Physical training depresses sinusal automatism and atrioventricular (AV) conduction not only by vagal tone dependent bu also by intrinsic mechanisms (Such L. et al. 2002 J Appl Physiol 92, 225-9). Nevertheless, the modifications on intrinsic AV conduction system refractoriness have been less studied. We investigate this question. Procedures were performed in accordance with the agreements of the European Convention of Strasbourg of March 18, 1986. Thirteen NZW rabbits were submitted to a six weeks endurance exercise-training program, and eleven controls were not trained. When the exercise program was finished, rabbits were anaesthetized (ketamine, 10 mg/kg i.v.), killed and the heart excised and isolated in a Langendorff system. An atrial pacing electrode and a ventricular recording 256-lead multiple electrode were positioned. AV conduction system effective and functional refractory periods were determined by atrial extrastimulus test. Three pacing cycle lengths were used (250, 200 and 150 ms) for each group. A two factor ANOVA analysis with repeated measures on one factor was performed for comparison between groups p< 0.05 was considered significant.

AV conduction system functional refractory periods obtained using the three pacing cycle lengths, were longer in trained (164+14 ms, n=11; 148+14 ms, n=11; 135+11 ms, n=12) than in control hearts (149+10 ms, n=8; 140+ 8 ms, n=10; 124+6 ms, n=10). The effective refractory periods were not different between groups. Mechanisms not vagal tone dependent also contributes to the lengthening of the AV conduction system refractoriness by training.

This work was supported by DEP 2006-56208-C03-01 grant of the Ministerio de Educación y Ciencia.