/or organelles). In isotonic "pulse medium," the membranes resealed after electropulsing with a time constant (tau R) of about 2 min. In 150 mOsm medium, resealing was faster (typically tau R approximately 0.5 min). The population distribution of PI uptake [coefficient of variation (CV) > 40%] was very broad and could not be accounted for by the radius dependence of pulse-induced voltage (CVradius approximately 10%). The variability in PI uptake could be explained if the electrical energy of the charged membrane, which depends on the whole-cell capacitance (Cc), was taken into account. Evaluation of the Cc values with single-cell resolution was based on measurement of the electrical charging time constant of the plasma membrane by electrorotation.

Radio-frequency microtools for particle and liver cell manipulation

Single particles can be manipulated by applying high frequencies to ultramicro electrode arrays fabricated on planar structures. Heat production can be reduced to the extent that intense electric fields can be applied even to unmodified cell culture media. Animal cells grow normally in the high field (up to 100 kV/m) between such continuously energized multielectrodes. As with laser tweezers [1-3], this technique can capture particles and cells in field traps, generate linear movement, and permit cell cultivation. It can also produce micropatterns of pH gradients, field-cast objects, and control cell adhesion. These microtools may be combined to develop cell separators, microsensors, and controlled-biocompatibility surfaces.

DNA, protein, and plasma-membrane incorporation by arrested mammalian cells

Incorporation of DNA, protein, and plasma membrane during blockage by aphidicolin or by doxorubicin was studied by flow cytometry and electrorotation of three cell lines (mouse-myeloma Sp2/0-Ag14, hybridoma H73C11, and fibroblast-like L929 cells). Drug-mediated arrest at the G1-S boundary (aphidicolin) or in G2/M (doxorubicin) did not arrest synthesis of either protein or total membrane area, the increases in which outstripped growth in cell volume and apparent cell area, respectively. Measurements of membrane capacity in normal and hypo-osmotic media showed that the drugs had not changed the fundamental bilayer, but that an increase in the number or size of microvilli must have occurred. Aphidicolin-arrested cells withstood hypo-osmotic stress better than untreated cells could, indicating that the membrane excess can be utilized as a reserve during rapid cell expansion. Hypo-osmotically treated cell populations exhibited only about half the coefficient of variance (CV) in membrane properties of cells at physiological osmolality. Populations of arrested cells exhibited the same high CV as asynchronous cells, indicating that chemical arrest does not give uniformly villated cell populations. However, the lowest CV values were given by some synchronized (aphidicolin-blocked, then released) populations. Removal of aphidicolin allowed most cells to progress through S and G2, and then divide. During these processes, the membrane excess was reduced. After removal of doxorubicin, the cells did not divide: some continued protein synthesis, grew abnormally large, and further increased their membrane excess. Membrane breakdown by electric pulsing (3 x 5kV/cm, 40 microseconds decay time) of aphidicolin-synchronized L cells in G2/M led to a 22% loss of plasma membrane (both the area-specific and the whole-cell capacitance were reduced), presumably via endocytosis-like vesiculation.

Effect of electric field pulses on the viability and on the membrane-bound immunoglobulins of LPS-activated murine B-lymphocytes: correlation with the cell cycle

The effects of microsecond electropulses (1-5 kV/cm) on the viability of murine B lymphocytes and on their binding of antibodies by surface immunoglobulin (Ig) were studied in relation to the cell cycle. Before electropulsing, cultures given 48 h mitogenic stimulation showed at least two cell subpopulations, which were distinguishable by their levels of surface-Ig expression as assessed with FITC-labelled antibodies against mouse Ig. The immunofluorescence intensity of cells in S and G2/M phases was higher than that of G0/G1 cells. After exposure of the mitogen-stimulated lymphocytes to three exponentially decaying (time constant tau = 5-40 microseconds) electric field pulses, dye exclusion assay showed that pulsing at 1 or 2 kV/cm (at 4 degrees C or 20 degrees C) did not cause permeabilization. Field strengths of 3, 4, or 5 kV/cm resulted in 20%, 45%, or 70% of dye-permeable cells, respectively, if the pulsed cells were transferred to phosphate-buffered saline on ice for 30 min. Incubation in full medium at 37 degrees C for 30 min ("resealing") significantly decreased the percentage of permeabilized cells. Electropulsed G0/G1 cells were not only more resistant to direct electric exposure (tolerated higher field strengths) than S + G2/M cells but also responded better to resealing. The surface Ig of lymphocytes pulsed at higher fields and low temperature (4 or 5 kV/cm, tau = 5 microseconds, three pulses, 4 degrees C) was less easily immunostained than in controls or in cells pulsed at 2 kV/cm or less. At 5 kV/cm those cells that were not permeabilized showed a greater reduction in immunostaining, especially if resealed.

A traveling-wave micropump for aqueous solutions: comparison of 1 g and microgram results

The operation of a micro-chip-based fluid circulator based on the interaction of a high-frequency traveling wave with a thermally generated inhomogeneity in an aqueous medium is investigated. The profiles of the electric field, the thermal gradient and the driving force within the device are derived numerically, and the dependence upon applied voltage under different experimental conditions is derived. The importance of convection in the operation of the device is assessed by operating it in various orientations, as well as under microgravity, and also by investigating the reversal and turn-on characteristics. The pump showed the unexpected ability to trap microparticles present in the driven fluid.

Dielectric measurements on electro-manipulation media

The permittivity and conductivity of solutions of sugars and sugar alcohols, such as are suitable for electric-field work on cells, were measured. The range of concentrations was 0-3 M (subject to solubility), and the frequency range was that commonly used in dielectrophoresis prior to electrofusion (200 kHz-2 MHz). This was widened to 30 kHz-12 MHz, when dispersive behaviour was found. A parallel-plate impedance chamber with variable electrode-spacing, suited to stable measurements in these low-conductivity aqueous media, was used. Errors due to electrode polarisation were effectively removed by linear regression of the series parameters against electrode distance, as well as by subtraction of data obtained on KCl solutions of comparable conductivity. The permittivity of the sugar solutions decreased as only approximately linear functions of concentration, so that a description in terms of both first- and second-order molar dielectric increments (delta 1 and delta 2) provided a better description of the behaviour than a single linear increment (delta). Sugar solutions of concentration of 1.2 M or less showed no change in permittivity or conductivity over the measured frequency range (i.e., were dispersion-free). On the other hand, various high-density media showed dispersion. The stabilised silica sol 'Percoll' showed dispersion over the whole frequency range; solutions of sucrose (at above 1.2 M concentration), as well as of proprietary high-density solutes ('Metrizamide' at above 0.5 M and 'Nycodenz' at above 0.75 M) showed dispersion above 0.6 MHz. Although these media are of interest for electro-manipulation (they can be used to prevent sedimentation), their dispersive properties may make them unsuitable for use with radio-frequency fields.

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.

Levitation, holding, and rotation of cells within traps made by high-frequency fields

Biological cells and other particles can be electrically manipulated by means of negative dielectrophoresis within microchambers whose electrode geometry is of the order of the cell size. Very-high-frequency fields (50 MHz and above) and media of increased relative permittivity are especially suitable for the purpose, as shown by experimental data on levitation and rotation. It appears to be possible to move and rotate cells or particles at will using this technology.

Divalent cation-sensitive pores formed by natural and synthetic melittin and by Triton X-100

Leakage of ions and low-molecular-weight metabolites from Lettre cells is induced by synthetic melittin, as effectively as by melittin isolated from bee venom; in each case leakage is inhibited by Ca2+, Zn2+ or H+. Inhibition of leakage by divalent cations is reversible in that Lettre cells incubated with melittin (or with Triton X-100) in the presence of inhibitory amounts of Zn2+, when freed of Zn2+ by EGTA or by centrifugation, begin to leak (in Zn2(+)-sensitive manner). Electrorotation of Lettre cells is altered by melittin, compatible with membrane permeabilization; melittin plus Zn2+ does not alter electrorotation until Zn2+ (and unbound melittin) are removed. Melittin or Triton X-100 added to calcein-loaded liposomes induces leakage of calcein; divalent cations inhibit. Energy transfer between liposome-associated melittin and 2-, 7- or 12-(9-anthroyloxy)stearate (AS) is maximal with 12-AS; addition of Zn2+ has little effect. Circular dichroism spectra of melittin plus liposomes are unaffected by Zn2+. These results show that the formation of divalent cation-sensitive pores is not dependent on the presence of endogenous membrane proteins and that the action of divalent cations is not by displacement of melittin (or Triton) from the lipid bilayer.

Alterations in the electrical properties of T and B lymphocyte membranes induced by mitogenic stimulation. Activation monitored by electro-rotation of single cells

Stimulation of either B or T lymphocytes using specific mitogens results in changes in the passive electrical properties of the cell surface. These effects can be related to growth and secretion. This was possible because the high resolution of the contra-field electro-rotation method, combined with the use of very low conductivity media, allowed accurate and analytically-derived values for the cell surface properties. Increases in effective CM (membrane capacity) and changes in apparent membrane conductivity (reflecting the additive effects of true membrane conductivity GM and surface conductance KS) were measured. After 72 h treatment with concanavalin A, thymocyte CM had increased from 0.76 muF/cm2 to 1.24-1.46 muF/cm2 (7.6 to 12.4-14.6 mF/m2). Allowing for the stimulation-induced size increase (cell radius increased from 2.8 to 4.4 micron) these data imply that the plasma membrane area per cell increases 5-fold during stimulation. Stimulation of B cells (by 3 days incubation with bacterial lipopolysaccharide) increased CM from 0.93 to 1.6-1.7 muF/cm2 (9.3 to 16-17 mF/m2). Incubation without mitogen gave no significant increase in CM or in radius. Control cells of different sizes showed no difference in membrane properties. The increases in effective CM are argued to reflect an increase in membrane ramification (microvilli, folding, etc.). The apparent membrane conductivity of T cells also increased during stimulation, from 5 to 21 mS/cm2 (50 to 210 S/m2). This increase is proportionately much greater than that in CM or in membrane area. It seems to be due to a real increase in GM, but a small increase in KS may also occur. The earliest changes in apparent membrane conductivity were evident between 3 and 5 h after stimulation, before the cells increased in size. This response parallels increases in transmembrane transport reported to follow mitogenic stimulation.

Differences in membrane properties between unfertilised and fertilised single rabbit oocytes demonstrated by electro-rotation. Comparison with cells from early embryos

The apparent membrane capacity of tubular rabbit oocytes increases from 1.7-2.0 microF/cm2 before fertilisation to 3.7-4.0 microF/cm2 after fertilisation. The membrane conductivity measured on single cells was also increased by fertilisation from less than 1 mS/cm2 to 14 mS/cm2. Cells obtained from 2-, 4- or 8-cell embryos exhibited intermediate values of membrane capacity (2.3-2.8 microF/cm2) and conductivity (5-22 mS/cm2). The values quoted are those effective between 1 and 10 kHz, the frequency of the rotating field used. The large apparent capacities are probably due to the presence of structures such as microvilli which cause the actual membrane area to exceed the smooth sphere area. It must be assumed that these structures change in form or number on fertilisation, and that they persist in embryos, at least up to the 8-cell stage. No difference was apparent between cells fertilised in vitro or in vivo. Comparison of the above zona-free data with measurements on zona-complete oocytes indicate how fertilised and unfertilised rabbit eggs may be distinguished from one another, even in the presence of the zona pellucida.

The comparative influence of substituted phenols (especially chlorophenols) on yeast cells assayed by electro-rotation and other methods

The toxicity of 31 phenols was studied by electro-rotation of yeast cells. Control yeast cells show both anti-field and co-field rotation, depending upon the field frequency applied. After treatment with supra-threshold amounts of phenols the anti-field rotation is weakened or abolished and a stronger co-field rotation can be seen. The proportion of cells showing the co-field rotation was found to be a sensitive measure of toxicity. Doses of 2.2 mumol/l of pentachlorophenol, or of 0.3 mumol/l of pentabromophenol were detectable after 3 h incubation at pH 4.0. At a given pH, the toxicity of the chlorophenols correlated extremely well with their octanol:water partition coefficients (Pow). The complete set of phenols showed fair overall correlation with Pow, but less good correlation with their acidity constants (pKa). In particular the toxicity of a given phenol was less than predicted from its pKa if the incubation pH was higher than the pKa. Biochemical assays on 23 of the phenols showed that the rotational sensitivity runs closely parallel to the sensitivities of cell growth rate and of the plasmamembrane ATPase, but less closely to the inhibition of purine incorporation. It appears that the electro-rotation method provides a useful and rapid test for the presence of organic ecotoxins. The test enables us to distinguish differences between single cells, and is comparable in sensitivity to biochemical tests that use vesicles or homogenates derived from a cell population.

The influence of tetraphenylborates (hydrophobic anions) on yeast cell electro-rotation

The action of a series of tetraphenylborate ion (TPB) derivatives on yeast cells was studied by electro-rotation of the pre-treated cells. TPB derivatives in which all four phenyl groups were substituted with fluorine, chlorine or trifluoromethyl were much more toxic than the unsubstituted compound, the effect increasing dramatically with increasing size of substituents. These observations suggest that the toxicity of these hydrophobic ions is determined mainly by their size and possibly also by the chemical inductivity of their substituent groups. The order of the toxicities of these ions was in fair agreement with literature values for their translocation rates across artificial bilayers. Incubation times of 3 h were used as standard, longer incubations (up to 48 h) showed that the number of cells affected by low doses of TPB increased with the logarithm of time after the first hour of incubation. Although measurements of the percentage of cells showing co-field rotation showed that controls were not adversely affected by incubations as long as 9 h, rotation spectra showed that some cells suffer loss of internal conductivity during extended incubations. Decrease of the pH of the incubation medium, or inclusion of high concentrations of NaCl or KCl, potentiated the effects of these hydrophobic ions. The toxicity developed slowly, and the sensitivity of the assay was only very weakly dependent on the cell suspension density.

Electro-rotation of mouse oocytes: single-cell measurements of zona-intact and zona-free cells and of the isolated zona pellucida

Passive electrical properties of oocytes and of zonae pellucidae, and the mechanical coupling between them, can be elucidated by means of rotating-field-induced rotation. In low-conductivity media (25-100 microS/cm) rotation of mouse oocytes (with or without their zonae) requires fields in the 1-100 kHz frequency range. However, an isolated zona shows weak rotation in the opposite direction to that of a cell, and in response to much higher field frequencies (approx. 1 MHz). In zona-intact mouse oocytes, the rotation of cell and zona are not rigidly coupled: thus rotation of the cell can still be induced when the zona is held stationary. However, rotation of freely suspended zona-intact cells is much slower than that of zona-free cells and requires an optimum field frequency that is approximately 1.5 kHz higher. These observations show that the electrical properties of the oocyte that are measured by rotation are altered by the presence of the zona pellucida, even though no such influence has been detected using micro-electrodes. The data are consistent with the zona acting as a porous shell with a conductivity of 40 microS/cm (preliminary estimate made at a single medium conductivity of 26 microS/cm). Measurements on cells from which the zonae had been removed gave values for the membrane capacity and resistivity of 1.2-1.3 microF/cm2 and 400 omega.cm2, respectively. These values may reflect the presence of plasmalemma microvilli. The results strongly suggest that the technique may be useful for studies of cell maturation and for in vitro fertilization, because the cells may be further cultured after measurement.

The effect of mercuric salts on the electro-rotation of yeast cells and comparison with a theoretical model

The rotational spectrum of yeast cells changed after pre-treatment of the cells with HgCl2 or Hg(NO3)2 and became indistinguishable from that of ultrasonically produced cell walls. The spectrum of the affected cells contained a peak which could only be explained by attributing a conductivity to the cell walls that was higher than that of the medium. Theoretical models of the rotational response are fully in accord with the experimental spectra. It is shown that the rotation method is capable of measuring even the low cell wall conductivity of yeast cells (which was found to be 33 microS/cm at 10 microS/cm medium conductivity). Knowledge of the spectra allowed a field frequency to be selected at which untreated cells showed no rotation, but at which cells affected by treatment with Hg(II) identified themselves by rotating in the same direction as the field. Calculation of the percentage of cells showing this co-field rotation gave an index (termed the co-field rotation value) of the proportion of the cells that were affected. Using this technique, effects of 25 nmol/l Hg(II) could be demonstrated. In media of low conductivity (10 microS/cm) the change in the rotational spectrum was usually 'all-or-none', whereas at 200 microS/cm a graded Hg(II)-mediated change became apparent. The co-field rotation method showed that the action of small quantities of Hg(II) was still increasing after 3 h of incubation and paralleled the Hg(II)-induced K+ release. A rapid reduction of the effects of Hg(II) was seen when 3-30 mM K+ (or Na+) or when 1 mM Ca2+ were present in the incubation medium, or as the pH was increased. At high incubation cell concentrations the toxic effect of Hg(II) was reduced, apparently due to binding by the cells.

Electro-fusion of cells: principles and potential for the future

Exposure of cells or liposomes to a brief pulse of a strong electrical field can result in a reversible breakdown of the outer membrane. Such breakdown results in an increase in permeability of the plasmalemma, which however re-seals after a short incubation (i.e. the original impermeability is restored). Two or more cells in contact can be made to fuse by this process, provided that the contact is close enough and that the pulse of the electrical field is short enough not to damage the cells. Methods of achieving this contact by electrical and magnetic fields are described. The magnetic method does not demand the use of the low conductivity media used earlier. Other possible modifications of this flexible technique are also described, and used to show how the technique can be modified in future, and how it may be applied to the fields of membrane research, medicine and plant breeding.