Effect of beta-adrenergic receptor blockade on development of training-induced bradycardia in rats.

Metoprolol-treated and untreated rats were subjected to the same treadmill running program for 12 weeks. After training and withdrawal of metoprolol, the treated rats had a more pronounced bradycardia during submaximal exercise than non-treated rats, although they had a lower heart weight/body weight ratio. The intrinsic heart rate (IHR) was slightly but not significantly lower in trained than in untrained untreated animals. The IHR was not altered by a period of metoprolol treatment neither alone nor in combination with training. It is concluded that chronic cardioselective beta-blockade during training augments the training-induced bradycardia but may impair the development of cardiac hypertrophy. The training-induced bradycardia is thus not related to the degree of heart rate increase during exercise for its development.