Ion-Induced Defect Permeation of Lipid Membranes

We have explored the mechanisms of uncatalyzed membrane ion permeation using atomistic simulations and electrophysiological recordings. The solubility-diffusion mechanism of membrane charge transport has prevailed since the 1960s, despite inconsistencies in experimental observations and its lack of consideration for the flexible response of lipid bilayers. We show that direct lipid bilayer translocation of alkali metal cations, Cl^–, and a charged arginine side chain analog occurs via an ion-induced defect mechanism. Contrary to some previous suggestions, the arginine analog experiences a large free-energy barrier, very similar to those for Na⁺, K⁺, and Cl^–. Our simulations reveal that membrane perturbations, due to the movement of an ion, are central for explaining the permeation process, leading to both free-energy and diffusion-coefficient profiles that show little dependence on ion chemistry and charge, despite wide-ranging hydration energies and the membrane’s dipole potential. The results yield membrane permeabilities that are in semiquantitative agreement with experiments in terms of both magnitude and selectivity. We conclude that ion-induced defect-mediated permeation may compete with transient pores as the dominant mechanism of uncatalyzed ion permeation, providing new understanding for the actions of a range of membrane-active peptides and proteins.     

The Split Personality of Glutamate Transporters: A Chloride Channel and a Transporter

Transporters and ion channels are conventionally categorised into distinct classes of membrane proteins. However, some membrane proteins have a split personality and can function as both transporters and ion channels. The excitatory amino acid transporters (EAATs) in particular, function as both glutamate transporters and chloride (Cl^?) channels. The EAATs couple the transport of glutamate to the co-transport of three Na⁺ ions and one H⁺ ion into the cell, and the counter-transport of one K⁺ ion out of the cell. The EAAT Cl^? channel is activated by the binding of glutamate and Na⁺, but is thermodynamically uncoupled from glutamate transport and involves molecular determinants distinct from those responsible for glutamate transport. Several crystal structures of an EAAT archaeal homologue, Glt_Ph, at different stages of the transport cycle, alongside numerous functional studies and molecular dynamics simulations, have provided extensive insights into the mechanism of substrate transport via these transporters. However, the molecular determinants involved in Cl^? permeation, and the mechanism by which this channel is activated are not entirely understood. Here we will discuss what is currently known about the molecular determinants involved in EAAT-mediated Cl^? permeation and the mechanisms that underlie their split personality.     

Optical Waveguide Lightmode Spectroscopic Techniques for Investigating Membrane-Bound Ion Channel Activities

Optical waveguide lightmode spectroscopic (OWLS) techniques were probed for monitoring ion permeation through channels incorporated into artificial lipid environment. A novel sensor set-up was developed by depositing liposomes or cell-derived membrane fragments onto hydrophilic polytetrafluoroethylene (PTFE) membrane. The fibrous material of PTFE membrane could entrap lipoid vesicles and the water-filled pores provided environment for the hydrophilic domains of lipid-embedded proteins. The sensor surface was kept clean from the lipid holder PTFE membrane by a water- and ion-permeable polyethylene terephthalate (PET) mesh. The sensor set-up was tested with egg yolk lecithin liposomes containing gramicidin ion channels and with cell-derived membrane fragments enriched in GABA-gated anion channels. The method allowed monitoring the move of Na⁺ and organic cations through gramicidin channels and detecting the Cl^–-channel functions of the (α₅β₂γ₂) GABA_A receptor in the presence or absence of GABA and the competitive GABA-blocker bicuculline.     

Multiscale Simulations Reveal Key Aspects of the Proton Transport Mechanism in the ClC-ec1 Antiporter

Multiscale reactive molecular dynamics simulations are used to study proton transport through the central region of ClC-ec1, a widely studied ClC transporter that enables the stoichiometric exchange of 2 Cl^– ions for 1 proton (H⁺). It has long been known that both Cl^– and proton transport occur through partially congruent pathways, and that their exchange is strictly coupled. However, the nature of this coupling and the mechanism of antiporting remain topics of debate. Here multiscale simulations have been used to characterize proton transport between E203 (Glu_in) and E148 (Glu_ex), the internal and external intermediate proton binding sites, respectively. Free energy profiles are presented, explicitly accounting for the binding of Cl^– along the central pathway, the dynamically coupled hydration changes of the central region, and conformational changes of Glu_in and Glu_ex. We find that proton transport between Glu_in and Glu_ex is possible in both the presence and absence of Cl^– in the central binding site, although it is facilitated by the anion presence. These results support the notion that the requisite coupling between Cl^– and proton transport occurs elsewhere (e.g., during proton uptake or release). In addition, proton transport is explored in the E203K mutant, which maintains proton permeation despite the substitution of a basic residue for Glu_in. This collection of calculations provides for the first time, to our knowledge, a detailed picture of the proton transport mechanism in the central region of ClC-ec1 at a molecular level.     

Cryo‐EM structure of the human NKCC1 transporter reveals mechanisms of ion coupling and specificity

The sodium–potassium–chloride transporter NKCC1 of the SLC12 family performs Na⁺‐dependent Cl⁻‐ and K⁺‐ion uptake across plasma membranes. NKCC1 is important for regulating cell volume, hearing, blood pressure, and regulation of hyperpolarizing GABAergic and glycinergic signaling in the central nervous system. Here, we present a 2.6 Å resolution cryo‐electron microscopy structure of human NKCC1 in the substrate‐loaded (Na⁺, K⁺, and 2 Cl⁻) and occluded, inward‐facing state that has also been observed for the SLC6‐type transporters MhsT and LeuT. Cl⁻ binding at the Cl1 site together with the nearby K⁺ ion provides a crucial bridge between the LeuT‐fold scaffold and bundle domains. Cl⁻‐ion binding at the Cl2 site seems to undertake a structural role similar to conserved glutamate of SLC6 transporters and may allow for Cl⁻‐sensitive regulation of transport. Supported by functional studies in mammalian cells and computational simulations, we describe a putative Na⁺ release pathway along transmembrane helix 5 coupled to the Cl2 site. The results provide insight into the structure–function relationship of NKCC1 with broader implications for other SLC12 family members.     

Non-Equilibrium Dynamics Contribute to Ion Selectivity in the KcsA Channel

The ability of biological ion channels to conduct selected ions across cell membranes is critical for the survival of both animal and bacterial cells. Numerous investigations of ion selectivity have been conducted over more than 50 years, yet the mechanisms whereby the channels select certain ions and reject others are not well understood. Here we report a new application of Jarzynski’s Equality to investigate the mechanism of ion selectivity using non-equilibrium molecular dynamics simulations of Na⁺ and K⁺ ions moving through the KcsA channel. The simulations show that the selectivity filter of KcsA adapts and responds to the presence of the ions with structural rearrangements that are different for Na⁺ and K⁺. These structural rearrangements facilitate entry of K⁺ ions into the selectivity filter and permeation through the channel, and rejection of Na⁺ ions. A mechanistic model of ion selectivity by this channel based on the results of the simulations relates the structural rearrangement of the selectivity filter to the differential dehydration of ions and multiple-ion occupancy and describes a mechanism to efficiently select and conduct K⁺. Estimates of the K⁺/Na⁺ selectivity ratio and steady state ion conductance for KcsA from the simulations are in good quantitative agreement with experimental measurements. This model also accurately describes experimental observations of channel block by cytoplasmic Na⁺ ions, the “punch through” relief of channel block by cytoplasmic positive voltages, and is consistent with the knock-on mechanism of ion permeation.     

Direct measurement of k channels in thylakoid membranes by incorporation of vesicles into planar lipid bilayers

Light-driven electron transfer reactions cause the active accumulation of protons inside thylakoids, yet at steady state the electrical potential difference across the thylakoid membrane is very small; therefore, there must be a flux of other ions to balance the charge that would otherwise be built up by the net movement of H⁺. This paper presents direct measurements of ion movements through channels in the thylakoid membrane. These were made possible by fusing thylakoid vesicles from spinach (Spinacia oleracea L.) into planar lipid bilayers, using techniques developed originally to study sarcoplasmic reticulum. No Mg²⁺ current was found, but voltage-dependent channels have been characterized, these being somewhat selective for K⁺ over Cl⁻. The data are consistent with a role for these channels in charge balance during light-driven H⁺ movements.     

Atomistic Detailed Mechanism and Weak Cation-Conducting Activity of HIV-1 Vpu Revealed by Free Energy Calculations

The viral protein U (Vpu) encoded by HIV-1 has been shown to assist in the detachment of virion particles from infected cells. Vpu forms cation-specific ion channels in host cells, and has been proposed as a potential drug target. An understanding of the mechanism of ion transport through Vpu is desirable, but remains limited because of the unavailability of an experimental structure of the channel. Using a structure of the pentameric form of Vpu – modeled and validated based on available experimental data – umbrella sampling molecular dynamics simulations (cumulative simulation time of more than 0.4 µs) were employed to elucidate the energetics and the molecular mechanism of ion transport in Vpu. Free energy profiles corresponding to the permeation of Na⁺ and K⁺ were found to be similar to each other indicating lack of ion selection, consistent with previous experimental studies. The Ser23 residue is shown to enhance ion transport via two mechanisms: creating a weak binding site, and increasing the effective hydrophilic length of the channel, both of which have previously been hypothesized in experiments. A two-dimensional free energy landscape has been computed to model multiple ion permeation, based on which a mechanism for ion conduction is proposed. It is shown that only one ion can pass through the channel at a time. This, along with a stretch of hydrophobic residues in the transmembrane domain of Vpu, explains the slow kinetics of ion conduction. The results are consistent with previous conductance studies that showed Vpu to be a weakly conducting ion channel.     

Mechanism of ion transport through lipid bilayer-membranes mediated by peptide cyclo-(d-Val-l-Pro-l-Val-d-Pro)3

The cyclic dodecapeptide PV, cyclo-(d-Val-l-Pro-l-Val-d-Pro)₃, a structural analogue of the ion-carier valinomycin, increase the cation permeability of lipid bilayer membranes. This paper reports the results of two types of relaxation experiments, namely relaxation of the membrane current after a voltage jump and decay of the membrane voltage after a charge pulse in lipid bilayer membranes exposed to PV. From the relaxation data, the rate constant for the translocation of the ion carrier complex across the membrane, as well as the partition coefficient of the complex between water and membrane solution interface were computed and found to be about one order of magnitude less than the comparable values for valinomycin (Val). Furthermore, the dependence of the initial membrane conductivity on ion concentration was used to evaluate the equilibrium constant, K, of complexation between PV and some monovalent cations in water. The values of K yield the following selectivity sequence of PV: Na⁺ < NH₄⁺ < K⁺ < Cs⁺ < Rb⁺. These and earlier results are consistent with the idea that PV promotes cation movement across membranes by the solution complexation mechanism which involves complexation between ion and carrier in the aqueous phase and transport of the carrier across the membrane. In the particular form of the solution complexation mechanism operating here, the PV present in the PV-cation complex carrying charge across the membrane derives from the side from which the current is flowing (cis-mechanism). As shown previously, valinomycin, in contrast to PV, acts by an interfacial complexation mechanism in which the Val in the Val-cation complex derives from the side toward which current is flowing (trans-mechanims). The comparison of the kinetic properties of these two closely related compounds yields interesting insights into the relationship between chemical structure and function of ion carriers.     

Electrostatic free energy landscapes for nucleic acid helix assembly

Metal ions are crucial for nucleic acid folding. From the free energy landscapes, we investigate the detailed mechanism for ion-induced collapse for a paradigm system: loop-tethered short DNA helices. We find that Na⁺ and Mg²⁺ play distinctive roles in helix–helix assembly. High [Na⁺] (>0.3 M) causes a reduced helix–helix electrostatic repulsion and a subsequent disordered packing of helices. In contrast, Mg²⁺ of concentration >1 mM is predicted to induce helix–helix attraction and results in a more compact and ordered helix–helix packing. Mg²⁺ is much more efficient in causing nucleic acid compaction. In addition, the free energy landscape shows that the tethering loops between the helices also play a significant role. A flexible loop, such as a neutral loop or a polynucleotide loop in high salt concentration, enhances the close approach of the helices in order to gain the loop entropy. On the other hand, a rigid loop, such as a polynucleotide loop in low salt concentration, tends to de-compact the helices. Therefore, a polynucleotide loop significantly enhances the sharpness of the ion-induced compaction transition. Moreover, we find that a larger number of helices in the system or a smaller radius of the divalent ions can cause a more abrupt compaction transition and a more compact state at high ion concentration, and the ion size effect becomes more pronounced as the number of helices is increased.     

A model of lysosomal pH regulation

Lysosomes must maintain an acidic luminal pH to activate hydrolytic enzymes and degrade internalized macromolecules. Acidification requires the vacuolar-type H⁺-ATPase to pump protons into the lumen and a counterion flux to neutralize the membrane potential created by proton accumulation. Early experiments suggested that the counterion was chloride, and more recently a pathway consistent with the ClC-7 Cl^–/H⁺ antiporter was identified. However, reports that the steady-state luminal pH is unaffected in ClC-7 knockout mice raise questions regarding the identity of the carrier and the counterion. Here, we measure the current–voltage characteristics of a mammalian ClC-7 antiporter, and we use its transport properties, together with other key ion regulating elements, to construct a mathematical model of lysosomal pH regulation. We show that results of in vitro lysosome experiments can only be explained by the presence of ClC-7, and that ClC-7 promotes greater acidification than Cl^–, K⁺, or Na⁺ channels. Our models predict strikingly different lysosomal K⁺ dynamics depending on the major counterion pathways. However, given the lack of experimental data concerning acidification in vivo, the model cannot definitively rule out any given mechanism, but the model does provide concrete predictions for additional experiments that would clarify the identity of the counterion and its carrier.     

K+ Block Is the Mechanism of Functional Asymmetry in Bacterial Nav Channels

Crystal structures of several bacterial Na_v channels have been recently published and molecular dynamics simulations of ion permeation through these channels are consistent with many electrophysiological properties of eukaryotic channels. Bacterial Na_v channels have been characterized as functionally asymmetric, and the mechanism of this asymmetry has not been clearly understood. To address this question, we combined non-equilibrium simulation data with two-dimensional equilibrium unperturbed landscapes generated by umbrella sampling and Weighted Histogram Analysis Methods for multiple ions traversing the selectivity filter of bacterial Na_vAb channel. This approach provided new insight into the mechanism of selective ion permeation in bacterial Na_v channels. The non-equilibrium simulations indicate that two or three extracellular K⁺ ions can block the entrance to the selectivity filter of Na_vAb in the presence of applied forces in the inward direction, but not in the outward direction. The block state occurs in an unstable local minimum of the equilibrium unperturbed free-energy landscape of two K⁺ ions that can be ‘locked’ in place by modest applied forces. In contrast to K⁺, three Na⁺ ions move favorably through the selectivity filter together as a unit in a loose “knock-on” mechanism of permeation in both inward and outward directions, and there is no similar local minimum in the two-dimensional free-energy landscape of two Na⁺ ions for a block state. The useful work predicted by the non-equilibrium simulations that is required to break the K⁺ block is equivalent to large applied potentials experimentally measured for two bacterial Na_v channels to induce inward currents of K⁺ ions. These results illustrate how inclusion of non-equilibrium factors in the simulations can provide detailed information about mechanisms of ion selectivity that is missing from mechanisms derived from either crystal structures or equilibrium unperturbed free-energy landscapes.     

Interaction of guanidinium and ammonium cations with phosphatidylcholine and phosphatidylserine lipid bilayers – Calorimetric,spectroscopic and molecular dynamics simulations study

The ability of arginine-rich peptides to cross the lipid bilayer and enter cytoplasm, unlike their lysine-based analogues, is intensively studied in the context of cell-penetrating peptides. Although the experiments have not yet reconstructed their internalization mechanism, the computational studies have shown that the type or charge of lipid polar groups is one of the crucial factors in their translocation. In order to gain more detailed insight into the interaction of guanidinium (Gdm⁺) and ammonium (NH₄⁺) cations, as important building blocks in arginine and lysine amino acids, with lipid bilayers, we conducted the experimental and computational study that tackles this phenomenon. The adsorption of Gdm⁺ and NH₄⁺ on lipid bilayers prepared from a zwitterionic (DPPC) and an anionic (DPPS) lipid was examined by thermoanalytic and spectroscopic techniques. Using temperature-dependent UV–Vis spectroscopy and DSC calorimetry we determined the impact of Gdm⁺ and NH₄⁺ on the thermotropic properties of lipid bilayers. FTIR data, along with molecular dynamics simulations, unraveled the molecular-level details on the nature of their interactions, showing the proton transfer between NH₄⁺ and DPPS, but not between Gdm⁺ and DPPS. The findings originated from this work imply that Gdm⁺ and NH₄⁺ form qualitatively different interactions with lipids of different charge which is reflected in the physico-chemical interactions that arginine-and lysine-based peptides establish at a complex and chemically heterogeneous environment such as the biological membrane.     

Voltage-dependent processes in the electroneutral amino acid exchanger ASCT2

Neutral amino acid exchange by the alanine serine cysteine transporter (ASCT)2 was reported to be electroneutral and coupled to the cotransport of one Na⁺ ion. The cotransported sodium ion carries positive charge. Therefore, it is possible that amino acid exchange is voltage dependent. However, little information is available on the electrical properties of the ASCT2 amino acid transport process. Here, we have used a combination of experimental and computational approaches to determine the details of the amino acid exchange mechanism of ASCT2. The [Na⁺] dependence of ASCT2-associated currents indicates that the Na⁺/amino acid stoichiometry is at least 2:1, with at least one sodium ion binding to the amino acid–free apo form of the transporter. When the substrate and two Na⁺ ions are bound, the valence of the transport domain is +0.81. Consistently, voltage steps applied to ASCT2 in the fully loaded configuration elicit transient currents that decay on a millisecond time scale. Alanine concentration jumps at the extracellular side of the membrane are followed by inwardly directed transient currents, indicative of translocation of net positive charge during exchange. Molecular dynamics simulations are consistent with these results and point to a sequential binding process in which one or two modulatory Na⁺ ions bind with high affinity to the empty transporter, followed by binding of the amino acid substrate and the subsequent binding of a final Na⁺ ion. Overall, our results are consistent with voltage-dependent amino acid exchange occurring on a millisecond time scale, the kinetics of which we predict with simulations. Despite some differences, transport mechanism and interaction with Na⁺ appear to be highly conserved between ASCT2 and the other members of the solute carrier 1 family, which transport acidic amino acids.     

Sodium chloride toxicity and the cellular basis of salt tolerance in halophytes

Background Halophytes are the flora of saline soils. They adjust osmotically to soil salinity by accumulating ions and sequestering the vast majority of these (generally Na⁺ and Cl⁻) in vacuoles, while in the cytoplasm organic solutes are accumulated to prevent adverse effects on metabolism. At high salinities, however, growth is inhibited. Possible causes are: toxicity to metabolism of Na⁺ and/or Cl⁻ in the cytoplasm; insufficient osmotic adjustment resulting in reduced net photosynthesis because of stomatal closure; reduced turgor for expansion growth; adverse cellular water relations if ions build up in the apoplast (cell walls) of leaves; diversion of energy needed to maintain solute homeostasis; sub-optimal levels of K⁺ (or other mineral nutrients) required for maintaining enzyme activities; possible damage from reactive oxygen species; or changes in hormonal concentrations.Scope This review discusses the evidence for Na⁺ and Cl⁻ toxicity and the concept of tissue tolerance in relation to halophytes.Conclusions The data reviewed here suggest that halophytes tolerate cytoplasmic Na⁺ and Cl⁻ concentrations of 100–200 mm, but whether these ions ever reach toxic concentrations that inhibit metabolism in the cytoplasm or cause death is unknown. Measurements of ion concentrations in the cytosol of various cell types for contrasting species and growth conditions are needed. Future work should also focus on the properties of the tonoplast that enable ion accumulation and prevent ion leakage, such as the special properties of ion transporters and of the lipids that determine membrane permeability.     

Permeability of sarcoplasmic reticulum membrane. The effect of changed ionic environments on Ca2+ release

Summary Permeability properties and the effects of a changed membrane potential on Ca²⁺ release of sarcoplasmic reticulum vesicles of rabbit skeletal muscle were investigated by Millipore filtration. The relative permeability of sarcoplasmic reticulum to solutes determined under conditions of isotope exchange at equilibrium and/or under conditions of net flow of solute and water into the vesicles was as follows: sucrose, Ca²⁺, Mn²⁺–, choline⁺, Tris⁺–+, Na⁺, Li⁺, Cl^–. Transient membrane potentials were induced by rapidly changing the ionic environment of the vesicles. Knowledge of the relative permeation rates of the above ions allowed prediction of the direction and extent of membrane polarization. Osmotic effects in the polarization measurements due to the rapid influx of solute and water into the vesicles were minimized by using media containing a fast (K⁺ or Cl^–) and a relatively slow (gluconate^– or choline⁺) penetrating ion.⁴⁵Ca²⁺ efflux from vesicles derived from different parts of the sarcoplasmic reticulum structure was not appreciably changed when vesicles were made more positive inside (choline chloride potassium gluconate) or more negative inside (potassium gluconate choline chloride). These studies suggest that part or all of the ion-induced changes in sarcoplasmic reticulum membrane permeability, previously interpreted to indicate depolarization-induced Ca²⁺ release, may be due to osmotic effects.     

Chloride movement across the basolateral membrane of proximal tubule cells

Summary Electrophysiologic and tracer experiments have shown that Cl^– entersNecturus proximal tubule cells from the tubule lumen by a process coupled to the flow of Na⁺, and that Cl^– entry is electrically silent. The mechanism of Cl^– exit from the cell across the basolateral membrane has not been directly studied. To evaluate the importance of the movement of Cl^– ions across the basolateral membrane, the relative conductance of Cl^– to K⁺ was determined by a new method. Single-barrel ion-selective microelectrodes were used to measure intracellular Cl^– and K⁺ as a function of basolateral membrane PD as it varied normally from tubule to tubule. Basolateral membrane Cl^– conductance was about 10% of K⁺ conductance by this method. A second approach was to voltage clamp the basolateral PD to 20 mV above and below the spontaneous PD, while sensing intracellular Cl^– activity with the second barrel of a double-barrel microelectrode. An axial wire electrode in the tubule lumen was used to pass current across the tubular wall and thereby vary the basolateral membrane PD. Cell Cl^– activity was virtually unaffected by the PD changes. We conclude that Cl^– leavesNecturus proximal tubule cells by a neutral mechanism, possibly coupled to the efflux of Na⁺ or K⁺.     

A missense mutation converts the Na+,K+-ATPase into an ion channel and causes therapy-resistant epilepsy

The ion pump Na⁺,K⁺-ATPase is a critical determinant of neuronal excitability; however, its role in the etiology of diseases of the central nervous system (CNS) is largely unknown. We describe here the molecular phenotype of a Trp931Arg mutation of the Na⁺,K⁺-ATPase catalytic α1 subunit in an infant diagnosed with therapy-resistant lethal epilepsy. In addition to the pathological CNS phenotype, we also detected renal wasting of Mg²⁺. We found that membrane expression of the mutant α1 protein was low, and ion pumping activity was lost. Arginine insertion into membrane proteins can generate water-filled pores in the plasma membrane, and our molecular dynamic (MD) simulations of the principle states of Na⁺,K⁺-ATPase transport demonstrated massive water inflow into mutant α1 and destabilization of the ion-binding sites. MD simulations also indicated that a water pathway was created between the mutant arginine residue and the cytoplasm, and analysis of oocytes expressing mutant α1 detected a nonspecific cation current. Finally, neurons expressing mutant α1 were observed to be depolarized compared with neurons expressing wild-type protein, compatible with a lowered threshold for epileptic seizures. The results imply that Na⁺,K⁺-ATPase should be considered a neuronal locus minoris resistentia in diseases associated with epilepsy and with loss of plasma membrane integrity.     

Intracellular compartmentation of Na+, K+ and Cl− in the Ehrlich ascites tumor cell: Correlation with the membrane potential

Summary The intracellular distribution of Na⁺, K⁺, Cl^– and water has been studied in the Ehrlich ascites tumor cell. Comparison of the ion and water contents of whole cells with those of cells exposed to La³⁺ and mechanical stress indicated that La³⁺ treatment results in selective damage to the cell membrane and permits evaluation of cytoplasmic and nuclear ion concentrations. The results show that Na⁺ is sequestered within the nucleus, while K⁺ and Cl^– are more highly concentrated in the cell cytoplasm. Reduction of the [Na⁺] of the incubation medium by replacement with K⁺ results in reduced cytoplasmic [Na⁺], increased [Cl^–] and no change in [K⁺]. Nuclear concentrations of these ions are virtually insensitive to the cation composition of the medium. Concomitant measurements of the membrane potential were made. The potential in control cells was –13.7 mV. Reduction of [Na⁺] in the medium caused significant depolarization. The measured potential is describable by the Cl^– equilibrium potential and can be accounted for in terms of cation distributions and permeabilities. The energetic implications of the intracellular compartmentation of ions are discussed.