On the molecular nature of the lidocaine receptor of cardiac Na+ channels. Modification of block by alterations in the alpha-subunit III-IV interdomain

Automated fabrication of a scalable heart-on-a-chip device by 3D printing of thermoplastic elastomer nanocomposite and hot embossing

The successful translation of organ-on-a-chip devices requires the development of an automated workflow for device fabrication, which is challenged by the need for precise deposition of multiple classes of materials in micro-meter scaled configurations. Many current heart-on-a-chip devices are produced manually, requiring the expertise and dexterity of skilled operators. Here, we devised an automated and scalable fabrication method to engineer a Biowire II multiwell platform to generate human iPSC-derived cardiac tissues. This high-throughput heart-on-a-chip platform incorporated fluorescent nanocomposite microwires as force sensors, produced from quantum dots and thermoplastic elastomer, and 3D printed on top of a polystyrene tissue culture base patterned by hot embossing. An array of built-in carbon electrodes was embedded in a single step into the base, flanking the microwells on both sides. The facile and rapid 3D printing approach efficiently and seamlessly scaled up the Biowire II system from an 8-well chip to a 24-well and a 96-well format, resulting in an increase of platform fabrication efficiency by 17,5000-69,000% per well. The device's compatibility with long-term electrical stimulation in each well facilitated the targeted generation of mature human iPSC-derived cardiac tissues, evident through a positive force-frequency relationship, post-rest potentiation, and well-aligned sarcomeric apparatus. This system's ease of use and its capacity to gauge drug responses in matured cardiac tissue make it a powerful and reliable platform for rapid preclinical drug screening and development.

Iontophoresis on Porcine and Human Gingiva

PURPOSE

Iontophoresis is a noninvasive method that enhances drug delivery using an electric field. This method can improve drug delivery to the tissues in the oral cavity. The effects of iontophoresis on gingival drug delivery have not been investigated. The objectives of this study were to (a) determine the flux enhancement of model permeants across porcine and human gingiva during iontophoresis, (b) examine the transport mechanisms of gingival iontophoresis, and (c) evaluate the potential of iontophoretically enhanced delivery for three model drugs lidocaine, ketorolac, and chlorhexidine.

METHODS

Passive and iontophoretic fluxes were determined with porcine and human gingiva using a modified Franz diffusion cell and model drugs and permeants. To investigate the transport mechanisms of iontophoresis, the enhancement from the direct-field effect was determined by positively and negatively charged model permeants. The electroosmosis enhancement effect was determined with neutral permeants of different molecular weight. The alteration of the gingival barrier due to electropermeabilization was evaluated using electrical resistance measurements.

RESULTS

Significant flux enhancement was observed during gingival iontophoresis. The direct-field effect was the major mechanism governing the iontophoretic transport of the charged permeants. Electroosmosis was from anode to cathode. The effective pore radius of the iontophoretic transport pathways in the porcine gingiva was ~0.68 nm. Irreversible electropermeabilization was observed after 2 and 4 h of iontophoresis under the conditions studied.

CONCLUSION

Iontophoresis could enhance drug delivery and reduce transport lag time, showing promise for gingival drug delivery.

Sodium Channels and Local Anesthetics-Old Friends With New Perspectives

The long history of local anesthetics (LAs) starts out in the late 19th century when the content of coca plant leaves was discovered to alleviate pain. Soon after, cocaine was established and headed off to an infamous career as a substance causing addiction. Today, LAs and related substances-in modified form-are indispensable in our clinical everyday life for pain relief during and after minor and major surgery, and dental practices. In this review, we elucidate on the interaction of modern LAs with their main target, the voltage-gated sodium channel (Navs), in the light of the recently published channel structures. Knowledge of the 3D interaction sites of the drug with the protein will allow to mechanistically substantiate the comprehensive data available on LA gating modification. In the 1970s it was suggested that LAs can enter the channel pore from the lipid phase, which was quite prospective at that time. Today we know from cryo-electron microscopy structures and mutagenesis experiments, that indeed Navs have side fenestrations facing the membrane, which are likely the entrance for LAs to induce tonic block. In this review, we will focus on the effects of LA binding on fast inactivation and use-dependent inhibition in the light of the proposed new allosteric mechanism of fast inactivation. We will elaborate on subtype and species specificity and provide insights into modelling approaches that will help identify the exact molecular binding orientation, access pathways and pharmacokinetics. With this comprehensive overview, we will provide new perspectives in the use of the drug, both clinically and as a tool for basic ion channel research.

Conformations of voltage-sensing domain III differentially define NaV channel closed- and open-state inactivation

Voltage-gated Na⁺ (Na_V) channels underlie the initiation and propagation of action potentials (APs). Rapid inactivation after Na_V channel opening, known as open-state inactivation, plays a critical role in limiting the AP duration. However, Na_V channel inactivation can also occur before opening, namely closed-state inactivation, to tune the cellular excitability. The voltage-sensing domain (VSD) within repeat IV (VSD-IV) of the pseudotetrameric Na_V channel α-subunit is known to be a critical regulator of Na_V channel inactivation. Yet, the two processes of open- and closed-state inactivation predominate at different voltage ranges and feature distinct kinetics. How inactivation occurs over these different ranges to give rise to the complexity of Na_V channel dynamics is unclear. Past functional studies and recent cryo-electron microscopy structures, however, reveal significant inactivation regulation from other Na_V channel components. In this Hypothesis paper, we propose that the VSD of Na_V repeat III (VSD-III), together with VSD-IV, orchestrates the inactivation-state occupancy of Na_V channels by modulating the affinity of the intracellular binding site of the IFMT motif on the III-IV linker. We review and outline substantial evidence that VSD-III activates in two distinct steps, with the intermediate and fully activated conformation regulating closed- and open-state inactivation state occupancy by altering the formation and affinity of the IFMT crevice. A role of VSD-III in determining inactivation-state occupancy and recovery from inactivation suggests a regulatory mechanism for the state-dependent block by small-molecule anti-arrhythmic and anesthetic therapies.

Functional cross-talk between phosphorylation and disease-causing mutations in the cardiac sodium channel Nav1.5

The cardiac sodium channel (Na_v1.5) is crucial for generating a regular heartbeat. It is thus not surprising that Na_v1.5 mutations have been linked to life-threatening arrhythmias. Interestingly, Na_v1.5 activity can also be altered by posttranslational modifications, such as tyrosine phosphorylation. Our combination of protein engineering and molecular modeling has revealed that the detrimental effect of a long QT3 patient mutation is only exposed when a proximal tyrosine is phosphorylated. This suggests a dynamic cross-talk between the genetic mutation and a neighboring phosphorylation, a phenomenon that could be important in other classes of proteins. Additionally, we show that phosphorylation can affect the channel’s sensitivity toward clinically relevant drugs, a finding that may prove important when devising patient-specific treatment plans.

The voltage-gated sodium channel Na_v1.5 initiates the cardiac action potential. Alterations of its activation and inactivation properties due to mutations can cause severe, life-threatening arrhythmias. Yet despite intensive research efforts, many functional aspects of this cardiac channel remain poorly understood. For instance, Na_v1.5 undergoes extensive posttranslational modification in vivo, but the functional significance of these modifications is largely unexplored, especially under pathological conditions. This is because most conventional approaches are unable to insert metabolically stable posttranslational modification mimics, thus preventing a precise elucidation of the contribution by these modifications to channel function. Here, we overcome this limitation by using protein semisynthesis of Na_v1.5 in live cells and carry out complementary molecular dynamics simulations. We introduce metabolically stable phosphorylation mimics on both wild-type (WT) and two pathogenic long-QT mutant channel backgrounds and decipher functional and pharmacological effects with unique precision. We elucidate the mechanism by which phosphorylation of Y1495 impairs steady-state inactivation in WT Na_v1.5. Surprisingly, we find that while the Q1476R patient mutation does not affect inactivation on its own, it enhances the impairment of steady-state inactivation caused by phosphorylation of Y1495 through enhanced unbinding of the inactivation particle. We also show that both phosphorylation and patient mutations can impact Na_v1.5 sensitivity toward the clinically used antiarrhythmic drugs quinidine and ranolazine, but not flecainide. The data highlight that functional effects of Na_v1.5 phosphorylation can be dramatically amplified by patient mutations. Our work is thus likely to have implications for the interpretation of mutational phenotypes and the design of future drug regimens.

Rate-dependent effects of lidocaine on cardiac dynamics: Development and analysis of a low-dimensional drug-channel interaction model

State-dependent sodium channel blockers are often prescribed to treat cardiac arrhythmias, but many sodium channel blockers are known to have pro-arrhythmic side effects. While the anti and proarrhythmic potential of a sodium channel blocker is thought to depend on the characteristics of its rate-dependent block, the mechanisms linking these two attributes are unclear. Furthermore, how specific properties of rate-dependent block arise from the binding kinetics of a particular drug is poorly understood. Here, we examine the rate-dependent effects of the sodium channel blocker lidocaine by constructing and analyzing a novel drug-channel interaction model. First, we identify the predominant mode of lidocaine binding in a 24 variable Markov model for lidocaine-sodium channel interaction by Moreno et al. Specifically, we find that (1) the vast majority of lidocaine bound to sodium channels is in the neutral form, i.e., the binding of charged lidocaine to sodium channels is negligible, and (2) neutral lidocaine binds almost exclusively to inactivated channels and, upon binding, immobilizes channels in the inactivated state. We then develop a novel 3-variable lidocaine-sodium channel interaction model that incorporates only the predominant mode of drug binding. Our low-dimensional model replicates an extensive amount of the voltage-clamp data used to parameterize the Moreno et al. model. Furthermore, the effects of lidocaine on action potential upstroke velocity and conduction velocity in our model are similar to those predicted by the Moreno et al. model. By exploiting the low-dimensionality of our model, we derive an algebraic expression for level of rate-dependent block as a function of pacing frequency, restitution properties, diastolic and plateau potentials, and drug binding rate constants. Our model predicts that the level of rate-dependent block is sensitive to alterations in restitution properties and increases in diastolic potential, but it is insensitive to variations in the shape of the action potential waveform and lidocaine binding rates.

Cardiac arrhythmias are often treated with drugs that block and alter the kinetics of membrane sodium channels. However, different drugs interact with sodium channels in different ways, and the complexity of the drug-channel interactions makes it difficult to predict whether a particular sodium channel blocker will reduce or increase the probability of cardiac arrhythmias. Here, we characterize the binding kinetics and effects on electrical signal propagation of the antiarrhythmic drug lidocaine, which is an archetypical example of a safe sodium channel blocker. Through analysis of a high-dimensional biophysically-detailed model of lidocaine-sodium channel interaction, we identify the predominant lidocaine binding pathway. We then incorporate only the key features of the predominant binding pathway into a novel low-dimensional model of lidocaine-sodium channel interaction. Our analysis of the low-dimensional model characterizes how the key binding properties of lidocaine affect electrical signal generation and propagation in the heart, and therefore our results are a step towards understanding the features that differentiate pro- and antiarrhythmic sodium channel blockers.

Identification of a critical binding site for local anaesthetics in the side pockets of Kv 1 channels

BACKGROUND AND PURPOSE

Local anaesthetics block sodium and a variety of potassium channels. Although previous studies identified a residue in the pore signature sequence together with three residues in the S6 segment as a putative binding site, the precise molecular basis of inhibition of K_v channels by local anaesthetics remained unknown. Crystal structures of K_v channels predict that some of these residues point away from the central cavity and face into a drug binding site called side pockets. Thus, the question arises whether the binding site of local anaesthetics is exclusively located in the central cavity or also involves the side pockets.

EXPERIMENTAL APPROACH

A systematic functional alanine mutagenesis approach, scanning 58 mutants, together with in silico docking experiments and molecular dynamics simulations was utilized to elucidate the binding site of bupivacaine and ropivacaine.

KEY RESULTS

Inhibition of K_v 1.5 channels by local anaesthetics requires binding to the central cavity and the side pockets, and the latter requires interactions with residues of the S5 and the back of the S6 segments. Mutations in the side pockets remove stereoselectivity of inhibition of K_v 1.5 channels by bupivacaine. Although binding to the side pockets is conserved for different local anaesthetics, the binding mode in the central cavity and the side pockets shows considerable variations.

CONCLUSION AND IMPLICATIONS

Local anaesthetics bind to the central cavity and the side pockets, which provide a crucial key to the molecular understanding of their K_v channel affinity and stereoselectivity, as well as their spectrum of side effects.

Novel G1481V and Q1491H SCN5A Mutations Linked to Long QT Syndrome Destabilize the Nav1.5 Inactivation State

Background

Na_v1.5, which is encoded by the SCN5A gene, is the predominant voltage-gated Na⁺ channel in the heart. Several mutations of this gene have been identified and reported to be involved in several cardiac rhythm disorders, including type 3 long QT interval syndrome, that can cause sudden cardiac death. We analyzed the biophysical properties of 2 novel variants of the Na_v1.5 channel (Q1491H and G1481V) detected in 5- and 12-week-old infants diagnosed with a prolonged QT interval.

Methods

The Na_v1.5 wild-type and the Q1491H and G1481V mutant channels were reproduced in vitro. Wild-type or mutant channels were cotransfected in human embryonic kidney (HEK) 293 cells with the beta 1 regulatory subunit. Na⁺ currents were recorded using the whole-cell configuration of the patch-clamp technique.

Results

The Q1491H mutant channel exhibited a lower current density, a persistent Na⁺ current, an enhanced window current due to a +20-mV shift of steady-state inactivation, a +10-mV shift of steady-state activation, a faster onset of slow inactivation, and a recovery from fast inactivation with fast and slow time constants of recovery. The G1481V mutant channel exhibited an increase in current density and a +7-mV shift of steady-state inactivation. The observed defects are characteristic of gain-of-function mutations typical of type 3 long QT interval syndrome.

Conclusions

The 5- and 12-week-old infants displayed prolonged QT intervals. Our analyses of the Q1491H and G1481V mutations correlated with the clinical diagnosis. The observed biophysical dysfunctions associated with both mutations were most likely responsible for the sudden deaths of the 2 infants.

Dexmedetomidine protects against lidocaine-induced neurotoxicity through SIRT1 downregulation-mediated activation of FOXO3a

Lidocaine, a typical local anesthetic, has been shown to directly induce neurotoxicity in clinical settings. Dexmedetomidine (DEX) is an alpha-2-adrenoreceptor agonist that has been used as anxiolytic, sedative, and analgesic agent which has recently found to protect against lidocaine-induced neurotoxicity. Nicotinamide adenine dinucleotide-dependent deacetylase sirtuin-1 (SIRT1)/forkhead box O3 (FOXO3a) signaling is critical for maintaining neuronal function and regulation of the apoptotic pathway. In the present study, we designed in vitro and in vivo models to investigate the potential effects of lidocaine and DEX on SIRT1 and FOXO3a and to verify whether SIRT1/FOXO3a-mediated regulation of apoptosis is involved in DEX-induced neuroprotective effects against lidocaine. We found that in both PC12 cells and brains of mice, lidocaine decreased SIRT1 level through promoting the degradation of SIRT1 protein. Lidocaine also increased FOXO3a protein level and increased the acetylation of SIRT1 through inhibiting SIRT1. Upregulation of SIRT1 or downregulation of FOXO3a significantly inhibited lidocaine-induced changes in both cell viability and apoptosis. DEX significantly inhibited the lidocaine-induced decrease of SIRT1 protein level and increase of FOXO3a protein level and acetylation of FOXO3a. Downregulation of SIRT1 or upregulation of FOXO3a suppressed DEX-induced neuroprotective effects against lidocaine. The data suggest that SIRT1/FOXO3a is a potential novel target for alleviating lidocaine-induced neurotoxicity and provide more theoretical support for the use of DEX as an effective adjunct to alleviate chronic neurotoxicity induced by lidocaine.

Towards chamber specific heart-on-a-chip for drug testing applications

Modeling of human organs has long been a task for scientists in order to lower the costs of therapeutic development and understand the pathological onset of human disease. For decades, despite marked differences in genetics and etiology, animal models remained the norm for drug discovery and disease modeling. Innovative biofabrication techniques have facilitated the development of organ-on-a-chip technology that has great potential to complement conventional animal models. However, human organ as a whole, more specifically the human heart, is difficult to regenerate in vitro, in terms of its chamber specific orientation and its electrical functional complexity. Recent progress with the development of induced pluripotent stem cell differentiation protocols, made recapitulating the complexity of the human heart possible through the generation of cells representative of atrial & ventricular tissue, the sinoatrial node, atrioventricular node and Purkinje fibers. Current heart-on-a-chip approaches incorporate biological, electrical, mechanical, and topographical cues to facilitate tissue maturation, therefore improving the predictive power for the chamber-specific therapeutic effects targeting adult human. In this review, we will give a summary of current advances in heart-on-a-chip technology and provide a comprehensive outlook on the challenges involved in the development of human physiologically relevant heart-on-a-chip.

Pharmacological Profile of the Sodium Current in Human Stem Cell-Derived Cardiomyocytes Compares to Heterologous Nav1.5+β1 Model

The cardiac Nav1.5 mediated sodium current (I_Na) generates the upstroke of the action potential in atrial and ventricular myocytes. Drugs that modulate this current can therefore be antiarrhythmic or proarrhythmic, which requires preclinical evaluation of their potential drug-induced inhibition or modulation of Nav1.5. Since Nav1.5 assembles with, and is modulated by, the auxiliary β1-subunit, this subunit can also affect the channel’s pharmacological response. To investigate this, the effect of known Nav1.5 inhibitors was compared between COS-7 cells expressing Nav1.5 or Nav1.5+β1 using whole-cell voltage clamp experiments. For the open state class Ia blockers ajmaline and quinidine, and class Ic drug flecainide, the affinity did not differ between both models. For class Ib drugs phenytoin and lidocaine, which are inactivated state blockers, the affinity decreased more than a twofold when β1 was present. Thus, β1 did not influence the affinity for the class Ia and Ic compounds but it did so for the class Ib drugs. Human stem cell-derived cardiomyocytes (hSC-CMs) are a promising translational cell source for in vitro models that express a representative repertoire of channels and auxiliary proteins, including β1. Therefore, we subsequently evaluated the same drugs for their response on the I_Na in hSC-CMs. Consequently, it was expected and confirmed that the drug response of I_Na in hSC-CMs compares best to I_Na expressed by Nav1.5+β1.

Mechanisms of Drug Binding to Voltage-Gated Sodium Channels

Voltage-gated sodium (Na⁺) channels are expressed in virtually all electrically excitable tissues and are essential for muscle contraction and the conduction of impulses within the peripheral and central nervous systems. Genetic disorders that disrupt the function of these channels produce an array of Na⁺ channelopathies resulting in neuronal impairment, chronic pain, neuromuscular pathologies, and cardiac arrhythmias. Because of their importance to the conduction of electrical signals, Na⁺ channels are the target of a wide variety of local anesthetic, antiarrhythmic, anticonvulsant, and antidepressant drugs. The voltage-gated family of Na⁺ channels is composed of α-subunits that encode for the voltage sensor domains and the Na⁺-selective permeation pore. In vivo, Na⁺ channel α-subunits are associated with one or more accessory β-subunits (β₁-β₄) that regulate gating properties, trafficking, and cell-surface expression of the channels. The permeation pore of Na⁺ channels is divided in two parts: the outer mouth of the pore is the site of the ion selectivity filter, while the inner cytoplasmic pore serves as the channel activation gate. The cytoplasmic lining of the permeation pore is formed by the S6 segments that include highly conserved aromatic amino acids important for drug binding. These residues are believed to undergo voltage-dependent conformational changes that alter drug binding as the channels cycle through the closed, open, and inactivated states. The purpose of this chapter is to broadly review the mechanisms of Na⁺ channel gating and the models used to describe drug binding and Na⁺ channel inhibition.

Probing kinetic drug binding mechanism in voltage-gated sodium ion channel: open state versus inactive state blockers

The kinetics and nonequilibrium thermodynamics of open state and inactive state drug binding mechanisms have been studied here using different voltage protocols in sodium ion channel. We have found that for constant voltage protocol, open state block is more efficient in blocking ionic current than inactive state block. Kinetic effect comes through peak current for mexiletine as an open state blocker and in the tail part for lidocaine as an inactive state blocker. Although the inactivation of sodium channel is a free energy driven process, however, the two different kinds of drug affect the inactivation process in a different way as seen from thermodynamic analysis. In presence of open state drug block, the process initially for a long time remains entropy driven and then becomes free energy driven. However in presence of inactive state block, the process remains entirely entropy driven until the equilibrium is attained. For oscillating voltage protocol, the inactive state blocking is more efficient in damping the oscillation of ionic current. From the pulse train analysis it is found that inactive state blocking is less effective in restoring normal repolarisation and blocks peak ionic current. Pulse train protocol also shows that all the inactive states behave differently as one inactive state responds instantly to the test pulse in an opposite manner from the other two states.

The Receptor Site and Mechanism of Action of Sodium Channel Blocker Insecticides

Sodium channels are excellent targets of both natural and synthetic insecticides with high insect selectivity. Indoxacarb, its active metabolite DCJW, and metaflumizone (MFZ) belong to a relatively new class of sodium channel blocker insecticides (SCBIs) with a mode of action distinct from all other sodium channel-targeting insecticides, including pyrethroids. Electroneutral SCBIs preferably bind to and trap sodium channels in the inactivated state, a mechanism similar to that of cationic local anesthetics. Previous studies identified several SCBI-sensing residues that face the inner pore of sodium channels. However, the receptor site of SCBIs, their atomic mechanisms, and the cause of selective toxicity of MFZ remain elusive. Here, we have built a homology model of the open-state cockroach sodium channel BgNav1-1a. Our computations predicted that SCBIs bind in the inner pore, interact with a sodium ion at the focus of P1 helices, and extend their aromatic moiety into the III/IV domain interface (fenestration). Using model-driven mutagenesis and electrophysiology, we identified five new SCBI-sensing residues, including insect-specific residues. Our study proposes the first three-dimensional models of channel-bound SCBIs, sheds light on the molecular basis of MFZ selective toxicity, and suggests that a sodium ion located in the inner pore contributes to the receptor site for electroneutral SCBIs.

Understanding Sodium Channel Function and Modulation Using Atomistic Simulations of Bacterial Channel Structures

Sodium channels are chief proteins involved in electrical signaling in the nervous system, enabling critical functions like heartbeat and brain activity. New high-resolution X-ray structures for bacterial sodium channels have created an opportunity to see how these proteins operate at the molecular level. An important challenge to overcome is establishing relationships between the structures and functions of mammalian and bacterial channels. Bacterial sodium channels are known to exhibit the main structural features of their mammalian counterparts, as well as several key functional characteristics, including selective ion conduction, voltage-dependent gating, pore-based inactivation and modulation by local anesthetic, antiarrhythmic and antiepileptic drugs. Simulations have begun to shed light on each of these features in the past few years. Despite deviations in selectivity signatures for bacterial and mammalian channels, simulations have uncovered the nature of the multiion conduction mechanism associated with Na(+) binding to a high-field strength site established by charged glutamate side chains. Simulations demonstrated a surprising level of flexibility of the protein, showing that these side chains are active participants in the permeation process. They have also uncovered changes in protein structure, leading to asymmetrical collapses of the activation gate that have been proposed to correspond to inactivated structures. These observations offer the potential to examine the mechanisms of state-dependent drug activity, focusing on pore-blocking and pore-based slow inactivation in bacterial channels, without the complexities of inactivation on multiple timescales seen in eukaryotic channels. Simulations have provided molecular views of the interactions of drugs, consistent with sites predicted in mammalian channels, as well as a wealth of other sites as potential new drug targets. In this chapter, we survey the new insights into sodium channel function that have emerged from studies of simpler bacterial channels, which provide an excellent learning platform, and promising avenues for mechanistic discovery and pharmacological development.

Cardiac Na Channels: Structure to Function

Heart rhythms arise from electrical activity generated by precisely timed opening and closing of ion channels in individual cardiac myocytes. Opening of the primary cardiac voltage-gated sodium (NaV1.5) channel initiates cellular depolarization and the propagation of an electrical action potential that promotes coordinated contraction of the heart. The regularity of these contractile waves is critically important since it drives the primary function of the heart: to act as a pump that delivers 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. Perturbations to NaV1.5 may alter the structure, and hence the function, of the ion channel and are associated downstream with a wide variety of cardiac conduction pathologies, such as arrhythmias.

Maturing human pluripotent stem cell-derived cardiomyocytes in human engineered cardiac tissues

Engineering functional human cardiac tissue that mimics the native adult morphological and functional phenotype has been a long held objective. In the last 5 years, the field of cardiac tissue engineering has transitioned from cardiac tissues derived from various animal species to the production of the first generation of human engineered cardiac tissues (hECTs), due to recent advances in human stem cell biology. Despite this progress, the hECTs generated to date remain immature relative to the native adult myocardium. In this review, we focus on the maturation challenge in the context of hECTs, the present state of the art, and future perspectives in terms of regenerative medicine, drug discovery, preclinical safety testing and pathophysiological studies.

Mechanism of Modification, by Lidocaine, of Fast and Slow Recovery from Inactivation of Voltage-Gated Na⁺ Channels

The clinically important suppression of high-frequency discharges of excitable cells by local anesthetics (LA) is largely determined by drug-induced prolongation of the time course of repriming (recovery from inactivation) of voltage-gated Na(+) channels. This prolongation may result from periodic drug-binding to a high-affinity binding site during the action potentials and subsequent slow dissociation from the site between action potentials ("dissociation hypothesis"). For many drugs it has been suggested that the fast inactivated state represents the high-affinity binding state. Alternatively, LAs may bind with high affinity to a native slow-inactivated state, thereby accelerating the development of this state during action potentials ("stabilization hypothesis"). In this case, slow recovery between action potentials occurs from enhanced native slow inactivation. To test these two hypotheses we produced serial cysteine mutations of domain IV segment 6 in rNav1.4 that resulted in constructs with varying propensities to enter fast- and slow-inactivated states. We tested the effect of the LA lidocaine on the time course of recovery from short and long depolarizing prepulses, which, under drug-free conditions, recruited mainly fast- and slow-inactivated states, respectively. Among the tested constructs the mutation-induced changes in native slow recovery induced by long depolarizations were not correlated with the respective lidocaine-induced slow recovery after short depolarizations. On the other hand, for long depolarizations the mutation-induced alterations in native slow recovery were significantly correlated with the kinetics of lidocaine-induced slow recovery. These results favor the "dissociation hypothesis" for short depolarizations but the "stabilization hypothesis" for long depolarizations.

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.

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.

Multiple pore conformations driven by asynchronous movements of voltage sensors in a eukaryotic sodium channel

Voltage-dependent Na(+) channels are crucial for electrical signalling in excitable cells. Membrane depolarization initiates asynchronous movements in four non-identical voltage-sensing domains of the Na(+) channel. It remains unclear to what extent this structural asymmetry influences pore gating as compared with outwardly rectifying K(+) channels, where channel opening results from a final concerted transition of symmetric pore gates. Here we combine single channel recordings, cysteine accessibility and voltage clamp fluorimetry to probe the relationships between voltage sensors and pore conformations in an inactivation deficient Nav1.4 channel. We observe three distinct conductance levels such that DI-III voltage sensor activation is kinetically correlated with formation of a fully open pore, whereas DIV voltage sensor movement underlies formation of a distinct subconducting pore conformation preceding inactivation in wild-type channels. Our experiments reveal that pore gating in sodium channels involves multiple transitions driven by asynchronous movements of voltage sensors. These findings shed new light on the mechanism of coupling between activation and fast inactivation in voltage-gated sodium channels.

The human Nav1.5 F1486 deletion associated with long QT syndrome leads to impaired sodium channel inactivation and reduced lidocaine sensitivity

The deletion of phenylalanine 1486 (F1486del) in the human cardiac voltage-gated sodium channel (hNav1.5) is associated with fatal long QT (LQT) syndrome. In this study we determined how F1486del impairs the functional properties of hNav1.5 and alters action potential firing in heterologous expression systems (human embryonic kidney (HEK) 293 cells) and their native cardiomyocyte background. Cells expressing hNav1.5-F1486del exhibited a loss-of-function alteration, reflected by an 80% reduction of peak current density, and several gain-of-function alterations, including reduced channel inactivation, enlarged window current, substantial augmentation of persistent late sodium current and an increase in ramp current. We also observed substantial action potential duration (APD) prolongation and prominent early afterdepolarizations (EADs) in neonatal cardiomyocytes expressing the F1486del channels, as well as in computer simulations of myocyte activity. In addition, lidocaine sensitivity was dramatically reduced, which probably contributed to the poor therapeutic outcome observed in the patient carrying the hNav1.5-F1486del mutation. Therefore, despite the significant reduction in peak current density, the F1486del mutation also leads to substantial gain-of-function alterations that are sufficient to cause APD prolongation and EADs, the predominant characteristic of LQTs. These data demonstrate that hNav1.5 mutations can have complex functional consequences and highlight the importance of identifying the specific molecular defect when evaluating potential treatments for individuals with prolonged QT intervals.

State-Dependent Inhibition of Sodium Channels by Local Anesthetics: A 40-Year Evolution

Knowledge about the mechanism of impulse blockade by local anesthetics has evolved over the past four decades, from the realization that Na⁺ channels were inhibited to affect the impulse blockade to an identification of the amino acid residues within the Na⁺ channel that bind the local anesthetic molecule. Within this period appreciation has grown of the state-dependent nature of channel inhibition, with rapid binding and unbinding at relatively high affinity to the open state, and weaker binding to the closed resting state. Slow binding of high affinity for the inactivated state accounts for the salutary therapeutic as well as the toxic actions of diverse class I anti-arrhythmic agents, but may have little importance for impulse blockade, which requires concentrations high enough to block the resting state. At the molecular level, residues on the S6 transmembrane segments in three of the homologous domains of the channel appear to contribute to the binding of local anesthetics, with some contribution also from parts of the selectivity filter. Binding to the inactivated state, and perhaps the open state, involves some residues that are not identical to those that bind these drugs in the resting state, suggesting spatial flexibility in the "binding site". Questions remaining include the mechanism that links local anesthetic binding with the inhibition of gating charge movements, and the molecular nature of the theoretical "hydrophobic pathway" that may be critical for determining the recovery rates from blockade of closed channels, and thus account for both therapeutic and cardiotoxic actions.

Voltage-gated sodium channel-associated proteins and alternative mechanisms of inactivation and block

Voltage-gated sodium channels mediate inward current of action potentials upon membrane depolarization of excitable cells. The initial transient sodium current is restricted to milliseconds through three distinct channel-inactivating and blocking mechanisms. All pore-forming alpha subunits of sodium channels possess structural elements mediating fast inactivation upon depolarization and recovery within milliseconds upon membrane repolarization. Accessory subunits modulate fast inactivation dynamics, but these proteins can also limit current by contributing distinct inactivation and blocking particles. A-type isoforms of fibroblast growth factor homologous factors (FHFs) bear a particle that induces long-term channel inactivation, while sodium channel subunit Navβ4 employs a blocking particle that rapidly dissociates upon membrane repolarization to generate resurgent current. Despite their different physiological functions, the FHF and Navβ4 particles have similarity in amino acid composition and mechanisms for docking within sodium channels. The three competing channel-inactivating and blocking processes functionally interact to regulate a neuron's intrinsic excitability.

Lidocaine reduces the transition to slow inactivation in Na(v)1.7 voltage-gated sodium channels

BACKGROUND AND PURPOSE

The primary use of local anaesthetics is to prevent or relieve pain by reversibly preventing action potential propagation through the inhibition of voltage-gated sodium channels. The tetrodotoxin-sensitive voltage-gated sodium channel subtype Na(v)1.7, abundantly expressed in pain-sensing neurons, plays a crucial role in perception and transmission of painful stimuli and in inherited chronic pain syndromes. Understanding the interaction of lidocaine with Na(v)1.7 channels could provide valuable insight into the drug's action in alleviating pain in distinct patient populations. The aim of this study was to determine how lidocaine interacts with multiple inactivated conformations of Na(v)1.7 channels.

EXPERIMENTAL APPROACH

We investigated the interactions of lidocaine with wild-type Na(v)1.7 channels and a paroxysmal extreme pain disorder mutation (I1461T) that destabilizes fast inactivation. Whole cell patch clamp recordings were used to examine the activity of channels expressed in human embryonic kidney 293 cells.

KEY RESULTS

Depolarizing pulses that increased slow inactivation of Na(v)1.7 channels also reduced lidocaine inhibition. Lidocaine enhanced recovery of Na(v)1.7 channels from prolonged depolarizing pulses by decreasing slow inactivation. A paroxysmal extreme pain disorder mutation that destabilizes fast inactivation of Na(v)1.7 channels decreased lidocaine inhibition.

CONCLUSIONS AND IMPLICATIONS

Lidocaine decreased the transition of Na(v)1.7 channels to the slow inactivated state. The fast inactivation gate (domain III-IV linker) is important for potentiating the interaction of lidocaine with the Na(v)1.7 channel.

A computational model to predict the effects of class I anti-arrhythmic drugs on ventricular rhythms

A long-sought, and thus far elusive, goal has been to develop drugs to manage diseases of excitability. One such disease that affects millions each year is cardiac arrhythmia, which occurs when electrical impulses in the heart become disordered, sometimes causing sudden death. Pharmacological management of cardiac arrhythmia has failed because it is not possible to predict how drugs that target cardiac ion channels, and have intrinsically complex dynamic interactions with ion channels, will alter the emergent electrical behavior generated in the heart. Here, we applied a computational model, which was informed and validated by experimental data, that defined key measurable parameters necessary to simulate the interaction kinetics of the anti-arrhythmic drugs flecainide and lidocaine with cardiac sodium channels. We then used the model to predict the effects of these drugs on normal human ventricular cellular and tissue electrical activity in the setting of a common arrhythmia trigger, spontaneous ventricular ectopy. The model forecasts the clinically relevant concentrations at which flecainide and lidocaine exacerbate, rather than ameliorate, arrhythmia. Experiments in rabbit hearts and simulations in human ventricles based on magnetic resonance images validated the model predictions. This computational framework initiates the first steps toward development of a virtual drug-screening system that models drug-channel interactions and predicts the effects of drugs on emergent electrical activity in the heart.

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.

Automated Patch Clamp on mESC-Derived Cardiomyocytes for Cardiotoxicity Prediction

Cardiovascular side effects are critical in drug development and have frequently led to late-stage project terminations or even drug withdrawal from the market. Physiologically relevant and predictive assays for cardiotoxicity are hence strongly demanded by the pharmaceutical industry. To identify a potential impact of test compounds on ventricular repolarization, typically a variety of ion channels in diverse heterologously expressing cells have to be investigated. Similar to primary cells, in vitro–generated stem cell–derived cardiomyocytes simultaneously express cardiac ion channels. Thus, they more accurately represent the native situation compared with cell lines overexpressing only a single type of ion channel. The aim of this study was to determine if stem cell–derived cardiomyocytes are suited for use in an automated patch clamp system. The authors show recordings of cardiac ion currents as well as action potential recordings in readily available stem cell–derived cardiomyocytes. Besides monitoring inhibitory effects of reference compounds on typical cardiac ion currents, the authors revealed for the first time drug-induced modulation of cardiac action potentials in an automated patch clamp system. The combination of an in vitro cardiac cell model with higher throughput patch clamp screening technology allows for a cost-effective cardiotoxicity prediction in a physiologically relevant cell system.

Role of voltage-gated sodium, potassium and calcium channels in the development of cocaine-associated cardiac arrhythmias

Cocaine is a highly active stimulant that alters dopamine metabolism in the central nervous system resulting in a feeling of euphoria that with time can lead to addictive behaviours. Cocaine has numerous deleterious effects in humans including seizures, vasoconstriction, ischaemia, increased heart rate and blood pressure, cardiac arrhythmias and sudden death. The cardiotoxic effects of cocaine are indirectly mediated by an increase in sympathomimetic stimulation to the heart and coronary vasculature and by a direct effect on the ion channels responsible for maintaining the electrical excitability of the heart. The direct and indirect effects of cocaine work in tandem to disrupt the co-ordinated electrical activity of the heart and have been associated with life-threatening cardiac arrhythmias. This review focuses on the direct effects of cocaine on cardiac ion channels, with particular focus on sodium, potassium and calcium channels, and on the contributions of these channels to cocaine-induced arrhythmias. Companion articles in this edition of the journal examine the epidemiology of cocaine use (Wood & Dargan) and the treatment of cocaine-associated arrhythmias (Hoffmann).

Molecular determinants of state-dependent block of voltage-gated sodium channels by pilsicainide

BACKGROUND AND PURPOSE

Pilsicainide, an anti-arrhythmic drug used in Japan, is described as a pure sodium channel blocker. We examined the mechanisms by which it is able to block open channels, because these properties may be especially useful to reduce hyperexcitability in pathologies characterized by abnormal sodium channel opening.

EXPERIMENTAL APPROACH

The effects of pilsicainide on various heterologously expressed human sodium channel subtypes and mutants were investigated using the patch clamp technique.

KEY RESULTS

Pilsicainide exhibited tonic and use-dependent effects comparable to those of mexiletine and flecainide on hNav1.4 channels. These use-dependent effects were abolished in the mutations F1586C and Y1593C within segment 6 of domain IV, suggesting that the interaction of pilsicainide with these residues is critical for its local anaesthetic action. Its affinity constants for closed channels (K(R)) and channels inactivated from the closed state (K(I)) were high, suggesting that its use-dependent block (UDB) requires the channel to be open for it to reach a high-affinity blocking site. Accordingly, basic pH, which slightly increased the proportion of neutral drug, dramatically decreased K(R) and K(I) values. Effects of pilsicainide were similar on skeletal muscle hNav1.4, brain hNav1.1 and heart hNav1.5 channels. The myotonic R1448C and G1306E hNav1.4 mutants were more and less sensitive to pilsicainide, respectively, due to mutation-induced gating modifications.

CONCLUSIONS AND IMPLICATIONS

Although therapeutic concentrations of pilsicainide may have little effect on resting and closed-state inactivated channels, it induces a strong UDB due to channel opening, rendering the drug ideally suited for inhibition of high-frequency action potential firing.

Autonomic modulation and antiarrhythmic therapy in a model of long QT syndrome type 3

Aims

Clinical observations in patients with long QT syndrome carrying sodium channel mutations (LQT3) suggest that bradycardia caused by parasympathetic stimulation may provoke torsades de pointes (TdP). β-Adrenoceptor blockers appear less effective in LQT3 than in other forms of the disease.

Methods and results

We studied effects of autonomic modulation on arrhythmias in vivo and in vitro and quantified sympathetic innervation by autoradiography in heterozygous mice with a knock-in deletion (ΔKPQ) in the Scn5a gene coding for the cardiac sodium channel and increased late sodium current (LQT3 mice). Cholinergic stimulation by carbachol provoked bigemini and TdP in freely roaming LQT3 mice. No arrhythmias were provoked by physical stress, mental stress, isoproterenol, or atropine. In isolated, beating hearts, carbachol did not prolong action potentials per se, but caused bradycardia and rate-dependent action potential prolongation. The muscarinic inhibitor AFDX116 prevented effects of carbachol on heart rate and arrhythmias. β-Adrenoceptor stimulation suppressed arrhythmias, shortened rate-corrected action potential duration, increased rate, and minimized difference in late sodium current between genotypes. β-Adrenoceptor density was reduced in LQT3 hearts. Acute β-adrenoceptor blockade by esmolol, propranolol or chronic propranolol in vivo did not suppress arrhythmias. Chronic flecainide pre-treatment prevented arrhythmias (all P < 0.05).

Conclusion

Cholinergic stimulation provokes arrhythmias in this model of LQT3 by triggering bradycardia. β-Adrenoceptor density is reduced, and β-adrenoceptor blockade does not prevent arrhythmias. Sodium channel blockade and β-adrenoceptor stimulation suppress arrhythmias by shortening repolarization and minimizing difference in late sodium current.

Using lidocaine and benzocaine to link sodium channel molecular conformations to state-dependent antiarrhythmic drug affinity

RATIONALE

Lidocaine and other antiarrhythmic drugs bind in the inner pore of voltage-gated Na channels and affect gating use-dependently. A phenylalanine in domain IV, S6 (Phe1759 in Na(V)1.5), modeled to face the inner pore just below the selectivity filter, is critical in use-dependent drug block.

OBJECTIVE

Measurement of gating currents and concentration-dependent availability curves to determine the role of Phe1759 in coupling of drug binding to the gating changes.

METHODS AND RESULTS

The measurements showed that replacement of Phe1759 with a nonaromatic residue permits clear separation of action of lidocaine and benzocaine into 2 components that can be related to channel conformations. One component represents the drug acting as a voltage-independent, low-affinity blocker of closed channels (designated as lipophilic block), and the second represents high-affinity, voltage-dependent block of open/inactivated channels linked to stabilization of the S4s in domains III and IV (designated as voltage-sensor inhibition) by Phe1759. A homology model for how lidocaine and benzocaine bind in the closed and open/inactivated channel conformation is proposed.

CONCLUSIONS

These 2 components, lipophilic block and voltage-sensor inhibition, can explain the differences in estimates between tonic and open-state/inactivated-state affinities, and they identify how differences in affinity for the 2 binding conformations can control use-dependence, the hallmark of successful antiarrhythmic drugs.

The pharmacology of voltage-gated sodium channels in sensory neurones

Voltage-gated sodium channels (VGSCs) are vital for the normal functioning of most excitable cells. At least nine distinct functional subtypes of VGSCs are recognized, corresponding to nine genes for their pore-forming alpha-subunits. These have different developmental expression patterns, different tissue distributions in the adult and are differentially regulated at the cellular level by receptor-coupled cell signalling systems. Unsurprisingly, VGSC blockers are found to be useful as drugs in diverse clinical applications where excessive excitability of tissue leads to pathological dysfunction, e.g. epilepsy or cardiac tachyarrhythmias. The effects of most clinically useful VGSC blockers are use-dependent, i.e. their efficacy depends on channel activity. In addition, many natural toxins have been discovered that interact with VGSCs in complex ways and they have been used as experimental probes to study the structure and function of the channels and to better understand how drugs interact with the channels. Here we have attempted to summarize the properties of VGSCs in sensory neurones, discuss how they are regulated by cell signalling systems and we have considered briefly current concepts of their physiological function. We discuss in detail how drugs and toxins interact with archetypal VGSCs and where possible consider how they act on VGSCs in peripheral sensory neurones. Increasingly, drugs that block VGSCs are being used as systemic analgesic agents in chronic pain syndromes, but the full potential for VGSC blockers in this indication is yet to be realized and other applications in sensory dysfunction are also possible. Drugs targeting VGSC subtypes in sensory neurones are likely to provide novel systemic analgesics that are tissue-specific and perhaps even disease-specific, providing much-needed novel therapeutic approaches for the relief of chronic pain.

Bupivacaine blocks N-type inactivating Kv channels in the open state: no allosteric effect on inactivation kinetics

Local anesthetics bind to ion channels in a state-dependent manner. For noninactivating voltage-gated K channels the binding mainly occurs in the open state, while for voltage-gated inactivating Na channels it is assumed to occur mainly in inactivated states, leading to an allosterically caused increase in the inactivation probability, reflected in a negative shift of the steady-state inactivation curve, prolonged recovery from inactivation, and a frequency-dependent block. How local anesthetics bind to N-type inactivating K channels is less explored. In this study, we have compared bupivacaine effects on inactivating (Shaker and K(v)3.4) and noninactivating (Shaker-IR and K(v)3.2) channels, expressed in Xenopus oocytes. Bupivacaine was found to block these channels time-dependently without shifting the steady-state inactivation curve markedly, without a prolonged recovery from inactivation, and without a frequency-dependent block. An analysis, including computational testing of kinetic models, suggests binding to the channel mainly in the open state, with affinities close to those estimated for corresponding noninactivating channels (300 and 280 microM for Shaker and Shaker-IR, and 60 and 90 microM for K(v)3.4 and K(v)3.2). The similar magnitudes of K(d), as well as of blocking and unblocking rate constants for inactivating and noninactivating Shaker channels, most likely exclude allosteric interactions between the inactivation mechanism and the binding site. The relevance of these results for understanding the action of local anesthetics on Na channels is discussed.

Late sodium current inhibition as a new cardioprotective approach

There is increasing evidence that the late sodium current of the sodium channel in myocytes plays a critical role in the pathophysiology of myocardial ischemia and thus is a potential therapeutic target in patients with ischemic heart disease. Ranolazine, an inhibitor of the late sodium current, reduces the frequency and severity of anginal attacks and ST-segment depression in humans, and unlike other antianginal drugs, ranolazine does not alter heart rate or blood pressure. In experimental animal models, ranolazine has been shown to reduce myocardial infarct size and to improve left ventricular function after acute ischemia and chronic heart failure. This article reviews published data describing the role of late sodium current and its inhibition by ranolazine in clinical and experimental studies of myocardial ischemia.

Outward stabilization of the S4 segments in domains III and IV enhances lidocaine block of sodium channels

The anti-arrhythmic drug lidocaine has been shown to have a lower affinity for block of voltage-gated sodium channels at hyperpolarized potentials compared to depolarized potentials. Concomitantly, lidocaine reduces maximum gating charge (Qmax) by 40% resulting from the complete stabilization of the S4 in domain III in an outward, depolarized position and partial stabilization of the S4 in domain IV in wild-type Na+ channels (Na(V)1.5). To investigate whether the pre-positioning of the S4 segments in these two domains in a depolarized conformation increases affinity for lidocaine block, a cysteine residue was substituted for the 3rd outermost charged residue in the S4 of domain III (R3C-DIII) and for the 2nd outermost Arg in S4 of domain IV (R2C-DIV) in Na(V)1.5. After biotinylation by exposure to extracellular MTSEA-biotin the mutated S4s became stabilized in an outward, depolarized position. For Na+ channels containing both mutations (R3C-DIII + R2C-DIV) the IC50 for rested-state lidocaine block decreased from 194 +/- 15 microM in control to 28 +/- 2 microM after MTSEA-biotin modification. To determine whether an intact inactivation gate (formed by the linker between domains III and IV) was required for local anaesthetic drugs to modify Na+ channel gating currents, a Cys was substituted for the Phe in the IFM motif of the inactivation gate (ICM) and then modified by intracellular MTSET (WT-ICM(MTSET)) before exposure to intracellular QX-222, a quarternary amine. Although WT-ICM(MTSET) required higher concentrations of drug to block I(Na) compared to WT, Qmax decreased by 35% and the V1/2 shifted leftward as previously demonstrated for WT. The effect of stabilization of the S4s in domains III and IV in the absence of an intact inactivation gate on lidocaine block was determined for R3C-DIII + ICM, R2C-DIV + ICM and R3C-DIII + R2C-DIV + ICM, and compared to WT-ICM. IC50 values were 1360 +/- 430 microM, 890 +/- 70 microM, 670 +/- 30 microM and 1920 +/- 60 microM, respectively. Thermodynamic mutant-cycle analysis was consistent with additive (i.e. independent) contributions from stabilization of the individual S4s in R3C-DIII + ICM and R2C-DIV + ICM. We conclude that the positions of the S4s in domains III and IV are major determinants of the voltage dependence of lidocaine affinity.

Pharmacogenetics and anti-arrhythmic drug therapy: a theoretical investigation

Pharmacological management of cardiac arrhythmias has been a long and widely sought goal. One of the difficulties in treating arrhythmia stems, in part, from incomplete understanding of the mechanisms of drug block and how intrinsic properties of channel gating affect drug access, binding affinity, and unblock. In the last decade, a plethora of genetic information has revealed that genetics may play a critical role in determining arrhythmia susceptibility and in efficacy of pharmacological therapy. In this context, we present a theoretical approach for investigating effects of drug-channel interaction. We use as an example open-channel or inactivated-channel block by the local anesthetics mexiletine and lidocaine, respectively, of normal and DeltaKPQ mutant Na(+) channels associated with the long-QT syndrome type 3. Results show how kinetic properties of channel gating, which are affected by mutations, are important determinants of drug efficacy. Investigations of Na(+) channel blockade are conducted at multiple scales (single channel and macroscopic current) and, importantly, during the cardiac action potential (AP). Our findings suggest that channel mean open time is a primary determinant of open state blocker efficacy. Channels that remain in the open state longer, such as the DeltaKPQ mutant channels in the abnormal burst mode, are blocked preferentially by low mexiletine concentrations. AP simulations confirm that a low dose of mexiletine can remove early afterdepolarizations and restore normal repolarization without affecting the AP upstroke. The simulations also suggest that inactivation state block by lidocaine is less effective in restoring normal repolarization and adversely suppresses peak Na(+) current.

Evidence for a multivalent interaction of symmetrical, N-linked, lidocaine dimers with voltage-gated Na+ channels

The interaction of symmetrical lidocaine dimers with voltage-gated Na+ channels (VGSCs) was examined using a FLIPR membrane potential assay and voltage-clamp. The dimers, in which the tertiary amines of the lidocaine moieties are linked by an alkylene chain (two to six methylene units), inhibited VGSC activator-evoked depolarization of cells heterologously-expressing rat (r) Na(v)1.2a, human (h) Na(v)1.5, and rNa(v)1.8, with potencies 10- to 100-fold higher than lidocaine (compound 1). The rank order of potency (C4 (compound 4) > C3 (compound 3) > or = C2 (compound 2) = C5 (compound 5) = C6 (compound 6) >> compound 1) was similar at each VGSC. Compound 4 exhibited strong use-dependent inhibition of hNa(v)1.5 with pIC50 values < 4.5 and 6.0 for tonic and phasic block, respectively. Coincubation with local anesthetics but not tetrodotoxin attenuated compound 4-mediated inhibition of hNa(v)1.5. These data suggest that the compound 4 binding site(s) is identical, or allosterically coupled, to the local anesthetic receptor. The dissociation rate of the dimers from hNa(v)1.5 was dependent upon the linker length, with a rank order of compound 1 > compound 5 = compound 6 > compound 2 >> compound 3. The observation that both the potency and dissociation rate of the dimers was dependent upon linker length is consistent with a multivalent interaction at VGSCs. hNa(v)1.5 VGSCs did not recover from inhibition by compound 4. However, "chase" with free local anesthetic site inhibitors increased the rate of dissociation of compound 4. Together, these data support the hypothesis that compound 4 simultaneously occupies two binding sites on VGSCs, both of which can be bound by known local anesthetic site inhibitors.

A mutation in the local anaesthetic binding site abolishes toluene effects in sodium channels

Toluene is a solvent of abuse that inhibits cardiac sodium channels in a manner that resembles the action of local anaesthetics. The purpose of this work was to analyze toluene effects on skeletal muscle sodium channels with and without beta1 subunit (Nav1.4+beta1 and Nav1.4-beta1, respectively) expressed in Xenopus laevis oocytes and to compare them with those produced in the F1579A mutant channel lacking a local anaesthetic binding site. Toluene inhibited Nav1.4 sodium currents (IC50=2.7 mM in Nav1.4+beta1 and 2.2 mM in Nav1.4-beta1 in a concentration dependent way. Toluene (3 mM) blocked sodium currents in Nav1.4 channels proportionally throughout the entire current-voltage relationship producing inactivation at more negative potentials. Minimal inhibition was produced by 3 mM toluene in F1579A mutant channels. Recovery from inactivation was slower both in Nav1.4 and F1579A channels in the presence of 3 mM toluene. The solvent blocked sodium currents in a use-dependent and frequency-dependent manner in Nav1.4 channels. A single mutation in the local anaesthetic binding site of Nav1.4 channels almost abolished toluene effects. These results suggest that this site is important for toluene action.

Sodium Channel Inactivation: Molecular Determinants and Modulation

Voltage-gated sodium channels open (activate) when the membrane is depolarized and close on repolarization (deactivate) but also on continuing depolarization by a process termed inactivation, which leaves the channel refractory, i.e., unable to open again for a period of time. In the “classical” fast inactivation, this time is of the millisecond range, but it can last much longer (up to seconds) in a different slow type of inactivation. These two types of inactivation have different mechanisms located in different parts of the channel molecule: the fast inactivation at the cytoplasmic pore opening which can be closed by a hinged lid, the slow inactivation in other parts involving conformational changes of the pore. Fast inactivation is highly vulnerable and affected by many chemical agents, toxins, and proteolytic enzymes but also by the presence of β-subunits of the channel molecule. Systematic studies of these modulating factors and of the effects of point mutations (experimental and in hereditary diseases) in the channel molecule have yielded a fairly consistent picture of the molecular background of fast inactivation, which for the slow inactivation is still lacking.

Interactions of local anesthetics with voltage-gated Na+ channels

Voltage-gated Na+ channels are dynamic transmembrane proteins responsible for the rising phase of the action potential in excitable membranes. Local anesthetics (LAs) and structurally related antiarrhythmic and anticonvulsant compounds target specific sites in voltage-gated Na+ channels to block Na+ currents, thus reducing excitability in neuronal, cardiac, or central nervous tissue. A high-affinity LA block is produced by binding to open and inactivated states of Na+ channels rather than to resting states and suggests a binding site that converts from a low- to a high-affinity conformation during gating. Recent findings using site-directed mutagenesis suggest that multiple S6 segments together form an LA binding site within the Na+ channel. While the selectivity filter may form the more extracellular-located part of this binding site, the role of the fast inactivation gate in LA binding has not yet been resolved. The receptor of the neurotoxin batrachotoxin (BTX) is adjacent to or even overlaps with the LA binding site. The close proximity of the LA and BTX binding sites to residues critical for inactivation, together with gating transitions through S6 segments, might explain the strong impact of LAs and BTX on inactivation of voltage-gated Na+ channels and might help elucidate the mechanisms underlying voltage- and frequency-dependent LA block.

Compound-specific Na+ channel pore conformational changes induced by local anaesthetics

Upon prolonged depolarizations, voltage-dependent Na+ channels open and subsequently inactivate, occupying fast and slow inactivated conformational states. Like C-type inactivation in K+ channels, slow inactivation is thought to be accompanied by rearrangement of the channel pore. Cysteine-labelling studies have shown that lidocaine, a local anaesthetic (LA) that elicits depolarization-dependent ('use-dependent') Na+ channel block, does not slow recovery from fast inactivation, but modulates the kinetics of slow inactivated states. While these observations suggest LA-induced stabilization of slow inactivation could be partly responsible for use dependence, a more stringent test would require that slow inactivation gating track the distinct use-dependent kinetic properties of diverse LA compounds, such as lidocaine and bupivacaine. For this purpose, we assayed the slow inactivation-dependent accessibility of cysteines engineered into domain III, P-segment (mu1: F1236C, K1237C) to sulfhydryl (MTSEA) modification using a high-speed solution exchange system. As expected, we found that bupivacaine, like lidocaine, protected cysteine residues from MTSEA modification in a depolarization-dependent manner. However, under pulse-train conditions where bupivacaine block of Na+ channels was extensive (due to ultra-slow recovery), but lidocaine block of Na+ channels was not, P-segment cysteines were protected from MTSEA modification. Here we show that conformational changes associated with slow inactivation track the vastly different rates of recovery from use-dependent block for bupivacaine and lidocaine. Our findings suggest that LA compounds may produce their kinetically distinct voltage-dependent behaviour by modulating slow inactivation gating to varying degrees.

State-Dependent Mibefradil Block of Na+ Channels

Mibefradil is a T-type Ca2+ channel antagonist with reported cross-reactivity with other classes of ion channels, including K+, Cl-, and Na+ channels. Using whole-cell voltage clamp, we examined mibefradil block of four Na+ channel isoforms expressed in human embryonic kidney cells: Nav1.5 (cardiac), Nav1.4 (skeletal muscle), Nav1.2 (brain), and Nav1.7 (peripheral nerve). Mibefradil blocked Nav1.5 in a use/frequency-dependent manner, indicating preferential binding to states visited during depolarization. Mibefradil blocked currents of all Na+ channel isoforms with similar affinity and a dependence on holding potential, and drug off-rate was slowed at depolarized potentials (k(off) was 0.024/s at -130 mV and 0.007/s at -100 mV for Nav1.5). We further probed the interaction of mibefradil with inactivated Nav1.5 channels. Neither the degree nor the time course of block was dependent on the stimulus duration, which dramatically changed the residency time of channels in the fast-inactivated state. In addition, inhibiting the binding of the fast inactivation lid (Nav1.5 ICM + MTSET) did not alter mibefradil block, confirming that the drug does not preferentially interact with the fast-inactivated state. We also tested whether mibefradil interacted with slow-inactivated state(s). When selectively applied to channels after inducing slow inactivation with a 60-s pulse to -10 mV, mibefradil (1 microM) produced 45% fractional block in Nav1.5 and greater block (88%) in an isoform (Nav1.4) that slow-inactivates more completely. Our results suggest that mibefradil blocks Na+ channels in a state-dependent manner that does not depend on fast inactivation but probably involves interaction with one or more slow-inactivated state(s).

Block of inactivation-deficient Na+ channels by local anesthetics in stably transfected mammalian cells: evidence for drug binding along the activation pathway

According to the classic modulated receptor hypothesis, local anesthetics (LAs) such as benzocaine and lidocaine bind preferentially to fast-inactivated Na⁺ channels with higher affinities. However, an alternative view suggests that activation of Na⁺ channels plays a crucial role in promoting high-affinity LA binding and that fast inactivation per se is not a prerequisite for LA preferential binding. We investigated the role of activation in LA action in inactivation-deficient rat muscle Na⁺ channels (rNav1.4-L435W/L437C/A438W) expressed in stably transfected Hek293 cells. The 50% inhibitory concentrations (IC₅₀) for the open-channel block at +30 mV by lidocaine and benzocaine were 20.9 ± 3.3 μM (n = 5) and 81.7 ± 10.6 μM (n = 5), respectively; both were comparable to inactivated-channel affinities. In comparison, IC₅₀ values for resting-channel block at −140 mV were >12-fold higher than those for open-channel block. With 300 μM benzocaine, rapid time-dependent block (τ ≈ 0.8 ms) of inactivation-deficient Na⁺ currents occurred at +30 mV, but such a rapid time-dependent block was not evident at −30 mV. The peak current at −30 mV, however, was reduced more severely than that at +30 mV. This phenomenon suggested that the LA block of intermediate closed states took place notably when channel activation was slow. Such closed-channel block also readily accounted for the LA-induced hyperpolarizing shift in the conventional steady-state inactivation measurement. Our data together illustrate that the Na⁺ channel activation pathway, including most, if not all, transient intermediate closed states and the final open state, promotes high-affinity LA binding.

Effects of lidocaine on shock-induced vulnerability

INTRODUCTION

Lidocaine is known to increase the defibrillation threshold (DFT) of monophasic shocks (MS) and have no effect on DFT of biphasic shocks (BS). The aim of this study was to enhance our understanding of the mechanisms of vulnerability and defibrillation through the investigation of this difference.

METHODS AND RESULTS

We studied the effect of 15 microM lidocaine on shock-induced vulnerability using fluorescent imaging of Langendorff-perfused rabbit hearts. Vulnerability was assessed as vulnerable window with shock strengths of 15 to 150 V and vulnerable period (VP) with shock delivery phase of 0% to 100% of action potential duration (% APD). With MS, lidocaine caused a significant increase in both the upper limit of vulnerability (ULV, 71 +/- 17 V vs 120 +/- 1.5 V, P < 0.01) and upper limit of VP (91 +/- 8.0% APD vs 110 +/- 4.2% APD, P < 0.01). With BS, lidocaine had no effect on ULV (40 +/- 3.4 V vs 45 +/- 4.5 V) and did not increase the upper limit of VP (78 +/- 8.9% APD vs 96 +/- 12% APD, P < 0.01). Lidocaine caused reduction of the conduction velocity during pacing (0.58 +/- 0.08 m/s vs 0.44 +/- 0.05 m/s, P < 0.01), shock-induced break excitation (0.82 +/- 0.17 m/s vs 0.30 +/- 0.07 m/s, P < 0.01), and postshock reentry (0.34 +/- 0.07 m/s vs 0.19 +/- 0.08 m/s, P < 0.01). Lidocaine had no effect on shock-induced virtual electrode polarization.

CONCLUSION

Lidocaine increased MS ULV due to slowing of shock-induced break-excitation wavefronts, which resulted in enhanced probability of survival of virtual electrode induced phase singularity. Lidocaine had no effect on BS ULV because no break excitation was induced by BS. Reduction of conduction velocity by lidocaine resulted in increased dispersion of repolarization and led to upper limit of VP increase for both MS and BS.

Closing and inactivation potentiate the cocaethylene inhibition of cardiac sodium channels by distinct mechanisms

Cocaethylene, a metabolite of cocaine and alcohol, is a potent inhibitor of the cardiac (Nav1.5) sodium channel heterologously expressed in Xenopus laevis oocytes. Cocaethylene produces minimal tonic block under resting conditions but causes a potent use-dependent inhibition during repetitive depolarization and a hyperpolarizing shift in the steady-state inactivation. The data are consistent with a state-dependent binding mechanism, which has high affinity for inactivated channels (KI = 17 microM) and low affinity for resting channels (KR = 185 micro). Mutations of the interdomain D3-D4 linker eliminated rapid inactivation and weakened the cocaethylene inhibition, consistent with an important role for fast inactivation in cocaethylene binding. A rapid component of cocaethylene inhibition was observed in a noninactivating mutant of Nav1.5 that was tightly linked to channel opening and displayed properties consistent with a pore blocking mechanism. Hyperpolarization caused the noninactivating mutant channel to close, trapping cocaethylene and slowing the recovery. Mutation of a conserved isoleucine (I1756C) located near the extracellular end of the D4S6 segment accelerated the recovery of the noninactivating channel, suggesting that this mutation facilitates cocaethylene untrapping, which seems to be the rate-limiting step in the recovery when the channel is closed. This contrasts with the rapidly inactivating channel, where the I1756C mutation did not alter the recovery from cocaethylene inhibition. The data suggest that additional mechanisms, such as more stable cocaethylene binding, may be a more important determinant of recovery kinetics when the channels are inactivated. The data indicate that deactivation and inactivation slow the recovery and potentiate the cocaethylene inhibition of the Nav1.5 channel by distinct mechanisms.

The sensitivity of sodium channels in immature and mature rat CA1 neurones to the local anaesthetics procaine and lidocaine

Sodium currents were recorded in CA1 hippocampal cells from new-born (P(4-10)) and older (P(>22)) rats, using whole-cell voltage clamp techniques. The effects of local anaesthetics (procaine and lidocaine) were studied in both cell populations. Parameters defining steady-state inactivation, removal of inactivation and the affinity of the anaesthetic molecules to the inactivated state were determined at both stages of maturation. Procaine and lidocaine induced a hyperpolarizing shift in steady-state inactivation curves, and slowed the rate of recovery from the inactivated state. Procaine disclosed differences between immature and older cells in what concerns block of the closed (resting) channels, drug affinity and binding to the inactivated state, i.e. the binding rate of procaine was found higher and the affinity lower in younger cells. The characteristics of procaine and lidocaine block on CA1 sodium currents differed in some particular aspects: magnitude of block on resting channels, shift in the voltage dependence and voltage sensitivity of steady-state inactivation, slow recovery from inactivation and use-dependent block.