Physical exercise inducing electrophysiological changes in the atria: sympathetic-mediated effects?

Rats were subjected to long-term physical exercise by swimming and their hearts were examined electrophysiologically in order to identify relevant factors as mechanisms of protection from rhythm disturbances. The role of catecholamines was also studied using a chronically applied beta-adrenergic blocking drug, Practolol, or a catecholamine depleting drug, Reserpine. The results were compared with those obtained in rats similarly injected but kept in sedentary life. Action potential duration was found to be prolonged in trained animals regardless of the drug employed. Trained animals showed increments in the myocardial mass. Prolongation of the action potential coincided with lower values for maximum follow frequencies. It is suggested that the effects elicited could protect the heart against rhythm disturbances. The sympathetic nervous system seemed to play no important role.