Molecular determinants of stereoselective bupivacaine block of hKv1.5 channels

Rational Design and Synthesis of 3-Morpholine Linked Aromatic-Imino-1H-Indoles as Novel Kv1.5 Channel Inhibitors Sharing Vasodilation Effects

Atrial fibrillation (AF) is the most common clinical sustained arrhythmia; clinical therapeutic drugs have low atrial selectivity and might cause more severe ventricle arrhythmias while stopping AF. As an anti-AF drug target with high selectivity on the atrial muscle cells, the undetermined crystal structure of Kv1.5 potassium channel impeded further new drug development. Herein, with the simulated 3D structure of Kv1.5 as the drug target, a series of 3-morpholine linked aromatic amino substituted 1H-indoles as novel Kv1.5 channel inhibitors were designed and synthesized based on target–ligand interaction analysis. The synthesis route was practical, starting from commercially available material, and the chemical structures of target compounds were characterized. It was indicated that compounds T16 and T5 (100 μM) exhibited favorable inhibitory activity against the Kv1.5 channel with an inhibition rate of 70.8 and 57.5% using a patch clamp technique. All compounds did not exhibit off-target effects against other drug targets, which denoted some selectivity on the Kv1.5 channel. Interestingly, twelve compounds exhibited favorable vasodilation activity on pre-contracted arterial rings in vitro using KCl or phenylephrine (PE) by a Myograph. The vasodilation rates of compounds T16 and T4 (100 μM) even reached over 90%, which would provide potential lead compounds for both anti-AF and anti-hypertension new drug development.

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.

Local anaesthetic refinement of pentobarbital euthanasia reduces abdominal writhing without affecting immunohistochemical endpoints in rats

Sodium pentobarbital is a commonly used agent for euthanizing laboratory rats, however its high pH can cause abdominal discomfort after intraperitoneal injection. Previous studies suggest that the addition of a local anaesthetic may alleviate this discomfort, but the practice has not been widely adopted. We examined the effect of combining lidocaine with pentobarbital on abdominal writhing, defecation, ultrasonic vocalizations, the rat grimace scale and immunohistochemical staining for c-Fos in the nucleus accumbens and basolateral amygdala of the brain. We also compared the amount of abdominal writhing following intraperitoneal administration of pentobarbital–lidocaine with that of pentobarbital–bupivacaine. Our results show that lidocaine reduces abdominal writhing and defecation without affecting immunohistochemistry for c-Fos or latency to loss of posture. However, scores on the rat grimace scale were low in both situations and almost no ultrasonic vocalizations were recorded. Additionally, we found that the amount of abdominal writhing was not significantly different when bupivacaine was used rather than lidocaine. Our results suggest that pentobarbital-induced euthanasia can be refined with the addition of lidocaine or other local anaesthetics.

Dynamics and modulation studies of human voltage gated Kv1.5 channel

The voltage gated Kv1.5 channels conduct the ultrarapid delayed rectifier current (IK_ur) and play critical role in repolarization of action potential duration. It is the most rapidly activated channel and has very little or no inactivated states. In human cardiac cells, these channels are expressed more extensively in atrial myocytes than ventricle. From the evidences of its localization and functions, Kv1.5 has been declared a selective drug target for the treatment of atrial fibrillation (AF). In this present study, we have tried to identify the rapidly activating property of Kv1.5 and studied its mode of inhibition using molecular modeling, docking, and simulation techniques. Channel in open conformation is found to be stabilized quickly within the dipalmitoylphosphatidylcholine membrane, whereas most of the secondary structure elements were lost in closed state conformation. The obvious reason behind its ultra-rapid property is possibly due to the amino acid alteration in S4-S5 linker; the replacement of Lysine by Glutamine and vice versa. The popular published drugs as well as newly identified lead molecules were able to inhibit the Kv1.5 in a very similar pattern, mainly through the nonpolar interactions, and formed sable complexes. V512 is found as the main contributor for the interaction along with the other important residues such as V505, I508, A509, V512, P513, and V516. Furthermore, two screened novel compounds show surprisingly better inhibitory potency and can be considered for the future perspective of antiarrhythmic survey.

Discovery and optimisation of potent and highly subtype selective Nav1.8 inhibitors with reduced cardiovascular liabilities

Potent and subtype selective Na_v1.8 inhibitors were designed and optimised for selectivity over hERG ion channel inhibition.

Elisidepsin Interacts Directly with Glycosylceramides in the Plasma Membrane of Tumor Cells to Induce Necrotic Cell Death

Plasma membrane integrity is essential for cell life. Any major break on it immediately induces the death of the affected cell. Different molecules were described as disrupting this cell structure and thus showing antitumor activity. We have previously defined that elisidepsin (Irvalec^®, PM02734) inserts and self-organizes in the plasma membrane of tumor cells, inducing a rapid loss of membrane integrity, cell permeabilization and necrotic death. Here we show that, in sensitive HCT-116 colorectal cells, all these effects are consequence of the interaction of elisidepsin with glycosylceramides in the cell membrane. Of note, an elisidepsin-resistant subline (HCT-116-Irv) presented reduced levels of glycosylceramides and no accumulation of elisidepsin in the plasma membrane. Consequently, drug treatment did not induce the characteristic necrotic cell death. Furthermore, GM95, a mutant derivative from B16 mouse melanoma cells lacking ceramide glucosyltransferase (UGCG) activity and thus the synthesis of glycosylceramides, was also resistant to elisidepsin. Over-expression of UGCG gene in these deficient cells restored glycosylceramides synthesis, rendering them sensitive to elisidepsin, at a similar level than parental B16 cells. These results indicate that glycosylceramides act as membrane targets of elisidepsin, facilitating its insertion in the plasma membrane and the subsequent membrane permeabilization that leads to drug-induced cell death. They also indicate that cell membrane lipids are a plausible target for antineoplastic therapy.

Interaction of propofol with voltage-gated human Kv1.5 channel through specific amino acids within the pore region

The intravenous anesthetic propofol affects the function of a diversity of ligand-gated and voltage-gated ion channels. However, there is little information as to whether propofol directly interacts with voltage-gated ion channel proteins to modulate their functions. The Kv1.5 channel is activated by membrane depolarization during action potentials and contributes to atrial repolarization in the human heart. This study was undertaken to examine the effect of propofol on voltage-gated human Kv1.5 (hKv1.5) channel and to elucidate the underlying molecular determinants. Site-directed mutagenesis was carried out through six amino acids that reside within the pore domain of hKv1.5 channel. Whole-cell patch-clamp technique was used to record membrane currents through the wild type and mutant hKv1.5 channels heterologously expressed in Chinese hamster ovary cells. Propofol (≥5 μM) reversibly and concentration-dependently (IC50 of 49.3±9.4 μM; n=6) blocked hKv1.5 current. Propofol-induced block of hKv1.5 current gradually progressed during depolarizing voltage-clamp steps and was enhanced by higher frequency of activation, consistent with a preferential block of the channels in their open state. The degree of current block by propofol was significantly attenuated in T480A, I502A, I508A and V516A, but not in H463C and L510A mutants of hKv1.5 channel. Thus, several amino acids near the selectivity filter (Thr480) or within S6 (Ile502, Ile508 and Val516) are found to be critically involved in the blocking action of propofol. This study provides the first evidence suggesting that direct interaction with specific amino acids underlies the blocking action of propofol on voltage-gated hKv1.5 channel.

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.

Homology modeling of Kv1.5 channel block by cationic and electroneutral ligands

The inner pore of potassium channels is targeted by many ligands of intriguingly different chemical structures. Previous studies revealed common and diverse characteristics of action of ligands including cooperativity of ligand binding, voltage- and use-dependencies, and patterns of ligand-sensing residues. Not all these data are rationalized in published models of ligand-channel complexes. Here we have used energy calculations with experimentally defined constraints to dock flecainide, ICAGEN-4, benzocaine, vernakalant, and AVE0118 into the inner pore of Kv1.5 channel. We arrived at ligand-binding models that suggest possible explanations for different values of the Hill coefficient, different voltage dependencies of ligands action, and effects of mutations of residues in subunit interfaces. Two concepts were crucial to build the models. First, the inner-pore block of a potassium channel requires a cationic "blocking particle". A ligand, which lacks a positively charged group, blocks the channel in a complex with a permeant ion. Second, hydrophobic moieties of a flexible ligand have a tendency to bind in hydrophobic subunit interfaces.

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.

Acacetin causes a frequency- and use-dependent blockade of hKv1.5 channels by binding to the S6 domain

We have demonstrated that the natural flavone acacetin selectively inhibits ultra-rapid delayed rectifier potassium current (I(Kur)) in human atria. However, molecular determinants of this ion channel blocker are unknown. The present study was designed to investigate the molecular determinants underlying the ability of acacetin to block hKv1.5 channels (coding I(Kur)) in human atrial myocytes using the whole-cell patch voltage-clamp technique to record membrane current in HEK 293 cells stably expressing the hKv1.5 gene or transiently expressing mutant hKv1.5 genes generated by site-directed mutagenesis. It was found that acacetin blocked hKv1.5 channels by binding to both closed and open channels. The blockade of hKv1.5 channels by acacetin was use- and frequency-dependent, and the IC(50) of acacetin for inhibiting hKv1.5 was 3.5, 3.1, 2.9, 2.1, and 1.7μM, respectively, at 0.2, 0.5, 1, 3, and 4Hz. The mutagenesis study showed that the hKv1.5 mutants V505A, I508A, and V512A in the S6-segment remarkably reduced the channel blocking properties by acacetin (IC(50), 29.5μM for V505A, 19.1μM for I508A, and 6.9μM for V512A). These results demonstrate the novel information that acacetin mainly blocks open hKv1.5 channels by binding to their S6 domain. The use- and rate-dependent blocking of hKv1.5 by acacetin is beneficial for anti-atrial fibrillation.

Irvalec inserts into the plasma membrane causing rapid loss of integrity and necrotic cell death in tumor cells

Irvalec is a marine-derived antitumor agent currently undergoing phase II clinical trials. In vitro, Irvalec induces a rapid loss of membrane integrity in tumor cells, accompanied of a significant Ca(2+) influx, perturbations of membrane conductivity, severe swelling and the formation of giant membranous vesicles. All these effects are not observed in Irvalec-resistant cells, or are significantly delayed by pretreating the cells with Zn(2+). Using fluorescent derivatives of Irvalec it was demonstrated that the compound rapidly interacts with the plasma membrane of tumor cells promoting lipid bilayer restructuration. Also, FRET experiments demonstrated that Irvalec molecules localize in the cell membrane close enough to each other as to suggest that the compound could self-organize, forming supramolecular structures that likely trigger cell death by necrosis through the disruption of membrane integrity.

Specificities of atrial electrophysiology: Clues to a better understanding of cardiac function and the mechanisms of arrhythmias

The electrical properties of the atria and ventricles differ in several aspects reflecting the distinct role of the atria in cardiac physiology. The study of atrial electrophysiology had greatly contributed to the understanding of the mechanisms of atrial fibrillation (AF). Only the atrial L-type calcium current is regulated by serotonine or, under basal condition, by phosphodiesterases. These distinct regulations can contribute to I(Ca) down-regulation observed during AF, which is an important determinant of action potential refractory period shortening. The voltage-gated potassium current, I(Kur), has a prominent role in the repolarization of the atrial but not ventricular AP. In many species, this current is based on the functional expression of K(V)1.5 channels, which might represent a specific therapeutic target for AF. Mechanisms regulating the trafficking of K(V)1.5 channels to the plasma membrane are being actively investigated. The resting potential of atrial myocytes is maintained by various inward rectifier currents which differ with ventricle currents by a reduced density of I(K1), the presence of a constitutively active I(KACh) and distinct regulation of I(KATP). Stretch-sensitive or mechanosensitive ion channels are particularly active in atrial myocytes and are involved in the secretion of the natriuretic peptide. Integration of knowledge on electrical properties of atrial myocytes in comprehensive schemas is now necessary for a better understanding of the physiology of atria and the mechanisms of AF.

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.

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.

TRESK-like potassium channels in leukemic T cells

In this study, we present patch-clamp characterization of the background potassium current in human lymphoma (Jurkat cells), generated by voltage-independent 16 pS channels with a high ( approximately 100-fold) K+/Na+ selectivity. Depending on the background K+ channels density, from few per cell up to approximately 1 open channel per microm2, resting membrane potential was in the range of -40 to -83 mV, approaching E (K) = -88 mV. The background K+ channels were insensitive to margotoxin (3 nM), apamine (3 nM), and clotrimazole (1 microM), high-affinity blockers of the lymphocyte Kv1.3, SKCa2, and IKCa1 channels. The current depended weakly on external pH. Arachidonic acid (20 microM) and Hg2+ (0.3-10 microM) suppressed background K+ current in Jurkat cells by 75-90%. Background K+ current was weakly sensitive to TEA+ (IC50 = 14 mM), and was efficiently suppressed by externally applied bupivacaine (IC50 = 5 microM), quinine (IC50 = 16 microM), and Ba2+ (2 mM). Our data, in particular strong inhibition by mercuric ions, suggest that background K+ currents expressed in Jurkat cells are mediated by TWIK-related spinal cord K+ (TRESK) channels belonging to the double-pore domain K+ channel family. The presence of human TRESK in the membrane protein fraction was confirmed by Western blot analysis.

Ligand binding to the voltage-gated Kv1.5 potassium channel in the open state--docking and computer simulations of a homology model

The binding of blockers to the human voltage-gated Kv1.5 potassium ion channel is investigated using a three-step procedure consisting of homology modeling, automated docking, and binding free energy calculations from molecular dynamics simulations, in combination with the linear interaction energy method. A reliable homology model of Kv1.5 is constructed using the recently published crystal structure of the Kv1.2 channel as a template. This model is expected to be significantly more accurate than earlier ones based on less similar templates. Using the three-dimensional homology model, a series of blockers with known affinities are docked into the cavity of the ion channel and their free energies of binding are calculated. The predicted binding free energies are in very good agreement with experimental data and the binding is predicted to be mainly achieved through nonpolar interactions, whereas the relatively small differences in the polar contribution determine the specificity. Apart from confirming the importance of residues V505, I508, V512, and V516 for ligand binding in the cavity, the results also show that A509 and P513 contribute significantly to the nonpolar binding interactions. Furthermore, we find that pharmacophore models based only on optimized free ligand conformations may not necessarily capture the geometric features of ligands bound to the channel cavity. The calculations herein give a detailed structural and energetic picture of blocker binding to Kv1.5 and this model should thus be useful for further ligand design efforts.

The molecular basis of high-affinity binding of the antiarrhythmic compound vernakalant (RSD1235) to Kv1.5 channels

Vernakalant (RSD1235) is an investigational drug recently shown to convert atrial fibrillation rapidly and safely in patients (J Am Coll Cardiol 44:2355-2361, 2004). Here, the molecular mechanisms of interaction of vernakalant with the inner pore of the Kv1.5 channel are compared with those of the class IC agent flecainide. Initial experiments showed that vernakalant blocks activated channels and vacates the inner vestibule as the channel closes, and thus mutations were made, targeting residues at the base of the selectivity filter and in S6, by drawing on studies of other Kv1.5-selective blocking agents. Block by vernakalant or flecainide of Kv1.5 wild type and mutants was assessed by whole-cell patch-clamp experiments in transiently transfected human embryonic kidney 293 cells. The mutational scan identified several highly conserved amino acids, Thr479, Thr480, Ile502, Val505, and Val508, as important residues for affecting block by both compounds. In general, mutations in S6 increased the IC50 for block by vernakalant; I502A caused an extremely local 25-fold decrease in potency. Specific changes in the voltage-dependence of block with I502A supported the crucial role of this position. A homology model of the pore region of Kv1.5 predicted that, of these residues, only Thr479, Thr480, Val505, and Val508 are potentially accessible for direct interaction, and that mutation at additional sites studied may therefore affect block through allosteric mechanisms. For some of the mutations, the direction of changes in IC50 were opposite for vernakalant and flecainide, highlighting differences in the forces that drive drug-channel interactions.

Mechanism of intracellular block of the KcsA K+ channel by tetrabutylammonium: insights from X-ray crystallography, electrophysiology and replica-exchange molecular dynamics simulations

The mechanism of intracellular blockade of the KcsA potassium channel by tetrabutylammonium (TBA) is investigated through functional, structural and computational studies. Using planar-membrane electrophysiological recordings, we characterize the binding kinetics as well as the dependence on the transmembrane voltage and the concentration of the blocker. It is found that the apparent affinity of the complex is significantly greater than that of any of the eukaryotic K(+) channels studied previously, and that the off-rate increases with the applied transmembrane voltage. In addition, we report a crystal structure of the KcsA-TBA complex at 2.9 A resolution, with TBA bound inside the large hydrophobic cavity located at the center of the channel, consistent with the results of previous functional and structural studies. Of particular interest is the observation that the presence of TBA has a negligible effect on the channel structure and on the position of the potassium ions occupying the selectivity filter. Inspection of the electron density corresponding to TBA suggests that the ligand may adopt more than one conformation in the complex, though the moderate resolution of the data precludes a definitive interpretation on the basis of the crystallographic refinement methods alone. To provide a rationale for these observations, we carry out an extensive conformational sampling of an atomic model of TBA bound in the central cavity of KcsA, using the Hamiltonian replica-exchange molecular dynamics simulation method. Comparison of the simulated and experimental density maps indicates that the latter does reflect at least two distinct binding orientations of TBA. The simulations show also that the relative population of these binding modes is dependent on the ion configuration occupying the selectivity filter, thus providing a clue to the nature of the voltage-dependence of the binding kinetics.

Binding site of a novel Kv1.5 blocker: a "foot in the door" against atrial fibrillation

Kv1.5 channel blockers prolong atrial action potentials and may prevent atrial flutter or fibrillation without affecting ventricular repolarization. Here we characterize the mechanisms of action of 2'-{[2-(4-methoxy-phenyl)-acetylamino]-methyl}-biphenyl-2-carboxylic acid (2-pyridin-3-yl-ethyl)-amide (AVE0118) on Kv1.5 channels heterologously expressed in Xenopus laevis oocytes. Whole cell currents in oocytes were recorded using the two-microelectrode voltage clamp technique. AVE0118 blocked Kv1.5 current in oocytes with an IC50 of 5.6 microM. Block was enhanced by higher rates of stimulation, consistent with preferential binding of the drug to the open state of the channel. Ala-scanning mutagenesis of the pore domain of Kv1.5 identified the amino acids Thr479, Thr480, Val505, Ile508, Val512, and Val516 as important residues for block by AVE0118. A homology model of the pore region of Kv1.5 predicts that these six residues face toward the central cavity of the channel. In addition, mutation of two other S6 residues (Ile502 and Leu510) that are predicted to face away from the central cavity also diminished drug block. All these putative drug-binding residues are highly conserved in other Kv channels, explaining our finding that AVE0118 also blocked Kv1.3, Kv2.1, Kv3.1, and Kv4.3 channels with similar potency. Docking of AVE0118 into the inner cavity of a Kv1.5 pore homology model predicted an unusual binding mode. The drug aligned with the inner S6 alpha-helical domain in a manner predicted to block the putative activation gate. This "foot-in-the-door" binding mode is consistent with the observation that the drug slowed the rate of current deactivation, causing a crossover of tail current traces recorded before and after drug treatment.

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'.

Comparative effects of levobupivacaine and racemic bupivacaine on excitotoxic neuronal death in culture and N-methyl-d-aspartate-induced seizures in mice

We compared the neurotoxic profile of racemic bupivacaine and levobupivacaine in: (i) a mouse model of N-methyl-D-aspartate (NMDA)-induced seizures and (ii) in an in vitro model of excitotoxic cell death. When used at high doses (36 mg/kg) both bupivacaine and levobupivacaine reduced the latency to NMDA-induced seizures and increased seizure severity. However, levobupivacaine-treated animals underwent less severe seizures as compared with bupivacaine-treated animals. Lower doses of levobupivacaine and bupivacaine had opposite effects on NMDA-induced seizures. At doses of 5 mg/kg, levobupivacaine increased the latency to partial seizures and prevented the occurrence of generalized seizures, whereas bupivacaine decreased the latency to partial seizures and did not influence the development of generalized seizures. In in vitro experiments, we exposed primary cultures of mouse cortical cells, containing both neurons and astrocytes, to 100 microM NMDA for 10 min for the induction of excitotoxic neuronal death. This treatment killed 70-80% of the neuronal population, as assessed 24 h after the excitotoxic pulse. In this particular model, both levobupivacaine and bupivacaine were neuroprotective against NMDA toxicity. However, neuroprotection by levobupivacaine was seen at lower concentrations (with respect to bupivacaine) and was maintained at concentrations of 3 mM, which are much higher than the plasma security threshold for the drug in vivo. In contrast, no protection against NMDA toxicity was detected when 3 mM concentrations of bupivacaine were applied to the cultures. Our data show a better neurotoxic profile of levobupivacaine as compared to racemic bupivacaine, and are indicative of a safer profile of levobupivacaine in clinical practice.

Structural basis for competition between drug binding and Kvbeta 1.3 accessory subunit-induced N-type inactivation of Kv1.5 channels

Kvbeta subunits are accessory proteins that modify gating of Kv1 channels. Kvbeta1.3 subunits bind to the N termini of Kv1.5 alpha-subunits and induce fast N-type inactivation, slow the rate of deactivation, and alter the voltage dependence and kinetics of channel activation. The N terminus of a Kvbeta subunit and quaternary ammonium compounds bind to the inner pore of Kv1 channels; however, it is unknown to what extent the pore binding sites for drugs and Kvbeta subunits overlap. Here, we used site-directed Ala mutagenesis to scan residues of the Kv1.5 pore to define the binding site for Kvbeta1.3 subunits. Individual mutations of five residues in the S6 domain (Val505, Ile508, Leu510, Val512, and Val516) greatly retarded or prevented Kvbeta1.3 induced inactivation, and reduced effects on Kv1.5 deactivation. Mutation of Thr479 and Thr480 enhanced Kvbeta1.3-induced N-type inactivation. None of the Ala mutations prevented the Kvbeta1.3-induced negative shifts in the voltage dependence of activation or slow C-type inactivation, suggesting that these gating effects are mediated by an interaction other than the one for N-type inactivation. Thr479, Thr480, Val505, Ile508, and Val512, of Kv1.5 channels are also important interaction sites for the anthranilic acid S0100176 (N-benzyl-N-pyridin-3-ylmethyl-2-(toluene-4-sulfonylamino)-benzamide hydrochloride). Leu510 and V516A prevented Kvbeta1.3-induced inactivation but did not alter drug block. Block of Kv1.5 by S0100176 was reduced and voltage-dependent in the presence of Kvbeta1.3 but not in the presence of an N-truncated form of the Kvbeta subunit. Thus, residues in the pore of Kv1.5 required for N-type inactivation overlap with but are not identical to the drug binding site.

Inhibition of the A-type K+ channels of dorsal root ganglion neurons by the long-duration anesthetic butamben

n-Butyl-p-aminobenzoate (BAB; butamben) is a long-duration anesthetic used for the treatment of chronic pain. Epidural administration of BAB is thought to reduce the electrical excitability of dorsal root nociceptor fibers by inhibiting voltage-gated ion channels. To further investigate this mechanism, we examined the effects of BAB on the potassium currents of acutely dissociated neurons from the rat dorsal root ganglion (DRG). These neurons express a rapidly inactivating A-type K(+) current (I(A)) that is resistant to tetraethylammonium (20 mM) but inhibited by 4-aminopyridine (5 mM). At low concentrations, BAB (< or =1 microM) selectively inhibited the I(A) component of DRG K(+) current. The voltage dependence of activation and inactivation, kinetics of recovery from inactivation, and the pharmacology of the DRG I(A) were similar to those of the Kv4 family of K(+) channels. Reverse transcription-polymerase chain reaction was used to establish that the messages encoding for all three of the mammalian Kv4 channel subunits (Kv4.1-Kv4.3) were present in the rat DRG. BAB produced a high-affinity, partial inhibition of heterologously expressed Kv4.2 channels (K(D) = 59 nM) but did not alter the kinetics or voltage sensitivity of gating. Substituting polar threonines for conserved hydrophobic residues of the S6 segment weakened BAB binding but did not alter the voltage-dependent gating of the Kv4.2 channel. At physiological pH, BAB is uncharged, suggesting that hydrophobic interactions may contribute to drug binding. The data support a mechanism in which BAB binds near the narrow cytoplasmic entrance of Kv4 channels and inhibits current by a pore blocking mechanism.

A Single Residue in the S6 Transmembrane Domain Governs the Differential Flecainide Sensitivity of Voltage-Gated Potassium Channels

Flecainide has been used to differentiate Kv4.2-based transient-outward K(+)-currents (flecainide-sensitive) from Kv1.4-based (flecainide-insensitive). We found that flecainide also inhibits ultrarapid delayed rectifier (I(Kur)) currents in Xenopus laevis oocytes carried by Kv3.1 subunits (IC(50), 28.3 +/- 1.3 microM) more strongly than Kv1.5 currents corresponding to human I(Kur) (IC(50), 237.1 +/- 6.2 microM). The present study examined molecular motifs underlying differential flecainide sensitivity. An initial chimeric approach pointed to a role for S6 and/or carboxyl-terminal sites in Kv3.1/Kv1.5 sensitivity differences. We then looked for homologous amino acid residues of the two sensitive subunits (Kv4.2 and Kv3.1) different from homologous residues for insensitive subunits (Kv1.4 and Kv1.5). Three candidate sites were identified: two in the S5-S6 linker and one in the S6 segment. Mutation of the proximal S5-S6 linker site failed to alter flecainide sensitivity. Mutation at the more distal site in Kv1.5 (V481L) modestly increased sensitivity, but the reciprocal Kv3.1 mutation (L401V) had no effect. S6 mutants caused marked changes: flecainide sensitivity decreased approximately 8-fold for Kv3.1 L422I (IC(50), 213 +/- 9 microM) and increased approximately 7-fold for Kv1.5 I502L (IC(50), 35.6 +/- 1.9 microM). Corresponding mutations reversed flecainide sensitivity of Kv1.4 and Kv4.2; L392I decreased Kv4.2 sensitivity by approximately 17-fold (IC(50) of 37.4 +/- 6.9 to 628 +/- 36 microM); I547L increased Kv1.4 sensitivity by approximately 15-fold (IC(50) of 706 +/- 37 to 40.9 +/- 7.3 microM). Our observations indicate that the flecainide sensitivity differences among these four voltage-gated K(+)-channels are determined by whether an isoleucine or a leucine is present at a specific amino acid location.

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.

Computational modelling of the open-state Kv1.5 ion channel block by bupivacaine

Binding of R(+)-bupivacaine to open-state homology models of the mammalian K(v)1.5 membrane ion channel is studied using automated docking and molecular dynamics (MD) methods. Homology models of K(v)1.5 are built using the 3D structures of the KcsA and MthK channels as a template. The packing of transmembrane (TM) alpha-helices in the KcsA structure corresponds to a closed channel state. Opening of the channel may be reached by a conformational transition yielding a bent structure of the internal S6 helices. Our first model of the K(v) open state involves a PVP-type of bending hinge in the internal helices, while the second model corresponds to a Gly-type of bending hinge as found in the MthK channel. Ligand binding to these models is probed using the common local anaesthetic bupivacaine, where blocker binding from the intracellular side of the channel is considered. Conformational properties and partial atomic charges of bupivacaine are determined from quantum mechanical HF/6-31G* calculations with inclusion of solvent effects. The automated docking and MD calculations for the PVP-bend model predict that bupivacaine could bind either in the central cavity or in the PVP region of the channel pore. Linear interaction energy (LIE) estimates of the binding free energies for bupivacaine predict strongest binding to the PVP region. Surprisingly, no binding is predicted for the Gly-bend model. These results are discussed in light of electrophysiological data which show that the K(v)1.5 channel is unable to close when bupivacaine is bound.

Molecular basis for Kv1.5 channel block: conservation of drug binding sites among voltage-gated K+ channels

Kv1.5 channels conduct the ultrarapid delayed rectifier current (IKur) that contributes to action potential repolarization of human atrial myocytes. Block of these channels has been proposed as a treatment for atrial arrhythmias. Here we report a novel and potent inhibitor of Kv1.5 potassium channels, N-benzyl-N-pyridin-3-yl-methyl-2-(toluene-4-sulfonylamino)-benzamide hydrochloride (S0100176), which exhibits features consistent with preferential block of the open state. The IC50 of S0100176 for Kv1.5 expressed in Xenopus oocytes was 0.7 microm. Ala-scanning mutagenesis within the pore helix and the S6 segment, regions that form the walls of the central cavity, was combined with voltage clamp analysis to identify point mutations that altered drug affinity. This approach identified Thr-479, Thr-480, Val-505, Ile-508, and Val-512 as the most important residues for block by S0100176. Mutations of these key residues to Ala or other amino acids caused marked changes in the IC50 of S0100176 (p<0.01). For example, the IC50 of S0100176 increased 362-fold for T480A, 26-fold for V505A, 150-fold for I508A, and 99-fold for V512A. We used modeling to dock S0100176 into the inner cavity of a Kv1.5 pore homology model that was generated based on the crystal structure of KcsA. The docking predicted that the five residues identified by the Ala scan were positioned less than 4.5 A from the compound. Based on the homology models, the positions of the five amino acids identified to interact with S0100176 face toward the central cavity and overlap with putative binding sites for other blockers and voltage-gated potassium channels.

Local anaesthetics block hyperpolarization-activated inward current in rat small dorsal root ganglion neurones

(1) Hyperpolarizing voltage steps evoke slowly activating inward currents in a variety of neurones and in cardiac cells. This hyperpolarization-activated inward current (I(h)) is thought to play a significant role in cell excitability, firing frequency, or in setting of the resting membrane potential in these cells. We studied the effects of lidocaine, mepivacaine, QX-314 and bupivacaine as well as its enantiomers on I(h) in the membrane of dorsal root ganglion neurones (DRG). (2) The patch-clamp technique was applied to small dorsal root ganglion neurones identified in 200 micro M thin slices of young rat DRGs. Under voltage-clamp conditions, the whole-cell I(h) current was recorded in the presence of different concentrations of the local anaesthetics. In current-clamp mode the resting membrane potential and the voltage response of DRG neurones to injected current pulses were investigated. (3) I(h) was reversibly blocked by bupivacaine, lidocaine and mepivacaine applied externally in clinically relevant concentrations. Concentration-response curves gave half-maximum inhibiting concentrations of 55, 99 and 190 micro M, respectively. Bupivacaine block of the I(h) current was not stereoselective. No significant effect was observed when QX-314 was applied to the external surface of the membrane. (4) In current-clamp experiments 60 micro M bupivacaine slightly hyperpolarized the membrane. The membrane stimulation by low-amplitude current pulses in the presence of bupivacaine showed an increase of the hyperpolarizing responses. (5) Our findings suggest an important role of the I(h)-block by local anaesthetics in the complex mechanism of drug action during epidural and spinal anaesthesia.

Amide local anesthetics potently inhibit the human tandem pore domain background K+ channel TASK-2 (KCNK5)

Blockade of voltage-gated sodium (Na+) channels by local anesthetics represents the main mechanism for inhibition of impulse propagation. Local anesthetic-induced potassium (K+) channel inhibition is also known to influence transmission of sensory impulses and to potentiate inhibition. K+ channels involved in this mechanism may belong to the emerging family of background tandem pore domain K+ channels (2P K+ channels). To determine more precisely the effects of local anesthetics on members of this ion channel family, we heterologously expressed the 2P K+ channels TASK-2 (KCNK5), TASK-1 (KCNK3), and chimeric TASK-1/TASK-2 channels in oocytes of Xenopus laevis. TASK-2 cDNA-transfected HEK 293 cells were used for single-channel recordings. Local anesthetic inhibition of TASK-2 was dose-dependent, agent-specific, and stereoselective. The IC50 values for R-(+)-bupivacaine and S-(-)-bupivacaine were 17 and 43 micro M and for R-(+)-ropivacaine and S-(-)-ropivacaine, 85 and 236 micro M. Lidocaine (1 mM) inhibited TASK-2 currents by 55 +/- 4%, whereas its quaternary positively charged analog N-ethyl lidocaine (QX314) had no effect. Bupivacaine (100 micro M) decreased channel open probability from 20.8 +/- 1.6% to 5.6 +/- 2.2%. Local anesthetics [300 micro M R-(+)-bupivacaine] caused significantly greater depolarization of the resting membrane potential of TASK-2-expressing oocytes compared with water-injected control oocytes (15.8 +/- 2.5 mV versus 0.1 +/- 0.05 mV; p < 0.001). Chimeric TASK-1/TASK-2 2P K+ channel subunits that retained pH sensitivity demonstrated that the carboxy domain of TASK-2 mediates the greater local anesthetic sensitivity of TASK-2. These results show that clinically achievable concentrations of local anesthetics inhibit background K+ channel function and may thereby enhance conduction blockade.

Local anesthetic block of Kv channels: role of the S6 helix and the S5-S6 linker for bupivacaine action

To gain insights in the molecular mechanisms of anesthesia, we analyzed the effects of bupivacaine on a series of voltage-gated K+ channels (Kv1.1, -1.2, -1.5, -2.1, -3.1, and -3.2) and various mutant channels derived from Kv2.1, using Xenopus laevis oocytes. Two phenomenologically different blocking effects were seen at room temperature: a time-dependent block of Kv1 and Kv3 channels (Kd between 110 and 240 microM), and a time-independent block on Kv2.1 (Kd = 220 microM). At 32 degrees C, however, Kv2.1 also showed a time-dependent block. Swapping the S6 helix between Kv1.2 and Kv2.1 introduced Kv1.2 features in Kv2.1. Critical residues were located in the N-terminal end of S6, positions 395 and 398. The triple substitution of residues 372, 373, and 374 in the S5-S6 linker decreased the bupivacaine affinity by 5-fold (Kd increased from 220 to 1170 microM). The results suggest that bupivacaine blocks Kv channels by an open-state-dependent mechanism and that Kv2.1 deviates from the other channels in allowing a partial closure of the channel with bupivacaine bound. The results also suggest that the binding site is located in the internal vestibule and that residues in the descending P-loop and the upper part of S6 are critical for the binding, most likely by allosteric mechanisms. A simple mechanistic scenario that explains the observations is presented. Thermodynamic considerations suggest that the interaction between bupivacaine and the channels is hydrophobic.

Molecular site of action of the antiarrhythmic drug propafenone at the voltage-operated potassium channel Kv2.1

The effects of the antiarrhythmic drug propafenone at Kv2.1 channels were studied with wild-type and mutated channels expressed in Xenopus laevis oocytes. Propafenone decreased the Kv2.1 currents in a time- and voltage-dependent manner (decrease of the time constants of current rise, increase of block with the duration of voltage steps starting from a block of less than 19%, increase of block with the amplitude of depolarization yielding a fractional electrical distance delta of 0.11 to 0.16). Block of Kv2.1 appeared with application to the intracellular, but not the extracellular, side of membrane patches. In mutagenesis experiments, all parts of the Kv2.1 channel were successively exchanged with those of the Kv1.2 channel, which is much more sensitive to propafenone. The intracellular amino and carboxyl terminus and the intracellular linker S4-S5 reduced the blocking effect of propafenone, whereas the linker S5-S6, as well as the segment S6 of the Kv1.2 channel, abolished it to the value of the Kv1.2 channel. In the linker S5-S6, this effect could be narrowed down to two groups of amino acids (groups 372 to 374 and 383 to 384), which also affected the sensitivity to tetraethylammonium. In segment S6, several amino acids in the intracellularly directed part of the helix significantly reduced propafenone sensitivity. The results suggest that propafenone blocks the Kv2.1 channel in the open state from the intracellular side by entering the inner vestibule of the channel. These results are consistent with a direct interaction of propafenone with the lower part of the pore helix and/or residues of segment S6.

Effects of irbesartan on cloned potassium channels involved in human cardiac repolarization

We studied the effects of irbesartan, a selective angiotensin II type 1 receptor antagonist, on human ether-a-go-go-related gene (HERG), KvLQT1+minK, hKv1.5, and Kv4.3 channels using the patch-clamp technique. Irbesartan exhibited a low affinity for HERG and KvLQT1+minK channels (IC(50) = 193.0 +/- 49.8 and 314.6 +/- 85.4 microM, respectively). In hKv1.5 channels, irbesartan produced two types of block, depending on the concentration tested. At 0.1 microM, irbesartan inhibited the current in a time-dependent manner (22 +/- 3.9% at +60 mV). The blockade increased steeply with channel activation increasing at more positive potentials. However, at 10 microM, irbesartan induced a time-independent blockade that occurred in the range of potentials of channel opening, reaching its maximum at approximately 0 mV, and remaining unchanged at more positive potentials (24.0 +/- 1.0% at +60 mV). In Kv4.3 currents, irbesartan produced a concentration-dependent block, which resulted in two IC(50) values (1.0 +/- 0.1 nM and 7.2 +/- 0.6 microM). At 1 microM, it inhibited the peak current and accelerated the time course of inactivation, decreasing the total charge crossing the membrane (36.6 +/- 7.8% at +50 mV). Irbesartan shifted the inactivation curve of Kv4.3 channels, the blockade increasing as the amount of inactivated channels increased. Molecular modeling was used to define energy-minimized dockings of irbesartan to hKv1.5 and HERG channels. In conclusion, irbesartan blocks Kv4.3 and hKv1.5 channels at therapeutic concentrations, whereas the blockade of HERG and KvLQT1+minK channels occurred only at supratherapeutic levels. In hKv1.5, a receptor site is apparent on each alpha-subunit of the channel, whereas in HERG channels a common binding site is present at the pore.

Effects of levobupivacaine, ropivacaine and bupivacaine on HERG channels: stereoselective bupivacaine block

1 Levobupivacaine and ropivacaine are the pure S(-) enantiomers of N-butyl- and N-propyl-2',6'-pipecoloxylidide, developed as less cardiotoxic alternatives to bupivacaine. In the present study, we have analysed the effects of levobupivacaine, ropivacaine and bupivacaine on HERG channels stably expressed in CHO cells. 2 The three drugs blocked HERG channels in a concentration-, time- and state-dependent manner. Block measured at the end of 5 s pulses to -10 mV induced by 20 microM bupivacaine (52.7+/-2.0%, n=15) and ropivacaine (55.5+/-2.7%, n=13) was similar (P>0.05) and both lower than that induced by levobupivacaine (67.5+/-4.2%, n=11) (P<0.05). 3 Dextrobupivacaine (20 microM) was less potent (47.2+/-5.2%, n=10) than levobupivacaine (P<0.05), indicating stereoselective HERG channel block. 4. Block induced by the three local anaesthetics exhibited a steep voltage dependence in the range of channel activation. In all cases, block measured at the maximum peak current at a test potential of 0 mV after promoting recovery from inactivation (I-->O) was lower than that observed at the end of 5-s pulses (I+O). 5. Levobupivacaine, ropivacaine and bupivacaine accelerated HERG inactivation kinetics, slowed the recovery from inactivation and shifted the inactivation curve towards more negative membrane potentials. The three local anaesthetics induced a rapid time-dependent decline after using a protocol that quickly activates HERG channels. 6. All these results suggest that: (1) these drugs bind to the open and the inactivated states of HERG channels, (2) they stabilize HERG channels in the inactivated state, and (3) block induced by bupivacaine enantiomers is stereoselective.

Assembly with the Kvbeta1.3 subunit modulates drug block of hKv1.5 channels

The assembly of voltage-gated potassium (Kv) channels with beta subunits modifies the electrophysiological characteristics of the alpha subunits. Kvbeta1.3 subunits shift the midpoint of the activation curve toward more negative voltages and slow the deactivation process. In addition, the Kvbeta1.3 subunit converts hKv1.5 from a delayed rectifier with a modest degree of slow inactivation to a channel with both fast and slow components of inactivation. In the present study, we have analyzed the effects of bupivacaine and a permanently charged analog [R(+)-N-methyl-bupivacaine (RB(+)1C)] on Kvalpha1.5 and Kvalpha1.5+Kvbeta1.3 channels expressed in human embryonic kidney 293 cells using the whole-cell configuration of the patch-clamp technique. Block induced by RB(+)1C binding to its external receptor site was not modified by the presence of this beta subunit. However, hKvalpha1.5+Kvbeta1.3 channels were ~4-fold less sensitive to bupivacaine than hKv1.5 channels in the absence of beta subunits (IC(50) = 47.5 +/- 5.1 versus 13.1 +/- 0.8 microM, respectively, p < 0.01). Quinidine was also less potent to block Kvalpha1.5+Kvbeta1.3 channels than Kvalpha1.5 channels (IC(50) = 49.6 microM versus 6.2 microM, respectively). These results suggest that the Kvbeta1.3 subunit does not modify the affinity of the charged bupivacaine for its external receptor site but markedly reduces the affinity of bupivacaine and quinidine for their internal receptor site in hKv1.5 channels.

Bupivacaine effects on hKv1.5 channels are dependent on extracellular pH

1. Bupivacaine-induced cardiotoxicity increases in hypoxic and acidotic conditions. We have analysed the effects of R(+)bupivacaine on hKv1.5 channels stably expressed in Ltk(-) cells using the whole-cell patch-clamp technique, at three different extracellular pH (pH(o)), 6.5, 7.4 and 10.0. 2. Acidification of the pH(o) from 7.4 to 6.5 decreased 4 fold the potency of R(+)bupivacaine to block hKv1.5 channels. At pH(o) 10.0, the potency of the drug increased approximately 2.5 fold. 3. Block induced by R(+)bupivacaine at pH(o) 6.5, 7.4 and 10.0, was voltage- and time-dependent in a manner consistent with an open state block of hKv1.5 channels. 4. At pH(o) 6.5, but not at pH(o) 7.4 or 10.0, R(+)bupivacaine increased by 95+/-3 % (n=6; P<0.05) the hKv1.5 current recorded at -10 mV, likely due to a drug-induced shift of the midpoint of activation (DeltaV=-8.5+/-1.4 mV; n=7). 5. R(+)bupivacaine development of block exhibited an 'instantaneous' component of block at the beginning of the depolarizing pulse, which averaged 12.5+/-1.8% (n=5) and 4.6+/-1.6% (n=6), at pH(o) 6.5 and 7.4, respectively, and that was not observed at pH(o) 10.0. 6. It is concluded that: (a) alkalinization of the pH(o) increases the potency of block of R(+)bupivacaine, and (b) at pH(o) 6.5, R(+)bupivacaine induces an 'agonist effect' of hKv1.5 current when recorded at negative membrane potentials.

Direct effects of candesartan and eprosartan on human cloned potassium channels involved in cardiac repolarization

In the present study, we analyzed the effects of two angiotensin II type 1 receptor antagonists, candesartan (0.1 microM) and eprosartan (1 microM), on hKv1.5, HERG, KvLQT1+minK, and Kv4.3 channels expressed on Ltk(-) or Chinese hamster ovary cells using the patch-clamp technique. Candesartan and eprosartan produced a voltage-dependent block of hKv1.5 channels decreasing the current at +60 mV by 20.9 +/- 2.3% and 14.3 +/- 1.5%, respectively. The blockade was frequency-dependent, suggesting an open-channel interaction. Eprosartan inhibited the tail amplitude of HERG currents elicited on repolarization after pulses to +60 mV from 239 +/- 78 to 179 +/- 72 pA. Candesartan shifted the activation curve of HERG channels in the hyperpolarizing direction, thus increasing the current amplitude elicited by depolarizations to potentials between -50 and 0 mV. Candesartan reduced the KvLQT1+minK currents elicited by 2-s pulses to +60 mV (38.7 +/- 6.3%). In contrast, eprosartan transiently increased (8.8 +/- 2.7%) and thereafter reduced the KvLQT1+minK current amplitude by 17.7 +/- 3.0%. Eprosartan, but not candesartan, blocked Kv4.3 channels in a voltage-dependent manner (22.2 +/- 3.5% at +50 mV) without modifying the voltage-dependence of Kv4.3 channel inactivation. Candesartan slightly prolonged the action potential duration recorded in guinea pig papillary muscles at all driving rates. Eprosartan prolonged the action potential duration in muscles driven at 0.1 to 1 Hz, but it shortened this parameter at faster rates (2--3 Hz). All these results demonstrated that candesartan and eprosartan exert direct effects on Kv1.5, HERG, KvLQT1+minK, and Kv4.3 currents involved in human cardiac repolarization.

Stereoselective effects of the enantiomers of a new local anaesthetic, IQB-9302, on a human cardiac potassium channel (Kv1.5)

1. The N-substituent of IQB-9302 has the same number of carbons as bupivacaine, but it exhibits a different spatial localization (n-butyl vs cyclopropylmethyl). Thus, the study of the effects of IQB-9302 enantiomers on hKv1.5 channels will lead to a better knowledge of the determinants of stereoselective block. 2. The effects of the IQB-9302 enantiomers were studied on hKv1.5 channels stably expressed in LTK:(-) cells using the whole-cell configuration of the patch-clamp technique. Drug molecular modelling was performed using Hyperchem software. 3. Block induced by IQB-9302 was stereoselective with the R(+) enantiomer being 3.2-fold more potent than the S(-) one (K(D) of 17.8+/-0.5 microM vs 58.6+/-4.0 microM). 4. S(-)- and R(+)IQB-9302 induced-block was time- and voltage-dependent consistent with an electrical distance from the cytoplasmic side of 0.173+/-0.022 (n=12) and 0.181+/-0.018 (n=10), respectively. 5. Potency of block of pipecoloxylidide local anaesthetics was linearly related to the length between the cationic tertiary amine and the end of the substituent. 6. Molecular modelling shows that only when S(-) and R(+) enantiomers are superimposed by their aromatic ring, their N-substituents are in opposite directions, which can explain the stereospecific block induced by bupivacaine and IQB-9302 with hKv1.5 channels. 7. These results suggest that: (a) IQB-9302 enantiomers block the open state of hKv1.5 channels, and (b) the length of the N-substituent in these local anaesthetics and not its volume determines the potency and degree of their stereoselective hKv1.5 channel block.

Effects of a quaternary bupivacaine derivative on delayed rectifier K(+) currents

Block of hKv1.5 channels by R-bupivacaine has been attributed to the interaction of the charged form of the drug with an intracellular receptor. However, bupivacaine is present as a mixture of neutral and charged forms both extra- and intracellularly. We have studied the effects produced by the R(+) enantiomer of a quaternary bupivacaine derivative, N-methyl-bupivacaine, (RB(+)1C) on hKv1.5 channels stably expressed in Ltk(-) cells using the whole-cell configuration of the patch-clamp technique. When applied from the intracellular side of the membrane, RB(+)1C induced a time- and voltage-dependent block similar to that induced by R-bupivacaine. External application of 50 microM RB(+)1C reduced the current at +60 mV by 24+/-2% (n=10), but this block displayed neither time- nor voltage-dependence. External RB(+)1C partially relieved block induced by R-bupivacaine (61+/-2% vs 56+/-3%, n=4, P<0.05), but it did not relieve block induced by internal RB(+)1C. In addition, it did not induce use-dependent block, but when applied in combination with internal RB(+)1C a use-dependent block that increased with pulse duration was observed. These results indicate that RB(+)1C induces different effects on hKv1.5 channels when applied from the intra or the extracellular side of the membrane, suggesting that the actions of bupivacaine are the resulting of those induced on the external and the internal side of hKv1.5 channels.

Effects of rupatadine, a new dual antagonist of histamine and platelet-activating factor receptors, on human cardiac kv1.5 channels

1. The effects of rupatadine, a new dual antagonist of both histamine H1 and platelet-activating factor receptors, were studied on human cloned hKv1.5 channels expressed in Ltk- cells using the whole-cell patch-clamp technique. 2. Rupatadine produced a use- and concentration-dependent block of hKv1.5 channels (KD=2.4+/-0.7 micronM) and slowed the deactivation of the tail currents, thus inducing the 'crossover' phenomenon. 3. Rupatadine-induced block was voltage-dependent increasing in the voltage range for channel opening suggesting an open channel interaction. At potentials positive to +10 mV the blockade decreased with a shallow voltage-dependence. Moreover, rupatadine also modified the voltage-dependence of hKv1.5 channel activation, which exhibited two components, the midpoint of the steeper component averaging -25. 2+/-2.7 mV. 4. When the intracellular K+ concentration ([K+]i) was lowered to 25% the voltage-dependent unblock observed at positive potentials was suppressed and the activation curve in the presence of rupatadine did not exhibit two components even when the midpoint of the activation curve was shifted to more negative potentials (-30. 3+/-1.3 mV). 5. On channels mutated on the residue R485 (R485Y) which is located on the external entryway of the pore the rupatadine-induced block did not decrease at potentials positive to +10 mV. In contrast, on V512M channels rupatadine reproduced all the features of the blockade observed on wild type channels. 6. All these results suggest that rupatadine blocks hKv1.5 channels binding to an external and to an internal binding site but only at concentrations much higher than therapeutic plasma levels in man. Efflux of K+ promotes the unbinding from the external site. Furthermore, rupatadine binds to an internal site and dramatically modifies the voltage-dependence of channel opening.