Sympathetic nerve activity during prolonged rhythmic forearm exercise

• Published in: Journal of applied physiology (1985) Vol. 76; no. 3; p. 1077
• Main Authors: Batman, B A, Hardy, J C, Leuenberger, U A, Smith, M B, Yang, Q X, Sinoway, L I
• Format: Journal Article
• Language: English
• Published: United States 01-03-1994
• Subjects: Adult; Blood Pressure - physiology; Exercise - physiology; Forearm - physiology; Heart Rate - physiology; Humans; Hydrogen-Ion Concentration; Isotonic Contraction - physiology; Magnetic Resonance Spectroscopy; Male; Muscle Contraction - physiology; Peroneal Nerve - physiology; Phosphates - blood; Phosphocreatine - blood; Pressoreceptors - physiology; Sympathetic Nervous System - physiology
• ISSN: 8750-7587
• DOI: 10.1152/jappl.1994.76.3.1077

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 fatigue, 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 perfused 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, 31P-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.