Connor J. Doherty

Validity and reliability of measuring resting muscle sympathetic nerve activity using short sampling durations in healthy humans

Resting muscle sympathetic nerve activity (MSNA) demonstrates high intraindividual reproducibility when sampled over 5-30 min epochs, although shorter sampling durations are commonly used before and during a stress to quantify sympathetic responsiveness. The purpose of the present study was to examine the intratest validity and reliability of MSNA sampled over 2 and 1 min and 30 and 15 s epoch durations. We retrospectively analyzed 68 resting fibular nerve microneurographic recordings obtained from 53 young, healthy participants (37 men; 23 ± 6 yr of age). From a stable 7-min resting baseline, MSNA (burst frequency and incidence, normalized mean burst amplitude, total burst area) was compared among each epoch duration and a standard 5-min control. Bland-Altman plots were used to determine agreement and bias. Three sequential MSNA measurements were collected using each sampling duration to calculate absolute and relative reliability (coefficients of variation and intraclass correlation coefficients). MSNA values were similar among each sampling duration and the 5-min control (all P > 0.05), highly correlated (r = 0.69-0.93; all P < 0.001), and demonstrated no evidence of fixed bias (all P > 0.05). A consistent proportional bias (P < 0.05) was present for MSNA burst frequency (all sampling durations) and incidence (1 min and 30 and 15 s), such that participants with low and high average MSNA underestimated and overestimated the true value, respectively. Reliability decreased progressively using the 30- and 15-s sampling durations. In conclusion, short 2 and 1 min and 30 s sampling durations can provide valid and reliable measures of MSNA, although increased sample size may be required for epochs ≤30 s, due to poorer reliability.

Muscle sympathetic nerve responses to passive and active one-legged cycling: insights into the contributions of central command

The contribution of central command to the peripheral vasoconstrictor response during exercise has been investigated using primarily handgrip exercise. The purpose of the present study was to compare muscle sympathetic nerve activity (MSNA) responses during passive (involuntary) and active (voluntary) zero-load cycling to gain insights into the effects of central command on sympathetic outflow during dynamic exercise. Hemodynamic measurements and contralateral leg MSNA (microneurography) data were collected in 18 young healthy participants at rest and during 2 min of passive and active zero-load one-legged cycling. Arterial baroreflex control of MSNA burst occurrence and burst area were calculated separately in the time domain. Blood pressure and stroke volume increased during exercise ( P < 0.0001) but were not different between passive and active cycling ( P > 0.05). In contrast, heart rate, cardiac output, and total vascular conductance were greater during the first and second minute of active cycling ( P < 0.001). MSNA burst frequency and incidence decreased during passive and active cycling ( P < 0.0001), but no differences were detected between exercise modes ( P > 0.05). Reductions in total MSNA were attenuated during the first ( P < 0.0001) and second ( P = 0.0004) minute of active compared with passive cycling, in concert with increased MSNA burst amplitude ( P = 0.02 and P = 0.005, respectively). The sensitivity of arterial baroreflex control of MSNA burst occurrence was lower during active than passive cycling ( P = 0.01), while control of MSNA burst strength was unchanged ( P > 0.05). These results suggest that central feedforward mechanisms are involved primarily in modulating the strength, but not the occurrence, of a sympathetic burst during low-intensity dynamic leg exercise. NEW & NOTEWORTHY Muscle sympathetic nerve activity burst frequency decreased equally during passive and active cycling, but reductions in total muscle sympathetic nerve activity were attenuated during active cycling. These results suggest that central command primarily regulates the strength, not the occurrence, of a muscle sympathetic burst during low-intensity dynamic leg exercise.

Ischemic preconditioning does not alter muscle sympathetic responses to static handgrip and metaboreflex activation in young healthy men

Ischemic preconditioning (IPC) has been hypothesized to elicit ergogenic effects by reducing feedback from metabolically sensitive group III/IV muscle afferents during exercise. If so, reflex efferent neural outflow should be attenuated. We investigated the effects of IPC on muscle sympathetic nerve activity (MSNA) during static handgrip (SHG) and used post‐exercise circulatory occlusion (PECO) to isolate for the muscle metaboreflex. Thirty‐seven healthy men (age: 24 ± 5 years [mean ± SD]) were randomized to receive sham (n = 16) or IPC (n = 21) interventions. Blood pressure, heart rate, and MSNA (microneurography; sham n = 11 and IPC n = 18) were collected at rest and during 2 min of SHG (30% maximal voluntary contraction) and 3 min of PECO before (PRE) and after (POST) sham or IPC treatment (3 × 5 min 20 mmHg or 200 mmHg unilateral upper arm cuff inflation). Resting mean arterial pressure was higher following sham (79 ± 7 vs. 83 ± 6 mmHg, P < 0.01) but not IPC (81 ± 6 vs. 82 ± 6 mmHg, P > 0.05), while resting MSNA burst frequency was unchanged (P > 0.05) with sham (18 ± 7 vs. 19 ± 9 bursts/min) or IPC (17 ± 7 vs. 19 ± 7 bursts/min). Mean arterial pressure, heart rate, stroke volume, cardiac output, and total vascular conductance responses during SHG and PECO were comparable PRE and POST following sham and IPC (All P > 0.05). Similarly, MSNA burst frequency, burst incidence, and total MSNA responses during SHG and PECO were comparable PRE and POST with sham and IPC (All P > 0.05). These findings demonstrate that IPC does not reduce hemodynamic responses or central sympathetic outflow directed toward the skeletal muscle during activation of the muscle metaboreflex using static exercise or subsequent PECO.

TRPV1 and BDKRB2 receptor polymorphisms can influence the exercise pressor reflex

The mechanisms responsible for the high inter‐individual variability in blood pressure responses to exercise remain unclear. Common genetic variants of genes related to the vascular transduction of sympathetic outflow have been investigated, but variants influencing skeletal muscle afferent feedback during exercise have not been explored. Single nucleotide polymorphisms in TRPV1 rs222747 and BDKRB2 rs1799722 receptors present in skeletal muscle were associated with differences in the magnitude of the blood pressure response to static handgrip exercise but not mental stress. The combined effects of TRPV1 rs222747 and BDKRB2 rs1799722 on blood pressure and heart rate responses during exercise were additive, and primarily found in men. Genetic differences in skeletal muscle metaboreceptors may be a risk factor for exaggerated blood pressure responses to exercise.

Interindividual variability in muscle sympathetic responses to static handgrip in young men: evidence for sympathetic responder types?

Negative and positive muscle sympathetic nerve activity (MSNA) responders have been observed during mental stress. We hypothesized that similar MSNA response patterns could be identified during the first minute of static handgrip and contribute to the interindividual variability throughout exercise. Supine measurements of multiunit MSNA (microneurography) and continuous blood pressure (Finometer) were recorded in 29 young healthy men during the first (HG1) and second (HG2) minute of static handgrip (30% maximal voluntary contraction) and subsequent postexercise circulatory occlusion (PECO). Responders were identified on the basis of differences from the typical error of baseline total MSNA: 7 negative, 12 positive, and 10 nonresponse patterns. Positive responders demonstrated larger total MSNA responses during HG1 ( P < 0.01) and HG2 ( P < 0.0001); however, the increases in blood pressure throughout handgrip exercise were similar between all groups, as were the changes in heart rate, stroke volume, cardiac output, total vascular conductance, and respiration (all P > 0.05). Comparing negative and positive responders, total MSNA responses were similar during PECO ( P = 0.17) but opposite from HG2 to PECO (∆40 ± 46 vs. ∆-21 ± 62%, P = 0.04). Negative responders also had a shorter time-to-peak diastolic blood pressure during HG1 (20 ± 20 vs. 44 ± 14 s, P < 0.001). Total MSNA responses during HG1 were associated with responses to PECO ( r = 0.39, P < 0.05), the change from HG2 to PECO ( r = -0.49, P < 0.01), and diastolic blood pressure time to peak ( r = 0.50, P < 0.01). Overall, MSNA response patterns during the first minute of static handgrip contribute to interindividual variability and appear to be influenced by differences in central command, muscle metaboreflex activation, and rate of loading of the arterial baroreflex.

Muscle sympathetic outflow during exercise: a tale of two limbs

Central sympathetic outflow to different skeletal muscle vascular beds (muscle sympathetic nerve activity; MSNA) during exercise is believed to be largely uniform (Ray et al. 1992), leaving the redistribution of blood flow to be regulated by local metabolic factors, including the disruption of neurally mediated vasoconstriction (i.e. functional sympatholysis). However, the assessment of MSNA in humans using microneurographic techniques is limited by the requirement to minimize limb movement and muscle activation. As a result, the majority of prior work has measured MSNA from an inactive limb (e.g. Ray et al. 1992). More recently, advancements in microneurographic signal denoising and sympathetic action potential identification have permitted the examination of the MSNA response in the active limb during exercise (Boulton et al. 2014). These results now provide the capacity to investigate whether sympathetic outflow can be controlled differentially between active and non-active limbs. In a recent publication in The Journal of Physiology, Boulton and colleagues (2018) have answered this question, studying the MSNA responses during low-intensity ipsilateral and contralateral isometric dorsiflexion of the foot. Contractions were performed unilaterally for 4 min at 10% of maximal volitional effort in three randomized conditions: (1) freely perfused contraction and recovery (termed no ischaemia), (2) freely perfused contraction and circulatory occluded recovery (termed post-exercise ischaemia) and (3) circulatory occluded contraction and recovery (termed continuous ischaemia). Circulatory occlusions were achieved by pneumatic pressure cuff inflation around the upper thigh. MSNA was assessed by identification of negative deflections from the raw neurogram, indicative of sympathetic action potentials, and quantified as spike frequency (spikes min−1). This method differs from conventional burst analysis of the integrated neurogram but is advantageous when recording from the active limb as it allows discrimination of positive deflecting neural activity related to motor activation or muscle spindle and Golgi tendon organ afferent feedback, all of which are active during contraction. The results demonstrated that MSNA to the non-active leg increased progressively throughout the contraction, becoming significantly elevated from baseline after 2 min, and remained elevated during ischaemia, while MSNA in the active leg increased rapidly (within the first minute of contraction), plateaued during contraction and returned to baseline immediately following contraction, even during ischaemia. As a result, the authors concluded that central command (input from higher order brain regions) was responsible for control of MSNA to the contracting muscles, while activation of metabolically sensitive afferents (muscle metaboreflex) was responsible for MSNA responses in the non-contracting leg. These results provide strong evidence for the existence of limb-specific differential control of sympathetic outflow and bring forth novel hypotheses for skeletal muscle blood flow regulation and the integrated control of the sympathetic nervous system during exercise. The strongest evidence that MSNA to the active limb is influenced predominantly by central command was the rapid increase from baseline in the active limb. Noteworthy, however, was the MSNA activation in the non-active limb after the first minute of non-ischaemic exercise, as well as an accentuated response during ischaemic exercise, indicating chemosensitive group III/IV afferent firing in all conditions (though no measurable onset of fatigue given consistent electromyographic recordings). Group III/IV afferent activation has been shown to elicit an inhibitory influence on central motor output (Sidhu et al. 2017), requiring central command to progressively increase over time during non-ischaemic exercise and during ischaemic exercise to maintain consistent force output. The observations of a plateau in MSNA after the first minute of contraction in the active limb and the similarities in MSNA magnitude between non-ischaemic and ischaemic exercise do not parallel this expected difference. Perhaps this was related to the use of low-intensity static dorsiflexion. Whether similar responses are present at higher intensities under stronger muscle metaboreflex activation or during other modes of exercise is unclear. For example, a change in heart rate would indicate a vagal withdrawal (a hallmark of central command), yet this was absent during the exercise protocol. How these results compare to other modes of exercise is an important issue. Static leg extension at 10% MVC was observed to elicit a reduction in MSNA in the non-active limb in the first minute of exercise and no change from baseline in the second minute of exercise (Ray et al. 1992). The progressive increase in non-active limb MSNA in the present study, however, more closely resembles the MSNA responses observed during static handgrip exercise (Ray et al. 1992). Additionally, our laboratory recently compared passive versus unloaded (minimal metaboreflex activation) one-legged cycling to probe the contributions of central command and demonstrated increases in multi-unit MSNA burst amplitude, but not occurrence, in the non-active leg during the first and second minute in the dynamic exercise condition (Doherty et al. 2017). Given that the authors established in a previous investigation that their measure of MSNA using spike frequency was most closely representative of MSNA burst amplitude, as opposed to burst frequency or total MSNA, the absence of a rapid increase in MSNA in the non-active limb during a static contraction is in contrast to our findings. Whether this reflects differences in the sympathetic response to static and dynamic exercise or muscle mass recruited is unclear. The most important consideration of the current findings is that the assumption of MSNA measured from the non-active limb during exercise as reflective of active limb sympathetic outflow can no longer be accepted. An emphasis should now be placed on describing the contribution of