Intracellular Ca2+ release underlies the development of phase 2 in mouse ventricular action potentials.

The ventricular action potential (AP) is characterized by a fast depolarizing phase followed by a repolarization that displays a second upstroke known as phase 2. This phase is generally not present in mouse ventricular myocytes. Thus we performed colocalized electrophysiological and optical recordings of APs in Langendorff-perfused mouse hearts founding a noticeable phase 2. Ryanodine as well as nifedipine reduced phase 2. Our hypothesis is that a depolarizing current activated by Ca(2+) released from the sarcoplasmic reticulum (SR) rather than the "electrogenicity" of the L-type Ca(2+) current is crucial in the generation of mouse ventricular phase 2. When Na(+) was partially replaced by Li(+) in the extracellular perfusate or the organ was cooled down, phase 2 was reduced. These results suggest that the Na(+)/Ca(2+) exchanger functioning in the forward mode is driving the depolarizing current that defines phase 2. Phase 2 appears to be an intrinsic characteristic of single isolated myocytes and not an emergent property of the tissue. As in whole heart experiments, ventricular myocytes impaled with microelectrodes displayed a large phase 2 that significantly increases when temperature was raised from 22 to 37°C. We conclude that mouse ventricular APs display a phase 2; however, changes in Ca(2+) dynamics and thermodynamic parameters also diminish phase 2, mostly by impairing the Na(+)/Ca(2+) exchanger. In summary, these results provide important insights about the role of Ca(2+) release in AP ventricular repolarization under physiological and pathological conditions.

Calsequestrin 2 deletion shortens the refractoriness of Ca²âº release and reduces rate-dependent Ca²âº-alternans in intact mouse hearts.

Calsequestrin (Casq2) is a low affinity Ca(2+)-binding protein located in sarcoplasmic reticulum (SR) of cardiac myocytes. Casq2 acts as a Ca(2+) buffer regulating free Ca(2+) concentration in the SR lumen and plays a significant role in the regulation of Ca(2+) release from this intracellular organelle. In addition, there is experimental evidence supporting the hypothesis that Casq2 also modulates the activity of the cardiac Ca(2+) release channels, ryanodine receptors (RyR2). In this study, Casq2 knockout mice (Casq2-/-) were used as a model to evaluate the effects of the Casq2 on the cytosolic and intra-SR Ca(2+) dynamics, and the electrical activity in the ventricular epicardial layer of intact beating hearts. Casq2-/- mice have accelerated intra-SR Ca(2+) refilling kinetics (76 ± 22 vs. 136.5 ± 15 ms) and a reduced refractoriness of Ca(2+) release (182 ± 32 ms Casq2+/+ and 111 ± 22 ms Casq2-/- ). In addition, mice display reduced Ca(2+) alternans (67% decline in the amplitude of Ca(2+) alternans at 7 Hz, 21oC) and less T-wave alternans at the electrocardiographic level. The results presented in this paper support the idea of Casq2 acting both as a buffer and a direct regulator of the Ca(2+) release process. Finally, we propose that alterations in Ca(2+) release refractoriness shown here could explain the relationship between Casq2 function and an increase in the risk for ventricular arrhythmias.

Transient Ca2+ depletion of the sarcoplasmic reticulum at the onset of reperfusion.

AIMS: Myocardial stunning is a contractile dysfunction that occurs after a brief ischaemic insult. Substantial evidence supports that this dysfunction is triggered by Ca2+ overload during reperfusion. The aim of the present manuscript is to define the origin of this Ca2+ increase in the intact heart. METHODS AND RESULTS: To address this issue, Langendorff-perfused mouse hearts positioned on a pulsed local field fluorescence microscope and loaded with fluorescent dyes Rhod-2, Mag-fluo-4, and Di-8-ANEPPS, to assess cytosolic Ca2+, sarcoplasmic reticulum (SR) Ca2+, and transmembrane action potentials (AP), respectively, in the epicardial layer of the hearts, were submitted to 12 min of global ischaemia followed by reperfusion. Ischaemia increased cytosolic Ca2+ in association with a decrease in intracellular Ca2+ transients and a depression of Ca2+ transient kinetics, i.e. the rise time and decay time constant of Ca2+ transients were significantly prolonged. Reperfusion produced a transient increase in cytosolic Ca2+ (Ca2+ bump), which was temporally associated with a decrease in SR-Ca2+ content, as a mirror-like image. Caffeine pulses (20 mM) confirmed that SR-Ca2+ content was greatly diminished at the onset of reflow. The SR-Ca2+ decrease was associated with a decrease in Ca2+ transient amplitude and a shortening of AP duration mainly due to a decrease in phase 2. CONCLUSION: To the best of our knowledge, this is the first study in which SR-Ca2+ transients are recorded in the intact heart, revealing a previously unknown participation of SR on cytosolic Ca2+ overload upon reperfusion in the intact beating heart. Additionally, the associated shortening of phase 2 of the AP may provide a clue to explain early reperfusion arrhythmias.

Transseptal dispersion of repolarization and its role in the development of Torsade de Pointes arrhythmias.

OBJECTIVE: This study was designed to quantitate transseptal dispersion of repolarization (DR) and delineate its role in arrhythmogenesis using the calcium agonist BayK 8644 to mimic the gain of function of calcium channel current responsible for Timothy syndrome. BACKGROUND: Amplification of transmural dispersion of repolarization (TDR) has been shown to contribute to development of Torsade de Pointes (TdP) arrhythmias under long-QT conditions. METHODS: An arterially perfused septal wedge preparation was developed via cannulation of the septal artery. Action potentials (APs) were recorded using floating microelectrodes together with a transseptal electrocardiogram (ECG). These data were compared to those recorded from arterially perfused canine left ventricular (LV) wedge preparations. RESULTS: Under control conditions, the shortest AP duration measured at 90% repolarization (APD(90)) was observed in right ventricular (RV) endocardium (181.8 +/- 15 ms), APD(90) peaked close to midseptum (278.0 +/- 32 ms), and abbreviated again as LV endocardium was approached (207.3 +/- 9 ms). Transseptal DR averaged 106 +/- 24 ms and T(peak)-T(end) 84 +/- 7 ms (n = 6). TDR and T(peak)-T(end) recorded from LV wedge were 36 +/- 9 ms and 34 +/- 19 ms, respectively (n = 30). BayK 8644 increased transseptal DR to 123.2 +/- 35 ms (n = 5) and induced early and delayed afterdepolarizations (3/5), rate-dependent ST-T-wave alternans (5/5), and TdP arrhythmias (3/5). CONCLUSIONS: Our data indicate that dispersion of repolarization across the interventricular septum is twice that of the LV free wall, predisposing to development of TdP under long-QT conditions. Our findings suggest that the coronary-perfused ventricular septal preparation may be a sensitive model in which to assess the potential arrhythmogenic effects of drugs and pathophysiological conditions.

Atrial-selective effects of chronic amiodarone in the management of atrial fibrillation.

BACKGROUND: Although amiodarone is one of the most effective pharmacologic agents used in clinical management of atrial fibrillation (AF), little is known about its differential effects in atrial and ventricular myocardium. OBJECTIVES: This study sought to compare the electrophysiological effects of chronic amiodarone in atria and ventricles. METHODS: We compared the electrophysiological characteristics of coronary-perfused atrial and ventricular wedge preparations isolated from untreated and chronic amiodarone-treated dogs (amiodarone, 40 mg/kg/day for 6 weeks, n = 12). RESULTS: Chronic amiodarone prolonged action potential duration (APD(90)) predominantly in atria compared to ventricles and prolonged the effective refractory period (ERP) more than APD(90) in both ventricular and atrial preparations (particularly in the latter) due to the development of postrepolarization refractoriness. Amiodarone reduced dispersion of APD(90) in both atria and ventricles. Although the maximum rate of increase of the action potential upstroke (V(max)) was significantly lower in both atria and ventricles of amiodarone-treated hearts versus untreated controls, the reduction of V(max) was much more pronounced in atria. Amiodarone prolonged P-wave duration more significantly than QRS duration, reflecting greater slowing of conduction in atria versus ventricles. These atrioventricular distinctions were significantly accentuated at faster activation rates. Persistent acetylcholine-mediated AF could be induced in only 1 of 6 atria from amiodarone-treated versus 10 of 10 untreated dogs. CONCLUSION: Our results indicate that under the conditions studied, chronic amiodarone has potent atrial-predominant effects to depress sodium channel-mediated parameters and that this action of the drug is greatly potentiated by its ability to prolong APD predominantly in the atria, thus contributing to its effectiveness to suppress AF.

Hemiblocks revisited.

The trifascicular nature of the intraventricular conduction system and the concept of trifascicular block and hemiblock were described by Rosenbaum and his coworkers in 1968. Since then, anatomic, pathological, electrophysiological, and clinical studies have confirmed the original description and scarce advances have been developed on the subject. In the present study, we attempt to review and redefine reliable criteria for the electrocardiographic and vectorcardiographic diagnosis of left anterior and posterior hemiblock. One of the most important problems related to hemiblocks is that they may simulate or conceal the electrocardiographic signs of myocardial infarction or myocardial ischemia and may mask or simulate ventricular hypertrophy. Illustrative examples of these associations are shown to help the interpretation of electrocardiograms. The incidence and prevalence of the hemiblocks is presented based on studies performed in hospital patients and general populations. One of the most common causes of hemiblocks is coronary artery disease, and there is a particularly frequent association between anteroseptal myocardial infarction and left anterior hemiblock. The second most important cause is arterial hypertension, followed by cardiomyopathies and Lev and Lenègre diseases. The hemiblocks may also occur in aortic heart disease and congenital cardiopathies. Left anterior hemiblock is more common in men and increases in frequency with advancing age. Evidence is presented regarding the relationship of spontaneous closure of ventricular septal defects, which may explain the finding of this and other conduction defects in young populations. Isolated left anterior hemiblock is a relatively frequent finding in subjects devoid of evidence of structural heart disease. Conversely, isolated left posterior hemiblock is a very rare finding; its prognostic significance is unknown and is commonly associated with right bundle-branch block. The most remarkable feature of this association is that the prognosis is much more serious with a great propensity to develop complete atrioventricular block and Adams-Stoke seizures.

In vitro effects of acute amiodarone and dronedarone on epicardial, endocardial, and M cells of the canine ventricle.

Amiodarone (AM) is an antiarrhythmic agent widely used in the treatment of ventricular and supraventricular arrhythmias. Dronedarone (DR) is a new compound with a pharmacological profile similar to that of AM, but iodine free. We previously demonstrated that chronic AM treatment reduces transmural dispersion of repolarization (TDR) in the canine heart. We used standard microelectrode technique to evaluate the effects of acute AM (100 microM) and DR (30 microM) on epicardial (EPI), endocardial (ENDO), and M region tissues obtained from the left ventricular wall of the canine heart. Amiodarone (100 microM, 120 min of exposure) produced little change in the action potential duration of ENDO and EPI tissues, but it shortened the action potential of M cells, especially at slow rates, leading to a decrease in TDR. Similar results were observed with DR. Acute AM (100 microM) and DR (30 microM) eliminated d-sotalol-induced early afterdepolarizations (EADs) and triggered activity in 3 of 3 and 2 of 6 M cell preparations, respectively. The reduction of TDR and the elimination of EAD-induced triggered activity differentiates AM and DR from other class III agents. These effects may explain the efficacy and low arrhythmogenicity of acute AM and suggest a potential safe use of DR as an antiarrhythmic agent.

El chequeo médico-deportivo en individuos asintomáticos: anomalías cardiológicas / Medical check-up in asymptomatic individuals: cardiac abnormalities

Objetivo: El presente trabajo se llevó a cabo con el propósito de evaluar la existencia de anomalías cardiológicas en pacientes asintomáticos que consultaron para la realización de un chequeo médico-deportivo, previo al ingreso en un programa controlado de actividad física. Material y métodos: Se realizó examen físico, electrocardiograma de reposo (ECG) y prueba ergométrica graduada (PEG) en 2.140 individuos asintomáticos (912 mujeres, media 41 años y 1.228 hombres, media 43 años), previo al ingreso en un programa controlado de actividad física. Según los antecentes y el examen físico, se realizó ecocardiograma bidimensional con Doppler color, registro Holter de 24 horas o estudio de perfusión miocárdica. Resultados: Se detectaron anomalías cardiológicas en 116 pacientes (5,4 por ciento): 3 con miocardiopatía hipertrófica (MCH), 4 con prolapso de válvula mitral (PVM), 1 con valvulopatía aórtica, 34 con anomalías en el ECG basal (29 con arritmia ventricular: extrasístoles ventriculares aisladas en 10 casos y frecuentes en 19, 1 con fibroaleteo auricular, 1 con preexcitación ventricular y 3 con síndrome de Brugada) y 74 con prueba ergométrica graduada (PEG) anormal por parámetros electrocardiográficos [53 con arritmia ventricular, entre los cuales se detectaron: taquicardia ventricular (TV) en 5, taquicardia paroxística supraventricular (TPSV) en 4, fibrilación auricular (FA) en 2, bloqueos AV de segundo grado en 3 y alteraciones isquémicas del segmento ST-T en 12, confirmada por estudio de perfusión miocárdica con 201 Tl SPECT en reposo y con esfuerzo, que demostró defecto reversible]. Cabe aclarar que las anormalidades en el ECG basal corresponden a diferentes pacientes que las observadas en el ECG de esfuerzo. Conclusión: La detección de anomalías cardiológicas en más del 5 por ciento de los pacientes asintomáicos indica la importancia de la realización de un chequeo médico-cardiológico previo al ingreso en un programa controlado de actividad física.

El chequeo médico-deportivo en individuos asintomáticos: anomalías cardiológicas / Medical check-up in asymptomatic individuals: cardiac abnormalities

Objetivo: El presente trabajo se llevó a cabo con el propósito de evaluar la existencia de anomalías cardiológicas en pacientes asintomáticos que consultaron para la realización de un chequeo médico-deportivo, previo al ingreso en un programa controlado de actividad física. Material y métodos: Se realizó examen físico, electrocardiograma de reposo (ECG) y prueba ergométrica graduada (PEG) en 2.140 individuos asintomáticos (912 mujeres, media 41 años y 1.228 hombres, media 43 años), previo al ingreso en un programa controlado de actividad física. Según los antecentes y el examen físico, se realizó ecocardiograma bidimensional con Doppler color, registro Holter de 24 horas o estudio de perfusión miocárdica. Resultados: Se detectaron anomalías cardiológicas en 116 pacientes (5,4 por ciento): 3 con miocardiopatía hipertrófica (MCH), 4 con prolapso de válvula mitral (PVM), 1 con valvulopatía aórtica, 34 con anomalías en el ECG basal (29 con arritmia ventricular: extrasístoles ventriculares aisladas en 10 casos y frecuentes en 19, 1 con fibroaleteo auricular, 1 con preexcitación ventricular y 3 con síndrome de Brugada) y 74 con prueba ergométrica graduada (PEG) anormal por parámetros electrocardiográficos [53 con arritmia ventricular, entre los cuales se detectaron: taquicardia ventricular (TV) en 5, taquicardia paroxística supraventricular (TPSV) en 4, fibrilación auricular (FA) en 2, bloqueos AV de segundo grado en 3 y alteraciones isquémicas del segmento ST-T en 12, confirmada por estudio de perfusión miocárdica con 201 Tl SPECT en reposo y con esfuerzo, que demostró defecto reversible]. Cabe aclarar que las anormalidades en el ECG basal corresponden a diferentes pacientes que las observadas en el ECG de esfuerzo. Conclusión: La detección de anomalías cardiológicas en más del 5 por ciento de los pacientes asintomáicos indica la importancia de la realización de un chequeo médico-cardiológico previo al ingreso en un programa controlado de actividad física. (AU)