Changes In Ionic Conductivity Research Articles

Grain boundary and grain interior conduction in γ′-Bi 2MoO 6

Impedance spectroscopy of fine grained (<10 μm) γ′-Bi 2MoO 6 samples, in the frequency range of 0.1 Hz–250 kHz, relevant to sensor applications, up to 800 °C, has been used to characterize grain boundary and grain interior contributions to conduction. Above 500 °C, the grain boundary contribution is no longer rate limiting and conduction is dominated by the grain interior component. The corresponding activation energies are 0.98 eV for grain boundary and 0.73 eV for grain interior components. The weak dependence of conductivity on oxygen partial pressure below 500 °C can be attributed to electrode–electrolyte interface phenomena, whereas the robust response to ethanol is commensurate with changes in intrinsic ionic conductivity.

Effect of Varying Electrolyte Conductivity on the Electrochemical Behavior of Porous Electrodes

Ionic conductivity in the air cathode of a polymer electrolyte membrane (PEM) fuel cell changes with distance depending on the ionomer loading [ J. Electrochem. Soc. , 150 , A1440 (2003) ]. When the ionic conductivity changes with distance inside a porous electrode, existing models in the literature fail to capture the behavior of porous electrodes. In this paper, we show how existing models in the literature can be modified to take care of varying ionic conductivity. The modified model is shown to be efficient in handling mathematical singularity arising from the distribution of ionic conductivity inside a porous electrode. In addition, the model developed was used to optimize the functionality of ionic conductivity (ionomer loading) to minimize the ohmic drop across a porous electrode. It is shown that depending on the system parameters, there might be an optimum way to distribute the ionomer loading inside the PEM fuel cell cathode.

Physicochemical Properties and Structures of Room Temperature Ionic Liquids. 2. Variation of Alkyl Chain Length in Imidazolium Cation

The alkyl chain length of 1-alkyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide ([Rmim][(CF(3)SO(2))(2)N], R = methyl (m), ethyl (e), butyl (b), hexyl (C(6)), and octyl (C(8))) was varied to prepare a series of room-temperature ionic liquids (RTILs), and the thermal behavior, density, viscosity, self-diffusion coefficients of the cation and anion, and ionic conductivity were measured over a wide temperature range. The self-diffusion coefficient, viscosity, ionic conductivity, and molar conductivity change with temperature following the Vogel-Fulcher-Tamman equation, and the density shows a linear decrease. The pulsed-field-gradient spin-echo NMR method reveals a higher self-diffusion coefficient for the cation compared to that for the anion over a wide temperature range, even if the cationic radius is larger than that of the anion. The summation of the cationic and anionic diffusion coefficients for the RTILs follows the order [emim][(CF(3)SO(2))(2)N] > [mmim][(CF(3)SO(2))(2)N] > [bmim][(CF(3)SO(2))(2)N] > [C(6)mim][(CF(3)SO(2))(2)N] > [C(8)mim][(CF(3)SO(2))(2)N], which greatly contrasts to the viscosity data. The ratio of molar conductivity obtained from impedance measurements to that calculated by the ionic diffusivity using the Nernst-Einstein equation quantifies the active ions contributing to ionic conduction in the diffusion components, in other words, ionicity of the ionic liquids. The ratio decreases with increasing number of carbon atoms in the alkyl chain. Finally, a balance between the electrostatic and induction forces has been discussed in terms of the main contribution factor in determining the physicochemical properties.

Role of up-regulation of in action potential shortening associated with atrial fibrillation in humans

Although previous studies in dogs have indicated a minimal role for changes in I(K1) in the shortening of action potential duration (APD) associated with atrial fibrillation (AF), in humans, there is evidence for significant AF-induced up-regulation of this current. In this computer model study, we investigated the relative contributions of the remodeling of I(K1), L-type calcium current, and other remodeled ionic channel currents to AF-induced APD reduction in human atrium. Two computer models of electrical activity of human atrial cell were modified by incorporating experimental data of AF-induced changes in human atrial ionic channel conductance and kinetics reported by Bosch et al. (I(CaL), I(to), I(K1), and I(Na)) (AF-1) and Workman et al. (I(CaL), I(to), and I(K1)) (AF-2). The roles and relative importance of individually remodeled ion channels in the APD reduction in human atrium were evaluated by the removal and exclusive methods, in which remodeling of specific currents was omitted, or considered in isolation, in the two models. When tested together, previously reported AF-induced changes in sarcolemmal ion currents result in marked shortening of atrial APD(90). With the AF-1 remodeled parameters, there is a 62% reduction in APD(90) for the Nygren et al. model, and a 68% reduction for the Courtemanche et al. model, which are comparable to experimental results of 60% reduction seen in humans. When tested individually, AF-1-induced changes in I(CaL), I(K1), or I(to) alone result in APD(90) reduction of 20%, 64%, and -10%, respectively, for the Nygren et al. model, and 27%, 40%, and 11.6%, respectively, for the Courtemanche et al. model. With the AF-2 remodeled parameters, there is a 47% reduction in APD(90) for the Nygren et al. model and a 49% reduction for the Courtemanche et al. model, which are also comparable to experimental results of 45% reduction. When tested individually, AF-2-induced changes in I(CaL) or I(K1) alone result in APD(90) reduction of 20% and 40%, respectively, for the Nygren et al. model, and 14% and 21%, respectively, for the Courtemanche et al. model. Previously reported changes in L-type Ca(2+) current are insufficient to account for the observed reduction in atrial APD associated with persistent AF. Up-regulation of I(K1) has a greater influence on atrial APD in the human model.

Strength-duration properties and glycemic control in human diabetic motor nerves

Objective To investigate the influences of hyperglycemia on axonal excitability in human diabetic nerves. Hyperglycemia results in decreased Na +–K + pump function, presumably leading to intra-axonal Na + accumulation and thereby, reduced Na + currents. Methods The strength-duration time constant ( τ SD), which partly depends on persistent Na + conductance active at the resting membrane potential, was measured in median motor axons of 79 diabetic patients. The relationship of τ SD with the state of glycemic control (hemoglobin A1c [HbA1c] levels) was analyzed. Results The mean τ SD was longer for diabetic patients than for normal controls, but the difference was not significant. Among diabetic patients, the subgroup of patients with good glycemic control (HbA1c<7%) had significantly longer τ SD than the patient group with poor control (HbA1c>9%; P=0.04). The mean τ SD was longest at the HbA1c level of 5–6%, gradually decreasing and reaching a plateau around the HbA1c level of 9%. There was an inverse relationship between HbA1c levels and τ SD, when the HbA1c levels ranged from 5 to 9% ( P=0.04). Conclusions In diabetic nerves, τ SD is generally longer than normal, but hyperglycemia is associated with paradoxically shortened τ SD, because of a decrease in axonal persistent Na + conductance, possibly related to reduced membranous Na + gradient, tissue acidosis, or other metabolic factors. Significance Measurements of τ SD could provide a new insight into changes in ionic conductance in human diabetic nerves.

Voltage-dependent ionic conductances in the human malignant astrocytoma cell line U87-MG.

Although the human malignant astrocytoma cell line U87-MG has been used in numerous studies, few findings are available on the properties of its membrane ion conductances. Characterization of the ion channels expressed in these cells will make it possible to study membrane ion conductance changes when a receptor is activated by its ligand. This will help to elucidate the functional properties of these receptors and their signal-transduction pathways in pathophysiological events. This work studied the voltage-dependent ionic conductances of U87-MG cells using the Whole-Cell Recording patch-clamp technique. Six types of voltage-dependent ionic currents were identified: (i) a TEA-, 4-AP- and CTX-sensitive Ca2+-dependent K+ current, (ii) a transient K+ current inhibited by 4-AP, (iii) an inwardly rectifying K+ current blocked by Ba2+ and 4-AP, (iv) a DIDS- and SITS-sensitive Cl- current, (v) a TTX-sensitive Na+ conductance and (vi) a L-type Ca2+ conductance activated by BayK-8644 and inhibited by Ni and the L-type Ca2+ channel inhibitor, nifedipine. In addition, electrical depolarizations elicited inward currents due to voltage-independent, Ca2+-dependent K+ influx against the electrochemical gradient, probably via an ouabain-sensitive Na+-K+ pump.

Ionic conductivity of LixB1−x/3PO4 ceramic electrolyte based on defect models

The ionic conductivity of LixB1−x/3PO4 is investigated in terms of defect models with a lithium doping level of 0<x<0.3. The crystalline solid electrolytes, LixB1−x/3PO4, are synthesized by a soft-chemical route by using H3BO3, H3PO4, LiOH·H2O. To determine the changes in ionic conductivity and activation energy of LixB1−x/3PO4 with lithium content x, AC-IS measurements are performed at temperatures between 25 and 90 °C. The maximum ionic conductivity is 3.74×10−6 S cm−1 at room temperature for x=0.10, and the activation energy increases from 0.3039 to 0.3385 eV with increasing lithium content.

The effects of tamoxifen and toremifene on bone cells involve changes in plasma membrane ion conductance.

Selective estrogen receptor modulators (SERMs), tamoxifen (Tam) and toremifene (Tor), are widely used in the treatment of breast cancer. In addition, they have been demonstrated to prevent estrogen deficiency-induced bone loss in postmenopausal women. These effects are thought to be caused by the interaction of the SERMs with the estrogen receptor, although SERMs have also been shown to conduct non-receptor-mediated effects such as rapid changes in membrane functions. We compared the effects of Tam, Tor, and 17beta-estradiol (E2) on the viability of rat osteoclasts and osteoblasts. Both Tam and Tor were found to cause osteoclast apoptosis in in vitro cultures, which was reversed by E2. In addition, at higher concentration (10 microM), both SERMs had an estrogen receptor-independent effect, which involved interaction with the plasma membrane as demonstrated with UMR-108 osteosarcoma cells by Tam and Tor, but not E2. A leak of protons leading to changes in intracellular pH was shown both in medullary bone derived membrane vesicles and in intact cells. These effects were followed by a rapid loss of cell viability and subsequent cell lysis. Our results show that both Tam and Tor have an ionophoric effect on the plasma membranes of bone cells and that these SERMs differed in this ability: Tor induced rapid membrane depolarization only in the presence of high concentration of potassium. These non-receptor-mediated effects may be involved in therapeutic responses and explain some clinical side effects associated with the treatment of patients with these SERMs.

Retinal bipolar neurons express the cyclic nucleotide-gated channel of cone photoreceptors.

Cyclic nucleotide-gated (CNG) channels link intracellular cyclic nucleotides to changes in membrane ionic conductance in a variety of physiological contexts. In the retina, in addition to their central role in phototransduction, CNG channels may be involved in nitric oxide signaling in bipolar neurons or in the hyperpolarizing synaptic response to glutamate in ON-type (depolarizing) bipolar cells. Despite their potential physiological significance, however, expression of CNG channels has not yet been demonstrated in bipolar cells. To identify CNG channel subtypes in retinal bipolar neurons, we used single-cell molecular biological techniques in morphologically distinctive ON bipolar cells from goldfish retina. Both single-cell in situ hybridization and single-cell RT-PCR demonstrated in ON bipolar cells the presence of mRNA for the CNG channel subtype that is also found in cone photoreceptors. Other bipolar cells, which likely represent OFF cells, did not express the cone CNG channel. Thus the CNG channel of cone photoreceptors is expressed in ON bipolar cells, where it may be involved in physiological responses to nitric oxide, or in the sign-inverting glutamatergic synapse that gives rise to the ON visual pathway.

Lysophosphatidic acid in neural signaling.

The physiological and pathological importance of lysophosphatidic acid (LPA) in the nervous system is underscored by its presence, as well as the expression of its receptors in neural tissues. In fact, LPA produces responses in a broad range of cell types related to the function of the nervous system. These cell types include neural cell lines, neural progenitors, primary neurons, oligodendrocytes, Schwann cells, astrocytes, microglia, and brain endothelial cells. LPA-induced cell type-specific effects include changes in cell morphology, promotion of cell proliferation and cell survival, induction of cell death, changes in ion conductance and Ca2+ mobilization, induction of pain transmission, and stimulation of vasoconstriction. These effects are mediated through a number of G protein-coupled LPA receptors that activate various downstream signaling cascades. This review provides a current summary of LPA-induced effects in neural cells in vitro or in vivo in combination with our current understanding of the signaling pathways responsible for these effects.

ELECTRORHEOLOGICAL EFFECT IN SUSPENSIONS OF AgI/Ag2O/V2O5/P2O5 GLASSES

The ER effect results from bulk and surface electric polarization processes in solid grains of ER suspensions but detailed mechanisms are not very clear. The aim of the present study was to find correlations between the character of bulk charge transport processes (electronic and/or ionic) in particles of a dispersion and parameters of the ER effect. As the dispersed phase we used glasses comprising oxides of silver, vanadium and phosphorus with addition of silver iodide. Bulk electric properties of this material could be modified without changing other properties influencing the ER effect like porosity, shape of grains, hardness, affinity to a liquid matrix etc. Variations of concentration of the components result in changes of ionic and electronic conductivity whilst other properties remain constant. The suspensions of the powdered glasses showed relatively high ER effect. The dynamic yield stress figured from 35 to 160 Pa at 2.0kV/mm for 12% concentration by volume. The highest values were observed for ionically conducting samples. The values of relaxation frequencies ( f R ) based on bulk properties of the glass sample were calculated and correlated with the yield stress whose maximum was obtained for samples of f R close to 100 kHz. High ER effect was observed also for samples of f R in the MHz range but in this case different polarization mechanism was postulated. The influence of polarization mechanisms on rheological behavior of the prepared fluids was discussed.

Multinuclear Solid-State NMR, DSC, and Conductivity Studies of Solid Polymer Electrolytes Based on Polyurethane/Poly(dimethylsiloxane) Segmented Copolymers

Solid polymer electrolytes based on polyurethane/poly(dimethylsiloxane) segmented copolymers (PS55) have been characterized by differential scanning calorimetry (DSC), ionic conductivity, and multinuclear solid-state NMR measurements. The results of DSC measurements indicate the formation of transient cross-links between Li+ ions and the ether oxygens on complexation with LiClO4, resulting in an increase in the soft segment Tg. However, the soft segment Tg remains almost invariant at high salt concentration. There is a conductivity jump at around 310−330 K that the behavior of ionic conductivity changes from Arrehnius- to Vogel−Tamman−Fulcher (VTF)-type behavior. Below this jump temperature, the conductivity follows Arrehnius-like behavior, implying a diffusing mechanism for transport of the charge carriers where the charge carriers are decoupled from the segmental motion of the polymer chain. By contrast, the diffusion of charge carrier is assisted by the segmental motions of the polymer chains above the...

Neural control of the heart: developmental changes in ionic conductances in mammalian intrinsic cardiac neurons

The expression and properties of ionic channels were investigated in dissociated neurons from neonatal and adult rat intracardiac ganglia. Changes in the hyperpolarization-activated and ATP-sensitive K + conductances during postnatal development and their role in neuronal excitability were examined. The hyperpolarization-activated nonselective cation current, I h, was observed in all neurons studied and displayed slow time-dependent rectification. An inwardly rectifying K + current, I K(IR), was present in a population of neurons from adult but not neonatal rats and was sensitive to block by extracellular Ba 2+. Using the perforated-patch recording configuration, an ATP-sensitive K + (K ATP) conductance was identified in ≥50% of intracardiac neurons from adult rats. Levcromakalim evoked membrane hyperpolarization, which was inhibited by the sulphonylurea drugs, glibenclamide and tolbutamide. Exposure to hypoxic conditions also activated a membrane current similar to that induced by levcromakalim and was inhibited by glibenclamide. Changes in the complement of ion channels during postnatal development may underlie observed differences in the function of intracardiac ganglion neurons during maturation. Furthermore, activation of hyperpolarization-activated and K ATP channels in mammalian intracardiac neurons may play a role in neural regulation of the mature heart and cardiac function during ischaemia–reperfusion.

Formation of Solid Electrolyte Interface in Lithium Nickel Mixed Oxide Electrodes during the First Cycling

We studied irreversible capacity and solid electrolyte interface (SEI) formation in lithium nickel mixed oxide in the first charge and discharge cycle. Initial capacity loss mainly originates from two irreversible processes; structural change and reactions between electrode active materials and electrolyte components. Upon contact of the electrode with electrolyte, the reactions spontaneously take place and result in an instantaneous formation of SEI film, whose ionic conductivity changes considerably in the subsequent charge and discharge cycle. According to the change of ionic conductivity, formation of SEI film during the first charge can be roughly divided into two voltage regions, below 3.4 V, at which a highly resistive SEI film is continuously grown, and between 3.4 and 3.8 V, at which a highly conductive SEI film is formed. The high conductivity of SEI film remains nearly invariant as the cell is cycled between 3.8 and 4.3 V, while falling significantly as the cell is discharged to 3.4 V and lower. © 2002 The Electrochemical Society. All rights reserved.

Spiral-wave meandering in reaction–diffusion models of ventricular muscle

Reentrant arrhythmias in cardiac tissue can be idealised as spiral-wave solutions of reaction–diffusion equations of excitable media. The dynamical behaviour of such solutions depends on the properties of the excitable medium. Here we characterise the effects of model-parameter changes for ionic conductances and concentrations on the meandering patterns and dynamics of spiral waves in a model of mammalian ventricular tissue. A two-dimensional reaction–diffusion system with kinetics described by the Oxsoft ® Heart equations for the single guinea-pig ventricular cell was used. We examine the development and stability of reentrant spiral-wave solutions as well as the effects of ionic conductance and concentration changes on spiral-wave meandering. Spiral-wave meandering behaviour provides a significant validation test for detailed biophysical models.

Changes in Molecular Dynamics during the Bulk Polymerization of an Epoxide/Diamine Mixture Containing Inert Diluents as Studied Using Dielectric Relaxation Spectroscopy

The evolution of the dielectric α relaxation and ionic conductivity with time tr during polymerization at 60 °C of mixtures of the diepoxide DGEBA, the diamine PACM, and the diluent Decalin (DEC) or di-n-butyl phthalate (DBP) has been studied using real-time dielectric relaxation spectroscopy (DRS). Data are presented as permittivity ε(ω,tr) and impedance Z(ω,tr). For cdil ≤ 20% (w/w), the main effect of diluent was to displace the α process to higher frequencies and, therefore, to increase the time required for glass formation over that for the unplasticized system. At higher values of cdil, the behavior of ε(ω,tr) for DGEBA/PACM/DBP systems was more complex as the properties became stationary at long times, showing that an elastomer rather than a glass was formed. The results are discussed in relation to the “floor temperature” TF for reaction introduced previously. The same data are presented as Z(ω,tr), which emphasizes changes in the ac ionic conductivity with time. As for the ε data, qualitative differences are observed for the behavior of Z(ω,tr) at low and high values of cdil. It is demonstrated that changes in molecular mobility with time for all systems can be judged by the behavior of ε(ω,tr) but not by that of Z(ω,tr) for these systems. The Fuoss−Kirkwood and KWW relaxation functions are used to express the half-width Δ log t of the plot of ε‘ ‘ vs log tr at fixed measuring frequencies for a reactive system. The changes in Δ log t with f and composition for the systems that became glasses are explained in terms of changes with time of (i) the width of the relaxation function and (ii) [Δ log <τα>/Δ log tr], where <τα> is the average relaxation time. The behavior of <τα(tr)> with time during reaction is considered in terms of the Angell fragility index m of the process. It is shown that (i) m increases during reaction for a given mixture and (ii) m, at fixed frequency, decreases as cdil is increased.

A Lithium-induced Conformational Change in Serotonin Transporter Alters Cocaine Binding, Ion Conductance, and Reactivity of Cys-109

Inactivation of serotonin transporter (SERT) expressed in HeLa cells by [2-(trimethylammonium)ethyl]methanethiosulfonate (MTSET) occurred much more readily when Na(+) in the reaction medium was replaced with Li(+). This did not result from a protective effect of Na(+) but rather from a Li(+)-specific increase in the reactivity of Cys-109 in the first external loop of the transporter. Li(+) alone of the alkali cations caused this increase in reactivity. Replacing Na(+) with N-methyl-d-glucamine (NMDG(+)) did not reduce the affinity of cocaine for SERT, as measured by displacement of a high affinity cocaine analog, but replacement of Na(+) with Li(+) led to a 2-fold increase in the K(D) for cocaine. The addition of either cocaine or serotonin (5-HT) protected SERT against MTSET inactivation. When SERT was expressed in Xenopus oocytes, inward currents were elicited by superfusing the cell with 5-HT (in the presence of Na(+)) or by replacing Na(+) with Li(+) but not NMDG(+). MTSET treatment of oocytes in Li(+) but not in Na(+) decreased both 5-HT and Li(+) induced currents, although 5-HT-induced currents were inhibited to a greater extent. Na(+) antagonized the effects of Li(+) on both inactivation and current. These results are consistent with Li(+) inducing a conformational change that exposes Cys-109, decreases cocaine affinity, and increases the uncoupled inward current.

Effect of lithium and sodium valproate ions on resting membrane potentials in neurons: an hypothesis

In an attempt to understand the therapeutic effects of lithium and sodium valproate in stabilizing the moods in manic depressive illness, the well-known Goldman–Hodgkin–Katz (G–H–K) equation is modified to include a fourth ion, such as a lithium ion or a sodium ion. The modified G–H–K equation is used to calculate the resting membrane potential in neurons. These calculations show that the resting membrane potential is depolarized depending upon the relative concentration of the lithium ion and upon its relative permeability. These calculations suggest that the resting membrane potential may be hyperpolarized in bipolar patients before treatment, and that the lithium ion perhaps depolarizes the resting membrane potential back to the normal level. They further support the prevailing hypothesis that manic-depressive illness may be caused by the hyperpolarization of the resting membrane potential, which, in turn, may be caused by the changes in ionic conductance (permeability) of the membranes. Sodium ions in sodium valproate do not significantly affect the resting membrane potential since they do not significantly change in the serum.

Molecular Basis of the Delayed Rectifier Current IKsin Heart

J. Kurokawa, H. Abriel and R. S. Kass. Molecular Basis of the Delayed Rectifier Current IKsin Heart. Journal of Molecular and Cellular Cardiology (2001) 33, 873–882. Electrical activity underlies the control of the frequency, strength, and duration of contraction of the heart. During the cardiac cycle, a regular rhythmic pattern must be established in time-dependent changes in ionic conductances in order to ensure events that underlie normal cardiac function. This pattern must be tightly regulated by sympathetic nervous activity to ensure a physiologically relevant relationship between diastolic filling and ejection times with variable heart rate. The duration of the ventricular action potential is controlled in part by a slowly activated potassium channel current, IKs. The molecular identity of the subunits that comprise the channels conducting this current is important, not only for understanding the fundamental mechanisms that control electrical activity in healthy individuals, but also for understanding the molecular basis of at least one inherited human disease, LQTS-1. This brief review summarizes key points of information regarding the molecular determinants of the activity of these channels, their relationship to human disease, and what is known, and yet to be discovered, about the molecular determinants of the regulation of this channel by sympathetic nervous activity.

Micellization parameters and electrostatic interactions in micellar solution of sodium dodecyl sulfate (SDS) at different temperatures

The effect of concentration on electrical conductance of SDS solution from 293.15–313.15 K above and below the CMC was studied. The micellization parameters, i.e. degrees of counterion binding ( β) and aggregation numbers ( n) were determined at different temperatures. The mass action model (MAM) was applied to calculate other micellization parameters like number of counterions per micelle ( m) and micellization constant ( K c), and was also used to obtain concentration changes of counterions [Na +], surfactant ions [DS −] and micelles [Mn] with respect to total surfactant concentration. The ionic conductance changes of the counterions λ Na + , surfactant ions λ DS − and micelles ( λ Mn) with respect to total ionic strength were evaluated by using Debye–Huckel–Onsager equations. The ionic conductivity and CMC of ionic micellar solutions and consequently all micellization parameters are influenced by temperature. The parameters like m, n and β decline, whereas ionic concentrations [Na +], [DS −] and [Mn] and ionic conductances λ Na + , λ DS − and ( λ Mn) increase with rise in temperature.