An Extracellular Ion Pathway Plays a Central Role in the Cooperative Gating of a K2P K+ Channel by Extracellular pH

Proton-gated TASK-3 K⁺ channel belongs to the K_2P family of proteins that underlie the K⁺ leak setting the membrane potential in all cells. TASK-3 is under cooperative gating control by extracellular [H⁺]. Use of recently solved K_2P structures allows us to explore the molecular mechanism of TASK-3 cooperative pH gating. Tunnel-like side portals define an extracellular ion pathway to the selectivity filter. We use a combination of molecular modeling and functional assays to show that pH-sensing histidine residues and K⁺ ions mutually interact electrostatically in the confines of the extracellular ion pathway. K⁺ ions modulate the pK_a of sensing histidine side chains whose charge states in turn determine the open/closed transition of the channel pore. Cooperativity, and therefore steep dependence of TASK-3 K⁺ channel activity on extracellular pH, is dependent on an effect of the permeant ion on the channel pH_o sensors.     

细胞内离子在气孔运动中的作用

气孔运动与植物水分代谢密切相关。保卫细胞中的无机离子作为第二信使(Ca²⁺)或者渗透调节物质(K⁺、Cl^–)在响应 外界理化因子的刺激、调节保卫细胞膨压过程中发挥重要作用。保卫细胞质膜和液泡膜上的离子通道作为各种刺激因素作 用的靶位点, 是保卫细胞离子转运的关键组分, 在气孔运动调控过程中扮演关键角色。该文对近年来保卫细胞离子的作用 和离子通道研究的进展进行了综述。     

Activity and Stoichiometry of Na+:HCO− 3 Cotransport in Immortalized Renal Proximal Tubule Cells

The proximal tubule Na⁺-HCO⁻ ₃ cotransporter is located in the basolateral plasma membrane and moves Na⁺, HCO⁻ ₃, and net negative charge together out of the cell. The presence of charge transport implies that at least two HCO⁻ ₃ anions are transported for each Na⁺ cation. The actual ratio is of physiological interest because it determines direction of net transport at a given membrane potential. To determine this ratio, a thermodynamic approach was employed that depends on measuring charge flux through the cotransporter under defined ion and electrical gradients across the basolateral plasma membrane. Cells from an immortalized rat proximal tubule line were grown as confluent monolayer on porous substrate and their luminal plasma membrane was permeabilized with amphotericin B. The electrical properties of these monolayers were measured in a Ussing chamber, and ion flux through the cotransporter was achieved by applying Na⁺ or HCO⁻ ₃ concentration gradients across the basolateral plasma membrane. Charge flux through the cotransporter was identified as difference current due to the reversible inhibitor dinitro-stilbene disulfonate. The cotransporter activity was Cl⁻ independent; its conductance ranged between 0.12 and 0.23 mS/cm² and was voltage independent between −60 and +40 mV. Reversal potentials obtained from current-voltage relations in the presence of Na⁺ gradients were fitted to the thermodynamic equivalent of the Nernst equation for coupled ion transport. The fit yielded a cotransport ratio of 3HCO⁻ ₃:1Na⁺. Received: 19 January 1996/Revised: 24 April 1996     

Laser spectroscopy measurements of the effective temperature of argon ions in the PNX-U plasma neutralizer

Laser spectroscopy measurements of the effective temperature of Ar¹⁺ ions in the PNX-U multipole trap, in which argon plasma is ionized and heated by microwaves under electron-cyclotron-resonance conditions, are performed using a narrow-band tunable dye laser. The absorption profile of the 611.5-nm line is examined. In a microwave power range of 5–50 kW, the observed behavior of the effective temperature of argon ions T_{i, eff} indicates an anomalous mechanism for ion heating. It is shown that certain information about the electron temperature can be deduced from measurements by the laser-induced fluorescence (LIF) technique. The measurements performed also serve to test the laser technique and apparatus that is presently being developed for diagnosing additives to the ITER divertor plasma by the LIF method.     

RF discharge-based plasma emitter

An injector of hydrogen atoms for plasma diagnostics in modern tokamaks has been developed at the Budker Institute of Nuclear Physics (Novosibirsk). The ion source of the injector produces a proton (helium ion) beam with a current of up to 2 A (1 A), an ion energy of up to 55 keV, a beam divergence of ～0.6\deg, and a pulse duration of up to 10 s. An RF discharge-based plasma emitter, which is one of the main parts of the ion source, is described. The emitter diameter is 72 mm, the ion current density is 120 mA/cm², and the inhomogeneity of the current density is ±6%. The beam is formed by a four-electrode ionoptical system with 163 round apertures. At a current of 2 A, the ion beam consists of 67% protons, 18% H ₂ ⁺ ions, and 15% H ₃ ⁺ ions, the total content of heavier ions in the beam being no higher than 2–3%.     

Ion temperature profiles along a hydrogen diagnostic beam in a TORE SUPRA tokamak plasma

The active particle diagnostic technique is used to study the ion temperature at five spatial points along the path of a hydrogen diagnostic beam. The temperature of the main ion plasma component (deuterium ions) measured by this diagnostic technique along the beam path is compared with the temperature of carbon ions (C⁺⁵). A study is made of the following characteristic features of the behavior of the ion temperature profiles T_i in various TORE SUPRA operating modes: the formation of flat and even hollow T_i profiles in ohmic discharges with q ～3 at the plasma edge, the change in T_i profiles in ergodic divertor discharges, and the difference between the temperature of the bulk ions measured by the active particle diagnostic technique and the temperature of C⁺⁵ ions in the plasma region r/a>0.5. The features revealed are explained at a qualitative level.     

Graded and All-or-None Electrogenesis in Arthropod Muscle : II. The effects of alkali-earth and onium ions on lobster muscle fibers

Conversion of graded responsiveness of lobster muscle fibers to all-or-none activity by alkali-earth and tetraethylammonium (TEA) ions appears to be due to a combination of effects. The membrane is hyperpolarized, its resistance is increased, and its sensitivity to external K⁺ is diminished, all effects which indicate diminished K⁺ conductance. While the spikes are prolonged, the conductance is higher throughout the response than it is in the resting membrane. Repetitive activity becomes prominent. These effects indicate maintained high conductance for an ion which causes depolarization. This is normally Na⁺, since its presence in low concentrations potentiates the effects of Ba⁺⁺, but the alkali-earth ions and TEA can also carry inward charge. Ba⁺⁺, Sr⁺⁺, and TEA appear to be more effective than is Ca⁺⁺ in its normal role, which is probably to depress K⁺ conductance and Na inactivation. Thus, conversion of graded to all-or-none responsiveness appears to occur because of the relative increase of depolarizing inward ion flux and decrease of repolarizing outward flux.     

Allosteric Effects of Permeating Cations on Gating Currents during K+ Channel Deactivation

K⁺ channel gating currents are usually measured in the absence of permeating ions, when a common feature of channel closing is a rising phase of off-gating current and slow subsequent decay. Current models of gating invoke a concerted rearrangement of subunits just before the open state to explain this very slow charge return from opening potentials. We have measured gating currents from the voltage-gated K⁺ channel, Kv1.5, highly overexpressed in human embryonic kidney cells. In the presence of permeating K⁺ or Cs⁺, we show, by comparison with data obtained in the absence of permeant ions, that there is a rapid return of charge after depolarizations. Measurement of off-gating currents on repolarization before and after K⁺ dialysis from cells allowed a comparison of off-gating current amplitudes and time course in the same cells. Parallel experiments utilizing the low permeability of Cs⁺ through Kv1.5 revealed similar rapid charge return during measurements of off-gating currents at E_Cs. Such effects could not be reproduced in a nonconducting mutant (W472F) of Kv1.5, in which, by definition, ion permeation was macroscopically absent. This preservation of a fast kinetic structure of off-gating currents on return from potentials at which channels open suggests an allosteric modulation by permeant cations. This may arise from a direct action on a slow step late in the activation pathway, or via a retardation in the rate of C-type inactivation. The activation energy barrier for K⁺ channel closing is reduced, which may be important during repetitive action potential spiking where ion channels characteristically undergo continuous cyclical activation and deactivation.     

Plasma decay in high-voltage nanosecond discharges in oxygen-containing mixtures

Plasma decay in high-voltage nanosecond discharges in CO₂: O₂ and Ar: O₂ mixtures at room gas temperature and a pressure of 10 Torr is studied experimentally and theoretically. The time dependence of the electron density during plasma decay is measured using microwave interferometry. The time evolution of the charged particle density, ion composition, and electron temperature is simulated numerically. It is shown that, under the given conditions, the discharge plasma is dominated for the most time by O ₂ ⁺ ions and plasma decay is determined by dissociative and three-body electron?ion recombination. As in the previous studies performed for air and oxygen plasmas, agreement between measurements and calculations is achieved only under the assumption that the rate of three-body recombination of molecular ions is much greater than that for atomic ions. The values of the rate constant of three-body recombination of electrons with О₂ ⁺ ions in a wide range of electron temperatures (500–5500 K), as well as for thermal (300 K) electrons, are obtained by processing the experimental results.     

Active ion uptake and maintenance of cation-anion balance: A critical examination of their role in regulating rhizosphere pH

The processes responsible for maintenance of cation-anion balance in plants and their relation to active ion accumulation and changes in rhizosphere pH are outlined and discussed. The major processes involved are: (1) accumulation and degradation of organic acids which occur in the plant mainly as organic acid anions (and their transfer within the plant) and (2) extrusion of H⁺ or OH^– into the rhizosphere. The relative importance of the two processes is determined by the size of the excess anion or cation uptake. Indeed, plants typically absorb unequal quantities of nutritive cations (NH₄ ⁺+Ca²⁺+ Mg²⁺+K⁺+Na⁺) and anions (NO₃ ^–+Cl^–+SO₄ ^2–+H₂PO₄ ^–) and charge balance is maintained by excretion of an amount of H⁺ or OH^– which is stoichiometrically equal to the respective excess cation or anion uptake. The mechanisms and processes by which H⁺ and in particular OH^– ions are excreted in response to unequal cation-anion uptake are, however, poorly understood.The contemporary view is that primary active extrusion of H⁺, catalyzed by a membrane-located ATPase, is the major driving force for secondary transport of cations and anions across the plasma membrane. However, the fact that net OH^– extrusion often occurs (since excess anion absorption commonly takes place) implies there is a yet-to-be characterized OH^– ion efflux mechanism at the plasma membrane that is associated with anion uptake. There is, therefore, a need for future studies of the uptake mechanisms and stoichiometry of anion uptake; particularly that of NO₃ ^– which is often the predominant anion absorbed. Another related phenonenon which requires detailed study in terms of cation-anion balance is localized rhizosphere acidification which can occur in response to deficiencies of Fe and P.     

Relationships Between Na+ Distribution,Concerted Migration,and Diffusion Properties in Rhombohedral NASICON

Rhombohedral NaZr₂(PO₄)₃ is the prototype of all the NASICON‐type materials. The ionic diffusion in these rhombohedral NASICON materials is highly influenced by the ionic migration channels and the bottlenecks in the channels which have been extensively studied. However, no consensus is reached as to which one is the preferential ionic migration channel. Moreover, the relationships between the Na⁺ distribution over the multiple available sites, concerted migration, and diffusion properties remain elusive. Using ab initio molecular dynamics simulations, here it is shown that the Na⁺ ions tend to migrate through the Na1–Na3–Na2–Na3–Na1 channels rather than through the Na2–Na3–Na3–Na2 channels. There are two types of concerted migration mechanisms: two Na⁺ ions located at the adjacent Na1 and Na2 sites can migrate either in the same direction or at an angle. Both mechanisms exhibit relatively low migration barriers owing to the potential energy conversion during the Na⁺ ions migration process. Redistribution of Na⁺ ions from the most stable Na1 sites to Na2 on increasing Na⁺ total content further facilitates the concerted migration and promotes the Na⁺ ion mobility. The work establishes a connection between the Na⁺ concentration in rhombohedral NASICON materials and their diffusion properties.     

Mechanism of Na+ Insertion in Alkali Vanadates and Its Influence on Battery Performance

Sodium‐ion batteries may become an alternative to the widespread lithium‐ion technology due to cost and kinetic advantages provided that cyclability is improved. For this purpose, the interplay between electrochemical and structural processes is key and is demonstrated in this work for Na_2.46V₆O₁₆ (NVO) and Li_2.55V₆O₁₆ employing operando synchrotron X‐ray diffraction. When NVO is cycled between 4.0 and 1.6 V, Na‐ions reversibly occupy two crystallographic sites, which results in remarkable cyclability. Upon discharge to 1.0 V, however, Na‐ions occupy also interstitial sites, inducing irreversible structural change with some loss of crystallinity concomitant with a decrease in capacity. Capacity fading increases with the ionic radius of the alkali ions (K⁺ > Na⁺ > Li⁺), suggesting that smaller ions stabilize the structure. This correlation of structural variation and electrochemical performance suggests a route toward improving cycling stability of a sodium‐ion battery. Its essence is a minor Li⁺‐retention in the A_2+xV₆O₁₆ structure. Even though the majority of Li‐ions are replaced by the abundant Na⁺, the residual Li‐ions (≈10%) are sufficient to stabilize the layered structure, diminishing the irreversible structural damage. These results pave the way for further exploitation of the role of small ions in lattice stabilization that increases cycling performance.     

Kinetics of Energetic O<Superscript>−</Superscript> Ions in the Discharge Plasmas of Water Vapor and H<Subscript>2</Subscript>O-Containing Mixtures

The process of relaxation of energetic O^– ions formed via dissociative attachment of electrons to molecules in the discharge plasmas of water vapor and H₂O: O₂ mixtures in a strong electric field is studied by the Monte Carlo method. The probability of energetic ions being involved in threshold ion–molecular processes is calculated. It is shown that several percent of energetic O^– ions formed via electron attachment to H₂O molecules in the course of plasma thermalization transform into OH^– ions via charge exchange or are destroyed with the formation of free electrons. The probabilities of charge exchange of O^– ions and electron detachment from them increase significantly (up to 90%) when O^– ions are formed via electron attachment to O₂ molecules in water vapor with an oxygen additive. This effect decreases with increasing oxygen fraction in the mixture but remains appreciable even when the fraction of H₂O molecules in the H₂O: O₂ mixture does not exceed several percent.     

Identification of the First Sodium Binding Site of the Phosphate Cotransporter NaPi-IIa (SLC34A1)

Transporters of the SLC34 family (NaPi-IIa,b,c) catalyze uptake of inorganic phosphate (P_i) in renal and intestinal epithelia. The transport cycle requires three Na⁺ ions and one divalent P_i to bind before a conformational change enables translocation, intracellular release of the substrates, and reorientation of the empty carrier. The electrogenic interaction of the first Na⁺ ion with NaPi-IIa/b at a postulated Na1 site is accompanied by charge displacement, and Na1 occupancy subsequently facilitates binding of a second Na⁺ ion at Na2. The voltage dependence of cotransport and presteady-state charge displacements (in the absence of a complete transport cycle) are directly related to the molecular architecture of the Na1 site. The fact that Li⁺ ions substitute for Na⁺ at Na1, but not at the other sites (Na2 and Na3), provides an additional tool for investigating Na1 site-specific events. We recently proposed a three-dimensional model of human SLC34a1 (NaPi-IIa) including the binding sites Na2, Na3, and P_i based on the crystal structure of the dicarboxylate transporter VcINDY. Here, we propose nine residues in transmembrane helices (TM2, TM3, and TM5) that potentially contribute to Na1. To verify their roles experimentally, we made single alanine substitutions in the human NaPi-IIa isoform and investigated the kinetic properties of the mutants by voltage clamp and ³²P uptake. Substitutions at five positions in TM2 and one in TM5 resulted in relatively small changes in the substrate apparent affinities, yet at several of these positions, we observed significant hyperpolarizing shifts in the voltage dependence. Importantly, the ability of Li⁺ ions to substitute for Na⁺ ions was increased compared with the wild-type. Based on these findings, we adjusted the regions containing Na1 and Na3, resulting in a refined NaPi-IIa model in which five positions (T200, Q206, D209, N227, and S447) contribute directly to cation coordination at Na1.     

Ion Trapping with Fast-Response Ion-Selective Microelectrodes Enhances Detection of Extracellular Ion Channel Gradients

Previously, functional mapping of channels has been achieved by measuring the passage of net charge and of specific ions with electrophysiological and intracellular fluorescence imaging techniques. However, functional mapping of ion channels using extracellular ion-selective microelectrodes has distinct advantages over the former methods. We have developed this method through measurement of extracellular K⁺ gradients caused by efflux through Ca²⁺-activated K⁺ channels expressed in Chinese hamster ovary cells. We report that electrodes constructed with short columns of a mechanically stable K⁺-selective liquid membrane respond quickly and measure changes in local [K⁺] consistent with a diffusion model. When used in close proximity to the plasma membrane (<4 μm), the ISMs pose a barrier to simple diffusion, creating an ion trap. The ion trap amplifies the local change in [K⁺] without dramatically changing the rise or fall time of the [K⁺] profile. Measurement of extracellular K⁺ gradients from activated rSlo channels shows that rapid events, 10-55 ms, can be characterized. This method provides a noninvasive means for functional mapping of channel location and density as well as for characterizing the properties of ion channels in the plasma membrane.     

Influence of the trapped ions on the screening effect and frictional force in a dusty plasma

The problem of screening of the charge acquired by a dust grain in a two-temperature plasma is considered. The influence of the trapped ions on the screening effect and on the frictional force exerted on a dust grain by an ion flow is investigated. It is shown that the ions trapped by a grain radically reduce the frictional force in the ion flow because their distribution is determined by the temperature of the cold buffer gas. The mechanism for the onset of the reactive force that accelerates the grain in the direction opposite to that of the flow is explained. It is based on the momentum transfer from the flow of the ions that are additionally accelerated in the grain field to the atoms of the buffer gas. As a result, the momentum carried by the charge-exchange atoms out of the “ions + grain” system exceeds the momentum they have carried into the system; this gives rise to a reactive force directed opposite to the ion flow (the negative frictional force). The magnitude of the reactive force is estimated.     

Unveiling the Unique Phase Transformation Behavior and Sodiation Kinetics of 1D van der Waals Sb2S3 Anodes for Sodium Ion Batteries

Carbon‐coated van der Waals stacked Sb₂S₃ nanorods (SSNR/C) are synthesized by facile hydrothermal growth as anodes for sodium ion batteries (SIBs). The sodiation kinetics and phase evolution behavior of the SSNR/C anode during the first and subsequent cycles are unraveled by coupling in situ transmission electron microscopy analysis with first‐principles calculations. During the first sodiation process, Na⁺ ions intercalate into the Sb₂S₃ crystals with an ultrafast speed of 146 nm s^?1. The resulting amorphous Na_x Sb₂S₃ intermediate phases undergo sequential conversion and alloying reactions to form crystalline Na₂S, Na₃Sb, and minor metallic Sb. Upon desodiation, Na⁺ ions extract from the nanocrystalline phases to leave behind the fully desodiated Sb₂S₃ in an amorphous state. Such unique phase evolution behavior gives rise to superb electrochemical performance and leads to an unexpectedly small volume expansion of ≈54%. The first‐principles calculations reveal distinctive phase evolution arising from the synergy between the extremely low Na⁺ ion diffusion barrier of 190 meV and the sharply increased electronic conductivity upon the formation of amorphous Na_x Sb₂S₃ intermediate phases. These findings highlight an anomalous Na⁺ ion storage mechanism and shed new light on the development of high performance SIB anodes based on van der Waals crystals.     

Examining the contributions of lipid shape and headgroup charge on bilayer behavior

To better understand bilayer property dependency on lipid electrostatics and headgroup size, we use atomistic molecular dynamics simulations to study negatively charged and neutral lipid membranes. We compare the negatively charged phosphatidic acid (PA), which at physiological pH and salt concentration has a negative spontaneous curvature, with the negatively charged phosphatidylglycerol (PG) and neutrally charged phosphatidylcholine (PC), both of which have zero spontaneous curvature. The PA lipids are simulated using two different sets of partial charges for the headgroup and the varied charge distribution between the two PA systems results in significantly different locations for the Na⁺ ions relative to the water/membrane interface. For one PA system, the Na⁺ ions are localized around the phosphate group. In the second PA system, the Na⁺ ions are located near the ester carbonyl atoms, which coincides with the preferred location site for the PG Na⁺ ions. We find that the Na⁺ ion location has a larger effect on bilayer fluidity properties than lipid headgroup size, where the A_lipid and acyl chain order parameter values are more similar between the PA and PG bilayers that have Na⁺ ions located near the ester groups than between the two PA bilayers.     

Differential Ion Stimulation of Plasmalemma Adenosine Triphosphatase from Leaf Epidermis and Mesophyll of Nicotiana rustica L

An ATPase preparation, presumed to be associated with plasma membrane due to the coincidence on isopycnic gradients of cellulase and β-glucan synthetase at high substrate, has been isolated from the epidermal and mesophyll of tobacco leaf. The ATPase from both tissues was found to prefer ATP over other nucleotides. The pH optimum was 7.0 in the presence of 3 millimolar MgCl₂ and pH 6.5 in the presence of 3 millimolar MgCl₂ and 100 millimolar KCl. Monovalent ion stimulation patterns of the ATPases from these tissues were found to differ and ion accumulation patterns in these tissues reflect this difference: mesophyll accumulated roughly equal amounts of Na⁺ and Rb⁺ and its plasma membrane ATPase is also equally stimulated by these ions; on the other hand, epidermal ATPase preparations showed a stronger stimulation by Rb⁺ than Na⁺ and this tissue was found to accumulate Rb⁺ in preference to Na⁺. Abscisic acid and fusicoccin affected both ATPase activity and ion uptake, the former inhibiting and the latter stimulating these parameters. These data support the hypothesis that the epidermal plasmalemma ATPase is involved in stomatal opening.     

Ionic processes underlying the decrease in osmotic potential of Chara L-cell fragments in dilute electrolyte solutions

Movements of ions are considered to be governed by the electroneutrality rule. Therefore, a cation moving across the cell membrane into the cell either passively or actively should move together with its counterion, an anion, in equal amounts of charge or in exchange for another cation inside the cell. This means that the net influx of the cation in question should be affected by the permeability of its counterion and/or another cation inside the cell. To examine osmotic and ionic regulation in Chara cells, cell fragments of Chara having a lower osmotic pressure than normal (L-cell fragments) were prepared. The L-cell fragments were individually put into various dilute electrolyte solutions and their osmotic potentials were measured with a turgor balance. Concentrations of K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^?, NO^?₃. and SO^2?₄. in the external electrolyte solutions in which L-cells had been incubated were also analysed by ion chromatography. The results showed that in 0.5 mM KCL + 0.1 mM CaCl₂ solution, Chara L-cell fragments absorbed K⁺ and Cl^? to maintain electroneutrality and then regained their osmotic potential very rapidly. When the anion was Cl, the cation absorbed at the highest rate was K⁺ On the other hand, when the cation was K, the anion absorbed at the highest rate was Cl, Other ions Ca²⁺, SO^2?₄ and NO^?₃ showed much less permeability than K⁺ and Cl ^?for the Chara plasma membrane. The conclusion from these findings was that due to the constraint of electroneutral transport, the uptake rate of a salt into L-cells is limited by the permeability of the least permeable ion.