Sympathetic nerve activity during prolonged rhythmic forearm exercise.
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.(ABSTRACT TRUNCATED AT 250 WORDS)
Batman
B A
BA
Division of Cardiology/Department of Medicine, Milton S. Hershey Medical Center, Pennsylvania State University, Hershey 17033.
Hardy
J C
JC
Leuenberger
U A
UA
Smith
M B
MB
Yang
Q X
QX
Sinoway
L I
LI
D000328:Adult; D001794:Blood Pressure / Q000502:physiology; D015444:Exercise / Q000502:physiology*; D005542:Forearm / Q000502:physiology*; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007551:Isotonic Contraction / Q000502:physiology; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D009119:Muscle Contraction / Q000502:physiology; D010543:Peroneal Nerve / Q000502:physiology; D010710:Phosphates / Q000097:blood; D010725:Phosphocreatine / Q000097:blood; D011311:Pressoreceptors / Q000502:physiology; D013564:Sympathetic Nervous System / Q000502:physiology*
D010710:Phosphates; D010725:Phosphocreatine