Effects of double-layer polarization on ion transport

Ultrasound-assisted electromembrane extraction of clonazepam from plasma and determination using capillary electrophoresis

In this work, ultrasound-assisted electromembrane extraction (UA-EME) coupled with capillary electrophoresis (CE) and diode array detection (DAD) was developed for the determination of clonazepam from plasma samples. A comparative study was carried out between conventional EME and UA-EME methods to investigate the influence of the ultrasound waves on the extraction efficiency. The central composite design was used for the optimization of the variables affecting these methods to achieve the best extraction efficiency. Under optimal extraction conditions, the UA-EME provided better extraction recovery in a shorter time (58% in 13 min) than the EME method (42% in 30 min). Ultrasound reduces the extraction time and increased recovery by reducing the thickness of the barrier layer. In addition, this method provided a higher pre-concentration factor (203) and a lower limit of detection (3 ng mL^-1) with good repeatability (RSDs were less than 10.11%).

Validating a Coarse-Grained Voltage Activation Model by Comparing Its Performance to the Results of Monte Carlo Simulations

Simulating the nature of voltage-activated systems is a problem of major current interest, ranging from the action of voltage-gated ion channels to energy storage batteries. However, fully microscopic converging molecular simulations of external voltage effects present a major challenge, and macroscopic models are associated with major uncertainties about the dielectric treatment and the underlying physical basis. Recently we developed a coarse-grained (CG) model that represents explicitly the electrodes, the electrolytes, and the membrane/protein system. The CG model provides a semimacroscopic way of capturing the microscopic physics of voltage-activated systems. Our method was originally validated by reproducing macroscopic and analytical results for key test cases and then used in modeling voltage-activated ion channels and related problems. In this work, we further establish the reliability of the CG voltage model by comparing it to the results of Monte Carlo (MC) simulations with a microscopic electrolyte model. The comparison explores different aspects of membrane, electrolyte, and electrode systems ranging from the Gouy-Chapman model to the determination of the electrolyte charge distribution in the solution between two electrodes (without and with a separating membrane), as well as the evaluation of gating charges. Overall the agreement is very impressive. This provides confidence in the CG model and also shows that the MC model can be used in realistic simulation of voltage activation of membrane proteins with sufficient computer time.

THE ROLE OF MEMBRANE ELECTROSTATICS IN THE REGULATION OF CELL VOLUME AND ION CONCENTRATIONS

We present a model to study how membrane surface negative charges can affect the electro-osmotic regulation properties of a cell. This model is based on the cellular analog proposed by Jakobsson, which includes passive and active ion transports; we further introduce the effect of membrane surface charges, using a generalized formulation of the Gouy–Chapman theory. We derive a system of nonlinear differential-algebraic equations (DAEs) which describes the dynamics of the cellular analog. The system admits a unique asymptotically stable stationary state, in which the Na-pump rate, which is crucial for electro-osmotic regulation, is inversely related to the Ca2+level in the extracellular milieu; numerical integration shows that this apparent inhibition of the Na-pump by external Ca2+results from a decrease in the electrostatic field produced by surface charges at the external side of the membrane. Furthermore, the degree of stability of the stationary state dramatically depends on the amount of negative charges on the membrane; a maximal stability is obtained for densities around - e /500 Å2, where the Na-pump is maximally activated by an increase in the Na content of the cytoplasm.

Coarse grained model for exploring voltage dependent ion channels

The relationship between the membrane voltage and the gating of voltage activated ion channels and other systems have been a problem of great current interest. Unfortunately, reliable molecular simulations of external voltage effects present a major challenge, since meaningful converging microscopic simulations are not yet available and macroscopic treatments involve major uncertainties in terms of the dielectric used and other key features. This work extends our coarse grained (CG) model to simulations of membrane/protein systems under external potential. Special attention is devoted to a consistent modeling of the effect of external potential due to the electrodes, emphasizing semimacroscopic description of the electrolytes in the solution regions between the membranes and the electrodes, as well as the coupling between the combined potential from the electrodes plus the electrolytes and the protein ionized groups. We also provide a clear connection to microscopic treatment of the electrolytes and thus can explore possible conceptual problems that are hard to resolve by other current approaches. For example, we obtain a clear description of the charge distribution in the entire electrolyte system, including near the electrodes in membrane/electrodes systems (where continuum models do not seem to provide the relevant results). Furthermore, the present treatment provides an insight on the distribution of the electrolyte charges before and after equilibration across the membrane, and thus on the nature of the gating charge. The different aspects of the model have been carefully validated by considering problems ranging for the simple Debye-Huckel, and the Gouy-Chapman models to the evaluation of the electrolyte distribution between two electrodes, as well as the effect of extending the simulation system by periodic replicas. Overall the clear connection to microscopic descriptions combined with the power of the CG modeling seems to offer a powerful tool for exploring the balance between the protein conformational energy and the interaction with the external potential in voltage activated channels. To illustrate these features we present a preliminary study of the gating charge in the voltage activated Kv1.2 channel, using the actual change in the electrolyte charge distribution rather than the conventional macroscopic estimate. We also discuss other special features of the model, which include the ability to capture the effect of changes in the protonation states of the protein residues during the close to open voltage induced transition. This article is part of a Special Issue entitled: Membrane protein structure and function.

The mechanism of ion conduction by valinomycin: analysis of charge pulse responses

Even though valinomycin has been employed and studied extensively for over 30 years, the attempts to explain its mechanism have not been entirely successful. The basic carrier model uses four rate constants that describe association of an ion and carrier, transfer of the complex across the membrane, dissociation of the complex, and transfer of the free carrier back across the membrane. If the basic model is correct all of these constants are independent of ion concentration. In previous work with rubidium the rate constants for transfer of free carrier, transfer of complexes, and dissociation were independent of the concentration, but the rate constant for association varied markedly. No satisfactory explanation for these observations was proposed. In this study current relaxations after charge pulses have been analyzed using digital data acquisition, a Bayesian algorithm, and inspection of linear plots of residuals. In agreement with previous results the relaxations for sufficiently high rubidium or potassium concentrations contain three exponential components, but the rate constants for association and dissociation decrease to similar extents as ion concentration increases. A simple extension of the carrier model to allow a more realistic description of association and dissociation is in good agreement with the rate constants fitted in the present study but not those for low ion concentrations found in previous work. At high ion concentrations the rate-limiting step in association appears to be a change in the conformation of the free carrier preceding the bimolecular association reaction. Transfer of neutral, free valinomycin between the surfaces is slower than the transfer of the charged ion-valinomycin complexes. Transfer of the complex may be hastened by deformation of the membrane, or transfer of the free carrier may be slowed by a need for conformation changes.

Absence of effects of low-frequency, low-amplitude magnetic fields on the properties of gramicidin A channels

The effects of static and low-frequency magnetic fields on gramicidin A channels have been investigated using bilayer patch clamp recording and a bridge technique capable of detecting 0.3% changes in the conductance of glyceryl monooleate membranes containing many channels. In the bridge technique the conductance was assessed using 10-ms voltage pulses applied at 10 Hz. Measurements were made for LiCl, KCl, and CsCl using magnetic fields of 50, 100, 500, and 5000 microT with the frequency scanned from 10-200 Hz. The combinations of static and low-frequency fields employed include the "cyclotron resonance" conditions at which effects had been predicted to occur. In no case was there any detectable change in conductance when the magnetic fields were applied or changed. Potassium currents through single gramicidin channels have been recorded for patches in which several channels may be open at once. Fields were applied for 2 min periods interleaved with 2 min controls. Methods have been developed to analyze the multichannel records to reveal the amplitude and duration of the channels together with the frequency, depth, and apparent period of flickers. No significant differences were observed between the control and field-exposed recording periods. The peak of the distribution of opening and closing transitions always coincided for fields on and off within the resolution, 0.4%, of the recordings. There are at least two types of flicker, one with typical period less than 0.1 ms, the other with typical period from 0.3-0.8 ms. Most of the latter were not complete closures with the conductance during a flicker 15-20% above the level for a full closure.

Diffusion around a cardiac calcium channel and the role of surface bound calcium

The diffusion of Ca as it converges to the external mouth of a Ca channel is examined. Diffusional limitation on Ca ions entering Ca channels during current flow, cause local extracellular Ca depletions. Such extracellular Ca depletions have been reported in cardiac muscle. The cardiac sarcolemma has a large number of low-affinity Ca binding sites that can buffer these local Ca depletions. For a hemisphere of extracellular space (of radius less than 0.33 microns) centered on the external mouth of a Ca channel the amount of Ca bound at the membrane surface exceeds that which is free within the associated hemisphere. The ratio of bound Ca/free Ca increases as r decreases, such that the [Ca] nearest the Ca channel is the most strongly buffered by sarcolemmal bound Ca. It is demonstrated that Ca ions coming from these sarcolemmal Ca binding sites contribute quantitatively to the integrated Ca current. The electric field generated by the local depletion of Ca near the channel mouth has little impact on the extent of Ca depletion, but if an additional electric field exists at the mouth of the channel, Ca depletion can be significantly altered. Other low-affinity Ca binding sites in the interstitium may also contribute to the buffering of extracellular Ca. The complex geometry of the extracellular space in cardiac muscle (e.g., transverse tubules and restrictions of extracellular space between cells) increases both the predicted Ca depletions (in the absence of binding) and the bound/free ratio. Thus, the impact of this surface Ca binding is greatly increased. By considering arrays of Ca channels in transverse tubules or in parallel planes (e.g., membranes of neighboring cells), extracellular Ca depletions are predicted which agree with those measured experimentally. Membrane Ca binding may also be expected to buffer increases in [Ca] around the inner mouth of Ca channels. It is demonstrated that in the absence of other intracellular systems most of the Ca entering the cell via Ca channels might be expected to be bound to the inner sarcolemmal surface. It is concluded that surface Ca binding may have a substantial impact on the processes of extracellular Ca depletion (and intracellular Ca accumulation).

Excimer fluorescence of pyrenyliodoacetamide-labeled tropomyosin: a probe of the state of tropomyosin in reconstituted muscle thin filaments

Rabbit skeletal tropomyosin (Tm) specifically labeled at cysteine groups with N-(1-pyrenyl)-iodoacetamide (PIA) exhibits excimer fluorescence. The excimer fluorescence was sensitive to the local conformation of Tm, to actin binding, and, in reconstituted thin filaments, to the Tm state change induced by binding of myosin subfragment 1 (S1). The properties of PIATm were similar to previously studied pyrenylmaleimide-labeled Tm (PMTm) [Ishii, Y., & Lehrer, S.S. (1985) Biochemistry 24, 6631] except that S1 binding to actin-Tm increased the excimer fluorescence in contrast to the time-dependent decrease seen for PMTm. The fluorescence properties of PIATm are sensitive to the Tm chain-chain interaction via equilibria among pyrene configurations and nonfluorescent dimer as well as the monomer and excimer-forming configurations. The effect of bound troponin (Tn) on the excimer fluorescence of PIATm in the reconstituted systems was dependent on ionic strength with a slight Ca2+ dependence. S1 titrations in the absence and presence of Tn and Ca2+ indicated that the excimer fluorescence probes the state change of Tm from the weak S1 binding state to the strong S1 binding state which is facilitated by Ca2+ [Hill et al. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 3186]. Binding of MgADP-S1 and MgAMPPNP-S1 produced the same total excimer fluorescence change as for nucleotide-free S1, showing that the strong S1 binding state of Tm-actin is independent of nucleotide.(ABSTRACT TRUNCATED AT 250 WORDS)

Voltage-dependent pore activity of the peptide alamethicin correlated with incorporation in the membrane: salt and cholesterol effects

Strong aggregation of incorporated alamethicin in the bilayer of lipid vesicles has been observed spectroscopically at aqueous peptide concentrations above a critical value c*. On the other hand, in conventional gating studies with planar lipid films, the onset of conducting pore formation can be characterized by a threshold voltage V.. We present experimental evidence of a direct correspondence between the effects on c* and V. when these parameters are modulated by adding NaCl (to the aqueous medium) or cholesterol (to the lipid moiety). A quantitative analysis supports the idea that the measured aggregation actually results in pore formation, the voltage-dependence being due to an electric field effect on the partition equilibrium of the peptide between the aqueous and the lipid phases.

Electrodiffusion of ions approaching the mouth of a conducting membrane channel

The movement of ions in the aqueous medium as they approach the mouth (radius a) of a conducting membrane channel is analyzed. Starting with the Nernst-Planck and Poisson equations, we derive a nonlinear integrodifferential equation for the electric potential, phi(r), a less than or equal to r less than infinity. The formulation allows deviations from charge neutrality and dependence of phi(r) on ion flux. A numerical solution is obtained by converting the equation to an integral equation that is solved by an iterative method for an assumed mouth potential, combined with a shooting method to adjust the mouth potential until the numerical solution agrees with an asymptotic expansion of the potential at r-a much greater than lambda (lambda = Debye length). Approximate analytic solutions are obtained by assuming charge neutrality (Läuger, 1976) and by linearizing. The linear approximation agrees with the exact solution under most physiological conditions, but the charge-neutrality solution is only valid for r much greater than lambda and thus cannot be used unless a much greater than lambda. Families of curves of ion flux vs. potential drop across the electrolyte, phi(infinity)-phi (a), and of permeant ion density at the channel mouth, n1(a), vs. flux are obtained for different values of a/lambda and S = a d phi/dr(a). If a much greater than lambda and S = O, the maximum flux (which is approached when n1(a)----0) is reduced by 50% compared to the value predicted by the charge-neutrality solution. Access resistance is shown to be a factor a/[2 (a + lambda)] times the published formula (Hille, 1968), which was derived without including deviations from charge neutrality and ion density gradients and hence does not apply when there is no counter-ion current. The results are applied to an idealized diffusion-limited channel with symmetric electrolytes. For S = O, the current/voltage curves saturate at a value dependent on a/lambda; for S greater than O, they increase linearly for large voltage.

Electrostatic radius of the gramicidin channel determined from voltage dependence of H+ ion conductance

The results of Decker and Levitt (1987) suggest that the conductance of H+ ion through the gramicidin channel is limited primarily by diffusion in the bulk solution at the channel mouth. It is assumed in this paper that the H+ conductance is 100% diffusion limited. This means that all the factors that influence the H+ flux are external to the channel and are presumed to be known. In particular, the diffusion coefficient of H+ in this region is assumed to be equal to the bulk solution value and the only force acting on the ion is that due to the applied voltage. A model of the H+ flux is derived, based on the Nernst-Planck equation. It has three adjustable parameters: the electrostatic radius, the capture distance, and the radius of the H+ ion. The acceptable range of the parameters was determined by comparing the predictions of the model with the experimental measurements of the H+ conductance at pH 3.75. The best fit was obtained for an electrostatic radius in the range 2.3-2.7 A. This is in good agreement with earlier predictions (2.5 A) based on the assumption that the dielectric constant of the channel water is equal to that of bulk water. The addition of 1 M choline Cl- (an impermeant) increases the H+ current at low voltage and decreases it at high voltage. The increase can be explained by the small surface charge that results from the separation of charge produced by exclusion of the large choline cation (relative to Cl-) from the membrane surface. The decrease at high voltages can be accounted for by the change in the profile of the applied potential produced by the increase in ionic strength.

Gramicidin-mediated currents at very low permeant ion concentrations

Current-voltage relations have been measured for the fluxes of caesium ions through pores formed by gramicidin in lipid bilayer membranes. The ionic currents have been separated from capacitative currents using a bridge circuit with an integrator as null-detector. The conductances during brief voltage pulses were small enough to avoid the effects of diffusion polarization and the ionic strength was raised using choline chloride or magnesium sulfate to reduce the effects of double-layer polarization. Under these conditions the current-voltage relations have the same shape at 0.1 and 1 mM, but different shapes for higher concentrations. These data demonstrate that the fluxes do not obey independence for concentrations above 10 mM, but they cannot be used in isolation to support a particular value of the binding constant. The shape observed at low concentrations suggests that entry of ions into the pore remains weakly potential dependent even at 300 mV.