New aspects on training bradycardia.

Rats were trained by treadmill running after chemical sympathectomy with 6-hydroxy-dopamine or during chronic beta receptor blockade. Contrary to untreated trained animals, sympathectomized rats did not get a reduction of the intrinsic heart rate after training despite an increased heart weight. In contrast, no cardiac hypertrophy occurred after training during beta adrenergic blockade but the heart rate during exercise was reduced in these animals. It is concluded that the training-induced bradycardia contains a lowering of the intrinsic heart rate and that this is not dependent on the stimulation of cardiac beta receptors or the magnitude of heart rate increase during exercise. The results also indicate that there is not a causal relationship between the training-induced bradycardia and cardiac hypertrophy. The latter conclusion is supported by an echocardiographic study in humans where no correlation was found between IHR and cardiac dimensions.