Differential sympathetic outflow elicited by active muscle in rats

The present study was undertaken to test the hypothesis that activation of the muscle reflex elicits less sympathetic activation in skeletal muscle than in internal organs. In decerebrate rats, we examined renal and lumbar (mainly innervating hindlimb blood vessels) sympathetic nerve activities (RSNA and LSNA, respectively) during 1 min of 1) repetitive (1- to 4-s stimulation-to- relaxation) contraction of the triceps surae muscle, 2) repetitive tendon stretch, and 3) repetitive contraction with hindlimb circulatory occlusion. During these interventions, RSNA and LSNA responded synchronously as tension developed. The increase was greater in RSNA than in LSNA [+51 ± 14 vs. +24 ± 5% (P < 0.05) with contraction, +46 ± 8 vs. +17 ± 4% (P < 0.05) with stretch, +76 ± 20 vs. 39 ± 7% (P < 0.05) with contraction during occlusion] during all three interventions: repetitive contraction (n = 10, +508 ± 48 g tension from baseline), tendon stretch (n = 12, +454 ± 34 g), and contraction during occlusion (n = 9, +473 ± 33 g). Additionally, hindlimb circulatory occlusion significantly enhanced RSNA and LSNA responses to contraction. These data demonstrate that RSNA responses to muscle contraction and stretch are greater than LSNA responses. We suggest that activation of the muscle afferents induces the differential sympathetic outflow that is directed toward the kidney as opposed to the limbs. This differential outflow contributes to the distribution of cardiac output observed during exercise. We further suggest that as exercise proceeds, muscle metabolites produced in contracting muscle sensitize muscle afferents and enhance sympathetic drive to limbs and renal beds.