SEARCH · Search PubMed

Results for “Electric Capacitance”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,531 records · Page 85Linked to original sources

PubMed1977

Capacitance studied of syntheric phospholipid Langmuir films.

Synthetic phosphatidylcholine Langmuir films have been incorporated into metal-insulator-metal (MIM) thin film junctions. The capacitance characteristics of these junctions have been studied as a function of temperature, the number of lipid layers in the insulating layer, and the length of the hydrocarbon chains of the lipid molecule. The thickness of the oxide layer on the base aluminum electrode has been determined to be larger than or equal to 11 A, and its effects on the capacitance characteristics have been considered in some detail. Indications of phase transitions in the temperature dependence of the capacitance imply that the basic lemellar arrangement of the lipid molecules is retained even after the samples are subjected to a dehydrating vacuum annealing process. An examination of the effects of varying the hydrocarbon chain length and salt content of the subphase during sample fabrication showed that capacitance characteristics of the MIM junction are very sensitive to small structural changes in the insulating layer.

Electric Conductivity↗

PubMed2006

Neural recording and stimulation of dissociated hippocampal cultures using microfabricated three-dimensional tip electrode array.

There is increasing interest in interfacing dissociated neuronal cultures with planar multielectrode arrays (MEAs) for the study of the dynamics of neuronal networks. Here we report on the successful use of three-dimensional tip electrode arrays (3D MEAs), originally developed for use with brain slices, for recording and stimulation of cultured neurons. We observed that many neurons grew directly on protruding electrode surface, appearing to make excellent contact. A larger than usual portion of extracellular spikes had large positive peaks, while most of the spikes from conventional two-dimensional electrode arrays had large negative spikes. This may be due to the direct capacitive coupling situation provided by relatively large electrode surface area.

Action Potentials↗

PubMed1993

Triple-frequency method for measuring blood impedance.

A method was developed for simultaneously measuring plasma resistance, cell interior resistance and cell membrane capacitance of blood. A three-element circuit model for simulating the impedance of blood was evaluated by comparing measured and calculated impedance frequency curves and was found feasible. The values of the three elements of the model could be determined by measuring the impedance amplitudes at three different frequencies. The cell membrane capacitance (Cm) was independent of temperature in the range 25-40 degrees C and was significantly higher for blood with ethylenediaminetetraacetic acid (EDTA) as anticoagulant than that for blood with sodium citrate and acid citrate dextrose (ACD). The Cm value of blood from patients with high erythrocyte sedimentation rate (ESR) was found to be significantly higher than that of blood from patients with low ESR. This suggests that the membrane capacitance might be a potential parameter for clinical applications.

Anticoagulants↗

PubMed1990

Single potassium channels in corneal epithelium.

The basal cell layers of the rabbit and human corneal epithelia contain a frequently occurring ionic channel whose unitary currents can be recorded in cell-attached or excised membrane patches by use of a patch voltage clamp. The channel is highly conductive (165 pS in 150 mM K+ salts) and is very selective for K+ over Na+ (PK/PNa greater than 40:1). Its open probability is increased by the application of suction to the recording pipette although its gating is less sensitive to suction than that of many other "stretch-activated" channels reported. The current through the channel is a saturating function of the K+ concentration in the bathing solutions with half saturation occurring at 480 mM and a single-channel current at saturation (imax) of 31 pA. In the absence of applied suction, the open probability is extremely variable from patch to patch and shows little voltage dependence over the physiologic voltage range. The channel also gates frequently to several subconductance levels. It is blocked by external Cs+ and Ba+2 in the 0.1-10 mM range but not by most other K+ channel blockers. It is also partially blocked by Ca+2 at both its internal and external surfaces. Because of its novel properties (stretch activation and large conductance), it can be used to measure the input resistance and total capacitance of single dissociated cells.

Animals↗

PubMed1993

Electrical impedance and haematocrit of human blood with various anticoagulants.

The relationship between the electrical impedance parameters and the haematocrit of normal human blood has been determined at 37 degrees C in the haematocrit range of 20-60% for four types of anticoagulant: ACD, EDTA, sodium heparin and buffered sodium citrate. For plasma resistance Rp and interior resistance R(i), the measured data could be fitted by both the exponential expression and the Maxwell-Fricke expressions, Rp = a/(1-h) + b and R(i) = a/h + b, where a and b are constants and h is the haematocrit expressed as a decimal. The membrane capacitance Cm was found to be directly proportional to the haematocrit and could be normalized to 100% haematocrit by the Fricke formula, C100 = C(m)/(1-rho p/Rp), where rho p is the resistivity of plasma. Observed differences for Rp and C(m) among samples with various anticoagulants indicate that the type of anticoagulant influences the electrical properties of the red cells and the plasma.

Anticoagulants↗

PubMed1998

The role of cytosolic Ca2+ in the secretion of NaCl in isolated in vitro perfused rectal gland tubules of Squalus acanthias.

In many exocrine glands cytosolic Ca2+ ([Ca2+]i) plays a pivotal role in stimulation-secretion coupling. In the rectal gland of the dogfish Squalus acanthias this appears not to be the case and it is believed that secretion is mainly controlled by the Cl- conductance of the luminal membrane. We have examined this question in a study of isolated in vitro perfused rectal gland tubules (RGT). Three types of measurements were performed: (1) measurements of [Ca2+]i by the fura-2 technique; (2) measurements of transepithelial electrical parameters, i.e. transepithelial voltage (Vte), transepithelial resistance (Rte), the equivalent short-circuit current (Isc) and the voltage across the basolateral membrane (Vbl), and (3) whole-cell patch-clamp measurements of cellular voltage (Vm), conductance (Gm) and membrane capacitance (Cm). The data indicates that carbachol (CCH) increases [Ca2+]i by increasing store release and Ca2+ influx. Other agonists, producing cytosolic cAMP, also increased [Ca2+] by enhancing Ca2+ influx. CCH hyperpolarized these cells and enhanced Gm significantly. The effect of CCH on Vte and Isc was most marked under control conditions and disappeared in RGT otherwise stimulated by agonists that lead to cAMP production. It is concluded that [Ca2+]i plays a major role in the stimulation of NaCl secretion in RGT by enhancing the basolateral K+ conductance. cAMP-producing agonists enhance [Ca2+]i by increased Ca2+ influx. CCH releases Ca2+ from respective stores. CCH, unlike the cAMP-producing agonists, only increases basolateral K+ conductance. It modulates secretion especially under conditions in which the cAMP pathway is not fully activated.

Adenosine Triphosphate↗

PubMed1991

K+ and Cl- currents in freshly isolated rat osteoclasts.

Membrane electrical properties of freshly isolated rat osteoclasts were studied using patch-clamp recording methods. Characterization of the passive membrane properties indicated that the osteoclast cell membrane behaved as an isopotential surface. The specific membrane capacitance was 1.2 +/- 0.3 microF/cm2 (mean +/- SD), with no difference between cells plated on glass and those adhering to a permeable collagen substrate. The current/voltage (I/V) relationship of all cells showed inward rectification and I/V curves shifted 51 mV positive per tenfold increase of [K+]out, indicating an inwardly rectifying K+ conductance. The voltage dependence of the K+ chord conductance (gK) also shifted positive along the voltage axis, and the maximum conductance increased, with elevation of [K+]out. gK for cells bathed in 4.7 mM [K+]out increased e-fold per 12 mV hyperpolarization, and half-maximal activation was at -89 mV. Approximately 18% (50 pS/pF) of the maximum gK was active at -70 mV. Inward single-channel currents were recorded in cell-attached patches at hyperpolarizing potentials. With symmetrical K+, channel conductance was 25 +/- 3 pS and reversal was close to the K+ equilibrium potential, consistent with this K+ channel underlying the whole-cell K+ currents. With both conventional whole-cell and perforated-patch recording, no voltage-activated Ca2+ current was detected. In approximately 30% of osteoclasts studied, an outwardly rectifying current was observed, which was reversibly blocked by 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS) and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS). This DIDS- and SITS-sensitive current reversed direction at the chloride equilibrium potential. We conclude that an inwardly rectifying K+ current is present in all rat osteoclasts and that some osteoclasts also exhibit an outwardly rectifying Cl- current. Both these membrane conductances may play an important physiological role by dissipating the potential that arises from the electrogenic transport of H+ across the ruffled membrane of the osteoclast.

Animals↗

PubMedDate not supplied

Cyclic-nucleotide-gated channels mediate synaptic feedback by nitric oxide.

Cyclic-nucleotide-gated (CNG) channels in outer segments of vertebrate photoreceptors generate electrical signals in response to changes in cyclic GMP concentration during phototransduction. CNG channels also allow the influx of Ca2+, which is essential for photoreceptor adaptation. In cone photoreceptors, cGMP triggers an increase in membrane capacitance indicative of exocytosis, suggesting that CNG channels are also involved in synaptic function. Here we examine whether CNG channels reside in cone terminals and whether they regulate neurotransmitter release, specifically in response to nitric oxide (NO), a retrograde transmitter that increases cGMP synthesis and potentiates synaptic transmission in the brain. Using intact retina, we show that endogenous NO modulates synapses between cones and horizontal cells. In experiments on isolated cones, we show directly that CNG channels occur in clusters and are indirectly activated by S-nitrosocysteine (SNC), an NO donor. Furthermore, both SNC and pCPT-cGMP, a membrane-permeant analogue of cGMP, trigger the release of transmitter from the cone terminals. The NO-induced transmitter release is suppressed by guanylate cyclase inhibitors and prevented by direct activation of CNG channels, indicating that their activation is required for NO to elicit release. These results expand our view of CNG channel function to include the regulation of synaptic transmission and mediation of the presynaptic effects of NO.

Ambystoma↗

PubMed1983

Radiofrequency-induced hyperthermia: computer simulation of specific absorption rate distributions using realistic anatomical models.

A description is given of a computer simulation technique which predicts the specific absorption rate (SAR) distribution within the human body resulting from the application of radiofrequency electromagnetic energy. The method uses an extension to the principle of over-relaxation of electric potentials and the basis of the simulation is a realistic three-dimensional model derived from both dielectric and anatomical data. Two of the principal means of applying radiofrequency hyperthermia, namely the use of capacitive electrodes and inductive coils, have been provided for. The accuracy of the simulation has been favourably tested using an agar split-phantom and an infrared thermograph camera. The simulations can be used to assist the design and clinical use of radiofrequency applicators, and examples are given of the application of both an inductive coil and switched capacitive electrodes to heat the thorax.

Computers↗

PubMed1992

Voltage-dependent and odorant-regulated currents in isolated olfactory receptor neurons of the channel catfish.

The electrical properties of olfactory receptor neurons, enzymatically dissociated from the channel catfish (Ictalurus punctatus), were studied using the whole-cell patch-clamp technique. Six voltage-dependent ionic currents were isolated. Transient inward currents (0.1-1.7 nA) were observed in response to depolarizing voltage steps from a holding potential of -80 mV in all neurons examined. They activated between -70 and -50 mV and were blocked by addition of 1 microM tetrodotoxin (TTX) to the bath or by replacing Na+ in the bath with N-methyl-D-glucamine and were classified as Na+ currents. Sustained inward currents, observed in most neurons examined when Na+ inward currents were blocked with TTX and outward currents were blocked by replacing K+ in the pipette solution with Cs+ and by addition of 10 mM Ba2+ to the bath, activated between -40 and -30 mV, reached a peak at 0 mV, and were blocked by 5 microM nimodipine. These currents were classified as L-type Ca2+ currents. Large, slowly activating outward currents that were blocked by simultaneous replacement of K+ in the pipette with Cs+ and addition of Ba2+ to the bath were observed in all olfactory neurons examined. The outward K+ currents activated over approximately the same range as the Na+ currents (-60 to -50 mV), but the Na+ currents were larger at the normal resting potential of the neurons (-45 +/- 11 mV, mean +/- SD, n = 52). Four different types of K+ currents could be differentiated: a Ca(2+)-activated K+ current, a transient K+ current, a delayed rectifier K+ current, and an inward rectifier K+ current. Spontaneous action potentials of varying amplitude were sometimes observed in the cell-attached recording configuration. Action potentials were not observed in whole-cell recordings with normal internal solution (K+ = 100 mM) in the pipette, but frequently appeared when K+ was reduced to 85 mM. These observations suggest that the membrane potential and action potential amplitude of catfish olfactory neurons are significantly affected by the activity of single channels due to the high input resistance (6.6 +/- 5.2 G omega, n = 20) and low membrane capacitance (2.1 +/- 1.1 pF, n = 46) of the cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1998

Functional characterization of a conducting polymer-based immunoassay system.

Experiments have been performed to characterize the electrical properties and functionality of a poly(3-hexylthiophene)-coated platinum electrode developed as a sensor for immunoassay read-out. Admittance measurements were performed on the coated electrodes as a function of frequency. The admittance spectra obtained show that the sensor is capacitive in nature. A circuit model is presented for comparison to other conducting polymer systems. Dynamic sensor response is characterized by oxidizing the polymer via a hydrogen peroxide-iodide pathway. Hydrogen peroxide is introduced either by direct injection or through a glucose-glucose oxidase reaction to determine electrode functionality and sensitivity. Sensor response to chemical oxidation is measured as a function of frequency and applied signal amplitude. System response is linear in frequency from 1 Hz to 70 Hz and in excitation amplitude up to approximately 600 mV. System sensitivity is analyzed based on oxidant generation from the enzyme-initiated pathway, sensor baseline drift, and the noise band on the quiescent sensor current.

Biosensing Techniques↗

PubMed1999

NOS inhibition potentiates norepinephrine but not sympathetic nerve-mediated co-transmission in resistance arteries.

OBJECTIVE: The in vitro interaction between sympathetic nerves and basal nitric oxide release was studied in a resistance artery, since these interact powerfully in large vessels. METHODS: The pharmacological interaction between L-NAME and vasoconstriction to field stimulation of sympathetic nerves or exogenous norepinephrine was studied in rabbit cutaneous resistance arteries in wire myographs. RESULTS: Relaxation of norepinephrine-induced tone by acetylcholine, but not sodium nitroprusside, was blocked by N omega-nitro-L-arginine methyl ester (L-NAME: 100 microM), indicating that the agonist-induced release of nitric oxide could oppose the vasoconstrictor effect of norepinephrine and confirming that L-NAME had no effect on endothelium-independent vasodilatation. L-NAME increased norepinephrine potency indicating basal NO release. With short bursts of electrical field stimulation purinergic transmission was dominant at low frequencies and adrenergic at high frequencies. L-NAME had no effect on nerve-mediated responses, even after blocking the purinergic component with alpha,beta-methylene ATP (3 microM), suggesting that the influence of spontaneously released nitric oxide does not extend to the vascular smooth muscle cells under adrenergic nervous control. CONCLUSION(S): This resistance artery exhibits a highly effective nitric oxide-mediated vasodilatation to acetylcholine. It has basal release of nitric oxide which antagonises exogenous norepinephrine. However, basal nitric oxide did not influence adrenergic nerve transmission, which contrasts with previous studies of larger arteries and veins. We speculate that in small resistance arteries there may be a spatial limitation to the zones of vascular smooth muscle influenced by the adrenergic nerves and by basal nitric oxide from the endothelium, respectively. The role of endogenous nitric oxide in modulating vascular tone may thus be less in resistance arteries than in conducting arteries or capacitance vessels and purinergic transmission appears to be particularly resistant.

Acetylcholine↗

PubMed2000

Tight coupling between electrical activity and exocytosis in mouse glucagon-secreting alpha-cells.

alpha-Cells were identified in preparations of dispersed mouse islets by immunofluorescence microscopy. A high fraction of alpha-cells correlated with a small cell size measured as the average cell diameter (10 microm) and whole-cell capacitance (<4 pF). The alpha-cells generated action potentials at a low frequency (1 Hz) in the absence of glucose. These action potentials were reversibly inhibited by elevation of the glucose concentration to 20 mmol/l. The action potentials originated from a membrane potential more negative than -50 mV, had a maximal upstroke velocity of 5 V/s, and peaked at +1 mV. Voltage-clamp experiments revealed the ionic conductances underlying the generation of action potentials. alpha-Cells are equipped with a delayed tetraethyl-ammonium-blockable outward current (activating at voltages above -20 mV), a large tetrodotoxin-sensitive Na+ current (above -30 mV; peak current 200 pA at +10 mV), and a small Ca2+ current (above -50 mV; peak current 30 pA at +10 mV). The latter flowed through omega-conotoxin GVIA (25%)- and nifedipine-sensitive (50%) Ca(2+)-channels. Mouse alpha-cells contained, on average, 7,300 granules, which undergo Ca(2+)-induced exocytosis when the alpha-cell is depolarized. Three functional subsets of granules were identified, and the size of the immediately releasable pool was estimated as 80 granules, or 1% of the total granule number. The maximal rate of exocytosis (1.5 pF/s) was observed 21 ms after the onset of the voltage-clamp depolarization, which is precisely the duration of Ca(2+)-influx during an action potential. Our results suggest that the secretory machinery of the alpha-cell is optimized for maximal efficiency in the use of Ca2+ for exocytosis.

Animals↗

PubMedDate not supplied

Modelling transthoracic defibrillation waveforms.

Recent investigations connected with implantable defibrillators yielded new data on heart electrophysiology, resulting in reassessment of existing and advancing of new types of electrical impulses. Different electrical equivalent circuits were proposed for modelling intracardiac and transthoracic defibrillation pulse waveforms, comprising generator, electrode interface and tissue resistances. We attempted modelling of the transmembrane voltage Vm time course, induced by different applied voltage Vs waveforms, taking into account only the shapes and the relative Vs and Vm amplitudes. The excitable cell membrane impedance Zm was modelled with higher resistance and lower capacitance, so that a shunting effect on the generator and tissue resistances was avoided. The result was a very simple equivalent circuit. We proposed criteria for efficient defibrillation pulse waveforms yielding a straightforward approach to model existing and new pulses and to assess their efficiency.

Biomedical Engineering↗

PubMed1992

The effect of baclofen and somatostatin on neuronal activity in the rat ventromedial hypothalamic nucleus in vitro.

The electrical properties of neurones within the ventromedial hypothalamic nucleus of the rat were studied in an in vitro slice preparation, using conventional intracellular recording techniques. A detailed analysis of 36 intracellular recordings appeared to suggest 3 cell types, based on membrane capacitance and resistance characteristics, confirming previous reports of a diversity of cell types within this nucleus. The responsiveness of each cell type to exogenously-applied baclofen and somatostatin was also investigated. The inhibitory responses to both of these drugs were concentration-related (over the range 100 nM to 1 microM), tetrodotoxin-resistant and consisted of a membrane hyperpolarization (mean +/- SEM = 6.7 +/- 1 and 10.7 +/- 1 mV for 1 microM somatostatin and baclofen, respectively) and an associated reduction in the firing frequency of spontaneously active cells. These agonist-evoked responses probably represented direct postsynaptic actions but they were not restricted to any single type of cell. Evidence for an additional presynaptic effect of baclofen was also obtained. Responses to baclofen were extremely robust and readily quantifiable, whereas those to somatostatin showed pronounced long-lasting desensitization, which was particularly marked a larger concentrations. These data support previous contentions, based on in vivo studies, that somatostatin and GABA are likely to participate in the control of complex functions by the ventromedial hypothalamic nucleus.

Action Potentials↗

PubMed1993

Focal exocytosis by eosinophils--compound exocytosis and cumulative fusion.

We have investigated the granule fusion events during exocytosis in horse eosinophils by time-resolved patch-clamp capacitance measurements. Stimulation with intracellular GTP gamma S leads to a stepwise capacitance increase by 4.0 +/- 0.9 pF. At GTP gamma S concentrations < 20 microM the step size distribution is in agreement with the granule size distribution in resting cells. Above 80 microM the number of steps is reduced and very large steps occur. The total capacitance increase, however, is unaffected. These results show that at high GTP gamma S concentrations granule--granule fusion occurs inside the cell forming large compound granules, which then fuse with the plasma membrane (compound exocytosis). The electrical equivalent circuit of the cell during degranulation indicates the formation of a degranulation sac by cumulative fusion events. Fusion of the first granule with the plasma membrane induces fusion of further granules with this granule directing the release of all the granular material to the first fusion pore. The physiological function of eosinophils is the killing of parasites. Compound exocytosis and cumulative fusion enable the cells to focus the release of cytotoxic proteins to well defined target regions and prevent uncontrolled diffusion of this material, which would damage intact host cells.

Animals↗

PubMed2002

Zeta potential of stearic acid monolayer at the air-aqueous solution interface.

The zeta (zeta) potential of an insoluble monolayer of stearic acid at the air-water interface was measured as a function of pH in the presence of 0.0001, 0.001, and 0.01 M NH4NO3. The zeta potential was measured by means of the plane interface technique which involved the determination of the electrophoretic velocity profile of reference (polystyrene latex) particles along the solution depth in a rectangular open quartz cell. The zeta vs pH relationship at 0.001 and 0.01 M NH4NO3 was analyzed in terms of the Gouy-Chapman-Stern-Grahame model for electrical double-layer incorporating a simple site-binding model used previously for many oxide and latex colloid studies. The dissociation constant (pKa) of stearic acid monolayer and double-layer parameters such as integral capacitances of inner and outer Helmholtz layers and the complexation constant of counterion complexes were also estimated.

Journal Article↗

PubMed2000

Loperamide: novel effects on capacitative calcium influx.

Loperamide is a widely used antidiarrheal that primarily acts at nanomolar concentrations through activation of opioid receptors in the gastrointestinal tract. At somewhat higher concentrations, loperamide blocks calmodulin activity, calcium channels, N-methyl-D-aspartate-receptor channels, and maitotoxin-elicited calcium influx. Loperamide at micromolar concentrations has now been shown to have a remarkable stimulatory effect on the capacitative calcium influx that is triggered in many cells by depletion of the inositol-trisphosphate-sensitive stores of calcium in the endoplasmic reticulum. The mechanism whereby loperamide enhances levels of intracellular calcium elevated by capacitative calcium influx is, as yet, undefined.

Adenosine Triphosphate↗