The effect of isoproterenol on the class III effect of azimilide in humans

Role of action potential configuration and the contribution of C²⁺a and K⁺ currents to isoprenaline-induced changes in canine ventricular cells

BACKGROUND AND PURPOSE

Although isoprenaline (ISO) is known to activate several ion currents in mammalian myocardium, little is known about the role of action potential morphology in the ISO-induced changes in ion currents. Therefore, the effects of ISO on action potential configuration, L-type Ca²⁺ current (I(Ca)), slow delayed rectifier K⁺ current (I(Ks)) and fast delayed rectifier K⁺ current (I(Kr)) were studied and compared in a frequency-dependent manner using canine isolated ventricular myocytes from various transmural locations.

EXPERIMENTAL APPROACH

Action potentials were recorded with conventional sharp microelectrodes; ion currents were measured using conventional and action potential voltage clamp techniques.

KEY RESULTS

In myocytes displaying a spike-and-dome action potential configuration (epicardial and midmyocardial cells), ISO caused reversible shortening of action potentials accompanied by elevation of the plateau. ISO-induced action potential shortening was absent in endocardial cells and in myocytes pretreated with 4-aminopyridine. Application of the I(Kr) blocker E-4031 failed to modify the ISO effect, while action potentials were lengthened by ISO in the presence of the I(Ks) blocker HMR-1556. Both action potential shortening and elevation of the plateau were prevented by pretreatment with the I(Ca) blocker nisoldipine. Action potential voltage clamp experiments revealed a prominent slowly inactivating I(Ca) followed by a rise in I(Ks) , both currents increased with increasing the cycle length.

CONCLUSIONS AND IMPLICATIONS

The effect of ISO in canine ventricular cells depends critically on action potential configuration, and the ISO-induced activation of I(Ks) - but not I(Kr) - may be responsible for the observed shortening of action potentials.

Modeling of the adrenergic response of the human IKs current (hKCNQ1/hKCNE1) stably expressed in HEK-293 cells

Stable coexpression of human (h)KCNQ1 and hKCNE1 in human embryonic kidney (HEK)-293 cells reconstitutes a nativelike slowly activating delayed rectifier K+ current (HEK-I(Ks)), allowing beta-adrenergic modulation of the current by stimulation of endogenous receptors in the host cell line. HEK-I(Ks) was enhanced two- to fourfold by isoproterenol (EC50 = 13 nM), forskolin (10 microM), or 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (50 microM), indicating an intact cAMP-dependent ion channel-regulating pathway analogous to the PKA-dependent regulation observed in native cardiac myocytes. Activation kinetics of HEK-I(Ks) were accurately fit with a novel modified second-order Hodgkin-Huxley (H-H) gating model incorporating a fast and a slow gate, each independent of each other in scale and adrenergic response, or a "heterodimer" model. Macroscopically, beta-adrenergic enhancement shifted the current activation threshold to more negative potentials and accelerated activation kinetics while leaving deactivation kinetics relatively unaffected. Modeling of the current response using the H-H model indicated that observed changes in gating could be explained by modulation of the opening rate of the fast gate. Under control conditions at nearly physiological temperatures (35 degrees C), rate-dependent accumulation of HEK-I(Ks) was observed only at pulse frequencies exceeding 3 Hz. Rate-dependent accumulation of I(Ks) at high pulsing rate had two phases, an initial staircaselike effect followed by a slower, incremental accumulation phase. These phases are readily interpreted in the context of a heterodimeric H-H model with two independent gates with differing closing rates. In the presence of isoproterenol after normalizing for its tonic effects, rate-dependent accumulation of HEK-I(Ks) appeared at lower pulse frequencies and was slightly enhanced (approximately 25%) over control.

Antibodies with beta-adrenergic activity from chronic chagasic patients modulate the QT interval and M cell action potential duration

AIMS

The aim of this study was to investigate whether the sera from chronic chagasic patients (CChPs) with beta-1 adrenergic activity (Ab-beta) can modulate ventricular repolarization. Beta-adrenergic activity has been described in CChP. It increases the L-type calcium current and heart rate in isolated hearts, but its effects on ventricular repolarization has not been described.

METHODS AND RESULTS

In isolated rabbit hearts, under pacing condition, QT interval was measured under Ab-beta perfusion. Beta-adrenergic activity was also tested in guinea pig ventricular M cells. Furthermore, the immunoglobulin fraction (IgG-beta) of the Ab-beta was tested on Ito, ICa, and Iks currents in rat, rabbit, and guinea pig myocytes, respectively. Beta-adrenergic activity shortened the QT interval. This effect was abolished in the presence of propranolol. In addition, sera from CChP without beta-adrenergic activity (Ab-beta) did not modulate QT interval. The M cell action potential duration (APD) was reversibly shortened by Ab-beta. Atenolol inhibited this effect of Ab-beta, and Ab- did not modulate the AP of M cells. Ito was not modulated by isoproterenol nor by IgG-beta. However, IgG-beta increased ICa and IKs.

CONCLUSION

The shortening of the QT interval and APD in M cells and the increase of IKs and ICa induced by IgG-beta contribute to repolarization changes that may trigger malignant ventricular arrhythmias observed in patients with chronic chagasic or idiopathic cardiomyopathy.

I(Ks) block by HMR 1556 lowers ventricular defibrillation threshold and reverses the repolarization shortening by isoproterenol without rate-dependence in rabbits

INTRODUCTION

The slow delayed rectifier K+ current (I(Ks)) contributes little to ventricular repolarization at rest. It is unclear whether I(Ks) plays a role during ventricular fibrillation (VF) or ventricular repolarization at rapid rates during beta-adrenergic stimulation.

METHODS AND RESULTS

In an in vivo rabbit model, we evaluated the effects of HMR 1556 (1 mg Kg(-1) + 1 mg kg(-1) hr(-1) i.v.), a selective I(Ks) blocker, on monophasic action potential duration at 90% repolarization (MAPD90), ventricular effective refractory period (VERP), and defibrillation threshold (DFT). In perfused rabbit hearts, the effects of HMR 1556 (10 and 100 nM) in the presence of isoproterenol (5 nM) on MAPD90 and VERP were studied at cycle lengths (CLs) 200-500 msec. In vivo, HMR 1556 prolonged MAPD90 by 6 +/- 1 msec at CL 200 msec (P < 0.01, n = 6), lowered DFT from 558 +/- 46 V to 417 +/- 31 V (P < 0.01), and decreased the coefficient of variation in the VF inter-beat deflection intervals from 8.9 +/- 0.6% to 6.5 +/- 0.4% (P < 0.05) compared with control. In perfused rabbit hearts, isoproterenol shortened MAPD90 by 5 +/- 1 msec at CL 200 msec and 11 +/- 4 msec at CL 500 msec (P < 0.05, n = 7). This shortening was reversed by HMR 1556 (P < 0.05), and both effects were rate-independent.

CONCLUSION

I(Ks) block increases VF temporal organization and lowers DFT, and I(Ks) that is activated following beta-adrenergic stimulation contributes to ventricular repolarization without rate dependence.

The Metabolic Profile of Azimilide in Man: In Vivo and in Vitro Evaluations

The metabolic fate of azimilide in man is unusual as it undergoes a cleavage in vivo resulting in the formation of two classes of structurally distinct metabolites. During a metabolite profiling study conducted in human volunteers to assess the contribution of all pathways to the clearance of (14)C-azimilide, greater than 82% of radioactivity was recovered in urine (49%-58%) and feces (33%). Urine, feces, and plasma were profiled for metabolites. A cleaved metabolite, 4-chloro-2-phenyl furoic acid was present at high concentration in plasma (metabolite/parent AUC ratio approx. 4), while other plasma metabolites, azimilide N-oxide (metabolite/parent AUC ratio 0.001), and a cleaved hydantoin metabolite (metabolite/parent AUC ratio = 0.3) were present at lower concentrations than azimilide. In urine, the cleaved metabolites were the major metabolites, (> 35% of the dose) along with phenols (as conjugates, 7%-8%), azimilide N-oxide (4%-10%), a butanoic acid metabolite (2%-3%), and desmethyl azimilide (2%). A limited investigation of fecal metabolites indicated that azimilide (3%-5%), desmethyl azimilide (1%-3%), and the butanoic acid metabolite (< 1%) were present. Contributing pathways for metabolism of azimilide, identified through in vitro and in-vivo studies, were CYPs 1A1 (est. 28%), 3A4/5 (est. 20%), 2D6 (< 1%), FMO (est. 14%), and cleavage (35%). Enzyme(s) involved in the cleavage of azimilide were not identified.

Azimilide decreases recurrent ventricular tachyarrhythmias in patients with implantable cardioverter defibrillators

OBJECTIVES

This study evaluated the effects of azimilide dihydrochloride (AZ) on anti-tachycardia pacing (ATP) and shock-terminated events in patients with implantable cardioverter defibrillators (ICDs).

BACKGROUND

Animal studies have shown the effectiveness of AZ for therapy of supraventricular and ventricular tachycardia (VT). Azimilide dihydrochloride was investigated as adjunctive treatment for reducing the frequency of VT and, thus, the need for ICD therapies, including ATP and cardioversion/defibrillation (ICD shocks) in patients with inducible monomorphic VT.

METHODS

A total of 172 patients were randomized to daily treatment with placebo, 35 mg, 75 mg, or 125 mg of oral AZ in this dose-ranging pilot study of patients with ICDs. The majority of patients had a history of documented remote myocardial infarction and congestive heart failure New York Heart Association class II or III.

RESULTS

The frequency of appropriate shocks and ATP were significantly decreased among AZ-treated patients compared with placebo patients. The incidence of ICD therapies per patient-year among the placebo group was 36, and it was 10, 12, and 9 among 35 mg, 75 mg, and 125 mg AZ patients, respectively (hazard ratio = 0.31, p = 0.0001). Azimilide dihydrochloride was generally well tolerated and did not affect left ventricular ejection fraction or minimal energy requirements for defibrillation or pacing.

CONCLUSIONS

Azimilide dihydrochloride may be a safe and effective drug for reducing the frequency of VT and ventricular fibrillation in patients with implanted ICDs.