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PubMed1978

The electrical response to light of bacteriorhodopsin in planar membranes.

We have measured the light-induced short-circuit current generated by a planar membrane containing bacteriorhodopsin incorporated by vesicle fusion. The experimental results are consistent with an equivalent electrical circuit analogue that assumes that the vesicles remain intact after fusion and that the current generator equivalent of the light-driven proton pump is linearly dependent on bias voltage. The transient response to light of the planar membrane has also been examined. Slow response times are seen to be associated with the capacitive charging and discharging of the fused vesicles. A study of the leading edge of the light response curve of the planar membrane yields information about the transient response of the light-driven proton pump. We propose that the translocation of protons across the membrane is associated with a first-order process characterized by a rate constant lambda.

Bacteriorhodopsins↗
PubMed2003

Alkyl monolayer passivated metal-semiconductor diodes: 2: Comparison with native silicon oxide.

To understand the electrical properties at passivated metal-semiconductor interfaces, two types of mercury-insulator-silicon (n-type) junctions, Hg\C10H21-Si and Hg\SiO2-Si, were fabricated and their current-voltage and capacitance-voltage characteristics compared. Both of them exhibited near-ideal rectifying characteristics with an excellent saturation current at reverse bias, which is in contrast to the previously reported ohmic behavior of an unmodified mercury-silicon junction. The experimental results also indicated that the n-decyl monolayer passivated junction possesses a higher effective barrier height, a lower ideality factor (that is, closer to unity), and better reproducibility than that of native silicon oxide. In addition, the dopant density and build-in potential, extracted from capacitance-voltage measurements of these passivated mercury-silicon junctions, revealed that alkyl monolayer derivatization does not alter the intrinsic properties of the silicon substrate. The calculated surface state density at the alkyl monolayer\silicon interface is lower than that of the silicon oxide\silicon interface. The present study increases the possibility of using advanced organic materials as ultrathin insulator layers for miniaturized, silicon-based microelectronic devices.

Journal Article↗
PubMed2000

Characterization of single-cell electroporation by using patch-clamp and fluorescence microscopy.

Electroporation of single NG108-15 cells with carbon-fiber microelectrodes was characterized by patch-clamp recordings and fluorescence microscopy. To minimize adverse capacitive charging effects, the patch-clamp pipette was sealed on the cell at a 90(o) angle with respect to the microelectrodes where the applied potential reaches a minimum. From transmembrane current responses, we determined the electric field strengths necessary for ion-permeable pore formation and investigated the kinetics of pore opening and closing as well as pore open times. From both patch-clamp and fluorescence microscopy experiments, the threshold transmembrane potentials for dielectric breakdown of NG108-15 cells, using 1-ms rectangular waveform pulses, was approximately 250 mV. The electroporation pulse preceded pore formation, and analyte entry into the cells was dictated by concentration, and membrane resting potential driving forces. By stepwise moving a cell out of the focused field while measuring the transmembrane current response during a supramaximal pulse, we show that cells at a distance of approximately 30 microm from the focused field were not permeabilized.

Animals↗
PubMed1969

The electrical characteristics of fixed charge membranes: solution of the field equations.

A theoretical analysis is made of the electrical characteristics of a membrane containing two fixed charge regions, of opposite sign, in contact. Profiles of ion concentrations, electrostatic potential, space charge density, as well as the voltage-current characteristics were obtained by numerical integration of the field equations on a computer. Comparison with the predictions of an earlier analysis of this system (Coster, 1965) shows that the latter is valid to a good approximation for membranes > 70 A in thickness. In particular the form of the electrical characteristics, including the punch-through effect, have been verified by the computer analysis. The range of useful validity of the earlier analysis, the use of Boltzmann statistics when currents are present, and variation of membrane capacitance with applied potential, are discussed in the light of the results obtained.

Computers↗
PubMed1993

Monitoring of segmental intra- and extracellular volume changes using electrical impedance spectroscopy.

Osmotically induced cellular volume changes in the perfused rat hindlimb were used to validate the use of bioelectrical impedance spectroscopy as a method for observing fluid shifts between the intracellular and extracellular spaces. Electrical impedance spectra were measured as cell volumes were manipulated by perfusion with Krebs-Henseleit solutions having different concentrations of NaCl. A simple equivalent circuit model of current conduction through the monitored tissue was fit to each measured spectrum to obtain segmental values of the equivalent intracellular resistance, membrane capacitance, and extracellular resistance. These parameters are theoretically governed by variations in the average cell volume fraction and ionic concentrations in the intra- and extracellular fluid spaces. In accord with this theoretical dependence, the parameters changed systematically and reversibly in conformance with both the magnitudes and directions of the perfusate concentration changes and the resultant cell volume changes. Results indicate that bioelectrical impedance spectroscopy, coupled with computer-aided equivalent circuit analysis, can be used to monitor segmental intercompartmental fluid shifts at minute-by-minute resolution.

Algorithms↗
PubMed1999

Multiple effects of 1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole hydrochloride (SKF 96365) on Ca2+ signaling in MDCK cells: depletion of thapsigargin-sensitive Ca2+ store followed by capacitative Ca2+ entry, activation of a direct Ca2+ entry, and inhibition of thapsigargin-induced capacitative Ca2+ entry.

The effect of 1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole hydrochloride (SKF 96365) on Ca2+ signaling in Madin Darby canine kidney (MDCK) cells was examined. SKF 96365 at 25-100 microM evoked a robust [Ca2+]i transient in a dose-dependent manner, measured by fura-2 fluorimetry. A concentration of 10 microM SKF 96365 did not have an effect. The transient consisted of a slow rise, a gradual decay, and a sustained plateau in physiological Ca2+ medium. Removal of extracellular Ca2+ reduced the Ca2+ signals evoked by 50-100 microM SKF 96365 by nearly half in the area under the curve, suggesting that SKF 96365 induced intracellular Ca2+ release and also extracellular Ca2+ influx. A concentration of 100 microM SKF 96365 caused significant Mn2+ quench of fura-2 fluorescence, which was partly inhibited by La3+ (1 mM) or Gd3+ (0.1 mM), indicating that the SKF 96365-induced Ca2+ influx had two components: one is sensitive to La3+ (1 mM) or Gd3+ (0.1 mM), the other is not. The internal Ca2+ source for the SKF 96365-induced [Ca2+]i transient was the endoplasmic reticulum Ca2+ store because, pretreatment with thapsigargin and cyclopiazonic acid, two inhibitors of the endoplasmic reticulum Ca2+ pump nearly abolished the SKF 96365-induced [Ca2+]i increase in Ca2+-free medium. In contrast, pretreatment with 100 microM SKF 96365 only partly depleted the thapsigargin-sensitive Ca2+ store. Addition of 10 mM Ca2+ induced a significant [Ca2+]i increase after prior incubation with 100 microM SKF 96365 in Ca2+-free medium, demonstrating that SKF 96365 induced capacitative Ca2+ entry. This capacitative Ca2+ entry was about 40% of that induced by 1 microM thapsigargin. Additional to inducing its own capacitative Ca2+ entry, 100 microM SKF 96365 partly inhibited thapsigargin- or uridine trisphosphate (UTP)-induced capacitative Ca2+ entry. We also investigated the mechanisms underlying the decay of the SKF 96365-induced [Ca2+]i transient. Inhibition of the plasma membrane Ca2+ pump with La3+ or Gd3+, or lowering extracellular Na+ level to 0.35 mM, significantly increased the SKF 96365-induced [Ca2+]i transient. In contrast, the mitochondrial uncoupler carbonylcyanide m-chlorophenylhydrazone had little effect. In Ca2+-free medium, the thapsigargin-induced [Ca2+]i increase was greatly reduced by pretreatment with SKF 96365. Collectively, we have found that besides its well-known inhibitory action on capacitative Ca2+ entry in many cell types, in MDCK cells SKF 96365 exerted multiple and complex effects on Ca2+ signaling. It induced a considerable increase in [Ca2+]i by releasing Ca2+ from the endoplasmic reticulum store followed by capacitative Ca2+ entry. It also caused a direct Ca2+ entry. The decay of the SKF 96365 response was significantly governed by efflux via the plasma membrane Ca2+ pump or Na+/Ca2+ exchange. Sequestration by mitochondria or the endoplasmic reticulum played a minor role. We caution use of SKF 96365 as an inhibitor of capacitative Ca2+ entry.

Animals↗
PubMed1985

An experimental comparison between the continuum and single jump descriptions of nonactin-mediated potassium transport through black lipid membranes.

The continuum and single jump treatments of ion transport through black lipid membranes predict experimentally distinguishable results, even when the same mechanistic assumptions are made and the same potential-distance profile is used. On the basis of steady-state current-voltage curves for nonactin-mediated transport of potassium ions, we find that the continuum model describes the data accurately, whereas the single jump model fails to do so, for all cases investigated in which capacitance measurements indicate that the membrane thickness varies little with applied potential.

Anti-Bacterial Agents↗
PubMed1998

Comparison of secretory responses as measured by membrane capacitance and by amperometry.

We have compared capacitance and amperometric measurements in bovine chromaffin cells when secretion was elicited by flash photolysis of caged-calcium or step depolarizations. Total amperometric charge depended linearly on the amount of capacitance increase in both types of experiments. Furthermore, the properties of resolvable amperometric spikes after flashes were comparable to those observed after depolarizations, and their timing was compatible with the rate of capacitance increase. For a more detailed comparison, we used Monte Carlo simulations of multiple amperometric events occurring randomly over the surface of a sphere and summing together, to generate a reference amperometric signal for a given measured capacitance increase. Even after correction for endocytotic processes, the time courses of the integrated experimental records lagged behind the integrated Monte Carlo records by approximately 50 ms in flash and depolarization experiments. This delay was larger by approximately 40 ms than what can be expected from the "pre-foot delay" or the foot duration. Possible sources for the remaining delay could be diffusional barriers like the patch-pipette and the chamber bottom, which are not taken into account in the model. We also applied a novel type of fluctuation analysis to estimate the relative quantum size of an amperometric event. On average the estimates from experimental amperometric traces, in both flash and depolarization experiments, were 3-5 times smaller than estimates from simulated ones. This discrepancy can be due to contributions to the amperometric current from small vesicles, preferred release from cellular regions orientated toward the chamber bottom, or abundance of "foot-only" events. In conclusion, amperometric signals in flash and depolarization experiments displayed similar delayed average time courses and a lower estimate for the relative quantum size compared to the modeled amperometric signals. However, individual amperometric spikes were in agreement with expectations derived from capacitance signals.

Adenosine Triphosphate↗
PubMed1998

Dielectric spectroscopy in the cultivation of suspended and immobilized hybridoma cells.

Dielectric spectroscopy, also referred to as capacitance measurement, was evaluated as a tool for on-line and real-time monitoring of hybridoma cell growth in suspension batch culture and in immobilized cell culture in porous glass carriers. The capacitance signal proved to be a lumped parameter influenced by cell concentration, cell size and culture conditions. During a batch culture the cell specific capacitance signal changes by about 45% having a maximum value at the maximum growth rate. An excellent correlation between the specific capacitance and the specific amount of nucleotidetriphosphates in the cells could be shown. Dielectric spectroscopy proved to be a useful tool for on-line monitoring of cell attachment and growth in open porous microcarriers in fluidized bed fermenters. Also, in this system only an approximate correlation with viable cell concentration appeared, whereas an exact correlation with the glutamine consumption rate, a measure of the metabolic activity of the cells, could be shown. This allowed a closed loop control of the medium feed rate, which was directly linked to the capacitance signal during the entire course of a continuous fermentation.

Animals↗
PubMed1998

Neuropeptide Y-induced potentiation of noradrenergic vasoconstriction in the human saphenous vein: involvement of endothelium generated thromboxane.

1. We investigated the potentiating effect of low concentrations of neuropeptide Y (NPY) on the vasoconstriction induced by transmural nerve stimulation (TNS) and noradrenaline (NA) in human saphenous veins. The effects of (i) endothelium removal; (ii) the addition of the NO pathway precursor L-arginine; (iii) the ET(A)/ET(B) endothelin receptor antagonist Ro 47-0203; (iv) the cyclo-oxygenase inhibitor, indomethacin; (v) the selective thromboxane A2 (TxA2) receptor antagonists Bay u3405 and ifetroban, and (vi) the TxA2 synthase inhibitor, UK 38485, were studied in order to gain information about the mechanisms of NPY-induced potentiation. 2. Contractile response curves for TNS (0.5-8 Hz) and for exogenously administered NA (0.1-3 microM) were obtained in superfused saphenous vein rings. The contractions induced by both TNS and NA at all tested frequencies and concentrations, respectively, were significantly potentiated by 50 nM NPY in endothelium intact veins. Conversely, in endothelium-denuded vessel rings the contractile-response curves to TNS and NA overlapped both in the absence and presence of NPY, thus suggesting that a release of vasoactive substances from endothelial cells could account for the noradrenergic NPY-induced potentiation. 3. In vessels with intact endothelium, the potentiating action of NPY on TNS and NA was unaffected by the presence of high concentrations of the NO precursor L-arginine (3-10 mM) or the non-selective ET(A)/ET(B) endothelin receptor antagonist, Ro 47-0203 (10 microM). These data indicate that the NPY-induced effect does not involve either the endothelium-derived vasodilator nitric oxide or the vasoconstrictor endothelin. Conversely, in the presence of the cyclo-oxygenase inhibitor, indomethacin (30 microM), NPY failed to potentiate the vasoconstrictions produced by either nerve stimulation or by exogenous NA, thus providing evidence that arachidonic acid metabolites through the cyclo-oxygenase pathway are mainly responsible for the potentiation evoked by NPY. 4. When the TxA2 receptor antagonists, Bay u 3405 (1 microM) and ifetroban (1 microM) were added to the superfusing medium, NPY did not alter either the frequency- or the concentration-response curves for either TNS or NA. Accordingly, both TNS- and NA-induced contractions were not potentiated by NPY in the presence of the TxA2 synthase inhibitor, UK 38485 (10 microM). This clearly demonstrates the pivotal role of TxA2 in NPY-induced potentiation. 5. In superfused vein rings with endothelium, a subthreshold concentration (0.2 nM) of the TxA2 mimetic U 46619 potentiated both TNS- and NA-induced vasoconstrictions. This potentiation was higher at low stimulation frequencies and low NA concentrations, and resembled that produced by NPY. 6. Our results indicate that in the human saphenous vein NPY potentiates the contractions produced by sympathetic nerve stimulation acting at the postjunctional level, primarily on endothelial cells. In particular, the NPY-induced release of a cyclo-oxygenase metabolite, namely TxA2, may have a synergistic effect on the vasoconstriction induced by the noradrenergic mediator. Thus, such a mechanism may play a key role in the maintenance of the sympathetic tone of large human capacitance vessels.

15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5↗
PubMed2005

Two-electrode biopotential measurements: power line interference analysis.

In this paper, an analysis of power line interference in two-electrode biopotential measurement amplifiers is presented. A model of the amplifier that includes its input stage and takes into account the effects of the common mode input impedance Z(C) is proposed. This approach is valid for high Z(C) values, and also for some recently proposed low-Z(C) strategies. It is shown that power line interference rejection becomes minimal for extreme Z(C) values (null or infinite), depending on the electrode-skin impedance's unbalance deltaZ(E). For low deltaZ(E) values, minimal interference is achieved by a low Z(C) strategy (Z(C) = 0), while for high deltaZ(E) values a very high Z(C) is required. A critical deltaZ(E) is defined to select the best choice, as a function of the amplifier's Common Mode Rejection Ratio (CMRR) and stray coupling capacitances. Conclusions are verified experimentally using a biopotential amplifier specially designed for this test.

Amplifiers, Electronic↗
PubMed1977

Determination of muscle cable parameters from a single membrane voltage response.

Computer techniques have been developed to achieve a least-squares fit of the Hodgkin and Rushton one time-constant cable equation to the recorded responses of muscle cells to the intracellular injection of square current pulses. In the rat diaphragm the response of the muscle fiber to square current pulses is well fit by a one time-constant model. This makes possible the estimation of the passive electrical properties of muscle sarcolemma using the response of the fiber to a single stimulus. A comparison of the results of this method and the older method of spatial decay in the rat diaphragm shows no significant differences. Average values obtained using the one-point method for estimating membrane resistance and capacitance are 520 omega cm2 and 4.6 micronF/cm2, respectively. An estimation of the average specific resistivity of the cytoplasm was made using this method, and it was found to be 213 omega cm. At levels of injected currents producing large membrane hyperpolarizations where delayed conductance changes make the spatial decay method useless, the one-point method continues to give consistent results since it utilizes only the early portion of the membrane response. Work with the frog sartorius muscle, which is known to display the characteristics of a two time-constant system, shows that this method is capable of estimating the slower components of this system by using only the later portion of the response curve. The immunity of this method to delayed conductance changes and its experimental facility make it useful in estimating the passive electrical properties of muscle fibers, particularly when working with tissues that are small and delicate or that have poor in vitro viability.

Animals↗
PubMed2006

Tumor necrosis factor-alpha downregulates the voltage gated outward K+ current in cultured neonatal rat cardiomyocytes: a possible cause of electrical remodeling in diseased hearts.

BACKGROUND: Inflammatory cytokines have been reported to contribute to the progression of cardiac remodeling in various heart diseases and a remarkable prolongation of the monophasic action potential duration and reductions in the expression of Kv4.2 and K+ channel-interacting protein-2 (KChIP-2) in a rat autoimmune myocarditis model have been documented. In this study, the effect of tumor necrosis factor-alpha (TNF-alpha) on cultured cardiomyocytes was evaluated, focusing on the change in the voltage-gated outward K+ current and expression of related molecules. METHODS AND RESULTS: Cardiomyocytes isolated from 1-day-old Lewis rats were cultured for 72 h and treated with TNF-alpha (50 ng/ml) for an additional 48 h. The myocytes treated with TNF-alpha showed a 22% reduction in the peak K+ current, which consisted of a transient outward K+ current (Ito) and 1.4-fold enhancement of the cell-capacitance in comparison with the control. Among the cardiac ion channel related molecules evaluated in this study, Kv4.2 and KChIP-2 mRNA exhibited remarkable reductions (p < 0.05). CONCLUSIONS: Treatment with TNF-alpha induced reductions in Ito as well as cellular hypertrophy in neonatal cultured myocytes, which indicates that TNF-alpha might play a role in promoting electrical remodeling of cardiomyocytes under inflammatory conditions.

Action Potentials↗
PubMed2006

AC electro-osmotic mixing induced by non-contact external electrodes.

We demonstrate efficient mixing in a micro-fluidic reservoir smaller than 10 microL using ac electro-osmosis driven by field-induced polarization. Our mixing device, of that electrodes are outside of the mixing unit, consists of three circular reservoirs (3mm in diameter) connected by a 1 mm x 1 mm channel. Unlike dc electro-osmosis, whose polarization is from charged substrate functional groups, this new mechanism uses the external field to capacitively charge the surface and the surface capacitance becomes the key factor in the electrokinetic mobility. The charging and mixing are enhanced at tailor-designed channel corners by exploiting the high normal fields at geometric singularities. The induced surface dielectric polarization and the resulting electric counter-ion double layer produce an effective Zeta potential in excess of 1 V, over one order of magnitude larger than the channel Zeta potential. The resulting ac electro-osmotic slip velocity scales quadratically with respect to the applied field, in contrast to the linear scaling of dc electro-osmosis and at 1cm/s and larger, exceeds the classical dc values by two orders of magnitude. The polarization is non-uniform at the corners due to field leakage to the dielectric substrate and the inhomogeneous slip velocity produces intense mixing vortices that effectively homogenize solutes in 30s in a 3mm reservoir, in contrast to hour-long mixing by pure diffusion.

Complex Mixtures↗
PubMed2001

Estimates of the intramembrane field through the harmonics of capacitive current in inhomogeneous bilayer lipid membranes.

Intramembrane field gives information about localisation of fixed charges or dipoles inside the lipid bilayer. There is systematic discrepancy between field estimates made by various methods. The possible reason of this discrepancy can be attributed to the misinterpretation of the data in the frames of the methods used. It stands for the method that is based on the compensation of the 2nd harmonic of capacitive current generated due to electrostriction phenomenon if sine voltage is applied to the bilayer. The theoretical grounds of the method mentioned are oversimplified because membrane heterogeneity has not been taken into consideration. The purpose of the work is the analysis of the generation of harmonics of capacitive current in inhomogeneous bilayer if intramembrane charges are located at different depth. The results of the study enable one to determine the position of intramembrane charges. The theoretical methods are used. The bilayer electrostriction induced by the electric field in the presence of intramembrane charges is computed. The intramembrane field depends upon localisation of the charges inside the bilayer like a sine curve; it goes to zero if the charges are located in the centre of the membrane. The charge discreteness affects the value of the compensation voltage due to nonlinearity of the bilayer deformations close to the charge. The probable appendices of outcomes are discussed for problems of intramembrane dye localisation and ion transport in the channel of sodium/potassium ATPase.

Lipid Bilayers↗
PubMedDate not supplied

[Heart fibroblasts, the mechanism of the appearance of their potentials and their possible role in regulating cardiac work].

Electrically non-excitable fibroblasts, which represent the other population of cells abundant in the sino-atrial node region, have been reported to be mechanosensitive in the frog and in the rat heart. It was shown that these cells respond to artificial or contraction-induced stretch of the atrial wall by a change in membrane potential. These changes could be explained by the operation of stretch-activated channels and intracellular calcium oscillation. Influences of cardiac fibroblasts on electrophysiological properties of cardiomyocytes would require interaction between these cells. In tissue culture studies, it has been shown, that fibroblasts and cardiomyocytes form nexus connections. In recent studies on fibroblast-cardiomyocyte junctions in the rabbit heart pacemaker region, non nexus-like contacts clearly dominated. These membrane non nexus-like contacts might promote capacitive interactions between heterologous cells, which has been demonstrated independently in electrophysiological studies. Through these contacts, the fibroblast membrane potential may affect the membrane potential of neighbouring myocytes in the right atrium which may play an important role for the chronotropic response of the heart to mechanical stretch of the right atrial wall. Electrically non-excitable but mechanosensitive cardiac fibroblasts can act as a substrate for an intracardiac mechano-electrical feedback mechanism by which mechanical changes, e.g. stretch, modulate the electrical activity. In the atria, fibroblasts may act as volume and mechanical sensors, respectively.

Animals↗