Modulation of muscle sympathetic nerve activity to muscle heating during dynamic exercise

Abstract

Previous studies from our laboratory have demonstrated that altering muscle temperature of the exercising forearm can elicit changes in muscle sympathetic nerve activity (MSNA) during ischemic isometric handgrip. The purpose of the current study was to determine the interactive effect of muscle temperature and blood flow on MSNA responses during dynamic handgrip (DHG). Eight subjects performed two bouts of graded DHG to fatigue followed by 2 min of postexercise muscle ischemia (PEMI). Local heating of the forearm increased muscle temperature from 33.6 +/- 0.3 to 38.3 +/- 0.5 degrees C (P < 0.05). Mean arterial pressure and heart rate increased in a linear fashion during graded DHG (P < 0.05) but were not affected by muscle temperature. MSNA (burst frequency and total activity) at fatigue and PEMI were elevated in all conditions (P < 0.05). However, MSNA responses were not different between temperature conditions. To ascertain the effect of blood flow, eight additional subjects completed two trials of ischemic DHG under control or warm conditions followed by 2 min of PEMI. MSNA, expressed as burst frequency and total activity, was significantly greater in warm compared with the control trial (Delta14 +/- 3 and Delta9 +/- 2 bursts/30 s, and Delta1,234 +/- 260 and Delta751 +/- 199 units/30 s, respectively). This finding supports the concept that muscle heating sensitizes skeletal muscle afferents during muscle contractions and augments MSNA in humans. However, on the basis of these findings, we conclude that muscle blood flow modulates the effect of muscle temperature on MSNA during exercise.

Fig. 1.

Muscle (top) and skin (bottom) temperatures in study 1. *Significant increase from baseline (P < 0.05). Temperature was significantly different between conditions (P < 0.05).

Fig. 2.

A: mean arterial pressure (MAP) and heart rate (HR) responses to dynamic handgrip (DHG) during the final 30 s and postexercise muscle ischemia (PEMI) from study 1. All values were significantly elevated from baseline (P < 0.05) except for HR during PEMI. There were no statistical differences between temperature trials. B: mean arterial pressure (MAP) and heart rate (HR) responses to ischemic DHG during 30 s prior to fatigue and PEMI from study 2. All values were significantly elevated from baseline (P < 0.05) except for HR during PEMI. There were no statistical differences between temperature trials.

Fig. 3.

A: muscle sympathetic nerve activity (MSNA) responses during the final 30 s (fatigue) of DHG and PEMI from study 1. There were no statistical differences between temperature trials for both DHG and PEMI. B: muscle sympathetic nerve activity (MSNA) responses during the final 30 s (fatigue) and PEMI from study 2. MSNA increased during ischemic DHG at fatigue and was elevated in the heat trial from the control trial (P = 0.004 for Δ burst frequency and P = 0.001 for Δ total activity). During PEMI, MSNA remained elevated from baseline but was not affected by temperature [interaction term (temperature × intervention) was P = 0.35 for Δ burst frequency and P = 0.20 for Δ total activity].

Fig. 4.

MSNA and MSNA total activity in arbitrary units (a.u.) from one subject at baseline and during the final 30 s of ischemic DHG during the control and warm muscle temperature conditions (study 2).