The assumption that tachycardia during light to moderate exercise was predominantly controlled by withdrawal of cardiac parasympathetic nerve activity but not by augmentation of cardiac sympathetic nerve activity (CSNA) was challenged by measuring CSNA during treadmill exercise (speed, 10-60 m/min) for 1 min in five conscious cats.
As soon as exercise started, CSNA and heart rate (HR) increased and mean arterial pressure (MAP) decreased; their time courses at the initial 12-s period of exercise were irrespective of the running speed. CSNA increased 168-297% at 7.1 +/- 0.4 s from the exercise onset, and MAP decreased 8-13 mmHg at 6.0 +/- 0.3 s, preceding the increase of 40-53 beats/min in HR at 10.5 +/- 0.4 s.
CSNA remained elevated during the later period of exercise, whereas HR and MAP gradually increased until the end of exercise. After the cessation of exercise, CSNA returned quickly to the control, whereas HR was slowly restored. In conclusion, cardiac sympathetic outflow augments at the onset of and during dynamic exercise even though the exercise intensity is low to moderate, which may contribute to acceleration of cardiac pacemaker rhythm.
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