SYMPATHETIC-NERVE ACTIVITY DURING PROLONGED RHYTHMIC FOREARM EXERCISE

Sympathetic nerve activity during prolonged rhythmic forearm exercise. J. Appl. Physiol. 76(3): 1077-1081, 1994. - Exercise is a potent stimulus to activate the sympathetic nervous system. Previous work suggests that metabolite sensitive muscle afferents are activated near the point of fa tigue, and, when activated, they determine the muscle sympathetic nerve activity (MSNA) response to isometric forearm exercise. Yet, studies using a more prolonged rhythmic exercise paradigm suggest that the sympathetic nervous system can be activated in a more graded fashion. The purpose of this study was to determine whether metaboreceptor stimulation would also be responsible for MSNA responses to prolonged rhythmic isotonic forearm exercise. Subjects (n = 16) performed rhythmic isotonic forearm exercise at 25% maximal voluntary contraction for 30 min as we measured MSNA (microneurography). We observed progressive increases in MSNA with a peak increase of 161 units from a baseline value of 180 units. We also performed posthandgrip circulatory arrest (PHG-CA) in nine of these subjects. This maneuver isolates the metaboreceptor contribution to MSNA. During PHG-CA, Delta MSNA values were not different from those observed during a freely per fused recovery period (n = 7). We also compared MSNA responses during the rhythmic paradigm with those seen during a static protocol at 40% of maximal voluntary contraction in five subjects. The two types of exercise caused similar increases in MSNA, but only the static paradigm was associated with a sustained MSNA response during PHG-CA. Finally, P-31-nuclear magnetic resonance was used to evaluate muscle metabolic responses during rhythmic and static forearm exercise (n = 6). Static exercise caused muscle acidosis and an increase in H2PO4-, whereas rhythmic exercise had no effect on muscle metabolism. We conclude that sympathoexcitation during rhythmic exercise is not dependent on engagement of muscle metabolite-sensitive afferents.