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PubMed2004

Magnetic response of metamaterials at 100 terahertz.

An array of single nonmagnetic metallic split rings can be used to implement a magnetic resonance, which arises from an inductor-capacitor circuit (LC) resonance, at 100-terahertz frequency. The excitation of the LC resonance in the normal-incidence geometry used in our experiments occurs through the coupling of the electric field of the incident light to the capacitance. The measured optical spectra of the nanofabricated gold structures come very close to the theoretical expectations. Additional numerical simulations show that our structures exhibit a frequency range with negative permeability for a beam configuration in which the magnetic field couples to the LC resonance. Together with an electric response that has negative permittivity, this can lead to materials with a negative index of refraction.

Journal Article↗
PubMed1990

Norepinephrine sensitivity of mesenteric veins in pregnant rats.

This study was designed to test the hypothesis that during the course of pregnancy there is a decrease in the venous response to adrenergic stimulation that is characterized by a decrease in venoconstriction to both exogenous norepinephrine (NE) and to transmural electric stimulation of endogenous sympathetic nerves. Capacitance-size mesenteric veins were removed from nonpregnant and early- (7/8 day) and late-pregnant (18/19 and 20/21 day) Sprague-Dawley rats and studied in vitro under pressurized conditions. Lumen diameter was measured continuously by a video-electronic method. There was a marked increase in the sensitivity of the veins to exogenous NE stimulation at the end of pregnancy, which was most dramatic at a transmural pressure of 6 compared with 2 mmHg. The increase in exogenous NE sensitivity was associated with a progressive decline in the response to transmural nerve stimulation during pregnancy. Cocaine, an inhibitor of neuronal uptake, resulted in a greater potentiation of the response to transmural nerve stimulation in the pregnant rats compared to controls, suggesting an increased reuptake mechanism during pregnancy. Additional studies in the pseudopregnant, lactating, and nonlactating rat suggested that the conceptus was necessary for the alterations in neural response but not for the increase in exogenous NE sensitivity. In conclusion there is a dramatic change in venous function during pregnancy in the rat that is characterized by a difference between endogenous and exogenous NE sensitivity. The fact that transmural pressure can profoundly affect exogenous NE sensitivity suggests that pregnancy-induced changes in venous volume could contribute to changes in venous reactivity.

Animals↗
PubMed2006

A mathematical model linking tree sap flow dynamics to daily stem diameter fluctuations and radial stem growth.

To date, models for simulating sap flow dynamics in individual trees with a direct link to stem diameter variation include only the diameter fluctuation driven by a change in stem water storage. This paper reports results obtained with a comprehensive flow and storage model using whole-tree leaf transpiration as the only input variable. The model includes radial stem growth based on Lockhart's equation for irreversible cell expansion. It was demonstrated that including growth is essential to obtaining good simulation results. To model sap flow dynamics, capacitance of storage tissues was assumed either constant (i.e., electrical analogue approach) or variable and dependent on the water content of the respective storage tissue (i.e., hydraulic system approach). These approaches resulted in different shapes for the desorption curve used to calculate the capacitance of storage tissues. Comparison of these methods allowed detection of specific differences in model simulation of sap flow at the stem base (F(stem)) and stem diameter variation (D). Sensitivity analysis was performed to select a limited subset of identifiable parameters driving most of the variability in model predictions of F(stem) and D Both the electrical analogue and the hydraulic system approach for the flow and storage model were successfully calibrated and validated for the case of a young beech tree (Fagus sylvatica L.). Use of an objective model selection criterion revealed that the flow and storage model based on the electrical analogue approach yielded better predictions.

Calibration↗
PubMed1988

Effect of packed cell volume on diastolic coronary artery pressure-flow relations in the dog.

To elucidate the role of the haemorheological properties of the perfusate in the coronary circulation, the diastolic pressure-flow relation was studied in nine open chest heart blocked dogs with minimal vasomotor tone when blood with various packed cell volumes (12-67%) was used as perfusate. An electrical analogue model with proximal resistance R1, capacitance C, distal resistance R2, and the zero flow pressure intercept Pint was derived from the observation of the pressure-flow relation to support the data analysis. The diastolic pressure decay was then determined after the perfusion line had been clamped to calculate stop flow coronary artery pressure (Psf). The stop flow coronary artery pressure decreased in relation to packed cell volume (r = 0.45, p less than 0.01), and the value for the lowest packed cell volume (10-29%) was slightly higher than the great cardiac vein pressure (about 3 mmHg). The zero flow pressure intercept of the steady state pressure-flow relation showed a close correlation with the stop flow coronary artery pressure (r = 0.87, p less than 0.001). The value of R1 + R2, which reflects the inverse of the steady state pressure-flow slope, decreased simultaneously with the packed cell volume (r = 0.62, p less than 0.001). The resistance ratio R2/(R1 + R2) by our model prediction decreased in relation to packed cell volume (r = 0.5, p less than 0.001). The values of stop flow coronary artery pressure, zero flow pressure intercept, and R1 + R2 for the highest packed cell volume (50-69%) were 17.8(1.1) mmHg, 25.1(1.3) mmHg, and 0.48(0.05) mmHg.ml-1.min.100 g-1 respectively, whereas those for the lowest packed cell volume (10-29%) were 13.4(0.8) mmHg, 19.7(1.0) mmHg, and 0.24(0.02) mmHg.ml-1.min.100 g-1. The pressure difference between the stop flow coronary artery pressure and the zero flow pressure intercept may be due to the non-linearity in the pressure-flow relation at a low perfusion pressure. The left ventricular end diastolic pressure and great cardiac vein pressure did not change in relation to the packed cell volume of the coronary perfusate. Thus it is concluded that packed cell volume is one factor determining the high zero flow pressure.

Animals↗
PubMed2003

A novel approach to in situ characterization of pancreatic beta-cells.

The tissue-slice technique has enabled major insights into neural and neuroendocrine physiology. Our aim was to adapt this technique to study the function of the endocrine pancreas. The preparation combines an in situ approach, as in gland perfusion, with a resolution characteristic of electrophysiological studies on single cells. The membrane potential in beta-cells in the slices recorded using the whole-cell patch-clamp was close to the calculated reversal potential for K+. With sufficient ATP in the recording pipette the beta-cells depolarized rapidly on exposure to an increased glucose concentration or stimulation with tolbutamide. The cells preserved bursting and spiking capacity for tens of minutes despite the whole-cell dialysis. In addition, the voltage clamp was used to monitor the changes in the membrane capacitance and to allow correlation of the electrical activity and the cytosolic calcium changes. The pancreatic tissue slice preparation is a novel method for studying the function of the beta- and other pancreatic endocrine and exocrine cells under near-physiological conditions.

Adenosine Triphosphate↗
PubMed1987

Design, construction, and use of an electroporator for plant protoplasts and animal cells.

We have designed and constructed an electroporation device capable of efficient transfer of DNA into both plant cell protoplasts and cultured murine lymphocytes. The electroporator design allows various combinations of voltage and capacitance to be used to optimize the electric pulse. Switching of large voltages and currents is accomplished with a silicon-controlled rectifier, yielding excellent reproducibility and long component life. A safety switch is provided to permit complete discharge of the device. Conditions suitable for high levels of transient expression and high frequencies of stable transformation for both plant and animal cell systems have been found.

Animals↗
PubMed1993

Electrophysiological analysis of transmission at the skeletal neuromuscular junction.

The review is divided into two sections. The first deals with methods and problems associated with performing electrophysiological experimentation on the skeletal muscle neuromuscular junction. The second section concentrates on the computer analysis of electrophysiological data. In the first section, the various techniques available for producing skeletal muscle paralysis are described. These include the use of pharmacological manipulations, such as an excess of magnesium ions or a competitive postjunctional nicotinic acetylcholine antagonist, physiological manipulations, such as cutting the muscle fibers, and the muscle fiber sodium channel toxin, mu-conotoxin. Also, in this section, a comparison is made of the use of voltage- and current-recording techniques, including descriptions of, and solutions to, the problems associated with membrane capacitance, nonlinear summation, membrane space constant, and electrical and mechanical interference. In the second section, details are given of the types of computer system commonly used for the analysis of electrophysiological data and also the requirements of the data analysis software. The use of computer algorithms for signal detection, signal evaluation, signal averaging, and curve fitting are qualitatively described, along with some of the problems and pitfalls often associated with these methods.

Animals↗
PubMed2006

A novel carbon electrode material for highly improved EDLC performance.

Porous materials, developed by grafting functional groups through chemical surface modification with a surfactant, represent an innovative concept in energy storage. This work reports, in detail, the first practical realization of a novel carbon electrode based on grafting of vinyltrimethoxysilane (vtmos) functional group for energy storage in electric double layer capacitor (EDLC). Surface modification with surfactant vtmos enhances the hydrophobisation of activated carbon and the affinity toward propylene carbonate (PC) solvent, which improves the wettability of activated carbon in the electrolyte solution based on PC solvent, resulting in not only a lower resistance to the transport of electrolyte ions within micropores of activated carbon but also more usable surface area for the formation of electric double layer, and accordingly, higher specific capacitance, energy density, and power capability available from the capacitor based on modified carbon. Especially, the effects from surface modification become superior at higher discharge rate, at which much better EDLC performance (i.e., much higher energy density and power capability) has been achieved by the modified carbon, suggesting that the modified carbon is a novel and very promising electrode material of EDLC for large current applications where both high energy density and power capability are required.

Journal Article↗
PubMed2005

Chaos supported stochastic resonance in a metal-ferroelectric-semiconductor heterostructure.

An experimental study is presented on a complex nonlinear system showing a particular type of dynamics that can be interpreted as stochastic resonance. The system consists of a metal-ferroelectric-semiconductor structure, which plays the role of a nonlinear element in an electric circuit with linear resistance, inductance, and capacitance connected in series (RLC series circuit) driven externally by a high-amplitude harmonic voltage source. The system presents various kinds of nonlinear behavior, of which the simplest, consisting of a period-doubling evolution to chaos, is of interest to this study. The broadband intrinsic chaos emerging after a period-doubling sequence exists for a large range of frequencies of the driving voltage. The appearance of the chaotic dynamics is associated with the promotion of a low-frequency harmonic spectral component. This is interpreted as stochastic resonance with intrinsic chaos replacing noise, the usual variable in regular SR.

Journal Article↗
PubMed2004

Microscopic view of charge injection in an organic semiconductor.

We have measured the chemical potential and capacitance in a disordered organic semiconductor by electric force microscopy, following the electric field and interfacial charge density microscopically as the semiconductor undergoes a transition from Ohmic to space-charge limited conduction. Electric field and charge density at the metal-organic interface are inferred from the chemical potential and current. The charge density at this interface increases with electric field much faster than is predicted by the standard diffusion-limited thermionic emission theories.

Journal Article↗
PubMed1984

The passive electrical properties of spheroidal aggregates cultured from neonatal rat heart cells.

Membrane specific resistance and capacitance of non-spontaneously active spheroidal aggregates, cultured from collagenase-dissociated neonatal rat heart cells, were calculated from changes in membrane potential due to intracellularly injected rectangular hyper- and depolarizing current pulses during diastole. The relation between steady-state membrane voltage displacement and injected current is linear for current pulses between +10 and -10 nA. No significant fall-off of electrotonic potential is measured in an aggregate at increasing distances from the site of current injection. The aggregate membrane resistance (input resistance) was best fitted by an inverse square function of the aggregate radius. This suggests selective current flow through the outer membranes of the spheroidal aggregate. Taking this into account the membrane specific resistance was calculated to be 753 +/- 38 omega cm2 (S.E. of mean; n = 39). The time course of the change in membrane potential is exponential with a time constant ranging from 5 to 26 ms, depending on the aggregate radius. The aggregate membrane capacitance is calculated from the exponential transients for each aggregate and appears to be a cubic function of the radius, indicating that the membrane area of all cells in the preparation equally contributes to the input capacitance. The membrane specific capacitance is calculated to be 0.97 +/- 0.02 microF/cm2 (S.E. of mean; n = 100). It is concluded that myocytes in aggregates are electrically well coupled and that a resistance in series with the inner membranes, if present, is negligible compared to the membrane resistance of the internal cells. In order to explain the finding that the membrane resistance was not inversely related to the cube of the aggregate radius, it is postulated that the membrane specific resistance might be a function of aggregate radius.

Action Potentials↗
PubMed2006

Control of mammalian cochlear amplification by chloride anions.

Chloride ions have been hypothesized to interact with the membrane outer hair cell (OHC) motor protein, prestin on its intracellular domain to confer voltage sensitivity (Oliver et al., 2001). Thus, we hypothesized previously that transmembrane chloride movements via the lateral membrane conductance of the cell, GmetL, could serve to underlie cochlear amplification in the mammal. Here, we report on experimental manipulations of chloride-dependent OHC motor activity in vitro and in vivo. In vitro, we focused on the signature electrical characteristic of the motor, the nonlinear capacitance of the cell. Using the well known ototoxicant, salicylate, which competes with the putative anion binding or interaction site of prestin to assess level-dependent interactions of chloride with prestin, we determined that the resting level of chloride in OHCs is near or below 10 mm, whereas perilymphatic levels are known to be approximately 140 mm. With this observation, we sought to determine the effects of perilymphatic chloride level manipulations of basilar membrane amplification in the living guinea pig. By either direct basolateral perfusion of the OHC with altered chloride content perilymphatic solutions or by the use of tributyltin, a chloride ionophore, we found alterations in OHC electromechanical activity and cochlear amplification, which are fully reversible. Because these anionic manipulations do not impact on the cation selective stereociliary process or the endolymphatic potential, our data lend additional support to the argument that prestin activity dominates the process of mammalian cochlear amplification.

Animals↗
PubMed2006

Electrical probing of endothelial cell behaviour on a fibronectin/polystyrene/thiol/gold electrode by Faradaic electrochemical impedance spectroscopy (EIS).

The electrochemical impedance spectroscopy (EIS) technique has been shown to be an effective tool for monitoring endothelial cell behaviour on a multilayer functionalised gold electrode. Polystyrene, a reproducible model substrate, is deposited as a thin layer on a thiol functionalised gold electrode. Fibronectin, a protein promoting endothelial cell adhesion, is then adsorbed on the polystyrene surface. The different steps of this multilayer assembly are characterized by Faradaic impedance. The charge transfer resistance and the capacitance for the total layer are modified at each step according to the electrical properties of each layer. This gives the endothelial cells' electrical state in terms of its resistive and capacitive properties. In this study, the endothelial cell layer presents a specific charge transfer resistance equal to 1.55 kOmega cm(2) with no large defects in the cell layer, and a specific capacitance equal to few microF cm(-2) explained by the existence of pseudopods. These electrical properties are correlated to the endothelial cell viability, adhesion and cytoskeleton organization.

Cell Adhesion↗
PubMedDate not supplied

Central nervous control of venous tone. III. Responses of capacitance and resistance vessels of skin to bulbar and hypothalamic stimulation.

Electrical stimulation of 350 points in the bulbar formation of 35 dogs under chloralose anaesthesia demonstrated the presence of sites producing increase or decrease of systemic arterial pressure (SAP) in the same general morphological limits of bulbar pressor and depressor regions as described by earlier authors. Simultaneous recording of pressure changes in the cutaneous vessels however demonstrated that pressure changes in these vessels did not correspond to the pressor or depressor effects of the SAP. Instead, responses were obtained in which pressures in cutaneous capacitance and resistance vessels followed a trend which was opposite in direction and magnitude to that of SAP. Thus there were 30 depressor sites which produce increase in cutaneous vessel pressure and 23 pressor sites which produced a fall in cutaneous vessel pressure. For a marked rise in the SAP, only 62 sites elicited equally marked increase in both capacitance and resistance vessel pressure, while another 52 elicited only a small increase of equivalent magnitude in the capacitance and resistance vessels. Stimulation of 84 points produced dissimilar effects on capacitance and resistance vessels out of which 38 elicited moderate increase in resistance vessel tone with minimal changes in the capacitance vessel tone, while 46 points elicited moderate increase in capacitance vessel tone with only a small increase in the resistance vessel tone. These points were diffusely admixed in the bulbar reticular formation. Effects which were exclusive to the capacitance and resistance vessels of skin, singly or in combination, without affecting the SAP were elicited from 12 points while another 28 points produced marked rise or fall of systemic arterial pressure without affecting the cutaneous vessels. These observations suggest that the neuronal organisation regulating cardiovascular activities at the bulbar level is quite complex having the capacity to generate varying activities in different components of the vascular circuits by differentially altering the discharge of the efferent sympathetic fibres on the one hand, and marked selectivity of action on any particular vascular bed or circulatory component on the other hand. Stimulation of 93 points in the hypothalamus produced similar patterns of response as obtained from medulla oblongata. In addition, stimulation of 6 points in the anterior hypothalamus produced a distinctive response accompanied by dilatation of cutaneous resistance and capacitance vessels with marked increase in respiratory rate and minimal changes in the SAP. This type of response which resembled the physiological response employed for heat loss was not obtained from any stimulation site in the medulla oblongata.

Animals↗
PubMed1975

Heat production non-myelinated nerves.

Experiments with the C fibres of the rabbit vagus nerve have established that heat is evolved during the depolarizing phase of the action potential and is absorbed during the repolarizing phase. Subsequent studies using the pike olfactory nerve indicate that the heat production begins at a high rate very early in the depolarizing phase and is completed in advance of the peak of the spike. This would be expected if the heat arised from the energy released by the discharge of the membrane capacitance which varies as the square of the membrane potential; but estimates of the stored energy fall short of the observed heat production by a factor of two or three times. The prominent cooling phase suggests that a substantial part of the heat may arise from an entropy change. Such an entropy change would be expected to result from the change in the electrical stress in the dielectric of the membrane capacitance, and may thus be manifestation of reversible changes in the molecular architecture of the insulating matrix of the membrane.

Action Potentials↗
PubMed2000

Capacitance cytometry: measuring biological cells one by one.

Measuring the DNA content of eukaryotic cells is a fundamental task in biology and medicine. We have observed a linear relationship between the DNA content of eukaryotic cells and the change in capacitance that is evoked by the passage of individual cells across a 1-kHz electric field. This relationship is species-independent; consequently, we have developed a microfluidic technique-"capacitance cytometry"-that can be used to quantify the DNA content of single eukaryotic cells and to analyze the cell-cycle kinetics of populations of cells. Comparisons with standard flow cytometry demonstrate the sensitivity of this new technique.

Animals↗
PubMed1998

Cochlear electrically evoked emissions modulated by mechanical transduction channels.

Cochlear outer hair cells are capable of both mechanical-to-electrical and electrical-to-mechanical transduction. Vibration of their stereocilia by sound is believed to stimulate somatic motility via a receptor potential developed across the basolateral membrane, thereby enhancing the mechanical vibration and increasing the sensitivity and frequency selectivity of the ear. Extrinsic electrical currents, applied at the tops of the cells, also appear to activate motility in vivo, presumably after entering the cell. Earlier experiments suggested such currents might enter through the transduction channels themselves, but an alternative shunt pathway through the membrane capacitance seems more likely on physical grounds. We therefore recorded electrically evoked oto-acoustic emissions while modulating the transduction channels by driving them with low-frequency sound. Recordings of the low-frequency cochlear microphonic provided a measure of the mean electrical conductance through the channels during sound stimulation. Emissions increased during displacement of the basilar membrane toward scala vestibuli, when the channels were biased open, and decreased on the opposite phase, and the modulation of the emission was in direct proportion to the cochlear microphonic. The results are the strongest evidence yet that electrically evoked emissions are generated directly by mechanisms related to cochlear transduction and lead to the surprising conclusion that, for frequencies up to at least 12 kHz, extrinsic electrical currents enter the hair cell predominantly by the resistive pathway through the transduction channels. Alternatively, the results might be consistent with direct modulation of a motility source driven by capacitive currents but whose output depends on the state of the channels.

Animals↗
PubMed1975

Release and uptake of haemoglobin and ions in red blood cells induced by dielectric breakdown.

External electric field strengths of the order of 10-3 minus10-4 v-cm-minus1 induce potassium release and concomitant sodium uptake in human and bovine red blood cells, as demonstrated in an electrolytic discharge chamber. The reversible increase of the membrane permeability once the critical membrane potential is reached is caused by dielectric breakdown of the membrane. The values of the critical membrane potential differences calculated from the potassium release and sodium uptake curves are close to those which were calculated from dielectric breakdown measurements in a hydrodynamic focussing Coulter Counter using the Laplace equation. With bovine red blood cells, the potassium release and the concomitant sodium uptake is coupled with haemoglobin release from the cells, while with human red blood cells much higher external electric field strengths are required for haemoglobin release. The external electric field strength required for solute release and uptake in bovine and human red blood cells depends on the pulse length, particularly below a value of about 10 mus, when a strong increase in the field strength occurs with decreasing pulse lengths. At 50-100 mus pulse lengths an asymptotic value of the critical electrical field strength of 2.6 kV-cm-minus1 for the modal volume of human red blood cells and 2.8 kV-cm-minus1 for the modal volume of bovine red blood cells is reached, corresponding to a critical membrane potential difference of about 1.1 V for both species. This value is close to that measured directly for dielectric breakdown of the membranes of Valonia utricularis (0.85 V, 20 degrees C). The increase in electric field strength with decreasing pulse length can be explained by the capacitance of the membrane, which becomes the rate limiting step for the temporal build-up of the electric potential across the membrane. The time constant of this process was determined to be approx. 10 mus. The critical membrane potential difference for breakdown is therefore pulse-length independent. The breakdown of the membrane can be interpreted by an electromechanical collapse of the membrane material. Numerical considerations of the dynamics of this membrane collapse predict that the breakdown time is a very rapid process.

Animals↗