Sustained depolarization-induced outward current in human atrial myocytes. Evidence for a novel delayed rectifier K+ current similar to Kv1.5 cloned channel currents

Impact of ancillary subunits on ventricular repolarization

Voltage-gated potassium (Kv) channels generate the outward K(+) ion currents that constitute the primary force in ventricular repolarization. Voltage-gated potassium channels comprise tetramers of pore-forming alpha subunits and, in probably most cases in vivo, ancillary or beta subunits that help define the properties of the Kv current generated. Ancillary subunits can be broadly categorized as cytoplasmic or transmembrane and can modify Kv channel trafficking, conductance, gating, ion selectivity, regulation, and pharmacology. Because of their often profound effects on Kv channel function, studies of the molecular correlates of ventricular repolarization must take into account ancillary subunits as well as alpha subunits. Cytoplasmic ancillary subunits include the Kv beta subunits, which regulate a range of Kv channels and may link channel gating to redox potential, and the KChIPs, which appear most often associated with Kv4 subfamily channels that generate the ventricular I(to) current. Transmembrane ancillary subunits include the MinK-related proteins (MiRPs) encoded by KCNE genes, which modulate members of most Kv alpha subunit subfamilies, and the putative 12-transmembrane domain KCR1 protein, which modulates hERG. In some cases, such as the ventricular I(Ks) channel complex, it is well established that the KCNQ1 alpha subunit must coassemble with the MinK (KCNE1) single-transmembrane domain ancillary subunit for recapitulation of the characteristic, unusually slowly-activating I(Ks) current. In other cases, it is not so clear-cut, and in particular, the roles of the other MiRPs (1-4) in regulating cardiac Kv channels such as KCNQ1 and hERG in vivo are under debate. MiRP1 alters hERG function and pharmacology, and inherited MiRP1 mutations are associated with inherited and acquired arrhythmias, but controversy exists over the native role of MiRP1 in regulating hERG (and therefore ventricular I(Kr)) in vivo. Some ancillary subunits may exhibit varied expression to shape spatial Kv current variation, for example, KChIP2 and the epicardial-endocardial I(to) current density gradient. Indeed, it is likely that most native ventricular Kv channels exhibit temporal and spatial heterogeneity of subunit composition, complicating both modeling of their functional impact on the ventricular action potential and design of specific current-targeted compounds. Here, we discuss current thinking and lines of experimentation aimed at resolving the complexities of the Kv channel complexes that repolarize the human ventricular myocardium.

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.

Homocysteine inhibits potassium channels in human atrial myocytes

1. A large body of evidence indicates that elevated homocysteine (Hcy) levels portend an increased risk for atrial fibrillation. However, little is known about the electrophysiological effects of Hcy on atrial myocytes. The present study was conducted to investigate the direct effects of Hcy on ion channels in human atria. 2. Whole-cell patch-clamp techniques were used to record potassium currents in human atrial cells. 3. In human atrial myocytes, transient outward potassium currents were significantly decreased by 24.8 +/- 5.9 and 38.4 +/- 10.4% in the presence of 50 and 500 micromol/L Hcy, respectively. The ultrarapid delayed rectifier potassium currents were decreased by approximately 30% when exposed to 500 micromol/L Hcy. The inward rectifier potassium currents were increased by approximately 40% in the presence of 500 micromol/L Hcy. 4. The results of the present study indicate that Hcy, an important risk factor for atrial fibrillation, could cause electrophysiological disturbances of potassium currents in human atrial myocytes.

Pharmacologic targets for atrial fibrillation

Despite advances in treatment, atrial fibrillation (AF) remains the most common arrhythmia in humans. Antiarrhythmic drug therapy continues to be a cornerstone of AF treatment, even in light of emerging non-pharmacologic therapies. Conventional antiarrhythmic drugs target cardiac ion channels and are often associated with modest AF suppression and the risk of ventricular proarrhythmia. Ongoing drug development has focused on targeting atrial-specific ion channels as well as novel non-ionic targets. Targeting non-ionic mechanisms may also provide new drugs directed towards the underlying mechanisms responsible for AF and possibly greater antiarrhythmic potency. Agents that act against these new targets may offer improved safety and efficacy in AF treatment.

Atrial fibrillation--a new cardiac channelopathy

Atrial fibrillation (AF) is a common complication of a wide range of cardiac and systemic diseases and is regarded generally as a sporadic, acquired disorder. Familial clustering of AF has been reported but definitive links of genetic factors with AF pathogenesis have been lacking. Genome-wide linkage studies and the discovery of mutations in families with AF have provided compelling evidence that genetic factors can have a role in the development of AF. Although relatively few disease genes have been identified, current data indicate that inherited defects in cardiac ion channel genes can predispose to AF by altering ion channel activation and atrial conduction properties. Mutations in the reported disease genes account for only a minority of all familial AF cases and further gene discovery studies are required. Characterisation of the genetic variants that cause AF in families provides a framework for elucidation of key disease pathways that underlie the more commonly-occurring complex forms of AF. A better understanding of the molecular and electrophysiological defects that promote AF in families and in the general population will facilitate new approaches to the diagnosis, prevention and treatment of AF.

The plateau outward current in canine ventricle, sensitive to 4-aminopyridine, is a constitutive contributor to ventricular repolarization

BACKGROUND AND PURPOSE

I(Kur) (Ultra-rapid delayed rectifier current) has microM sensitivity to 4-aminopyridine (4-AP) and is an important modulator of the plateau amplitude and action potential duration in canine atria. Kv1.5 encodes I(Kur) and is present in both atria and ventricles in canines and humans. We hypothesized that a similar plateau outward current with microM sensitivity to 4-AP is present in canine ventricle.

EXPERIMENTAL APPROACH

We used established voltage clamp protocols and used 4-AP (50 and 100 microM) to measure a plateau outward current in normal canine myocytes isolated from the left ventricular mid-myocardium.

KEY RESULTS

Action potential recordings in the presence of 4-AP showed significant prolongation of action potential duration at 50 and 90% repolarization at 0.5 and 1 Hz (P<0.05), while no prolongation occurred at 2 Hz. Voltage clamp experiments revealed a rapidly activating current, similar to current characteristics of canine atrial I(Kur), in approximately 70% of left ventricular myocytes. The IC(50) of 4-AP for this current was 24.2 microM. The concentration of 4-AP used in our experiments resulted in selective blockade of an outward current that was not I(to) or I(Kr). Beta-adrenergic stimulation with isoprenaline significantly increased the 4-AP sensitive outward current density (P<0.05), suggesting a role for this current during increased sympathetic stimulation. In silico incorporation into a canine ventricular cell model revealed selective AP prolongation after current blockade.

CONCLUSIONS AND IMPLICATIONS

Our results support the existence of a canine ventricular plateau outward current sensitive to micromolar 4-AP and its constitutive role in ventricular repolarization.

The Effect of Vernakalant (RSD1235), an Investigational Antiarrhythmic Agent, on Atrial Electrophysiology in Humans

OBJECTIVES

To determine the acute effects of vernakalant (RSD1235) on electrophysiologic (EP) properties in humans.

BACKGROUND

Vernakalant is an investigational mixed ion channel blocker that can terminate acute atrial fibrillation (AF) in humans at 2 to 5 mg/kg and may be more "atrial-selective" than available agents.

METHODS

Patients (N=19; 53% male; age, 48+/-11 years) underwent EP study before and after 25 minutes of intravenous vernakalant administration: 2 mg/kg over 10 min+0.5 mg/kg/hr for 35 min or 4 mg/kg over 10 min+1 mg/kg/hr for 35 min. EP measurements, including atrial refractory period (AERP) and ventricular refractory period (VERP), were obtained.

RESULTS

The lower dose prolonged AERP at 600, but not at 400 or 300 msec paced cycle length. The higher dose significantly prolonged AERP from 203+/-31 msec to 228+/-24 msec at 600 msec, 182+/-30 msec to 207+/-27 msec at 400 msec, and 172 msec+/-24 to 193+/-21 msec at 300 msec. There was no significant prolongation of VERP at either dose or at any cycle length. There was a small but significant prolongation of AV nodal refractoriness; Wenckebach cycle length prolonged by 18+/-12 msec (from baseline 343+/-54 msec) at the higher dose (P<0.05). Sinus node recovery time also increased by 123+/-158 msec (from baseline 928+/-237 msec) at the higher dose (P<0.05). There was a slight prolongation of QRS duration at the higher dose, during ventricular pacing at CL=400 msec (15+/-15 msec, P=0.0547). QT and HV intervals were unchanged.

CONCLUSIONS

At doses similar to those tested clinically, vernakalant dose-dependently prolonged atrial refractoriness, prolonged AV nodal conduction and refractoriness, and slightly prolonged QRS duration, but it had no effect on ventricular refractoriness.

Effects of phloretin and phloridzin on Ca2+ handling, the action potential, and ion currents in rat ventricular myocytes

The effects of the phytoestrogens phloretin and phloridzin on Ca(2+) handling, cell shortening, the action potential, and Ca(2+) and K(+) currents in freshly isolated cardiac myocytes from rat ventricle were examined. Phloretin increased the amplitude and area and decreased the rate of decline of electrically evoked Ca(2+) transients in the myocytes. These effects were accompanied by an increase in the Ca(2+) load of the sarcoplasmic reticulum, as determined by the area of caffeine-evoked Ca(2+) transients. An increase in the extent of shortening of the myocytes in response to electrically evoked action potentials was also observed in the presence of phloretin. To further examine possible mechanisms contributing to the observed changes in Ca(2+) handling and contractility, the effects of phloretin on the cardiac action potential and plasma membrane Ca(2+) and K(+) currents were examined. Phloretin markedly increased the action potential duration in the myocytes, and it inhibited the Ca(2+)-independent transient outward K(+) current (I(to)). The inwardly rectifying K(+) current, the sustained outward delayed rectifier K(+) current, and L-type Ca(2+) currents were not significantly different in the presence and absence of phloretin, nor was there any evidence that the Na(+)/Ca(2+) exchanger was affected. The effects of phloretin on Ca(2+) handling in the myocytes are consistent with its effects on I(to). Phloridzin did not significantly alter the amplitude or area of electrically evoked Ca(2+) transients in the myocytes, nor did it have detectable effects on the sarcoplasmic reticulum Ca(2+) load, cell shortening, or the action potential.

Membrane cholesterol modulates Kv1.5 potassium channel distribution and function in rat cardiomyocytes

Membrane lipid composition is a major determinant of cell excitability. In this study, we assessed the role of membrane cholesterol composition in the distribution and function of Kv1.5-based channels in rat cardiac membranes. In isolated rat atrial myocytes, the application of methyl-beta-cyclodextrin (MCD), an agent that depletes membrane cholesterol, caused a delayed increase in the Kv1.5-based sustained component, I(kur), which reached steady state in approximately 7 min. This effect was prevented by preloading the MCD with cholesterol. MCD-increased current was inhibited by low 4-aminopyridine concentration. Neonatal rat cardiomyocytes transfected with Green Fluorescent Protein (GFP)-tagged Kv1.5 channels showed a large ultrarapid delayed-rectifier current (I(Kur)), which was also stimulated by MCD. In atrial cryosections, Kv1.5 channels were mainly located at the intercalated disc, whereas caveolin-3 predominated at the cell periphery. A small portion of Kv1.5 floated in the low-density fractions of step sucrose-gradient preparations. In live neonatal cardiomyocytes, GFP-tagged Kv1.5 channels were predominantly organized in clusters at the basal plasma membrane. MCD caused reorganization of Kv1.5 subunits into larger clusters that redistributed throughout the plasma membrane. The MCD effect on clusters was sizable 7 min after its application. We conclude that Kv1.5 subunits are concentrated in cholesterol-enriched membrane microdomains distinct from caveolae, and that redistribution of Kv1.5 subunits by depletion of membrane cholesterol increases their current-carrying capacity.

Benzopyran sulfonamides as KV1.5 potassium channel blockers

K(V)1.5 blockers have the potential to be atrium-selective agents for treatment of atrial fibrillation. The benzopyrans provide a template for the synthesis of potent and selective K(V)1.5 blockers.

In Vivo Cardiac Electrophysiologic and Antiarrhythmic Effects of an Isoquinoline IKur Blocker, ISQ-1, in Rat, Dog, and Nonhuman Primate

The cardiac electrophysiologic effects of ISQ-1, an isoquinolinone I(Kur) blocker, were characterized in vivo. In rat, ISQ-1 elicited maximal 33% to 36% increases in atrial and ventricular refractoriness at a plasma concentration of 11.5 microM. In African green monkey, ISQ-1 increased atrial refractory period (maximal 17% at plasma concentration up to 20 microM) with no effect on ventricular refractory period or ECG QTc. Likewise in dog, ISQ-1 increased atrial refractory period (maximal 16% at plasma concentration up to 2 microM) with no effect on ventricular refractory period or QTc. In contrast, studies with ibutilide in nonhuman primate and dog demonstrated concomitant increases in atrial and ventricular refractoriness and QTc. Additionally, in a dog model of atrial flutter, ISQ-1 terminated ongoing flutter at doses (2.5 +/- 0.5 mg/kg IV) that selectively prolonged atrial refractoriness (13% increase), whereas flutter termination with ibutilide occurred at doses that increased both atrial and ventricular refractoriness as well as QTc. Of note, the cardiac electrophysiologic profiles displayed by ISQ-1 in these species were similar to those reported previously by our lab with a structurally distinct I(Kur) blocker. Taken together, these results further support the inhibition of I(Kur) as an approach to terminate atrial arrhythmia.

Synthesis of psoralen derivatives and their blocking effect of hKv1.5 channel

Previously, we found that a furocoumarin derivative, psoralen (7H-furo[3,2-g][1]benzopyran-7-one), blocked a human Kv1.5 potassium channel (hKv1.5) and has a potential antiarrhythmic effect. In the present study, to develop more potent hKv1.5 blockers or antiarrhythmic drugs, we synthesized ten psoralen derivatives and examined their blocking effects on hKv1.5 stably expressed in Ltk cells. Among the newly synthesized psoralen derivatives, three derivatives (Compounds 5, 9 and 10) showed the open channel-blocking effect. Compound 9 among them was the most potent in blocking hKv1.5. We found that compound 9, one of the psoralen derivatives, inhibited the hKv1.5 current in a concentration-, use- and voltage-dependent manner with an IC50 value of 27.4 +/- 5.1 nM at +60 mV. Compound 9 accelerated the inactivation kinetics of the hKv1.5 channel, slowed the deactivation kinetics of hKv1.5 current resulting in a tail crossover phenomenon. Compound 9 inhibited hKv1.5 current in a use-dependent manner. These results indicate that compound 9, one of psoralen derivatives, acts on hKv1.5 channel as an open channel blocker and is much more potent than psoralen in blocking hKv1.5 channel. If further studies were done, compound 9 might be an ideal antiarrhythmic drug for atrial fibrillation.

Evolution of thiazolidine-based blockers of human Kv1.5 for the treatment of atrial arrhythmias

Blockade of the Kv1.5 ion channel is a potentially atrial-selective avenue for the treatment of atrial fibrillation and atrial flutter. The development and biological evaluation of a series of thiazolidine-based blockers of Kv1.5 is described.

Innovative approaches to anti-arrhythmic drug therapy

Normal cardiac function requires an appropriate and regular beating rate (cardiac rhythm). When the heart rhythm is too fast or too slow, cardiac function can be impaired, with derangements that vary from mild symptoms to life-threatening complications. Irregularities, particularly those involving excessively fast or slow rates, constitute cardiac 'arrhythmias'. In the past, drug treatment of cardiac arrhythmias has proven difficult, both because of inadequate effectiveness and a risk of serious complications. However, a variety of recent advances have opened up exciting possibilities for the development of novel and superior approaches to arrhythmia therapy. This article will review recent progress and future prospects for treating two particularly important cardiac arrhythmias: atrial fibrillation and ventricular fibrillation.

Effects of carvedilol on transient outward and ultra-rapid delayed rectifier potassium currents in human atrial myocytes

Carvedilol is a beta- and alpha(1)-adrenoceptor antagonist. It is widely used in the treatment of cardiovascular diseases including atrial arrhythmias. However, it is unclear whether carvedilol may affect the repolarization currents, transient outward K(+) current (I(to)) and ultra-rapid delayed rectifier K(+) current (I(Kur)) in the human atrium. The present study evaluated effects of carvedilol on I(to) and I(Kur) in isolated human atrial myocytes by whole-cell patch-clamp recording technique. We found that carvedilol reversibly inhibited I(to) and I(Kur) in a concentration-dependent manner. Carvedilol (0.3 microM) suppressed I(to) from 9.2+/-0.5 pA/pF to 4.8+/-0.5 pA/pF (P<0.01) and I(Kur) from 3.6+/-0.5 pA/pF to 1.9+/-0.3 pA/pF (P<0.01) at +50 mV. I(to) was inhibited in a voltage-dependent manner, being significantly attenuated at test potentials from +10 to +50 mV, whereas the inhibition of I(Kur) was independent. The concentration giving a 50% inhibition was 0.50 microM for I(to) and 0.39 microM for I(Kur). Voltage-dependence of activation, inactivation and time-dependent recovery from inactivation of I(to) were not altered by carvedilol. However, time to peak and time-dependent inactivation of I(to) were significantly accelerated, indicating an open channel blocking action. The findings indicate that carvedilol significantly inhibits the major repolarization K(+) currents I(to) and I(Kur) in human atrial myocytes.

Discovery and synthesis of tetrahydroindolone derived semicarbazones as selective Kv1.5 blockers

A novel class of tetrahydroindolone-derived semicarbazones has been discovered as potent Kv1.5 blockers. In in vitro studies, several compounds exhibited very good potency for blockade of Kv1.5. Compound 8i showed good selectivity for blockade of Kv1.5 vs hERG and L-type calcium channels. In an anesthetized pig model, compounds 8i and 10c increased atrial ERP about 28%, 18%, respectively, in the right atrium without affecting ventricular ERP.

Potent antagonists of the Kv1.5 potassium channel: Synthesis and evaluation of analogous N,N-diisopropyl-2-(pyridine-3-yl)acetamides

This letter describes the discovery of a novel series of potent Kv1.5 ion channel antagonists based on a diisopropyl amide scaffold. Structure-activity relationships of functionalized analogs are discussed. Key compound 1-(3-(diisopropylcarbamoyl)-2-phenyl-3-(pyridin-3-yl)propyl)-3-(2-fluorobenzyl)urea (10) exhibits significant atrial-selective effects in an in vivo model.

Torilin fromTorilis japonica (Houtt.) DC. Blocks hKv1.5 channel current

Torilin was purified from Torilis japonica (Houtt.) DC., and its effects on a rapidly activating delayed rectifier K+ channel (hKv1.5), cloned from human heart and stably expressed in Ltk- cells, as well as the corresponding K+ current (the ultrarapid delayed rectifier, I(KUR)) were assessed in human atrial myocytes. Using the whole cell configuration of the patch-clamp technique, torilin was found to inhibit the hKv1.5 current in time and voltage-dependent manners, with an IC50 value of 2.51+/-0.34 microM at +60 mV. Torilin accelerated the inactivation kinetics of the hKv1.5 channel, and slowed the deactivation kinetics of the hKv1.5 current, resulting in a tail crossover phenomenon. Additionally, torilin inhibited the hKv1.5 current in a use-dependent manner. These results strongly suggest that torilin is a type of open-channel blocker of the hKv1.5 channel.

Discovery and in vitro/in vivo studies of tetrazole derivatives as Kv1.5 blockers

A novel class of tetrazole-derived Kv1.5 blockers is disclosed. In in vitro studies, several compounds had IC(50)s ranging from 180 to 550 nM. In vivo studies indicated that compounds 2f and 2j increased right atrial ERP about 40% without affecting ventricular ERP.

Synthesis and evaluation of (2-phenethyl-2H-1,2,3-triazol-4-yl)(phenyl)methanones as Kv1.5 channel blockers for the treatment of atrial fibrillation

A series of novel (2-phenethyl-2H-1,2,3-triazol-4-yl)(phenyl)methanones were prepared and examined for utility as Kv1.5 channel blockers for the treatment of atrial fibrillation.

Restoring depressed HERG K+ channel function as a mechanism for insulin treatment of abnormal QT prolongation and associated arrhythmias in diabetic rabbits

Abnormal QT prolongation (QT-P) in diabetic patients has become a nonnegligible clinical problem and has attracted increasing attention from basic scientists, because it increases the risk of lethal ventricular arrhythmias. Correction of QT-P may be an important measure in minimizing sudden cardiac death in diabetic patients. Here we report the efficacy of insulin in preventing QT-P and the associated arrhythmias and the mechanisms underlying the effects in a rabbit model of type 1 insulin-dependent diabetes mellitus (IDDM). The heart rate-corrected QT (QTc) interval and action potential duration were considerably prolonged, with frequent ventricular tachycardias. The rapid delayed rectifier K+ current (IKr) was markedly reduced in IDDM hearts, and hyperglycemia depressed the function of the human ether-a-go-go-related gene (HERG), which conducts IKr. The impairment was primarily ascribed to the enhanced oxidative damage to the myocardium, as indicated by the increased intracellular level of reactive oxygen species and simultaneously decreased endogenous antioxidant reserve and by the increased lipid peroxidation and protein oxidation. Moreover, IDDM or hyperglycemia resulted in downregulation of HERG protein level. Insulin restored the depressed IKr/HERG and prevented QTc/action potential duration prolongation and the associated arrhythmias, and the beneficial actions of insulin are partially due to its antioxidant ability. Our study represents the first documentation of oxidative stress as the major metabolic mechanism for HERG K+ dysfunction, which causes diabetic QT-P, and suggests IKr/HERG as a potential therapeutic target for treatment of the disorder.

Discovery and synthesis of tetrahydroindolone-derived carbamates as Kv1.5 blockers

A novel class of tetrahydroindolone-derived carbamates has been discovered whose members are potent Kv1.5 blockers. The in vitro data show that compounds 6 and 29 are quite potent. They are also very selective over hERG (>450-fold) and L-type calcium channels (>450-fold).

Atrial-specific drug AVE0118 is free of torsades de pointes in anesthetized dogs with chronic complete atrioventricular block

BACKGROUND

The novel compound AVE0118 has been shown to prevent and terminate persistent atrial fibrillation. AVE0118 blocks I(Kur), I(KAch), and I(to), leading to prolongation of atrial repolarization with no change in ventricular repolarization. This finding suggests that AVE0118 may be devoid of proarrhythmic side effects. Experimentally, AVE0118 has been antiarrhythmic against some specific ventricular arrhythmias.

OBJECTIVES

The purpose of this study was to investigate the proarrhythmic and antiarrhythmic effects of AVE0118 in anesthetized dogs with chronic complete AV block, known for a high proclivity for torsades de pointes (TdP).

METHODS

AVE0118 was administered intravenously as a fast infusion (0.5 mg/kg/5 min) and a slow infusion (3 or 10 mg/kg/60 min). Dofetilide was given to induce TdP. ECG and monophasic action potentials were recorded. Short-term beat-to-beat variability (STV) of the left ventricular monophasic action potential duration (MAPD) was calculated. We examined whether AVE0118 (1) caused ventricular proarrhythmia (both infusions), (2) prevented dofetilide-induced TdP (slow infusion + dofetilide after 30 minutes), and (3) abolished TdP (fast infusion).

RESULTS

At 0.55 +/- 0.10 microg/mL (fast infusion at 10 minutes), AVE0118 did not increase ventricular repolarization or induce TdP; however, right atrial MAPD(50) and MAPD(90) were significantly increased by 26% +/- 9% and 10% +/- 5%, respectively (P <.05 vs baseline). At 1.9 +/- 0.5 microg/mL and 6.1 +/- 1.2 microg/mL (30 minutes of 3 or 10 mg/kg/h), AVE0118 did not induce TdP (0/6 and 0/4) nor prevent dofetilide-induced TdP (6/6 and 2/2). Dofetilide significantly increased all repolarization parameters, including STV from 2.1 +/- 0.4 ms to 4.6 +/- 1.8 ms (P <.05 vs baseline), which were not changed by AVE0118 (to 2.1 +/- 0.3 ms after 30 minutes). Rapid infusion of AVE0118 did not suppress dofetilide-induced TdP.

CONCLUSION

In the anesthetized chronic complete AV block dog, the atrial-specific drug AVE0118 is free of TdP and has no antiarrhythmic properties against dofetilide-induced torsades de pointes.

Kv1.5 channelopathy due to KCNA5 loss-of-function mutation causes human atrial fibrillation

Atrial fibrillation is a rhythm disorder characterized by chaotic electrical activity of cardiac atria. Predisposing to stroke and heart failure, this common condition is increasingly recognized as a heritable disorder. To identify genetic defects conferring disease susceptibility, patients with idiopathic atrial fibrillation, lacking traditional risk factors, were evaluated. Genomic DNA scanning revealed a nonsense mutation in KCNA5 that encodes Kv1.5, a voltage-gated potassium channel expressed in human atria. The heterozygous E375X mutation, present in a familial case of atrial fibrillation and absent in 540 unrelated control individuals, introduced a premature stop codon disrupting the Kv1.5 channel protein. The truncation eliminated the S4-S6 voltage sensor, pore region and C-terminus, preserving the N-terminus and S1-S3 transmembrane domains that secure tetrameric subunit assembly. Heterologously expressed recombinant E375X mutant failed to generate the ultrarapid delayed rectifier current I(Kur) vital for atrial repolarization and exerted a dominant-negative effect on wild-type current. Loss of channel function translated into action potential prolongation and early after-depolarization in human atrial myocytes, increasing vulnerability to stress-provoked triggered activity. The pathogenic link between compromised Kv1.5 function and susceptibility to atrial fibrillation was verified, at the organism level, in a murine model. Rescue of the genetic defect was achieved by aminoglycoside-induced translational read-through of the E375X premature stop codon, restoring channel function. This first report of Kv1.5 loss-of-function channelopathy establishes KCNA5 mutation as a novel risk factor for repolarization deficiency and atrial fibrillation.

Over-expression of Kv1.5 in rat cardiomyocytes extremely shortens the duration of the action potential and causes rapid excitation

BACKGROUND

Genetically abnormal action potential duration (APD) can be a cause of arrhythmias that include long and short QT interval syndrome.

PURPOSE

The aim of this study was to evaluate the arrhythmogenic effect of short QT syndrome induced by the over-expression of Kv1.5 in rat.

METHODS

From Sprague-Dawley rats on fetal days 18-19, cardiomyocytes were excised and cultured with and without transfection with the Kv-1.5 gene using an adenovirus vector. The expression of Kv1.5 was proven by immunohistochemistry and Western blot analysis. In the culture dish and in the whole cells, the electrical activities were recorded using the whole-cell patch-clamp technique and the effects of 4-AP and verapamil were tested.

RESULTS

After transfection with Kv1.5 for 12h, immunohistochemical staining and Western blot analysis were positive for Kv1.5 while they were negative in the control transfected with only Lac-Z. In the culture dish, the myocytes showed spontaneous beating at 115beats/min (bpm) just prior to the transfection with Kv1.5 and increased to 367bpm at 24h. The control myocytes showed stable beating rates during culturing. 4-AP at 200microM slowed down the rate and verapamil abolished the beating. In the whole cells, the maximal resting membrane potential was slightly depolarized and APD was extremely abbreviated both at 50% and 90% of repolarization compared with those of the control. Rapid spontaneous activities were found in a single myocyte with Kv1.5 transfection and 4-AP slowed down the frequency of the activities with a reversal of the shortened APD.

CONCLUSION

The over-expression of Kv1.5 induced short APD and triggered activities in rat cardiomyocytes. This model can be used to study the arrhythmogenic substrate of short QT syndrome.

Novel, potent inhibitors of human Kv1.5 K+ channels and ultrarapidly activating delayed rectifier potassium current

We have identified a series of diphenyl phosphine oxide (DPO) compounds that are potent frequency-dependent inhibitors of cloned human Kv1.5 (hKv1.5) channels. DPO inhibited hKv1.5 expressed in Chinese hamster ovary cells in a concentration-dependent manner preferentially during channel activation and slowed the deactivating tail current, consistent with a predominant open-channel blocking mechanism. Varying kinetics of DPO interaction with Kv1.5 channels resulted in differing potencies and frequency dependencies of inhibition that were comparable for both expressed hKv1.5 current and native ultrarapidly activating delayed rectifier potassium current (IKur) in human atrial myocytes. Selectivity of DPO versus other cardiac K+ channels was demonstrated in human atrial myocytes (IKur versus transient outward potassium current) and guinea pig ventricular myocytes [IKur versus rapidly activating delayed rectifier potassium current (IKr), slowly activating delayed rectifier potassium current (IKs) and inward rectifier potassium current (IK1), and one compound (DPO-1) was shown to be 15-fold more selective for Kv1.5 versus Kv3.1 channels expressed in Xenopus oocytes. DPO-1 also prolonged action potentials of isolated human atrial but not ventricular myocytes, in contrast to the effect of a selective IKr blocker. The selectivity and kinetics of inhibition hKv1.5 and IKur by DPO and the resulting selective prolongation of atrial repolarization could provide an effective profile for treatment of supraventricular arrhythmias.

In vivo cardiac electrophysiologic effects of a novel diphenylphosphine oxide IKur blocker, (2-Isopropyl-5-methylcyclohexyl) diphenylphosphine oxide, in rat and nonhuman primate

The voltage-gated potassium channel, Kv1.5, which underlies the ultrarapid delayed rectifier current, I(Kur), is reported to be enriched in human atrium versus ventricle, and has been proposed as a target for novel atrial antiarrhythmic therapy. The administration of the novel I(Kur) blocker (2-isopropyl-5-methyl-cyclohexyl) diphenylphosphine oxide (DPO-1) (0.06, 0.2, and 0.6 mg/kg/min i.v. x 20 min; total doses 1.2, 4.0, and 12.0 mg/kg, respectively) to rat, which exhibits I(Kur) in both atria and ventricle, elicited significant, dose-dependent increases in atrial and ventricular refractory period (9-42%) at all doses tested, with no changes in cardiac rate or indices of cardiac conduction. Plasma levels achieved in rat at the end of the three infusions were 1.1, 4.1, and 7.7 microM. Reverse transcription-polymerase chain reaction analysis of African green monkey atria and ventricle demonstrated an atrial preferential distribution of Kv1.5 transcript. The administration of DPO-1 (1.0, 3.0, and 10.0 mg/kg i.v.; 5-min infusions) to African green monkey elicited significant increases in atrial refractoriness (approximately 15% increase at the 10.0 mg/kg dose), with no change in ventricular refractory period, ECG intervals, heart rate, or blood pressure. Plasma levels of DPO-1 achieved in African green monkey were 0.58, 1.12, and 5.43 microM. The concordance of effect of DPO-1 on myocardial refractoriness with distribution of Kv1.5 in these two species is consistent with the I(Kur) selectivity of DPO-1 in vivo. Moreover, the selective increase in atrial refractoriness in primate supports the concept of I(Kur) blockade as an approach for the development of atrial-specific antiarrhythmic agents.

Electrophysiological characterization of three non-synonymous single nucleotide polymorphisms (R87Q, A251T, and P307S) found in hKv1.5

Non-synonymous single nucleotide polymorphisms (SNPs) in the KCNA5/hKv1.5 gene, which encodes for a voltage-gated K+ channel responsible for the I (Kur) current in the human atria, have been recently reported. To gain further knowledge on potential influence of hKv1.5 SNPs, we searched for their presence in a specific population of 96 French-Canadians and characterized electrophysiological properties of the variants in two cell lines. The presumed promoter (-83 bp) and coding regions were sequenced. We found three heterozygous SNPs: R87Q, A251T, and P307S. Functional analysis of SNPs transfected in Chinese hamster ovary (CHO) cells showed that both R87Q and P307S diminished the inactivation amplitude (e.g., at +60 mV, amplitudes were 89+/-26, 23+/-4, and 22+/-7 pA/pF for the wild type, R87Q and P307S, respectively; n=8, 6, and 8, respectively). Inactivation was slowed with these variants (e.g., tau (fast) at +50 mV were 270+/-48, 490+/-66, and 340+/-45 ms for the wild type, R87Q, and P307S, respectively) while R87Q additionally accelerated the rate of hKv1.5 channel opening. A dominant-negative effect was observed for R87Q but not for P307S. SNPs properties were not reproduced when expressed in the HEK293 cell line, suggesting that the regulatory beta-subunit present in CHO cells (and the human heart) is essential for the SNPs effects that we have observed.

Spironolactone and its main metabolite canrenoic acid block hKv1.5, Kv4.3 and Kv7.1 + minK channels

Both spironolactone (SP) and its main metabolite, canrenoic acid (CA), prolong cardiac action potential duration and decrease the Kv11.1 (HERG) current. We examined the effects of SP and CA on cardiac hKv1.5, Kv4.3 and Kv7.1+minK channels that generate the human I(Kur), I(to1) and I(Ks), which contribute to the control of human cardiac action potential duration.hKv1.5 currents were recorded in stably transfected mouse fibroblasts and Kv4.3 and Kv7.1 + minK in transiently transfected Chinese hamster ovary cells using the whole-cell patch clamp. SP (1 microM) and CA (1 nM) inhibited hKv1.5 currents by 23.2 +/- 3.2 and 18.9 +/- 2.7%, respectively, shifted the midpoint of the activation curve to more negative potentials and delayed the time course of tail deactivation.SP (1 microM) and CA (1 nM) inhibited the total charge crossing the membrane through Kv4.3 channels at +50 mV by 27.1 +/- 6.4 and 27.4 +/- 5.7%, respectively, and accelerated the time course of current decay. CA, but not SP, shifted the inactivation curve to more hyperpolarised potentials (V(h)-37.0 +/- 1.8 vs -40.8 +/- 1.6 mV, n = 10, P < 0.05).SP (10 microM) and CA (1 nM) also inhibited Kv7.1 + minK currents by 38.6 +/- 2.3 and 22.1 +/- 1.4%, respectively, without modifying the voltage dependence of channel activation. SP, but not CA, slowed the time course of tail current decay.CA (1 nM) inhibited the I(Kur) (29.2 +/- 5.5%) and the I(to1) (16.1 +/- 3.9%) recorded in mouse ventricular myocytes and the I(K) (21.8 +/- 6.9%) recorded in guinea-pig ventricular myocytes.A mathematical model of human atrial action potentials demonstrated that K(+) blocking effects of CA resulted in a lengthening of action potential duration, both in normal and atrial fibrillation simulated conditions. The results demonstrated that both SP and CA directly block hKv1.5, Kv4.3 and Kv7.1 + minK channels, CA being more potent for these effects. Since peak free plasma concentrations of CA ranged between 3 and 16 nM, these results indicated that blockade of these human cardiac K(+) channels can be observed after administration of therapeutic doses of SP. Blockade of these cardiac K(+) currents, together with the antagonism of the aldosterone proarrhythmic effects produced by SP, might be highly desirable for the treatment of supraventricular arrhythmias.

In vivo antiarrhythmic and cardiac electrophysiologic effects of a novel diphenylphosphine oxide IKur blocker (2-isopropyl-5-methylcyclohexyl) diphenylphosphine oxide

The antiarrhythmic efficacy of the novel ultrarapid delayed rectifier potassium current (IKur) blocker (2-isopropyl-5-methylcyclohexyl) diphenylphosphine oxide (DPO-1) was compared with efficacies of the standard class III rapidly activating component of delayed rectifier potassium current (IKr) blockers [+-N-[1'-(6-cyano-1,2,3,4-tetrahydro-2-napthalenyl)-3,4-dihydro-4-hydroxyspiro[2H-1-benzopyran-2,4'-piperidin]-6-yl] methanesulfonamide hydrochloride (MK499) and ibutilide and the class IC agent propafenone in a canine model of Y-shaped intracaval and right atrial free wall surgical lesions producing the substrate for reentrant atrial flutter. Electrocardiographic and cardiac electrophysiologic effects also were assessed at the effective antiarrhythmic doses of test agents. DPO-1 terminated atrial arrhythmia (six/six preparations; 5.5 +/- 2.0 mg/kg i.v.) while significantly increasing atrial relative and effective refractory periods (+15.7 and +15.2%, respectively) but having no significant effects on ventricular refractory periods or electrocardiogram (ECG) intervals. Effective antiarrhythmic doses of MK499 (five/five preparations; 0.004 +/- 0.002 mg/kg i.v.) and ibutilide (five/five preparations; 0.003 +/- 0.001 mg/kg i.v.) similarly increased atrial relative (+23.2 and +25.1%, respectively) and effective (+21.6 and +31.9%, respectively) refractory periods. However, antiarrhythmic doses of MK499 and ibutilide also consistently and significantly increased ventricular relative (+9.9 and +7.6%, respectively) and effective (+10.4 and +9.9%, respectively) refractory periods, rate-corrected ECG QTc (+6.7 and +7.8%, respectively), and paced QT (+7.3 and +8.5%, respectively) intervals. Doses of propafenone that terminated atrial arrhythmia (five/five preparations; 0.94 +/- 0.54 mg/kg i.v.) significantly increased ECG QRS interval (+11.1%). These findings support the approach of atrial selective modulation of refractoriness through block of IKur for the development of potentially safer and more effective atrial antiarrhythmic agents.

Role of angiotensin system and effects of its inhibition in atrial fibrillation: clinical and experimental evidence

Atrial fibrillation (AF) is a common arrhythmia that is difficult to treat. Anti-arrhythmic drug therapy, to maintain sinus-rhythm, is limited by inadequate efficacy and potentially serious adverse effects. There is increasing interest in novel therapeutic approaches that target AF-substrate development. Recent trials suggest that angiotensin converting-enzyme (ACE)-inhibitors and angiotensin-receptor blockers (ARBs) may be useful, particularly in patients with left ventricular hypertrophy or failure. The clinical potential and mechanisms of this approach are under active investigation. Angiotensin-II is involved in remodelling and may have direct electrophysiological actions. Experimental studies show protection from atrial structural and possibly electrical remodelling with ACE-inhibitors and ARBs, as well as potential effects on cardiac ion-channels. This article reviews information pertaining to the clinical use and mechanism of action of ACE-inhibitors and ARBs in AF. A lack of prospective randomized double-blind trials data limits their application in AF patients without another indication for their use, but studies under way may alter this in the near future. This exciting field of investigation may lead to significant improvements in therapeutic options for AF patients.

Drug therapy for atrial fibrillation: where do we go from here?

Atrial fibrillation, the most common cardiac arrhythmia requiring medical attention, has effects that range from mild symptoms to devastating stroke. Although treatments have evolved since the foxglove plant (later identified as containing digitalis) was first administered to slow the heart rate, satisfactory drug therapy has not been developed. In this review we describe present-day medical options and developments of future therapies to treat atrial fibrillation and maintain normal sinus rhythm.

Molecular physiology of cardiac repolarization

The heart is a rhythmic electromechanical pump, the functioning of which depends on action potential generation and propagation, followed by relaxation and a period of refractoriness until the next impulse is generated. Myocardial action potentials reflect the sequential activation and inactivation of inward (Na(+) and Ca(2+)) and outward (K(+)) current carrying ion channels. In different regions of the heart, action potential waveforms are distinct, owing to differences in Na(+), Ca(2+), and K(+) channel expression, and these differences contribute to the normal, unidirectional propagation of activity and to the generation of normal cardiac rhythms. Changes in channel functioning, resulting from inherited or acquired disease, affect action potential repolarization and can lead to the generation of life-threatening arrhythmias. There is, therefore, considerable interest in understanding the mechanisms that control cardiac repolarization and rhythm generation. Electrophysiological studies have detailed the properties of the Na(+), Ca(2+), and K(+) currents that generate cardiac action potentials, and molecular cloning has revealed a large number of pore forming (alpha) and accessory (beta, delta, and gamma) subunits thought to contribute to the formation of these channels. Considerable progress has been made in defining the functional roles of the various channels and in identifying the alpha-subunits encoding these channels. Much less is known, however, about the functioning of channel accessory subunits and/or posttranslational processing of the channel proteins. It has also become clear that cardiac ion channels function as components of macromolecular complexes, comprising the alpha-subunits, one or more accessory subunit, and a variety of other regulatory proteins. In addition, these macromolecular channel protein complexes appear to interact with the actin cytoskeleton and/or the extracellular matrix, suggesting important functional links between channel complexes, as well as between cardiac structure and electrical functioning. Important areas of future research will be the identification of (all of) the molecular components of functional cardiac ion channels and delineation of the molecular mechanisms involved in regulating the expression and the functioning of these channels in the normal and the diseased myocardium.

Sulfur dioxide derivative modulation of potassium channels in rat ventricular myocytes

The effects of sulfur dioxide (SO2) derivatives (bisulfite and sulfite, 1:3 M/M) on voltage-dependent potassium current in isolated adult rat ventricular myocyte were investigated using the whole cell patch-clamp technique. SO2 derivatives (10 microM) increased transient outward potassium current (I(to)) and inward rectifier potassium current (I(K1)), but did not affect the steady-state outward potassium current (I(ss)). SO2 derivatives significantly shifted the steady-state activation curve of I(to) toward the more negative potential at the V(h) point, but shifted the inactivation curve to more positive potential. SO2 derivatives markedly shifted the curve of time-dependent recovery of I(to) from the steady-state inactivation to the left, and accelerated the recovery of I(to) from inactivation. In addition, SO2 derivatives also significantly change the inactivation time constants of I(to) with increasing fast time constant and decreasing slow time constant. These results indicated a possible correlation between the change of properties of potassium channel and SO2 inhalation toxicity, which might cause cardiac myocyte injury through increasing extracellular potassium via voltage-gated potassium channels.

Direct block of cloned hKv1.5 channel by cytochalasins, actin-disrupting agents

The action of cytochalasins, actin-disrupting agents on human Kv1.5 channel (hKv1.5) stably expressed in Ltk−cells was investigated using the whole cell patch-clamp technique. Cytochalasin B inhibited hKv1.5 currents rapidly and reversibly at +60 mV in a concentration-dependent manner with an IC50of 4.2 μM. Cytochalasin A, which has a structure very similar to cytochalasin B, inhibited hKv1.5 (IC50of 1.4 μM at +60 mV). Pretreatment with other actin filament disruptors cytochalasin D and cytochalasin J, and an actin filament stabilizing agent phalloidin had no effect on the cytochalasin B-induced inhibition of hKv1.5 currents. Cytochalasin B accelerated the decay rate of inactivation for the hKv1.5 currents. Cytochalasin B-induced inhibition of the hKv1.5 channels was voltage dependent with a steep increase over the voltage range of the channel's opening. However, the inhibition exhibited voltage independence over the voltage range in which channels are fully activated. Cytochalasin B produced no significant effect on the steady-state activation or inactivation curves. The rate constants for association and dissociation of cytochalasin B were 3.7 μM/s and 7.5 s−1, respectively. Cytochalasin B produced a use-dependent inhibition of hKv1.5 current that was consistent with the slow recovery from inactivation in the presence of the drug. Cytochalasin B (10 μM) also inhibited an ultrarapid delayed rectifier K+current ( IK,ur) in human atrial myocytes. These results indicate that cytochalasin B primarily blocks activated hKv1.5 channels and endogenous IK,urin a cytoskeleton-independent manner as an open-channel blocker.

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.

Mechanisms of Atrial Fibrillation: Lessons From Animal Models

Studies in animal models have provided extremely important insights about atrial fibrillation (AF). The classic mechanisms that still form the framework for our understanding of AF (focal activity, single-circuit or "mother wave" reentry, and multiple circuit reentry) were established based on animal studies almost 100 years ago. The past 10 years have witnessed a tremendous acceleration of animal work in this area, including the development of a range of AF models in clinically relevant pathological substrates (eg, atrial tachycardia remodeling, congestive heart failure, pericarditis, ischemic heart disease, mitral valve disease, volume overload states, respiratory failure) and the establishment of an increasing number of genetically defined transgenic mouse models. This article reviews the contribution of animal models to our knowledge about AF mechanisms and to clinical management, dealing with such issues as the theory of reentry; the specific applications of various animal models and their contribution to our understanding of electrophysiologic, ionic, and molecular mechanisms; the role of the autonomic nervous system and regional factors; and the development of novel therapeutic approaches. The complementary nature of animal research and clinical investigation is emphasized and the clinical relevance of findings in experimental models is highlighted.

Blocking Characteristics of hKv1.5 and hKv4.3/hKChIP2.2 After Administration of the Novel Antiarrhythmic Compound AZD7009

AZD7009 is a novel antiarrhythmic compound in early clinical development for management of atrial fibrillation. Electrophysiological studies in animals have shown high antiarrhythmic efficacy, predominant action on atrial electrophysiology, and low proarrhythmic activity. AZD7009 has previously been shown to inhibit hERG and hNav1.5 currents. The main objective of the present study was to characterize the effects of AZD7009 on hKv1.5 and hKv4.3/hKChIP2.2 currents to get a deeper understanding of the ion channel-blocking properties of the compound. hKv1.5 and hKv4.3/hKChIP2.2 currents were expressed in CHO cells. Currents were measured using the whole-cell configuration of the voltage-clamp technique. AZD7009 inhibited hKv1.5 and hKv4.3/hKChIP2.2 currents with equal potency: the IC50 for hKv1.5 block was 27.0 +/- 1.6 muM (n = 6), and the IC50 for hKv4.3/hKChIP2.2 block was 23.7 +/- 4.4 muM (n = 5). Block of the hKv4.3/hKChIP2.2 current was frequency dependent with larger block at higher frequency, whereas block of the hKv1.5 current was slightly decreased at higher frequency. In conclusion, AZD7009 inhibits both the hKv1.5 and the hKv4.3/hKChIP2.2 currents. These effects likely contribute to the effects described in animals in vivo.

Effects of diltiazem and nifedipine on transient outward and ultra-rapid delayed rectifier potassium currents in human atrial myocytes

1. It is unknown whether the widely used L-type Ca(2+) channel antagonists diltiazem and nifedipine would block the repolarization K(+) currents, transient outward current (I(to1)) and ultra-rapid delayed rectifier K(+) current (I(Kur)), in human atrium. The present study was to determine the effects of diltiazem and nifedipine on I(to1) and I(Kur) in human atrial myocytes with whole-cell patch-clamp technique. 2. It was found that diltiazem substantially inhibited I(to1) in a concentration-dependent manner, with an IC(50) of 29.2+/-2.4 microM, and nifedipine showed a similar effect (IC(50)=26.8+/-2.1 muM). The two drugs had no effect on voltage-dependent kinetics of the current; however, they accelerated I(to1) inactivation significantly, suggesting an open channel block. 3. In addition, diltiazem and nifedipine suppressed I(Kur) in a concentration-dependent manner (at +50 mV, IC(50)=11.2+/-0.9 and 8.2+/-0.8 microM, respectively). These results indicate that the Ca(2+) channel blockers diltiazem and nifedipine substantially inhibit I(to1) and I(Kur) in human atrial myocytes.

Atria selective prolongation by NIP-142, an antiarrhythmic agent, of refractory period and action potential duration in guinea pig myocardium

NIP-142 is a novel benzopyran compound that was shown to prolong the atrial effective refractory period and terminate experimental atrial fibrillation in the dog. In the present study, we examined the effects of NIP-142 on isolated guinea pig myocardium and on the G-protein-coupled inwardly rectifying potassium channel current (acetylcholine-activated potassium current; I(KACh)) expressed in Xenopus oocytes. NIP-142 (10 and 100 microM) concentration-dependently prolonged the refractory period and action potential duration in the atrium but not in the ventricle. E-4031 and 4-aminopyridine prolonged action potential duration in both left atrium and right ventricle. Prolongation by NIP-142 of the atrial action potential duration was observed at stimulation frequencies between 0.5 and 5 Hz. In contrast, the prolongation by E-4031 was not observed at higher frequencies. Tertiapin, a blocker of I(KACh), prolonged action potential duration in the atrium but not in the ventricle. NIP-142 completely reversed the carbachol-induced shortening of atrial action potential duration. NIP-142 (1 to 100 microM), as well as tertiapin (0.1 to 100 nM), concentration-dependently blocked I(KACh) expressed in Xenopus oocytes; the blockade by NIP-142 was not affected by membrane voltage. In conclusion, NIP-142 was shown to prolong atrial refractory period and action potential duration through blockade of I(KACh) which may possibly explain its previously described antiarrhythmic activity. NIP-142 has pharmacological properties that are different from classical class III antiarrhythmic agents such as atria specificity and lack of reverse frequency dependence, and thus appears promising for the treatment of supraventricular arrhythmia.

Risperidone reduces K+ currents in human atrial myocytes and prolongs repolarization in human myocardium

The antipsychotic agent risperidone has been shown to cause QT prolongation. In rabbit heart preparations, we have demonstrated that risperidone markedly lengthened action potential duration and blocked the delayed rectifier current, I(Kr.) The current study was designed to investigate the risperidone effects: (i) on the main K(+) repolarizing currents on human atrial myocytes, using whole-cell patch clamp recordings; (ii) on action potentials recorded from human atrial and ventricular myocardium using conventional microelectrodes. We found that: (1) risperidone (3-30 microM) reduced significantly the sustained current, I(sus), and 30 microM decreased significantly the transient outward current I(to) but was without effect on the inward rectifier current I(K1); (2) risperidone (0.3-10 microM) lengthened significantly the final repolarization of the atrial action potential and risperidone (10 microM) markedly lengthened the final repolarization in ventricular myocardium. This study showed that risperidone exerts direct electrophysiological effects on human preparations but only at relatively high concentration.

Open channel block of hKv1.5 by psoralen fromHeracleum moellendorffii hance

A furocoumarin derivative, psoralen (7H-furo[3,2-g][1]benzopyran-7-one), was isolated from the n-hexane fraction of Heracleum moellendorffii Hance. We examined the effects of psoralen on a human Kv1.5 potassium channel (hKv1.5) cloned from human heart and stably expressed in Ltk- cells. We found that psoralen inhibited the hKv1.5 current in a concentration-, use- and voltage-dependent manner with an IC50 value of 180 +/- 21 nM at +60 mV. Psoralen accelerated the inactivation kinetics of the hKv1.5 channel, and it slowed the deactivation kinetics of the hKv1.5 current resulting in a tail crossover phenomenon. These results indicate that psoralen acts on the hKv1.5 channel as an open channel blocker. Furthermore, psoralen prolonged the action potential duration of rat atrial muscles in a dose-dependent manner. Taken together, the present results strongly suggest that psoralen may be an ideal antiarrhythmic drug for atrial fibrillation.

Cross-talk between beta(1)-adrenoceptors and ET(A) receptors in modulation of the slow component of delayed rectifier K(+) currents

Delayed rectifier K(+) currents (I(K)) play a critical role in determining cardiac action potential duration (APD). Modulation of I(K) affects cardiac excitability critically. There are three components of cardiac delayed rectifier, and the slowly activating component (I(Ks)) is influenced strongly by a variety of stimuli. Plasma levels of noradrenaline and endothelin are elevated in heart failure, and arrhythmias are promoted by such humoral abnormalities through modulation of ion channels. It has been reported that protein kinase A (PKA) and protein kinase C (PKC) modulate I(Ks) from human minK in a complex manner. In the present study, we coexpressed human minK with the human beta(1)-adrenoceptor (hbeta(1)AR) and the endothelin receptor subtype A (hET(A)R) in Xenopus oocytes and investigated the effects of receptor activation on the currents (I(Ks)) flowing through the oocytes. ET-1 modulated I(Ks) biphasically: a transient increase followed by a decrease. The PKC inhibitor chelerythrine completely inhibited the effects of ET-1. Intracellular EGTA abolished the transient increase by ET-1 and partially inhibited the subsequent decrease in the currents. When I(Ks) was increased by 10(-6) M isoproterenol (ISO), ET-1 did not increase but rather decreased the current to an even greater extent than under control conditions. In addition, the effects of ISO on I(Ks) were suppressed by ET(A)R stimulation. These data indicate that I(Ks) can be regulated by cross-talk between the ET(A)R and beta(1)AR systems in addition to direct regulation by each receptor system.

Specific pattern of ionic channel gene expression associated with pacemaker activity in the mouse heart

Even though sequencing of the mammalian genome has led to the discovery of a large number of ionic channel genes, identification of the molecular determinants of cellular electrical properties in different regions of the heart has been rarely obtained. We developed a high-throughput approach capable of simultaneously assessing the expression pattern of ionic channel repertoires from different regions of the mouse heart. By using large-scale real-time RT-PCR, we have profiled 71 channels and related genes in the sinoatrial node (SAN), atrioventricular node (AVN), the atria (A) and ventricles (V). Hearts from 30 adult male C57BL/6 mice were microdissected and RNA was isolated from six pools of five mice each. TaqMan data were analysed using the threshold cycle (C(t)) relative quantification method. Cross-contamination of each region was checked with expression of the atrial and ventricular myosin light chains. Two-way hierarchical clustering analysis of the 71 genes successfully classified the six pools from the four distinct regions. In comparison with the A, the SAN and AVN were characterized by higher expression of Nav beta 1, Nav beta 3, Cav1.3, Cav3.1 and Cav alpha 2 delta 2, and lower expression of Kv4.2, Cx40, Cx43 and Kir3.1. In addition, the SAN was characterized by higher expression of HCN1 and HCN4, and lower expression of RYR2, Kir6.2, Cav beta 2 and Cav gamma 4. The AVN was characterized by higher expression of Nav1.1, Nav1.7, Kv1.6, Kvbeta1, MinK and Cav gamma 7. Other gene expression profiles discriminate between the ventricular and the atrial myocardium. The present study provides the first genome-scale regional ionic channel expression profile in the mouse heart.

Intracellular Ca2+ concentration and rate adaptation of the cardiac action potential

Influx of Ca(2+) ions through the cardiac plasma membrane contributes to the shaping of the action potential plateau and acts as trigger for the release of Ca(2+) ions from the sarcoplasmic reticulum and the initiation of the contractile process. The increased intracellular Ca(2+) concentration feeds back on the channels and transporters in the plasma membrane and modulates the electrical activity. This interaction and its change with rate of pacing is the topic of this review, which is subdivided in three parts. In part I a description is given of different channels and transporters that carry Ca(2+) ions, or are activated-modulated by intracellular Ca(2+) ions. In part II an analysis is given of the changes in action potential duration and shape when stimuli are applied in the relative refractory period (electrical restitution) and when rate is suddenly increased and kept at the higher level until steady-state is obtained. A description of experimental findings in each case is followed by a discussion of possible mechanisms. Part III deals with physiopathological aspects of Ca(2+) handling and discusses recent information on hypertrophy, heart failure and atrial fibrillation.

Kv1.3-blocking 5-phenylalkoxypsoralens: a new class of immunomodulators

The lymphocyte potassium channel Kv1.3 is widely regarded as a promising new target for immunosuppression. To identify a potent small-molecule Kv1.3 blocker, we synthesized a series of 5-phenylalkoxypsoralens and tested them by whole-cell patch clamp. The most potent compound of this series, 5-(4-phenylbutoxy)psoralen (Psora-4), blocked Kv1.3 in a use-dependent manner, with a Hill coefficient of 2 and an EC50 value of 3 nM, by preferentially binding to the C-type inactivated state of the channel. Psora-4 is the most potent small-molecule Kv1.3 blocker known. It exhibited 17- to 70-fold selectivity for Kv1.3 over closely related Kv1-family channels (Kv1.1, Kv1.2, Kv1.4, and Kv1.7) with the exception of Kv1.5 (EC50, 7.7 nM) and showed no effect on human ether-a-go-go-related channel, Kv3.1, the calcium-activated K+ channels (IKCa1, SK1-SK3, and BKCa), or the neuronal NaV1.2 channel. In a test of in vivo toxicity in rats, Psora-4 did not display any signs of acute toxicity after five daily subcutaneous injections at 33 mg/kg body weight. Psora-4 selectively suppressed the proliferation of human and rat myelin-specific effector memory T cells with EC50 values of 25 and 60 nM, respectively, without persistently suppressing peripheral blood naive and central memory T cells. Because autoantigen-specific effector memory T cells contribute to the pathogenesis of T cell-mediated autoimmune diseases such as multiple sclerosis, Psora-4 and other Kv1.3 blockers may be useful as immunomodulators for the therapy of autoimmune disorders.