Abstract
Summary
The intrinsic rate of contraction of isolated atria from male rats, exercised for 15 wk, was considerably lower than that of sedentary controls. There was a marked sub-sensitivity to acetylcholine of atria from exercised rats. This subsensitivity may be related to an increase in the tonic release of acetylcholine by vagal cardio-inhibitory fibers in the exercised rat. It is concluded that the bradycardia of training probably involves at least two mechanisms; a decrease in the intrinsic rate of the sinoatrial node and an increase in cardiac parasympathetic activity.
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