How does vestibule surface charge affect ion conduction and toxin binding in a sodium channel?

A Quantum Corrected Poisson-Nernst-Planck Model for Biological Ion Channels

A quantum corrected Poisson-Nernst-Planck (QCPNP) model is proposed for simulating ionic currents through biological ion channels by taking into account both classical and quantum mechanical effects. A generalized Gummel algorithm is also presented for solving the model system. Compared with the experimental results of X-ray crystallography, it is shown that the quantum PNP model is more accurate than the classical model in predicting the average number of ions in the channel pore. Moreover, the electrostatic potential has been found to reach as high as 19% difference between two models around the charged vestibule which has been shown to play a significant role in the permeation of ions through ion-selective ligand-gated or voltage-activated channels. These results indicate that the QCPNP model may be considered as a more refined continuum model that can be incorporated into a multi-scale electrophysiology modeling.

Voltage-gated calcium channels: Determinants of channel function and modulation by inorganic cations

Voltage-gated calcium channels (VGCCs) represent a key link between electrical signals and non-electrical processes, such as contraction, secretion and transcription. Evolved to achieve high rates of Ca(2+)-selective flux, they possess an elaborate mechanism for selection of Ca(2+) over foreign ions. It has been convincingly linked to competitive binding in the pore, but the fundamental question of how this is reconcilable with high rates of Ca(2+) transfer remains unanswered. By virtue of their similarity to Ca(2+), polyvalent cations can interfere with the function of VGCCs and have proven instrumental in probing the mechanisms underlying selective permeation. Recent emergence of crystallographic data on a set of Ca(2+)-selective model channels provides a structural framework for permeation in VGCCs, and warrants a reconsideration of their diverse modulation by polyvalent cations, which can be roughly separated into three general mechanisms: (I) long-range interactions with charged regions on the surface, affecting the local potential sensed by the channel or influencing voltage-sensor movement by repulsive forces (electrostatic effects), (II) short-range interactions with sites in the ion-conducting pathway, leading to physical obstruction of the channel (pore block), and in some cases (III) short-range interactions with extracellular binding sites, leading to non-electrostatic modifications of channel gating (allosteric effects). These effects, together with the underlying molecular modifications, provide valuable insights into the function of VGCCs, and have important physiological and pathophysiological implications. Allosteric suppression of some of the pore-forming Cavα1-subunits (Cav2.3, Cav3.2) by Zn(2+) and Cu(2+) may play a major role for the regulation of excitability by endogenous transition metal ions. The fact that these ions can often traverse VGCCs can contribute to the detrimental intracellular accumulation of metal ions following excessive release of endogenous Cu(2+) and Zn(2+) or exposure to non-physiological toxic metal ions.

SAP97 regulates Kir2.3 channels by multiple mechanisms

We examined the impact of coexpressing the inwardly rectifying potassium channel, Kir2.3, with the scaffolding protein, synapse-associated protein (SAP) 97, and determined that coexpression of these proteins caused an approximately twofold increase in current density. A combination of techniques was used to determine if the SAP97-induced increase in Kir2.3 whole cell currents resulted from changes in the number of channels in the cell membrane, unitary channel conductance, or channel open probability. In the absence of SAP97, Kir2.3 was found predominantly in a cytoplasmic, vesicular compartment with relatively little Kir2.3 localized to the plasma membrane. The introduction of SAP97 caused a redistribution of Kir2.3, leading to prominent colocalization of Kir2.3 and SAP97 and a modest increase in cell surface Kir2.3. The median Kir2.3 single channel conductance in the absence of SAP97 was approximately 13 pS, whereas coexpression of SAP97 led to a wide distribution of channel events with three distinct peaks centered at 16, 29, and 42 pS. These changes occurred without altering channel open probability, current rectification properties, or pH sensitivity. Thus association of Kir2.3 with SAP97 in HEK293 cells increased channel cell surface expression and unitary channel conductance. However, changes in single channel conductance play the major role in determining whole cell currents in this model system. We further suggest that the SAP97 effect results from SAP97 binding to the Kir2.3 COOH-terminal domain and altering channel conformation.

Intrinsic electrostatic potential in the BK channel pore: role in determining single channel conductance and block

The internal vestibule of large-conductance Ca²⁺ voltage-activated K⁺ (BK) channels contains a ring of eight negative charges not present in K⁺ channels of lower conductance (Glu386 and Glu389 in hSlo) that modulates channel conductance through an electrostatic mechanism (Brelidze, T.I., X. Niu, and K.L. Magleby. 2003. Proc. Natl. Acad. Sci. USA. 100:9017–9022). In BK channels there are also two acidic amino acid residues in an extracellular loop (Asp326 and Glu329 in hSlo). To determine the electrostatic influence of these charges on channel conductance, we expressed wild-type BK channels and mutants E386N/E389N, D326N, E329Q, and D326N/E329Q channels on Xenopus laevis oocytes, and measured the expressed currents under patch clamp. Contribution of E329 to the conductance is negligible and single channel conductance of D326N/E329Q channels measured at 0 mV in symmetrical 110 mM K⁺ was 18% lower than the control. Current–voltage curves displayed weak outward rectification for D326N and the double mutant. The conductance differences between the mutants and wild-type BK were caused by an electrostatic effect since they were enhanced at low K⁺ (30 mM) and vanished at high K⁺ (1 M K⁺). We determine the electrostatic potential change, Δφ, caused by the charge neutralization using TEA⁺ block for the extracellular charges and Ba²⁺ for intracellular charges. We measured 13 ± 2 mV for Δφ at the TEA⁺ site when turning off the extracellular charges, and 17 ± 2 mV for the Δφ at the Ba²⁺ site when the intracellular charges were turned off. To understand the electrostatic effect of charge neutralizations, we determined Δφ using a BK channel molecular model embedded in a lipid bilayer and solving the Poisson-Boltzmann equation. The model explains the experimental results adequately and, in particular, gives an economical explanation to the differential effect on the conductance of the neutralization of charges D326 and E329.

On the nature of antimicrobial activity: a model for protegrin-1 pores

We conducted over 150 ns of simulation of a protegrin-1 octamer pore in a lipid bilayer composed of palmitoyloleoyl-phosphatidylethanolamine (POPE) and palmitoyloleoyl-phosphatidylglycerol (POPG) lipids mimicking the inner membrane of a bacterial cell. The simulations improve on a model of a pore proposed from recent NMR experiments and provide a coherent understanding of the molecular mechanism of antimicrobial activity. Although lipids tilt somewhat toward the peptides, the simulated protegrin-1 pore more closely follows the barrel-stave model than the toroidal-pore model. The movement of ions is investigated through the pore. The pore selectively allows negatively charged chloride ions to pass through at an average rate of one ion every two nanoseconds. Only two events are observed of sodium ions crossing through the pore. The potential of mean force is calculated for the water and both ion types. It is determined that the chloride ions move through the pore with ease, similarly to the water molecules with the exception of a zone of restricted movement midway through the pore. In bacteria, ions moving through the pore will compromise the integrity of the transmembrane potential. Without the transmembrane potential as a countermeasure, water will readily flow inside the higher osmolality cytoplasm. We determine that the diffusivity of water through a single PG-1 pore is sufficient to cause fast cell death by osmotic lysis.

Isoform-dependent interaction of voltage-gated sodium channels with protons

Protons are potent physiological modifiers of voltage-gated Na(+) channels, shifting the voltage range of channel gating and reducing current magnitude (pK(a) approximately 6). We recently showed that proton block of the skeletal muscle isoform (Na(V)1.4) resulted from protonation of the four superficial carboxylates in the outer vestibule of the channel. We concluded that the large local negative electrostatic field shifted the outer vestibule carboxylate pK(a) into the physiological range. However, block was not complete; the best-fit titration curves yielded an acid pH asymptote of 10-15%, suggesting that the selectivity filter carboxylates may not be protonated. Using HEK 293 cells stably expressing different isoforms, each with varying channel density, we demonstrate that a pH-independent current is found in Na(V)1.4, but not in the cardiac isoform (Na(V)1.5). Mutational studies showed that absence of the pH-independent current in Na(V)1.5 could be ascribed to the cysteine in domain I, just above the selectivity filter aspartate (Cys373). We suggest that this cysteine can be protonated in acid solution to produce a positive charge that blocks the pore. Competition between protons and Na(+) did not exist for Na(+) concentrations between 1 and 140 mm. The residual current in acid solution, when the cysteine is absent, confirms that over the range of pH values that can be achieved physiologically, the selectivity filter carboxylates are not protonated. The pH-independent current helps to protect activation of skeletal muscle during the acidosis that occurs during exercise.

Three computational methods for studying permeation, selectivity and dynamics in biological ion channels

The cell membrane, confining some ions and molecules on one side and exchanging others with the other side, is the ultimate unit of the physiology of life. The delicate task of regulating the transport of ions across the membrane is carried out by biological nanotubes called 'ion channels'. Recently, there have been enormous strides in our understanding of the structure-function relationships of biological ion channels. The molecular structures of several ion channels have been determined from crystallographic analysis, including potassium channels, mechanosensitive channels, a chloride channel, as well as gramicidin channels and porins. It is expected that the X-ray structures of other ion channels will soon follow these discoveries, ushering in a new era of ion channel studies in which predicting the function of channels from their atomic structures will become the main quest. In parallel to these experimental findings, there have been important advances in computational biophysics. Here we summarize three theoretical approaches that have been utilized to understand the dynamics of ion permeation across bio-nanotubes, highlighting their advantages and shortcomings, and briefly describe some of the salient properties of ion channels uncovered through computational studies.

Conduction through the inward rectifier potassium channel, Kir2.1, is increased by negatively charged extracellular residues

Ion channel conductance can be influenced by electrostatic effects originating from fixed “surface” charges that are remote from the selectivity filter. To explore whether surface charges contribute to the conductance properties of Kir2.1 channels, unitary conductance was measured in cell-attached recordings of Chinese hamster ovary (CHO) cells transfected with Kir2.1 channels over a range of K⁺ activities (4.6–293.5 mM) using single-channel measurements as well as nonstationary fluctuation analysis for low K⁺ activities. K⁺ ion concentrations were shown to equilibrate across the cell membrane in our studies using the voltage-sensitive dye DiBAC₄(5). The dependence of γ on the K⁺ activity (a_K) was fit well by a modified Langmuir binding isotherm, with a nonzero intercept as a_K approaches 0 mM, suggesting electrostatic surface charge effects. Following the addition of 100 mM N-methyl-d-glucamine (NMG⁺), a nonpermeant, nonblocking cation or following pretreatment with 50 mM trimethyloxonium (TMO), a carboxylic acid esterifying agent, the γ–a_K relationship did not show nonzero intercepts, suggesting the presence of surface charges formed by glutamate or aspartate residues. Consistent with surface charges in Kir2.1 channels, the rates of current decay induced by Ba²⁺ block were slowed with the addition of NMG or TMO. Using a molecular model of Kir2.1 channels, three candidate negatively charged residues were identified near the extracellular mouth of the pore and mutated to cysteine (E125C, D152C, and E153C). E153C channels, but not E125C or D152C channels, showed hyperbolic γ–a_K relationships going through the origin. Moreover, the addition of MTSES to restore the negative charges in E53C channels reestablished wild-type conductance properties. Our results demonstrate that E153 contributes to the conductance properties of Kir2.1 channels by acting as a surface charge.

CFTR: a cysteine at position 338 in TM6 senses a positive electrostatic potential in the pore

We investigated the accessibility to protons and thiol-directed reagents of a cysteine substituted at position 338 in transmembrane segment 6 (TM6) of CFTR to test the hypothesis that T338 resides in the pore. Xenopus oocytes expressing T338C CFTR exhibited pH-dependent changes in gCl and I-V shape that were specific to the substituted cysteine. The apparent pKa of T338C CFTR was more acidic than that expected for a cysteine or similar simple thiols in aqueous solution. The pKa was shifted toward alkaline values when a nearby positive charge (R334) was substituted with neutral or negatively charged residues, consistent with the predicted influence of the positive charge of R334, and perhaps other residues, on the titration of a cysteine at 338. The relative rates of chemical modification of T338C CFTR by MTSET+ and MTSES- were also altered by the charge at 334. These observations support a model for CFTR that places T338 within the anion conduction path. The apparent pKa of a cysteine substituted at 338 and the relative rates of reaction of charged thiol-directed reagents provide a crude measure of a positive electrostatic potential that may be due to R334 and other residues near this position in the pore.

Time-resolved dynamics of proton transfer in proteinous systems

The dynamics of proton dissociation from an acidic moiety and its subsequent dispersion in the bulk is regulated by the physical chemical properties of the solvent. The solvent has to provide a potential well to accommodate the discharged proton, screen it from the negative charge of the conjugated base, and provide an efficient mode for the diffusion of the proton to the bulk. On measuring the dynamics of proton dissociation in the time-resolved domain, the kinetic analysis of the reaction can quantitate the properties of the immediate environment. In this review we implement the kinetic analysis for evaluating the properties of small cavities in proteins and the diffusion of protons within narrow channels. On the basis of this analysis,we discuss how the clustering of proton-binding sites on a surface can endow the surface with enhanced capacity to attract protons and to funnel them toward a specific site.

A ring of eight conserved negatively charged amino acids doubles the conductance of BK channels and prevents inward rectification

Large-conductance Ca2+-voltage-activated K+ channels (BK channels) control many key physiological processes, such as neurotransmitter release and muscle contraction. A signature feature of BK channels is that they have the largest single channel conductance of all K+ channels. Here we examine the mechanism of this large conductance. Comparison of the sequence of BK channels to lower-conductance K+ channels and to a crystallized bacterial K+ channel (MthK) revealed that BK channels have a ring of eight negatively charged glutamate residues at the entrance to the intracellular vestibule. This ring of charge, which is absent in lower-conductance K+ channels, is shown to double the conductance of BK channels for outward currents by increasing the concentration of K+ in the vestibule through an electrostatic mechanism. Removing the ring of charge converts BK channels to inwardly rectifying channels. Thus, a simple electrostatic mechanism contributes to the large conductance of BK channels.

Voltage-gated proton channels and other proton transfer pathways

Proton channels exist in a wide variety of membrane proteins where they transport protons rapidly and efficiently. Usually the proton pathway is formed mainly by water molecules present in the protein, but its function is regulated by titratable groups on critical amino acid residues in the pathway. All proton channels conduct protons by a hydrogen-bonded chain mechanism in which the proton hops from one water or titratable group to the next. Voltage-gated proton channels represent a specific subset of proton channels that have voltage- and time-dependent gating like other ion channels. However, they differ from most ion channels in their extraordinarily high selectivity, tiny conductance, strong temperature and deuterium isotope effects on conductance and gating kinetics, and insensitivity to block by steric occlusion. Gating of H(+) channels is regulated tightly by pH and voltage, ensuring that they open only when the electrochemical gradient is outward. Thus they function to extrude acid from cells. H(+) channels are expressed in many cells. During the respiratory burst in phagocytes, H(+) current compensates for electron extrusion by NADPH oxidase. Most evidence indicates that the H(+) channel is not part of the NADPH oxidase complex, but rather is a distinct and as yet unidentified molecule.

Role of outer ring carboxylates of the rat skeletal muscle sodium channel pore in proton block

Voltage-gated Na+ current is reduced by acid solution. Protons reduce peak Na+ conductance by lowering single channel conductance and shift the voltage range of gating by neutralizing surface charges. Structure-function studies identify six carboxyls and a lysine in the channel's outer vestibule. We examined the roles of the superficial ring of carboxyls in acid block of Na(v)1.4 (the rat skeletal muscle Na+ channel isoform) by measuring the effects of their neutralization or their substitution by lysine on sensitivity to acid solutions, using the two-micropipette voltage clamp in Xenopus oocytes. Alteration of the outer ring of carboxylates had little effect on the voltage for half-activation of Na+ current, as if they are distant from the channels' voltage sensors. The mutations did not abolish proton block; rather, they all shifted the pK(a) (-log of the dissociation constant) in the acid direction. Effects of neutralization on pK(a) were not identical for different mutations, with E758Q > D1241A > D1532N > E403Q. E758K showed double the effect of E758Q, and the other lysine mutations all produced larger effects than the neutralizing mutations. Calculation of the electrostatic potential produced by these carboxylates using a pore model showed that the pK(a) values of carboxylates of Glu-403, Glu-758, and Asp-1532 are shifted to values similar to the experimentally measured pK(a). Calculations also predict the experimentally observed changes in pK(a) that result from mutational neutralization or introduction of a positive charge. We propose that proton block results from partial protonation of these outer ring carboxylates and that all of the carboxylates contribute to a composite Na+ site.

Surface potentials and the calculated selectivity of ion channels

Ion channels catalyze the transport of ions across biological membranes. A proper understanding of ion-channel functioning is essential to our knowledge of cell physiology, and, in this context, ion-channel selectivity is a key concept. The extent to which a channel permeates two ion species, a and b, is expressed by the permeability ratio, P(a)/P(b). This paper addresses a complication in the calculation of P(a)/P(b) that is related to the existence of surface potentials (psi) and that so far has not been fully appreciated. This paper shows the rather surprising effect of psi on the calculated P(a)/P(b) of a channel that is permeable to two ion species of different valence. If we ignore psi, we conclude, for instance, P(a) > P(b). If we implement psi in the calculation of P(a)/P(b), we may, however, conclude exactly the reverse, i.e., P(a) < P(b). Because electrostatic potentials arise at the surface of essentially all biological membranes, this paper argues for a more critical evaluation of ion channel selectivity measurements.

Tests of continuum theories as models of ion channels. I. Poisson-Boltzmann theory versus Brownian dynamics

Continuum theories of electrolytes are widely used to describe physical processes in various biological systems. Although these are well-established theories in macroscopic situations, it is not clear from the outset that they should work in small systems whose dimensions are comparable to or smaller than the Debye length. Here, we test the validity of the mean-field approximation in Poisson-Boltzmann theory by comparing its predictions with those of Brownian dynamics simulations. For this purpose we use spherical and cylindrical boundaries and a catenary shape similar to that of the acetylcholine receptor channel. The interior region filled with electrolyte is assumed to have a high dielectric constant, and the exterior region representing protein a low one. Comparisons of the force on a test ion obtained with the two methods show that the shielding effect due to counterions is overestimated in Poisson-Boltzmann theory when the ion is within a Debye length of the boundary. As the ion gets closer to the boundary, the discrepancy in force grows rapidly. The implication for membrane channels, whose radii are typically smaller than the Debye length, is that Poisson-Boltzmann theory cannot be used to obtain reliable estimates of the electrostatic potential energy and force on an ion in the channel environment.

The influence of surface charges on the conductance of the human connexin37 gap junction channel

The single-channel conductance of the hCx37 homotypic gap junction channel does not saturate with transjunctional voltages up to +/-75 mV, nor does it depend linearly on the intracellular electrolyte concentration. The average maximum unitary conductances measured in KCl were 175 pS (30 mM), 236 pS (55 mM), 343 pS (110 mM), and 588 pS (270 mM) in the presence of 0.1 mM MgCl(2). The unexpectedly high unitary conductance at low salt concentrations can be explained by fixed charge groups within or near the channel orifice. Fixed cytoplasmic surface charges (3.4 e) positioned adjacent (15 A) to the channel pore adequately model the data (surface charge density of 0.24 e/(nm)(2)). In other experiments, high Mg(2+) reduced the unitary conductance of hCx37 homotypic gap junction channels more than predicted by screening alone, consistent with specific effects of Mg(2+) on the channel.

Mechanisms of cation permeation in cardiac sodium channel: description by dynamic pore model

The selective permeability to monovalent metal cations, as well as the relationship between cation permeation and gating kinetics, was investigated for native tetrodotoxin-insensitive Na-channels in guinea pig ventricular myocytes using the whole-cell patch clamp technique. By the measurement of inward unidirectional currents and biionic reversal potentials, we demonstrate that the cardiac Na-channel is substantially permeable to all of the group Ia and IIIa cations tested, with the selectivity sequence Na(+) >/= Li(+) > Tl(+) > K(+) > Rb(+) > Cs(+). Current kinetics was little affected by the permeant cation species and concentrations tested (</=160 mM), suggesting that the permeation process is independent of the gating process in the Na-channel. The permeability ratios determined from biionic reversal potentials were concentration and orientation dependent: the selectivity to Na(+) increased with increasing internal [K(+)] or external [Tl(+)]. The dynamic pore model describing the conformational transition of the Na-channel pore between different selectivity states could account for all the experimental data, whereas conventional static pore models failed to fit the concentration-dependent permeability ratio data. We conclude that the dynamic pore mechanism, independent of the gating machinery, may play an important physiological role in regulating the selective permeability of native Na-channels.

Selectivity and permeation in calcium release channel of cardiac muscle: alkali metal ions

Current was measured from single open channels of the calcium release channel (CRC) of cardiac sarcoplasmic reticulum (over the range +/-180 mV) in pure and mixed solutions (e.g., biionic conditions) of the alkali metal ions Li+, K+, Na+, Rb+, Cs+, ranging in concentration from 25 mM to 2 M. The current-voltage (I-V) relations were analyzed by an extension of the Poisson-Nernst-Planck (PNP) formulation of electrodiffusion, which includes local chemical interaction described by an offset in chemical potential, which likely reflects the difference in dehydration/solvation/rehydration energies in the entry/exit steps of permeation. The theory fits all of the data with few adjustable parameters: the diffusion coefficient of each ion species, the average effective charge distribution on the wall of the pore, and an offset in chemical potential for lithium and sodium ions. In particular, the theory explains the discrepancy between "selectivities" defined by conductance sequence and "selectivities" determined by the permeability ratios (i.e., reversal potentials) in biionic conditions. The extended PNP formulation seems to offer a successful combined treatment of selectivity and permeation. Conductance selectivity in this channel arises mostly from friction: different species of ions have different diffusion coefficients in the channel. Permeability selectivity of an ion is determined by its electrochemical potential gradient and local chemical interaction with the channel. Neither selectivity (in CRC) seems to involve different electrostatic interaction of different ions with the channel protein, even though the ions have widely varying diameters.

Charged amino acids near the pore entrance influence ion-conduction of a human L-type cardiac calcium channel

Voltage-dependent L-type Ca2+ channels form highly selective pores for Ca2+ ions in the membranes of excitable cells. We investigated the functional role of negatively charged residues, within or near the selectivity region, in ion permeation of a human cardiac L-type Ca2+ channel. Glutamates in each of the four repeats, and an aspartate in repeat IV, were substituted with positively charged lysine. Wild-type and mutant Ca2+ channels were expressed in Xenopus oocytes. Block by Ca2+ and Mg2 of inward Li+ currents through the channels was used to assess the effects of amino acid substitutions on high-affinity divalent cation binding. The rank order of IC50's for Ca2+ block of I(Li) was: E677K > E1086K > E334K > E1387K > D1391K > or approximately wild-type. The order of IC50's for Mg2+ block of I(Li) indicated differential involvement of the same residues in Mg2+ binding: E 1387K > E334K > E1086K > E677K > D 1391K = wild-type. Mutants E1387K and D1391K effectively permeated Ba2+, but exhibited a decreased single-channel conductance. The unitary current amplitude carried by Na+, in the absence of external divalent cations, was slightly decreased in the E1387K mutant but not in the D1391K mutant. The results confirm that each of the four glutamates participate unequally in high-affinity Ca2+ binding. Additionally, our results indicate that these glutamate residues participate in Mg2+ binding. The glutamate at position 1387 may be only peripherally involved in the formation of a high-affinity Ca2+ -binding site but is central to a Mg2+ binding site accessible from the external side of the pore. The aspartate at position 1391 is most likely located just external to the selectivity region.

Ion conductance of the Ca(2+)-activated maxi-K+ channel from the embryonic rat brain

By using single-channel recording techniques, we measured the conductance (gK) of the Ca(2+)-activated Maxi-K+ channel from the embryonic rat brain, and examined its dependence on K+ ions present in equimolar concentrations on both sides of the membrane patch. With ionic strength maintained constant by substitution of N-methyl-D-glucamine for K+, gK has a sigmoidal dependence upon [K+]. This result has been obscured in previous work by variations in ionic strength, which has a marked effect on single-channel conductance, especially in the limit for which this variable approaches zero. The gK versus [K+] relationship is described, theoretically, by a three-barrier, two-binding-site model in which the barrier that an ion must cross to leave the channel is decreased as [K+] is increased.

pH- and voltage-dependent conductances in toad skin

The present study focuses on two closely related topics on ion conductance in toad skins: (i) the interaction of apical protons with the apical voltage-dependent Cl(-)-activated channels of the mitochondria-rich cells, and (ii) the description and characterization of a novel subject, a voltage-dependent H(+)-activated conductance. The Cl- conductance (GCl) is activated by tissue hyperpolarization (which leads to apical membrane depolarization) and the presence of Cl- ions in the apical solution. Increasing apical proton concentration (from pH 8 to pH 4) impairs the process of activation of the Cl- conductive pathway, slowing the kinetics of It activation and reducing the steady-stage values of Gt and It. This effect is markedly voltage-dependent since no effect is seen at Vt = -100 mv and is fully present at -50 mV. The voltage-dependence of the pH effect suggests that the critical protonation sites of the apical Cl- channels are not freely exposed to the apical solution but dwell within the membrane electric field. An also coherent interpretation is that titration of apical proton binding sites affects the gating of the voltage-dependent Cl- channels, shifting the conductance-vs.-voltage curve to more negative clamping potentials. Tissue conductance in the absence of apical Cl- ions can be importantly affected by the pH of the apical solution (pHa), the effect being markedly dependent on the clamping potential. Generally speaking, the effect of rising apical proton concentration can be conspicuous at negative clamping potentials, while at positive potentials changes in tissue conductance were never observed. For a clamping potential of -100 mV, a turning point somewhere between pHa = 4 and pHa = 3 was observed. Apical acidification to pH 4 has no effect upon tissue conductance while apical acidification to pH 3 leads to a marked, slow and reversible increase of tissue conductance. A striking similitude exists between the voltage-dependent Cl(-)-gated conductance and the voltage-dependent proton-gated conductance regarding: (i) slow time courses of activation and deactivation, (ii) requirement for a negative clamping potential and the presence of a specific ion species in the apical solution for activation to take place, (iv) instantaneous ohmic behavior, and (v) steady-state rectification. However, so far the results do not permit one to conclude definitely that the voltage-dependent Cl(-)-gated conductance and the voltage-dependent proton-gated conductance share a common pathway.

Trimethyloxonium modification of batrachotoxin-activated Na channels alters functionally important protein residues

The extracellular side of single batrachotoxin-activated voltage-dependent Na channels isolated from rat skeletal muscle membranes incorporated into neutral planar lipid bilayers were treated in situ with the carboxyl methylating reagent, trimethyloxonium (TMO). These experiments were designed to determine whether TMO alters Na channel function by a general through-space electrostatic mechanism or by methylating specific carboxyl groups essential to channel function. TMO modification reduced single-channel conductance by decreasing the maximal turnover rate. Modification increased channel selectivity for sodium ions relative to potassium ions as measured under biionic conditions. TMO modification increased the mu-conotoxin (muCTX) off-rate by three orders of magnitude. Modification did not alter the muCTX on-rate at low ionic strength or Na channel voltage-dependent gating characteristics. These data demonstrate that TMO does not act via a general electrostatic mechanism. Instead, TMO targets protein residues specifically involved in ion conduction, ion selectivity, and muCTX binding. These data support the hypothesis that muCTX blocks open-channel current by physically obstructing the ion channel pore.

Surface charge and calcium channel saturation in bullfrog sympathetic neurons

Currents carried by Ba2+ through calcium channels were recorded in the whole-cell configuration in isolated frog sympathetic neurons. The effect of surface charge on the apparent saturation of the channel with Ba2+ was examined by varying [Ba2+]o and ionic strength. The current increased with [Ba2+]o, and the I-V relation and the activation curve shifted to more positive voltages. The shift of activation could be described by Gouy-Chapman theory, with a surface charge density of 1 e-/140 A2, calculated from the Grahame equation. Changes in ionic strength (replacing N-methyl-D-glucamine with sucrose) shifted the activation curve as expected for a surface charge density of 1 e-/85 A2, in reasonable agreement with the value from changing [Ba2+]o. The instantaneous I-V for fully activated channels also changed with ionic strength, which could be described either by a low surface charge density (less than 1 e-/1,500 A2), or by block by NMG with Kd approximately 300 mM (assuming no surface charge). We conclude that the channel permeation mechanism sees much less surface charge than the gating mechanism. The peak inward current saturated with an apparent Kd = 11.6 mM for Ba2+, while the instantaneous I-V saturated with an apparent Kd = 23.5 mM at 0 mV. This discrepancy can be explained by a lower surface charge near the pore, compared to the voltage sensor. After correction for a surface charge near the pore of 1 e-/1,500 A2, the instantaneous I-V saturated as a function of local [Ba2+]o, with Kd = 65 mM. These results suggest that the channel pore does bind Ba2+ in a saturable manner, but the current-[Ba2+]o relationship may be significantly affected by surface charge.

Differential effects of sulfhydryl reagents on saxitoxin and tetrodotoxin block of voltage-dependent Na channels

We have probed a cysteine residue that confers resistance to tetrodotoxin (TTX) block in heart Na channels, with membrane-impermeant, cysteine-specific, methanethiosulfonate (MTS) analogs. Covalent addition of a positively charged group to the cysteinyl sulfhydryl reduced pore conductance by 87%. The effect was selectively prevented by treatment with TTX, but not saxitoxin (STX). Addition of a negatively charged group selectively inhibited STX block without affecting TTX block. These results agree with models that place an exposed cysteinyl sulfhydryl in the TTX site adjacent to the mouth of the pore, but do not support the contention that STX and TTX are interchangeable. The surprising differences between the two toxins are consistent with the hypothesis that the toxin-receptor complex can assume different conformations when STX or TTX bound.

Defensive roles for secondary metabolites from marine sponges and sponge-feeding nudibranchs

In the marine environment sponges (Porifera) constitute one of the most interesting sources of bioactive natural products. The high frequency of bioactive components in these primitive filter-feeders is interpreted as chemical defence of sponges against environmental stress factors such as predation, overgrowth by fouling organisms or competition for space. The highest incidence of toxic or deterrent sponge metabolites is consequently found in habitats such as coral reefs that are characterized by intense competition and feeding pressure due, for example, to carnivorous fish. Further support for the adaptive significance of sponge constituents is derived from the observation that sponges which are growing exposed are usually more toxic than those growing unexposed. Whereas the chemical defence of sponges seems to be highly effective against most species of fish, a group of shell-less gastropods, the nudibranchs, has specialized on sponges. While feeding on sponges the nudibranchs sequester the effective chemical armoury of their prey, which is subsequently employed for their own protection. Some nudibranchs, however, have become independent of this interspecific flow of natural products and are able to accumulate defensive compounds through de novo synthesis.

Permeation of Na+ through open and Zn(2+)-occupied conductance states of cardiac sodium channels modified by batrachotoxin: exploring ion-ion interactions in a multi-ion channel

Mammalian heart sodium channels inserted into planar bilayers exhibit a distinctive subconductance state when single batrachotoxin-modified channels are exposed to external Zn2+. The current-voltage behavior of the open state and the Zn(2+)-induced substate was characterized in the presence of symmetrical Na+ ranging from 2 to 3000 mM. The unitary conductance of the open state follows a biphasic dependence on [Na+] that can be accounted for by a 3-barrier-2-site model of Na+ permeation that includes double occupancy and Na(+)-Na+ repulsion. The unitary conductance of the Zn2+ substate follows a monophasic dependence on [Na+] that can be explained by a similar 3-barrier-2-site model with low affinity for Na+ and single occupancy due to repulsive interaction with a Zn2+ ion bound near the external entrance to the pore. The apparent association rate of Zn2+ derived from dwell-time analysis of flickering events is strongly reduced as [Na+] is raised from 50 to 500 mM. The apparent dissociation rate of Zn2+ is also enhanced as [Na+] is increased. While not excluding surface charge effects, such behavior is consistent with two types of ion-ion interactions: 1) A competitive binding interaction between Zn2+ and Na+ due to mutual competition for high affinity sites in close proximity. 2) A noncompetitive, destabilizing interaction resulting from simultaneous occupancy by Zn2+ and Na+. The repulsive influence of Zn2+ on Na+ binding in the cardiac Na+ channel is similar to that which has been proposed to occur between Ca2+ and Na+ in structurally related calcium channels. Based on recent mutagenesis data, a schematic model of functionally important residues in the external cation binding sites of calcium channels and cardiac sodium channels is proposed. In this model, the Zn(2+)-induced subconductance state results from Zn2+ binding to a site in the external vestibule that is close to the entrance of the pore but does not occlude it.

Ion permeation, divalent ion block, and chemical modification of single sodium channels. Description by single- and double-occupancy rate-theory models

Calcium ions, applied internally, externally, or symmetrically, have been used in conjunction with rate-theory modeling to explore the energy profile of the ion-conducting pore of sodium channels. The block, by extracellular and/or intracellular calcium, of sodium ion conduction through single, batrachotoxin-activated sodium channels from rat brain was studied in planar lipid bilayers. Extracellular calcium caused a reduction of inward current that was enhanced by hyperpolarization and a weaker block of outward current. Intracellular calcium reduced both outward and inward sodium current, with the block being weakly dependent on voltage and enhanced by depolarization. These results, together with the dependence of single-channel conductance on sodium concentration, and the effects of symmetrically applied calcium, were described using single- or double-occupancy, three-barrier, two-site (3B2S), or single-occupancy, 4B3S rate-theory models. There appear to be distinct outer and inner regions of the channel, easily accessed by external or internal calcium respectively, separated by a rate-limiting barrier to calcium permeation. Most of the data could be well fit by each of the models. Reducing the ion interaction energies sufficiently to allow a small but significant probability of two-ion occupancy in the 3B2S model yielded better overall fits than for either 3B2S or 4B3S models constrained to single occupancy. The outer ion-binding site of the model may represent a section of the pore in which sodium, calcium, and guanidinium toxins, such as saxitoxin or tetrodotoxin, compete. Under physiological conditions, with millimolar calcium externally, and high potassium internally, the model channels are occupied by calcium or potassium much of the time, causing a significant reduction in single-channel conductance from the value measured with sodium as the only cation species present. Sodium conductance and degree of block by external calcium are reduced by modification of single channels with the carboxyl reagent, trimethyloxonium (TMO) (Worley et al., 1986) Journal of General Physiology. 87:327-349). Elevations of only the outermost parts of the energy profiles for sodium and calcium were sufficient to account for the reductions in conductance and in efficacy of calcium block produced by TMO modification.

A structural model of the tetrodotoxin and saxitoxin binding site of the Na+ channel

Biophysical evidence has placed the binding site for the naturally occurring marine toxins tetrodotoxin (TTX) and saxitoxin (STX) in the external mouth of the Na+ channel ion permeation pathway. We developed a molecular model of the binding pocket for TTX and STX, composed of antiparallel beta-hairpins formed from peptide segments of the four S5-S6 loops of the voltage-gated Na+ channel. For TTX the guanidinium moiety formed salt bridges with three carboxyls, while two toxin hydroxyls (C9-OH and C10-OH) interacted with a fourth carboxyl on repeats I and II. This alignment also resulted in a hydrophobic interaction with an aromatic ring of phenylalanine or tyrosine residues for the brainII and skeletal Na+ channel isoforms, but not with the cysteine found in the cardiac isoform. In comparison to TTX, there was an additional interaction site for STX through its second guanidinium group with a carboxyl on repeat IV. This model satisfactorily reproduced the effects of mutations in the S5-S6 regions and the differences in affinity by various toxin analogs. However, this model differed in important ways from previously published models for the outer vestibule and the selectivity region of the Na+ channel pore. Removal of the toxins from the pocket formed by the four beta-hairpins revealed a structure resembling a funnel that terminated in a narrowed region suitable as a candidate for the selectivity filter of the channel. This region contained two carboxyls (Asp384 and Glu942) that substituted for molecules of water from the hydrated Na+ ion. Simulation of mutations in this region that have produced Ca2+ permeation of the Na+ channel created a site with three carboxyls (Asp384, Glu942, and Glu1714) in proximity.

A simple model for surface charge on ion channel proteins

We present a simple two-parameter model for surface charge directly associated with ion channels. A spherically symmetric "charged shell" models a distribution of surface charge arrayed about the channel entrance, with a corresponding set of image charges behind the plane of the membrane. The transition between a regime of buffered conductance and a regime of rapidly falling conductance at very low ionic strength is found to depend on the magnitude of the surface charge as well as the separation between the charge and the channel entrance. This resolves an apparent discrepancy between the experimental findings of Naranjo and Latorre (1993. Biophys. J. 64:1038-1050) and previous theoretical computations. The charged-shell model is used in a comparative study of the toad skeletal muscle conductance data of Naranjo and Latorre, the rat skeletal muscle conductances of Ravindran et al. (1992. Biophys. J. 61:494-508), and a second set of rat muscle conductances presented in this paper.

Correlations between changes in membrane capacitance induced by changes in ionic environment and the conductance of channels incorporated into bilayer lipid membranes

The action of metal polycations and pH on ionic channels produced in bilayer lipid membranes (BLM) by three different toxins was studied by measuring membrane capacitance and channel conductance. Here, we show that critical concentrations of Cd2+, La3+ or Tb3+ induce complex changes in membrane capacitance. The time course of capacitance changes is similar to the time course of channel blocking by these ions at low concentration. No changes in BLM capacitance or conductance were observed in the range of pH 5.8-9.0. A pH shift from 7.4 to 3-4 or 11-12 induced large changes in BLM capacitance and channel conductance. For all studied channel-forming proteins, the initial capacitance increase preceded the conductance decrease caused by addition of polycations or by a change in pH. A close relationship between membrane lipid packing and ion channel protein is suggested.

Ion conduction in substates of the batrachotoxin-modified Na+ channel from toad skeletal muscle

Batrachotoxin-modified Na+ channels from toad muscle were inserted into planar lipid bilayers composed of neutral phospholipids. Single-channel conductances were measured for [Na+] ranging between 0.4 mM and 3 M. When membrane preparations were made in the absence of protease inhibitors, two open conductance states were identified: a fully open state (16.6 pS in 200 mM symmetrical NaCl) and a substate that was 71% of the full conductance. The substate was predominant at [Na+] > 65 mM, whereas the presence of the fully open state was predominant at [Na+] < 15 mM. Addition of protease inhibitors during membrane preparation stabilized the fully open state over the full range of [Na+] studied. In symmetrical Na+ solutions and in biionic conditions, the ratio of amplitudes remained constant and the two open states exhibited the same permeability ratios of PLi/PNa and PCs/PNa. The current-voltage relations for both states showed inward rectification only at [Na+] < 10 mM, suggesting the presence of asymmetric negative charge densities at both channel entrances, with higher charge density in the external side. An energy barrier profile that includes double ion occupancy and asymmetric charge densities at the channel entrances was required to fit the conductance-[Na+] relations and to account for the rectification seen at low [Na+]. Energy barrier profiles differing only in the energy peaks can give account of the differences between both conductance states. Estimation of the surface charge density at the channel entrances is very dependent on the ion occupancy used and the range of [Na+] tested. Independent evidence for the existence of a charged external vestibule was obtained at low external [Na+] by identical reduction of the outward current induced by micromolar additions of Mg2+ and Ba2+.

Divalent cation competition with [3H]saxitoxin binding to tetrodotoxin-resistant and -sensitive sodium channels. A two-site structural model of ion/toxin interaction

Monovalent and divalent cations competitively displace tetrodotoxin and saxitoxin (STX) from their binding sites on nerve and skeletal muscle Na channels. Recent studies of cloned cardiac (toxin-resistant) and brain (toxin-sensitive) Na channels suggest important structural differences in their toxin and divalent cation binding sites. We used a partially purified preparation of sheep cardiac Na channels to compare monovalent and divalent cation competition and pH dependence of binding of [3H]STX between these toxin-resistant channels and toxin-sensitive channels in membranes prepared from rat brain. The effects of several chemical modifiers of amino acid groups were also compared. Toxin competition curves for Na+ in heart and Cd2+ in brain yielded similar KD values to measurements of equilibrium binding curves. The monovalent cation sequence for effectiveness of [3H]STX competition is the same for cardiac and brain Na channels, with similar KI values for each ion and slopes of -1. The effectiveness sequence corresponds to unhydrated ion radii. For seven divalent cations tested (Ca2+, Mg2+, Mn2+, Co2+, Ni2+, Cd2+, and Zn2+) the sequence for [3H]STX competition was also similar. However, whereas all ions displaced [3H]STX from cardiac Na channels at lower concentrations, Cd2+ and Zn2+ did so at much lower concentrations. In addition, and by way of explication, the divalent ion competition curves for both brain and cardiac channels (except for Cd2+ and Zn2+ in heart and Zn2+ in brain) had slopes of less than -1, consistent with more than one interaction site. Two-site curves had statistically better fits than one-site curves. The derived values of KI for the higher affinity sites were similar between the channel types, but the lower affinity KI's were larger for heart. On the other hand, the slopes of competition curves for Cd2+ and Zn2+ were close to -1, as if the cardiac Na channel had one dominant site of interaction or more than one site with similar values for KI. pH titration of [3H]STX binding to cardiac channels showed a pKa of 5.5 and a slope of 0.6-0.9, compared with a pKa of 5.1 and slope of 1 for brain channels. Tetramethyloxonium (TMO) treatment abolished [3H]STX binding to cardiac and brain channels and STX protected channels, but the TMO effect was less dramatic for cardiac channels. Trinitrobenzene sulfonate preferentially abolished [3H]STX binding to brain channels by action at an STX protected site.(ABSTRACT TRUNCATED AT 400 WORDS)

Theoretical perspectives on ion-channel electrostatics: continuum and microscopic approaches

Peter Läuger introduced me (P.C.J.) to the field of ion-channel electrostatics while I was a sabbatical visitor at Konstanz in 1978–79. Läuger pointed out that the relative conductance of hydrophobic ions through phosphatidyl choline (PC) and glyceryl monooleate (GMO) membranes differed by a factor of about 100 (Hladky & Haydon, 1973), quite consistent with the difference in the water-membrane potential differences in the two systems (Pickar & Benz, 1978). However, cation conductance through gramicidin channels spanning these membranes only differs by a factor of 2–3 (Bamberg et al. 1976). Why? It is the pursuit of an answer to this question which led me into my researches in this field.

Intra and extracellular surface charges near Ca2+ channels in neurons and neuroblastoma cells

The properties of low (LVA) and high (HVA) voltage-activated calcium currents were investigated in rat sensory neurons and a murine neuroblastoma cell line exposed to various concentrations of intra- or extracellular monovalent ([c+]i/o) and trivalent ([c3+]i/o) cations. In neurons, when [c+]i was changed from 150 to 20 mM, positive shifts of 18-28 mV were observed in activation curves of both LVA and HVA currents, as well as in LVA inactivation curves. Extracellularly, in divalent-free solutions, [c+]o of 20-50 mM produced medium (12-22 mV) negative shifts of the LVA channel properties. These data were used to estimate, by a "screening" model, a negative surface charge density around neuron's calcium channels of 1/1,000 and 1/1,325 eA-2 at the outside or inside face, respectively. In the presence of physiological concentrations of divalent cations, [c+]o of 20-60 mM caused smaller (4-11 mV) negative shifts of the activation and inactivation curves, which can be explained by assuming a partial neutralization of negative charges by divalent cations. By applying the above procedure to LVA channels of neuroblastoma cells, the ratio of extra- to intracellular surface charge density turned out to be more than tenfold higher than in neurons. Effects produced by [c3+]i/o were not in agreement with expectations based on screening or binding models.

Extracellular divalent and trivalent cation effects on sodium current kinetics in single canine cardiac Purkinje cells

1. The effects of the extracellular divalent cations barium, calcium, cadmium, cobalt, magnesium, manganese, nickel and zinc and the trivalent cation lanthanum on macroscopic sodium current (INa) were characterized in enzymatically isolated single canine cardiac Purkinje cells under voltage clamp at 9-14 degrees C. 2. All di(tri)valent cations produced depolarizing shifts in the conductance-voltage relationship. The order of efficacy, taken as the concentration required to produce a 5 mV shift in the mid-point of peak INa conductance, from least to most effective was (mM): Ca2+ (2.97) approximately Mg2+ (2.67) approximately Ba2+ (1.93) > CO2+ (1.02) approximately Mn2+ (0.88) > Ni2+ (0.54) > La3+ (0.095) approximately Cd2+ (0.083) approximately Zn2+ (0.076). 3. Addition of di(tri)valent cations also produced depolarizing shifts in voltage-dependent availability. The order of efficacy from the least to most effective was (mM): Cd2+ (7.70) approximately Mg2+ (6.86) approximately Ba2+ (4.50) > Ca2+ (2.47) approximately CO2+ (1.87) approximately Mn2+ (1.24) approximately Ni2+ (1.20) > Zn2+ (0.300) > La3+ (0.060). 4. The Gouy-Chapman-Stern equations were used to evaluate di(tri)valent cation efficacy in binding to surface charges. Surface charge density was estimated as 0.72 sites nm-2, and it was assumed that Mg2+, the divalent cation that produced the smallest shift, screened but did not bind to surface charges. Based on voltage-dependent availability, KD from lowest to highest affinity were (mM): Ba2+ (2500) > CO2+ (1670) approximately Mn2+ (1430) approximately Ca2+ = Cd2+ = Ni2+ (1200) > Zn2+ (250) > La3+ (30). 5. All di(tri)valent cations also produced a concentration-dependent acceleration of INa tail current relaxation. The addition of Ca2+ and La3+ produced acceleration of tail current relaxations that could be accounted for by the surface charge effects predicted from the shift in voltage-dependent availability. Cd2+, which produced almost no change in voltage-dependent availability, dramatically accelerated tail current relaxation. Zn2+, Ni2+, Mn2+ and CO2+ also produced greater acceleration of tail current relaxation than could be accounted for by surface charge effects. 6. Di(tri)valent cations delayed time to peak INa in a concentration-dependent manner. The time to peak INa-voltage relationship was well described by an exponential plus a constant, and di(tri)valent cations did not affect the slope factor or constant but shifted the relationship in the depolarizing direction. Similar to their effect on tail currents, addition of some di(tri)valent cations produced larger effects on time to peak INa than expected from the shift of voltage-dependent availability.(ABSTRACT TRUNCATED AT 400 WORDS)

Lipid surface charge does not influence conductance or calcium block of single sodium channels in planar bilayers

We have studied the effects of membrane surface charge on Na+ ion permeation and Ca2+ block in single, batrachotoxin-activated Na channels from rat brain, incorporated into planar lipid bilayers. In phospholipid membranes with no net charge (phosphatidylethanolamine, PE), at low divalent cation concentrations (approximately 100 microM Mg2+), the single channel current-voltage relation was linear and the single channel conductance saturated with increasing [Na+] and ionic strength, reaching a maximum (gamma max) of 31.8 pS, with an apparent dissociation constant (K0.5) of 40.5 mM. The data could be approximated by a rectangular hyperbola. In negatively charged bilayers (70% phosphatidylserine, PS; 30% PE) slightly larger conductances were observed at each concentration, but the hyperbolic form of the conductance-concentration relation was retained (gamma max = 32.9 pS and K0.5 = 31.5 mM) without any preferential increase in conductance at lower ionic strengths. Symmetrical application of Ca2+ caused a voltage-dependent block of the single channel current, with the block being greater at negative potentials. For any given voltage and [Na+] this block was identical in neutral and negatively charged membranes. These observations suggest that both the conduction pathway and the site(s) of Ca2+ block of the rat brain Na channel protein are electrostatically isolated from the negatively charged headgroups on the membrane lipids.

Modeling ion permeation through batrachotoxin-modified Na+ channels from rat skeletal muscle with a multi-ion pore

The mechanism of ion permeation through Na+ channels that have been modified by batrachotoxin (BTX) and inserted into planar bilayers has been generally described by models based on single-ion occupancy, with or without an influence of negative surface charge, depending on the tissue source. For native Na+ channels there is evidence suggestive of a multi-ion conduction mechanism. To explore the question of ion occupancy, we have reexamined permeation of Na+, Li+, and K+ through BTX-modified Na+ channels from rat skeletal muscle. Single-channel current-voltage (I-V) behavior was studied in neutral lipid bilayers in the presence of symmetrical Na+ concentrations ranging from 0.5 to 3,000 mM. The dependence of unitary current on the mole fraction of Na+ was also examined in symmetrical mixtures of Na(+)-Li+ and Na(+)-K+ at a constant total ionic strength of 206 and 2,006 mM. The dependence of unitary conductance on symmetrical Na+ concentration does not exhibit Michaelis-Menten behavior characteristic of single-ion occupancy but can be simulated by an Eyring-type model with three barriers and two sites (3B2S) that includes double occupancy and ion-ion repulsion. Best-fit energy barrier profiles for Na+, Li+, and K+ were obtained by nonlinear curve fitting of I-V data using the 3B2S model. The Na(+)-Li+ and Na(+)-K+ mole-fraction experiments do not exhibit an anomalous mole-fraction effect. However, the 3B2S model is able to account for the biphasic dependence of unitary conductance on symmetrical [Na+] that is suggestive of multiple occupancy and the monotonic dependence of unitary current on the mole fraction of Na+ that is compatible with single or multiple occupancy. The best-fit 3B2S barrier profiles also successfully predict bi-ionic reversal potentials for Na(+)-Li+ and Na(+)-K+ in both orientations across the channel. Our experimental and modeling results reconcile the dual personality of ion permeation through Na+ channels, which can display features of single or multiple occupancy under various conditions. To a first approximation, the 3B2S model developed for this channel does not require corrections for vestibule surface charge. However, if negative surface charges of the protein do influence conduction, the conductance behavior in the limit of low [Na+] does not correspond to a Gouy-Chapman model of planar surface charge.

Permeant ion effects on the gating kinetics of the type L potassium channel in mouse lymphocytes

Permeant ion species was found to profoundly affect the gating kinetics of type l K+ currents in mouse T lymphocytes studied with the whole-cell or on-cell patch gigaohm-seal techniques. Replacing external K+ with Rb+ (as the sole monovalent cation, at 160 mM) shifted the peak conductance voltage (g-V) relation by approximately 20 mV to more negative potentials, while NH4+ shifted the g-V curve by 15 mV to more positive potentials. Deactivation (the tail current time constant, tau tail) was slowed by an average of 14-fold at -70 mV in external Rb+, by approximately 8-fold in Cs+, and by a factor of two to three in NH4+. Changing the external K+ concentration, [K+]o, from 4.5 to 160 mM or [Rb+]o from 10 to 160 mM had no effect on tau tail. With all the internal K+ replaced by Rb+ or Cs+ and either isotonic Rb+ or K+ in the bath, tau tail was indistinguishable from that with K+ in the cell. With the exception of NH4+, activation time constants were insensitive to permeant ion species. These results indicate that external permeant ions have stronger effects than internal permeant ions, suggesting an external modulatory site that influences K+ channel gating. However, in bi-ionic experiments with reduced external permeant ion concentrations, tau tail was sensitive to the direction of current flow, indicating that the modulatory site is either within the permeation pathway or in the outer vestibule of the channel. The latter interpretation implies that outward current through an open type l K+ channel significantly alters local ion concentrations at the modulatory site in the outer vestibule, and consequently at the mouth of the channel. Experiments with mixtures of K+ and Rb+ in the external solution reveal that deactivation kinetics are minimally affected by addition of Rb+ until the Rb+ mole fraction approaches unity. This relationship between mole fraction and tau tail, together with the concentration independence of tau tail, was hard to reconcile with simple models in which occupancy of a site within the permeation pathway prevents channel closing, but is consistent with a model in which a permeant ion binding site in the outer vestibule modulates gating depending on the species of ion occupying the site. A description of the ionic selectivity of the type l K+ channel is presented in the companion paper (Shapiro and DeCoursey, 1991b).

On the interaction of bovine pancreatic trypsin inhibitor with maxi Ca(2+)-activated K+ channels. A model system for analysis of peptide-induced subconductance states

Bovine pancreatic trypsin inhibitor (BPTI) is a 58-residue basic peptide that is a representative member of a widely distributed class of serine protease inhibitors known as Kunitz inhibitors. BPTI is also homologous to dendrotoxin peptides from mamba snake venom that have been characterized as inhibitors of various types of voltage-dependent K+ channels. In this study we compared the effect of DTX-I, a dendrotoxin peptide, and BPTI on large conductance Ca(2+)-activated K+ channels from rat skeletal muscle using planar bilayer methodology. As previously found for DTX-I (1990. Neuron. 2:141-148), BPTI induces the appearance of distinct subconductance events when present on the internal side of maxi K(Ca) channels. The single channel kinetics of substate formation follow the predictions of reversible binding of the peptide to a single site or class of sites with a Kd of 4.6 microM at 0 mV and 50 mM symmetrical KCl. The apparent association rate of BPTI binding decreases approximately 1,000-fold per 10-fold increase in ionic strength, suggestive of a strong electrostatic interaction between the basic peptide and negative surface charge in the vicinity of the binding site. The equilibrium Kd for BPTI and DTX-I is also voltage dependent, decreasing e-fold per 30 mV of depolarization. The unitary subconductance current produced by BPTI binding exhibits strong inward rectification in the presence of symmetrical KCl, corresponding to 15% of open channel current at +60 mV and 70% of open state at -40 mV. In competition experiments, the internal pore-blocking ions, Ba2+ and TEA+, readily block the substate with the same affinity as that for blocking the normal open state. These results suggest that BPTI does not bind near the inner mouth of the channel so as to directly interfere with cation entry to the channel. Rather, the mechanism of substate production appears to involve a conformational change that affects the energetics of K+ permeation.

Competitive binding interaction between Zn2+ and saxitoxin in cardiac Na+ channels. Evidence for a sulfhydryl group in the Zn2+/saxitoxin binding site

Mammalian heart Na+ channels exhibit approximately 100-fold higher affinity for block by external Zn2+ than other Na+ channel subtypes. With batrachotoxin-modified Na+ channels from dog or calf heart, micromolar concentrations of external Zn2+ result in a flickering block to a substate level with a conductance of approximately 12% of the open channel at -50 mV. We examined the hypothesis that, in this blocking mode, Zn2+ binds to a subsite of the saxitoxin (STX) binding site of heart Na+ channels by single-channel analysis of the interaction between Zn2+ and STX and also by chemical modification experiments on single heart Na+ channels incorporated into planar lipid bilayers in the presence of batrachotoxin. We found that external Zn2+ relieved block by STX in a strictly competitive fashion. Kinetic analysis of this phenomenon was consistent with a scheme involving direct binding competition between Zn2+ and STX at a single site with intrinsic equilibrium dissociation constants of 30 nM for STX and 30 microM for Zn2+. Because high-affinity Zn2(+)-binding sites often include sulfhydryl groups as coordinating ligands of this metal ion, we tested the effect of a sulfhydryl-specific alkylating reagent, iodoacetamide (IAA), on Zn2+ and STX block. For six calf heart Na+ channels, we observed that exposure to 5 mM IAA completely abolished Zn2+ block and concomitantly modified STX binding with at least 20-fold reduction in affinity. These results lead us to propose a model in which Zn2+ binds to a subsite within or near the STX binding site of heart Na+ channels. This site is also presumed to contain one or more cysteine sulfhydryl groups.

Divalent cation selectivity for external block of voltage-dependent Na+ channels prolonged by batrachotoxin. Zn2+ induces discrete substates in cardiac Na+ channels

The mechanism of block of voltage-dependent Na+ channels by extracellular divalent cations was investigated in a quantitative comparison of two distinct Na+ channel subtypes incorporated into planar bilayers in the presence of batrachotoxin. External Ca2+ and other divalent cations induced a fast voltage-dependent block observed as a reduction in unitary current for tetrodotoxin-sensitive Na+ channels of rat skeletal muscle and tetrodotoxin-insensitive Na+ channels of canine heart ventricular muscle. Using a simple model of voltage-dependent binding to a single site, these two distinct Na+ channel subtypes exhibited virtually the same affinity and voltage dependence for fast block by Ca2+ and a number of other divalent cations. This group of divalent cations exhibited an affinity sequence of Co congruent to Ni greater than Mn greater than Ca greater than Mg greater than Sr greater than Ba, following an inverse correlation between binding affinity and ionic radius. The voltage dependence of fast Ca2+ block was essentially independent of CaCl2 concentration; however, at constant voltage the Ca2+ concentration dependence of fast block deviated from a Langmuir isotherm in the manner expected for an effect of negative surface charge. Titration curves for fast Ca2+ block were fit to a simplified model based on a single Ca2+ binding site and the Gouy-Chapman theory of surface charge. This model gave similar estimates of negative surface charge density in the vicinity of the Ca2+ blocking site for muscle and heart Na+ channels. In contrast to other divalent cations listed above, Cd2+ and Zn2+ are more potent blockers of heart Na+ channels than muscle Na+ channels. Cd2+ induced a fast, voltage-dependent block in both Na+ channel subtypes with a 46-fold higher affinity at 0 mV for heart (KB = 0.37 mM) vs. muscle (KB = 17 mM). Zn2+ induced a fast, voltage-dependent block of muscle Na+ channels with low affinity (KB = 7.5 mM at 0 mV). In contrast, micromolar Zn2+ induced brief closures of heart Na+ channels that were resolved as discrete substate events at the single-channel level with an apparent blocking affinity of KB = 0.067 mM at 0 mV, or 110-fold higher affinity for Zn2+ compared with the muscle channel. High-affinity block of the heart channel by Cd2+ and Zn2+ exhibited approximately the same voltage dependence (e-fold per 60 mV) as low affinity block of the muscle subtype (e-fold per 54 mV), suggesting that the block occurs at structurally analogous sites in the two Na+ channels.(ABSTRACT TRUNCATED AT 400 WORDS)