Effects of endurance training on cholinergic and adrenergic receptors of rat heart

To test the hypothesis that alterations in adrenergic or cholinergic receptors occur in response to physical training, and that changes in receptor properties could be mechanistically important in producting the altered cardiovascular physiology of the trained state, we studied the effects of endurance training by swimming upon beta adrenergic, alpha adrenergic, and muscarinic cholinergic receptors of rat heart. Because of previously reported sex-related differences in the cardiac adaptation to training, male and female rats were studied separately. Despite the occurrence of demonstrable training bradycardia in males, and of cardiac hypertrophy in females, there were no discernible effects of the training program upon the properties of cardiac beta adrenergic receptors. However, hearts from swimmers of both sexes demonstrated fewer numbers of muscarinic cholinergic and alpha adrenergic receptors than sedentary controls, without differences in the receptor affinities for antagonist or agonist compounds. These findings are inconsistent with the hypothesis that altered cardiac sensitivity to neurotransmitters contributes directly to training bradycardia.