Lance D. Wilson

Redox modification of ryanodine receptors underlies calcium alternans in a canine model of sudden cardiac death

AIMS Although cardiac alternans is a known predictor of lethal arrhythmias, its underlying causes remain largely undefined in disease settings. The potential role of, and mechanisms responsible for, beat-to-beat alternations in the amplitude of systolic Ca(2+) transients (Ca(2+) alternans) was investigated in a canine post-myocardial infarction (MI) model of sudden cardiac death (SCD). METHODS AND RESULTS Post-MI dogs had preserved left ventricular (LV) function and susceptibility to ventricular fibrillation (VF) during exercise. LV wedge preparations from VF dogs were more susceptible to action potential (AP) alternans and the frequency-dependence of Ca(2+) alternans was shifted towards slower rates in myocytes isolated from VF dogs relative to controls. In both groups of cells, cytosolic Ca(2+) transients ([Ca(2+)](c)) alternated in phase with changes in diastolic Ca(2+) in sarcoplasmic reticulum ([Ca(2+)](SR)), but the dependence of [Ca(2+)](c) amplitude on [Ca(2+)](SR) was steeper in VF cells. Abnormal ryanodine receptor (RyR) function in VF cells was indicated by increased fractional Ca(2+) release for a given amplitude of Ca(2+) current and elevated diastolic RyR-mediated SR Ca(2+) leak. SR Ca(2+) uptake activity did not differ between VF and control cells. VF myocytes had an increased rate of reactive oxygen species production and increased RyR oxidation. Treatment of VF myocytes with reducing agents normalized parameters of Ca(2+) handling and shifted the threshold of Ca(2+) alternans to higher frequencies. CONCLUSION Redox modulation of RyRs promotes generation of Ca(2+) alternans by enhancing the steepness of the Ca(2+) release-load relationship and thereby providing a substrate for post-MI arrhythmias.

Mechanoelectrical Feedback as Novel Mechanism of Cardiac Electrical Remodeling

Background— Altered electrical activation of the heart by pacing or disease induces profound ventricular electrical remodeling (VER), manifested electrocardiographically as T-wave memory and ultimately as deleterious mechanical remodeling from heterogeneous strain. Although T-wave memory is associated with altered expression of sarcolemmal ion channels, the biophysical mechanisms responsible for triggering remodeling of cardiac ion channels are unknown. Methods and Results— To test the hypothesis that mechanoelectrical feedback triggered by regional strain is a mechanism for VER, dogs (n=6) underwent 4 weeks of ventricular pacing to induce VER. Multisegment transmural optical action potential imaging of left ventricular wedges revealed profound and selective prolongation of action potential duration in late-activated (288±29 ms) compared with early-activated (250±9 ms) myocardial segments (P<0.05), providing the first experimental evidence that amplification of repolarization gradients between segments of left ventricle is the electrophysiological basis for T-wave memory. In vivo tagged magnetic resonance imaging revealed a 2-fold and preferential increase in circumferential strain in late-activated segments of myocardium, which exactly coincided with segments undergoing VER. VER could not be attributed to structural remodeling because it occurred without any histological evidence of cellular hypertrophy. Conclusions— The mechanism responsible for triggering remodeling of ion channel function in VER was locally enhanced circumferential strain. These data suggest a novel mechanoelectrical feedback mechanism for inducing physiological and potentially deleterious electrical heterogeneities in the heart.

Heart failure enhances susceptibility to arrhythmogenic cardiac alternans

BACKGROUND Although heart failure (HF) is closely associated with susceptibility to sudden cardiac death (SCD), the mechanisms linking contractile dysfunction to cardiac electrical instability are poorly understood. Cardiac alternans has also been closely associated with SCD, and has been linked to a mechanism for amplifying electrical heterogeneities in the heart. However, previous studies have focused on alternans in normal rather than failing myocardium. OBJECTIVE This study sought to investigate the hypothesis that HF enhances susceptibility to arrhythmogenic cardiac alternans. METHODS High-resolution transmural optical mapping was performed in canine wedge preparations from normal (n = 8) and HF (n = 8) hearts produced by rapid ventricular pacing. RESULTS HF significantly (P < .004) lowered the heart rate (HR) threshold for action potential duration alternans (APD-ALT) from 236 +/- 25 beats/min to 185 +/- 25 beats/min. In dual optical mapping of action potentials and intracellular Ca experiments (n = 16), HF lowered the HR threshold for Ca-ALT (beat-to-beat alternations of cellular Ca cycling) from 238 +/- 35 to 177 +/- 26 beats/min (P < .005). Importantly: (1) Ca-ALT always either developed at slower HR or simultaneously with APD-ALT in the same cells, and (2) the magnitude of Ca-ALT and APD-ALT were closely correlated (P < .05). HF similarly lowered the HR threshold for Ca-ALT in isolated myocytes under nonalternating action potential clamp, indicating that HF enhances susceptibility to cellular alternans independent of HF-associated changes in repolarization. Importantly, HF significantly (P < .02) lowered the HR threshold for spatially discordant arrhythmogenic alternans (different regions of cells alternating in opposite phase, DIS-ALT). Ventricular fibrillation (VF) was induced in 88% of HF preparations, but only 12% of normal preparations (P < .003) and was uniformly preceded by development of DIS-ALT. CONCLUSION Heart failure increases the susceptibility to arrhythmogenic cardiac alternans, which arises from HF-induced impairment in calcium cycling.

Spontaneous calcium release in tissue from the failing canine heart

Abnormalities in calcium handling have been implicated as a significant source of electrical instability in heart failure (HF). While these abnormalities have been investigated extensively in isolated myocytes, how they manifest at the tissue level and trigger arrhythmias is not clear. We hypothesize that in HF, triggered activity (TA) is due to spontaneous calcium release from the sarcoplasmic reticulum that occurs in an aggregate of myocardial cells (an SRC) and that peak SCR amplitude is what determines whether TA will occur. Calcium and voltage optical mapping was performed in ventricular wedge preparations from canines with and without tachycardia-induced HF. In HF, steady-state calcium transients have reduced amplitude [135 vs. 170 ratiometric units (RU), P < 0.05] and increased duration (252 vs. 229 s, P < 0.05) compared with those of normal. Under control conditions and during beta-adrenergic stimulation, TA was more frequent in HF (53% and 93%, respectively) compared with normal (0% and 55%, respectively, P < 0.025). The mechanism of arrhythmias was SCRs, leading to delayed afterdepolarization-mediated triggered beats. Interestingly, the rate of SCR rise was greater for events that triggered a beat (0.41 RU/ms) compared with those that did not (0.18 RU/ms, P < 0.001). In contrast, there was no difference in SCR amplitude between the two groups. In conclusion, TA in HF tissue is associated with abnormal calcium regulation and mediated by the spontaneous release of calcium from the sarcoplasmic reticulum in aggregates of myocardial cells (i.e., an SCR), but importantly, it is the rate of SCR rise rather than amplitude that was associated with TA.

Benzyl Alcohol as an Alternative Local Anesthetic

STUDY OBJECTIVES Benzyl alcohol has been used as a local anesthetic for brief superficial skin procedures; however, its efficacy for long-term cutaneous anesthesia has not been established. We sought to compare the cutaneous anesthetic effects of benzyl alcohol with epinephrine with the effects of lidocaine with epinephrine and with placebo. METHODS This study was a prospective, randomized, double-blind, placebo-controlled clinical trial of 30 healthy paid adult volunteers. Subjects received 1-mL intradermal injections of benzyl alcohol.9% with 1:100,000 epinephrine, lidocaine 1% with 1:100,000 epinephrine, and physiologic saline solution without benzyl alcohol as placebo in a randomized, double-blind fashion. Pain on injection and degree of anesthesia at 5, 15, 30, and 45 minutes was assessed with a 10-cm graded visual analog scale (VAS). Statistical significance was determined by repeated measures ANOVA between groups with a Neuman-Keuls test for post hoc comparison of means and Students t test for paired means. RESULTS Benzyl alcohol was 48% less painful on injection than placebo (P <.008) and 42% less painful on injection than lidocaine with epinephrine (P <.05). Lidocaine with epinephrine and placebo were equally painful on injection. After the 5-minute measurement, benzyl alcohol provided significantly better anesthesia than placebo during the remaining observation period (VAS score 48%, 49%, and 51% decreased from baseline at 15, 30, and 45 minutes, respectively, all P <.02 versus placebo). However, benzyl alcohol provided less effective anesthesia than lidocaine with epinephrine (VAS score 72%, 76%, 84%, and 88% decreased from baseline at 5, 15, 30, and 45 minutes, respectively, all P <.001 versus placebo) throughout most of the observation period. CONCLUSION Benzyl alcohol with epinephrine provides prolonged cutaneous anesthesia, although it is not as effective as lidocaine with epinephrine. However, benzyl alcohol is significantly less painful on injection than lidocaine with epinephrine, and it may offer an alternative for local anesthesia.

Enhanced Dispersion of Repolarization Explains Increased Arrhythmogenesis in Severe Versus Therapeutic Hypothermia

Background—Hypothermia is proarrhythmic, and, as the use of therapeutic hypothermia (TH) increases, it is critically important to understand the electrophysiological effects of hypothermia on cardiac myocytes and arrhythmia substrates. We tested the hypothesis that hypothermia-enhanced transmural dispersion of repolarization (DOR) is a mechanism of arrhythmogenesis in hypothermia. In addition, we investigated whether the degree of hypothermia, the rate of temperature change, and cooling versus rewarming would alter hypothermia-induced arrhythmia substrates. Methods and Results—Optical action potentials were recorded from cells spanning the transmural wall of canine left ventricular wedge preparations at baseline (36°C), during cooling and during rewarming. Electrophysiological parameters were examined while varying the depth of hypothermia. On cooling to 26°C, DOR increased from 26±4 ms to 93±18 ms (P=0.021); conduction velocity decreased from 35±5 cm/s to 22±5 cm/s (P=0.010). On rewarming to 36°C, DOR remained prolonged, whereas conduction velocity returned to baseline. Conduction block and reentry was observed in all severe hypothermia preparations. Ventricular fibrillation/ventricular tachycardia was seen more during rewarming (4/5) versus cooling (2/6). In TH (n=7), cooling to 32°C mildly increased DOR (31±6 to 50±9, P=0.012), with return to baseline on rewarming and was associated with decreased arrhythmia susceptibility. Increased rate of cooling did not further enhance DOR or arrhythmogenesis. Conclusions—Hypothermia amplifies DOR and is a mechanism for arrhythmogenesis. DOR is directly dependent on the depth of cooling and rewarming. This provides insight into the clinical observation of a low incidence of arrhythmias in TH and has implications for protocols for the clinical application of TH.

Cocaethylene Causes Dose-dependent Reductions in Cardiac Function in Anesthetized Dogs

Twelve million Americans abuse both cocaine and ethanol each year because this drug combination produces a pronounced and prolonged euphoria. However, these substances in combination are substantially more toxic than either drug alone. This toxicity may be due to cocaethylene, which has been detected in the serum of patients who have used cocaine and ethanol and two require emergency treatment. Cocaethylene is a pharmacologically active cocaine metabolite formed in the liver only in the presence of ethanol. To investigate the cardiovascular effects of cocaethylene, we randomized 15 mongrel dogs to receive 11.25 mg/kg (n = 4), 7.5 mg/kg (n = 6), or 3.75 mg/kg (n = 5) of cocaethylene as an intravenous (i.v.) bolus. These doses were chosen to achieve serum concentrations of cocaethylene consistent with those observed in patients with cocaine and ethanol toxicity. The ECG and the femoral arterial, left ventricular (LV), and pulmonary artery pressure were measured continuously, and cardiac output (CO) and serum levels of cocaethylene were monitored at specific intervals before and after drug administration. The maximal rate of increase and decrease in LV pressure (LVP), i.e., (dP/dt)max and (dP/dt)min, were determined as our indexes of ventricular contractility and relaxation. Cocaethylene concentrations peaked 2-4 min after each bolus and then decreased in a curvilinear manner. Cocaethylenes half-life (t1/2) was 150 +/- 15.1 min (mean +/- SEM). The greatest hemodynamic changes occurred at the peak cocaethylene serum concentrations in each group. In comparison with control measurements, cocaethylene in concentrations of 11.25 and 7.5 mg/kg decreased (dP/dt)max by 81 and 43% and decreased (dP/dt)min by 80 and 36%, respectively. In these two groups, cocaethylene decreased stroke volume (SV) by 29 and 33% and reduced mean arterial pressure (MAP) by 65 and 30%, respectively. Cocaethylene increased pulmonary artery wedge pressure (PAWP) by 70 and 67% in the 11.25- and 7.5-mg/kg groups. These hemodynamic changes persisted for 60 min after the bolus administration. In each of the three groups, cocaethylene increased the QRS interval duration by 60, 32, and 44% and the QTc interval by 38, 21, and 17%. These ECG changes persisted for 120 min. These experiments suggest cocaethylene depresses the myocardium. Cocaethylene may be a major contributor to the delayed but substantial cardiotoxicity that occurs in individuals who use both cocaine and ethanol.

Cellular alternans : A mechanism linking calcium cycling proteins to cardiac arrhythmogenesis

Abstract:  Essentially all previous research on alternans has been restricted to normal myocardium, whereas sudden cardiac death (SCD) occurs most commonly in patients with ventricular dysfunction (i.e., heart failure), which is associated with marked disruption of proteins responsible for normal calcium cycling in myocytes. Several lines of evidence from studies in normal hearts suggest a link between impaired calcium cycling which characterizes ventricular mechanical dysfunction and impaired calcium cycling that is responsible for alternans. In normal myocardium, cells which exhibit the slowest calcium cycling, and not the slowest repolarization, are most susceptible to alternans. Decreased expression of key calcium cycling proteins is observed in alternans‐prone cells. Sarcoplasmic reticulum ATPase (SERCA2a) expression is decreased, suggesting a mechanism for the slower sarcoplasmic reticulum (SR) calcium reuptake observed in alternans‐prone cells. In addition, diminished ryanodine receptor (RyR) function leading to abnormal calcium release from the SR is also linked to cellular alternans. Although impaired contractile function clearly predisposes to SCD, the mechanisms linking mechanical to electrophysiological dysfunction in the heart are unclear. We propose that cellular calcium alternans may be an important mechanism linking mechanical dysfunction to cardiac arrhythmogenesis.

Point: M cells are present in the ventricular myocardium

Sicouri and Antzelevitch 1 described a subpopulation of cells in the deep subepicardium that displayed distinct electrophysiologic properties. These midmyocardial cells (i.e., M cells) exhibited action potential duration (APD) that was substantially longer than the APD of epicardial or endocardial myocytes (Figure 1), 1 which was attributed to their weaker IKs current but stronger late sodium and sodium/ calcium exchanger currents. The M cell has been proposed to underlie the electrophysiologic basis for transmural dispersion of repolarization (DOR) 1,2 and the T wave of the electrocardiogram. 3 This represented an apparent departure from classic teaching, which stated that APD shortens monotonically in the direction of propagation (i.e., endocardium to epicardium), resulting in a repolarization vector in the opposite direction to depolarization, hence accounting for the upright (concor

Transmural dispersion of repolarization as a preclinical marker of drug-induced proarrhythmia.

Abstract: Torsade de Pointes (TdP) proarrhythmia is a major complication of therapeutic drugs that block the delayed rectifier current. QT interval prolongation, the principal marker used to screen drugs for proarrhythmia, is both insensitive and nonspecific. Consequently, better screening methods are needed. Drug-induced transmural dispersion of repolarization (TDR) is mechanistically linked to TdP. Therefore, we hypothesized that drug-induced enhancement of TDR is more predictive of proarrhythmia than QT interval. High-resolution transmural optical action potential mapping was performed in canine wedge preparations (n = 19) at baseline and after perfusion with 4 different QT prolonging drugs at clinically relevant concentrations. Two proarrhythmic drugs in patients (bepridil and E4031) were compared with 2 nonproarrhythmic drugs (risperidone and verapamil). Both groups prolonged the QT (all P < 0.02), least with the proarrhythmic drug bepridil, reaffirming that QT is a poor predictor of TdP. In contrast, TDR was enhanced only by proarrhythmic drugs (P < 0.03). Increased TDR was due to a preferential prolongation of midmyocardial cell, relative to epicardial cell, APD, whereas nonproarrhythmic drugs similarly prolonged both cell types. In contrast to QT prolongation, augmentation of TDR was induced by proarrhythmic but not nonproarrhythmic drugs, suggesting TDR is a superior preclinical marker of proarrhythmic risk during drug development.