Previous work has shown that exercise training increases the ventricular fibrillation threshold of the isolated perfused rat heart. The aim of our study was to determine whether exercise training that begins after myocardial infarction can similarly increase the ventricular fibrillation threshold. Rats that had suffered an experimental myocardial infarction were subject to a running training program. Thereafter, the ventricular fibrillation threshold was measured before and after the onset of acute reinfarction induced by a second coronary artery ligation. Ventricular fibrillation thresholds were significantly elevated in trained rats during normoxia (13.7±2.2 vs. 4.7±0.8 mA, p<0.01) and during acute ischemia (6.8±1.6 vs. 3.0±0.7 mA, p<0.02). The myocardial cyclic AMP level was lower in the nonischemic zone of the trained hearts (0.21±0.01 vs. 0.28±0.01 nmol/g,p<0.05), which also had lower cyclic AMP levels after epinephrine challenge (0.50±0.05 vs. 0.73±0.09 nmol/g, p<0.01; 1.41±0.11 vs. 1.85±0.09 nmol/g, p<0.02 after epinephrine 10`M and 5 X 10-6 M injection, trained vs. untrained). Both propranolol 10-6 M and epinephrine 5 x 10-7 M attenuated the difference in ventricular fibrillation thresholds before and after second coronary artery ligation and eliminated any difference in cyclic AMP content of both the nonischemic and ischemic myocardial tissue. We conclude that exercise training increases the ventricular fibrillation threshold of the previously infarcted isolated rat heart before and after the onset of reinfarction and that the training effect may be mediated by a decrease in myocardial sympathetic tone, an increase in parasympathetic tone, or both. (Circulation 1989;80:138-145) E pidemiologic evidence shows an association between exercise and a reduced incidence of sudden cardiac death.1-4 We have previously shown that the exercise-trained rat heart has an increased resistance to ventricular fibrillaFrom the MRC Ischaemic

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