Ion selectivity of carriers and channels

Dynamics of ionic flows via Poisson-Nernst-Planck systems with local hard-sphere potentials: Competition between cations

We study a quasi-one-dimensional steady-state Poisson-Nernst-Planck type model for ionic flows through a membrane channel with three ion species, two positively charged with the same valence and one negatively charged. Bikerman's local hard-sphere potential is included in the model to account for ion sizes. The problem is treated as a boundary value problem of a singularly perturbed differential system. Under the framework of a geometric singular perturbation theory, together with specific structures of this concrete model, the existence and uniqueness of solutions to the boundary value problem for small ion sizes is established. Furthermore, treating the ion sizes as small parameters, we derive an approximation of individual fluxes, from which one can further study the qualitative properties of ionic flows and extract concrete information directly related to biological measurements. Of particular interest is the competition between two cations due to the nonlinear interplay between finite ion sizes, diffusion coefficients and boundary conditions, which is closely related to selectivity phenomena of open ion channels with given protein structures. Furthermore, we are able to characterize the distinct effects of the nonlinear interplays between these physical parameters. Numerical simulations are performed to identify some critical potentials which play critical roles in examining properties of ionic flows in our analysis.

Phenanthroline-Based Polyarylate Porous Membranes with Rapid Water Transport for Metal Cation Separation

The selective separation of ions in terms of extremely similar size and properties remains an important challenge in water purification. We innovated a kind of porous nanofilm via interfacial polymerization using rigid heterocyclic ligands to achieve high valent cation selectivity and rapid water/ion transport. The interconnected microporosity and uniformly distributed cation-affinitive sites of the ultrathin membranes enabled water permeation (7.5 L m^-2 h^-1 bar^-1), ion permeance of Na⁺ (1.5 mol m^-2 h^-1 bar^-1), and Mg²⁺/Na⁺ permselectivity (2.1) during nanofiltration. The forward osmosis exhibited a prominent water flux of 95 LMH at 1 M NaCl draw solution, which expanded various applications. The polyarylate membranes comprising 4,7-diphenyl-1,10-phenanthroline showed a higher water permeation and ion selectivity than the planar monomers, e.g., resorcinol. A distinct fluorescence responsiveness existed between membranes and cations for the interaction characterization. Host-guest nuclear magnetic resonance (NMR) spectroscopy and solid-state nuclear magnetic resonance spectroscopy characterized the preferential affinitive of divalent/high-valent cations in the interconnected microporous powders; an ultraviolet spectrophotometer characterized the light responsiveness of the porous nanofilms. Such an active membrane has potential applications in selective separation and adsorption of cations, photocatalytic materials, photosensors, and other fields.

Cs + ADC in rat brain decreases markedly at death

Spectroscopic resolution of intracellular and extracellular compartments can be used to probe the kinetic environment of those spaces and the compartment-specific changes that occur following injury. This is important for understanding the biophysical mechanisms that underlie the remarkable diffusion-weighted MRI contrast of injured central nervous system (CNS) tissue. Cesium-133 is a physiologic analog of potassium that is actively taken up by cells and resides primarily in the intracellular space. The (133)Cs(+) signal can, thus, be exploited to probe the kinetic environment of the intracellular space. Two principal (133)Cs(+) resonances were observed at 11.74 T. These resonances arise separately from (133)Cs(+) in brain and temporalis muscle. The apparent diffusion coefficient (ADC) of Cs(+) in brain decreased from 1.0 +/- 0.2 microm(2)/ms in healthy tissue to 0.24 +/- 0.04 microm(2)/ms following global ischemia (average ADC +/- average uncertainty), while there was no significant change in the ADC of Cs(+) in temporalis muscle after injury. This finding underscores the tissue-specific nature of the decrease in ADC that accompanies brain injury. Further, as the Cs(+) ADC should reflect water ADC in the intracellular space, these results strongly support the hypothesis that the decrease in water ADC associated with CNS injury arises largely from kinetic changes taking place in the intracellular space.

Steric selectivity in Na channels arising from protein polarization and mobile side chains

Monte Carlo simulations of equilibrium selectivity of Na channels with a DEKA locus are performed over a range of radius R and protein dielectric coefficient epsilon(p). Selectivity arises from the balance of electrostatic forces and steric repulsion by excluded volume of ions and side chains of the channel protein in the highly concentrated and charged (approximately 30 M) selectivity filter resembling an ionic liquid. Ions and structural side chains are described as mobile charged hard spheres that assume positions of minimal free energy. Water is a dielectric continuum. Size selectivity (ratio of Na+ occupancy to K+ occupancy) and charge selectivity (Na+ to Ca2+) are computed in concentrations as low as 10(-5) M Ca2+. In general, small R reduces ion occupancy and favors Na+ over K+ because of steric repulsion. Small epsilon(p) increases occupancy and favors Na+ over Ca2+ because protein polarization amplifies the pore's net charge. Size selectivity depends on R and is independent of epsilon(p); charge selectivity depends on both R and epsilon(p). Thus, small R and epsilon(p) make an efficient Na channel that excludes K+ and Ca2+ while maximizing Na+ occupancy. Selectivity properties depend on interactions that cannot be described by qualitative or verbal models or by quantitative models with a fixed free energy landscape.

NAD(P)H fluorescence imaging of postsynaptic neuronal activation in murine hippocampal slices

We examined mechanisms contributing to stimulus-evoked changes in NAD(P)H fluorescence as a marker of neuronal activation in area CA1 of murine hippocampal slices. Three types of stimuli (electrical, glutamate iontophoresis, bath-applied kainate) produced biphasic fluorescence changes composed of an initial transient decrease ("initial component," 1-3%), followed by a longer-lasting transient increase ("overshoot," 3-8%). These responses were matched by inverted biphasic flavin adenine dinucleotide (FAD) fluorescence transients, suggesting that these transients reflect mitochondrial function rather than optical artifacts. Both components of NAD(P)H transients were abolished by ionotropic glutamate receptor block, implicating postsynaptic neuronal activation as the primary event involved in generating the signals, and not presynaptic activity or reuptake of synaptically released glutamate. Spatial analysis of the evoked signals indicated that the peak of each component could arise in different locations in the slice, suggesting that there is not always obligatory coupling between the two components. The initial NAD(P)H response showed a strong temporal correspondence to intracellular Ca+ increases and mitochondrial depolarization. However, despite the fact that removal of extracellular Ca2+ abolished neuronal cytosolic Ca2+ transients to exogenous glutamate or kainate, this procedure did not reduce slice NAD(P)H responses evoked by either of these agonists, implying that mechanisms other than neuronal mitochondrial Ca2+ loading underlie slice NAD(P)H transients. These data show that, in contrast to previous proposals, slice NAD(P)H transients in mature slices do not reflect neuronal Ca2+ dynamics and demonstrate that these signals are sensitive indicators of both the spatial and temporal characteristics of postsynaptic neuronal activation in these preparations.

NAD(P)H fluorescence imaging of postsynaptic neuronal activation in murine hippocampal slices

We examined mechanisms contributing to stimulus-evoked changes in NAD(P)H fluorescence as a marker of neuronal activation in area CA1 of murine hippocampal slices. Three types of stimuli (electrical, glutamate iontophoresis, bath-applied kainate) produced biphasic fluorescence changes composed of an initial transient decrease ("initial component," 1-3%), followed by a longer-lasting transient increase ("overshoot," 3-8%). These responses were matched by inverted biphasic flavin adenine dinucleotide (FAD) fluorescence transients, suggesting that these transients reflect mitochondrial function rather than optical artifacts. Both components of NAD(P)H transients were abolished by ionotropic glutamate receptor block, implicating postsynaptic neuronal activation as the primary event involved in generating the signals, and not presynaptic activity or reuptake of synaptically released glutamate. Spatial analysis of the evoked signals indicated that the peak of each component could arise in different locations in the slice, suggesting that there is not always obligatory coupling between the two components. The initial NAD(P)H response showed a strong temporal correspondence to intracellular Ca+ increases and mitochondrial depolarization. However, despite the fact that removal of extracellular Ca2+ abolished neuronal cytosolic Ca2+ transients to exogenous glutamate or kainate, this procedure did not reduce slice NAD(P)H responses evoked by either of these agonists, implying that mechanisms other than neuronal mitochondrial Ca2+ loading underlie slice NAD(P)H transients. These data show that, in contrast to previous proposals, slice NAD(P)H transients in mature slices do not reflect neuronal Ca2+ dynamics and demonstrate that these signals are sensitive indicators of both the spatial and temporal characteristics of postsynaptic neuronal activation in these preparations.

Cesium uptake studies on human erythrocytes

The uptake of Cs+ ions into the erythrocytes of abstemious volunteers and of alcoholic patients was followed using 133Cs NMR. The uptake rates are approximately linear with a rate of 0.33 mM.h-1 at an extracellular Cs+ concentration of 10 mM. There is no discernible difference in the uptake rate between the two classes of subject despite earlier reports that Cs+ distribution is different and consequently that Cs+ transport might be anomalous in alcoholics. There is no evidence of saturation of the input rate and Cs(+)-loaded cells retain their Cs+ when incubated in a Cs(+)-free buffer, strongly suggesting that there is no transport mechanism for the removal of Cs+ from the erythrocyte. Experiments designed to ascertain which intracellular ion is being replaced by Cs+ indicate that it predominantly displaces K+.

Modification of cesium toxicity by calcium in mammalian system

The interaction between cesium chloride CsCl and calcium chloride CaCl2 was observed in bone marrow chromosomes of mice. The two salts were administered orally to laboratory bred Swiss albino mice in vivo singly or one followed by the other, or both simultaneously. CsCl induced chromosomal aberrations in frequencies directly proportional to the dose administered. The frequency of aberrations was reduced significantly when the two chemicals were administered simultaneously or when CaCl2 was given 2 h before CsCl. Thus, CaCl2 is able to protect against the cytotoxicity of CsCl.

Microscopic model for selective permeation in ion channels

Ionic permeation in the selectivity filter of ion channels is analyzed by a microscopic model based on molecular kinetic theory. The energy and flux equations are derived by assuming that: (a) the selectivity filter is formed by a symmetrical array of carbonyl groups; (b) ion movement is near the axis of the channel; (c) a fraction of water molecules is separated from the ion while it moves across the selectivity filter; (d) the applied voltage drops linearly across the selectivity filter; (e) ions move independently. Energy profiles, single channel conductances, and the degree of hydration of K+ in a hypothetical K+ channel are examined by varying the following microscopic parameters: ion radius and mass, channel radius, number of effective water dipoles, and number of carbonyl groups. The i-V curve is linear up to +/- 170 mV. If the positions of energy maxima and minima are fixed, this linear range is reduced to +/- 50 mV. Channel radius and ion-water interactions are found to be two major channel structural determinants for selectivity sequences. Both radius and mass of an ion are important in selectivity mediated by these interactions. The theory predicts a total of 15 possible kinetic selectivity sequences for alkali cations in ion channels with a single selectivity filter.

Ion permeation through single channels activated by acetylcholine in denervated toad sartorius skeletal muscle fibers: effects of alkali cations

The gigaohm seal technique was used to study ion permeation through acetylcholine-activated channels in cell-attached patches of the extrajunctional membrane of chronically denervated, enzyme-treated cells from the sartorius muscle of the toad Bufo marinus. The most frequently occurring channel type (greater than 95% of channel openings), provisionally classified as 'extrajunctional,' had a chord conductance of approximately 25 pS under normal conditions (-70 mV, 11 degrees C, Normal Toad Ringer's). The less frequently observed channel type (less than 5% of channel openings), classified as a 'junctional' type, had a conductance of 35 pS under the same conditions, and a similar null potential. In many patches, a small percentage (usually less than 2%) of openings of the extrajunctional channel displayed a lower conductance state. The shape of the I-V curves obtained for the extrajunctional channel depended on the predominant extracellular cation. For Cs and K, the I-V curves were essentially linear over the voltage range +50 to -150 mV across the patch, suggesting that the potential independent component of the energy profile within the channel was symmetrical. For Li, the I-V curve was very nonlinear, displaying a significant sublinearity at hyperpolarized potentials. Both an electrodiffusion and a symmetrical uniform four-barrier, three-site rate-theory model provided reasonable fits to the data, whereas symmetrical two-barrier, single-site rate-theory models did not. For the alkali cations examined, the relative permeability sequence was PCs greater than PK greater than PNa greater than PLi--a "proportional" selectivity sequence. This was different from the single channel conductance sequence which was found to be gamma K greater than gamma Cs greater than gamma Na greater than gamma Li implying that ions do not move independently through the channel. The relative binding constant sequence for the channel sites was found to be a "polarizability" sequence, i.e., KLi greater than KCs greater than KNa greater than KK. There was an inverse relationship between the relative binding constant and the relative mobility for the cations examined. Under conditions when the single-channel conductance was relatively high, the conductance at depolarized potentials was lower than that predicted by both electrodiffusion and rate theory models, suggesting that there was a rate-limiting access step for ions, from the intracellular compartment into the channel.

Ion movement through gramicidin A channels. Studies on the diffusion-controlled association step

The permeability characteristics of gramicidin A channels are generally considered to reflect accurately the intrinsic properties of the channels themselves; i.e., the aqueous convergence regions are assumed to be negligible barriers for ion movement through the channels. The validity of this assumption has been examined by an analysis of gramicidin A single-channel current-voltage characteristics up to very high potentials (500 mV). At low permeant ion concentrations the currents approach a voltage-independent limiting value, whose magnitude is proportional to the permeant ion concentration. The magnitude of this current is decreased by experimental maneuvers that decrease the aqueous diffusion coefficient of the ions. It is concluded that the magnitude of this limiting current is determined by the diffusive ion movement through the aqueous convergence regions up to the channel entrance. It is further shown that the small-signal (ohmic) permeability properties also reflect the existence of the aqueous diffusion limitation. These results have considerable consequences for the construction of kinetic models for ion movement through gramicidin A channels. It is shown that the simple two-site-three-barrier model commonly used to interpret gramicidin A permeability data may lead to erroneous conclusions, as biionic potentials will be concentration dependent even when the channel is occupied by at most one ion. The aqueous diffusion limitation must be considered explicitly in the analysis of gramicidin A permeability characteristics. Some implications for understanding the properties of ion-conducting channels in biological membranes will be considered.

Ion selectivity of the nerve membrane sodium channel incorporated into liposomes

Tetrodotoxin-sensitive sodium channels of lobster nerve membranes were incorporated into soybean liposomes by the freeze-thaw-sonication procedure and their ionic selectivity was studied. Veratridine and grayanotoxin-I were used to activate the sodium channels and the increment of the ionic flux through them was specifically abolished by tetrodotoxin. The drug-sensitive 22Na+, 42K+, 86Rb+ and 137Cs+ influxes were measured. The permeability ratios calculated directly from ion fluxes showed that the channels preferably allow the passage of Na+. No anion influx ([32P]phosphate, [35S]sulfate, 36Cl) sensitive to the drugs was observed. The data reveal that the sodium channels incorporated into liposomes remain cation-selective and discriminate among different cations.

Ion selectivity of the cation transport system of isolated intact cattle rod outer segments: evidence for a direct communication between the rod plasma membrane and the rod disk membranes

The ion selectivity of cation transport through the plasma membrane of isolated intact cattle rod outer segments (rods) is investigated by means of 45Ca-exchange experiments and light-scattering experiments. These techniques appear to provide complementary information: the 45Ca experiments (45Ca fluxes in rods) describe electroneutral antiport, whereas the light-scattering experiments (shrinkage and swelling of rods upon hypertonic shocks with various electrolytes) reveal electrogenic uniport. Electroneutral symport of ions (salt transport) does not take place without addition of external ionophores and application of salts of weak acids. 1. Intact rods recover from a hypertonic shock in the presence of FCCP when lithium, sodium and potassium acetate are applied, but not when ammonium chloride, calcium and magnesium acetate are used. This indicates that the plasma membrane of isolated intact cattle rods is relatively permeable to net transport of Na+, Li+ and K+, and relatively impermeable to net transport of Cl-, Mg2+ and Ca2+ under conditions that do not give rise to diffusion potentials. 2. Rapid (t1/2 < 1 min) efflux of 45Ca from preloaded intact rods is observed when Na+, Ca2+, Sr2+, and under certain conditions also Ba2+, are added to the external medium. Li+, K+, Rb+, Cs+, Mg2+ and Mn2+ are ineffective in this respect as well as protons at pH 7.4. It is concluded that 45Ca efflux reflects electroneutral exchange diffusion of internal 45Ca with external Na+, Ca2+, Sr2+ and Ba2+, respectively. 3. All tested cations lower the rate of 45Ca uptake. The latter can be described by a single rate constant indicating a homogeneous rod preparation and a homogeneous endogenous Ca2+ pool. However, only those cations which stimulate 45Ca efflux from preloaded rods lower the final equilibrium of 45Ca uptake. Except for the effects of K+, Rb+ and Cs+ the reduction of the rate of 45Ca uptake by external cations appears to arise from competition for a common site on the plasms membrane. The observed affinities for this site do not correlate with actual transport (as indicated by the ability to stimulate 45Ca efflux). 4. K+ increases the affinity of the exchange diffusion system to Ca2+ from 1 microM to 0.15 microM and changes the relative affinities with respect to Ca2+ for the other cations (Na+, Mg2+, Mn2+, Sr2+, Ba2+). Furthermore, the maximal rate of Ba-Ca exchange is strongly stimulated by K+, whereas the maximal rate of Ca-Ca exchange is reduced at saturating Ca2+ concentrations. 5. The exchange diffusion transport mode can be turned off by external Na+ in a process that is not of a stochastic nature, which implies interdependence of individual transport entities and which results in an inhomogeneity of the endogenous Ca2+ pool. K+ acts as antagonist of Na+ in this effect. The revelence of these findings is discussed in relation to the generally accepted view, that a diffusable transmitter in the rod cytosol communicates the photochemical event in the disk membrane to the electrical properties of the plasma membrane...

Lithium efflux through the Na/K pump in human erythrocytes

ACTIVE LI EFFLUX FROM HUMAN ERYTHROCYTES WAS SHOWN TO BE MEDIATED BY THE NA/K PUMP: (i) intracellular Li (Li(c)) activated ouabain-sensitive K influx, and (ii) a portion of the Li efflux required external K and was inhibited by ouabain. In activating K influx, Li(c) interacts with the pump like Na rather than like K-depleting the cells of orthophosphate inhibited activation of K influx by intracellular K (K/K exchange) but did not inhibit Li-activated K influx. (To show these interactions of Li(c) with the Na/K pump, p-chloromercuribenzenesulfonate or nystatin was used to allow replacement of intracellular Na and K with Li and choline.) From kinetic studies of the pump, it was shown that the apparent affinity of the intracellular aspect of the Na/K pump for Li was an order of magnitude less than that for Na. From simultaneous measurements of ouabain-sensitive net fluxes of Li and K in Na-free cells, it was shown that the pump-mediated K influx and Li efflux were coupled. The stoichiometry of the coupling ratio was close to 1:1 for Li:K, different from the coupling ratio of 3:2 for Na:K in the pump's normal mode of operation. It had been shown previously that the Na/K pump in human erythrocytes mediates active Li influx. Because it also mediates active Li efflux, the molecular mechanisms for distinguishing between Na and K must be qualitatively different at the internal and external aspects of the pump. The possible relevance of the results of this study to manic depressive illness and Li therapy is discussed.

Permeability of the endplate membrane activated by acetylcholine to some organic cations

The ability of various organic cations to depolarize the ACh-activated endplate membrane in the absence of Na ions was examined on frog sartorius muscle by measuring the endplate potential on the muscle surface with the moving electrode technique. The ACh-activated endplate membrane was very permeable to ammonium and its methyl and hydroxy derivatives, and moderately permeable to guanidine derivatives and Tris (hydroxymethyl) aminomethane. The permeability of alkylol derivatives of ammonium diminished progressively with increase in molecular size. The present results suggested that the endplate ionic channels can be represented by a pore of about 6.4 A in diameter.