Stereoselective block of a human cardiac potassium channel (Kv1.5) by bupivacaine enantiomers

Rational design, synthesis, and evaluation of novel polypharmacological compounds targeting NaV1.5, KV1.5, and K2P channels for atrial fibrillation

Atrial fibrillation (AF) involves electrical remodeling of the atria, with ion channels such as NaV1.5, KV1.5, and TASK-1 playing crucial roles. This study investigates acetamide-based compounds designed as multi-target inhibitors of these ion channels to address AF. Compound 6f emerged as the most potent in the series, demonstrating a strong inhibition of TASK-1 (IC50 ∼ 0.3 μM), a moderate inhibition of NaV1.5 (IC50 ∼ 21.2 μM) and a subtle inhibition of KV1.5 (IC50 ∼ 81.5 μM), alongside unexpected activation of TASK-4 (∼ 40% at 100 μM). Functional assays on human atrial cardiomyocytes from sinus rhythm (SR) and patients with AF revealed that 6f reduced action potential amplitude in SR (indicating NaV1.5 block), while in AF it increased action potential duration (APD), reflecting high affinity for TASK-1. Additionally, 6f caused hyperpolarization of the resting membrane potential in AF cardiomyocytes, consistent with the observed TASK-4 activation. Mathematical modeling further validated its efficacy in reducing AF burden. Pharmacokinetic analyses suggest favorable absorption and low toxicity. These findings identify 6f as a promising multi-target therapeutic candidate for AF management.

Liberal use of local anaesthetic and the risk of toxicity in elective arthroplasties at a tertiary teaching hospital

Background

Local anaesthetic systemic toxicity (LAST) is a life-threatening potential complication that may follow the administration of local anaesthetic (LA) drugs, and is cumulative across the drug class. Local anaesthetics are commonly administered via different routes for elective orthopaedic procedures – both by anaesthetists and surgeons. We hypothesized that total doses of LA may be routinely encroaching upon toxicity.

Methods

All total hip or knee arthroplasties (THAs and TKAs) performed within a 3 month period at the John Hunter Hospital (tertiary referral centre and teaching hospital) were audited to assess total administration of LA. Demographics, surgical characteristics, use of general anaesthesia or sedation, and use of local anaesthetic via any route of administration was recorded. For each patient, a weight-based theoretical maximum safe dose was calculated and compared against the dose they received. Data is presented as mean ± SD, percentages. Statistical significance was determined at p < 0.05.

Results

130 THAs and TKAs were identified within the audit period. 52 patients exceeded their drug-class theoretical maximum safe dose. 49 patients exceeded their weight-based maximum dose for a single LA agent, in all cases ropivacaine. Non-obese individuals receive significantly higher mean dose than obese individuals (119.4% [98.6–140.3] vs 78.82% [65.95–91.69], p = 0.001). No LAST events were identified.

Conclusions

Patients who received elective total hip or knee arthroplasties were exposed to concerningly high total doses of local anaesthetic, suggesting that greater awareness of the additive toxicity of drugs within this class is required.

Local anesthetic systemic toxicity: A narrative review for emergency clinicians

INTRODUCTION

Emergency clinicians utilize local anesthetics for a variety of procedures in the emergency department (ED) setting. Local anesthetic systemic toxicity (LAST) is a potentially deadly complication.

OBJECTIVE

This narrative review provides emergency clinicians with the most current evidence regarding the pathophysiology, evaluation, and management of patients with LAST.

DISCUSSION

LAST is an uncommon but potentially life-threatening complication of local anesthetic use that may be encountered in the ED. Patients at extremes of age or with organ dysfunction are at higher risk. Inadvertent intra-arterial or intravenous injection, as well as repeated doses and higher doses of local anesthetics are associated with greater risk of developing LAST. Neurologic and cardiovascular manifestations can occur. Early recognition and intervention, including supportive care and intravenous lipid emulsion 20%, are the mainstays of treatment. Using ultrasound guidance, aspirating prior to injection, and utilizing the minimal local anesthetic dose needed are techniques that can reduce the risk of LAST.

CONCLUSIONS

This focused review provides an update for the emergency clinician to manage patients with LAST.

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.

Challenges Faced with Small Molecular Modulators of Potassium Current Channel Isoform Kv1.5

The voltage-gated potassium channel Kv1.5, which mediates the cardiac ultra-rapid delayed-rectifier (IKur) current in human cells, has a crucial role in atrial fibrillation. Therefore, the design of selective Kv1.5 modulators is essential for the treatment of pathophysiological conditions involving Kv1.5 activity. This review summarizes the progress of molecular structures and the functionality of different types of Kv1.5 modulators, with a focus on clinical cardiovascular drugs and a number of active natural products, through a summarization of 96 compounds currently widely used. Furthermore, we also discuss the contributions of Kv1.5 and the regulation of the structure-activity relationship (SAR) of synthetic Kv1.5 inhibitors in human pathophysiology. SAR analysis is regarded as a useful strategy in structural elucidation, as it relates to the characteristics that improve compounds targeting Kv1.5. Herein, we present previous studies regarding the structural, pharmacological, and SAR information of the Kv1.5 modulator, through which we can assist in identifying and designing potent and specific Kv1.5 inhibitors in the treatment of diseases involving Kv1.5 activity.

Choline and NMDG directly reduce outward currents: reduced outward current when these substances replace Na+ is alone not evidence of Na+-activated K+ currents

Choline chloride is often, and N-methyl-d-glucamine (NMDG) sometimes, used to replace sodium chloride in studies of sodium-activated potassium channels. Given the high concentrations used in sodium replacement protocols, it is essential to test that it is not the replacement substances themselves, as opposed to the lack of sodium, that cause any observed effects. We therefore compared, in lobster stomatogastric neurons and leech Retzius cells, the effects of applying salines in which choline chloride replaced sodium chloride, and in which choline hydroxide or sucrose was added to normal saline. We also tested, in stomatogastric neurons, the effect of adding NMDG to normal saline. These protocols allowed us to measure the direct effects (i.e., effects not due to changes in sodium concentration or saline osmolarity or ionic strength) of choline on stomatogastric and leech currents, and of NMDG on stomatogastric currents. Choline directly reduced transient and sustained depolarization-activated outward currents in both species, and NMDG directly reduced transient depolarization-activated outward currents in stomatogastric neurons. Experiments with lower choline concentrations showed that adding as little as 150 mM (stomatogastric) or 5 mM (leech) choline reduced at least some depolarization-activated outward currents. Reductions in outward current with choline chloride or NMDG replacement alone are thus not evidence of sodium-activated potassium currents. NEW & NOTEWORTHY We show that choline or N-methyl-d-glucamine (NMDG) directly (i.e., not due to changes in extracellular sodium) decrease outward currents. Prior work studying sodium-activated potassium channels in which sodium was replaced with choline or NMDG without an addition control may therefore be artifactual.

The Mechanisms Underlying Lipid Resuscitation Therapy

The experimental use of lipid emulsion for local anesthetic toxicity was originally identified in 1998. It was then translated to clinical practice in 2006 and expanded to drugs other than local anesthetics in 2008. Our understanding of lipid resuscitation therapy has progressed considerably since the previous update from the American Society of Regional Anesthesia and Pain Medicine, and the scientific evidence has coalesced around specific discrete mechanisms. Intravenous lipid emulsion therapy provides a multimodal resuscitation benefit that includes both scavenging (eg, the lipid shuttle) and nonscavenging components. The intravascular lipid compartment scavenges drug from organs susceptible to toxicity and accelerates redistribution to organs where drug (eg, bupivacaine) is stored, detoxified, and later excreted. In addition, lipid exerts nonscavenging effects that include postconditioning (via activation of prosurvival kinases) along with cardiotonic and vasoconstrictive benefits. These effects protect tissue from ischemic damage and increase tissue perfusion during recovery from toxicity. Other mechanisms have diminished in favor based on lack of evidence; these include direct effects on channel currents (eg, calcium) and mass-effect overpowering a block in mitochondrial metabolism. In this narrative review, we discuss these proposed mechanisms and address questions left to answer in the field. Further work is needed, but the field has made considerable strides towards understanding the mechanisms.

Local anesthetic systemic toxicity: current perspectives

Local anesthetic systemic toxicity (LAST) is a life-threatening adverse event that may occur after the administration of local anesthetic drugs through a variety of routes. Increasing use of local anesthetic techniques in various healthcare settings makes contemporary understanding of LAST highly relevant. Recent data have demonstrated that the underlying mechanisms of LAST are multifactorial, with diverse cellular effects in the central nervous system and cardiovascular system. Although neurological presentation is most common, LAST often presents atypically, and one-fifth of the reported cases present with isolated cardiovascular disturbance. There are several risk factors that are associated with the drug used and the administration technique. LAST can be mitigated by targeting the modifiable risk factors, including the use of ultrasound for regional anesthetic techniques and restricting drug dosage. There have been significant developments in our understanding of LAST treatment. Key advances include early administration of lipid emulsion therapy, prompt seizure management, and careful selection of cardiovascular supportive pharmacotherapy. Cognizance of the mechanisms, risk factors, prevention, and therapy of LAST is vital to any practitioner using local anesthetic drugs in their clinical practice.

Evaluation of the cardiotoxicity and resuscitation of rats of a newly developed mixture of a QX-314 analog and levobupivacaine

Objective

This study was designed to evaluate the cardiotoxicity of a QX-314 analog (QX-OH) and a mixture of QX-OH and levobupivacaine (LL-1) and to compare the ability to resuscitate rats after asystole induced by levobupivacaine (Levo-BUP), QX-314, QX-OH, and LL-1.

Methods

First, we used the “up-and-down” method to determine median dose resulting in appearance of cardiotoxicity (CD_50C) and asystole (CD_50A) of Levo-BUP, QX-314, QX-OH, and LL-1 in rats. Safety index (SI; ratio of CD_50C compared with 2-fold median effective dose needed to produce sensory blockade) of the 4 drugs was calculated. Isobolograms were used for drug interaction analysis. Second, rats received 1.2-fold CD_50A in the 4 groups. When asystole occurred, standard cardiopulmonary resuscitation was started and continued for 30 min or until return of spontaneous circulation (ROSC) with native rate–pressure product ≥30% baseline for 5 min.

Results

Ranking of CD_50C was Levo-BUP < QX-314 ≈ QX-OH. Ranking of CD_50A was Levo-BUP < QX-314 < QX-OH. However, the SI of Levo-BUP was significantly higher than that of QX-314 (10.60 vs. 1.20) or QX-OH (10.60 vs. 1.44). The SI of LL-1 was similar to that of Levo-BUP. Nonsynergistic interaction was observed for cardiac effects between QX-OH and Levo-BUP. ROSC was attained initially by 8 of 8 rats in the Levo-BUP group, 3 of 8 in the QX-314 group, 6 of 8 in the QX-OH group, and 8 of 8 in the LL-1 group. Sustained recovery was achieved in the Levo-BUP group but not in the other groups.

Conclusion

Levo-BUP and LL-1 are safer than QX-314 or QX-OH. Cardiac effects between QX-OH and Levo-BUP were nonsynergistic. Initial successful resuscitation could be achieved in the QX-OH- and LL-1-induced asystole, but advanced life support might be needed.

Clinical Concentrations of Local Anesthetics Bupivacaine and Lidocaine Differentially Inhibit Human Kir2.x Inward Rectifier K+ Channels

BACKGROUND

Inward rectifier K channels of the Kir2.x subfamily are widely expressed in neuronal tissues, controlling neuronal excitability. Previous studies reported that local anesthetics (LAs) do not affect Kir2 channels. However, the effects have not been studied at large concentrations used in regional anesthesia.

METHODS

This study used the patch-clamp technique to examine the effects of bupivacaine and lidocaine on Kir2.1, Kir2.2, and Kir2.3 channels expressed in human embryonic kidney 293 cells.

RESULTS

When applied extracellularly in whole-cell recordings, both LAs inhibited Kir2.x currents in a voltage-independent manner. Inhibition with bupivacaine was slow and irreversible, whereas that with lidocaine was fast and reversible. Kir2.3 displayed a greater sensitivity to bupivacaine than Kir2.1 and Kir2.2 (50% inhibitory concentrations at approximately 5 minutes, 0.6 vs 8-10 mM), whereas their sensitivities to lidocaine were similar (50% inhibitory concentrations, 1.5-2.7 mM). Increases in the charged/neutral ratio of the LAs at an acidic extracellular pH attenuated their inhibitory effects, and a permanently charged lidocaine derivative QX-314 exhibited no effects when applied extracellularly. Inside-out experiments demonstrated that inhibition of Kir2.1 with cytoplasmic lidocaine and QX-314 was rapid and reversible, whereas that induced by bupivacaine was slow and irreversible. Furthermore, dose-inhibition relations for the charged form of bupivacaine and lidocaine obtained at different cytoplasmic pHs could be approximated by a single relation for each LA.

CONCLUSIONS

The results indicate that both LAs at clinical concentrations equilibrated rapidly with the intracellular milieu, differentially inhibiting Kir2.x channel function from the cytoplasmic side.

Development of functional hindbrain oculomotor circuitry independent of both vascularization and neuronal activity in larval zebrafish

We investigated the contribution of blood vessel formation and neuronal excitability to the development of functional neural circuitry in larval zebrafish by analyzing oculomotor performance in response to visual and vestibular stimuli. To address the dependence of neuronal function on the presence of blood vessels, we compared wild type embryos to reck and cloche mutants that lacked intracerebral blood vessels. To test how neuronal excitability impacts neuronal development and intracerebral vascularization, we blocked neural activity using Tetraodotoxin (TTX) and Tricaine. In reck mutants, we found both slow phase horizontal tracking and fast phase resets with only a slightly reduced amplitude and bandwidth. Spontaneous saccades, eye position holding and vestibular gravitoinertial induced eye rotation were also present. All of these behaviors except for visual tracking were observed in cloche mutants that lacked any head vasculature. Thus, numerous oculomotor neuronal circuits spanning the forebrain, midbrain and hindbrain compartments, ending in motor innervations of the eye muscles, were correctly formed and generated appropriate oculomotor behaviors without blood vessels. However, our observations indicate that beginning at approximately six days, circulation was required for sustained behavioral performance. We further found that blocking neuronal excitability with either TTX or Tricaine up to 4-5 days post fertilization did not noticeably interfere with intracerebral blood vessel formation in wild type larvae. After removal of drug treatments, the oculomotor behaviors returned within hours. Thus, development of neuronal circuits that drive oculomotor performance does not require neuronal spiking or activity. Together these findings demonstrate that neither vascularization nor neuronal excitability are essential for the formation of numerous oculomotor nuclei with intricately designed connectivity and signal processing. We conclude that a genetic blueprint specifies early larval structural and physiological features, and this developmental strategy may be viewed as a unique adaptation required for early survival.

Structural Insights into GIRK Channel Function

G protein-gated inwardly rectifying potassium (GIRK; Kir3) channels, which are members of the large family of inwardly rectifying potassium channels (Kir1-Kir7), regulate excitability in the heart and brain. GIRK channels are activated following stimulation of G protein-coupled receptors that couple to the G(i/o) (pertussis toxin-sensitive) G proteins. GIRK channels, like all other Kir channels, possess an extrinsic mechanism of inward rectification involving intracellular Mg(2+) and polyamines that occlude the conduction pathway at membrane potentials positive to E(K). In the past 17 years, more than 20 high-resolution atomic structures containing GIRK channel cytoplasmic domains and transmembrane domains have been solved. These structures have provided valuable insights into the structural determinants of many of the properties common to all inward rectifiers, such as permeation and rectification, as well as revealing the structural bases for GIRK channel gating. In this chapter, we describe advances in our understanding of GIRK channel function based on recent high-resolution atomic structures of inwardly rectifying K(+) channels discussed in the context of classical structure-function experiments.

PKC inhibition results in a Kv 1.5 + Kv β1.3 pharmacology closer to Kv 1.5 channels

BACKGROUND AND PURPOSE

The Kv β1.3 subunit modifies the gating and pharmacology of Kv 1.5 channels in a PKC-dependent manner, decreasing channel sensitivity to bupivacaine- and quinidine-mediated blockade. Cardiac Kv 1.5 channels associate with receptor for activated C kinase 1 (RACK1), the Kv β1.3 subunit and different PKC isoforms, resulting in the formation of a functional channelosome. The aim of the present study was to investigate the effects of PKC inhibition on bupivacaine and quinidine block of Kv 1.5 + Kv β1.3 channels.

EXPERIMENTAL APPROACH

HEK293 cells were transfected with Kv 1.5 + Kv β1.3 channels, and currents were recorded using the whole-cell configuration of the patch-clamp technique. PKC inhibition was achieved by incubating the cells with either calphostin C or bisindolylmaleimide II and the effects of bupivacaine and quinidine were analysed.

KEY RESULTS

The voltage-dependent inactivation of Kv 1.5 + Kv β1.3 channels and their pharmacological behaviour after PKC inhibition with calphostin C were similar to those displayed by Kv 1.5 channels alone. Indeed, the IC50 values for bupivacaine were similar in cells whose PKC was inhibited with calphostin C or bisindolylmaleimide II. Similar results were also observed in the presence of quinidine.

CONCLUSIONS AND IMPLICATIONS

The finding that the voltage-dependence of inactivation and the pharmacology of Kv 1.5 + Kv β1.3 channels after PKC inhibition resembled that observed in Kv 1.5 channels suggests that both processes are dependent on PKC-mediated phosphorylation. These results may have clinical relevance in diseases that are characterized by alterations in kinase activity.

Interaction of local anesthetics with biomembranes consisting of phospholipids and cholesterol: mechanistic and clinical implications for anesthetic and cardiotoxic effects

Despite a long history in medical and dental application, the molecular mechanism and precise site of action are still arguable for local anesthetics. Their effects are considered to be induced by acting on functional proteins, on membrane lipids, or on both. Local anesthetics primarily interact with sodium channels embedded in cell membranes to reduce the excitability of nerve cells and cardiomyocytes or produce a malfunction of the cardiovascular system. However, the membrane protein-interacting theory cannot explain all of the pharmacological and toxicological features of local anesthetics. The administered drug molecules must diffuse through the lipid barriers of nerve sheaths and penetrate into or across the lipid bilayers of cell membranes to reach the acting site on transmembrane proteins. Amphiphilic local anesthetics interact hydrophobically and electrostatically with lipid bilayers and modify their physicochemical property, with the direct inhibition of membrane functions, and with the resultant alteration of the membrane lipid environments surrounding transmembrane proteins and the subsequent protein conformational change, leading to the inhibition of channel functions. We review recent studies on the interaction of local anesthetics with biomembranes consisting of phospholipids and cholesterol. Understanding the membrane interactivity of local anesthetics would provide novel insights into their anesthetic and cardiotoxic effects.

Interaction of local anesthetics with the K (+) channel pore domain: KcsA as a model for drug-dependent tetramer stability

Local anesthetics and related drugs block ionic currents of Na (+) , K (+) and Ca ( 2+) conducted across the cell membrane by voltage-dependent ion channels. Many of these drugs bind in the permeation pathway, occlude the pore and stop ion movement. However channel-blocking drugs have also been associated with decreased membrane stability of certain tetrameric K (+) channels, similar to the destabilization of channel function observed at low extracellular K (+) concentration. Such drug-dependent stability may result from electrostatic repulsion of K (+) from the selectivity filter by a cationic drug molecule bound in the central cavity of the channel. In this study we used the pore domain of the KcsA K (+) channel protein to test this hypothesis experimentally with a biochemical assay of tetramer stability and theoretically by computational simulation of local anesthetic docking to the central cavity. We find that two common local anesthetics, lidocaine and tetracaine, promote thermal dissociation of the KcsA tetramer in a K (+) -dependent fashion. Docking simulations of these drugs with open, open-inactivated and closed crystal structures of KcsA yield many energetically favorable drug-channel complexes characterized by nonbonded attraction to pore-lining residues and electrostatic repulsion of K (+) . The results suggest that binding of cationic drugs to the inner cavity can reduce tetramer stability of K (+) channels.

The contribution of hydrophobic residues in the pore-forming region of the ryanodine receptor channel to block by large tetraalkylammonium cations and Shaker B inactivation peptides

Although no high-resolution structural information is available for the ryanodine receptor (RyR) channel pore-forming region (PFR), molecular modeling has revealed broad structural similarities between this region and the equivalent region of K⁺ channels. This study predicts that, as is the case in K⁺ channels, RyR has a cytosolic vestibule lined with predominantly hydrophobic residues of transmembrane helices (TM10). In K⁺ channels, this vestibule is the binding site for blocking tetraalkylammonium (TAA) cations and Shaker B inactivation peptides (ShBPs), which are stabilized by hydrophobic interactions involving specific residues of the lining helices. We have tested the hypothesis that the cytosolic vestibule of RyR fulfils a similar role and that TAAs and ShBPs are stabilized by hydrophobic interactions with residues of TM10. Both TAAs and ShBPs block RyR from the cytosolic side of the channel. By varying the composition of TAAs and ShBPs, we demonstrate that the affinity of both species is determined by their hydrophobicity, with variations reflecting alterations in the dissociation rate of the bound blockers. We investigated the role of TM10 residues of RyR by monitoring block by TAAs and ShBPs in channels in which the hydrophobicity of individual TM10 residues was lowered by alanine substitution. Although substitutions changed the kinetics of TAA interaction, they produced no significant changes in ShBP kinetics, indicating the absence of specific hydrophobic sites of interactions between RyR and these peptides. Our investigations (a) provide significant new information on both the mechanisms and structural components of the RyR PFR involved in block by TAAs and ShBPs, (b) highlight important differences in the mechanisms and structures determining TAA and ShBP block in RyR and K⁺ channels, and (c) demonstrate that although the PFRs of these channels contain analogous structural components, significant differences in structure determine the distinct ion-handling properties of the two species of channel.

Polyunsaturated Fatty acids modify the gating of kv channels

Polyunsaturated fatty acids (PUFAs) have been reported to exhibit antiarrhythmic properties, which are attributed to their capability to modulate ion channels. This PUFAs ability has been reported to be due to their effects on the gating properties of ion channels. In the present review, we will focus on the role of PUFAs on the gating of two Kv channels, Kv1.5 and Kv11.1. Kv1.5 channels are blocked by n-3 PUFAs of marine [docosahexaenoic acid (DHA) and eicosapentaenoic acid] and plant origin (alpha-linolenic acid, ALA) at physiological concentrations. The blockade of Kv1.5 channels by PUFAs steeply increased in the range of membrane potentials coinciding with those of Kv1.5 channel activation, suggesting that PUFAs-channel binding may derive a significant fraction of its voltage sensitivity through the coupling to channel gating. A similar shift in the activation voltage was noted for the effects of n-6 arachidonic acid (AA) and DHA on Kv1.1, Kv1.2, and Kv11.1 channels. PUFAs-Kv1.5 channel interaction is time-dependent, producing a fast decay of the current upon depolarization. Thus, Kv1.5 channel opening is a prerequisite for the PUFA-channel interaction. Similar to the Kv1.5 channels, the blockade of Kv11.1 channels by AA and DHA steeply increased in the range of membrane potentials that coincided with the range of Kv11.1 channel activation, suggesting that the PUFAs-Kv channel interactions are also coupled to channel gating. Furthermore, AA regulates the inactivation process in other Kv channels, introducing a fast voltage-dependent inactivation in non-inactivating Kv channels. These results have been explained within the framework that AA closes voltage-dependent potassium channels by inducing conformational changes in the selectivity filter, suggesting that Kv channel gating is lipid dependent.

Update on local anesthetics: focus on levobupivacaine

In recent years levobupivacaine, the pure S (−)-enantiomer of bupivacaine, emerged as a safer alternative for regional anesthesia than its racemic parent. It demonstrated less affinity and strength of depressant effects onto myocardial and central nervous vital centers in pharmacodynamic studies, and a superior pharmacokinetic profile. Clinically, levobupivacaine is well tolerated in a variety of regional anesthesia techniques both after bolus administration and continuous postoperative infusion. Reports of toxicity with levobupivacaine are scarce and occasional toxic symptoms are usually reversible with minimal treatment with no fatal outcome. Yet, levobupivacaine has not entirely replaced bupivacaine in clinical practice. In anesthesia and analgesia practice, levobupivacaine and bupivacaine produce comparable surgical sensory block with similar adverse side effects, and equal labor pain control with comparable maternal and fetal outcome. The equipotency of the two drugs has been recently questioned, prompting clinicians to increase the dose of levobupivacaine in an attempt to ensure adequate anesthesia and analgesia and offsetting, therefore, the advantages of less motor block with levobupivacaine. In this review we aim to discuss the pharmacological essentials of the safer profile of levobupivacaine, and analyze the evidence regarding the current clinical indications.

The influence of age on bupivacaine cardiotoxicity

BACKGROUND

The susceptibility of children and newborns to cardiotoxicity from racemic bupivacaine, RS(±)-bupivacaine, is controversial. Some studies indicate that newborns can sustain higher bupivacaine plasma levels than adults, without severe toxicity. In this study, we compared the influence of age on cardiotoxicity from RS(±)-bupivacaine and S(-)-bupivacaine in rats. The effects of these local anesthetics (LAs) on the regulation of intracellular Ca(2+) concentrations in cardiac fibers were also investigated.

METHODS

The lethal dose was determined in ventilated male Wistar rats at 2, 4, 8, and 16 weeks of age by monitoring when cardiac electrical activity stopped after infusion of RS(±)-bupivacaine and S(-)-bupivacaine (4 mg · kg(-1) · min(-1)). The effects on cardiac muscle contraction were investigated by in vitro measurement of papillary muscle twitches in the presence and absence of RS(±)-bupivacaine or S(-)-bupivacaine. Skinned ventricular fibers were used to investigate the intracellular effects on Ca(2+) regulation induced by both LAs.

RESULTS

The lethal dose for RS(±)-bupivacaine and S(-)-bupivacaine in 2-week-old animals (46.0 ± 5.2 and 91.3 ± 4.9 mg · kg(-1), respectively) was higher than in 16-week-old animals (22.7 ± 1.3 and 22.0 ± 2.7 mg · kg(-1), respectively). Papillary muscle twitches were reduced in a dose-dependent manner, with significant difference between young and adult hearts. In adults, the muscle twitches were reduced to 8.6% ± 0.8% of control by RS(±)-bupivacaine, and to 18.1% ± 2.7% of control by S(-)-bupivacaine (100 μM). S(-)-bupivacaine had a positive inotropic effect at <10 μM, but only in 2-week-old animals. In chemically skinned ventricular fibers, RS(±)-bupivacaine and S(-)-bupivacaine induced similar increases in Ca(2+) release from the sarcoplasmic reticulum (SR) preactivated with caffeine (1 mM), and this effect was greater in younger rats than adults. In 16-week-old rats, caffeine-induced tension was 53.9% ± 1.7% of the maximal fiber response with RS(±)-bupivacaine, and 54.1% ± 3.2% with S(-)-bupivacaine. The caffeine response in 2-week-old rats was 81.1% ± 3.7% of the maximal response with RS(±)-bupivacaine, and 78.1% ± 4.5% with S(-)-bupivacaine. The Ca(2+) sensitivity of contractile proteins was equally increased at both ages tested, with RS(±)-bupivacaine or S(-)-bupivacaine. Ca(2+) uptake from the SR was not altered by the LA or by age.

CONCLUSIONS

Differences in the mechanisms for regulating intracellular SR Ca(2+) may contribute to the decreased susceptibility of young animals to cardiodepression induced by RS(±)-bupivacaine and S(-)-bupivacaine.

Stereoselective Inhibition of the hERG1 Potassium Channel

A growing number of drugs have been shown to prolong cardiac repolarization, predisposing individuals to life-threatening ventricular arrhythmias known as Torsades de Pointes. Most of these drugs are known to interfere with the human ether à-gogo related gene 1 (hERG1) channel, whose current is one of the main determinants of action potential duration. Prolonged repolarization is reflected by lengthening of the QT interval of the electrocardiogram, as seen in the suitably named drug-induced long QT syndrome. Chirality (presence of an asymmetric atom) is a common feature of marketed drugs, which can therefore exist in at least two enantiomers with distinct three-dimensional structures and possibly distinct biological fates. Both the pharmacokinetic and pharmacodynamic properties can differ between enantiomers, as well as also between individuals who take the drug due to metabolic polymorphisms. Despite the large number of reports about drugs reducing the hERG1 current, potential stereoselective contributions have only been scarcely investigated. In this review, we present a non-exhaustive list of clinically important molecules which display chiral toxicity that may be related to hERG1-blocking properties. We particularly focus on methadone cardiotoxicity, which illustrates the importance of the stereoselective effect of drug chirality as well as individual variations resulting from pharmacogenetics. Furthermore, it seems likely that, during drug development, consideration of chirality in lead optimization and systematic assessment of the hERG1 current block with all enantiomers could contribute to the reduction of the risk of drug-induced LQTS.

The acute toxicity of local anesthetics

IMPORTANCE OF THE FIELD

Systemic toxicity, usually from overdose or intravascular dose, is feared because it mainly affects the heart and brain, and may be acutely life-threatening.

AREAS COVERED IN THIS REVIEW

Pharmacological studies of local anesthetic toxicity have largely been reviewed primarily relating to the evaluation of ropivacaine and levobupivacaine during the past decade. This review/opinion focuses more on the principles and concepts underlying the main models used, from chemical pharmacological and pharmacokinetic perspectives.

WHAT THE READER WILL GAIN

Research models required to produce pivotal toxicity data are discussed. The potencies for neural blockade and systemic toxicity are associated across virtually all models, with some deviations through molecular stereochemistry. These models show that all local anesthetics can produce direct cardiovascular system toxicity and CNS excitotoxicity that may further affect the cardiovascular system response. Whereas the longer-acting local anesthetics are more likely to cause cardiac death by malignant arrhythmias, the shorter-acting agents are more likely to cause cardiac contraction failure. In most models, equi-anesthetic doses of ropivacaine and levobupivacaine are less likely to produce serious toxicity than bupivacaine.

TAKE HOME MESSAGE

Of the various models, this reviewer favors a whole-body large animal preparation because of the comprehensive data collection possible. The conscious sheep preparation has contributed more than any other, and may be regarded as the de facto 'standard' experimental model for concurrent study of local anesthetic toxicity ± pharmacokinetics, using experimental designs that can reproduce the toxicity seen in clinical accidents.

Overlapping binding sites of structurally different antiarrhythmics flecainide and propafenone in the subunit interface of potassium channel Kv2.1

Kv2.1 channels, which are expressed in brain, heart, pancreas, and other organs and tissues, are important targets for drug design. Flecainide and propafenone are known to block Kv2.1 channels more potently than other Kv channels. Here, we sought to explore structural determinants of this selectivity. We demonstrated that flecainide reduced the K(+) currents through Kv2.1 channels expressed in Xenopus laevis oocytes in a voltage- and time-dependent manner. By systematically exchanging various segments of Kv2.1 with those from Kv1.2, we determined flecainide-sensing residues in the P-helix and inner helix S6. These residues are not exposed to the inner pore, a conventional binding region of open channel blockers. The flecainide-sensing residues also contribute to propafenone binding, suggesting overlapping receptors for the drugs. Indeed, propafenone and flecainide compete for binding in Kv2.1. We further used Monte Carlo-energy minimizations to map the receptors of the drugs. Flecainide docking in the Kv1.2-based homology model of Kv2.1 predicts the ligand ammonium group in the central cavity and the benzamide moiety in a niche between S6 and the P-helix. Propafenone also binds in the niche. Its carbonyl group accepts an H-bond from the P-helix, the amino group donates an H-bond to the P-loop turn, whereas the propyl group protrudes in the pore and blocks the access to the selectivity filter. Thus, besides the binding region in the central cavity, certain K(+) channel ligands can expand in the subunit interface whose residues are less conserved between K(+) channels and hence may be targets for design of highly desirable subtype-specific K(+) channel drugs.

I(Kur)/Kv1.5 channel blockers for the treatment of atrial fibrillation

Atrial fibrillation (AF) is the most common sustained arrhythmia. Anti-arrhythmic drugs remain the mainstay of therapy, but the available class I and III anti-arrhythmic drugs are only moderately effective in long-term restoring/maintaining sinus rhythm (SR) and can produce potentially fatal ventricular pro-arrhythmia. In an attempt to identify safer and more effective anti-arrhythmic drugs, drug discovery efforts have focused on 'atrial selective drugs' that target cardiac ion channel(s) that are exclusively or predominantly expressed in the atria. The ultra-rapid activating delayed rectifier K(+) current (I(Kur)), carried by Kv1.5 channels, is a major repolarizing current in human atria, but seems to play no role in the ventricle. This finding offers the possibility of developing selective I(Kur) blockers to restore and maintain SR without a risk of ventricular pro-arrhythmia. Several I(Kur) blockers are now being developed but clinical data are still limited, so the precise role of these agents in the treatment of AF remains to be defined. In this review we analyze the possible advantages and disadvantages of the developmental I(Kur) blockers as they represent the first step for the development of potential atrial selective drugs for a more effective and safer treatment and prevention of AF.

Effects of bupivacaine and ropivacaine on field potential in rat hippocampal slices

BACKGROUND

In spite of more than 20 yr of research, the mechanism whereby local anaesthetics act on the brain to mediate anaesthesia still remains unclear. Furthermore, the effect of local anaesthetics on neuronal excitability and synaptic transmission in the hippocampus has not been reported. Thus, the purpose of the present study was to find out the differences between the local anaesthetics, bupivacaine and ropivacaine, in their actions on synaptic transmission of brain in the context of hippocampal field potential.

METHODS

Brains were removed from 3- to 4-week-old rats and transverse slices (300 microm thick) were prepared using a microslicer. A slice was then placed on the centre on a multielectrode dish probe. To record evoked field potentials at 64 sites, a pair of single planar microelectrodes delivering bipolar constant current pulses (45-90 microA, 0.1 ms) was applied. Electrophysiological recordings were measured using the 64-channel multielectrode dish.

RESULTS

The amplitude of field potential in the rat CA1 region was inhibited by both bupivacaine and ropivacaine. The inhibitory effects of bupivacaine and ropivacaine on field potential amplitudes in CA1 were similar. For bupivacaine 10 microg ml(-1), inhibited field potentials were incompletely recovered; in contrast, for 10 ropivacaine microg ml(-1), inhibited field potentials were completely recovered after washing out with incubation solution.

CONCLUSIONS

Inhibitory effects of bupivacaine and ropivacaine on hippocampal field potential amplitude and recovery rate after washout after bupivacaine or ropivacaine treatment represent the underlying mechanisms of the systemic toxicity of local anaesthetics.

The toxicity of local anesthetics: the place of ropivacaine and levobupivacaine

PURPOSE OF REVIEW

Ropivacaine and levobupivacaine were developed after evidence of bupivacaine-related severe toxicity. Despite a comparable analgesic profile, quantitative differences become evident with regard to their specific rate of systemic toxicity. The present article provides a concise review of the toxic potencies of levobupivacaine and ropivacaine.

RECENT FINDINGS

As lipophilicity is known to be a major determinant in local anesthetic toxicity, the clinical safety profile of ropivacaine seems to be more favorable than that of levobupivacaine. Experimental studies and case reports confirm this hypothesis, showing that ropivacaine is characterized by fewer (cardio) toxic effects and, most probably, a greater margin of safety. Both agents also may dose dependently damage neurons and skeletal muscle tissue at the injection site. Although their specific rate of neurotoxicity appears to be rather low, levobupivacaine is characterized by an outstanding myotoxic potential.

SUMMARY

Compared with bupivacaine, both agents may be considered as 'more well tolerated' but not as 'totally well tolerated', as they are still capable of inducing systemic and local toxicity. However, ropivacaine seems to have the greatest margin of safety of all long-acting local anesthetics at present.

Kv1.5 open channel block by the antiarrhythmic drug disopyramide: molecular determinants of block

Kv1.5 is considered to be a potential molecular target for treatment of atrial fibrillation or flutter. Disopyramide is widely used in the treatment of atrial flutter and/or atrial fibrillation. The present study was undertaken to characterize the effects of disopyramide on currents mediated by Kv1.5 channels and to determine the putative binding site involved in the inhibitory effects of disopyramide. Experiments were carried out on wild-type and site directed mutated hKv1.5 channels expressed on HEK 293 cells using the patch-clamp technique. Disopyramide acting from the cytoplasmic side of the membrane produced blocking effects on Kv1.5 that exhibited several features typical of an open channel blocker. Ala-scanning mutagenesis of the Kv1.5 pore domain combined with macroscopic current analysis suggested that disopyramide interacted only with the Val512 residue that faces to the central cavity of the channel. Mutation of this key residue to Ala caused marked change in the IC(50) of disopyramide (22-fold). The single interaction between disopyramide and Val512 in the PVP region is able to change the mechanism of channel closure, reproducing the "foot-in-the-door" phenomenon.

Role of protein kinase Czeta and its adaptor protein p62 in voltage-gated potassium channel modulation in pulmonary arteries

Voltage-gated potassium (K(V)) channels play an essential role in regulating pulmonary artery function, and they underpin the phenomenon of hypoxic pulmonary vasoconstriction. Pulmonary hypertension is characterized by inappropriate vasoconstriction, vascular remodeling, and dysfunctional K(V) channels. In the current study, we aimed to elucidate the role of PKCzeta and its adaptor protein p62 in the modulation of K(V) channels. We report that the thromboxane A(2) analog 9,11-dideoxy-11alpha,9alpha-epoxymethano-prostaglandin F(2alpha) methyl acetate (U46619) inhibited K(V) currents in isolated mice pulmonary artery myocytes and the K(V) current carried by human cloned K(V)1.5 channels expressed in Ltk(-) cells. Using protein kinase C (PKC)zeta(-/-) and p62(-/-) mice, we demonstrate that these two proteins are involved in the K(V) channel inhibition. PKCzeta coimmunoprecipitated with K(V)1.5, and this interaction was markedly reduced in p62(-/-) mice. Pulmonary arteries from PKCzeta(-/-) mice also showed a diminished [Ca(2+)](i) and contractile response, whereas genetic inactivation of p62(-/-) resulted in an absent [Ca(2+)](i) response, but it preserved contractile response to U46619. These data demonstrate that PKCzeta and its adaptor protein p62 play a key role in the modulation of K(V) channel function in pulmonary arteries. These observations identify PKCzeta and/or p62 as potential therapeutic targets for the treatment of pulmonary hypertension.

Effects of capsaicin on VGSCs in TRPV1-/- mice

Two different mechanisms by which capsaicin blocks voltage-gated sodium channels (VGSCs) were found by using knockout mice for the transient receptor potential V1 (TRPV1(-/-)). Similar with cultured rat trigeminal ganglion (TG) neurons, the amplitude of tetrodotoxin-resistant (TTX-R) sodium current was reduced 85% by 1 muM capsaicin in capsaicin sensitive neurons, while only 6% was blocked in capsaicin insensitive neurons of TRPV1(+/+) mice. The selective effect of low concentration capsaicin on VGSCs was reversed in TRPV1(-/-) mice, which suggested that this effect was dependent on TRPV1 receptor. The blockage effect of high concentration capsaicin on VGSCs in TRPV1(-/-) mice was the same as that in capsaicin insensitive neurons of rats and TRPV1(+/+) mice. It is noted that non-selective effect of capsaicin on VGSCs shares many similarities with local anesthetics. That is, firstly, both blockages are concentration-dependent and revisable. Secondly, being accompanied with the reduction of amplitude, voltage-dependent inactivation curve shifts to hyperpolarizing direction without a shift of activation curve. Thirdly, use-dependent blocks are induced at high stimulus frequency.

Local anesthetics

Local anesthetics are used broadly to prevent or reverse acute pain and treat symptoms of chronic pain. This chapter, on the analgesic aspects of local anesthetics, reviews their broad actions that affect many different molecular targets and disrupt their functions in pain processing. Application of local anesthetics to peripheral nerve primarily results in the blockade of propagating action potentials, through their inhibition of voltage-gated sodium channels. Such inhibition results from drug binding at a site in the channel's inner pore, accessible from the cytoplasmic opening. Binding of drug molecules to these channels depends on their conformation, with the drugs generally having a higher affinity for the open and inactivated channel states that are induced by membrane depolarization. As a result, the effective potency of these drugs for blocking impulses increases during high-frequency repetitive firing and also under slow depolarization, such as occurs at a region of nerve injury, which is often the locus for generation of abnormal, pain-related ectopic impulses. At distal and central terminals the inhibition of voltage-gated calcium channels by local anesthetics will suppress neurogenic inflammation and the release of neurotransmitters. Actions on receptors that contribute to nociceptive transduction, such as TRPV1 and the bradykinin B2 receptor, provide an independent mode of analgesia. In the spinal cord, where local anesthetics are present during epidural or intrathecal anesthesia, inhibition of inotropic receptors, such as those for glutamate, by local anesthetics further interferes with neuronal transmission. Activation of spinal cord mitogen-activated protein (MAP) kinases, which are essential for the hyperalgesia following injury or incision and occur in both neurons and glia, is inhibited by spinal local anesthetics. Many G protein-coupled receptors are susceptible to local anesthetics, with particular sensitivity of those coupled via the Gq alpha-subunit. Local anesthetics are also infused intravenously to yield plasma concentrations far below those that block normal action potentials, yet that are frequently effective at reversing neuropathic pain. Thus, local anesthetics modify a variety of neuronal membrane channels and receptors, leading to what is probably a synergistic mixture of analgesic mechanisms to achieve effective clinical analgesia.

Regulation of Intracellular Calcium by Bupivacaine Isomers in Cardiac Myocytes from Wistar Rats

In this study we investigated the effects of a racemic mixture of bupivacaine (RS(+/-)bupivacaine) and its isomers (S(-)bupivacaine and R(+)bupivacaine) on the Ca2+ handling by ventricular myocytes from Wistar rats. Single ventricular myocytes were enzymatically isolated and loaded with the fluorescent Ca2+ indicator fura 2-am to estimate intracellular Ca2+ concentration during contraction and relaxation cycles. S(-)bupivacaine (10 muM) significantly increased peak amplitude and the rate of increase of Ca2+ transients in 155% +/- 54% (P < 0.05) and 194% +/- 94% (P < 0.01) of control. However, exposure to R(+)bupivacaine had no effect on either peak amplitude or rate of increase at any concentration tested. Saponin-skinned ventricular fibers were used to investigate the effect of bupivacaine on the intracellular Ca2+ regulation by sarcoplasmic reticulum (SR) and on the Ca2+ sensitivity of contractile system. S(-), R(+), and RS(+/-)bupivacaine induced Ca2+ release from SR (P < 0.01). In SR-disrupted skinned ventricular cells, bupivacaine and its isomers (5 mM) increased the sensitivity of contractile system to Ca(2+). S(-), RS(+/-), and R(+)bupivacaine significantly increased pCa50 from 5.8 +/- 0.1, 5.8 +/- 0.1, and 5.8 +/- 0.1, to 6.1 +/- 0.1 (P < 0.05), 6.0 +/- 0.1 (P < 0.05), and 6.1 +/- 0.1 (P < 0.05). Ca2+ release from SR through RyR2 activation could explain the increase of Ca2+ transients in cardiac cells. Increased intracellular Ca2+ in cardiac myocytes display a stereoselectivity to S(-)bupivacaine.

Interactions of anesthetics with their targets: non-specific, specific or both?

What makes a general anesthetic a general anesthetic? We shall review first what general anesthesia is all about and which drugs are being used as anesthetics. There is neither a unique definition of general anesthesia nor any consensus on how to measure it. Diverse drugs and combinations of drugs generate general anesthetic states of sometimes very different clinical quality. Yet the principal drugs are still considered to belong to the same class of 'general anesthetics'. Effective concentrations of inhalation anesthetics are in the high micromolar range and above, and even for intravenous anesthetics they do not go below the micromolar range. At these concentrations, many molecular and higher level targets are affected by inhalation anesthetics, fewer probably by intravenous anesthetics. The only physicochemical characteristic shared by anesthetics is the correlation of their anesthetic potencies with hydrophobicity. These correlations depend on the group of general anesthetics considered. In this review, anesthetic potencies for many different targets are plotted against octanol/water partition coefficients as measure of hydrophobicity. Qualitatively, similar correlations result, suggesting several but weak interactions with proteins as being characteristic of anesthetic actions. The polar interactions involved are weak, being roughly equal in magnitude to hydrophobic interactions. Generally, intravenous anesthetics are noticeably more potent than inhalation anesthetics. They differ considerably more between each other in their interactions with various targets than inhalation anesthetics do, making it difficult to come to a decision which of these should be used in future studies as representative 'prototypical general anesthetics'.

A double-blind comparison of 0.5% bupivacaine with 1:200,000 epinephrine and 0.5% levobupivacaine with 1:200,000 epinephrine for the inferior alveolar nerve block

This double-blind cross-over study compared the anesthetic success and onset and duration of lip and pulpal anesthesia of 0.5% bupivacaine and levobupivacaine solutions, both with 1:200,000 epinephrine, when administered for inferior alveolar nerve anesthesia. Thirty healthy volunteers were randomly anesthetized using one of the solutions. The inferior canine, second premolar, and molar were tested with electric stimulation. The pulpal anesthetic success rates for bupivacaine and levobupivacaine were 80% and 76.66%, respectively, for molars, 76.66% (both solutions) for premolars, and 70% (both solutions) for canines. At least 250 minutes of pulpal anesthesia was achieved. There were no significant differences between the solutions considering the measured parameters (P > .05). Because of the similar anesthetic behavior of the 2 solutions in this study and the low toxicity related in the literature for levobupivacaine, there is justification for replacing bupivacaine with levobupivacaine for inferior alveolar nerve local anesthesia.

Expression and characterization of delayed rectifying K+channels in anterior rat taste buds

Delayed rectifying K+(DRK) channels in taste cells have been implicated in the regulation of cell excitability and as potential targets for direct and indirect modulation by taste stimuli. In the present study, we have used patch-clamp recording to determine the biophysical properties and pharmacological sensitivity of DRK channels in isolated rat fungiform taste buds. Molecular biological assays at the taste bud and single-cell levels are consistent with the interpretation that taste cells express a variety of DRK channels, including members from each of the three major subfamilies: KCNA, KCNB, and KCNC. Real-time PCR assays were used to quantify expression of the nine DRK channel subtypes. While taste cells express a number of DRK channels, the electrophysiological and molecular biological assays indicate that the Shaker Kv1.5 channel (KCNA5) is the major functional DRK channel expressed in the anterior rat tongue.

Bupivacaine, levobupivacaine and ropivacaine: are they clinically different?

Two new, long-acting local anaesthetics have been developed after the evidence of bupivacaine-related severe toxicity: levobupivacaine and ropivacaine. Both these agents are pure left-isomers and, based on their three-dimensional structure, they have less toxic potential both on the central nervous system and on the heart. Several clinical studies have evaluated their toxicology and clinical profiles: theoretically and experimentally, some differences can be seen, but the reflections of these characteristics into clinical practice have not been evident. Evaluating randomised, controlled trials that have compared these three local anaesthetics, this chapter supports the evidence that both levobupivacaine and ropivacaine have a clinical profile similar to that of racemic bupivacaine, and that the minimal differences observed between the three agents are mainly related to the slightly different anaesthetic potency, with racemic bupivacaine>levobupivacaine>ropivacaine. However, the reduced toxic potential of the two pure left-isomers supports their use in those clinical situations in which the risk of systemic toxicity related to either overdosing or unwanted intravascular injection is high, such as during epidural or peripheral nerve blocks.

A proposed mechanism of bupivacaine-induced contraction of human umbilical artery smooth muscle cells

This in vitro study using ring preparations of human umbilical vessels and cultured human umbilical artery smooth muscle cells was designed to determine: (a) the mechanism of bupivacaine-induced contraction of ring preparations, and (b) whether similar concentrations of bupivacaine release Cal(2+) in cultured smooth muscle cells. Isometric tension was recorded from ring preparations of human umbilical veins and arteries in an isolated tissue chamber. Separate fluorescence and electrophysiology studies were done with cultured human umbilical artery smooth muscle cells. Bupivacaine-evoked contractions of ring preparations were either tonic or twitch in nature. The contraction of ring preparations was dependent on extracellular Cal(2+) and sensitive to nifedipine inhibition. Bupivacaine also increased intracellular Cal(2+) in patterns consistent with tonic or phasic tension responses seen in isometric recordings. In addition, the membrane-resting potential was depolarized by bupivacaine. Since similar concentrations of bupivacaine caused both contraction and a rise in intracellular Ca(2+), the bupivacaine-evoked contraction was the result of increased cell Cal(2+) and the source of this Ca(2+) was the extracellular space.

Levobupivacaine: a new safer long acting local anaesthetic agent

The choice of local anaesthetic is influenced by several factors; it must provide effective anaesthesia and analgesia for the duration of the procedure and meet the expectations for post-operative pain management. Of primary concern is patient safety. Bupivacaine, currently the most widely used long acting local anaesthetic agent in both surgery and obstetrics, generally has a good safety record but its use has resulted in fatal cardiotoxicity, usually after accidental intravascular injection. Hence, for several years there has been a need for a long acting local anaesthetic, similar to bupivacaine, but with an improved cardiovascular safety profile. Levobupivacaine, the single enantiomer version of bupivacaine, offers a new long acting local anaesthetic, clinically equivalent in anaesthetic potency to bupivacaine, but with a reduced toxicity profile. Preclinical studies, from in vitro in single ion channels to whole large animal models, have unquestionably demonstrated that levobupivacaine is significantly less CNS toxic and cardiotoxic than bupivacaine. Cardiotoxicity is less easy to study in man, as the clinical signs are not usually seen until the CNS toxicity is marked, and well beyond that which is tolerable to volunteers or patients. Nevertheless, levobupivacaine has been shown to have less effect on myocardial contractility and QTc prolongation, early signs of cardiotoxicity, than bupivacaine in healthy subjects. In clinical use levobupivacaine has been shown to be equally efficacious as bupivacaine at comparable doses and concentrations, and has been found to produce similar anaesthetic characteristics (onset, duration and density of block). As levobupivacaine now becomes commercially available, the database available with which to make efficacy and safety comparisons with other local anaesthetics will increase, and the true value of this new long acting local anaesthetic should become even more apparent.

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.

Potassium, sodium, calcium and glutamate-gated channels: pore architecture and ligand action

In the last decade, the idea of common organization of certain ion channel families exhibiting diverse physiological and pharmacological properties has received strong experimental support. Transmembrane topologies and patterns of the pore-facing residues are conserved in P-loop channels that include high-selective cation channels and certain ligand-gated channels. X-ray structures of bacterial K+ channels, KcsA, MthK and KvAP, help to understand structure-function relationships of other P-loop channels. Data on binding sites and mechanisms of action of ligands of K+, Na+, Ca2+ and glutamate gated ion channels are considered in view of their possible structural similarity to the bacterial K+ channels. Emphasized are structural determinants of ligand-receptor interactions within the channels and mechanisms of state-dependent action of the ligands.