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Autonomic and Cardiovascular Control Unit, Department of Exercise Science, University of Georgia, Athens, Georgia 30602
Received 15 October 1996; accepted in final form 30 December 1996.
Ray, Chester A., and Kathryn H. Gracey. Augmentation of
exercise-induced muscle sympathetic nerve activity during muscle heating. J. Appl. Physiol. 82(6):
1719-1725, 1997.
The muscle metabo- and mechanoreflexes have been
shown to increase muscle sympathetic nerve activity (MSNA) during
exercise. Group III and IV muscle afferents, which are believed to
mediate this response, have been shown to be thermosensitive in
animals. The purpose of the present study was to evaluate the effect of
muscle temperature on MSNA responses during exercise. Eleven subjects
performed ischemic isometric handgrip at 30% of maximal voluntary
contraction to fatigue, followed by 2 min of postexercise muscle
ischemia (PEMI), with and without local heating of the forearm. Local
heating of the forearm increased forearm muscle temperature from 34.4 ± 0.2 to 38.9 ± 0.3°C
(P = 0.001). Diastolic and
mean arterial pressures were augmented during exercise in the heat.
MSNA responses were greater during ischemic handgrip with local heating
compared with control (no heating) after the first 30 s. MSNA responses
at fatigue were greater during local heating. MSNA increased by 16 ± 2 and 20 ± 2 bursts per 30 s for control and heating,
respectively (P = 0.03). When
expressed as a percent change in total activity (total burst
amplitude), MSNA increased 531 ± 159 and 941 ± 237% for
control and heating, respectively (P = 0.001). However, MSNA was not different during PEMI between trials.
This finding suggests that the augmentation of MSNA during exercise
with heat was due to the stimulation of mechanically sensitive muscle
afferents. These results suggest that heat sensitizes skeletal muscle
afferents during muscle contraction in humans and may play a role in
the regulation of MSNA during exercise.
exercise pressor reflex; isometric contraction; muscle ischemia; muscle temperature; group III and IV afferents
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