Muscle metabolites accentuate muscle sympathetic nerve activity responses to passive muscle stretch

Abstract

It was speculated that muscle metabolites can sensitize muscle mechanoreceptors, and accentuate muscle sympathetic nerve activity (MSNA) responses to mechanoreceptors stimulation in exercise. However, this hypothesis has not been verified directly in healthy subjects. To test whether the MSNA response to passive stretch could be accentuated with increasing metabolites in the muscles, MSNA, ECG and blood pressure (BP) were recorded in 14 young healthy subjects. Subjects performed static isometric handgrip at 30% maximum voluntary contraction (MVC) until fatigue followed by 4 min post exercise ischemia (PEMI). After 2 min of PEMI, the hand was passively stretched (dorsiflexion). In 10 of the subjects, the stretch protocol was also performed without preceding handgrip or ischemia, ischemia alone, and following non-fatigue handgrip at 30% MVC followed by ischemia. During the PEMI after fatigue handgrip, stretch induced significant increase in MSNA (26.4±2.6 to 30.7±2.8 bursts/min, P=0.002) and mean BP (106±2 to 108±2 mmHg, P=0.01). Stretch performed under the other 3 conditions had no significant effect on MSNA. The present data suggest that muscle metabolites accentuate MSNA response to mechanoreceptors stimulation, probably via sensitizing the mechanoreceptors. Supported by NIH P01 HL077670 and M01 RR010732, and AHA 0565399U, 0635245N.