The cardio-protective features of tricyclic antidepressants

Transient ventricular fibrillation and myosin heavy chain isoform profile

The present paper deals with spontaneous ventricular defibrillation in mammals and the possibility to facilitate its occurrence. Clinical and experimental evidence suggest that in the majority of cases, ventricular fibrillation (VF) is permanent, requiring defibrillation by electric shock. However, a growing number of reports show that VF can terminate spontaneously in various mammals, including human beings.The mechanisms involved in spontaneous ventricular defibrillation are controversial. Available reports imply that intracellular Ca²⁺ overload is the key event triggering VF and preventing its reversal. Since the sarcoplasmatic reticulum is the main intracellular Ca²⁺ regulating organelle and the activity of the cardiac SR Ca²⁺ ATPase (SERCA 2a) is its prime element of Ca²⁺ sequestration, spontaneous ventricular defibrillation likely requires high level of SERCA 2a activity. We suggest that mammalian hearts with high SERCA 2a activity defibrillate spontaneously and those with low activity only after its enhancement. Since high SERCA 2a activity is co-expressed with the myosin heavy chain (MHC) isoform V1, we assumed that those hearts preferentially expressing V1 MHC are able to defibrillate spontaneously. Hearts with small amounts of V1 MHC and correspondingly lower level of SERCA 2a activity can only defibrillate following administration of compounds that augment SERCA 2a activity and prevent intracellular Ca²⁺ overload.

A new class of antiarrhythmic-defibrillatory agents

Novel dibenzoazepine and 11-oxo-dibenzodiazepine derivatives are shown to be effective ventricular defibrillating drug candidates. They exhibit significant in vivo defibrillatory activity with no observed changes in ECG either before or after the VF event. These compounds also exhibit antifibrillatory activity by elevating the fibrillation threshold potential, all suggesting that such drugs could be used to treat VF either by themselves or together with electrical defibrillators.

Sotalol facilitates spontaneous ventricular defibrillation by enhancing intercellular coupling. An entirely new mechanism for its antiarrhythmic action

We have previously shown that sotalol, a class III antiarrhythmic agent, helps spontaneous ventricular defibrillation in various mammalian species. Since we hypothesized that self ventricular defibrillation depends on a high degree of intercellular synchronization, and since the major electrophysiological action of sotalol causing prolongation of action potential duration (APD), cannot fully explain its defibrillating property, we carried out a series of studies to examine the effect of sotalol on intercellular myocardial coupling. Guinea pig right ventricular muscle preparations were superfused in a tissue bath and the spread of intracellularly injected fluorescent dye (Lucifer yellow CH) to the neighboring cells was studied under various conditions. When either the Ca2+ concentration of Tyrode's solution was elevated to 6 mM or the solution was made hypoxic by not bubbling O2 (n = 3 each), no spread of the injected dye was observed. The addition of 1 microM sotalol to the high Ca2+ solution or 0.5 microM to the hypoxic superfusate (n = 3 each) caused a wide spreading of the dye, thus strongly suggesting a marked improvement in the intercellular coupling. These results show an entirely new property of sotalol, i.e., enhancement of cellular synchronization, which may better explain its ability to cause spontaneous ventricular defibrillation than its class III action. Our previous demonstration of successful spontaneous ventricular defibrillation by several other agents that are known to enhance intercellular coupling but have contrasting actions on APD further substantiates our hypothesis.

How can we facilitate spontaneous termination of ventricular fibrillation and prevent sudden cardiac death? A working hypothesis

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 to the patient's 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.

The role of catecholamines on intercellular coupling, myocardial cell synchronization and self ventricular defibrillation

Ventricular fibrillation (VF) is one of the most life threatening events. Although in humans VF is generally sustained (SVF) requiring artificial defibrillation, in various mammals and in some cases in humans VF terminates by itself, reverting spontaneously into sinus rhythm. Since VF is one of the main causes of sudden death, one of the important clinical problems today is if and how we can transform the fatal SVF into a self limited transient one (TVF). From electrophysiological studies carried out on anaesthetized open chest animals, we have found that TVF requires a high degree of intercellular coupling and synchronization. Cardiac myocytes are electrically coupled with adjacent cells. The intercellular coupling is a focus of low electrical resistance which allows rapid transmission of electrical impulses between cells. Any decrease in intercellular coupling decreases the ability of the heart for self defibrillation. The cell-to-cell coupling decreases with age, ischemia, VF and variations in physiological conditions probably due to an increase in intercellular resistance (Ri), widening in the internexal gaps, decrease in electrotonic space constant (lambda) etc. All of these factors are known to be affected by intracellular concentration of free Ca++ ([Ca++]). On the basis of studies carried out on various mammals at different ages, we hypothesized that the ability of the heart to defibrillate depends on the cardiac catecholamine level [CA], during VF. This hypothesis is supported by the facts, known from the literature, that increase in [CA] decreases intracellular free Ca++ concentration, decreases Ri and increases lambda.(ABSTRACT TRUNCATED AT 250 WORDS)

A self-protecting servo-model for explanation of the mechanism involved in spontaneous ventricular defibrillation

Ventricular fibrillation (VF) is the most life-threatening arrhythmia. It has been suggested that VF in humans is always sustained. Recent publications indicated that VF can be either sustained (SVF) or transient (TVF), reverting spontaneously into sinus rhythm. In previous studies we have hypothesized that TVF requires, during VF, a high cardiac catecholamine level ([CA]). Since during VF sympathetic activity is enhanced, the question arises of why VF is sustained in the majority of cases. Looking on the living body as a self-protecting servo-mechanism, we propose a servo-model that on the one hand describes the mechanism involved in TVF and on the other proposes a therapeutic procedure which can help the heart in its effort to transform VF into TVF. Our model has been examined by various experimental studies. The results obtained strongly support our hypothesis.

Ethmozine and ethacizine--new antiarrhythmic drugs with defibrillating properties

Ventricular fibrillation (VF) is a life-threatening arrhythmia that leads to death unless electrical defibrillation is applied in time. Recent publications indicate that VF can be either sustained (SVF), requiring electrical defibrillation, or transient (TVF), reverting spontaneously into sinus rhythm. Since VF cannot be totally prevented by drugs, a new antiarrhythmic therapeutic approach has been proposed: drug-induced enhancement of the ability of the heart to defibrillate by itself. In this study we examined the defibrillating potency of two antiarrhythmic phenothiazines, ethmozine (ETM) and ethacizine (ETA), as well as their effects on catecholamine uptake and on the electrophysiological properties of the myocardial cell membrane. The antiarrhythmic-defibrillatory activity was examined in cats; the inhibitory effect on [3H]-norepinephrine (NE) uptake was examined in rat brain synaptosomes, and the electrophysiological membrane effects were examined by microelectrode recordings in perfused strips of heart ventricle from guinea-pigs. The results indicate that: 1. ETA exhibits similar but stronger antiarrhythmic-defibrillating and NE reuptake inhibitory effects than ETM; 2. ETA at 10-6 M decreases ventricular conduction time and increases Vmax while ETM at this concentration does not change them; 3. The defibrillating ability of the drugs can be related to their inhibitory potency on NE reuptake. We suggest that the risk of sympathomimetic arrhythmogenicity is prevented by the previously described, membrane stabilizing Class 1 antiarrhythmic properties of these drugs.

Pharmacology and clinical use of a new group of antiarrhythmic drugs: derivatives of tricyclic nitrogen-containing systems

This paper is concerned with a new group of antiarrhythmic drugs: derivatives of phenothiazine and dibenzazepine which belong to tricyclic nitrogen-containing systems. It has been discovered that the transition from omega-aminoalkyl to omega-aminoacyl phenothiazine derivatives results in a decrease in psychotropic activity and an increase in cardiovascular, particularly antiarrhythmic, action. Based on structural and antiarrhythmic activity studies, the new and effective drugs ethmozine, ethacizine and bonnecor have been selected and extensively studied. Data on spectra and mechanisms of their action, obtained in different arrhythmia models, have been confirmed by clinical studies. Correlation between pharmacological properties and electrophysiological mechanisms, and differences in action of the drugs studied on brain and heart plasma membrane receptors may serve as a starting point for the further directed search for new antiarrhythmic drugs among tricyclic nitrogen-containing structures.