It has been reported that chronic physical exercise protects against cardiac sudden death. Several authors have attributed this protective effect to the increase of vagal parasympathetic tone produced by the trained state. Nevertheless, the exact underlying mechanisms are not completely clear. On the other hand, several authors have pointed out that physical training decreases ventricular heterogeneity, which is a very important factor in the initiation and maintenance of ventricular fibrillation (VF). However, the role of ventricular parasympathetic postganglionic neurons on the modifications of electrophysiological heterogeneity by training are not known. Our aim was to investigate the role of these neurons on the training-induced changes of ventricular heterogeneity in isolated rabbit heart.

For this purpose, 25 New Zealand rabbits were divided in two groups: trained group (n=8) and control group (n=17). Animals in trained group were submitted to 6 weeks of chronic physical exercise on treadmill, while control group was housed in the animal quarter during the same period. After these 6 weeks, rabbits were anaesthetized, killed and their hearts excised and isolated in a Langendorff system. A pacing electrode and a plaque with 256 recording electrodes were positioned on left ventricle. VF was induced by pacing at increasing frequencies (2Hz^.min^-1), and maintained without interrupting perfusion, before and after the infusion of atropine (1mM). In each experiment, dominant frequency (DF) of VF was determined (Welch´s method) in multiple points of ventricular myocardium. Mean DF and its standard deviation were used in order to calculate the coefficient of variation (CV). CV was compared (control vs trained group) before cholinergic blockade to evaluate the training effect on ventricular heterogeneity with an ANOVA test (repeated measures). We also used an ANOVA test (repeated measures) to compare CV in trained group (before vs after atropine) for analyzing the role of cholinergic postganglionic neurons on this training-induced modification.

CV significantly decreased (p<0.05) in trained group (8.8±4% vs 13.6±5% at 30 seconds, 14.03±6% vs 10.02±2% at 60 seconds, 14.5±6% vs 9.7±3% at 90 seconds and 13.1±5% vs 10.1±3% at 120 seconds). Parasympathetic blockade did not modify the decrease of CV observed in trained group.

In conclusion, since parasympathetic postganglionic neurons do not seem to play any role in the decrease of ventricular heterogeneity, this training-induced modification could be intrinsic in nature.

Supported by MEC (DEP2007-73234-C03-01) and Generalitat Valenciana (BFPI/2008/003).