Reflex stimulation of cardiac sympathetic nerve activity during static muscle contraction in cats

Reflex response of cardiac sympathetic nerve activity (CSNA) during static contraction of the triceps surae muscle was studied using anesthetized cats. A 1-min contraction was evoked by stimulating the peripheral ends of the cut L7 and S1 ventral roots. CSNA increased 48 +/- 13% immediately after the onset of contraction, which was abolished by cutting the L4-S1 dorsal roots. This rapid increase in CSNA preceded rises in heart rate (13 +/- 1 beats/min) and arterial blood pressure (33 +/- 6 mmHg). When tension development was altered by changing the frequency of ventral root stimulation or the initial muscle length, the CSNA increase depended on the tension developed. Passive stretch of the muscle, which primarily activates mechanoreceptors, increased CSNA by 41 +/- 22%. When the contraction was sustained for 5 min, CSNA remained elevated throughout the contraction despite a fall in tension, suggesting that the later increase in CSNA is caused by factors other than a mechanical event of contraction (e.g., metabolic products). Thus it is suggested that cardiac sympathetic outflow is stimulated due to a reflex arising from the contracting muscle. The increase in CSNA at the initiation of contraction is likely to be caused by a reflex from muscle mechanoreceptors, which is followed by a subsequent increase due to a reflex from muscle metaboreceptors.