Blockade of cardiac sodium channels by lidocaine. Single-channel analysis

Functional effects of drugs and toxins interacting with NaV1.4

NaV1.4 is a voltage-gated sodium channel subtype that is predominantly expressed in skeletal muscle cells. It is essential for producing action potentials and stimulating muscle contraction, and mutations in NaV1.4 can cause various muscle disorders. The discovery of the cryo-EM structure of NaV1.4 in complex with β1 has opened new possibilities for designing drugs and toxins that target NaV1.4. In this review, we summarize the current understanding of channelopathies, the binding sites and functions of chemicals including medicine and toxins that interact with NaV1.4. These substances could be considered novel candidate compounds or tools to develop more potent and selective drugs targeting NaV1.4. Therefore, studying NaV1.4 pharmacology is both theoretically and practically meaningful.

Electrophysiological Properties of Tetraploid Cardiomyocytes Derived from Murine Pluripotent Stem Cells Generated by Fusion of Adult Somatic Cells with Embryonic Stem Cells

Most cardiomyocytes (CMs) in the adult mammalian heart are either binucleated or contain a single polyploid nucleus. Recent studies have shown that polyploidy in CMs plays an important role as an adaptive response to physiological demands and environmental stress and correlates with poor cardiac regenerative ability after injury. However, knowledge about the functional properties of polyploid CMs is limited. In this study, we generated tetraploid pluripotent stem cells (PSCs) by fusion of murine embryonic stem cells (ESCs) and somatic cells isolated from bone marrow or spleen and performed a comparative analysis of the electrophysiological properties of tetraploid fusion-derived PSCs and diploid ESC-derived CMs. Fusion-derived PSCs exhibited characteristics of genuine ESCs and contained a near-tetraploid genome. Ploidy features and marker expression were also retained during the differentiation of fusion-derived cells. Fusion-derived PSCs gave rise to CMs, which were similar to their diploid ESC counterparts in terms of their expression of typical cardiospecific markers, sarcomeric organization, action potential parameters, response to pharmacologic stimulation with various drugs, and expression of functional ion channels. These results suggest that the state of ploidy does not significantly affect the structural and electrophysiological properties of murine PSC-derived CMs. These results extend our knowledge of the functional properties of polyploid CMs and contribute to a better understanding of their biological role in the adult heart.

Alteration of Ligamento-Muscular Reflex Patterns After Cutaneous and Periarticular Desensitization of the Basal Thumb Joint: An Electromyographic Study

PURPOSE

Stimulation of the dorsoradial ligament (DRL) of the first carpometacarpal joint (CMC-1) has shown a ligamento-muscular reflex pathway between the DRL and CMC-1 stabilizing muscles in healthy volunteers. However, it remains unclear how this ligamento-muscular reflex pattern is altered after anesthetizing sensory skin receptors and administering a further periarticular block around the CMC-1 joint, which may influence the dynamic aspects of joint stability.

METHODS

Ligamento-muscular reflexes were obtained from the extensor pollicis longus, abductor pollicis longus, abductor pollicis brevis, and the first dorsal interosseous muscles in 10 healthy participants after establishing superficial anesthesia of the skin around the CMC-1. The DRL was stimulated with a fine wire electrode while EMG activities were recorded during isometric tip, key, and palmar pinch. The measurements were repeated after an additional periarticular CMC-1 block using 5 ml of 1% lidocaine. Average EMG values were analyzed to compare the prestimulus and poststimulus activity.

RESULTS

Statistically significant changes in poststimulus EMG activity were observed in all 4 muscles and all 3 tested thumb positions. A markedly reduced activity in all 4 muscles was observed in the palmar position, followed by the tip and key pinch positions. Almost no reactions were observed in the first 20 ms poststimulus for all muscles in all positions.

CONCLUSIONS

Superficial skin anesthesia and an additional periarticular CMC-1 block anesthesia resulted in a reduced ligamento-muscular reflex pattern in all 4 muscles.

CLINICAL RELEVANCE

Ligamento-muscular reflexes play an important role in dynamic CMC-1 joint stability. The elimination of early reactions, those considered joint-protective reflexes, is a potential risk factor for developing osteoarthritis or injury because it results in an inability to adequately protect and stabilize the joint in sudden movements.

Hydrophobic Drug/Toxin Binding Sites in Voltage-Dependent K+ and Na+ Channels

In the Na_v channel family the lipophilic drugs/toxins binding sites and the presence of fenestrations in the channel pore wall are well defined and categorized. No such classification exists in the much larger K_v channel family, although certain lipophilic compounds seem to deviate from binding to well-known hydrophilic binding sites. By mapping different compound binding sites onto 3D structures of Kv channels, there appear to be three distinct lipid-exposed binding sites preserved in K_v channels: the front and back side of the pore domain, and S2-S3/S3-S4 clefts. One or a combination of these sites is most likely the orthologous equivalent of neurotoxin site 5 in Na_v channels. This review describes the different lipophilic binding sites and location of pore wall fenestrations within the K_v channel family and compares it to the knowledge of Na_v channels.

Ion Channels as Therapeutic Targets for Type 1 Diabetes Mellitus

Ion channels are integral proteins expressed in almost all living cells and are involved in muscle contraction and nutrient transport. They play a critical role in the normal functioning of the excitable tissues of the nervous system and regulate the action potential and contraction events. Dysfunction of genes encodes ion channel proteins, which disrupt the channel function and lead to a number of diseases, among which is type 1 diabetes mellitus (T1DM). Therefore, understanding the complex mechanism of ion channel receptors is necessary to facilitate the diagnosis and management of treatment. In this review, we summarize the mechanism of important ion channels and their potential role in the regulation of insulin secretion along with the limitations of ion channels as therapeutic targets. Furthermore, we discuss the recent investigations of the mechanism regulating the ion channels in pancreatic beta cells, which suggest that ion channels are active participants in the regulation of insulin secretion.

A novel NaV1.5 voltage sensor mutation associated with severe atrial and ventricular arrhythmias

BACKGROUND

Inherited autosomal dominant mutations in cardiac sodium channels (NaV1.5) cause various arrhythmias, such as long QT syndrome and Brugada syndrome. Although dozens of mutations throughout the protein have been reported, there are few reported mutations within a voltage sensor S4 transmembrane segment and few that are homozygous. Here we report analysis of a novel lidocaine-sensitive recessive mutation, p.R1309H, in the NaV1.5 DIII/S4 voltage sensor in a patient with a complex arrhythmia syndrome.

METHODS AND RESULTS

We expressed the wild type or mutant NaV1.5 heterologously for analysis with the patch-clamp and voltage clamp fluorometry (VCF) techniques. p.R1309H depolarized the voltage-dependence of activation, hyperpolarized the voltage-dependence of inactivation, and slowed recovery from inactivation, thereby reducing the channel availability at physiologic membrane potentials. Additionally, p.R1309H increased the "late" Na(+) current. The location of the mutation in DIIIS4 prompted testing for a gating pore current. We observed an inward current at hyperpolarizing voltages that likely exacerbates the loss-of-function defects at resting membrane potentials. Lidocaine reduced the gating pore current.

CONCLUSIONS

The p.R1309H homozygous NaV1.5 mutation conferred both gain-of-function and loss-of-function effects on NaV1.5 channel activity. Reduction of a mutation-induced gating pore current by lidocaine suggested a therapeutic mechanism.

Mechanisms underlying atrial-selective block of sodium channels by Wenxin Keli: Experimental and theoretical analysis

INTRODUCTION

Atrial-selective inhibition of cardiac sodium channel current (INa) and INa-dependent parameters has been shown to contribute to the safe and effective management of atrial fibrillation. The present study was designed to examine the basis for the atrial-selective actions of Wenxin Keli.

METHODS

Whole cell INa was recorded at room temperature in canine atrial and ventricular myocytes. Trains of 40 pulses were elicited over a range of pulse durations and interpulse intervals to determine tonic and use-dependent block. A Markovian model for INa that incorporates interaction of Wenxin Keli with different states of the channel was developed to examine the basis for atrial selectivity of the drug.

RESULTS

Our data indicate that Wenxin Keli does not bind significantly to either closed or open states of the sodium channel, but binds very rapidly to the inactivated state of the channel and dissociates rapidly from the closed state. Action potentials recorded from atrial and ventricular preparations in the presence of 5g/L Wenxin Keli were introduced into the computer model in current clamp mode to simulate the effects on maximum upstroke velocity (Vmax). The model predicted much greater inhibition of Vmax in atrial vs. ventricular cells at rapid stimulation rates.

CONCLUSION

Our findings suggest that atrial selectivity of Wenxin Keli to block INa is due to more negative steady-state inactivation, less negative resting membrane potential, and shorter diastolic intervals in atrial vs. ventricular cells at rapid activation rates. These actions of Wenxin Keli account for its relatively safe and effective suppression of atrial fibrillation.

Rate-dependent activation failure in isolated cardiac cells and tissue due to Na+ channel block

While it is well established that class-I antiarrhythmics block cardiac sodium channels, the mechanism of action of therapeutic levels of these drugs is not well understood. Using a combination of mathematical modeling and in vitro experiments, we studied the failure of activation of action potentials in single ventricular cells and in tissue caused by Na(+) channel block. Our computations of block and unblock of sodium channels by a theoretical class-Ib antiarrhythmic agent predict differences in the concentrations required to cause activation failure in single cells as opposed to multicellular preparations. We tested and confirmed these in silico predictions with in vitro experiments on isolated guinea-pig ventricular cells and papillary muscles stimulated at various rates (2-6.67 Hz) and exposed to various concentrations (5 × 10(-6) to 500 × 10(-6) mol/l) of lidocaine. The most salient result was that whereas large doses (5 × 10(-4) mol/l or higher) of lidocaine were required to inhibit action potentials temporarily in single cells, much lower doses (5 × 10(-6) mol/l), i.e., therapeutic levels, were sufficient to have the same effect in papillary muscles: a hundredfold difference. Our experimental results and mathematical analysis indicate that the syncytial nature of cardiac tissue explains the effects of clinically relevant doses of Na(+) channel blockers.

The role of late I Na in development of cardiac arrhythmias

Late I Na is an integral part of the sodium current, which persists long after the fast-inactivating component. The magnitude of the late I Na is relatively small in all species and in all types of cardiomyocytes as compared with the amplitude of the fast sodium current, but it contributes significantly to the shape and duration of the action potential. This late component had been shown to increase in several acquired or congenital conditions, including hypoxia, oxidative stress, and heart failure, or due to mutations in SCN5A, which encodes the α-subunit of the sodium channel, as well as in channel-interacting proteins, including multiple β subunits and anchoring proteins. Patients with enhanced late I Na exhibit the type-3 long QT syndrome (LQT3) characterized by high propensity for the life-threatening ventricular arrhythmias, such as Torsade de Pointes (TdP), as well as for atrial fibrillation. There are several distinct mechanisms of arrhythmogenesis due to abnormal late I Na, including abnormal automaticity, early and delayed after depolarization-induced triggered activity, and dramatic increase of ventricular dispersion of repolarization. Many local anesthetic and antiarrhythmic agents have a higher potency to block late I Na as compared with fast I Na. Several novel compounds, including ranolazine, GS-458967, and F15845, appear to be the most selective inhibitors of cardiac late I Na reported to date. Selective inhibition of late I Na is expected to be an effective strategy for correcting these acquired and congenital channelopathies.

Computational approaches to understand cardiac electrophysiology and arrhythmias

Cardiac rhythms arise from electrical activity generated by precisely timed opening and closing of ion channels in individual cardiac myocytes. These impulses spread throughout the cardiac muscle to manifest as electrical waves in the whole heart. Regularity of electrical waves is critically important since they signal the heart muscle to contract, driving the primary function of the heart to act as a pump and deliver blood to the brain and vital organs. When electrical activity goes awry during a cardiac arrhythmia, the pump does not function, the brain does not receive oxygenated blood, and death ensues. For more than 50 years, mathematically based models of cardiac electrical activity have been used to improve understanding of basic mechanisms of normal and abnormal cardiac electrical function. Computer-based modeling approaches to understand cardiac activity are uniquely helpful because they allow for distillation of complex emergent behaviors into the key contributing components underlying them. Here we review the latest advances and novel concepts in the field as they relate to understanding the complex interplay between electrical, mechanical, structural, and genetic mechanisms during arrhythmia development at the level of ion channels, cells, and tissues. We also discuss the latest computational approaches to guiding arrhythmia therapy.

The sodium channel as a target for local anesthetic drugs

Na channels are the source of excitatory currents for the nervous system and muscle. They are the target for a class of drugs called local anesthetics (LA), which have been used for local and regional anesthesia and for excitatory problems such as epilepsy and cardiac arrhythmia. These drugs are prototypes for new analgesic drugs. The drug-binding site has been localized to the inner pore of the channel, where drugs interact mainly with a phenylalanine in domain IV S6. Drug affinity is both voltage- and use-dependent. Voltage-dependency is the result of changes in the conformation of the inner pore during channel activation and opening, allowing high energy interaction of drugs with the phenylalanine. LA drugs also reduce the gating current of Na channels, which represents the movement of charged residues in the voltage sensors. Specifically, drug binding to phenylalanine locks the domain III S4 in its outward (activated) position, and slows recovery of the domain IV S4. Although strongly affecting gating, LA drugs almost certainly also block by steric occlusion of the pore. Molecular definition of the binding and blocking interactions may help in new drug development.

Mechanisms of atrial-selective block of Na⁺ channels by ranolazine: I. Experimental analysis of the use-dependent block

Atrial-selective inhibition of cardiac Na(+) channel current (I(Na)) and I(Na)-dependent parameters has been shown to contribute to the safe and effective management of atrial fibrillation. The present study examined the basis for the atrial-selective actions of ranolazine. Whole cell I(Na) was recorded at 15°C in canine atrial and ventricular myocytes and in human embryonic kidney (HEK)-293 cells expressing SCN5A. Tonic block was negligible at holding potentials from -140 to -100 mV, suggesting minimal drug interactions with the closed state. Trains of 40 pulses were elicited over a range of holding potentials to determine use-dependent block. Guarded receptor formalism was used to analyze the development of block during pulse trains. Use-dependent block by ranolazine increased at more depolarized holding potentials, consistent with an interaction of the drug with either preopen or inactivated states, but was unaffected by longer pulse durations between 5 and 200 ms, suggesting a weak interaction with the inactivated state. Block was significantly increased at shorter diastolic intervals between 20 and 200 ms. Responses in atrial and ventricular myocytes and in HEK-293 cells displayed a similar pattern. Ranolazine is an open state blocker that unbinds from closed Na(+) channels unusually fast but is trapped in the inactivated state. Kinetic rates of ranolazine interactions with different states of atrial and ventricular Na(+) channels were similar. Our data suggest that the atrial selectivity of ranolazine is due to a more negative steady-state inactivation curve, less negative resting membrane potential, and shorter diastolic intervals in atrial cells compared with ventricular cells at rapid rates.

Sodium channel molecular conformations and antiarrhythmic drug affinity

Class I cardiac antiarrhythmic drugs, for example, lidocaine, mexiletine, flecainide, quinidine, and procainamide, continue to play an important role in the therapy for cardiac arrhythmias because of the presence of use-dependent block. Lidocaine, as well as related drugs such as mepivacaine, bupivacaine, and cocaine, also belong to the class of medications referred to as local anesthetics. In this review, we will consider lidocaine as the prototypical antiarrhythmic drug because it continues to be widely used both as an antiarrhythmic drug (first used as an antiarrhythmic drug in 1950) as well as a local anesthetic agent. Both of these clinical uses depend upon block of sodium current (I(Na)), but it is the presence of use-dependent I(Na) block, that is, an increasing amount of block at faster heart rates, which enables a local anesthetic agent to be a useful antiarrhythmic drug. Although many early studies investigated the action of antiarrhythmic drugs on Na currents, the availability of site-directed mutant Na channels has enabled for major advances in understanding their mechanisms of action based upon molecular conformations of the Na channel.

Functional characterization of cardiomyocytes derived from murine induced pluripotent stem cells in vitro

Several types of terminally differentiated somatic cells can be reprogrammed into a pluripotent state by ectopic expression of Klf4, Oct3/4, Sox2, and c-Myc. Such induced pluripotent stem (iPS) cells have great potential to serve as an autologous source of cells for tissue repair. In the process of developing iPS-cell-based therapies, the major goal is to determine whether differentiated cells derived from iPS cells, such as cardiomyocytes (CMs), have the same functional properties as their physiological in vivo counterparts. Therefore, we differentiated murine iPS cells to CMs in vitro and characterized them by RT-PCR, immunocytochemistry, and electrophysiology. As key markers of cardiac lineages, transcripts for Nkx2.5, alphaMHC, Mlc2v, and cTnT could be identified. Immunocytochemical stainings revealed the presence of organized sarcomeric actinin but the absence of mature atrial natriuretic factor. We examined characteristics and developmental changes of action potentials, as well as functional hormonal regulation and sensitivity to channel blockers. In addition, we determined expression patterns and functionality of cardiac-specific voltage-gated Na+, Ca2+, and K+ channels at early and late differentiation stages and compared them with CMs derived from murine embryonic stem cells (ESCs) as well as with fetal CMs. We conclude that iPS cells give rise to functional CMs in vitro, with established hormonal regulation pathways and functionally expressed cardiac ion channels; CMs generated from iPS cells have a ventricular phenotype; and cardiac development of iPS cells is delayed compared with maturation of native fetal CMs and of ESC-derived CMs. This difference may reflect the incomplete reprogramming of iPS cells and should be critically considered in further studies to clarify the suitability of the iPS model for regenerative medicine of heart disorders.

The role of late I and antiarrhythmic drugs in EAD formation and termination in Purkinje fibers

Multiple components of cardiac Na current play a role in determining electrical excitation in the heart. Recently, the role of nonequilibrium components in controlling cardiac action potential plateau duration, and their importance in regulating the occurrence of afterdepolarizations and arrhythmias have garnered more attention. In particular, late Na current (late I(Na)) has been shown to be important in LQT2 and LQT3 arrhythmias. Class III agents like dofetilide, clofilium, and sotalol, which can all cause a drug-induced form of LQT2, significantly lengthen action potential duration at 50% and 90% repolarization in isolated rabbit Purkinje fibers, and can initiate the formation of early afterdepolarizations, and extra beats. These actions can lead to the development of a serious ventricular tachycardia, torsades de pointes, in animal models and patients. However, pretreatment with agents that block late I(Na), like lidocaine, mexiletine, and RSD1235, a novel mixed ion channel blocker for the rapid pharmacologic conversion of atrial fibrillation, significantly attenuates the prolonging effects of Class III agents or those induced by ATX-II, a specific toxin that delays Na channel inactivation and amplifies late I(Na) greatly, mimicking LQT3. The Na channel block caused by lidocaine and RSD1235 can be through the open or inactivated states of the channel, but both equivalently inhibit a late component of Na current (I(Na)), recorded at 22 degrees C using whole-cell patch clamp of Nav 1.5 expressed in HEK cells. These protective actions of lidocaine, mexiletine, and RSD1235 may result, at least in part, from their ability to inhibit late I(Na) during action potential repolarization, and inhibition of the inward currents contributing to EAD and arrhythmia formation.

Molecular action of lidocaine on the voltage sensors of sodium channels

Block of sodium ionic current by lidocaine is associated with alteration of the gating charge-voltage (Q-V) relationship characterized by a 38% reduction in maximal gating charge (Q(max)) and by the appearance of additional gating charge at negative test potentials. We investigated the molecular basis of the lidocaine-induced reduction in cardiac Na channel-gating charge by sequentially neutralizing basic residues in each of the voltage sensors (S4 segments) in the four domains of the human heart Na channel (hH1a). By determining the relative reduction in the Q(max) of each mutant channel modified by lidocaine we identified those S4 segments that contributed to a reduction in gating charge. No interaction of lidocaine was found with the voltage sensors in domains I or II. The largest inhibition of charge movement was found for the S4 of domain III consistent with lidocaine completely inhibiting its movement. Protection experiments with intracellular MTSET (a charged sulfhydryl reagent) in a Na channel with the fourth outermost arginine in the S4 of domain III mutated to a cysteine demonstrated that lidocaine stabilized the S4 in domain III in a depolarized configuration. Lidocaine also partially inhibited movement of the S4 in domain IV, but lidocaine's most dramatic effect was to alter the voltage-dependent charge movement of the S4 in domain IV such that it accounted for the appearance of additional gating charge at potentials near -100 mV. These findings suggest that lidocaine's actions on Na channel gating charge result from allosteric coupling of the binding site(s) of lidocaine to the voltage sensors formed by the S4 segments in domains III and IV.

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.

Block of wild-type and inactivation-deficient cardiac sodium channels IFM/QQQ stably expressed in mammalian cells

The role of inactivation as a central mechanism in blockade of the cardiac Na(+) channel by antiarrhythmic drugs remains uncertain. We have used whole-cell and single channel recordings to examine the block of wild-type and inactivation-deficient mutant cardiac Na(+) channels, IFM/QQQ, stably expressed in HEK-293 cells. We studied the open-channel blockers disopyramide and flecainide, and the lidocaine derivative RAD-243. All three drugs blocked the wild-type Na(+) channel in a use-dependent manner. There was no use-dependent block of IFM/QQQ mutant channels with trains of 20 40-ms pulses at 150-ms interpulse intervals during disopyramide exposure. Flecainide and RAD-243 retained their use-dependent blocking action and accelerated macroscopic current relaxation. All three drugs reduced the mean open time of single channels and increased the probability of their failure to open. From the abbreviation of the mean open times, we estimated association rates of approximately 10(6)/M/s for the three drugs. Reducing the burst duration contributed to the acceleration of macroscopic current relaxation during exposure to flecainide and RAD-243. The qualitative differences in use-dependent block appear to be the result of differences in drug dissociation rate. The inactivation gate may play a trapping role during exposure to some sodium channel blocking drugs.

Lidocaine induces a slow inactivated state in rat skeletal muscle sodium channels

1. Local anaesthetics such as lidocaine (lignocaine) interact with sodium channels in a manner that is exquisitely sensitive to the voltage-dependent conformational state of the ion channel. When depolarized in the presence of lidocaine, sodium channels assume a long-lived quiescent state. Although studies over the last decade have localized the lidocaine receptor to the inner aspect of the aqueous pore, the mechanistic basis of depolarization-induced 'use-dependent' lidocaine block remains uncertain. 2. Recent studies have shown that lowering the extracellular Na+ concentration ([Na+]o) and mutations in the sodium channel outer P-loop modulate occupancy of a quiescent 'slow' inactivated state with intermediate kinetics (termed IM) that involves structural rearrangements in the outer pore. 3. Site-directed mutagenesis and ion-replacement experiments were performed using voltage-clamped Xenopus oocytes and cultured (HEK-293) cells expressing wild-type and mutant rat skeletal muscle (mu1) sodium channels. 4. Our results show that lowering [Na+]o potentiates use-dependent lidocaine block. The effect of [Na+]o is maintained despite a III-IV linker mutation that partially disrupts fast inactivation (F1304Q). In contrast, the effect of lowering [Na+]o on lidocaine block is reduced by a P-loop mutation (W402A) that limits occupancy of IM. 5. Our findings are consistent with a simple allosteric model where lidocaine binding induces channels to occupy a native slow inactivated state that is inhibited by [Na+]o.

Effects of epoxyeicosatrienoic acids on the cardiac sodium channels in isolated rat ventricular myocytes

1. Whole-cell Na+ currents (holding potential, -80 mV; test potential, -30 mV) in rat myocytes were inhibited by 8, 9-epoxyeicosatrienoic acid (8,9-EET) in a dose-dependent manner with 22+/-4% inhibition at 0.5 microM, 48+/-5% at 1 microM, and 73+/-5% at 5 microM (mean +/- S.E.M., n = 10, P<0.05 for each dose vs. control). Similar results were obtained with 5,6-, 11,12-, and 14,15-EETs, while 8,9-dihydroxyeicosatrienoic acid (DHET) was 3-fold less potent and arachidonic acid was 10- to 20-fold less potent. 2. 8,9-EET produced a dose-dependent, hyperpolarized shift in the steady-state membrane potential at half-maximum inactivation (V ), without changing the slope factor. 8,9-EET had no effect on the steady-state activation of Na+ currents. 3. Inhibition of Na+ currents by 8,9-EET was use dependent, and channel recovery was slowed. The effects of 8,9-EET were greater at depolarized potentials. 4. Single channel recordings showed 8,9-EET did not change the conductance or the number of active Na+ channels, but markedly decreased the probability of Na+ channel opening. These results were associated with a decrease in the channel open time and an increase in the channel closed times. 5. Incubation of cultured cardiac myocytes with 1 microM [3H]8,9-EET showed that 25% of the radioactivity was taken up by the cells over a 2 h period, and most of the uptake was incorporated into phospholipids, principally phosphatidylcholine. Analysis of the medium after a 2 h incubation indicated that 86% of the radioactivity remained as [3H]8,9-EET while 13% was converted into [3H]8,9-DHET. After a 30 min incubation, 1-2% of the [3H]8,9-EET uptake by cells remained as unesterified EET. 6. These results demonstrate that cardiac cells have a high capacity to take up and metabolize 8,9-EET. 8,9-EET is a potent use- and voltage-dependent inhibitor of the cardiac Na+ channels through modulation of the channel gating behaviour.

Antiarrhythmic drugs and cardiac ion channels: mechanisms of action

In this review a description and an analysis are given of the interaction of antiarrhythmic drugs with their molecular target, i.e. ion channels and receptors. Our approach is based on the concept of vulnerable parameter, i.e. the electrophysiological property which plays a crucial role in the genesis of arrhythmias. To prevent or stop the arrhythmia a drug should modify the vulnerable parameter by its action on channel or receptor targets. In the first part, general aspects of the interaction between drugs channel molecules are considered. Drug binding depends on the state of the channel: rested, activated pre-open, activated open, or inactivated state. The change in channel behaviour with state is presented in the framework of the modulated-receptor hypothesis. Not only inhibition but also stimulation can be the result of drug binding. In the second part a detailed and systematic description and an analysis are given of the interaction of drugs with specific channels (Na+, Ca2+, K+, "pacemaker") and non-channel receptors. Emphasis is given to the type of state-dependent block involved (rested, activated and inactivated state block) and the change in channel kinetics. These properties vary and determine the voltage- and frequency-dependence of the change in ionic current. Finally, the question is asked as to whether the available drugs by their action on channels and receptors modify the vulnerable parameter in the desired way to stop or prevent arrhythmias.

The molecular interaction between local anesthetic/antiarrhythmic agents and voltage-gated sodium channels

Local anesthetic/antiarrhythmic agents render their therapeutic effects via suppression of ionic current through voltage-gated Na channels. Recent work to understand the molecular basis of this drug-receptor interaction has exploited the combined technologies of molecular biology and electrophysiology. Despite the complexity of the effects of site-directed mutations on Na channel function and local anesthetic action, some consistent themes are emerging. Recent studies suggest that the local anesthetic compounds actively promote channel inactivation and, in doing so, function as allosteric effectors. Although the charged moiety may enter the Na channel pore, the primary mechanism whereby local anesthetic agents reduce excitability may be to induce channel inactivation.

Pharmacological targeting of long QT mutant sodium channels

The congenital long QT syndrome (LQTS) is an inherited disorder characterized by a delay in cardiac cellular repolarization leading to cardiac arrhythmias and sudden death often in young people. One form of the disease (LQT3) involves mutations in the voltage-gated cardiac sodium channel. The potential for targeted suppression of the LQT defect was explored by heterologous expression of mutant channels in cultured human cells. Kinetic and steady state analysis revealed an enhanced apparent affinity for the predominantly charged, primary amine compound, mexiletine. The affinity of the mutant channels in the inactivated state was similar to the wild type (WT) channels (IC50 approximately 15-20 microM), but the late-opening channels were inhibited at significantly lower concentrations (IC50 = 2-3 microM) causing a preferential suppression of the late openings. The targeting of the defective behavior of the mutant channels has important implications for therapeutic intervention in this disease. The results provide insights for the selective suppression of the mutant phenotype by very low concentrations of drug and indicate that mexiletine equally suppresses the defect in all three known LQT3 mutants.

Potentiation of beta-adrenergic signaling by adenoviral-mediated gene transfer in adult rabbit ventricular myocytes

Our laboratory has been testing the hypothesis that genetic modulation of the beta-adrenergic signaling cascade can enhance cardiac function. We have previously shown that transgenic mice with cardiac overexpression of either the human beta2-adrenergic receptor (beta2AR) or an inhibitor of the beta-adrenergic receptor kinase (betaARK), an enzyme that phosphorylates and uncouples agonist-bound receptors, have increased myocardial inotropy. We now have created recombinant adenoviruses encoding either the beta2AR (Adeno-beta2AR) or a peptide betaARK inhibitor (consisting of the carboxyl terminus of betaARK1, Adeno-betaARKct) and tested their ability to potentiate beta-adrenergic signaling in cultured adult rabbit ventricular myocytes. As assessed by radioligand binding, Adeno-beta2AR infection led to approximately 20-fold overexpression of beta-adrenergic receptors. Protein immunoblots demonstrated the presence of the Adeno-betaARKct transgene. Both transgenes significantly increased isoproterenol-stimulated cAMP as compared to myocytes infected with an adenovirus encoding beta-galactosidase (Adeno-betaGal) but did not affect the sarcolemmal adenylyl cyclase response to Forskolin or NaF. beta-Adrenergic agonist-induced desensitization was significantly inhibited in Adeno-betaARKct-infected myocytes (16+/-2%) as compared to Adeno-betaGal-infected myocytes (37+/-1%, P < 0.001). We conclude that recombinant adenoviral gene transfer of the beta2AR or an inhibitor of betaARK-mediated desensitization can potentiate beta-adrenergic signaling.

Local anesthetics as effectors of allosteric gating. Lidocaine effects on inactivation-deficient rat skeletal muscle Na channels

Time- and voltage-dependent local anesthetic effects on sodium (Na) currents are generally interpreted using modulated receptor models that require formation of drug-associated nonconducting states with high affinity for the inactivated channel. The availability of inactivation-deficient Na channels has enabled us to test this traditional view of the drug-channel interaction. Rat skeletal muscle Na channels were mutated in the III-IV linker to disable fast inactivation (F1304Q: FQ). Lidocaine accelerated the decay of whole-cell FQ currents in Xenopus oocytes, reestablishing the wild-type phenotype; peak inward current at -20 mV was blocked with an IC50 of 513 microM, while plateau current was blocked with an IC50 of only 74 microM (P < 0.005 vs. peak). In single-channel experiments, mean open time was unaltered and unitary current was only reduced at higher drug concentrations, suggesting that open-channel block does not explain the effect of lidocaine on FQ plateau current. We considered a simple model in which lidocaine reduced the free energy for inactivation, causing altered coupling between activation and inactivation. This model readily simulated macroscopic Na current kinetics over a range of lidocaine concentrations. Traditional modulated receptor models which did not modify coupling between gating processes could not reproduce the effects of lidocaine with rate constants constrained by single-channel data. Our results support a reinterpretation of local anesthetic action whereby lidocaine functions as an allosteric effector to enhance Na channel inactivation.

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.

Distinct local anesthetic affinities in Na+ channel subtypes

Lidocaine is a widely used local anesthetic and antiarrhythmic drug that is believed to exert its clinically important action by blocking voltage-gated Na+ channels. Studies of Na+ channels from different species and tissues and the complexity of the drug-channel interaction create difficulty in understanding whether there are Na+ channel isoform specific differences in the affinity for lidocaine. Clinical usage suggests that lidocaine selectively targets cardiac Na+ channels because it is effective for the treatment of arrhythmias with few side effects on muscle or neuronal channels except at higher concentrations. One possibility for this selectivity is an intrinsically higher drug-binding affinity of the cardiac isoform. Alternatively, lidocaine may appear cardioselective because of preferential interactions with the inactivated state of the Na+ channel, which is occupied much longer in cardiac cells. Recombinant skeletal muscle (hSkM1) and cardiac sodium channels (hH1) were studied under identical conditions, with a whole-cell voltage clamp used to distinguish the mechanisms of lidocaine block. Tonic block at high concentrations of lidocaine (0.1 mM) was greater in hH1 than in hSkM1. This was also true for use-dependent block, for which 25-microM lidocaine produced an inhibition in hH1 equivalent to 0.1 mM in the skeletal muscle isoform. Pulse protocols optimized to explore inactivated-state block revealed that hSkM1 was five to eight times less sensitive to block by lidocaine than was hH1. The results also indicate that relatively more open-state block occurs in hSkM1. Thus, the cardiac sodium channel is intrinsically more sensitive to inhibition by lidocaine.

Three distinct components of the negative inotropic action of lidocaine in dog Purkinje fiber

1. The negative inotropic action of 10 microM lidocaine and 100 microM nicorandil was compared as a function of the pacing cycle length, ranging from 300-3000 ms, in isolated canine Purkinje fiber preparations. 2. The applied concentrations of lidocaine and nicorandil produced similar shortening of action potential duration; however, lidocaine compromised contractility stronger than nicorandil at each cycle length. 3. Normalizing the inotropic action of the drugs to their shortening effect on action potential duration, the negative inotropic action of lidocaine can be regarded as a sum of three distinct components: negative inotropy associated to the shortening of action potential duration per se, reduction of contractility likely due to direct inhibition of the fast sodium current and of the "window" sodium current by lidocaine.

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.

Basic concepts in cellular cardiac electrophysiology: Part II: Block of ion channels by antiarrhythmic drugs

Antiarrhythmic drugs have relative specificity for blocking each of the major classes of ion channels that control the action potential. The kinetics of block is determined by the state of the channel. Those channel states occupied at depolarized potentials generally have greater affinity for the blocking drugs. The kinetics of the drug-channel interaction is important in determining the blocking profile observed clinically. The increased mortality resulting from drug treatment in CAST and several atrial fibrillation trials has resulted in a shift in antiarrhythmic drug development from the Na+ channel blocking (Class I) drugs to the K+ channel blocking (Class III) drugs. While both Classes of drugs have a proarrhythmic potential, this may be less for the Class III agents. Their lack of negative inotropy also make them more attractive. It is important that the potential advantages of these agents be evaluated in controlled clinical trials. In several laboratories, the techniques of molecular biology and biophysics are being combined to determine the block site of available drugs. This information will aid in the future development of agents with greater specificity, and hopefully greater efficacy and safety than those currently in clinical use.

Lidocaine inhibits choline uptake and phosphatidylcholine biosynthesis in human leukemic monocyte-like U937 cells

The effect of lidocaine on [3H]choline uptake and the incorporation of label into phosphatidylcholine (PC) in human monocyte-like U937 cells was investigated. Lidocaine inhibited the rate of choline uptake in a dose-dependent manner; at 3.2 mM it resulted in a drastic reduction, by as much as 65 per cent (n = 10; p < 0.0005) or 55 per cent (n = 10; p < 0.0006) in a 3- or 6-h incubation, respectively. Lidocaine also decreased the rate of choline incorporation into PC in a dose-dependent manner. At the highest dose, nearly 70 per cent or 45 per cent reduction was seen in a 3- or 6-h incubation, respectively. Analysis of choline-containing metabolites showed that the major label association with phosphocholine and PC was reduced to a similar extent which was also parallel to the inhibition of choline uptake. At 3.2 mM lidocaine, the reduction of choline uptake was shown to follow a competitive inhibition. In the case of [3H] choline incorporation into PC, the inhibitory pattern was shown to be of a mixed type. The pulse-chase study dissecting the effect on choline metabolism from that on total choline uptake indicated that lidocaine exerted an additionally inhibitory effect on intracellular choline metabolism into PC. In a separate protocol in which the labelled cells were first allowed to be chased until 3H-incorporation into PC reached a steady state, lidocaine no longer showed any effect. These results seem to exclude the possibility of enhanced PC breakdown and further suggest that the main inhibitory effect is on the CDP-choline pathway for PC biosynthesis. After a 3-h treatment, CTP: cholinephosphate cytidylyltransferase (CYT) in both the cytosolic and microsomal fractions was inhibited by approximately 20 per cent, while choline kinase (CK) and choline phosphotransferase (CPT) remain relatively unchanged. There was no evidence for translocation of CYT between cytosol and microsomes. Taken together, we have demonstrated a dual inhibitory function of lidocaine which inhibits PC biosynthesis in addition to its ability to block choline uptake profoundly in U937 cells.

Chemically modified cardiac Na+ channels and their sensitivity to antiarrhythmics: is there a hidden drug receptor?

Elementary Na+ currents were recorded at 19 degrees C in inside-out patches from cultured neonatal rat cardiocytes. In analyzing the sensitivity of chemically modified Na+ channels to several class 1 antiarrhythmic drugs, the hypothesis was tested that removal of Na+ inactivation may be accompanied by a distinct responsiveness to these drugs, open channel blockade. Iodate-modified and trypsin-modified cardiac Na+ channels are noninactivating but strikingly differ from each other by their open state kinetics, a O1-O2 reaction (tau open(1) 1.4 +/- 0.3 msec; tau open(2) 5.4 +/- 1.1 msec; at -40 mV) in the former and a single open state (tau open 3.0 +/- 0.5 msec; at -40 mV) in the latter. Lidocaine (150 mumol/liter) like propafenone (10 mumol/liter), diprafenone (10 mumol/liter) and quinidine (20 mumol/liter) in cytoplasmic concentrations effective to depress NPo significantly can interact with both types of noninactivating Na+ channels to reduce the dwell time in the conducting configuration. Iodate-modified Na+ channels became drug sensitive during the O2 state. At -40 mV, for example, lidocaine reduced tau open(2) to 62 +/- 5% of the control without detectable changes in tau open(1). No evidence could be obtained that these inhibitory molecules would flicker-block the open Na+ pore. Drug-induced shortening of the open state, thus, is indicative for a distinct mode of drug action, namely interference with the gating process. Lidocaine proved less effective to reduce tau open(2) when compared with the action of diprafenone. Both drugs apparently interacted with individual association rate constants, a(lidocaine) was 0.64 x 10(6) mol-1 sec-1 and a(diprafenone) 13.6 x 10(6) mol-1 sec-1. Trypsin-modified Na+ channels also appear capable of discriminating among these antiarrhythmics, the ratio a(diprafenone)/a(lidocaine) even exceeded the value in iodate-modified Na+ channels. Obviously, this antiarrhythmic drug interaction with chemically modified Na+ channels is receptor mediated: drug occupation of such a hypothetical hidden receptor that is not available in normal Na+ channels may facilitate the exit from the open state.

Pharmacological modulation of human cardiac Na+ channels

Pharmacological modulation of human sodium current was examined in Xenopus oocytes expressing human heart Na+ channels. Na+ currents activated near -50 mV with maximum current amplitudes observed at -20 mV. Steady-state inactivation was characterized by a V1/2 value of -57 +/- 0.5 mV and a slope factor (k) of 7.3 +/- 0.3 mV. Sodium currents were blocked by tetrodotoxin with an IC50 value of 1.8 microM. These properties are consistent with those of Na+ channels expressed in mammalian myocardial cells. We have investigated the effects of several pharmacological agents which, with the exception of lidocaine, have not been characterized against cRNA-derived Na+ channels expressed in Xenopus oocytes. Lidocaine, quinidine and flecainide blocked resting Na+ channels with IC50 values of 521 microM, 198 microM, and 41 microM, respectively. Use-dependent block was also observed for all three agents, but concentrations necessary to induce block were higher than expected for quinidine and flecainide. This may reflect differences arising due to expression in the Xenopus oocyte system or could be a true difference in the interaction between human cardiac Na+ channels and these drugs compared to other mammalian Na+ channels. Importantly, however, this result would not have been predicted based upon previous studies of mammalian cardiac Na+ channels. The effects of DPI 201-106, RWJ 24517, and BDF 9148 were also tested and all three agents slowed and/or removed Na+ current inactivation, reduced peak current amplitudes, and induced use-dependent block. These data suggest that the alpha-subunit is the site of interaction between cardiac Na+ channels and Class I antiarrhythmic drugs as well as inactivation modifiers such as DPI 201-106.

Kinetics of interaction of disopyramide with the cardiac sodium channel: fast dissociation from open channels at normal rest potentials

Block of cardiac sodium channels is enhanced by repetitive depolarization. It is not clear whether the changes in drug binding result from a change in affinity that is dependent on voltage or on the actual state of the channel. This question was examined in rabbit ventricular myocytes by analyzing the kinetics of block of single sodium channel currents with normal gating kinetics or channels with inactivation and deactivation slowed by pyrethrin toxins. At -20 and -40 mV, disopyramide 100 microM blocked the unmodified channel. Mean open time decreased 45 and 34% at -20 and -40 mV during exposure to disopyramide. Exposure of cells to the pyrethrin toxins deltamethrin or fenvalrate caused at least a tenfold increase in mean open time, and prominent tail currents could be recorded at the normal resting potential. The association rate constant of disopyramide for the normal and modified channel at -20 mV was similar, approximately 10 x 10(6)/M/sec. During exposure to disopyramide, changes in open and closed times and in open channel noise at -80 and -100 mV are consistent with fast block and unblocking events at these potentials. This contrasts with the slow unbinding of drug from resting channels at similar potentials. We conclude that the sodium channel state is a critical determinant of drug binding and unbinding kinetics.

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.

Block of voltage-dependent sodium currents by the substance P receptor antagonist (+/-)-CP-96,345 in neurones cultured from rat cortex

1. Whole cell patch clamp recordings of voltage- and tetrodotoxin-sensitive Na+ currents were made from cultured rat neocortical neurones (E18). The effects of the non-peptide NK1 receptor antagonist, (+/-)-CP-96,345 on Na+ currents was examined, relative to the effect of the local anaesthetic lignocaine and tetrodotoxin. 2. Sodium currents were reversibly depressed by bath application of (+/-)-CP-96,345 with a half-maximally effective concentration of 18 +/- 2 microM at a stimulation frequency of 0.1 Hz. Likewise the concentrations required to half-maximally inhibit sodium currents by tetrodotoxin and lignocaine were 10 +/- 2 nM and 1.3 +/- 0.2 mM respectively. 3. The depression of sodium currents by (+/-)-CP-96,345 (10 microM) was use-dependent in that raising the stimulus frequency from 0.1 Hz to 10 Hz further decreased the magnitude of sodium currents from 60 +/- 5% to 37 +/- 5% of control values respectively. Similarly, the depression of sodium currents by lignocaine (500 microM) and tetrodotoxin (30 nM) was also accentuated by raising the stimulus frequency from 0.1 Hz to 10 Hz. 4. The effect of (+/-)-CP-96,345 was not associated with a change in either the activation or steady-state inactivation characteristics of these currents, suggesting that its mechanism of action was via open channel blockade. 5. These data demonstrate that in addition to antagonizing NK1 receptors, (+/-)-CP-96,345 also acts as a channel blocker on sodium channels at micromolar concentrations, an effect which should be taken into consideration when examining the antinociceptive or anti-inflammatory action of this compound.

Reversal of lidocaine effects on sodium currents by isoproterenol in rabbit hearts and heart cells

We demonstrated recently that isoproterenol enhanced the cardiac voltage-dependent sodium currents (INa) in rabbit ventricular myocytes through dual G-protein regulatory pathways. In this study, we tested the hypothesis that isoproterenol reverses the sodium channel blocking effects of class I antiarrhythmic drugs through modulation of INa. The experiments were performed in rabbit ventricular myocytes using whole-cell patch-clamp techniques. Reversal of lidocaine suppression of INa by isoproterenol (1 microM) was significant at various concentrations of lidocaine (20, 65, and 100 microM, P < 0.05). The effects of isoproterenol were voltage dependent, showing reversal of INa suppression by lidocaine at normal and hyperpolarized potentials (negative to -80 mV) but not at depolarized potentials. Isoproterenol enhanced sodium channel availability but did not alter the steady state activation or inactivation of INa nor did it improve sodium channel recovery in the presence of lidocaine. The physiological significance of the single cell INa findings were corroborated by measurements of conduction velocities using an epicardial mapping system in isolated rabbit hearts. Lidocaine (10 microM) significantly suppressed epicardial impulse conduction in both longitudinal (theta L, 0.430 +/- 0.024 vs. 0.585 +/- 0.001 m/s at baseline, n = 7, P < 0.001) and transverse (theta T, 0.206 +/- 0.012 vs. 0.257 +/- 0.014 m/s at baseline, n = 8, P < 0.001) directions. Isoproterenol (0.05 microM) significantly reversed the lidocaine effects with theta L of 0.503 +/- 0.027 m/s and theta T of 0.234 +/- 0.015 m/s (P = 0.014 and 0.004 compared with the respective lidocaine measurements). These results suggest that enhancement of INa is an important mechanism by which isoproterenol reverses the effects of class I antiarrhythmic drugs.

Na channel kinetics remain stable during perforated-patch recordings

The results of studies on modulation of Na channel function are often difficult to interpret due to time-dependent changes in channel kinetics. Although the "tight-seal" whole cell voltage-clamp technique has proved very useful in studying the properties of the cardiac Na current, the spontaneous shift of parameters of inactivation and activation gating to more negative potential is a serious limitation to the use of the technique. The shifts are believed to result from changes in the intracellular milieu effected by dialysis; moreover, use of a variety of different anions and cations in the internal micropipette solution has not obviated the problem. The perforated-patch technique permits low-resistance intracellular access without free dialysis between the intracellular solution and the recording micropipette. We have compared steady-state inactivation and peak current-voltage relationship of whole cell Na currents measured with the conventional whole cell and perforated-patch techniques in rabbit atrial myocytes at 17 degrees C. Although gating parameters shifted to more negative potentials when recorded with the conventional technique, stable kinetics could be observed for up to 150 min with the perforated-patch technique. The potential for one-half Na channel inactivation was -73 +/- 5.1 mV and is consistent with measurements made using indirect techniques such as upstroke velocity measurements. The fact that the intracellular milieu is left relatively intact makes the approach attractive for studying modulation of the Na current by neurotransmitters and hormones.

Blockade of cardiac sodium channels. Competition between the permeant ion and antiarrhythmic drugs

A number of basic and clinical studies suggest that elevation of external sodium concentrations, [Na]o, may reverse the cardiotoxic effect of local anesthetic-class drugs. The mechanisms of reversal are uncertain. The blocking action of lidocaine and disopyramide were studied over a range of [Na]o. Both whole-cell voltage clamp and single-channel recordings were performed on isolated rabbit myocytes at 17 and 22 degrees C, respectively. In the presence of lidocaine, an inactivated channel blocker, the level of steady-state block in response to pulse train stimulation was not affected by variations in [Na]o from 20 to 150 mM. Estimates of the rate of dissociation of drug from the channel also were unaffected. In contrast, steady-state block by disopyramide, a drug that blocks open channels, was decreased as [Na]o was increased. Single-channel measurements suggest that the influence of [Na]o on channel current amplitude was small, 12% for a 25 mM increase in [Na]o. This increase in single-channel current amplitude would affect drug-free channels only, in that our studies suggest that drug-associated channels do not conduct. The association rate constant of disopyramide with open single sodium channels was decreased from 10 x 10(6) to 5 x 10(6)/M per s by an increase in [Na]o from 120 to 180 mM. Elevation of [Na]o may reverse the blocking action of local anesthetic-class drugs by an increase in single-channel current amplitude or by a decrease in drug association rate with the sodium channel. The occurrence of the latter action depends on the mode of block of the specific agent.

Block and modulation of cardiac Na+ channels by antiarrhythmic drugs, neurotransmitters and hormones

The Na+ channel is an important target for the action of antiarrhythmic drugs. Application of contemporary biophysical, biochemical and molecular biological techniques have added considerably to our knowledge of its structure, function, modulation and block by antiarrhythmic drugs. The increased mortality from the use of these drugs for prophylaxis of cardiac arrhythmias has forced a re-evaluation of their use and of the entire pharmacological strategy of arrhythmia management. Gus Grant and David Wendt review recent studies on the block and modulation of cardiac Na+ channels and the place of Na+ channel blockers in future antiarrhythmic drug development.

Responsiveness of cardiac Na+ channels to antiarrhythmic drugs: the role of inactivation

Elementary Na+ currents were recorded at 9 degrees C in inside-out patches from cultured neonatal rat heart myocytes. In characterizing the sensitivity of cooled, slowly inactivating cardiac Na+ channels to several antiarrhythmic drugs including propafenone, lidocaine and quinidine, the study aimed to define the role of Na+ inactivation for open channel blockade. In concentrations (1-10 mumol/liter) effective to depress NPo significantly, propafenone completely failed to influence the open state of slowly inactivating Na+ channels. With 1 mumol/liter, tau open (at -45 mV) in cooled, (-)-DPI-modified, noninactivating Na+ channels proved to be drug resistant and could not be flicker-blocked by 10 mumol/liter propafenone. The same drug concentration induced in (-)-DPI-modified Na+ channels a discrete block with association and dissociation rate constants of 16.1 +/- 5.3 x 10(6) mol-1 sec-1 and 675 +/- 25 sec-1, respectively. Quinidine, known to have a considerable affinity for activated Na+ channels, in lower concentrations (5 mumol/liter) left tau open unchanged or reduced, in higher concentrations (10 mumol/liter) tau open only slightly to 81% of the predrug value whereas NPo declined to 30%, but repetitive blocking events during the conducting state could never be observed. Basically the same drug resistance of the open state was seen in cardiac Na+ channels whose open-state kinetics had been modulated by the cytoplasmic presence of F- ions. But in this case, propafenone reduced reopening and selectively abolished a long-lasting open state. This drug action is unlikely related to the inhibitory effect on NPo since hyperpolarization and the accompanying block attenuation did not restore the channel kinetics. It is concluded that cardiac Na+ channels cannot be flicker-blocked by antiarrhythmic drugs unless Na+ inactivation is removed.

Na+ channel blockade by cyclic AMP and other 6-aminopurines in neonatal rat heart

Elementary Na+ currents were recorded at 19 degrees C in cell attached and inside-out patches from cultured neonatal rat cardiocytes in order to study the effect of cAMP and other 6-aminopurines. The treatment of the cardiocytes with db-cAMP (1 x 10(-3) mol/liter) led to a decline of reconstructed macroscopic peak INa to 62 +/- 7.6% of the initial control value. This reduction in NPo was mostly accompanied by a decrease in burst activity. Open-state kinetics were preserved even in DPI-modified, noninactivating Na+ channels. Since the stimulator of the adenylate cyclase, forskolin (1 x 10(-6) mol/liter), evoked a similar pattern of response, the NPo decrease can be considered as the functional correlate of Na+ channel phosphorylation brought about by cAMP-dependent protein kinase. As found in inside-out patches, cAMP (1 x 10(-3) mol/liter) remained effective under cell-free conditions and reduced reconstructed macroscopic peak INa to about 50% of the initial control value when the absence of Mg-ATP at the cytoplasmic membrane surface prevents phosphorylation reactions. A very similar response developed in the cytoplasmic presence of other 6-aminopurines including ATP (1 x 10(3) mol/liter), adenosine (1 x 10(-4) mol/liter), adenine (1 x 10(-5) mol/liter) and hypoxanthine (1 x 10(-5) mol/liter). This susceptibility to adenine suggests that cardiac Na+ channels in situ could sense intracellular fluctuations of adenine nucleotides, most likely of ATP.