Kinetic properties of single sodium channels in rat heart and rat brain

Spontaneously active NaV1.5 sodium channels may underlie odor sensitivity

The olfactory system is remarkably sensitive to airborne odor molecules, but precisely how very low odor concentrations bordering on just a few molecules per olfactory sensory neuron can trigger graded changes in firing is not clear. This report reexamines signaling in olfactory sensory neurons in light of the recent account of NaV1.5 sodium channel-mediated spontaneous firing. Using a model of spontaneous channel activity, the study shows how even submillivolt changes in membrane potential elicited by odor are expected to cause meaningful changes in NaV1.5-dependent firing. The results suggest that the random window currents of NaV1.5 channels may underpin not only spontaneous firing in olfactory sensory neurons but the cellular response to odor as well, thereby ensuring the robustness and sensitivity of signaling that is especially important for low odor concentrations.

Persistent human cardiac Na+ currents in stably transfected mammalian cells: Robust expression and distinct open-channel selectivity among Class 1 antiarrhythmics

Miniature persistent late Na(+) currents in cardiomyocytes have been linked to arrhythmias and sudden death. The goals of this study are to establish a stable cell line expressing robust persistent cardiac Na(+) currents and to test Class 1 antiarrhythmic drugs for selective action against resting and open states. After transient transfection of an inactivation-deficient human cardiac Na(+) channel clone (hNav1.5-CW with L409C/A410W double mutations), transfected mammalian HEK293 cells were treated with 1 mg/ml G-418. Individual G-418-resistant colonies were isolated using glass cylinders. One colony with high expression of persistent Na(+) currents was subjected to a second colony selection. Cells from this colony remained stable in expressing robust peak Na(+) currents of 996 ± 173 pA/pF at +50 mV (n = 20). Persistent late Na(+) currents in these cells were clearly visible during a 4-second depolarizing pulse albeit decayed slowly. This slow decay is likely due to slow inactivation of Na(+) channels and could be largely eliminated by 5 μM batrachotoxin. Peak cardiac hNav1.5-CW Na(+) currents were blocked by tetrodotoxin with an IC(50) value of 2.27 ± 0.08 μM (n = 6). At clinic relevant concentrations, Class 1 antiarrhythmics are much more selective in blocking persistent late Na(+) currents than their peak counterparts, with a selectivity ratio ranging from 80.6 (flecainide) to 3 (disopyramide). We conclude that (1) Class 1 antiarrhythmics differ widely in their resting- vs. open-channel selectivity, and (2) stably transfected HEK293 cells expressing large persistent hNav1.5-CW Na(+) currents are suitable for studying as well as screening potent open-channel blockers.

Distribution and function of sodium channel subtypes in human atrial myocardium

Voltage-gated sodium channels composed of a pore-forming α subunit and auxiliary β subunits are responsible for the upstroke of the action potential in cardiac muscle. However, their localization and expression patterns in human myocardium have not yet been clearly defined. We used immunohistochemical methods to define the level of expression and the subcellular localization of sodium channel α and β subunits in human atrial myocytes. Nav1.2 channels are located in highest density at intercalated disks where β1 and β3 subunits are also expressed. Nav1.4 and the predominant Nav1.5 channels are located in a striated pattern on the cell surface at the z-lines together with β2 subunits. Nav1.1, Nav1.3, and Nav1.6 channels are located in scattered puncta on the cell surface in a pattern similar to β3 and β4 subunits. Nav1.5 comprised approximately 88% of the total sodium channel staining, as assessed by quantitative immunohistochemistry. Functional studies using whole cell patch-clamp recording and measurements of contractility in human atrial cells and tissue showed that TTX-sensitive (non-Nav1.5) α subunit isoforms account for up to 27% of total sodium current in human atrium and are required for maximal contractility. Overall, our results show that multiple sodium channel α and β subunits are differentially localized in subcellular compartments in human atrial myocytes, suggesting that they play distinct roles in initiation and conduction of the action potential and in excitation-contraction coupling. TTX-sensitive sodium channel isoforms, even though expressed at low levels relative to TTX-sensitive Nav1.5, contribute substantially to total cardiac sodium current and are required for normal contractility. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

Advances in targeting voltage-gated sodium channels with small molecules

Blockade of voltage-gated sodium channels (VGSCs) has been used successfully in the clinic to enable control of pathological firing patterns that occur in conditions as diverse as chronic pain, epilepsy, and arrhythmias. Herein we review the state of the art in marketed sodium channel inhibitors, including a brief compendium of their binding sites and of the cellular and molecular biology of sodium channels. Despite the preferential action of this drug class toward over-excited cells, which significantly limits potential undesired side effects on other cells, the need to develop a second generation of sodium channel inhibitors to overcome their critical clinical shortcomings is apparent. Current approaches in drug discovery to deliver novel and truly innovative sodium channel inhibitors is next presented by surveying the most recent medicinal chemistry breakthroughs in the field of small molecules and developments in automated patch-clamp platforms. Various strategies aimed at identifying small molecules that target either particular isoforms of sodium channels involved in specific diseases or anomalous sodium channel currents, irrespective of the isoform by which they have been generated, are critically discussed and revised.

Subcellular heterogeneity of sodium current properties in adult cardiac ventricular myocytes

BACKGROUND

Sodium channel α-subunits in ventricular myocytes (VMs) segregate either to the intercalated disc or to lateral membranes, where they associate with region-specific molecules.

OBJECTIVE

To determine the functional properties of sodium channels as a function of their location in the cell.

METHODS

Local sodium currents were recorded from adult rodent VMs and Purkinje cells by using the cell-attached macropatch configuration. Electrodes were placed either in the cell midsection (M) or at the cell end (area originally occupied by the intercalated disc [ID]). Channels were identified as tetrodotoxin (TTX)-sensitive (TTX-S) or TTX-resistant (TTX-R) by application of 100 nM of TTX.

RESULTS

Average peak current amplitude was larger in ID than in M and largest at the site of contact between attached cells. TTX-S channels were found only in the M region of VMs and not in Purkinje myocytes. TTX-R channels were found in both M and ID regions, but their biophysical properties differed depending on recording location. Sodium current in rat VMs was upregulated by tumor necrosis factor-alpha. The magnitude of current increase was largest in the M region, but this difference was abolished by application of 100 nM of TTX.

CONCLUSIONS

Our data suggest that (a) a large fraction of TTX-R (likely Na(v)1.5) channels in the M region of VMs are inactivated at normal resting potential, leaving most of the burden of excitation to TTX-R channels in the ID region; (b) cell-cell adhesion increases functional channel density at the ID; and (c) TTX-S (likely non-Na(v)1.5) channels make a minimal contribution to sodium current under control conditions, but they represent a functional reserve that can be upregulated by exogenous factors.

Functional protein expression of multiple sodium channel alpha- and beta-subunit isoforms in neonatal cardiomyocytes

Voltage-gated sodium channels are composed of pore-forming alpha- and auxiliary beta-subunits and are responsible for the rapid depolarization of cardiac action potentials. Recent evidence indicates that neuronal tetrodotoxin (TTX) sensitive sodium channel alpha-subunits are expressed in the heart in addition to the predominant cardiac TTX-resistant Na(v)1.5 sodium channel alpha-subunit. These TTX-sensitive isoforms are preferentially localized in the transverse tubules of rodents. Since neonatal cardiomyocytes have yet to develop transverse tubules, we determined the complement of sodium channel subunits expressed in these cells. Neonatal rat ventricular cardiomyocytes were stained with antibodies specific for individual isoforms of sodium channel alpha- and beta-subunits. alpha-actinin, a component of the z-line, was used as an intracellular marker of sarcomere boundaries. TTX-sensitive sodium channel alpha-subunit isoforms Na(v)1.1, Na(v)1.2, Na(v)1.3, Na(v)1.4 and Na(v)1.6 were detected in neonatal rat heart but at levels reduced compared to the predominant cardiac alpha-subunit isoform, Na(v)1.5. Each of the beta-subunit isoforms (beta1-beta4) was also expressed in neonatal cardiac cells. In contrast to adult cardiomyocytes, the alpha-subunits are distributed in punctate clusters across the membrane surface of neonatal cardiomyocytes; no isoform-specific subcellular localization is observed. Voltage clamp recordings in the absence and presence of 20 nM TTX provided functional evidence for the presence of TTX-sensitive sodium current in neonatal ventricular myocardium which represents between 20 and 30% of the current, depending on membrane potential and experimental conditions. Thus, as in the adult heart, a range of sodium channel alpha-subunits are expressed in neonatal myocytes in addition to the predominant TTX-resistant Na(v)1.5 alpha-subunit and they contribute to the total sodium current.

Axonal Na+ channels ensure fast spike activation and back-propagation in cerebellar granule cells

In most neurons, Na+ channels in the axon are complemented by others localized in the soma and dendrites to ensure spike back-propagation. However, cerebellar granule cells are neurons with simplified architecture in which the dendrites are short and unbranched and a single thin ascending axon travels toward the molecular layer before bifurcating into parallel fibers. Here we show that in cerebellar granule cells, Na+ channels are enriched in the axon, especially in the hillock, but almost absent from soma and dendrites. The impact of this channel distribution on neuronal electroresponsiveness was investigated by multi-compartmental modeling. Numerical simulations indicated that granule cells have a compact electrotonic structure allowing excitatory postsynaptic potentials to diffuse with little attenuation from dendrites to axon. The spike arose almost simultaneously along the whole axonal ascending branch and invaded the hillock the activation of which promoted spike back-propagation with marginal delay (<200 micros) and attenuation (<20 mV) into the somato-dendritic compartment. These properties allow granule cells to perform sub-millisecond coincidence detection of pre- and postsynaptic activity and to rapidly activate Purkinje cells contacted by the axonal ascending branch.

The cardiac persistent sodium current: an appealing therapeutic target?

The sodium current in the heart is not a single current with a mono-exponential decay but rather a mixture of currents with different kinetics. It is not clear whether these arise from distinct populations of channels, or from modulation of a single population. A very slowly inactivating component, [(INa(P))] I(Na(P)) is usually about 1% of the size of the peak transient current [I(Na(T))], but is enhanced by hypoxia. It contributes to Na(+) loading and cellular damage in ischaemia and re-perfusion, and perhaps to ischaemic arrhythmias. Class I antiarrhythmic agents such as flecainide, lidocaine and mexiletine generally block I(NA(P)) more potently than block of I(Na(T)) and have been used clinically to treat LQT3 syndrome, which arises because mutations in SCN5A produce defective inactivation of the cardiac sodium channel. The same approach may be useful in some pathological situations, such as ischaemic arrhythmias or diastolic dysfunction, and newer agents are being developed with this goal. For example, ranolazine blocks I(Na(P)) about 10 times more potently than I(Na(T)) and has shown promise in the treatment of angina. Alternatively, the combination of I(Na(P)) block with K(+) channel block may provide protection from the induction of Torsades de Pointe when these agents are used to treat atrial arrhythmias (eg Vernakalant). In all of these scenarios, an understanding of the role of I(Na(P)) in cardiac pathophysiology, the mechanisms by which it may affect cardiac electrophysiology and the potential side effects of blocking I(Na(P)) in the heart and elsewhere will become increasingly important.

Fine gating properties of channels responsible for persistent sodium current generation in entorhinal cortex neurons

The gating properties of channels responsible for the generation of persistent Na(+) current (I(NaP)) in entorhinal cortex layer II principal neurons were investigated by performing cell-attached, patch-clamp experiments in acutely isolated cells. Voltage-gated Na(+)-channel activity was routinely elicited by applying 500-ms depolarizing test pulses positive to -60 mV from a holding potential of -100 mV. The channel activity underlying I(NaP) consisted of prolonged and frequently delayed bursts during which repetitive openings were separated by short closings. The mean duration of openings within bursts was strongly voltage dependent, and increased by e times per every approximately 12 mV of depolarization. On the other hand, intraburst closed times showed no major voltage dependence. The mean duration of burst events was also relatively voltage insensitive. The analysis of burst-duration frequency distribution returned two major, relatively voltage-independent time constants of approximately 28 and approximately 190 ms. The probability of burst openings to occur also appeared largely voltage independent. Because of the above "persistent" Na(+)-channel properties, the voltage dependence of the conductance underlying whole-cell I(NaP) turned out to be largely the consequence of the pronounced voltage dependence of intraburst open times. On the other hand, some kinetic properties of the macroscopic I(NaP), and in particular the fast and intermediate I(NaP)-decay components observed during step depolarizations, were found to largely reflect mean burst duration of the underlying channel openings. A further I(NaP) decay process, namely slow inactivation, was paralleled instead by a progressive increase of interburst closed times during the application of long-lasting (i.e., 20 s) depolarizing pulses. In addition, long-lasting depolarizations also promoted a channel gating modality characterized by shorter burst durations than normally seen using 500-ms test pulses, with a predominant burst-duration time constant of approximately 5-6 ms. The above data, therefore, provide a detailed picture of the single-channel bases of I(NaP) voltage-dependent and kinetic properties in entorhinal cortex layer II neurons.

G protein modulates thyroid hormone-induced Na(+) channel activation in ventricular myocytes

To evaluate the effects of liothyronine (3,5,3'-triiodo-L-thyronine, T(3)) on Na(+) channel current (I(Na)) properties, I(Na) was recorded in adult guinea pig ventricular myocytes. T(3) (1 nM) acutely increased whole cell I(Na) and shifted the steady-state I(Na) inactivation curve dose dependently. When the pipette solution contained 100 microM GTP or GTPgammaS, the effect of T(3) on the whole cell I(Na) was increased two- to threefold. This effect was almost completely abolished by pertussis toxin preincubation. In the cell-attached patch, T(3) increased the open probability of single I(Na) by reducing the null probability. In the inside-out patch, T(3) effect was 10 times faster than that in whole cell and cell-attached patches while GTPgammaS was present and could be completely washed out. T(3) alone slightly increased the channel open probability by increasing the closed state to open state rate constant (k(CO)) and reducing the null probability. GTPgammaS exposure only increased the number of functional channels. T(3) and GTPgammaS synergistically enhanced the channel open probability 5.8 +/- 0.5-fold by increasing k(CO), decreasing the open state to absorbing inactivated state rate constant, and greatly reducing the null probability. These results demonstrate that T(3) acts on the cytosolic side of the membrane and acutely activates I(Na). Pertussis toxin-sensitive G protein modulation greatly magnifies the T(3) effects on the channel kinetics and null probability, thereby increasing the channel open probability.

Sodium channel currents in maturing acutely isolated rat hippocampal CA1 neurones

Sodium channel currents were recorded in excised inside-out patches from immature (P(4-10)) and older (P(20-46)) rat CA1 neurones. Channel conductance was 16.6+/-0.013 pS (P(20-46)) and 19.0+/-0.031 pS (P(4-10)). Opening patterns varied with step voltage and with age. In some patches bursting was apparent at voltages positive to -30 mV. Non-bursting behaviour was more dominant in patches from younger animals. In older animals mean open time (m.o.t.) was best described by two exponentials especially in the older cells; in the immature, there were fewer cases with two exponentials. The time constant of inactivation (tau(h)) estimated in ensemble averages was best described by two exponentials (tau(hf) and tau(hs)) in most patches from older cells. tau(hf) decreased with depolarization; tau(hs) increased in the range -30 to 0 mV. The voltage dependence of tau(hf) in the older cells is identical to that of the single tau(h) found in the younger; the results indicate a dominance of tau(hf) in the younger. Patches from younger cells more often showed one apparent active channel; in such cases, m.o.t. was described by a single exponential. However, in two cases, channels showed bursting behaviour with one of these channels showing a shift between bursting and non-bursting modes. Our findings are consistent with a heterogeneous channel population and with changes in the population in the course of maturation.

Role of the C-terminal domain in inactivation of brain and cardiac sodium channels

Inactivation is a fundamental characteristic of Na(+) channels, and small changes cause skeletal muscle paralysis and myotonia, epilepsy, and cardiac arrhythmia. Brain Na(v)1.2a channels have faster inactivation than cardiac Na(v)1.5 channels, but minor differences in inactivation gate structure are not responsible. We constructed chimeras in which the C termini beyond the fourth homologous domains of Na(v)1.2a and Na(v)1.5 were exchanged. Replacing the C-terminal domain (CT) of Na(v)1.2a with that of Na(v)1.5 (Na(v)1.2/1.5CT) slowed inactivation at +40 mV approximately 2-fold, making it similar to Na(v)1.5. Conversely, replacing the CT of Na(v)1.5 with that of Na(v)1.2a (Nav1.5/1.2CT) accelerated inactivation, making it similar to Na(v)1.2a. Activation properties were unaffected. The voltage dependence of steady-state inactivation of Na(v)1.5 is 16 mV more negative than that of Na(v)1.2a. The steady-state inactivation curve of Na(v)1.2a was shifted +12 mV in Na(v)1.2/1.5CT, consistent with destabilization of the inactivated state. Conversely, Na(v)1.5/1.2CT was shifted -14 mV relative to Na(v)1.5, consistent with stabilization of the inactivated state. Although these effects of exchanging C termini were consistent with their effects on inactivation kinetics, they magnified the differences in the voltage dependence of inactivation between brain and cardiac channels rather than transferring them. Thus, other parts of these channels determine the basal difference in steady-state inactivation. Deletion of the distal half of either the Na(v)1.2 or Na(v)1.5 CTs accelerated open-state inactivation and negatively shifted steady-state inactivation. Thus, the C terminus has a strong influence on kinetics and voltage dependence of inactivation in brain Na(v)1.2 and cardiac Na(v)1.5 channels and is primarily responsible for their differing rates of channel inactivation.

Na(+) current contribution to the diastolic depolarization in newborn rabbit SA node cells

Isolated newborn, but not adult, rabbit sinoatrial node (SAN) cells exhibit spontaneous activity that (unlike adult) are highly sensitive to the Na(+) current (I(Na)) blocker TTX. To investigate this TTX action on automaticity, cells were voltage clamped with ramp depolarizations mimicking the pacemaker phase of spontaneous cells (-60 to -20 mV, 35 mV/s). Ramps elicited a TTX-sensitive current in newborn (peak density 0.89 +/- 0.14 pA/pF, n = 24) but not adult (n = 5) cells. When depolarizing ramps were preceded by steplike depolarizations to mimic action potentials, ramp current decreased 54.6 +/- 8.0% (n = 3) but was not abolished. Additional experiments demonstrated that ramp current amplitude depended on the slope of the ramp and that TTX did not alter steady-state holding current at pacemaker potentials. This excluded a steady-state Na(+) window component and suggested a kinetic basis, which was investigated by measuring TTX-sensitive I(Na) during long step depolarizations. I(Na) exhibited a slow but complete inactivation time course at pacemaker voltages (tau = 33.9 +/- 3.9 ms at -50 mV), consistent with the rate-dependent ramp data. The data indicate that owing to slow inactivation of I(Na) at diastolic potentials, a small TTX-sensitive current flows during the diastolic depolarization in neonatal pacemaker myocytes.

Sodium channel isoform-specific effects of halothane: protein kinase C co-expression and slow inactivation gating

The modulatory effect of protein kinase C (PKC) on the response of Xenopus oocyte-expressed Na channel alpha-subunits to halothane (2-bromo-2-chloro-1,1,1-trifluroethane) was studied. Na currents through rat skeletal muscle, rat brain and human cardiac muscle Na channels were assessed using cell-attached patch clamp recordings. PKC activity was increased by co-expression of a constitutively active PKC alpha-isozyme. Decay of macroscopic Na currents could be separated into fast and slow exponential phases. PKC co-expression alone slowed Na current decay in neuronal channels, through enhancement of the amplitude of the slower phase of decay. Halothane (1.0 mM) was without effect on any of the three isoforms expressed alone but, after co-expression of PKC, there was enhancement of Na current decay with reduction in charge movement through skeletal muscle and neuronal channels. Cardiac channels were relatively insensitive to halothane. Enhanced Na current decay resulted from suppression of the slow phase, without effect on the faster phase or on either decay tau. Suppression of Na current through skeletal muscle channels was concentration-dependent over the therapeutic range and was described by third order reaction kinetics, with an IC(50) of 0.55 mM. We conclude that the halothane suppresses skeletal muscle and brain Na channel activity in this preparation through a reduction in the slow mode of inactivation gating, but only after PKC co-expression. Cardiac Na channels were relatively insensitive to halothane. The mechanism is likely to involve phosphorylation of the channel inactivation gate, although phosphorylation of other sites in the channel may account for the isoform specific differences.

High conductance sustained single-channel activity responsible for the low-threshold persistent Na(+) current in entorhinal cortex neurons

Stellate cells from entorhinal cortex (EC) layer II express both a transient Na(+) current (I(Na)) and a low-threshold persistent Na(+) current (I(NaP)) that helps to generate intrinsic theta-like oscillatory activity. We have used single-channel patch-clamp recording to investigate the Na(+) channels responsible for I(NaP) in EC stellate cells. Macropatch (more than six channels) recordings showed high levels of transient Na(+) channel activity, consisting of brief openings near the beginning of depolarizing pulses, and lower levels of persistent Na(+) channel activity, characterized by prolonged openings throughout 500 msec long depolarizations. The persistent activity contributed a noninactivating component to averaged macropatch recordings that was comparable with whole-cell I(NaP) in both voltage dependence of activation (10 mV negative to the transient current) and amplitude (1% of the transient current at -20 mV). In 14 oligochannel (less than six channels) patches, the ratio of transient to persistent channel activity varied from patch to patch, with 10 patches exhibiting exclusively transient openings and one patch showing exclusively persistent openings. In two patches containing only a single persistent channel, prolonged openings were observed in >50% of test depolarizations. Moreover, persistent openings had a significantly higher single-channel conductance (19.7 pS) than transient openings (15.6 pS). We conclude that this stable high-conductance persistent channel activity is responsible for I(NaP) in EC stellate cells. This persistent channel behavior is more enduring and has a higher conductance than the infrequent and short-lived transitions to persistent gating modes that have been described previously in brain neurons.

High conductance sustained single-channel activity responsible for the low-threshold persistent Na(+) current in entorhinal cortex neurons

Stellate cells from entorhinal cortex (EC) layer II express both a transient Na(+) current (I(Na)) and a low-threshold persistent Na(+) current (I(NaP)) that helps to generate intrinsic theta-like oscillatory activity. We have used single-channel patch-clamp recording to investigate the Na(+) channels responsible for I(NaP) in EC stellate cells. Macropatch (more than six channels) recordings showed high levels of transient Na(+) channel activity, consisting of brief openings near the beginning of depolarizing pulses, and lower levels of persistent Na(+) channel activity, characterized by prolonged openings throughout 500 msec long depolarizations. The persistent activity contributed a noninactivating component to averaged macropatch recordings that was comparable with whole-cell I(NaP) in both voltage dependence of activation (10 mV negative to the transient current) and amplitude (1% of the transient current at -20 mV). In 14 oligochannel (less than six channels) patches, the ratio of transient to persistent channel activity varied from patch to patch, with 10 patches exhibiting exclusively transient openings and one patch showing exclusively persistent openings. In two patches containing only a single persistent channel, prolonged openings were observed in >50% of test depolarizations. Moreover, persistent openings had a significantly higher single-channel conductance (19.7 pS) than transient openings (15.6 pS). We conclude that this stable high-conductance persistent channel activity is responsible for I(NaP) in EC stellate cells. This persistent channel behavior is more enduring and has a higher conductance than the infrequent and short-lived transitions to persistent gating modes that have been described previously in brain neurons.

Cardiac sodium channel Markov model with temperature dependence and recovery from inactivation

A Markov model of the cardiac sodium channel is presented. The model is similar to the CA1 hippocampal neuron sodium channel model developed by Kuo and Bean (1994. Neuron. 12:819-829) with the following modifications: 1) an additional open state is added; 2) open-inactivated transitions are made voltage-dependent; and 3) channel rate constants are exponential functions of enthalpy, entropy, and voltage and have explicit temperature dependence. Model parameters are determined using a simulated annealing algorithm to minimize the error between model responses and various experimental data sets. The model reproduces a wide range of experimental data including ionic currents, gating currents, tail currents, steady-state inactivation, recovery from inactivation, and open time distributions over a temperature range of 10 degrees C to 25 degrees C. The model also predicts measures of single channel activity such as first latency, probability of a null sweep, and probability of reopening.

Volatile anesthetics significantly suppress central and peripheral mammalian sodium channels

1. Voltage-dependent sodium channels are important for neuronal signal propagation and integration. 2. Non-mammalian preparations, such as squid giant axon, have sodium channels which have been found to be insensitive to clinical anesthetic concentrations. 3. On the other hand, sodium channels from mammalian neurons are much more sensitive to block by volatile anesthetics. 4. Due to a significant hyperpolarizing shift in steady-state inactivation, IC50s for sodium channel block at potentials close to the resting membrane potential overlapped with clinical anesthetic concentrations. 5. Hence, sodium channels in mammalian neurons may be sensitive molecular targets of volatile anesthetics.

Tetrodotoxin-blockable calcium currents in rat ventricular myocytes; a third type of cardiac cell sodium current

1. Whole-cell patch clamp currents from freshly isolated adult rat ventricular cells, recorded in external Ca2+ (Ca2+o) but no external Na+ (Na+o), displayed two inward current components: a smaller component that activated over more negative potentials and a larger component (L-type Ca2+ current) that activated at more positive potentials. The smaller component was not generated by Ca2+ channels. It was insensitive to 50 microM Ni2+ and 10 microM La3+ but suppressed by 10 microM tetrodotoxin (TTX). We refer to this component as ICa(TTX). 2. The conductance-voltage, g(V), relation in Ca2+o only was well described by a single Boltzmann function (half-maximum potential, V1/2, of -44.5; slope factor, k, of -4.49 mV, means of 3 cells). g(V) in Ca2+o plus Na+o was better described as the sum of two Boltzmann functions, one nearly identical to that in Ca2+o only (mean V1/2 of -45.1 and k of -3.90 mV), and one clearly distinct (mean V1/2 of -35.6 and k of -2.31 mV). Mean maximum conductance for ICa(TTX) channels increased 23.7% on adding 1 mM Na+o to 3 mM Ca2+o. ICa(TTX) channels are permeable to Na+ ions, insensitive to Ni2+ and La3+ and blocked by TTX. They are Na+ channels. 3. ICa(TTX) channels are distinct from classical cardiac Na+ channels. They activate and inactivate over a more negative range of potentials and have a slower time constant of inactivation than the classical Na+ channels. They are also distinct from yet another rat ventricular Na+ current component characterized by a much higher TTX sensitivity and by a persistent, non-fast-inactivating fraction. That ICa(TTX) channels activate over a more negative range of potentials than classical cardiac Na+ channels suggests that they may be critical for triggering the ventricular action potential and so of importance for cardiac arrhythmias.

Late sodium channel openings underlying epileptiform activity are preferentially diminished by the anticonvulsant phenytoin

Late openings of sodium channels were observed in outside-out patch recordings from hippocampal neurons in culture. In previous studies of such neurons, a persistent sodium current appeared to underlie the ictal epileptiform activity. All the channel currents were blocked by tetrodotoxin. In addition to the transient openings of sodium channels making up the peak sodium current, there were two types of late channel openings: brief late and burst openings. These late channel openings occurred throughout voltage pulses that lasted 750 ms, producing a persistent sodium current. At -30 mV, this current was 0.4% of the peak current. The late channel openings occurred throughout the physiological range of trans-membrane voltages. The anticonvulsant phenytoin reduced the late channel openings more than the peak currents. The effect on the persistent current was greatest at more depolarized voltages, whereas the effect on peak currents was not substantially voltage dependent. In the presence of 60 microM phenytoin, peak sodium currents at -30 mV were 40-41% of control, as calculated using different methods of analysis. Late currents were 22-24% of control. Phenytoin primarily decreased the number of channel openings, with less effect on the duration of channel openings and no effect on open channel current. This set of findings is consistent with models in which phenytoin binds to the inactivated state of the channel. The preferential effect of phenytoin on the persistent sodium current suggests that an important pharmacological mechanism for a sodium channel anticonvulsant is to reduce late openings of sodium channels, rather than reducing all sodium channel openings. We hypothesize that pharmacological interventions that are most selective in reducing late openings of sodium channels, while leaving early channel openings relatively intact, will be those that produce an anticonvulsant effect while interfering minimally with normal function.

Effects of clamp rise-time on rat brain IIA sodium channels in Xenopus oocytes

The kinetic properties of wild-type rat brain IIa sodium channels in excised macropatches were studied using step depolarizations and ramp depolarizations to imitate the slow settling-time of voltage in two-electrode voltage clamp. Ramp depolarizations longer than 1 ms produce an increasing suppression of peak sodium current (I[Na]). Two rates of inactivation can be seen in macroscopic sodium current records from excised patches following both step and ramp depolarizations. During slow ramp depolarizations, reduction in peak I[Na] is associated with selective loss of the fastest rate of test-pulse inactivation. This change can be interpreted as resulting from inactivation of a separate sub-population of 'fast mode' channels. The slow rate of test-pulse inactivation is relatively unaffected by changing ramp durations. These results are sufficient to explain the typically slow inactivation kinetics seen in two-electrode voltage clamp recordings of sodium channels in Xenopus oocytes. Thus, the kinetics of sodium channels expressed in Xenopus oocytes are not readily characterizable by two-electrode clamp because of the large membrane capacitance and resulting slow clamp settling time which artifactually selects for slow mode channels.

Blockade of cardiac Na+ channels by a charged class I antiarrhythmic agent, bisaramil: possible interaction of the drug with a pre-open closed state

The mechanism of cardiac Na+ channel block by a charged class I antiarrhythmic agent, bisaramil, was studied in guinea-pig ventricular myocytes using patch-clamp techniques of whole-cell, cell-attached and inside-out configurations. Bath application of bisaramil caused the use-dependent block of whole cell Na+ current (INa) in a concentration-dependent manner and EC30 value was 2.0 microM. At 5 microM bisaramil, the degree of the use-dependent block of INa with a short (5 ms) pulse protocol (44.9 +/- 5.7% of the first pulse INa) was comparable to that with a long (200 ms) pulse protocol (42.8 +/- 5.9%). In cell-attached patches, bisaramil applied to the bath solution (external application) concentration dependently blocked macropatch Na+ currents (50.3 +/- 3.1% inhibition with 10 microM bisaramil). Internal application of bisaramil decreased the inside-out macropatch currents (82.6 +/- 1.3% inhibition with 10 microM bisaramil). Blocking effects of bisaramil applied to the bath solution were greater than those seen on the pipette application in all of the whole-cell, cell-attached and inside-out configurations. In cell-attached patches containing a single active channel, bath application of 10 microM bisaramil increased the null sweeps with a significant (P < 0.001) nonrandom clustering and decreased the total number of openings, whereas no changes in the number of openings per active sweep, unitary current amplitude, mean open time and mean closed time were observed. While the peak average current was decreased by 51.0 +/- 5.6% with 10 microM bisaramil, the number of active sweeps was decreased by 31.4 +/- 6.2%. In the presence of 10 microM bisaramil, the mean values of first latencies were significantly (P < 0.05) increased and the peak value of the first latency density function was decreased by 15.8 +/- 3.6%. From these results, we conclude that a charged tertiary amine, bisaramil interacts with cardiac Na+ channels preferentially in the activated state. Interactions with pre-open closed states might contribute to the activated channel block by the drug.

Multiple mechanisms of Na+ channel--linked long-QT syndrome

Inheritable long-QT syndrome (LQTS) is a disease in which delayed ventricular repolarization leads to cardiac arrhythmias and the possibility of sudden death. In the chromosome 3-linked disease, one mutation of the cardiac Na+ channel gene results in a deletion of residues 1505 to 1507 (Delta KPQ), and two mutation result in substitutions (N1325S and R1644H). We compared all three mutant-channel phenotypes by heterologous expression in Xenopus oocytes. Each produced a late phase of inactivation-resistant, mexiletine- and tetrodotoxin-sensitive whole-cell currents, but the underlying mechanisms were different at the single-channel level. N1325S and R1644H showed dispersed reopenings after the initial transient, whereas Delta KPQ showed both dispersed reopenings and long-lasting bursts. Thus, two distinct biophysical defects underlie the in vitro phenotype of persistent current in Na+ channel-linked LQTS, and the additive effects of both are responsible for making the Delta KPQ phenotype the most severe.

Single-channel analysis of two types of Na+ currents in rat dorsal root ganglia

The properties of voltage-gated Na+ channels were studied in neurones isolated from rat dorsal root ganglia using the outside-out configuration of the patch-clamp technique. Two types of single-channel currents were identified from the difference in unit amplitudes. Neither type was evoked in the medium in which extracellular Na+ ions were replaced by an equimolar amount of tetramethylammonium ions. The two types of single-channel currents differed in their sensitivity to tetrodotoxin (TTX). The smaller channel current was insensitive to 1 microM TTX (referred to as TTX-I), while the larger channel current was blocked by 1 nM TTX (TTX-S). The unit amplitudes measured during a step depolarization to -30 mV (1.4 mM internal and 250 mM external Na+ concentrations) were 1.16 pA for TTX-S and 0.57 pA for TTX-I, respectively. The slope conductance measured at -30 mV was 16.3 pS for TTX-S and 8.5 pS for TTX-I. TTX-S could be activated by step depolarizations positive to -60 mV, while TTX-I could be activated at potentials positive to -40 mV. When the test pulse was preceded by a depolarizing prepulse, the prepulse positive to -50 mV preferentially inactivated TTX-S with a minimal effect on TTX-I. Activation and inactivation time courses of the averaged ensemble currents computed from TTX-S showed remarkable resemblances to the time courses of the macroscopic TTX-sensitive Na+ current. Similarly, the ensemble currents of TTX-I mimicked the macroscopic TTX-insensitive Na+ current. It was concluded that the two types of Na+ channels in rat dorsal root ganglia differ not only in their sensitivity to TTX, but also in their single-channel conductances.

Unitary conductance of Na+ channel isoforms in cardiac and NB2a neuroblastoma cells

Unitary conductances of native Na+ channel isoforms (gamma Na) have been determined under a variety of conditions, making comparisons of gamma Na difficult. To allow direct comparison, we measured gamma Na in cell-attached patches on NB2a neuroblastoma cells and rabbit ventricular myocytes under identical conditions [pipette solution (in mM): 280 Na+ and 2 Ca2+, pH 7.4; 10 degrees C]. gamma Na of NB2a channels, 22.9 +/- 0.9 pS, was 21% greater than that of cardiac channels, 18.9 +/- 0.9 pS. In contrast, respective extrapolated reversal potentials, +62.4 +/- 4.6 and +57.9 +/- 5.1 mV, were not significantly different. Several kinetic differences between the channel types were also noted. Negative to -20 mV, mean open time (MOT) of the NB2a isoform was significantly less than that of cardiac channels, and, near threshold, latency to channel opening decayed more rapidly in NB2a. On the basis of analysis of MOT between -60 and 0 mV, the rate constants at 0 mV for the open-to-closed (O-->C) and open-to-inactivated (O-->I) transitions were 0.42 +/- 0.11 and 0.47 +/- 0.11 ms-1 in NB2a and 0.10 +/- 0.06 and 1.19 +/- 0.07 ms-1 in myocytes. The slope factors were -38.9 +/- 8.7 and +10.7 +/- 6.1 mV in NB2a and -27.3 +/- 7.1 and +23.7 +/- 4.9 mV in myocytes. Transition rate constants were significantly different in NB2a and cardiac cells, but voltage dependence was not.

Interaction between external Na+ and mexiletine on Na+ channel in guinea-pig ventricular myocytes

To assess the modulation of Na+ channel block with local anaesthetics by the change of external Na+ concentration ([Na+]o), we examined the block by mexiletine at different [Na+]o using the whole-cell and the cell-attached configurations of the patch-clamp technique. Lowering [Na+]o increased the degree of use-dependent block of the whole-cell Na+ current. The external Na+ dependence of the Na+ current block was caused by the interaction of mexiletine with the activated Na+ channel, but not with the inactivated channel. In single-Na+ channel current recordings at a reduced [Na+]o of 70 mM, mexiletine shortened the mean open time of the channels (1.32 +/- 0.06 ms in the control vs. 0.86 +/- 0.12 ms with the drug, P < 0.05) without changes in the unitary current amplitude, whereas the drug did not affect mean open time at a [Na+]o of 140 mM. Moreover, the open time distributions during drug exposure at the reduced [Na+]o were better fitted to a double exponential than to a single exponential in four out of six experiments. These data suggest that mexiletine induces two conductive states: the native open state and a state representing the first step of open channel block. The transition from the former to the latter is dependent on [Na+]o, suggesting an antagonistic interaction of external Na+ with mexiletine.

Characterization of single voltage-gated Na+ and Ca2+ channels in apical dendrites of rat CA1 pyramidal neurons

1. We have used dendrite-attached patch-clamp techniques to record single Na+ and Ca2+ channel activity from the apical dendrites (up to 350 microns away from soma) of CA1 pyramidal neurons in rat hippocampal slices (ages: 2-8 weeks). 2. Na+ channels were found in every patch examined (range: 2 to > 20 channels per patch). Channel openings, which had a slope conductance of 15 +/- 0.3 pS (mean +/- S.E.M.), began with test commands to around -50 mV and consisted of both early transient channel activity and also later occurring prolonged openings of 5-15 ms. All Na+ channel activity was suppressed by inclusion of TTX (1 microM) in the recording pipette. 3. Ca2+ channel activity was recorded in about 80% of the patches examined (range: 1 to > 10 channels per patch). Several types of channel behaviour were observed in these patches. Single channel recordings in 110 mM BaCl2, revealed an approximately 10 pS channel of small unitary current amplitude (-0.5 pA at -20 mV). These channels began activating at relatively hyperpolarized potentials (-50 mV) and ensemble averages of this low voltage-activated (LVA) channel activity showed rapid inactivation. 4. A somewhat heterogeneous population of high voltage-activated, moderate conductance (HVAm; approximately 17 pS), Ca2+ channel activity was also encountered. These channels exhibited a relatively large unitary amplitude (-0.8 pA at 0 mV) and ensemble averages demonstrated moderate inactivation. The HVAm population of channels could be tentatively subdivided into two separate groups based upon mean channel open times. 5. Less frequently, HVA, large conductance (27 pS) Ca2+ channel activity (HVA1) was also observed. This large unitary amplitude (-1.5 pA at 0 mV) channel activity began with steps to approximately 0 mV and ensemble averages did not show any time-dependent inactivation. The dihydropyridine Ca2+ channel agonist Bay K 8644 (0.5 or 1 microM) was found to characteristically prolong these channel openings. 6. omega-Conotoxin MVIIC (10 microM), did not significantly reduce the amount of channel activity recorded from the LVA, HVAm or HVA1 channel types in dendritic patches. In patches from somata, omega-conotoxin MVIIC was effective in eliminating a significant amount of HVAm Ca2+ channel activity. Inclusion of 50 or 100 microM NiCl2 to the recording solution significantly reduced the amount of channel activity recorded from LVA and HVAm channel types in dendritic patches. A subpopulation of HVAm channels was, however, found to be Ni2+ insensitive. Dendritic HVA, channel activity was unaffected by these low concentrations of Ni2+.(ABSTRACT TRUNCATED AT 400 WORDS)

A persistent sodium current in acutely isolated histaminergic neurons from rat hypothalamus

Histamine neurons acutely dissociated from the tuberomammillary nucleus of the rat hypothalamus were studied in whole-cell and cell-attached patch-clamp experiments. Electrophysiological properties of dissociated cells were found to be similar to those recorded in slice experiments using microelectrodes. Tuberomammillary neurons fired spontaneously and this activity persisted when Cs+ (1.5 mM) was added to, or when K+ was removed from the extracellular solution. In whole-cell experiments a persistent tetrodotoxin-sensitive inward current was recorded. In cell attached recordings voltage-gated sodium channels displayed either normal or non-inactivating behavior. These results provide a further analysis of the properties of histaminergic neurons and indicate that spontaneous activity is intrinsic to individual neurons. Evidence for a non-inactivating tetrodotoxin-sensitive sodium current is presented. Single channel recordings indicate that this current is the result of non-inactivating behavior of sodium channels. Such a current is well suited for biasing tuberomammillary neurons toward spontaneous activity.

Convertible modes of inactivation of potassium channels in Xenopus myocytes differentiating in vitro

1. Voltage-dependent inactivating single-channel potassium currents were recorded in cell-attached and inside-out patches from embryonic Xenopus myocytes differentiating in culture. 2. Channels with rapid inactivation (time constants < 25 ms) and with slow inactivation (time constants > 80 ms) recorded after one day in vitro appear to belong to two functionally different classes. Rapidly and slowly inactivating channels show steady-state inactivation with potentials of half-inactivation of -74 +/- 7 and -44 +/- 9 mV. They exhibit voltage-dependent activation, with times to half-maximal activation of 0.79 +/- 0.09 and 1.17 +/- 0.22 ms when stepped from -120 to +40 mV. Rapidly inactivating channels also have a lower open probability than slowly inactivating ones. The channels have similar conductances of 23 +/- 6 and 17 +/- 4 pS and extrapolated reversal potentials close to the potassium equilibrium potential. 3. In cell-attached patches, inactivation behaviours of channels with rapid or slow inactivation do not change during recording. After patch excision, rapidly inactivating channels usually switch to a slow inactivation mode. Slowly inactivating channels derived from rapidly inactivating channels after patch excision retain their conductance and extrapolated reversal potential, but are not distinguishable from native slowly inactivating channels with respect to steady-state inactivation, activation and inactivation times, as well as open probabilities. 4. The change in inactivation behaviour of rapidly inactivating channels after patch excision is reversed by application of reduced dithiothreitol (DTT). In contrast, channels with slow inactivation in the cell-attached mode do not change in to rapidly inactivating channels after application of DTT in the excised configuration, suggesting that these channels belong to a structurally different class. 5. Frequent observation of superposing channel openings indicates clustering of inactivating potassium channels in the myocyte membrane, since many patches lack channel activity. Clustering does not depend on the presence of differentiating neurones. 6. Channels with rapid inactivation increase 6-fold in density during the first day in culture in the presence of neurones; channel density decreases in their absence. Channels with slow inactivation increase 2-fold in density in the presence or absence of differentiating neurones during this period. 7. Channels with rapid or slow inactivation in cell-attached membrane belong to functionally distinct classes that are developmentally regulated differently. Reversible changes from rapid to slow inactivation mode after patch excision suggest that the channels may be structurally related.

Voltage-dependent sodium channels in human small-cell lung cancer cells: role in action potentials and inhibition by Lambert-Eaton syndrome IgG

Sodium channels of human small-cell lung cancer (SCLC) cells were examined with whole-cell and single-channel patch clamp methods. In the tumor cells from SCLC cell line NCI-H146, the majority of the voltage-gated Na+ channels are only weakly tetrodotoxin (TTX)-sensitive (Kd = 215 nM). With the membrane potential maintained at -60 to -80 mV, these cells produced all-or-nothing action potentials in response to depolarizing current injection (> 20 pA). Similar all-or-nothing spikes were also observed with anodal break excitation. Removal of external Ca2+ did not affect the action potential production, whereas 5 microM TTX or substitution of Na+ with choline abolished it. Action potentials elicited in the Ca(2+)-free condition were reversibly blocked by 4 mM MnCl2 due to the Mn(2+)-induced inhibition of voltage-dependent sodium currents (INa). Therefore, Na+ channels, not Ca2+ channels, underlie the excitability of SCLC cells. Whole-cell INa was maximal with step-depolarizing stimulations to 0 mV, and reversed at +45.2 mV, in accord with the predicted Nernst equilibrium potential for a Na(+)-selective channel. INa evoked by depolarizing test potentials (-60 to +40 mV) exhibited a transient time course and activation/inactivation kinetics typical of neuronal excitable membranes; the plot of the Hodgkin-Huxley parameters, m infinity and h infinity, also revealed biophysical similarity between SCLC and neuronal Na+ channels. The single channel current amplitude, as measured with the inside-out patch configuration, was 1.0 pA at -20 mV with a slope conductance of 12.1 pS. The autoantibodies implicated in the Lambert-Eaton myasthenic syndrome (LES), which are known to inhibit ICa and INa in bovine adrenal chromaffin cells, also significantly inhibited INa in SCLC cells. These results indicate that (i) action potentials in human SCLC cells result from the regenerative increase in voltage-gated Na+ channel conductance; (ii) fundamental characteristics of SCLC Na+ channels are the same as the classical sodium channels found in a variety of excitable cells; and (iii) in some LES patients, SCLC Na+ channels are an additional target of the pathological IgG present in the patients' sera.

Voltage dependence of beta-adrenergic modulation of conduction in the canine Purkinje fiber

Although recent voltage-clamp and microelectrode studies have demonstrated beta-adrenergic modulation of Na+ current (INa) the modulation of conduction by catecholamines and the voltage dependence of that process have not been elucidated. To determine whether voltage-dependent modulation of conduction occurs in the presence of a beta-adrenergic agonist, the effect of 1 mumol/L isoproterenol on impulse propagation in canine Purkinje fibers was examined by using a dual-microelectrode technique. At physiological membrane potentials ([K+]o 5.4 mmol/L), isoproterenol increased squared conduction velocity [theta 2, 0.39 +/- 0.25 (m/s)2 (mean +/- SD)] from 3.46 +/- 0.86 to 3.85 +/- 0.98 (m/s)2 (P < .011), an 11% change, without altering the maximum first derivative of the upslope of phase 0 of the action potential (Vmax, 641 +/- 50 versus 657 +/- 47 V/s, P = NS). At transmembrane potential of -65 mV, produced by 12 mmol/L [K+]o titration, theta 2 declined 79% to 0.73 +/- 0.44 (m/s)2 as Vmax decreased 85% to 95 +/- 43 V/s (P < .02). The addition of isoproterenol further decreased theta 2 to 0.49 +/- 0.33 (m/s)2 (P = .02) in parallel with a further decline in Vmax to 51 +/- 25 V/s (P < .05). Isoproterenol produced a 3-mV hyperpolarizing shift of apparent Na+ channel availability curves generated from both theta 2 and Vmax, used as indexes of the fast inward INa, without changing the slopes of the relation. The relation between normalized theta 2 and Vmax over a range of depolarized potentials was linear and was not appreciably altered by isoproterenol.(ABSTRACT TRUNCATED AT 250 WORDS)

Single-channel analysis of tetrodotoxin-sensitive and tetrodotoxin-resistant sodium channels in rat dorsal root ganglion neurons

Tetrodotoxin-sensitive and tetrodotoxin-resistant single sodium channel currents were recorded from rat dorsal root ganglion neurons. The two types of sodium channel currents could be distinguished by the effects of predepolarization, 10 nM tetrodotoxin, and the inactivation during depolarization. Single-channel conductances were calculated to be 6.3 and 3.4 pS in the tetrodotoxin-sensitive and tetrodotoxin-resistant channels, respectively.

Kinetic mode switch of rat brain IIA Na channels in Xenopus oocytes excised macropatches

Na currents recorded from outside-out macropatches excised from Xenopus oocytes expressing the alpha subunit of the rat brain Na channel IIA show at least two distinguishable components in their inactivation time course, with time constants differing about tenfold (tau h1 = approx. 150 microseconds and tau h2 = approx. 2 ms). In excised patches, the inactivation properties of Na currents changed with time, favoring the faster inactivation kinetics. Analysis of the fast and slow current kinetics shows that only the relative magnitudes of tau h1 and tau h2 components are altered without significant changes in the time constants of activation or inactivation. In addition, voltage dependence of both activation and steady-state inactivation of Na currents are shifted to more negative potentials in patches with predominantly fast inactivation, although reversal potentials and valences remained unaltered. We conclude that the two inactivation modes discerned in this study are conferred by two states of Na channel the interconversion of which are regulated by an as yet unknown mechanism that seems to involve cytosolic factors.

Effects of III-IV linker mutations on human heart Na+ channel inactivation gating

Na+ channel inactivation, a critical determinant of refractoriness, differs in cardiomyocytes and neurons. In rat brain type IIa (rB2a) Na+ channels, a critical residue in the cytoplasmic linker between domains III and IV regulates fast inactivation such that a Phe-->Gln substitution (F1489Q) inhibits inactivation by at least 85%. Since this residue is conserved in voltage-gated Na+ channels, we tested whether F1485Q, the analogous mutation in human heart (hH1a) Na+ channels, has a similar functional effect. We found that fast inactivation in wild-type (WT) channels expressed in Xenopus oocytes was complete within 15 milliseconds at a test potential of 0 mV, and its time course was biexponential with time constants of 0.4 and 2 milliseconds. But in contrast to rB2a, the FQ mutation inhibited inactivation by < 50% and increased mean single-channel open time by only twofold. Residual fast inactivation was monoexponential, with a time constant similar to that of the slower phase of normal inactivation (2 milliseconds). In the mutant channels, unlike WT, null tracings were absent at holding potentials in the range of -140 to -120 mV, and the voltage range of steady-state inactivation coincided exactly with that of activation, suggesting that residual inactivation was tightly coupled to the open state. As in rB2a, simultaneous mutations of I1484Q and M1486Q, in addition to mutation F1485Q, completely inhibited fast inactivation. Our results show that in heart Na+ channels, the IFM cluster controls the stability of both open- and closed-channel inactivation in a manner qualitatively similar to that in the brain. Structural differences in the putative inactivation receptor may explain the distinct gating patterns in channel subtypes.

Gating of cardiac Na+ channels in excised membrane patches after modification by alpha-chymotrypsin

Single cardiac Na+ channels were investigated after intracellular proteolysis to remove the fast inactivation process in an attempt to elucidate the mechanisms of channel gating and the role of slow inactivation. Na+ channels were studied in inside-out patches excised from guinea-pig ventricular myocytes both before and after very brief exposure (2-4 min) to the endopeptidase, alpha-chymotrypsin. Enzyme exposure times were chosen to maximize removal of fast inactivation and to minimize potential nonspecific damage to the channel. After proteolysis, the single channel current-voltage relationship was approximately linear with a slope conductance of 18 +/- 2.5 pS. Na+ channel reversal potentials measured before and after proteolysis by alpha-chymotrypsin were not changed. The unitary current amplitude was not altered after channel modification suggesting little or no effect on channel conductance. Channel open times were increased after removal of fast inactivation and were voltage-dependent, ranging between 0.7 (-70 mV) and 3.2 (-10 mV) ms. Open times increased with membrane potential reaching a maximum at -10 mV; at more positive membrane potentials, open times decreased again. Fast inactivation appeared to be completely removed by alpha-chymotrypsin and slow inactivation became more apparent suggesting that fast and slow inactivation normally compete, and that fast inactivation dominates in unmodified channels. This finding is not consistent with a slow inactivated state that can only be entered through the fast inactivated state, since removal of fast inactivation does not eliminate slow inactivation. The data indicate that cardiac Na+ channels can enter the slow inactivated state by a pathway that bypasses the fast inactivated state and that the likelihood of entering the slow inactivated state increases after removal of fast inactivation.

Microscopic heterogeneity in unitary N-type calcium currents in rat sympathetic neurons

1. Single N-type calcium (Ca2+) channels in rat superior cervical ganglion neurons display complex patterns of activity in both inactivating and non-inactivating gating modes. Unitary currents were elicited by holding the patch at -90 mV and stepping to +30 mV for 740 ms. Barium (110 mM) was used as the charge carrier. The dihydropyridine agonist (+)-202-791 was included in the bath to ensure that single channel recordings showed no L-type Ca2+ channel mode 2 activity. Using this protocol, we characterized three additional patterns of N-type Ca2+ channel activity named: (1) LLP for large unitary current amplitude (i = -0.92 pA) and low open probability (Po = 0.26); (2) SLP for small unitary current amplitude (i = -0.77 pA) and low open probability (Po = 0.25); and (3) SHP for its small unitary current (i = -0.77 pA) and higher open probability (Po = 0.39). 2. Transitions among these patterns of activity occur more slowly than transitions between closed and open states, resulting in significant clustering of like sweeps. Thus, the complicated gating of single N-type Ca2+ channels can be dissected into multiple, independent modes, each with the same reproducible pattern of activity. 3. This heterogeneous activity is not unique to sympathetic neurons, for inactivating (4), non-inactivating (4), SLP (4) and SHP (3 patches) gating modes were also observed in cell-attached patch recordings (n = 4) of single N-type Ca2+ channels in differentiated phaeochromocytoma (PC12) cells. 4. The 1568 sweeps from four single N-type Ca2+ channel recordings that used the same voltage protocol were categorized by mode to determine the frequency of occurrence of each. Of the 54% of sweeps that showed activity, 42% were inactivating and 58% were non-inactivating. The contribution by each mode to the sustained current was estimated using the equation: I = NPoi, where N is the frequency of occurrence of each mode and Po and i are the mean values of open probability and unitary current amplitude respectively. The LLP mode contributed 18%, the SLP mode 16%, and the SHP mode 66% of the sustained whole cell N-type Ba2+ current. 5. The variability in the incidence among these modes in other cell types may resolve some of the controversy surrounding the characterization of N- and L-type whole cell Ca2+ current components in peripheral neurons. In addition, the number of different modes provides a source of plasticity that may be a target of modulation by neurotransmitters and cellular signals.

Modification of sodium channel inactivation by alpha-chymotrypsin in canine cardiac Purkinje cells

INTRODUCTION

Studies of tetrodotoxin-sensitive sodium current (INa) after modification of inactivation by intracellular enzymes in mammalian cells have demonstrated a marked increase in peak INa at test potentials near current threshold causing a large, negative shift of the peak INa conductance-voltage relationship by approximately -20 mV. These findings support a kinetic model in which the unmodified Na channel has rapid and voltage-independent inactivation from the open state. However, the kinetics of cardiac Na channels differ from those of mammalian neuronal Na channels. In particular, inactivation of cardiac Na channels has been proposed to be more voltage dependent than that of tetrodotoxin-sensitive Na channels. To help understand the role of inactivation in cardiac Na channel kinetic behavior, we studied Na currents before and after modification of inactivation by the proteolytic enzyme, alpha-chymotrypsin.

METHODS AND RESULTS

Whole cell INa was measured in single canine cardiac Purkinje cells that were voltage clamped and internally perfused with a large-bore suction pipette. The decay of INa in response to step depolarizations was dramatically slowed after perfusion with intracellular alpha-chymotrypsin consistent with modification of inactivation. In contrast to mammalian tetrodotoxin-sensitive Na current, Boltzmann distribution fits to peak INa conductance-voltage (GNa-V) relationships after alpha-chymotrypsin showed no change in either the potential at half maximum conductance (V 1/2), after correction for the spontaneous background shift of INa kinetics, or in the voltage-dependence of conductance (i.e., slope factor of GNa-V relationships). Maximal peak INa conductance increased by 18%. INa tail-current relaxations at potentials < or = -110 mV, after correction for spontaneous shifts in Na channel kinetics, were also similar before and after modification by alpha-chymotrypsin.

CONCLUSION

alpha-chymotrypsin modified inactivation of cardiac INa with little or no change in activation, and cardiac Na channel inactivation was slow near threshold and played little role in determining V1/2 for peak INa conductance-voltage relationships.

Use-dependent block of Na+ currents by mexiletine at the single channel level in guinea-pig ventricular myocytes

1. The mechanism of use-dependent block of Na+ current by mexiletine was studied at the single channel level in guinea-pig ventricular myocytes by the patch-clamp techniques. All experiments were performed using stimulation protocols to enable us to analyze the strict dependence of changes in channel properties on channel use. 2. In cell-attached patches, bath or pipette application of mexiletine (40 microM) produced a use-dependent reduction of the peak average current without changes in single channel conductance. Null sweeps were increased and the number of openings per sweep decreased with successive pulses, whereas no significant change in the mean open time was detected during the train. 3. Block by mexiletine became greater when pulse duration was extended beyond the period in which channels were open, suggesting that block progressed without channel opening. 4. At near threshold potentials, mexiletine decreased the later occurrence of first openings. Additionally, late openings were reduced in a use-dependent way. 5. We conclude that mexiletine binds to the inactivated closed states of the Na+ channel and then causes a failure of late openings as well as early, which results in null sweeps on subsequent depolarization.

Bursting of cardiac sodium channels after acute exposure to 3,5,3'-triiodo-L-thyronine

Physiological concentrations of 3,5,3'-triiodo-L-thyronine (T3) acutely increased burst-mode gating of Na+ channels in rabbit ventricular myocytes. Bursting was measured as the ratio of long events to the total number of events multiplied by 100 (%LE); a long event was defined as a set of openings or a single opening with a total duration greater than or equal to five times the control mean open time (MOT) for cell-attached patches. In the cell-attached configuration, adding either 5 or 50 nM T3 to the pipette increased the %LE. %LE had a biphasic voltage dependence and peaked at -50 mV, although the largest percentage change from control occurred between -30 and -40 mV. Neither unitary conductance nor the overall MOT was altered by T3-induced bursting. However, the MOT of openings within bursts increased, implying a kinetically distinct mode of channel gating during bursts. Long events sometimes were grouped into runs, but the more usual pattern suggested that modal shifts occurred in approximately 1 second. Similar behavior was observed with triiodothyroacetic acid, a T3 analogue that does not elicit protein synthesis. To investigate involvement of soluble second messengers, cell-attached recordings were made with and without T3 in the bath. Placed outside the pipette, 50 and 100 nM T3 failed to alter MOT, unitary current, or %LE. Na+ channel gating also was unaffected by patch excision and by exposing the cytoplasmic face of inside-out patches to 50 nM T3. Nevertheless, excision to the inside-out configuration with 5 nM T3 in the pipette dramatically increased the %LE and lengthened MOT. These results suggest that T3 induced Na+ channel bursting by an extranuclear mechanism that requires proximity of T3 to the extracellular face of the Na+ channel. Furthermore, T3 was not membrane permeant on the time scale of these experiments. Na+ channel bursting may contribute to the propensity for arrhythmias in hyperthyroidism and to the positive inotropic effect of acute T3 administration in the stunned and ischemic myocardium.

Loss of Na+ channel inactivation by anemone toxin (ATX II) mimics the myotonic state in hyperkalaemic periodic paralysis

1. Mutations that impair inactivation of the sodium channel in skeletal muscle have recently been postulated to cause several heritable forms of myotonia in man. A peptide toxin from Anemonia sulcata (ATX II) selectively disrupts the inactivation mechanism of sodium channels in a way that mimics these mutations. We applied ATX II to rat skeletal muscle to test the hypothesis that myotonia is inducible by altered sodium channel function. 2. Single-channel sodium currents were measured in blebs of surface membrane that arose from the mechanically disrupted fibres. ATX II impaired inactivation as demonstrated by persistent reopenings of sodium channels at strongly depolarized test potentials. A channel failed to inactivate, however, in only a small proportion of the depolarizing steps. With micromolar amounts of ATX II, the ensemble average open probability at the steady state was 0.01-0.02. 3. Ten micromolar ATX II slowed the relaxation of tension after a single twitch by an order of magnitude. Delayed relaxation is the in vitro analogue of the stiffness experienced by patients with myotonia. However, peak twitch force was not affected within the range of 0-10 microM ATX II. 4. Intracellular injection of a long-duration, constant current pulse elicited a train of action potentials in ATX II-treated fibres. After-depolarizations and repetitive firing often persisted beyond the duration of the stimulus. Trains of action potentials varied spontaneously in amplitude and firing frequency in a similar way to the electromyogram of a myotonic muscle. Both the after-depolarization and the post-stimulus firing were abolished by detubulating the fibres with glycerol. 5. We conclude that a loss of sodium channel inactivation alone, without changes in resting membrane conductance, is sufficient to produce the electrical and mechanical features of myotonia. Furthermore, in support of previous studies on myotonic muscle from patients, this model provides direct evidence that only a small proportion of sodium channels needs to function abnormally to cause myotonia.

Modification of cardiac sodium channels by carboxyl reagents. Trimethyloxonium and water-soluble carbodiimide

In TTX-sensitive nerve and skeletal muscle Na+ channels, selective modification of external carboxyl groups with trimethyloxonium (TMO) or water-soluble carbodiimide (WSC) prevents voltage-dependent Ca2+ block, reduces unitary conductance, and decreases guanidinium toxin affinity. In the case of TMO, it has been suggested that all three effects result from modification of a single carboxyl group, which causes a positive shift in the channel's surface potential. We studied the effect of these reagents on Ca2+ block of adult rabbit ventricular Na+ channels in cell-attached patches. In unmodified channels, unitary conductance (gamma Na) was 18.6 +/- 0.9 pS with 280 mM Na+ and 2 mM Ca2+ in the pipette and was reduced to 5.2 +/- 0.8 pS by 10 mM Ca2+. In contrast to TTX-sensitive Na+ channels, Ca2+ block of cardiac Na+ channels was not prevented by TMO; after TMO pretreatment, gamma Na was 6.1 +/- 1.0 pS in 10 mM Ca2+. Nevertheless, TMO altered cardiac Na+ channel properties. In 2 mM Ca2+, TMO-treated patches exhibited up to three discrete gamma Na levels: 15.3 +/- 1.7, 11.3 +/- 1.5, and 9.8 +/- 1.8 pS. Patch-to-patch variation in which levels were present and the absence of transitions between levels suggests that at least two sites were modified by TMO. An abbreviation of mean open time (MOT) accompanied each decrease in gamma Na. The effects on channel gating of elevating external Ca2+ differed from those of TMO pretreatment. Increasing pipette Ca2+ from 2 to 10 mM prolonged the MOT at potentials positive to approximately -35 mV by decreasing the open to inactivated (O-->I) transition rate constant. On the other hand, even in 10 mM Ca2+ TMO accelerated the O-->I transition rate constant without a change in its voltage dependence. Ensemble averages after TMO showed a shortening of the time to peak current and an acceleration of the rate of current decay. Channel modification with WSC resulted in analogous effects to those of TMO in failing to show relief from block by 10 mM Ca2+. Further, WSC caused a decrease in gamma Na and an abbreviation of MOT at all potentials tested. We conclude that a change in surface potential caused by a single carboxyl modification is inadequate to explain the effects of TMO and WSC in heart. Failure of TMO and WSC to prevent Ca2+ block of the cardiac Na+ channel is a new distinction among isoforms in the Na+ channel multigene family.

Neuroanatomical distribution and binding properties of saxitoxin sites in the rat and turtle CNS

Since saxitoxin (STX) binds to voltage-sensitive sodium channels and blocks their function, it has been widely used in the study of these channels. There is, however, limited information on STX binding properties and the neuroanatomical distribution of the Na+ channel as a function of brain region in the rat and in lower vertebrates such as the turtle. In the present study, we used a broad range of 3H-STX concentration (up to 64 nM) to examine saturation profiles and density distribution in both adult rat and turtle brains. We found that (1) STX sites do not vary greatly in affinity (most Kds = 2 to 5 nM) in various regions of the adult rat brain; (2) STX binding distribution was very heterogeneous in the rat with much higher density in the cortex, hippocampus, amygdala, and cerebellum than in the brainstem and spinal cord; (3) STX sites are mostly localized in layers made mostly of neurons with low density in white matter; and (4) turtle brain STX sites had similar binding properties, but its brain had much fewer STX sites than the rat, especially in the cerebellum and rostral areas such as the cortex. We conclude that (a) adult brain sodium channels have similar STX binding affinity in spite of the existence of multiple sodium channel subtypes; (b) the brainstem is very different from rostral brain areas in channel density; and (c) the turtle brain has a much lower sodium channel density than the rat brain.

Three types of sodium channels in adult rat dorsal root ganglion neurons

Several types of Na+ currents have previously been demonstrated in dorsal root ganglion (DRG) neurons isolated from neonatal rats, but their expression in adult neurons has not been studied. Na+ current properties in adult dorsal root ganglion (DRG) neurons of defined size class were investigated in isolated neurons maintained in primary culture using a combination of microelectrode current clamp, patch voltage clamp and immunocytochemical techniques. Intracellular current clamp recordings identified differing relative contributions of TTX-sensitive and -resistant inward currents to action potential waveforms in DRG neuronal populations of defined size. Patch voltage clamp recordings identified three distinct kinetic types of Na+ current differentially distributed among these size classes of DRG neurons. 'Small' DRG neurons co-express two types of Na+ current: (i) a rapidly-inactivating, TTX-sensitive 'fast' current and (ii) a slowly-activating and -inactivating, TTX-resistant 'slow' current. The TTX-sensitive Na+ current in these cells was almost completely inactivated at typical resting potentials. 'Large' cells expressed a single TTX-sensitive Na+ current identified as 'intermediate' by its inactivation rate constants. 'Medium'-sized neurons either co-expressed 'fast' and 'slow' current or expressed only 'intermediate' current. Na+ channel expression in these size classes was also measured by immunocytochemical techniques. An antibody against brain-type Na+ channels (Ab7493)10 labeled small and large neurons with similar intensity. These results demonstrate that three types of Na+ currents can be detected which correlate with electrogenic properties of physiologically and anatomically distinct populations of adult rat DRG neurons.

Sodium channel gene defects in the periodic paralyses

Abnormal Na+ currents that produce membrane depolarization have been associated with the episodes of muscle weakness that are the hallmark of the periodic paralyses. There is now strong evidence that various point mutations in the gene encoding the adult skeletal muscle voltage-dependent Na+ channel produce these abnormal currents, and are responsible for the expression of the disease phenotype.

The cloned cardiac Na channel alpha-subunit expressed in Xenopus oocytes show gating and blocking properties of native channels

The neonatal rat cardiac Na channel alpha-subunit directed currents in oocytes show characteristic cardiac relative resistance to tetrodotoxin (TTX) block. TTX-sensitive currents obtained by expression in Xenopus oocytes of the alpha-subunits of the rat brain (BrnIIa) and adult skeletal muscle (microI) Na channels show abnormally slow decay kinetics. In order to determine if currents directed by the cardiac alpha-subunit (RHI) exhibit kinetics in oocytes like native currents, we compared RHI-directed currents in oocytes to Na currents in freshly isolated neonatal rat myocytes. The decay rate of RHI currents approached that of neonatal myocytes and was faster than BrnIIa and microI currents in oocytes. The voltage dependence of availability and activation was the same as that in the rat myocytes except for a 12-19 mV shift in the depolarizing direction. The RHI Na currents were sensitive to Cd2+ block, and they showed use dependence of TTX and lidocaine block similar to native currents. The current expressed in oocytes following injection of the cRNA encoding for the alpha-subunit of the cardiac Na channel possesses most of the characteristic kinetic and pharmacological properties of the native cardiac Na current.

Na channels that remain open throughout the cardiac action potential plateau

In this paper we report the direct measurement of rare Na channel events that occur during the cardiac action potential, viz., channels that open at the upstroke and remain open throughout the plateau and early repolarization phase. The technique we use allows us to record channel activity and action potentials at the same time; thus, we are certain of when the Na channels open and when they finally close. The slow Na channels have the same voltage dependence, single-channel conductance, and TTX sensitivity as the fast Na channels, and they conduct Li. It therefore seems likely that the fast and the slow currents flow through the same channel. If this interpretation is correct, then the Na channel not only initiates the action potential but also helps to maintain its plateau. It is possible that the slow Na currents represent a separate collection of channels rather than a low-probability state of the fast Na channels. Regardless of which interpretation is correct, the present experiments allow us to assess the effect of the slow currents on action potential shape and on sustained Na entry.

Alkaloid-modified sodium channels from lobster walking leg nerves in planar lipid bilayers

Alkaloid-modified, voltage-dependent sodium channels from lobster walking leg nerves were studied in planar neutral lipid bilayers. In symmetrical 0.5 M NaCl the single channel conductance of veratridine (VTD) (10 pS) was less than that of batrachotoxin (BTX) (16 pS) modified channels. At positive potentials, VTD- but not BTX-modified channels remained open at a flickery substate. VTD-modified channels underwent closures on the order of milliseconds (fast process), seconds (slow process), and minutes. The channel fractional open time (f(o)) due to the fast process, the slow process, and all channel closures (overall f(o)) increased with depolarization. The fast process had a midpoint potential (V(a)) of -122 mV and an apparent gating charge (z(a)) of 2.9, and the slow process had a V(a) of -95 mV and a z(a) of 1.6. The overall f(o) was predominantly determined by closures on the order of minutes, and had a V(a) of about -24 mV and a shallow voltage dependence (z(a) approximately 0.7). Augmenting the VTD concentration increased the overall f(o) without changing the number of detectable channels. However, the occurrence of closures on the order of minutes persisted even at super-saturating concentrations of VTD. The occurrence of these long closures was nonrandom and the level of nonrandomness was usually unaffected by the number of channels, suggesting that channel behavior was nonindependent. BTX-modified channels also underwent closures on the order of milliseconds, seconds, and minutes. Their characterization, however, was complicated by the apparent low BTX binding affinity and by an apparent high binding reversibility (channel disappearance) of BTX to these channels. VTD- but not BTX-modified channels inactivated slowly at high positive potentials (greater than +30 mV). Single channel conductance versus NaCl concentrations saturated at high NaCl concentrations and was non-Langmuirian at low NaCl concentrations. At all NaCl concentrations the conductance of VTD-modified channels was lower than that of BTX-modified channels. However, this difference in conductance decreased as NaCl concentrations neared zero, approaching the same limiting value. The permeability ratio of sodium over potassium obtained under mixed ionic conditions was similar for VTD (2.46)- and BTX (2.48)-modified channels, whereas that obtained under bi-ionic conditions was lower for VTD (1.83)- than for BTX (2.70)-modified channels. Tetrodotoxin blocked these alkaloid-modified channels with an apparent binding affinity in the nanomolar range.

Dose-dependent modulation of the cardiac sodium channel by sea anemone toxin ATXII

The effects of sea anemone toxin ATXII on single sodium channels were studied in cell-attached patches on rabbit ventricular myocytes at 20-22 degrees C. Exposure of patches to 1,000 nM ATXII induced long-lasting bursts of openings, which were more dramatically different from control at -20 mV than at -50 mV. Mean open duration, which had a biphasic dependence on voltage in control patches, was monotonically dependent on voltage in toxin-exposed patches, being 3.5 times longer than control at -20 mV and 4.5 times longer at -10 mV. Multiple mean open durations were detected at depolarized potentials. To test whether the multiple mean open durations resulted from a mixture of modified and unmodified openings, histograms of late openings (when unmodified channels would be inactivated) were constructed. Because in most cases these fit a single exponential with a mean open duration like that of modified channels, we conclude that voltage-dependent toxin unbinding produced a mixed population of unmodified and modified openings. Consistent with this hypothesis, lower concentrations of toxin most often produced open-duration histograms best fit with two exponentials. Ensembles revealed complex decay kinetics, which could be interpreted within the context of the toxin-induced increase in mean open duration and burst duration and the summation of modified and unmodified events. Analysis of the numbers of early versus late events at -20 mV for patches exposed to 20 nM, 100 nM, and 1,000 nM ATXII predicted the ED50 for ATXII block to be 285 nM at this potential. Using a five-state Markovian model, the action of ATXII could be explained as a reduction of the open-to-inactivated rate constant without effect on inactivation from closed states or other rate transitions.