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At least 1,369 records · Page 76Linked to original sources
PubMed2001

Frequency response of alternating currents through the Staphylococcus aureus alpha-hemolysin ion channel.

Alternating currents were measured through transmembrane ion channels formed by Staphylococcus aureus alpha-hemolysin proteins in planar bilayer membranes as part of an investigation to determine the channel's frequency response and the appropriateness of an equivalent circuit commonly used to model electrical interactions at the surface of cells. The experimental approach includes a novel method for separating the alternating current through one or more channels, which is conductive in nature, from the capacitively coupled current through the membrane. Separation of the conductive and capacitive alternating currents made it possible to measure the frequency response of the alpha-hemolysin channels. The results of the study are consistent with an equivalent circuit of a membrane capacitor in parallel with one or more channel resistors over the frequency range 30-120 Hz. The possible usefulness of frequency response data for ion channels in cell membranes during investigations of biological effects of time-varying magnetic fields is briefly discussed.

Bacterial Toxins↗
PubMed1999

Electrophysiological and morphological diversity of mouse sympathetic neurons.

We have used multiple-labeling immunohistochemistry, intracellular dye-filling, and intracellular microelectrode recordings to characterize the morphological and electrical properties of sympathetic neurons in the superior cervical, thoracic, and celiac ganglia of mice. Neurochemical and morphological characteristics of neurons varied between ganglia. Thoracic sympathetic ganglia contained three main populations of neurons based on differential patterns of expression of immunoreactivity to tyrosine hydroxylase, neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP). In the celiac ganglion, nearly all neurons contained immunoreactivity to both tyrosine hydroxylase and NPY. Both the overall size of the dendritic tree and the number of primary dendrites were greater in neurons from the thoracic and celiac ganglia compared with those from the superior cervical ganglion. The electrophysiological properties of sympathetic neurons depended more on their ganglion of origin rather than their probable targets. All neurons in the superior cervical ganglion had phasic firing properties and large afterhyperpolarizations (AHPs). In addition, 34% of these neurons displayed an afterdepolarization preceding the AHP. Superior cervical ganglion neurons had prominent I(M), I(A), and I(H) currents and a linear current-voltage relationship between -60 and -110 mV. Neurons from the thoracic ganglia had significantly smaller action potentials, AHPs, and apparent cell capacitance compared with superior cervical ganglion neurons, and only 18% showed an afterdepolarization. All neurons in superior cervical ganglia and most neurons in celiac ganglia received at least one strong preganglionic input. Nearly one-half the neurons in the celiac ganglion had tonic firing properties, and another 15% had firing properties intermediate between those of tonic and phasic neurons. Most celiac neurons showed significant inward rectification below -90 mV. They also expressed I(A), but with slower inactivation kinetics than that of superior cervical or thoracic neurons. Both phasic and tonic celiac ganglion neurons received synaptic inputs via the celiac nerves in addition to strong inputs via the splanchnic nerves. Multivariate statistical analysis revealed that the properties of the action potential, the AHP, and the apparent cell capacitance together were sufficient to correctly classify 80% of neurons according to their ganglion of origin. These results indicate that there is considerable heterogeneity in the morphological, neurochemical, and electrical properties of sympathetic neurons in mice. Although the morphological and neurochemical characteristics of the neurons are likely to be related to their peripheral projections, the expression of particular electrophysiological traits seems to be more closely related to the ganglia within which the neurons occur.

Action Potentials↗
PubMed1997

Quantitative correlations among fibrinogen concentration, sedimentation rate and electrical impedance of blood.

Blood cells from 8 normal subjects, anticoagulated with EDTA, were washed twice with saline solution (100 mmol/l) and resuspended into saline solutions of different concentrations of human plasma fibrinogen. The erythrocyte sedimentation rates (ESRs) of these suspensions were determined by the Westergren method, and their resistance Rp and capacitance Cm were determined by measuring the impedances at three frequencies 100 kHz, 800 kHz and 1.2 MHz. The results showed that the logarithm of the ESR was proportional to the concentration of fibrinogen, Fb (g/l), expressed as: ln(ESR) = 0.468Fb - 0.10, (r = 0.96); and the haematocrit-normalised resistance R'p and capacitance C'm were both directly proportional to the concentration of fibrinogen, R'p = 3.99Fb + 465.0, (r = 0.87), and C'm = 49.7Fb + 628.2, (r = 0.96). The influence of fibrinogen on the impedance might partly be the result of interfacial polarisation and/or of red cell rouleaux formation. The results are useful for understanding the mechanism of the association between the impedance and the ESR of blood, and for finding means of improving the accuracy when estimating the ESR by the electrical impedance method.

Blood Physiological Phenomena↗
PubMed1987

Time-resolved protonation dynamics of a black lipid membrane monitored by capacitative currents.

The laser-induced proton pulse (Gutman, M. (1986) Methods Enzymol. 127, 522-538) was used for transient protonation of one side of a black lipid membrane. The charging of the membrane drives an electric (voltage or current) signal selectively representing the fast proton exchange at the membrane/electrolyte interface. The sensitivity of the electric signal to the presence of buffer indicates that proton transfer is measured, not some dyes or membrane photoelectric artifact. The same event can be visualized in an analogous system consisting of a pH indicator adsorbed to neutral detergent-phospholipid mixed micelles. The time-resolved light absorption transient is equivalent to the electrically determined transient charging of the membrane surface. The sensitivity of the current measurement exceeds the spectrophotometric method by 6-8 orders of magnitudes. As little as 10(-18) mol of H+ reacting with 0.75 mm2 of the membrane surface can be monitored in a time-resolved observation. Both types of observed transients were accurately reconstructed by the numerical solution of coupled, non-linear, differential equations describing the system. The rate constants of the various proton transfer reactions were calculated and found to be of diffusion controlled reactions. There is no evidence for any barrier at the interface which either prevents protons from reaching the membrane, or keeps proton on the interface. The electric measurements can be applied for monitoring proton transfer kinetics of complex biomembrane preparations.

Hydrogen-Ion Concentration↗
PubMed1996

Modelling of cardiac-related changes in lung resistivity measured with EITS.

Resistivity data from 9.6 kHZ to 1.2 MHz were recorded from eight normal subjects using an electrical impedance tomographic spectroscopy (EITS) system and then averaged to a mean cardiac cycle using the ECG gating technique. The Cole-Cole model, that is, extracellular resistance R connected in parallel with intracellular resistance S and membrane capacitance C in series, with a distribution parameter a, was applied to model the frequency characteristics and to produce parametric images. During systole, SC and RC were found to decrease and FR increase. The changes in R/S were not consistent among the subjects. We estimated the peak changes in R, S and C to be -2.5%, -3.3% and -7.6% respectively. The results can be explained by considering the blood vessels as spheres of different sizes with blood inside them. The decrease in R during systole might be caused by the increased blood content in relatively large vessels, whereas that in S by the increased blood volume in relatively small vessels. The capacitance of blood is normally smaller than that of lung tissue, whereas FR blood is higher than that of lung tissue. Hence, as blood content increases, C should decrease and FR increase.

Blood Volume↗
PubMed2004

The effects of biphasic waveform design on post-resuscitation myocardial function.

OBJECTIVES: This study examined the effects of biphasic truncated exponential waveform design on survival and post-resuscitation myocardial function after prolonged ventricular fibrillation (VF). BACKGROUND: Biphasic waveforms are more effective than monophasic waveforms for successful defibrillation, but optimization of energy and current levels to minimize post-resuscitation myocardial dysfunction has been largely unexplored. We examined a low-capacitance waveform typical of low-energy application (low-energy biphasic truncated exponential [BTEL]; 100 microF, < or =200 J) and a high-capacitance waveform typical of high-energy application (high-energy biphasic truncated exponential [BTEH]; 200 microF, > or =200 J). METHODS: Four groups of anesthetized 40- to 45-kg pigs were investigated. After 7 min of electrically induced VF, a 15-min resuscitation attempt was made using sequences of up to three defibrillation shocks followed by 1 min of cardiopulmonary resuscitation. Animals were randomized to BTEL at 150 J or 200 J or to BTEH at 200 J or 360 J. RESULTS: Resuscitation was unsuccessful in three of the five animals treated with BTEH at 200 J. All other attempts were successful. Significant therapy effects were observed for survival (p = 0.035), left ventricular ejection fraction (p < 0.001), stroke volume (p < 0.001), fractional area change (p < 0.001), cardiac output (p = 0.044), and mean aortic pressure (p < 0.001). Hemodynamic outcomes were negatively associated with energy and average current but positively associated with peak current. Peak current was the only significant predictor of survival (p < 0.001). CONCLUSIONS: Maximum survival and minimum myocardial dysfunction were observed with the low-capacitance 150-J waveform, which delivered higher peak current while minimizing energy and average current.

Animals↗
PubMed2006

Storage effects on the Cole-Cole parameters of erythrocyte suspensions.

Chemical alterations of red blood cells (RBCs) during storage eventually affect the electrical properties of blood. In this study, the physiological parameters such as extracellular (SAGM + CPD + residual plasma) Na(+), K(+), Cl(-), pH, 2,3-DPG and ATP together with the Cole-Cole parameters were measured using erythrocyte suspensions from 51 male donors (31 donors form the training set and 20 donors are used for testing), on the 0th, 10th, 21st, 35th and 42nd days of storage. During storage, while the surrounding fluid resistance (R(e)) and the effective cell membrane capacitance (C(m)) increased progressively with time, the intracellular fluid resistance (R(i)) has decreased. Storage of RBCs resulted in a rise in K(+) and a fall in Na(+), Cl(-), pH, 2,3-DPG and ATP. Accordingly, electrical parameters were all correlated with Na(+), K(+), Cl(-), pH and ATP at varying levels. By applying multi-regression analysis, it is concluded that R(i), R(e) and C(m) are appropriate for modeling Na(+), K(+), Cl(-), pH and ATP during storage.

2,3-Diphosphoglycerate↗
PubMed1986

Ephaptically generated potentials in CA1 neurons of rat's hippocampus in situ.

Passive (ephaptic) transmembrane currents generated by antidromically evoked electrical fields were studied in CA1 hippocampal neurons of urethan-anesthetized rats. Recording was mostly from the stratum pyramidale where the fields have a maximum (negative) amplitude. The antidromic population spike was consistently smaller when recorded inside a noninvaded neuron rather than extracellularly. This indicated a substantial transmembrane potential (Vm), which was revealed and quantified by subtracting from the intracellular record a just-extracellular one. In neurons that have spikes greater than or equal to 40 mV (mean 60 mV), the average Vm was 41.1% of the extracellular field (Ve; mean 6.7 mV). Typically, Vm was very brief (mean duration 1.1 ms), predominantly monophasic, and in a depolarizing direction. Though almost synchronous with Ve, the peak of Vm was most often delayed slightly. Its amplitude varied with the intensity of antidromic stimulation, bearing an approximately constant relation to Ve, and it was not markedly sensitive to large changes in membrane potential. Most of these features confirm its ephaptic nature. By contrast, no consistent Vm was recorded from unresponsive cells, presumed to be glia. When combined with subthreshold depolarizing pulses, antidromic fields increased the firing probability of cells not activated by the antidromic stimulus. The ephaptic nature of this excitation was indicated by its very short latency, too early to be of synaptic origin, a much greater jitter of spike latency than was seen with antidromic spikes, and its inability to follow repetitive stimulation at frequencies as low as 2 Hz. In addition, juxta-threshold ephaptic excitations showed the random patterns of firing and very steep relation to intensity of stimulation expected of single-unit responses to electrical stimulation. In general, much larger excitatory effects could be demonstrated in neurons that had a high threshold for antidromic activation. The correlation between Vm and the increase in firing probability (r = 0.85) was strongly positive. A significant excitatory effect was detectable with antidromic fields as small as 1 mV. These ephaptically generated transmembrane potentials are probably of functional significance, even under physiological conditions, particularly in promoting synchronized firing of CA1 neurons. In the APPENDIX, the predictions of a simple neuronal model as a lumped resistance and capacitance circuit are shown to agree quite well with the observations.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

RF current density imaging in homogeneous media.

MRI has proven capable of imaging quasistatic volume current densities in electrolytic and biological media. In this paper, the feasibility of extending the method to image RF current density at the Larmor frequency is studied. RF current imaging could be relevant to MR power absorption and safety and to hyperthermia analysis, as well as creating dielectric and conductivity-dependent tissue contrast. The approach is to deliberately induce or inject RF currents in a sample synchronous with an MR pulse sequence and measure the resulting transverse RF magnetic field components. Current density is extracted by computing the curl of the magnetic fields. The preliminary theory has been developed for uniform media where both displacement and conduction currents exist while skin effects or eddy currents are absent. If the derivative in the B0 direction of the RF magnetic field component parallel to B0 is negligible, then sufficient information exists to reconstruct the RF current density component that is parallel to B0 without rotating the sample. The relative phase of the current can also be estimated. The method has been proven feasible by successfully imaging a uniform 85.6-MHz current density in a salt water phantom. The experiment conforms closely to capacitively coupled hyperthermia heating.

Artifacts↗
PubMed1996

Regulation of intracellular calcium is closely linked to glucose metabolism in J774 macrophages.

The effects of 2-deoxy-D-glucose (2dGlc) and glucose deprivation were investigated in the J774 murine macrophage-like cell line. 2dGlc addition or glucose deprivation for 4 min led to an inhibition in the transient increase in cytoplasmic free Ca2+ ([Ca2+]i) that otherwise occurs in response to three different agonists: IgG, ATP and platelet activating factor. This inhibition was preceded by a partial release of Ca2+ from intracellular, thapsigargin-sensitive stores. In contrast, the transition from 5 to 30 mM glucose caused a decrease in [Ca2+]i and a corresponding increase in thapsigargin-sensitive sequestered Ca2+. The effects of an alternate glycolytic inhibitor, NaF, and a mitochondrial inhibitor, rotenone, were also tested. These inhibitors caused neither a release of Ca2+ from intracellular stores nor an inhibition in any of the agonist responses. The capacitative influx of extracellular Ca2+ following depletion of intracellular stores was also found to be selectively inhibited by the prior addition of 2dGlc or with glucose deprivation. In addition, when an elevated plateau of [Ca2+]i was established by the irreversible depletion of intracellular Ca2+ stores, the addition of 2dGlc caused a decrease in the on-going capacitative entry of Ca2+.

Adenosine Triphosphate↗
PubMed1999

[The advances of the research work on lens cells using patch clamp technique].

The study of lens cells by patch clamp technique began in the early 90's. It shows that the membrane of lens cells possess different types of ionic channels. These channels are responsible for the modulation of the hydration state and transparency of lens. Several patch clamp configurations have been used to study the activity of ionic channels. The resting potential and capacitance as well as resistance of different lens cells were also measured.

Aerospace Medicine↗
PubMed1977

Computerized brain impedograph.

Design of an on-line system utilizing a minicomputer and a specific dedicated hardware for 2T brain tissue impedance is provided. The prototype system, which can operate from 2 Hz to 10 kHz, covers most of the frequency range of interest in the encephalographic research. Driving the brain tissue with constant current and sampling the response current and voltage, the coefficients for a Fourier series expansion can easily be obtained. Real (resistance) and imaginary (capacitance) impedance components at each chosen frequency are printed on a Teletype printer.

Brain↗
PubMed1992

A model study of electric field interactions between cardiac myocytes.

The transmission of excitation via electric field coupling was studied in a model comprising two myocytes abutted end-to-end and placed in an unbounded volume conductor. Each myocyte was modeled as a small cylinder of membrane (10 microns in diameter and 100 microns in length) capped at both ends. A Beeler-Reuter model modified for the Na+ current dynamics served to simulate the membrane ionic current. There was no resistive coupling between the myocytes and the intercellular junction consisted of closely apposed pre- and post-junctional membranes, separated by a uniform cleft distance. The membrane current crossing the prejunctional membrane during the action potential upstroke tends to flow out of the cleft, but it is partly prevented from doing so by the shunt resistance constituted by the cleft volume conductor. The prejunctional upstroke gives rise to a pulse of positive potential within the cleft which induces a small capacitive current across the post-junctional membrane to yield a small positive change in the intracellular potential in the post-junctional cell. The net result is an hyperpolarization of the post-junctional cleft membrane and a slight depolarization of the rest of the cell membrane since the extracellular potential outside of the cell is zero. The magnitude of this depolarization is quite small for a flat junctional membrane and it can be increased by membrane folding and interdigitation, so as to increase the junctional membrane area by a factor of 10 or more. Even then the post-junctional depolarization does not reach threshold when the extracellular potential around the post-junctional cell is effectively zero. Threshold depolarization occurs in the presence of a large decrease of post-junctional load, by increasing the junctional membrane capacitance and/or decreasing the volume of the post-junctional cell. Assuming that the normal resistive coupling between two cardiac myocytes is 1-4 M omega, our model study indicates that electric field coupling would then be about two orders of magnitude smaller. However, substantial enhancement of the efficacy of electric field transmission was observed in the case of cells with substantial junctional membrane folding.

Action Potentials↗