It is well-known that aerobic endurance training modifies the balance between sympathetic and parasympathetic nervous system, increasing basal parasympathetic tone. This nervous mechanism produces modifications on the electrophysiological properties of the myocardium, such as a decrease in resting heart rate and a depression of atrioventricular (AV) conduction. However, the influence of the intrinsic cardiac nervous system on the depression of AV dromotropism is not well-known. Thus, our aim is to determine the role of postganglionic parasympathetic neurons in the depression of atrioventricular conduction produced by physical training in isolated rabbit heart.

The study was carried out in 28 New Zealand rabbits which were assigned to control (n=9), sham operated (n=10) and trained (n=9) group. The trained group was submitted to 6 weeks of chronic physical exercise on treadmill, while the other groups were 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. Pacing and recording electrodes were positioned on left ventricle. We evaluated Wenckebach cycle length (WCL) and atrioventricular functional refractory period (AVFRP), a parameter closely related to AV conduction, before and after the infusion of atropine (1mM). WCL was determined by pacing at increasing frequencies. To determine AVFRP, we used several extrastimuli tests at four different pacing cycle lengths (10% less than the basal cycle length, 250, 200 and 150ms). An ANOVA test (repeated measures) was applied to analyze the effects of parasympathetic blockade on atrioventricular refractoriness. A Student-t test was used to evaluate changes in WCL. In sham operated group, time and other manoeuvers as stimulation protocol did not modify WCL and AVFRP. WCL was longer (p<0.05) in trained animals (135.1±13 versus 122.4±10ms). AVFRP was also longer (p<0.05) in trained animals than controls (i.e. 160.3±14 versus 144.7±6ms at 250ms of pacing cycle length). There were no differences in WCL and AVFRP after atropine administration within both groups.

In conclusion, the depression of atrioventricular conduction produced by physical training does not seem to be dependent on parasympathetic postganglionic neurons activity.

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