ACUTE EXERCISE ATTENUATES CARDIAC AUTONOMIC REGULATION IN HYPERTENSIVE RATS

Dynamic exercise may be used as a safe, therapeutic approach to reduce sympathetic nerve activity at rest and thus may be beneficial for individuals with hypertension. Therefore, we tested the hypothesis that a single bout of mild to moderate dynamic exercise would decrease cardiac sympathetic tonus at rest. We designed two experimental protocols to test this hypothesis in male spontaneously hypertensive rats. In protocol 1 (n=6) cardiac sympathetic tonus and parasympathetic tonus were determined before and after a single bout of dynamic exercise. We developed protocol 2 (n=5) to determine the component of the autonomic nervous system responsible for the postexercise reduction in heart rate. Rats were instrumented with catheters inserted into the descending aorta for measurements of arterial pressure, mean arterial pressure, and heart rate and into the jugular vein for infusion of drugs. A single bout of mild to moderate dynamic treadmill exercise (12 m/min, 10% grade for 42+/-1 minutes, representing approximately 74% to 79% of maximal heart rate) resulted in a postexercise reduction in mean arterial pressure (163+/-7 to 149+/-5 mm Hg; P<.05). Associated with the postexercise by potension was a reduction in sympathetic and parasympathetic tonus (47+/-12% and 71+/-12%, respectively). The reduction in heart rate during the early recovery phase was due to a withdrawal of sympathetic tonus, because beta(1)-adrenergic receptor blockade significantly enhanced the postexercise reduction in heart rate, and muscarinic-cholinergic receptor blockade did not affect the postexercise decrease in heart rate until 20 minutes after exercise. These results demonstrate that (1) sympathetic tonus and parasympathetic tonus are attenuated after a single bout of dynamic exercise and (2) the reduction in sympathetic tonus and parasympathetic tonus mediated cardiodeceleration during the early recovery period.