The role of the autonomic nervous system in exercise bradycardia in rats.

The origin of the relative bradycardia found in athletes and in chronically exercised animals is still controversial. To further investigate the role of the autonomic nervous system in exercise bradycardia three groups of Sprague-Dawley rats performed swimming exercises daily for 1.5 hr for a total of 45 hrs. The contribution of the sympathetic and parasympathetic systems to the resultant bradycardia was elucidated by differential autonomic blockade using propranolol hydrochloride and atropine sulphate respectively. This procedure permits the determination of the intrinsic heart rates so that it is possible to observe the contributions of the two divisions of the autonomic nervous system in exercise bradycardia. The results showed that both sympathetic and parasympathetic tones of the trained rats were reduced, being respectively 13% and 5% lower than the control groups. It was concluded that the reduction in heart rate in trained rats was mainly due to a reduction in the sympathetic tone.