Role of the Adrenergic Nervous System in Development of Training‐Induced Bradycardia

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Abstract
Sprague-Dawley rats, normal and chemically sympathectomized with 6-hydroxy-dopamine, were trained by treadmill running. The normal rats, unlike the sympathectomized animals, showed reduction of the exercise heart rate after the training period. Compared to a sedentary control group, the sympathectomized rats showed no difference in intrinsic heart rate after pithing and denervation and no increase in heart weight. The increase of the heart weight/body weight ratio after training was smaller in the sympathectomized group than in the normal one. A functioning adrenergic nervous system is apparently necessary for an efficient adaptation to physical training. Administration of noradrenaline [norepinephine] to pithed trained and untrained rats showed that .beta.-adrenergic receptor sensitivity was not altered by physical training. The intrinsic heart rate of normal trained rats was lower than that of normal control rats.