Physical training increases ventricular fibrillation thresholds of isolated rat hearts during normoxia, hypoxia and regional ischemia.

The effect of exercise training on cardiovascular mortality is controversial. The purpose of this study was to determine the effect of a period of treadmill training on the ventricular fibrillation threshold of the isolated rat heart. Trained hearts had higher threshold values during standard, control perfusion conditions, and when exposed to hypoxia, hypoxia plus isoproterenol infusion, or when subjected to coronary artery ligation. Myocardial metabolic studies failed to define the mechanism for the effect of running training. However, in coronary ligated hearts, the content of the arrhythmogenic substance 3',5' cyclic adenosine monophosphate (cyclic AMP) was reduced in the ischemic zone of hearts from trained rats. Cyclic AMP levels were also lower in trained hearts during control perfusions. We conclude that running training increases the resistance of the heart to ventricular fibrillation by mechanisms that are largely unknown, although they may involve cyclic AMP.