Augmentation of exercise-induced muscle sympathetic nerve activity during muscle heating.

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 degree 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.