Reflex Responses in Plasma Catecholamines Caused by Static Contraction of Skeletal Muscle.

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Abstract
To examine a hypothesis of whether static muscle contraction produces a release of catecholamines from the adrenal medulla via reflex stimulation of preganglionic adrenal sympathetic nerve activity induced by receptors in the contracting muscle, we compared the reflex responses in a concentration of epinephrine (Ep) and norepinephrine (NEp) in arterial plasma during static contraction and during a mechanical stretch of the hindlimb triceps surae muscle in anesthetized cats. Static contraction was evoked by electrically stimulating the peripheral ends of the cut L7 and S1 ventral roots at 20 or 40 Hz. Mean arterial pressure (MAP) and heart rate (HR) increased 23 ± 3.1 mmHg and 19 ± 4.3 beats/min during static contraction. Ep in arterial plasma increased 0.18 ± 0.072 ng/ml over the control of 0.14 ± 0.051 ng/ml within 1 min from the onset of static contraction, and NEp increased 0.47 ± 0.087 ng/ml over the control of 0.71 ± 0.108 ng/ml. Following a neuromuscular blockade, although the same ventral root stimulation failed to produce the cardiovascular and plasma catecholamine responses, the mechanical stretch of the muscle increased MAP, HR, and plasma Ep, but not plasma NEp. With bilateral adrenalectomy, the baseline Ep became negligible (0.012 ± 0.001 ng/ml) and the baseline NEp was lowered to 0.52 ± 0.109 ng/ml. Neither static contraction nor mechanical stretch produced significant responses in plasma Ep and NEp following the adrenalectomy. These results suggest that static muscle contraction augments preganglionic adrenal sympathetic nerve activity, which in turn secretes epinephrine from the adrenal medulla into plasma. A muscle mechanoreflex from the contracting muscle may play a role in stimulation of the adrenal sympathetic nerve activity.