Role of sympathoadrenergic mechanisms in arrhythmogenesis

Cardiac Sympathetic Nerve Sprouting and Susceptibility to Ventricular Arrhythmias after Myocardial Infarction

Ventricular arrhythmogenesis is thought to be a common cause of sudden cardiac death following myocardial infarction (MI). Nerve remodeling as a result of MI is known to be an important genesis of life-threatening arrhythmias. It is hypothesized that neural modulation might serve as a therapeutic option of malignant arrhythmias. In fact, left stellectomy or β-blocker therapy is shown to be effective in the prevention of ventricular tachyarrhythmias (VT), ventricular fibrillation (VF), and sudden cardiac death (SCD) after MI both in patients and in animal models. Results from decades of research already evidenced a positive relationship between abnormal nerve density and ventricular arrhythmias after MI. In this review, we summarized the molecular mechanisms involved in cardiac sympathetic rejuvenation and mechanisms related to sympathetic hyperinnervation and arrhythmogenesis after MI and analyzed the potential therapeutic implications of nerve sprouting modification for ventricular arrhythmias and SCD control.

Acute risk factors for myocardial infarction

Increased knowledge concerning the triggering of acute cardiovascular diseases has yielded a change in philosophical approach to this field. During the last decade, clinical evidence suggested that the term acute risk factors can be used for the activities and events that suddenly and transiently increase the risk of acute cardiac diseases. External triggers, such as heavy physical activity, emotional stress, eating, cold or heat exposure, coffee or alcohol consumption, cocaine or marijuana use and sexual intercourse are recognized as most important acute risk factors. It is likely that the morning hours may be considered as an endogenous, external triggering independent acute risk factor related to physiological sympathetic arousal. The features of triggering have been best described for an acute myocardial infarction whose moment of onset appears to be the result of a dynamic interaction between an endogenous response to acute risk factors and patient vulnerability. In this article, pathophysiological changes implicated as internal triggering mechanisms are summarized and the terms sympathetic and parasympathetic triggering patterns are introduced. A highly individual approach tailored both to protect against acute risk factors and to reduce patient vulnerability could provide a more complete protection from myocardial infarction and other coronary incidents. Lifestyle modifications, regular physical activity and adequate drug regimens may at least prove able to defer the occurrence of coronary thrombosis, thereby providing time for the development of collateral vessels, plaque stabilization or invasive/surgical treatment.

Vulnerability to ventricular arrhythmias [corrected] and heterogeneity of action potential duration in normal rats

In normal rats, we analysed the arrhythmogenic role of intrinsic action potential duration (APD) heterogeneity. In each animal, ventricular arrhythmic events (VAEs) occurring spontaneously and during the exposure to an acute social challenge were telemetrically recorded. Action potentials were recorded from isolated left ventricular myocytes, at a pacing rate of 5 Hz (patch clamp: current-clamp mode). APDs were measured at -20 mV, -30 mV, -40 mV, -50 mV and -60 mV. The difference between the shortest and the longest APD was also computed, as an index of individual APD heterogeneity. Animals predisposed to stress-induced arrhythmias showed higher values of APD and APD heterogeneity as compared with the remaining rats. We concluded that, in the normal heart, a large intrinsic APD heterogeneity resulting from specific electrophysiological properties of ventricular myocytes is not in itself arrhythmogenic, but can predispose towards arrhythmia development under certain conditions, such as autonomic activation.

Sympathetic nervous system activity and ventricular tachyarrhythmias: recent advances

Sympathetic nervous system activity (SNSA) is believed to participate in the genesis of ventricular tachyarrhythmias (VTA) but understanding has been impeded by the number and complexity of effects and the paucity of data from humans. New information from studies of genetic disorders, animal models, and spontaneous human arrhythmias indicates the importance of the temporal pattern of SNSA in arrhythmia development. The proarrhythmic effects of short-term elevations of SNSA are exemplified by genetic disorders and include enhancement of early and delayed afterdepolarizations and increased dispersion of repolarization. The role of long-term elevations of SNSA is suggested by animal models of enhanced SNSA signaling that results in apoptosis, hypertrophy, and fibrosis, and sympathetic nerve sprouting caused by infusion of nerve growth factor. Processes that overlap short- and long-term effects are suggested by changes in R-R interval variability (RRV) that precede VTA in patients by several hours. SNSA-mediated alterations in gene expression of ion channels may account for some intermediate-term effects. The propensity for VTA is highest when short-, intermediate, and long-term changes are superimposed. Because the proarrhythmic effects are related to the duration and intensity of SNSA, normal regulatory processes such as parasympathetic activity that inhibits SNSA, and oscillations that continuously vary the intensity of SNSA may provide vital antiarrhythmic protection that is lost in severe heart failure and other disorders. These observations may have therapeutic implications. The recommended use of beta-adrenergic receptor blockers to achieve a constant level of inhibition does not take into account the temporal patterns and regional heterogeneity of SNSA, the proarrhythmic effects of alpha-adrenergic receptor stimulation, or the potential proarrhythmic effects of beta-adrenergic receptor blockade. Further research is needed to determine if other approaches to SNSA modulation can enhance the antiarrhythmic effects.

The antiarrhythmic effects of ischaemic preconditioning in anaesthetized dogs are prevented by atropine; role of changes in baroreceptor reflex sensitivity

1. Dogs, anaesthetized with chloralose and urethane, were subjected to a 25 min occlusion of the left anterior descending coronary artery. This resulted in ventricular ectopic activity, a reduction in baroreflex sensitivity (BRS, measured following the intravenous administration of phenylephrine), elevations in the epicardial ST-segment and increases in the degree of inhomogeneity of electrical activation, both measured from the ischaemic region of the left ventricular wall. 2. These changes were markedly reduced when the 25 min occlusion was preceded, 20 min earlier, by a 5 min (preconditioning) occlusion of the same coronary artery (e.g. VF during ischaemia reduced from 40% in the controls to 0%; P<0.05; BRS increased from 1.22+/-0.23 pre-occlusion to 1.61+/-0.25 mmHg ms(-1) post-occlusion in preconditioned dogs; cf. 1.28+/-0.29 to 0.45+/-0.12 mmHg ms(-1) respectively in the controls, P<0.05). 3. These beneficial effects of preconditioning were prevented by the administration, 10 min prior to the 25 min coronary artery occlusion, of atropine (1 mg kg(-1) i.v. followed by a continuous infusion of 0.04 mg kg(-1) h(-1)). For example, VF during occlusion was increased from 0% in the preconditioned dogs to 40% (P<0.05) in the presence of atropine and BRS was again reduced during occlusion (from 1.75+/-0.29 to 0.30+/-0.08 mmHg ms(-1); P<0.05). 4. We conclude that preconditioning reduces arrhythmia severity during ischaemia by favourably modifying cardiac autonomic receptor mechanism through enhancing vagal influences.

Enhanced in vivo and in vitro contractile responses to beta(2)-adrenergic receptor stimulation in dogs susceptible to lethal arrhythmias

The response to beta-adrenergic receptor (beta-AR) stimulation was evaluated in both isolated cardiomyocytes (video edge detection) and the intact animal (echocardiography) in dogs either susceptible (S) or resistant (R) to ventricular fibrillation induced by a 2-min coronary occlusion during the last minute of exercise. In the intact animal, velocity of circumferential fiber shortening (Vcf) was evaluated both before (n = 27, S = 12 and R = 15) and after myocardial infarction. Before infarction, increasing doses of isoproterenol provoked similar contractile and heart rate responses in each group of dogs. Either beta(1)-AR (bisoprolol) or beta(2)-AR (ICI-118551) antagonists reduced the isoproterenol response, with a larger reduction noted after the beta(1)-AR blockade. In contrast, after infarction, isoproterenol induced a significantly larger Vcf and heart rate response in the susceptible animals that was eliminated by beta(2)-AR blockade. The single-cell isotonic shortening response to isoproterenol (100 nM) was also larger in cells obtained from susceptible compared with resistant dogs and was reduced to a greater extent by beta(2)-AR blockade in the susceptible dog myocytes (S, -48%, n = 6; R, -15%, n = 9). When considered together, these data suggest that myocardial infarction provoked an enhanced beta(2)-AR response in susceptible, but not resistant, animals.