Muscle sympathetic single‐unit responses during rhythmic handgrip exercise and isocapnic hypoxia in males: The role of sympathoexcitation magnitude

Abstract

A small proportion of postganglionic muscle sympathetic single units can be inhibited during sympathoexcitatory stressors in humans. However, whether these responses are dependent on the specific stressor or the level of sympathoexcitation remains unclear. We hypothesize that, when matched by sympathoexcitatory magnitude, different stressors can evoke similar proportions of inhibited single units. Multiunit and single-unit muscle sympathetic nerve activity (MSNA) were recorded in seven healthy, young males at baseline and during: 1) rhythmic handgrip exercise (40% of maximum voluntary contraction) and 2) acute isocapnic hypoxia (partial pressure of end-tidal O2 of 47±3 mmHg). Single units were classified as activated, nonresponsive, or inhibited if the spike frequency was above, within, or below the baseline variability, respectively. By design, rhythmic handgrip and isocapnic hypoxia similarly increased multiunit total MSNA (delta 273±208 vs. 254±193 AU, P=0.84) and single-unit spike frequency (delta 7±9 vs. 11±15 spikes/min, P=0.29). Among 19 identified single units, the proportion of activated (58% vs. 74%) non-responsive (21% vs. 16%) and inhibited (21% vs. 10%) single units were not different between rhythmic handgrip and isocapnic hypoxia (P=0.57). However, only 9 (47%) single units behaved with concordant response patterns across both stressors (8 activated and 1 non-responsive during both stressors). During the 1-min epoch with the highest increase in total MSNA during hypoxia (delta 595±282 AU, P<0.01) none of the single units were inhibited. These findings suggest that the proportion of muscle sympathetic single units inhibited during stress are associated with the level of sympathoexcitation and not the stressor per se in healthy young males.