role of autonomic nervous system in exercise bradycardia in rats

ABSTRACT

It has been a commonly held belief that the relative bradycardia found in athletes and in chronically exercised animals is due to an excessive vagal tone, although the experimental evidence for this is both indirect and unconvincing. Daily swimming of 1.5 hours up to a total of 45 hours produced bradycardia. The contribution of sympathetic and parasympathetic activity to the resting heart rate of trained and control rats was investigated by means of differential blockade of the autonomic nervous system by atropine sulphate and propranolol hydrochloride. This procedure permits the calculation of the intrinsic heart rate and the contribution of the autonomic tone to the control of heart rate. The calculated sympathetic tone [as% of intrinsic heart rate] in the trained rats was 87% that of the control. Reduction in the parasympathetic tone in the trained rats was only 5% that of control. It was concluded, from this study, that training bradycardia resulted from greater reduction in sympathetic than parasympathetic tone, although the activity of both sympathetic and parasympathetic was reduced in the trained rats