IKr Blocker Research Articles

Aging-related changes of QT and RR intervals in conscious guinea pigs

Introduction Although guinea pigs are suitable for in vivo QT assessment of newly discovered drugs at the pre-clinical stage because of the similarity of the ion channels between the guinea pig heart and the human, there is limited data available regarding the characteristics of QT interval in conscious guinea pigs. Aging is one of several factors which have been shown to affect the QT interval in humans and animals. In the present study, we examined the influence of age on QT and RR intervals in conscious guinea pigs. Methods Electrocardiograms were recorded from female Hartley guinea pigs at the age of 6 weeks (young; n = 6) and 23 months (old; n = 4) via a telemetry system. The QT and RR intervals were measured during daytime and nighttime, and following intravenous bolus injection of E-4031 (0.1 mg/kg) or terfenadine (4 mg/kg). Comparisons were made to determine group differences in: (1) the normal values of the QT and RR intervals, (2) the best-fit QT-correction formula, (3) the circadian rhythm of QT and RR intervals, and (4) drug effects on repolarization. Results The normal values of QT and RR intervals in the old group were significantly longer than those in the young group. The best-fit formula for correcting QT interval was a modified Bazett's formula for both young and old groups. The old group did not show the nocturnal variation of either QT or RR interval. Terfenadine caused significantly greater QTc prolongation in the old group compared to the young. Discussion Aging affects resting QT and RR intervals in conscious female guinea pigs, a factor which should be considered when examining the effects of compounds on cardiac repolarization. Also, the present study suggests a possibility that age can affect QTc prolongation induced by some IKr blockers.

Reduction of Dispersion of Repolarization and Prolongation of Postrepolarization Refractoriness Explain the Antiarrhythmic Effects of Quinidine in a Model of Short QT Syndrome

Short QT syndrome (SQTS) is a newly described ion channelopathy, characterized by a short QT interval resulting from an accelerated cardiac repolarization, associated with syncope, atrial fibrillation, and sudden cardiac death due to ventricular fibrillation. As therapeutic options in SQTS are still controversial, we examined antiarrhythmic mechanisms in an experimental model of SQTS. Pinacidil, an I(K-ATP) channel opener, was administered in increasing concentrations (50-100 microM) in 48 Langendorff-perfused rabbit hearts and led to a significant reduction of action potential duration and QT interval, thereby mimicking SQTS. Eight simultaneously recorded monophasic action potentials demonstrated an increase in dispersion of repolarization, especially between the left and the right ventricle. During programmed ventricular stimulation with up to two extrastimuli, pinacidil significantly increased the inducibility of ventricular fibrillation (1 heart under baseline conditions, 29 hearts during pinacidil administration; P = 0.0001). Additional treatment with the I(Kr) blocker sotalol (100 microM) and the class I antiarrhythmic drugs flecainide (2 microM) and quinidine (0.5 microM) randomly assigned to three groups of 16 hearts led to prolongation of repolarization as well as refractory period. Sotalol or flecainide did not reduce the rate of inducibility of ventricular fibrillation significantly (P = 0.63; P = 0.219). However, quinidine reduced the inducibility of ventricular fibrillation by 73% (P = 0.008). The antiarrhythmic potential of quinidine was associated with a significantly greater prolongation of MAP duration, refractoriness, and postrepolarization refractoriness (PRR) as compared with sotalol and flecainide. Moreover, quinidine, in contrast to sotalol and flecainide, reduced dispersion of repolarization. Pinacidil mimics SQTS via increasing potassium outward currents, thereby facilitating inducibility of ventricular fibrillation. Quinidine demonstrates superior antiarrhythmic properties in the treatment of ventricular fibrillation in this model as compared with sotalol and flecainide because of its effects on refractoriness, PRR, and by reducing dispersion of repolarization.

The effect of ATX-II on torsade de pointes induced by E-4031 in vivo

Torsade de pointes (TdP) is associated with long QT (LQT) syndromes which can be precipitated by altering cardiac potassium currents e.g. IKr (LQT2) or the cardiac sodium current INa (LQT3). The aim of this study was to test the hypothesis that ATX-II will potentiate E-4031-induced TdP. Experiments were performed in pentobarbital-anaesthetized α1-adrenoceptor-stimulated male New Zealand White rabbits. Rabbits were given increasing doses of either the INa opener ATX-II (n=6), the IKr blocker E-4031 (n=6), or both drugs in combination (n=6).

NIP-141, a multiple ion channel blocker, terminates aconitine-induced atrial fibrillation and prevents the rapid pacing-induced atrial effective refractory period shortening in dogs

NIP-141 is a novel multiple ion channel blocker with atrial selective effects. In this study, we examined the effects of NIP-141 on aconitine-induced atrial fibrillation (AF) and rapid atrial pacing-induced atrial effective refractory period (ERP) shortening in dogs. Aconitine AF was induced by the application of aconitine on the right appendage. NIP-141 (10 mg/kg) converted AF to sinus rhythm in 5 of 6 dogs. The Na(+) channel blockers disopyramide (1 mg/kg) and phenytoin (10 mg/kg) also terminated AF, but the I(Kr) blocker (d-sotalol; 4 mg/kg) and a Ca(2+) channel blocker (verapamil; 0.3 mg/kg) did not terminate AF in this model. To clarify the mechanism of AF termination, we examined the effects on ERP and conduction time, but NIP-141 (10 mg/kg) had no significant effects. In a short-term rapid atrial pacing model, NIP-141 (2.5 mg/kg/10 min, followed by 0.033 mg/kg/min) prevented atrial ERP shortening. We also found NIP-141 bound to Na(+) channel site 2 receptor and L-type Ca(2+) channel, but not to Na(+) channel site 1 receptor using radioligands binding assay. NIP-141 terminated AF in aconitine-induced AF and prevented the atrial remodelling by short-term rapid pacing in dogs, possibly via the blocking of Na(+) and Ca(2+) channels.

QT long acquis : un problème envahissant ?

QT prolongation is essentially of pharmacologic origin. It is principally linked to a block of the outward potassium current Ikr, with as a consequence a prolongation of the repolarisation causing early after potentials and re-entry. The term “repolarisation reserve” expresses the variable risk of arrhythmia among individuals under the same drug blocking Ikr. This reserve can the altered under various pathologic or genetic conditions. A series of risk factors (bradycarTorsades de pointes were described in 1966 by Dessertenne. They are due to a perturbation of ventricular repolarisation causing QT prolongation on surface ECG. Acquired dia, electrolytic disorders, cardiac disease, neurologic disorders, nutrition troubles, female gender) can play a role as well as the metabolic processing of pharmacological agents by Cytochrome P450 and various inhibitors or inductors of this system which can influence the half life of drugs. The list of drugs involved is continuously increasing: antiarrhythmics, antihistamines, psychotropics, anti-infectious are the main categories involved. Risk prediction is difficult particularly for non cardiovascular drugs and a low risk incidence. An other risk is to exclude patients from the benefit of an efficient drug for a serious but unfrequent risk, at last an industrial risk for the manufacturer when a drug is withdrawn lately when important quantities of money have already been invested for its development. The diagnosis of torsades is easy on standard ECG although QT measurement and its heart rate variation remain uneasy. The treatment of the arrhythmias is based on heart rate acceleration by Isoprenaline or intravenous pacing and on intravenous administration of magnesium.

Cumulative Experience of Azimilide-Associated Torsades de Pointes Ventricular Tachycardia in the 19 Clinical Studies Comprising the Azimilide Database

The purpose of this study was to assess the incidence, temporal characteristics, and risk factors associated with azimilide-associated torsades de pointes (TdP) ventricular tachycardia. Azimilide dihydrochloride is a class III antiarrhythmic drug possessing Ikr and Iks channel-blocking properties. Oral azimilide (75 to 125 mg/day) was taken by 5,375 patients in 19 clinical trials conducted at 775 international centers. Of 3,964 patients in double-blind studies, 1,427 had a history of atrial fibrillation or other supraventricular arrhythmia, 510 had an implantable cardioverter-defibrillator, and 2,027 were post-myocardial infarction patients with a left ventricular ejection fraction < or =35%. The TdP occurred in 56 patients assigned to azimilide, was dose-related, and tended to occur earlier with an azimilide-loading regimen. Forty-three percent of TdP patients had a QT interval corrected by Bazett's formula, for heart rate, (QTc) > or =500 ms at the time of or before the TdP occurrence. Significant risk factors using logistic regression were increasing age, female gender, diuretic use, and lack of aspirin use. Azimilide-associated TdP has characteristics and risk factors similar to other Ikr blockers. However, there is a distinctive temporal profile. The TdP events are not concentrated in the first week. The azimilide-associated TdP rate is 1% (95% confidence interval 0.78 to 1.35) and is not increased in patients with low left ventricular ejection fraction, even in women.

Reduced repolarization reserve due to anthracycline therapy facilitates torsade de pointes induced by IKr blockers

Cytostatic agents such as anthracyclines may cause changes in the electrophysiologic properties of the heart. We hypothesized that anthracyclines facilitate life-threatening proarrhythmic side effects of cardiovascular and non-cardiovascular repolarization prolonging drugs. The electrophysiologic effects of chronic administration of doxorubicin (Dox) were studied in ten rabbits, which were treated with Dox twice a week (1.5 mg/kg i.v.). A control group (11 rabbits) was given NaCl solution. Two of ten Dox rabbits died suddenly, the remaining animals showed mild clinical signs of heart failure after a period of six weeks. Echocardiography demonstrated a decrease in ejection fraction (pre treatment: 74 +/- 23% to post treatment: 63 +/- 16% (p <0.05)). In isolated hearts, action potential duration measured by eight simultaneously recorded monophasic action potentials (MAP) was similar in Dox and control hearts. However, in Dox rabbits, administration of the I(Kr)-blocker erythromycin (150-300 microM) led to a significant greater prolongation of the mean MAP duration (63 +/- 21ms vs 29 +/- 12 ms, p <0.05) and the QT interval (100 +/- 32ms vs 58 +/- 17 ms, p <0.05) as compared to control. Moreover, I(Kr)-block led to a more marked increase of dispersion of MAP(90) in the Dox group as compared to control hearts (23 +/- 7ms vs. 9 +/- 4 ms). In the presence of hypokalemia more episodes of early afterdepolarizations and torsade de pointes occurred (p <0.05). Even during the early phase of chemotherapeutic treatment,before significant QT-prolongation is present,anthracyclines lead to an increased sensitivity to the proarrhythmic potency of I(Kr)-blocking drugs. Thus, anthracycline therapy reduces repolarization reserve and thereby represents a novel contributing factor for the development of life-threatening proarrhythmia.

Blockade of IKs by HMR 1556 increases the reverse rate‐dependence of refractoriness prolongation by dofetilide in isolated rabbit ventricles

1. The rate-dependent contributions of the rapid and slow components of the cardiac delayed rectifier K+ current (IKr and IKs, respectively) to repolarization are not fully understood. It is unclear whether the addition of IKs block will attenuate reverse rate-dependence seen after IKr block. 2. The individual and combined electrophysiological effects of selective IKr and IKs blockers, dofetilide and HMR 1556, respectively, were evaluated using Langendorff-perfused rabbit hearts. Monophasic action potential duration at 90% repolarization (MAPD90) and ventricular effective refractory period (VERP) were determined at cycle lengths (CLs) of 200-500 ms (at 50 ms intervals). 3. Dofetilide (1-100 nM) prolonged MAPD90 in a concentration-dependent manner (P < 0.001, n = 6) with reverse rate-dependence (P < 0.0001). In contrast, HMR 1556 (10-240 nM) alone did not prolong MAPD90. However, in the presence of 7.5 nM dofetilide, HMR 1556 (100 nM) increased the extent of reverse rate-dependence by further prolonging MAPD90 at CLs of 400, 450 and 500 ms (P < 0.05, n = 9) and, to a lesser extent, at shorter CLs (e.g. by 17 +/- 4 ms at CL 500 vs 2 +/- 3 ms at CL 200 ms). 4. Effects of dofetilide and HMR 1556 on VERP were similar to those on MAPD90. The slope of the VERP vs CL relation was steeper after the combination (0.081 +/- 0.013) than after dofetilide alone (0.028 +/- 0.018, P < 0.01, n = 9). 5. Blockade of rabbit IKs increased reverse rate-dependence of IKr block.

P1-35: The common Angiotensinogen (AGT) polymorphism T207M protects from drug-induced QT prolongation

IKr block is a well-described mechanism for drug-induced QT prolongation and development of life-threatening arrhythmias. QT prolongation by drug is highly variable among individuals, and a genetic component to risk has been suggested. Polymorphisms in the angiotensinogen gene (AGT) have been associated with arrhythmias previously.

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.

The influence of extracellular acidosis on the effect of Type III antiarrhythmic agents on the cardiac potassium channel (IKr)

Ischemia may cause major changes in pH. The change in pH could influence the effect of type III agents on the rapid delayed rectifier potassium channel (IKr). We evaluated the effects of acidosis on IKr inhibition caused by amiodarone (AM), azimilide (AZ), dofetilide (D) and quinidine (Q). IKr was studied at room temperature by using HERG gene expressed in Xenopus oocytes and two electrodes voltage clamp technique was applied. The recording bath solution contained in mmol/L: 96 NaCl, 5.0 KCl, 2.0 CaCl2, 1.0 MgCl2, 5 HEPES; and pH adjusted with NaOH to 6.2 or 7.4. AZ, AM, Q at 1, 3, 10, 30, 100 μM and D at 0.03, 0.1, 0.3, 1, 3 μM caused dose dependent block in HERG current. The IC50 was 5.8 ± 0.3 μM for AZ, 9.9 ±1.01 μM for Q and 0.5 ± 0.02 μM for D. When extracellular pH was decreased from 7.4 to 6.2, the IC50 increased to 96 ± 11 μM for AZ, 203 ± 16 μM for Q and 13 ± 1 μM for D. The block effect was decreased 32% for AZ, 42% for D, 29% for Q when pH was decreased from 7.4 to 6.2. For AM, the IC50 was 38 ± 9 μM at pH 7.4 and 27 ± 2 μM at and pH 6.2. There was no significant difference in the percentage of current block by AM between pH 6.2 and 7.4. Our data shows that the Ikr blocking effect of AZ, D and Q was attenuated at acidic pH while this was not the case for AM. The effect of AZ, D and Q at low pH creates heterogeneity of repolarization between ischemic and normal regions, but no change in current block would be expected with AM with regional acidosis. These observations may explain the reduced proarrhythmic effect of AM.

OIII-A-3Lengthening of cardiac repolarization during the sequential administration of two IKR blockers: Order of administration as a major determinant

BACKGROUND/AIMS Block of IKrcauses significant lengthening of cardiac repolarization and is of major concern for drug safety. The objective of the present study was to assess prolongation of cardiac repolarization during the combined use of two IKr blockers when administered sequentially in different orders. METHODS Hearts (n=84) from Hartley male guinea-pigs were isolated and buffer-perfused in the Langendorff mode. Monophasic action potential duration was measured at 90% repolarization (MAPD90). After a 5 min perfusion with buffer, hearts were perfused with[domperidone (50, 150 and 300 nM) followed by erythromycin (10 μM) or by ketoconazole (10 μM)], or inversely with[ketoconazole administered first (10 μM) followed by domperidone (50, 150 and 300 nM)]. RESULTS Increasing concentrations of domperidone caused a concentration-dependent prolongation of MAPD90. Sequential administration of erythromycin or ketoconazole caused an additive effect. In contrast, MAPD90 prolongation caused by domperidone was no longer concentration-dependent when ketoconazole was administered first. CONCLUSION Our results demonstrate that the order of the sequential administration of two IKr blockers seems to be an important factor on lengthening of cardiac repolarization. Further studies are necessary to elucidate the responsible mechanism. Clinical Pharmacology & Therapeutics (2005) 79, P29–P29; doi: 10.1016/j.clpt.2005.12.108

PII-8Evaluation of gender-related differences in the prolongation of action potential duration during concomitant block of iKR and iKS

PII-8Evaluation of gender-related differences in the prolongation of action potential duration during concomitant block of iKR and iKS

Comparison of Four Rate-Correction Algorisms for the Ventricular Repolarization Period in Assessing Net Effects of IKr Blockers in Dogs

The utility of corrected and uncorrected QT interval changes for assessing net repolarization delay by IKr (a rapid component of delayed rectifier K+ currents) blockers was assessed in halothane-anesthetized dogs using the electrocardiogram and monophasic action potential (MAP) recordings with electrical ventricular pacing. Intravenous administration of dl-sotalol (0.2 – 2 mg/kg) prolonged the MAP duration and RR interval, while terfenadine (3 mg/kg) increased the MAP duration but transiently shortened RR interval. The order of correlation coefficient between the MAP duration at a pacing cycle length of 400 ms and MAP duration itself or that with arithmetical correction was uncorrected > Van de Water = Matsunaga > Fridericia > Bazett. These results suggest that Matsunaga’s and Van de Water’s formulae would better predict the net repolarization delay in the in vivo canine model. Also, the risk of drug candidates that may prolong the QT interval should be judged by change in uncorrected QT interval as well as corrected QT interval.

The congenital long QT syndromes from genotype to phenotype: clinical implications

The long QT syndrome (LQTS) is a genetic disorder responsible for many sudden deaths before age 20. The identification of several LQTS genes, all encoding cardiac ion channels, has had a major impact on the management strategy for both patients and family members. Genotype-guided therapy allows more effective individually tailored therapy. Therapeutic options, including beta-blockers, left cardiac sympathetic denervation, and implantable defibrillators are discussed for patients of known and of unknown genotype. The recent identification of modifier genes which amplify the effect of an LQTS mutation may change the approach to risk stratification.

Pharmacological Modification of Spiral-type Reentry in the Ventricle

Spiral-type reentry is a principal mechanism for the genesis of life-threatening ventricular tachyarrhythmias. We investigated differential effects of Na channel blockers, IKr blocker (nifekalant), ICa,L blocker (verapamil), and amiodarone on the spiral reentry in rabbit hearts by using a high-resolution optical mapping system. Two-dimensional subepicardial layer of left ventricular myocardium with uniform anisotropy was prepared by endocardial cryoablation. During ventricular tachycardia (VT) elicited by cross-field stimulation, spiral-type excitations rotating around functional block lines (FBLs) were visualized.

Isolation and characterization of IKr in cardiac myocytes by Cs+ permeation

Isolation of the rapidly activating delayed rectifier potassium current (I(Kr)) from other cardiac currents has been a difficult task for quantitative study of this current. The present study was designed to separate I(Kr) using Cs+ in cardiac myocytes. Cs+ have been known to block a variety of K+ channels, including many of those involved in the cardiac action potential such as inward rectifier potassium current I(K1) and the transient outward potassium current I(to). However, under isotonic Cs+ conditions (135 mM Cs+), a significant membrane current was recorded in isolated rabbit ventricular myocytes. This current displayed the voltage-dependent onset of and recovery from inactivation that are characteristic to I(Kr). Consistently, the current was selectively inhibited by the specific I(Kr) blockers. The biophysical and pharmacological properties of the Cs+-carried human ether-a-go-go-related gene (hERG) current were very similar to those of the Cs+-carried I(Kr) in ventricular myocytes. The primary sequence of the selectivity filter in hERG was in part responsible for the Cs+ permeability, which was lost when the sequence was changed from GFG to GYG, characteristic of other, Cs+-impermeable K+ channels. Thus the unique high Cs+ permeability in I(Kr) channels provides an effective way to isolate I(Kr) current. Although the biophysical and pharmacological properties of the Cs+-carried I(Kr) are different from those of the K+-carried I(Kr), such an assay enables I(Kr) current to be recorded at a level that is large enough and sufficiently robust to evaluate any I(Kr) alterations in native tissues in response to physiological or pathological changes. It is particularly useful for exploring the role of reduction of I(Kr) in arrhythmias associated with heart failure and long QT syndrome due to the reduced hERG channel membrane expression.

Female Gender is a Risk Factor for Drug-Induced Long QT and Torsade de Point (TdP) in Anesthetized, Beagle Dogs

The female gender is known to be more sensitive to drug-induced long QT and cardiac arrhythmia, based on clinical observations, experimental in vitro data, in isolated rabbit and dog Purkinje fibers, and also in vivo data in rabbits. However, to-date there has been no demonstration of a gender difference in vivo in dogs. As such, the aim of this study was to try and elucidate whether a gender difference is present in this species in vivo. This knowledge is important to drug safety evaluation since conventionally the male dog is the chosen sex for safety pharmacology studies. We evaluated potential gender differences in the following variables in neuromuscular blocked, mechanically ventilated anesthetized dogs: ventricular repolarization (ECG lead II, QT and qtcv; right ventricular endocardial monophasic action potential (MAP), duration at 90% repolarization, (APD90), spatial dispersion of the T wave (Tp-Te), instability of qtend (QT TI: total instability), occurrence of eads and/or tdps. These parameters were derived from continuous beat-to-beat recordings of ECG and MAP signals. 20 female and 18 male dogs were treated with dofetilide (0.05 mg/kg i.v. Infusion over 10 min). At baseline, no statistically significant gender difference was observed on the various parameters, whilst dofetilide infusion produced greater electrophysiological alterations in females than in males. The respective median maximum percentage changes from baseline in females versus males were: heart rate (+3% / +6%, ns), QT (+23% / +14%, p = 0.003), qtcv (+20% / +15%, p = 0.03) and APD90 (+26% / +17%, p = 0.006). Dofetilide tended to differently increase Tp-Te (+40% vs +23%, ns) and QT TI (+150% vs +104%), more in female than in male animals. Additionally, dofetilide infusion elicited more incidences of eads on the MAP signal in female than in male dogs (73% versus 48%). Furthermore, the ikr blocker dofetilide (0.05 mg/kg iv) combined with Iks blocker HMR1556 (0.5 mg/kg iv) elicited tdps in 3 out of 4 female dogs in our preliminary investigations. Similar to man, the present study suggests that the female gender may be a risk factor for drug-induced long QT. Indeed, since significant alterations in additional markers beyond QT interval itself appear to be more pronounced in female dogs and eads occur more frequently in this sex, female dogs could be more sensitive to induction of polymorphic ventricular tachycardia (tdp). As such, consideration should be given to incorporation of female dogs into standard cardiovascular safety evaluations, or at least be the chosen single sex for drug safety evaluation, since the use of males may potentially lead to an underestimate of the QT-related pro-arrhythmogenic risk of a new chemical entity.

Novel Method to Assess Cardiac Electrophysiology in the Rat

The rat continues to be an important tool to assess cardiac electrophysiologic (EP) effects of test agents and to study the distribution/role of ion channels in cardiovascular diseases. However, no data have been described that accurately measure discrete cardiac EP parameters in rats in vivo. Therefore, we developed a method to assess cardiac EP in rats and then profiled several ion channel agents. Briefly, rats were instrumented with endocardially placed electrodes to assess cardiac refractoriness and conduction. Administration of class I agents resulted in a dose-dependent slowing of ventricular conduction. The potassium channel blocker 4-aminopyridine caused significant increases in atrial and ventricular refractoriness. An IKr blocker had little or no effect on atrial and ventricular refractoriness but significantly increased AV nodal refractoriness. Additionally, an IKs blocker had little effect on rat cardiac EP. The L-type blocker diltiazem caused a dose-dependent delay in AV node conduction and an increase in AV node refractoriness. Overall, this study provides normative data that describe the roles of Na, Ca, and K channels in rat cardiac electrophysiology, in vivo. Furthermore, the model provides a method to assess changes in cardiac electrophysiology in the setting of disease by using well-established rat models of induced or genetic cardiovascular disease.

A Mechanism for the Potential Proarrhythmic Effect of Acidosis, Bradycardia, and Hypokalemia on the Blockade of Human Ether-a-go-go-Related Gene (HERG) Channels

Many drugs are proarrhythmic by inhibiting the cardiac rapid delayed rectifier potassium channel (IKr). In this study, we use quinidine as an example of highly proarrhythmic agent to investigate the risk factors that may facilitate the proarrhythmic effects of drugs. We studied the influence of pacing, extracellular potassium, and pH on quinidine's IKr blocking effect, all potential factors influencing quinidine's cardiac toxicity. Since the HERG gene encodes IKr, we studied quinidine's effect on HERG expressed in Xenopus oocytes by the 2-electrode voltage clamp technique. When extracellular K+ was 5 mmol/L, quinidine blocked the HERG current dose dependently, with an IC50 of 6.3 +/- 0.2 micromol/L. The blockade was much more prominent at more positive membrane potentials. The inhibition of HERG by quinidine was not use dependent. There was no significant difference between block with or without pacing. When extracellular K+ was lowered to 2.5 mmol/L, the current inhibition by quinidine was enhanced, and IC50 decreased to 4.6 +/- 0.5 micromol/L. At 10 mmol/L extracellular K+, there was less inhibition by quinidine and the IC50 was 11.2 +/- 3.1 micromol/L. Extracellular acidification decreased both steady state and tail currents of HERG. We conclude that the inhibitory effect of quinidine on IKr was decreased with extracellular acidification, which may produce heterogeneity in the repolarization between normal and ischemic cardiac tissue. Thus, the use-independent blockade of IKr by QT-prolonging agents such as quinidine may contribute to cardiac toxicity with bradycardia, hypokalemia, and acidosis further exaggerating the proarrhythmic potential of these agents.