Mordechai Manoach

Influence of hemorrhage on the QRS complex of the electrocardiogram

Abstract The removal of blood from anesthetized cats or dogs invariably produces an immediate reduction of the amplitude of the QRS complex, which can be restored to normal by reinjecting the blood. In the present study it was demonstrated that the QRS changes are directly related to the changes of the heart volume, viz., reduction of QRS amplitude after bleeding or after clamping of the inferior vena cava or increase after overfilling of the heart. Anoxia, changes of lung volume, or rotation of the heart had not contributed to the registered QRS changes.

Acute, nongenomic effect of thyroid hormones in preventing calcium overload in newborn rat cardiocytes

In this study, we examined the acute effects of thyroid hormones (TH) T3 and T4, leading to improvement of myocardial function through activation of Ca2+ extrusion mechanisms and, consequently, prevention of intracellular calcium overload. Extracellular calcium elevation from 1.8 to 3.8 mM caused immediate increase in intracellular calcium level ([Ca2+]i) in newborn cardiomyocyte cultures. Administration of 10 or 100 nM T3 or T4 rapidly (within 10 sec) decreased [Ca2+]i to its control level. Similar results were obtained when [Ca2+]i was elevated by decreasing extracellular Na+ concentration, causing backward influx of Ca2+ through Na+/Ca2+ exchanger, or by administration of caffeine, releasing Ca2+ from the sarcoplasmic reticulum (SR). Under these conditions, T3 or T4 decreased [Ca2+]i. T3 and T4 also exhibited protective effects during ischemia. T3 or T4 presence during hypoxia for 120 min in culture medium restricted the increase of [Ca2+]i and prevented the pathological effects of its overload. An inhibitor of SR Ca2+‐ATPase (SERCA2a), thapsigargin, increases [Ca2+]i and in its presence neither T3 nor T4 had any effect on the [Ca2+]i level. The reduction of [Ca2+]i level by T3 and T4 was also blocked in the presence of H‐89 (a PKA inhibitor), and by calmodulin inhibitors. The effect of TH on the reduction of [Ca2+]i was prevented by propranolol, indicating that the hormones exert their effect through interaction with adrenergic receptors. These results support our hypothesis that TH prevent calcium overload in newborn rat cardiomyocytes, most likely by a direct, acute, and nongenomic effect on Ca2+ transport into the SR. J. Cell. Physiol. 207: 220–231, 2006.

Hypertension-related intermyocyte junction remodelling is associated with a higher incidence of low-K(+)-induced lethal arrhythmias in isolated rat heart.

The aim of this study was to characterise the arrhythmogenic mechanisms involved in hypokalaemia‐induced sustained ventricular fibrillation (SVF), in hypertensive rats. The hearts from rats with hypertension induced by the nitric oxide synthase inhibitor L‐NAME, and age‐matched normotensive controls, were perfused in Langendorff mode with oxygenated Krebs‐Henseleit solution followed by a K+‐deficient solution. In additional experiments, free intracellular Ca2+ concentration ([Ca2+]i) was measured using fura‐2 in conjunction with an epicardial optical probe. The epicardial electrocardiogram was continuously monitored during all experiments. The gap junction protein connexin‐43 and the ultrastructure of the cardiomyocytes were examined, and selected enzyme activities were measured in situ. There was a higher incidence of low‐K+‐induced SVF in the hearts of hypertensive compared to normotensive rats (83% vs. 33%, P < 0.05). Perfusion with a low‐K+‐containing solution lead to elevation of diastolic [Ca2+]i that was accompanied by premature beats, bigeminy, ventricular tachycardia and transient ventricular fibrillation. These events occurred earlier with increased incidence and duration in the hearts of hypertensive rats (arrhythmia scores: hypertensive, 4.9 ± 0.7; normotensive, 3.1 ± 0.1; P < 0.05), which exhibited apparent remodelling accompanied by a significant decrease in the density of connexin‐43‐positive gap junctions. Moreover, low‐K+‐related myocardial changes, including local impairment of intermyocyte junctions, ultrastructural alterations due to Ca2+ overload and intercellular uncoupling, and decreased enzyme activities were more pronounced and more dispersed in hypertensive than normotensive rats. In conclusion, nitric oxide‐deficient hypertension is associated with decreased myocardial coupling at gap junctions. The further localised deterioration of junctional coupling, due to low‐K+‐induced Ca2+ disturbances, as well as spatial heterogeneity of myocardial alterations including interstitial fibrosis, probably provide the mechanisms for re‐entry and sustaining ventricular fibrillation.

Gap junction remodelling is involved in the susceptibility of diabetic rats to hypokalemia-induced ventricular fibrillation.

The objective of the present study was to examine the susceptibility of diabetic rats with cardiomyopathy to hypokalemia-induced ventricular fibrillation and to localize gap junction protein connexin-43 as well as subcellular changes that may be involved in the development of severe arrhythmia. Our results showed a significantly higher incidence of sustained ventricular fibrillation in diabetic hearts as compared with control hearts, 80% vs 20%, respectively. Diabetic cardiomyopathy itself was accompanied by a distinct decrease in connexin-43-immunopositive gap junctions. Moreover, interstitial fibrosis and subcellular alterations to various degrees were observed in diabetic hearts, and a further deterioration of the ultrastructure and impairment of intercellular junctions, and a stronger local decrease in connexin-43 levels due to hypokalemia were found. These changes were heterogeneously distributed throughout the myocardium and occurred earlier and were more pronounced in diabetic hearts than control hearts. In conclusion, our results indicate that diabetic cardiomyopathy is associated with down-regulation of gap junction proteins and may account for the higher vulnerability of diabetic rats to ventricular fibrillation in combination with impairment of intercellular communication due to hypokalemia.

Enhanced connexin-43 and alpha-sarcomeric actin expression in cultured heart myocytes exposed to triiodo-L-thyronine.

This study examined whether triiodo-l-thyronine (T3) affects the expression of the major intercellular channel protein, connexin-43, and contractile protein α-sarcomeric actin. Cultured cardiomyocytes from newborn rats were treated on day three in culture with 10 or 100 nM T3 and examined 48 and 72 h thereafter. Treated and untreated cells were examined by immunofluorescence and electron microscopy. Expression levels of Cx43 and sarcomeric α-actin were monitored by Western blot analysis. Immunofluorescence labeling showed cell membrane location of Cx43 in punctuate gap junctions, whereby fluorescence signal area was significantly higher in cultured cardiomyocytes exposed to T3. This correlated with electron microscopical findings showing increased numbers and size of gap junction profiles, as well as with a significant dose-dependent increase of Cx43 expression detected by Western blot. Immunofluorescence of sarcomeric α-actin was enhanced and its expression increased dose- and time-dependently in T3-treated cultured heart myocytes. However, exposure to the higher dosage (100 nM) of T3 caused mild disintegration of sarcomeric α-actin in some myocytes, suggesting an over-dosage. The results indicate that T3 up-regulates Cx43 and accelerates gap junction formation in cultured neonatal cardiomyocytes. They suggest that thyroid status cannot only modulate the mechanical function of cardiomyocytes but also cell-to-cell communication essential for myocardial electrical and metabolic synchronizations.

The protective effect of d-sotalol against hypoxia-induced myocardial uncoupling

SummaryThe effects ofd-sotalol on intercellular electrical coupling and ultrastructure under hypoxic conditions were investigated in myocardial samples from eight young (1–2 months) and four older (10–12 months) guinea pigs. A right ventricular muscle strip was kept simultaneously in two divided chambers and superfused with normoxic and/or hypoxic (97% N2+ 3% CO2) Krebs solution. Hypoxia caused shortening of action potential duration (APD) and electrical cellto-cell uncoupling. If the uncoupling appeared after short-term hypoxia (less than 30 min), administration of 3.10−7M ofd-sotalol to the hypoxic perfusate led to a recovery of electrical coupling. Transmission electron microscopy revealed moderate reversible ultrastructural alterations of the cardiomyocytes. No apparent changes in intercellular junctions were observed. The recoupling effect of sotalol decreased with the time of hypoxia as the ultrastructural damage progressed. After prolonged hypoxia (more than 30min), cardiomyocytes were markedly injured, intercellular junctions were severely affected, and gap junctions occurred less frequently. In these cases, administration ofd-sotalol caused only transient recoupling. After 1h of hypoxia, no recoupling was observed. Pretreatment withd-sotalol prevented hypoxia-induced electrical uncoupling and markedly attenuated ultrastructural damage, although shortening of APD still persisted. Our results indicate that the cardioprotective effect ofd-sotalol on electrical intercellular coupling is closely associated with sotalol-induced prevention of the ultrastructural damage. Considering previous results, we suggest that this protective effect ofd-sotalol may be related to its ability to increase intracellular cyclic adenosine monophosphate and, thereby, to decrease cytosolic free Ca. These effects can explain the antiarrhythmic and defibrillating properties ofd-sotalol.

The structural-functional basis of spontaneous ventricular defibrillation.

Ventricular fibrillation is, worldwide, one of the main causes of sudden death. In humans, ventricular fibrillation is generally sustained and, in order to save life, it requires electrical defibrillation. Studies on experimental ventricular fibrillation were started in 1850 by Hoffa and Ludwig [l]. The mechanism presumed to be involved in sustained fibrillation was then revised by MacWilliams in 1918 [2]. He suggested that fibrillation started when the relation between the refractory period and conduction time was altered, either by shortening of the refractory period, by lengthening of the conduction time, or a combination of both.

How Can We Facilitate Spontaneous Termination of Ventricular Fibrillation and Prevent Sudden Cardiac Death? A Working Hypothesis

Spontaneous Ventricular Defibrillation. Ventricular fibrillation (VF) is one of the most life‐threatening arrhythmias encountered in daily clinical practice. Its occurrence cannot be completely prevented by currently used antiarrhythmic drugs, and, in most instances, VF is sustained and leads lo the patients death unless a successful DC defibrillation is applied. However, spontaneous reversion of VF to sinus rhythm has been observed in various animals and occasionally even in man. Hence, facilitation of self ventricular defibrillation must be explored as an alternative therapeutic approach. In experimental studies using several mammalian species, we have found that self ventricular defibrillation requires a good intercellular coupling and well synchronized electrical activity in the ventricles, which, in untreated animals, depend on their myocardial catecholamine content. It can then be hypothesized that any agent that elevates the catecholamine level during VF would facilitate spontaneous ventricular defibrillation, and drugs inhibiting extraneuronal catecholamine reuptake have indeed been shown to possess this ability. It is suggested that their effects are mediated by an increase in the intracellular cAMP level, and any compounds sharing this property could well prove efficacious in making VF transient and in reducing sudden cardiac death.

A new group of defibrillatory drugs in the classification of antiarrhythmic agents

Ventricular antiarrhythmic therapy is aimed traditionally at preventing arrhythmias and ventricular fibrillation. Recently, a new approach has been introduced, where drug therapy facilitates the ability of the heart for spontaneous defibrillation. The chemical features and the electrophysiologic properties are discussed below. The introduction of this new group of defibrillating antiarrhythmic drugs implies the extension of the common classification of antiarrhythmic drugs.

Is cyclic AMP involved in the defibrillating effect of sotalol

Ventricular fibrillation induced in animals pretreated with sotalol, a class III antiarrhythmic agent, would spontaneously terminate and revert into a sinus rhythm. This phenomenon has been attributed to the class III action of this drug, i.e., prolongation of myocardial action potential duration and effective refractory period. Since various observations suggested that these alone cannot explain the defibrillating phenomenon, we hypothesised that sotalol affected ventricular intercellular synchronization by increasing intercellular coupling. Our recent experimental studies have shown that sotalol antagonized the cellular decoupling to guinea pig ventricular muscle strip caused by perfusion with either a hypoxic normal Tyrodes solution or an oxygenated high Ca2+ Tyrodes solution. We assumed that the most likely mechanism for the restoration of intercellular coupling would be increasing intracellular cAMP concentration. In order to test this hypothesis, we studied the modification of this sotalol-induced recoupling by a cAMP dependent protein kinase inhibitor. The results clearly supported our assumption since the addition of Arg-Gly-Tyr-Ala-Leu- Gly (pure A- kinase inhibitor) prevented the aforementioned cellular recoupling action of sotalol in a dose-dependent manner. It can thus be concluded that changes in intracellular cAMP level are involved in the synchronizing / defibrillating effect of sotalol.