Bum-Rak Choi

Cytosolic Ca2+ triggers early afterdepolarizations and torsade de pointes in rabbit hearts with type 2 long QT syndrome

The role of intracellular Ca2+ (Ca12+) in triggering early afterdepolarizations (EADs), the origins of EADs and the mechanisms underlying Torsade de Pointes (TdP) were investigated in a model of long QT syndrome (Type 2). Perfused rabbit hearts were stained with RH327 and Rhod‐2/AM to simultaneously map membrane potential (Vm) and Ca12+ with two photodiode arrays. The IKr blocker E4031 (0.5 μM) together with 50% reduction of [K+]o and [Mg2+]o elicited long action potentials (APs), Vm oscillations on AP plateaux (EADs) then ventricular tachycardia (VT). Cryoablation of both ventricular chambers eliminated Purkinje fibres as sources of EADs. E4031 prolonged APs (0.28 to 2.3 s), reversed repolarization sequences (base→apex) and enhanced repolarization gradients (30 to 230 ms, n= 12) indicating a heterogeneous distribution of IKr. At low [K+]o and [Mg2+]o, E4031 elicited spontaneous Ca12+and Vm spikes or EADs (3.5 ± 1.9 Hz) during the AP plateau (n= 6). EADs fired ‘out‐of‐phase’ from several sites, propagated, collided then evolved to TdP. Phase maps (Ca12+vs. Vm) had counterclockwise trajectories shaped like a ‘boomerang’ during an AP and like ellipses during EADs, with Vm preceding Ca12+ by 9.2 ± 1.4 (n= 6) and 7.2 ± 0.6 ms (n= 5/6), respectively. After cryoablation, EADs from surviving epicardium (∼1 mm) fired at the same frequency (3.4 ± 0.35 Hz, n= 6) as controls. At the origins of EADs, Ca12+ preceded Vm and phase maps traced clockwise ellipses. Away from EAD origins, Vm coincided with or preceded Ca12+. In conclusion, overload elicits EADs originating from either ventricular or Purkinje fibres and ‘out‐of‐phase’ EAD activity from multiple sites generates TdP, evident in pseudo‐ECGs.

Simultaneous maps of optical action potentials and calcium transients in guinea‐pig hearts: mechanisms underlying concordant alternans

1 The mechanisms underlying electro‐mechanical alternans caused by faster heart rates were investigated in perfused guinea‐pig hearts stained with RH237 and Rhod‐2 AM to simultaneously map optical action potentials (APs) and intracellular free Ca2+ (Ca2+i). 2 Fluorescence images of the heart were focused on two 16 × 16 photodiode arrays to map Ca2+i (emission wavelength (λem) = 585 ± 20 nm) and APs (λem > 715 nm) from 252 sites. Spatial resolution was 0·8 mm × 0·8 mm per diode and temporal resolution 4000 frames s−1. 3 The mean time‐to‐peak for APs and [Ca2+]i was spatially homogeneous (8·8 ± 0·5 and 25·6 ± 5·0 ms, respectively; n= 6). The durations of APs (APDs) and Ca2+i transients were shorter at the apex and progressively longer towards the base, indicating a gradient of ventricular relaxation. 4 Restitution kinetics revealed increasingly longer delays between AP and Ca2+i upstrokes (9·5 ± 0·4 to 11·3 ± 0·4 ms) with increasingly shorter S1‐S2 intervals, particularly at the base, despite nearly normal peak [Ca2+]i. 5 Alternans of APs and Ca2+i transients were induced by a decrease in cycle length (CL), if the shorter CL captured at the pacing site and was shorter than refractory periods (RPs) near the base, creating heterogeneities of conduction velocity. 6 Rate‐induced alternans in normoxic hearts were concordant (long APD with large [Ca2+]i) across the epicardium, with a magnitude (difference between odd‐even signals) that varied with the local RP. Alternans were initiated by gradients of RP, producing alternans of conduction that terminated spontaneously without progressing to fibrillation.

Sex, Age, and Regional Differences in L-Type Calcium Current Are Important Determinants of Arrhythmia Phenotype in Rabbit Hearts With Drug-Induced Long QT Type 2

In congenital and acquired long QT type 2, women are more vulnerable than men to Torsade de Pointes. In prepubertal rabbits (and children), the arrhythmia phenotype is reversed; however, females still have longer action potential durations than males. Thus, sex differences in K+ channels and action potential durations alone cannot account for sex-dependent arrhythmia phenotypes. The L-type calcium current (ICa,L) is another determinant of action potential duration, Ca2+ overload, early afterdepolarizations (EADs), and Torsade de Pointes. Therefore, sex, age, and regional differences in ICa,L density and in EAD susceptibility were analyzed in epicardial left ventricular myocytes isolated from the apex and base of prepubertal and adult rabbit hearts. In prepubertal rabbits, peak ICa,L at the base was 22% higher in males than females (6.4±0.5 versus 5.0±0.2 pA/pF; P<0.03) and higher than at the apex (6.4±0.5 versus 5.0±0.3 pA/pF; P<0.02). Sex differences were reversed in adults: ICa,L at the base was 32% higher in females than males (9.5±0.7 versus 6.4±0.6 pA/pF; P<0.002) and 28% higher than the apex (9.5±0.7 versus 6.9±0.5 pA/pF; P<0.01). Apex–base differences in ICa,L were not significant in adult male and prepubertal female hearts. Western blot analysis showed that Cav1.2&agr; levels varied with sex, maturity, and apex–base, with differences similar to variations in ICa,L; optical mapping revealed that the earliest EADs fired at the base. Single myocyte experiments and Luo–Rudy simulations concur that ICa,L elevation promotes EADs and is an important determinant of long QT type 2 arrhythmia phenotype, most likely by reducing repolarization reserve and by enhancing Ca2+ overload and the propensity for ICa,L reactivation.

Life Span of Ventricular Fibrillation Frequencies

Abstract— The nature and organization of electrical activity during ventricular fibrillation (VF) are important and controversial subjects dominated by 2 competing theories: the wavebreak and the dominant mother rotor hypothesis. To investigate spatiotemporal characteristics of ventricular fibrillation (VF), transmembrane potentials (Vm) were recorded from multiple sites of perfused rabbit hearts using a voltage-sensitive dye and a photodiode array or a CCD camera, and the time-frequency characteristics of Vm were analyzed by short-time fast Fourier transform (FFT) or generalized time-frequency representation with a cone-shaped kernel. The analysis was applied to all pixels to track VF frequencies in time and space. VF consisted of blobs, which are groups of contiguous pixels with a common frequency and an ill-defined shape. At any time t, several VF frequency blobs coexisted in the field of view, and the number of coexisting blobs was on average 5.9±2.1 (n=8 hearts) as they appeared and disappeared discontinuously with time and were not fixed in space. The life span of frequency blobs from birth to either annihilation or breakup to another frequency had a half-life of 0.39±0.13 second (n=4 hearts). The Ca2+ channel blocker nifedipine increased the stability of VF frequencies and reduced the number of frequency blobs progressing to a single frequency. In conclusion, VF consists of dynamically changing frequency blobs, which have a short life span and can be modified by pharmacological interventions, suggesting that VF is maintained by dynamically changing multiple wavelets.

Sex Modulates the Arrhythmogenic Substrate in Prepubertal Rabbit Hearts with Long QT 2

Females have a greater susceptibility to Torsade de Pointes in congenital and drug‐induced long QT syndrome (LQTS) that has been attributed to the modulation of ion channel expression by sex hormones. However, little is known regarding sex differences in pre‐puberty, that is, before the surge of sexual hormones. In patients with congenital LQTS types 1 and 2, male children tend to have a greater occurrence of adverse events, especially in 10–15 year olds, than their female counterpart. To evaluate whether the rabbit model of drug‐acquired LQTS exhibits similar age dependences, hearts of prepubertal rabbits were perfused, mapped optically to record action potentials (APs) and treated with an IKr blocker, E4031 to elicit LQTS2. As expected, AP durations (APD) were significantly longer in female (n = 18) than male hearts (n = 10), at long cycle length. Surprisingly, E4031 (50–250 nM) induced a greater prolongation of APDs in male than in female hearts, and in both genders reversed the direction of repolarization (apex → base to base → apex), enhancing dispersions of repolarization. Furthermore, in male hearts, E4031 (0.5 μM) elicited early afterdepolarizations (EADs) that progressed to polymorphic ventricular tachycardia (PVT) (n = 7/10) and were interrupted by isoproterenol (40 nM) and prevented by propranolol (0.5–2.5 μM). In female hearts, E4031 (0.5 μM) produced marked prolongations of APDs yet few EADs with no progression to PVT (n = 16/18). Thus, sex differences are opposite in prepubertal versus adult rabbits with respect to E4031‐induced APD prolongation, EADs and PVT, underscoring the fact that APD prolongation alone is insufficient to predict arrhythmia susceptibility.

Adaptation of Cardiac Action Potential Durations to Stimulation History with Random Diastolic Intervals

Introduction: The restitution hypothesis proposes that adaptation of cardiac action potential duration (APD) to rate changes is a predictor of ventricular fibrillation (VF). Conventional restitution kinetics plots the APD of a premature beat as a function of the previous diastolic interval (DI), and VF vulnerability is related to how rapidly APD shortens with decreasing DI. However, APD depends not only on the previous DI but also on the history of previous APDs and DIs. For a comprehensive understanding of APD restitution, we developed a random stimulation protocol and curve fitted each APD with the previous DIs and APDs using multiple autoregressive analyses.

Prevention of adverse electrical and mechanical remodeling with biventricular pacing in a rabbit model of myocardial infarction

BACKGROUND Biventricular (BIV) pacing can improve cardiac function in heart failure (HF). OBJECTIVE This study sought to investigate the mechanisms of benefit of BIV pacing using a rabbit model of myocardial infarction (MI). METHODS New Zealand White rabbits were divided into 4 groups (sham-operated [C], MI with no pacing [MI], MI with right ventricular pacing [MI+RV], and MI with BIV pacing [MI+BIV]) and underwent serial electrocardiograms and echocardiograms. At 4 weeks, hearts were excised and tissue was extracted from various areas of the left ventricle (LV). RESULTS Four weeks after coronary ligation, BIV pacing prevented systolic and diastolic dilation of the LV as well as the reduction in its fractional shortening, restored the QRS width and the rate-dependent QT intervals to their baseline values, and prevented the decline of the ether-a-go-go (Erg) protein levels. This prevention of remodeling was not documented in the MI+RV groups. CONCLUSION In this rabbit model of BIV pacing and MI, we show prevention of adverse mechanical and electrical remodeling of the heart. These changes may underlie some of the benefits seen with BIV pacing in HF patients with more severe LV dysfunction.

Dual‐Dye Optical Mapping after Myocardial Infarction: Does the Site of Ventricular Stimulation Alter the Properties of Electrical Propagation?

Introduction: Myocardial infarction (MI) disrupts electrical conduction in affected ventricular areas. We investigated the effect of MI on the regional voltage and calcium (Ca) signals and their propagation properties, with special attention to the effect of the site of ventricular pacing on these properties.

Fiber orientation and cell-cell coupling influence ventricular fibrillation dynamics.

Introduction: The structure of ventricular fibrillation (VF) is influenced by regional differences in action potential durations and perhaps restitution kinetics and fiber anisotropy. The spatial organization of VF was investigated by measuring the cross‐correlation (CC) and mutual information (MI) of membrane potential (Vm) oscillations recorded from multiple sites.