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At least 253 records · Page 14Linked to original sources
PubMed2003

Effect of polylysine on transformations and permeability of negative vesicular membranes.

Small (40-60 nm in diameter) and large (300-350 nm) negative vesicles were complexed with a cationic polypeptide, poly-L-lysine (PL). Laser microelectrophoresis experiments showed that in small vesicles rendered anionic with the addition of cardiolipin (CL(2-)), only the CL(2-) in the outer leaflet is involved in the complexation with PL. Calorimetric and other data demonstrate that the binding of PL to the membrane surface causes domains ("rafts") of CL(2-) to form in the outer leaflet, and it is these domains that electrostatically bind the polymer. The kinetics of transmembrane permeation of doxorubicin (Dox, a fluorescent anti-tumor drug) was monitored with and without PL binding to the outer surface of the vesicles. It was found that PL mediates the permeation of Dox into the vesicle interior. In the absence of PL, the Dox molecule (possessing an amino group of pK(a)=8.6) binds to the anionic vesicles in the protonated form and, consequently, suffers an impaired mobility through the membrane. On the other hand, when the PL covers the vesicle surface, Dox passes though the membrane with greater ease. The effects of salt and polyanion on the stability of PL-vesicle complexes and the PL-mediated Dox permeation are also discussed.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1969

Effect of divalent cations on potassium conductance of squid axons: determination of surface charge.

Potassium conductance-voltage curves have been determined for a squid axon in high external potassium solution for a wide range of divalent cation concentrations. A decrease in divalent ion concentration shifts the conductance-voltage curve along the voltage axis in the direction of more hyperpolarized voltages by as much as 9 mv for an e-fold change in concentration. When the divalent ion concentration is less than about 5 mM, a further decrease does not cause a significant shift of the conductance-voltage curve. These results can be explained by assuming that on the outer surface of the membrane there is a negative fixed charge which can bind calcium ions, and that the axon is sensitive to the resulting double-layer potential. From our data, the best value for charge density was found to be one electronic charge per 120 square angstroms, and a lower limit to be one electronic charge per 280 square angstroms.

Animals↗
PubMed1971

Kinetic theory model for ion movement through biological membranes. I. Field-dependent conductances in the presence of solution symmetry.

A model for ion movement through specialized sites in the plasma membrane is presented and analyzed using techniques from nonequilibrium kinetic theory. It is assumed that ions traversing these specialized regions interact with membrane molecules through central conservative forces. The membrane molecules are approximated as massive spherical scattering centers so that ionic fractional energy losses per collision are much less than one. Equations for steady-state membrane ionic currents and conductances as functions of externally applied electric field strength are derived and numerically analyzed, under the restriction of identical solutions on each size of the membrane and constant electric fields within the membrane. The analysis is carried through for a number of idealized ion-membrane molecule central force interactions. For any interaction leading to a velocity-dependent ion-membrane molecule collision frequency, the membrane chord conductance is a function of the externally applied electric field. Interactions leading to a collision frequency that is an increasing (decreasing) function of ionic velocity are characterized by chord conductances that are decreasing (increasing) functions of field strength. For ion-neutral molecule interactions, the conductance is such a rapidly decreasing function of field strength that the slope conductance becomes negative for all field strengths above a certain value.

Biological Transport↗
PubMed1971

A stored charge model for the sodium channel.

A new model is proposed to account for the apparent conductance changes of the sodium, or early, channel in nerve fiber membranes. In this model it is assumed that the channels are gated at the interior side of the membrane and are resistively limited at the exterior side by sodium selective barriers of high resistance to ion flow. Under resting conditions the closed channels accumulate a store of sodium ions, dependent on the exterior sodium concentration. With the application of a depolarizing clamp the interior gates open allowing the stored ions to discharge into the interior low sodium concentration solution. In this model the initial rise in the early current results from the opening of more and more gates in response to the depolarizing clamp. The subsequent fall in the early current results from the "capacitative" discharge of the opened channels, limited by the high resistive barrier at the exterior end. Upon repolarization, the gates reclose and sodium ions reaccumulate in the channels from the high concentration external solution, but at a slow rate determined by the resistive barrier. Preliminary tests of this model, using a number of simplifying assumptions, show that it has the ability to account, at least semiquantitatively, for the major characteristics of the experimental clamp results.

Axons↗
PubMed1973

Model studies of the magnetocardiogram.

A general expression is developed for the quasi-static magnetic field outside an inhomogeneous nonmagnetic volume conductor containing internal electromotive forces. Multipole expansions for both the electric and magnetic fields are derived. It is shown that the external magnetic field vanishes under conditions of axial symmetry. The magnetic field for a dipole current source in a sphere is derived, and the effect of an eccentric spherical inhomogeneity is analyzed. Finally the magnetic dipole moment is calculated for a current dipole in a conducting prolate spheroid.

Conductometry↗
PubMed1975

Axon voltage-clamp simulations. I. Methods and tests.

This is the first in a series of four papers in which we present the numerical simulation of the application of the voltage clamp technique to excitable cells. In this paper we describe the application of the Crank-Nicolson (1947) method for the solution of the parabolic partial differential equations that describe a cylindrical cell in which the ionic conductances are functions of voltage and time (Hodgkin and Huxley, 1952). This method is compared with other methods in terms of accuracy and speed of solution for a propagated action potential. In addition, differential equations representing a simple voltage-clamp electronic circuit are presented. Using the voltage clamp circuit equations, we simulate the voltage clamp of a single isopotential membrane patch and show how the parameters of the circuit affect the transient response of the patch to a step change in the control potential. The stimulation methods presented in this series of papers allow the evaluation of voltage clamp control of an excitable cell or a syncytium of excitable cells. To the extent that membrane parameters and geometrical factors can be determined, the methods presented here provide solutions for the voltage profile as a function of time.

Action Potentials↗
PubMed1975

Axon voltage-clamp simulations. II. Double sucrose-gap method.

This is the second in a series of four papers on the simulation of the voltage clamp of cylindrical excitable cells. In this paper we evaluate the double sucrose-gap voltage-clamp technique for the squid and lobster giant axons. Using the Crank-Nicolson method of solution of the cable equations and differential equations representing the voltage clamp circuit we studied the effect of length of the sucrose gap "node" on the voltage profile along an excitable cell during a simulated voltage clamp. The voltage gradients along the region of the cell within the node produce "notches" in the current recording as well as changes in the magnitude of the sodium and potassium current for a given voltage step. Our results show that good voltage clamp control requires node lengths less than one-half the axon diameter.

Action Potentials↗
PubMed1975

Axon voltage-clamp simulations. III. Postsynaptic region.

This is the third in a series of four papers in which we present the numerical simulations of the application of the voltage clamp technique to excitable cells. In this paper we discuss the problem of voltage clamping a region of a cylindrical cell using microelectrodes for current injection and voltage recording. A recently developed technique (Llinás et al., 1974) of internal application of oil drops to electrically insulate a short length of the postsynaptic region of the squid giant synapse is evaluated by simulation of the voltage clamp of an excitable cylindrical cell of finite length with variable placement of the current and voltage electrodes. Our results show that ENa can be determined quite accurately with feasible oil gap lengths but that the determination of the reversal potential for the synaptic conductance, ES, can be considerably in error. The error in the determination of ES dependp, and especially the membrane resistance at the time the synaptic conductance occurs. It is shown that the application of tetraethylammonium chloride to block the active potassium conductance very significantly reduces the error in the determination of ES. In addition we discuss the effects of cable length and electrode position on the apparent amplitude and time course of the syn aptic conductance change. These results are particularly relevant to the application of the voltage clamp technique to cells with nonsomatic synapses. The method of simulation presented here provides a tool for evaluation of voltage clamp analysis of synaptic transmission for any cell with known membrane parameters and geometry.

Action Potentials↗
PubMed1975

Axon voltage-clamp simulations. A multicellular preparation.

In this paper we extend the simulation of the voltage clamp of a single nerve fiber to a bundle of axons. These simulations included not only the description of the voltage clamp circuit and a single unidimensional cable to represent the preparation in the "node" region of a double sucrose gap used previously but also a series resistance and a shunt pathway. The output of the voltage control amplifier is applied across the membrane plus the series resistance, producing a voltage drop across the series resistance due to the current generated by the membrane in response to a depolarizing voltage step. Since the membrane current has an inward and an outward phase, voltage drops of opposite sign are produced across the series resistance. During the transient current and at all points along an axon, the potential deviation produced by the series resistance is opposite to the deviation produced by the longitudinal gradient. Only at a command potential equal to the sodium equilibrium potential, the membrane potential transiently matches the command potential. For the attempted voltage clamp of an axon, values of series resistance larger than 50 omega-cm2 allowed propagated action potentials in the membrane. In spite of the presence of propagated action potentials at the calbe membrane, the recorded current does not show "notches" and it has a phase of inward current and a phase of outward current. It is concluded that, in a multicellular preparation with series resistance, the recording of a square voltage pulse does not indicate voltage control of the transmembrane potential. The presence of a shunt pathway produces inaccurate values of current density. Neither series or shunt resistance produce "notches" in the current records.

Action Potentials↗
PubMed1986

A simple conductimetric method as an alternative to the colorimetric p-nitrobenzylpyridine test for the measurement of the reactivity of potentially mutagenic alkylating compounds.

A simple method for the measurement of the kinetics of reaction of potentially mutagenic alkyl halides with amines, based on the direct conductimetric monitoring of the quaternary ammonium salt produced in these reactions, is proposed and applied to the alkylation of p-nitrobenzylpyridine (NBP) and triethylamine (TEA) in different solvents. With respect to the classical colorimetric NBP-test, this method has the advantage that the rates can be measured continuously over the entire course of the reactions and the kinetic order and constants can be easily obtained. It is also shown that the previously proposed, NBP modified test', using simultaneously NBP and TEA, gives actually the sum of the rate constants for the reactions of the alkylating reagent with the two amines.

Alkylation↗
PubMed1999

Correlation between changes in morphology, electrical properties, and angiotensin-converting enzyme activity in the failing heart.

Evidence is available that morphologic and electrophysiologic abnormalities are present in the failing heart. In the present work, the progressive changes in electrical properties and morphology of the failing heart of Syrian cardiomyopathic hamsters (TO2) were investigated at different stages of the pathological process, and the possible role of the renin-angiotensin system was studied. Cardiomyopathic hamsters 2 and 11 months of age were used. Age-matched normal hamsters (F1B) were utilized as controls. Measurements of membrane potential, conduction velocity and refractoriness were made with conventional intracellular electrodes connected to a high impedance DC amplifier. Serum and cardiac angiotensin-converting enzyme (ACE) activities were measured in controls and cardiomyopathic animals. The results indicated that interstitial fibrosis and calcification were present in the heart of 2-month old Syrian cardiomyopathic hamsters. Measurements of the resting potential performed in the isolated right ventricle of 2-month old Syrian cardiomyopathic hamsters indicated an average value of -66.7 +/- 0.96 mV (n = 25); in the controls of the same age was -78.5 +/- 1 mV (n = 25, P < 0.05); and in 11-month old cardiomyopathic hamsters was -67.8 +/- 0.83 mV (n = 10). The duration of the action potential measured at 50 and 90% of repolarization in 2-month old hamsters was well above the controls. The conduction velocity measured in the isolated right ventricle of 2-month old Syrian cardiomyopathic hamsters (44.2 +/- 1.6 cm/s, n = 12) was not different from the control (43.7 +/- 1.1 cm/s, n = 7, P > 0.05) but was significantly larger than that recorded from the ventricle of 11-month old animals (37.8 +/- 2.9 cm/s, n = 11, P < 0.05). ACE activity was 0.26 +/- 0.01 nmol/mg x min in the heart of controls at 2 months of age and did not change with age. Although in the 2-month old cardiomyopathic hamsters the enzyme activity (0.28 +/- 0.04 nmol/mg x min) was not different from the controls (P > 0.05), in myopathic animals at 11 months of age, the enzyme activity (0.56 +/- 0.027 nmol/mg x min) was greater than controls (P < 0.05). The ACE activity in plasma followed the same pattern. The conclusion from these experiments is, that some parameters like resting potential, action potential duration, and morphological abnormalities appeared quite early in the failing process. The decline in conduction velocity, however, appeared later on, concurrently with the activation of plasma and cardiac renin-angiotensin systems.

Age Factors↗
PubMed2000

Determination of thiosulfate, thiocyanate and polythionates in a mixture by ion-pair chromatography with ultraviolet absorbance detection.

A sensitive ion chromatographic method has been developed for the determination of mixtures of thiosulfate, thiocyanate and polythionates (tri-, tetra-, penta- and hexathionate). The proposed method is based on the separation of the sulfur anions on an octadecylsilica (ODS) column with an acetonitrile-water mobile phase containing tetrapropylammonium salt (TPA) as an ion-pairing reagent and the ultraviolet absorption detection of the sulfur anions. When an acetonitrile-water (20:80, v/v) solution (pH 5.0) containing 6 mM TPA was used as a mobile phase at flow-rate of 0.6 ml min(-1), the sulfur anions were resolved within 22 min. The detection limits defined at S/N=3 and 230 nm were very low for all anions, except trithionate: 30 nM for thiosulfate, 60 nM for thiocyanate, 20 nM for tetrathionate, 15 nM for pentathionate and 18 nM for hexathionate. The proposed method gave recoveries ranging from 95.0 to 105.0% when applied to the determination of polythionates added to hot spring waters.

Anions↗
PubMed2000

Determination of epichlorohydrin by ion chromatography.

In this work we developed a new method for epichlorohydrin determination with suppressed ion chromatography. The technique is based on a reaction between the analyte and sulfur(IV) to form a product with a terminal sulfonate group that can be analyzed by anion-exchange chromatography. The reaction conditions were optimized as a function of temperature, type and concentrations of reagents and pH. Due to the characteristics of the product formed, the columns used were an lonPac AS 11 and AS II-HC with a NaOH eluent. The eluent concentration was optimized in order to achieve a complete separation of epichlorohydrin, chloride and nitrate ions, commonly occurring in drinking waters. In order to improve the detection limits, a preconcentration step, using reversed-phase materials, has been optimized. The method developed was suitable for epichlorohydrin determination in drinking water.

Chromatography, Liquid↗
PubMed2000

Determination of some pesticides and intermediates by ion chromatography.

We explored the possibility of determining some pesticides and process intermediates by ion chromatography. Some applications of this technique, standardized and adopted to meet the requirements of Gharda Chemicals (which is a leading producer of agrochemicals in India), will be presented in this communication. These include analysis of the finished products [(a) dicamba dimethylamine (DMA)/potassium/sodium salt acid and (b) 2,4-dichlorophenoxyacetic acid (2,4-D)], and the process intermediates [(c) tetrachlorobutyric acid and (d) glyoxylic acid/glycollic acid/formic acid]. Ion chromatographic analysis of sub-ppm levels of 2,5-dibromo- and monochlorobromo-phenols, in 2,5-dichlorophenol, after adequate sample preparation steps, is also being presented.

Chromatography, Liquid↗
PubMed2000

Enhanced conductometric detection of cyanide in suppressed ion chromatography.

Weak-acid anions such as borate and cyanide, present problems in chemically suppressed ion chromatography, since the acids are weakly ionised, giving low conductivity and therefore decreased sensitivity. For borate this problem was overcome by converting the weak acid to its sodium salt, by the use of a second anion micromembrane suppressor (AMMS) as an ion exchange reactor (IER), flushed by EDTA reagent, to supply sodium ions for the conversion. This paper will discuss the use of this IER system to determine cyanide, which is also a very weak acid, with a similar pKa to borate. The detection limit for cyanide of 50 microM compare favourably with indirect conductivity detection.

Chromatography, Ion Exchange↗
PubMed2000

Improved method for the determination of trace perchlorate in ground and drinking waters by ion chromatography.

Ammonium perchlorate, a key ingredient in solid rocket propellants, has been found in ground and surface waters in a number of U.S. states, and perchlorate contamination of public drinking water wells is now a serious problem in California. Perchlorate poses a health risk and preliminary data from the U.S. EPA reports that exposure to less than 4-18 microg/l provides adequate human health protection. An improved ion chromatographic method was developed for the determination of low microg/l levels of perchlorate in ground and drinking waters based on a Dionex IonPac AS16 column, an hydroxide eluent generated using an EG40 automated eluent generator, large loop (1000 microl) injection, and suppressed conductivity detection. The method is free of interferences from common inorganic anions, linear over the range of 2-100 microg/l perchlorate, and quantitative recoveries are obtained for low microg/l levels of perchlorate in spiked ground and drinking water samples. The MDL of 150 ng/l permits quantification of perchlorate below the levels that ensure adequate health protection.

Chromatography, Liquid↗
PubMed2000

Novel three-dimensional capillary electrophoresis system for complex and trace analysis.

A novel triple column capillary electrophoresis system is described. Design specifications facilitate method development and analyses by providing on-line, selective, pre-concentration and clean-up of both high (ml) and low (microl) volumes of specific analytes in two dimensions and separation via an additional third dimension. The system described additionally provides four distinct detection capabilities via both contactless conductivity and UV. The addition of a third dimension to the previously reported "coupled-column" systems, and further modifications made, has allowed for optimal identification, separation, and quantitation of micro-components in complex mixtures. The ability to perform both capillary zone electrophoretic and isotachophoretic separations on-line and in any combination enhances the scope for rapid analytical method development and analysis of complex or trace sample components.

Conductometry↗