Spontaneous bursts of muscle sympathetic nerve activity decrease leg vascular conductance in resting humans

Abstract

Previous studies in humans attempting to assess sympathetic vascular transduction have related large reflex-mediated increases in muscle sympathetic nerve activity (MSNA) to associated changes in limb vascular resistance. However, such procedures do not provide insight into the ability of MSNA to dynamically control vascular tone on a beat-by-beat basis. Thus we examined the influence of spontaneous MSNA bursts on leg vascular conductance (LVC) and how variations in MSNA burst pattern (single vs. multiple bursts) and burst size may affect the magnitude of the LVC response. In 11 young men, arterial blood pressure, common femoral artery blood flow, and MSNA were continuously recorded during 20 min of supine rest. Signal averaging was used to characterize percent changes in LVC for 15 cardiac cycles following heartbeats associated with and without MSNA bursts. LVC significantly decreased following MSNA bursts, reaching a nadir during the 6th cardiac cycle (single bursts, −2.9 ± 1.1%; and multiple bursts, −11.0 ± 1.4%; both, P < 0.001). Individual MSNA burst amplitudes and the total amplitude of consecutive bursts were related to the magnitude of peak decreases in LVC. In contrast, cardiac cycles without MSNA bursts were associated with a significant increase in LVC (+3.1 ± 0.5%; P < 0.001). Total vascular conductance decreased in parallel with LVC also reaching a nadir around the peak rise in arterial blood pressure following an MSNA burst. Collectively, these data are the first to assess beat-by-beat sympathetic vascular transduction in resting humans, demonstrating robust and dynamic decreases in LVC following MSNA bursts, an effect that was absent for cardiac cycles without MSNA bursts.

the generation and regulation of central sympathetic outflow has been a primary focus of autonomic cardiovascular research for decades (5, 10). However, the ability of the peripheral vasculature to respond to changes in sympathetic activity has received less attention. Indeed, for sympathetic nerve impulses to influence vascular tone, there must be successful release of neurotransmitter(s) into the synaptic cleft (2), abundant receptor binding (35), and widespread signal transduction within vascular smooth muscle (34) to result in a vasomotor response (29). Although this paradigm is widely accepted (3, 12, 26), limited data are available in humans. Furthermore, previous studies attempting to demonstrate sympathetic vascular transduction in humans have related large reflex-mediated increases in muscle sympathetic nerve activity (MSNA) to associated changes in vascular resistance (6, 17, 23). While such procedures reveal that elevated MSNA increases vascular resistance, they do not provide insight into the ability of MSNA to dynamically control vascular tone on a beat-by-beat basis, nor do they represent sympathetic vascular transduction under normal resting conditions.

An important, but rarely studied, feature of human autonomic physiology is the substantial variability present even under resting conditions. In this regard, the bursting pattern and size of individual MSNA bursts is quite variable within and among subjects (15, 27). We speculate that these irregular variations in MSNA may be important for increasing the efficacy of vascular responses. Indeed, random patterns of sympathetic nerve stimulation have been shown to elicit greater individual contractile responses in small rat arteries than regularly delivered stimuli (18). Moreover, animal preparations have consistently demonstrated that elevations in the frequency of sympathetic nerve stimulation (analogous to MSNA burst size) augments the magnitude of vasoconstriction (16, 25, 30). Although such variability is known to influence vasoconstrictor responsiveness in animal blood vessel preparations, whether, and how the spontaneous variability of MSNA influences sympathetic vascular transduction in humans have not been tested.

With this background in mind, we sought to determine the ability of spontaneous bursts of MSNA to produce beat-by-beat changes in leg vascular conductance (LVC) in resting humans. Accordingly, we used a spike-triggered averaging approach similar to that used by Wallin and Nerhed when they demonstrated for the first time that arterial blood pressure (BP) transiently increased following MSNA bursts (33). Our laboratory recently extended this work by showing that the BP responses following MSNA bursts are primarily mediated by decreases in total vascular conductance (TVC) and that the magnitude of the pressor response is positively related to the size of MSNA bursts (31). In the present study, we determined to what degree the skeletal muscle vasculature contributes to TVC decreases and arterial BP rises following spontaneous MSNA bursts. We hypothesized that 1) LVC would transiently decrease following MSNA bursts, whereas cardiac cycles lacking MSNA would exhibit minimal or no changes and 2) the size of individual bursts and number of consecutive MSNA bursts would be positively associated with a greater decrease in LVC and subsequent rise in arterial BP.

METHODS

Eleven healthy men with a mean age of 25 ± 1 yr, height of 176 ± 2 cm, and weight of 79 ± 2 kg (means ± SE) were recruited for voluntary participation in this study. All subjects were normotensive (resting BP < 140/80 mmHg), of normal stature (body mass index < 30 kg/m²), and nonsmokers and were free of any known neurological, pulmonary, metabolic, or cardiovascular disease. All subjects received a verbal and written explanation of the measurement procedures and study objectives. After acknowledging the risks and benefits associated with the investigation, each subject provided written informed consent before participation. The University of Missouri Health Sciences Institutional Review Board approved all procedures. On the experimental day, subjects were instructed to report to the laboratory ≥3 h following a meal and abstaining from alcohol, caffeine, and strenuous physical activity for the preceding 12 h.

Experimental measurements.

Heart rate was monitored using a lead II electrocardiogram (ECG; Quinton Q710, Bothell, WA). Beat-by-beat changes in arterial BP were measured noninvasively by a servo-controlled finger photoplethysmograph (Finometer; Finapres Medical Systems, Amsterdam, The Netherlands) placed on the middle finger of the left hand. In addition, an automated sphygmomanometer (Welch Allyn, Skaneatles Falls, NY) provided absolute brachial artery BP values to verify absolute Finometer readings. Respiratory movements were monitored using a strain-gauge pneumograph placed in a stable position over the abdomen (Pneumotrace; UFI, Morro Bay, CA).

Multiunit postganglionic MSNA was recorded using standard microneurographic techniques, as previously described in detail (21, 28, 36). Briefly, a unipolar tungsten microelectrode was inserted into muscle fascicles of the peroneal nerve near the fibular head of the left leg. Neural signals were amplified, filtered (bandwidth, 700–2,000 Hz), rectified, and integrated (0.1 s time constant) to obtain mean voltage neurograms. MSNA recordings were identified by the presence of spontaneously occurring bursts with characteristic pulse synchronicity, responsiveness to an end-expiratory breath hold, and lack of response to arousal or skin stimulation. Time averaged values for MSNA were quantified over the entire recording period as burst frequency (in bursts/min) and burst incidence (in bursts/100 cardiac cycles).

Common femoral artery (CFA) blood flow was obtained in the right leg using a duplex Doppler ultrasound (Logiq 7, GE Medical Systems, Milwaukee, WI), equipped with a linear array transducer operating at a frequency of 12 MHz. Blood velocity was simultaneously obtained with diameter in pulsed-wave mode at an insonation angle of 60° and operating at a linear frequency of 5 MHz. The CFA was imaged 2 to 3 cm proximal to the bifurcation into the superficial and deep femoral arteries. The transducer was stabilized with a custom-designed clamp, which was confirmed by its maintained proximity to markings on the skin.

Experimental protocol.

Subjects were comfortably positioned supine in a quiet, dimly lit, climate-controlled room (23 ± 1°C). Each subject was instrumented for the simultaneous measurements of heart rate, BP, respiration, MSNA, and CFA blood flow. Before data collection, signals were acquired for 10 min to verify stability of the MSNA recording. Subsequently, all variables were continuously recorded for 20 min while the subject lay resting quietly and awake. Pilot experiments indicated that this recording duration provided an adequate representation of resting MSNA variability.

Data analysis.

All experimental measurements were acquired into a custom LabVIEW program interfaced with video output of the Doppler ultrasound machine as previously described (20, 24). Briefly, the ECG, BP, and MSNA signals were sampled at 1 kHz and embedded as data streams into an AVI file containing video images output from the ultrasound at an effective sampling rate of 30 Hz. Offline analysis involving edge detection protocols and threshold algorithms determined the diameter (in cm) and weighted mean velocity (V_mean; in cm/s) from the captured video output. These data were processed using a second custom LabVIEW program, which generated synchronized beat-by-beat data of all recorded variables gated by the R wave of the ECG. Stroke volume was estimated using Modelflow software (7) and was aligned with the LabVIEW program output via changes in cardiac interval time. The product of stroke volume and heart rate was used to estimate cardiac output (CO). TVC was calculated as CO divided by mean arterial pressure (MAP). CFA blood flow (in ml/min) was calculated using the following equation: V_mean (in cm/s)·π·[mean CFA diameter (in cm) ÷ 2]²·60 s/min. MAP was calculated as the integral of the arterial BP waveform. LVC was calculated as CFA blood flow divided by MAP.

Spike-triggered averaging was used to characterize beat-by-beat sympathetic vascular transduction and determine the systematic influence of MSNA bursts on LVC. To do this, LVC was calculated for each heartbeat containing a MSNA burst and for the following 15 heartbeats. Percent changes in LVC following all MSNA bursts were averaged for each subject, and a group mean was determined. The peak or nadir of this mean response was used to provide an estimate of the overall vascular response, whereas the entire 15-heartbeat period was characterized beat by beat to provide a more detailed evaluation of the time course and magnitude of the responses. Because spontaneous MSNA bursts may either occur in isolation or in sequence with other MSNA bursts, we examined the effect of burst patterning on the magnitude of the LVC responses. All MSNA bursts were segregated as either single bursts (directly bordered by ≥1 heartbeat lacking MSNA) or multiple bursts (any burst positioned directly adjacent to another MSNA burst). The percent change in LVC was then calculated following every burst belonging to a given pattern and an average response was determined for each pattern. Single bursts and multiple bursts represented 39 ± 4 and 61 ± 4% of all MSNA bursts, respectively. We also determined the impact of normalized burst size on LVC response magnitude. For this analysis, the heights of all MSNA bursts were first expressed as a percentage of the three largest bursts during the 20-min recording, which were assigned a value of 100 (in arbitrary units). Bursts were then ranked and divided equally into four quartiles (Q1–Q4; smallest to largest), independent of burst pattern. The average percent change in LVC following all MSNA bursts pertaining to a quartile was calculated for each subject, and the group mean was determined.

To examine the combined effect of MSNA burst pattern and size, MSNA bursts were segregated into clusters of uninterrupted sympathetic activity. A burst cluster was defined as either a single burst or a consecutive series of bursts separated on each side by ≥1 cardiac cycle without MSNA. The number of bursts contained within each cluster was used to produce four groups: 1) single burst clusters, 2) couplet burst clusters, 3) triplet burst clusters, and 4) quadruplet burst clusters (≥4 consecutive bursts). Clusters containing four or five bursts represented 80 ± 9% of all quadruplet groupings, whereas greater consecutive burst numbers occurred exponentially less, indicating that we effectively encompassed the upper limit of burst patterns in these young healthy men. For each cluster group, the sum of burst heights contained within the cluster was determined and ranked into four quartiles (Q1–Q4). Thus, for this analysis, 16 MSNA burst cluster categories were designated (4 cluster groups × 4 quartiles each). Because each cluster of bursts was considered a single event, percent changes in LVC were calculated following only the first burst of the cluster (not the average change following each burst of the cluster).

We used two types of controls to assess the specificity of LVC responses to MSNA bursts. First, percent changes in LVC were determined for the 15 cardiac cycles following each heartbeat without an MSNA burst (i.e., nonbursts). Second, a white-noise control in which randomly selected cardiac cycles were followed for 15 heartbeats was performed to remove any systematic relationship with MSNA occurrence. The number of cardiac cycles selected for the white noise control was equal to each subject's MSNA burst count, keeping the number of observations consistent within a subject. To compare the influence of MSNA bursts on LVC with that of systemic cardiovascular variables, these analyses were also used to calculate changes in TVC, CO, and MAP following MSNA bursts, as described in detail for LVC.

Statistics.

Statistical analyses were performed using the Sigmastat (version 3.0) statistical package. Comparisons between burst pattern, size, and cluster amplitude were considered using two-way repeated-measures ANOVA to test for differences between cardiac cycles, between conditions, and for an interaction between conditions and cardiac cycles. A Pearson correlation was used to assess the relationship between the nadir changes in LVC and total amplitude of each MSNA burst cluster. Post hoc differences were determined using the Student-Newman-Keuls test. Significance was set at P < 0.05, and data are expressed as means ± SE.

RESULTS

Time-averaged values for cardiovascular measurements, MSNA, and CFA blood flow are presented in Table 1.

Table 1.

Resting time-averaged neural cardiovascular variables

Cardiovascular and neural measurements
SBP, mmHg	124 ± 3
DBP, mmHg	70 ± 2
MAP, mmHg	90 ± 2
HR, beats/min	61 ± 2
CO, l/min	7.0 ± 0.4
TVC, ml·min−1·mmHg−1	78 ± 5
Burst frequency, bursts/min	17 ± 2
Burst incidence, bursts/100 cardiac cycles	29 ± 3
Common femoral artery measurements
Mean diameter, cm	0.94 ± 0.03
Mean velocity, cm/s	7.4 ± 0.9
Mean flow, ml/min	300 ± 34
Mean conductance, ml·min−1·mmHg−1	3.4 ± 0.4

Values are means ± SE.

SBP, systolic blood pressure; DBP, diastolic blood pressure; MAP, mean arterial blood pressure; HR, heart rate; CO, cardiac output; TVC, total vascular conductance.

Effect of MSNA burst pattern on LVC.

Beat-by-beat changes in LVC following various MSNA burst patterns are shown in Fig. 1. LVC significantly decreased following all MSNA bursts, changing from 3.45 ± 0.39 to a subsequent nadir of 3.19 ± 0.36 ml·min⁻¹·mmHg⁻¹, which represented a −7.7 ± 1.1% reduction (P < 0.001). The magnitude of the response was graded in that when all MSNA bursts were segregated into burst patterns, single bursts evoked a slight decrease in LVC (−2.9 ± 1.1%; P < 0.001), whereas multiple bursts evoked more robust reductions in LVC (−11.0 ± 1.4%; P < 0.001). In contrast to heartbeats with MSNA bursts, cardiac cycles lacking MSNA (nonbursts) were followed by a significant increase in LVC, changing from 3.36 ± 0.38 to a subsequent peak of 3.46 ± 0.38 ml·min⁻¹·mmHg⁻¹, which represented a +3.1 ± 0.5% increase (P < 0.001). Importantly, for the white noise control, LVC demonstrated only small, inconsistent changes following these randomly selected cardiac cycles resulting in no appreciable changes in LVC (P > 0.05).

Fig. 1.

Summary data showing beat-by-beat percent changes in leg vascular conductance (LVC) following spontaneous muscle sympathetic nerve activity (MSNA) bursts. A: changes in LVC following all bursts, nonbursts, and white noise. B: changes following single and multiple MSNA bursts. Brackets denote significant difference from percent changes in white noise. Values are means ± SE.

Effect of MSNA burst size on LVC.

Beat-by-beat changes in LVC following MSNA bursts of varying amplitude are shown in Fig. 2. In general, the changes in LVC were graded to the height of the MSNA burst. In this regard, the smallest 25% of MSNA burst heights (Q1) were followed by a significant decrease in LVC, reaching a nadir of −3.5 ± 1.4% (P < 0.05). The nadir change in LVC following Q2 MSNA bursts was greater in magnitude at −6.2 ± 1.1%; however, this change was not statistically different from that of Q1. LVC following Q3 MSNA bursts decreased to a significantly greater magnitude than that of Q1 and Q2 bursts, reaching a nadir of −9.8 ± 1.2% (P < 0.001). Finally, MSNA bursts in the largest quartile (Q4) were followed by a reduction in LVC with a nadir of −12.4 ± 1.8% (P < 0.001), representing the greatest magnitude of change.

Fig. 2.

Summary data showing beat-by-beat percent changes in LVC following MSNA bursts of varying height grouped into quartiles from smallest to largest (Q1–Q4). Brackets denote significant difference from percent changes in white noise. Values are means ± SE.

Effect of MSNA cluster size on LVC.

To account for variations in MSNA burst pattern in combination with burst size, MSNA bursts were evaluated as clusters of uninterrupted MSNA. The beat-by-beat changes in LVC following MSNA burst clusters of various sizes are shown in Fig. 3. These clusters were divided into four groups, segregated by their number of consecutive bursts [singlets (Fig. 3A), couplets (Fig. 3B), triplets (Fig. 3C), and quadruplets (Fig. 3D)]. For singlet burst clusters, the smallest 75% of clusters (Q1–Q3) were not associated with significant decreases in LVC (P > 0.05). However, the largest quartile of singlet burst clusters (Q4) significantly decreased LVC to a nadir of −9.3 ± 2.0% (P < 0.001). LVC did not significantly decrease following the smallest quartile (Q1) of couplet burst clusters. However, the largest 75% of couplet MSNA burst clusters (Q2–Q4) were followed by significant LVC decreases progressively reaching greater magnitudes in the nadir changes (Q2, −11.2 ± 2.7%; Q3, −11.6 ± 1.8%; and Q4, −15.8 ± 3.0%; all, P < 0.05). In the consideration of triplet and quadruplet burst clusters, a more pronounced biphasic change in LVC emerged as the length of a cluster increased beyond two bursts wherein LVC increased before a robust and sustained decrease was observed. This biphasic response likely resulted from examining LVC changes from the initiating burst of a cluster (i.e., LVC typically continued to increase until the final bursts of the cluster). Also, there was a lower occurrence of triplet and quadruplet burst clusters during the 20-min recording periods in these young healthy subjects (Table 2), warranting some caution in interpreting these results. Indeed, statistical significance was not observed for the decreases in LVC following the smallest 75% (Q1–Q3) of these longer burst clusters. Nevertheless, the largest quartile (Q4) of both triplet and quadruplet burst clusters exhibited significant and robust decreases in LVC, reaching a nadir of −16.4 ± 3.2% for triplets and −15.8 ± 5.2% for quadruplets (both, P < 0.001). A strong negative relationship resulted between the nadir decreases in LVC and the total amplitude of each of the 16 MSNA burst clusters (r = 0.81, P < 0.001; Fig. 4).

Fig. 3.

Summary data showing beat-by-beat percent changes in LVC following MSNA burst clusters grouped into singlets (A), couplets (B), triplets (C), and quadruplets (D). Brackets denote significant difference from percent changes in white noise. Values are means ± SE.

Table 2.

MSNA burst distribution during 20-min recording period

	Count (Range)	Percentage, %
All bursts	320 ± 41 (122–530)	100 ± 0
Single bursts	116 ± 13 (58–181)	39 ± 4
Multiple bursts	204 ± 32 (56–366)	61 ± 4
Burst clusters
Singlet	116 ± 13 (58–181)	60 ± 3
Couplet	58 ± 8 (17–86)	28 ± 2
Triplet	17 ± 3 (6–42)	9 ± 1
Quadruplet	7 ± 2 (0–21)	3 ± 1

Values are means ± SE.

Fig. 4.

Summary data relating the nadir percent decrease in LVC and the total amplitude of each MSNA burst cluster. Color denotes the cluster grouping (single, red; couplet, green; triplet, yellow; and quadruplet, blue), whereas symbols differentiate the cluster quartile (Q1–Q4) of each cluster group. Brackets denote significant difference from white noise percent changes. Values are means ± SE. AU, arbitrary unit.

Effect of MSNA bursts on systemic cardiovascular variables.

Beat-by-beat changes in CO, TVC, and MAP following all MSNA bursts are presented in Fig. 5. CO exhibited a rapid and significant increase following all MSNA bursts reaching a peak of +1.6 ± 0.5% (P <0.001) during the second cardiac cycle. In contrast, a slight but significant decrease in CO (−0.6 ± 0.2%; P < 0.001) was observed following cardiac cycles without a MSNA burst (i.e., nonbursts). These changes in CO were transient, returning to baseline by the fourth heartbeat. Following all MSNA bursts, TVC significantly decreased to a nadir of −2.8 ± 0.3% (P < 0.001), whereas an increase in TVC was observed for nonbursts (+1.1 ± 0.2%; P < 0.001). For MAP, a significant increase was observed following all MSNA bursts, reaching a peak of +3.2 ± 0.5% (P < 0.001). In contrast, a significant decrease in MAP followed cardiac cycles without a MSNA burst (−1.1 ± 0.1%; P < 0.001). Importantly, the nadir of TVC and the peak of MAP occurred during the seventh cardiac cycle, one heartbeat following the LVC nadir (see Fig. 1). As observed with LVC, no significant changes in CO, TVC or MAP were observed following randomly selected heartbeats for the white noise control.

Fig. 5.

Summary data showing beat-by-beat percent changes in cardiac output (top), total vascular conductance (middle), and mean arterial pressure (bottom) following all MSNA bursts, nonbursts, and white noise. Asterisks and brackets denote significant difference from white noise percent changes. Values are means ± SE.

DISCUSSION

The major novel finding of the current study is that individual, spontaneously occurring MSNA bursts produce transient decreases in skeletal muscle vascular conductance. This represents the first evidence in humans of beat-by-beat sympathetic vascular transduction resulting from unprovoked MSNA and demonstrates robust and dynamic ensuing vascular responses. Furthermore, natural variations in the amount of spontaneous sympathetic activity (i.e., pattern, size, clustering) exhibited a strong influence on the magnitude of sympathetic vascular transduction. Interestingly, LVC increased following cardiac cycles without MSNA bursts, which highlights the importance of MSNA bursts to maintain vascular tone. Lastly, we found that LVC changes occurred in temporal synchronicity with changes in TVC and MAP, indicating that beat-by-beat sympathetic vascular transduction in skeletal muscle plays an important role in the regulation of resting systemic cardiovascular hemodynamics.

Previous studies assessing sympathetic vascular transduction in humans have characterized the relationship between large reflex-mediated increases in MSNA and mean vascular resistance quantified over periods ranging from 30 s to 2 min (17, 23, 33). We reasoned that sympathetic transduction is a dynamic process and that time-averaging approaches are not capable of providing information about how MSNA controls vascular tone on a beat-by-beat basis. In this regard, Wallin and Nerhed (33) identified clear pressor responses following spontaneous MSNA bursts using a spike-triggered averaging approach to assess beat-by-beat changes in diastolic BP following individual MSNA bursts. We recently extended these findings to describe TVC decreases associated with the BP rise following MSNA bursts as well as clear blunting of these responses with advancing age (31). In the present study, we adapted the spike-triggered averaging method to include simultaneously acquired beat-by-beat femoral vascular conductance measures. Indeed, we provide the first beat-by-beat sympathetic vascular transduction measures in humans, demonstrating robust and dynamic decreases in LVC following MSNA bursts. Importantly, these measures are characterized under unprovoked, closed-loop conditions, providing information on the normal effect of sympathetic bursts on the skeletal muscle vasculature at rest. These findings extend the results of Vissing et al. (32), wherein 15-s epochs of spontaneous MSNA were strongly related to calf vascular resistance measured every 15 s with venous occlusion plethysmography. Indeed, we now demonstrate a clear influence of individual MSNA bursts on vascular conductance, revealing a transient and maximal decrease in LVC after approximately 6 heartbeats. Also, by having beat-by-beat data, we were able to comprehensively examine the effects of variations in MSNA burst patterns and sizes on LVC responses.

In regard to burst patterning, the magnitude of decrease in LVC following MSNA bursts was graded in that single MSNA bursts evoked a slight decrease, whereas multiple bursts evoked more robust reductions in LVC. Such a grading effect in LVC is likely due, in part, to additional consecutive MSNA bursts releasing greater amounts of norepinephrine for adrenergic receptor binding. This notion is supported by studies demonstrating that regional norepinephrine spillover is correlated with resting MSNA burst frequency (8, 9). Furthermore, increases in MSNA with lower body negative pressure are strongly related to elevations in interstitial norepinephrine within skeletal muscle (14). We also found that the changes in LVC following MSNA bursts were graded to the amplitude (i.e., height) of each burst. In this regard, sympathetic burst amplitude has been shown to be reflective of the number of activated postganglionic sympathetic fibers (19). Thus larger MSNA burst heights likely result in a larger portion of each peroneal nerve relaying action potentials and consequently releasing greater overall norepinephrine. Our burst cluster analyses accounted for variations in both pattern and height by summing the amplitude of uninterrupted MSNA bursts and also revealed a clear graded reduction in LVC in association with the total amount of MSNA. These graded and robust decreases in LVC are likely due to the combination of greater recruitment of sympathetic fibers and more norepinephrine release at sympathetic nerve terminals. Interestingly, when clusters were made up of similar total amplitude but a different number of bursts, the clusters composed of fewer MSNA bursts and larger burst heights were the most effective in decreasing LVC. This suggests that MSNA burst height may be more effective than burst pattern in evoking leg vasoconstriction. These findings warrant further investigation.

Of interest, the nadir of LVC responses following MSNA bursts occurred in the heartbeat just before the nadir of the decrease in TVC and the peak of the MAP response, indicating that sympathetic-induced oscillations in skeletal muscle blood flow influence systemic cardiovascular variables at rest. Indeed, this association suggests that MSNA and the magnitude of the ensuing sympathetic vascular transduction importantly contribute to the support of resting BP. In agreement, a nearly identical time line has been found for increases in MSNA and subsequent alterations in vascular tone and BP during carotid baroreceptor perturbation (13, 22). Importantly, we also found that cardiac cycles without MSNA bursts (nonbursts) are followed by a subsequent increase in LVC and decrease in BP. These findings further highlight the transient nature of MSNA's effects. Together, our findings indicate both a dynamic influence of sympathetic vascular transduction and a tonic requirement for MSNA bursts to maintain systemic vascular tone and BP. To validate that these fluctuations were exclusively associated with MSNA, randomly selected heartbeats without any relationship to MSNA bursts (white noise) were characterized in a similar manner and no systematic changes in any measured variable was found.

These findings have important implications for sympathetic control of the vasculature and its potential decline with advancing age and disease (1, 4). We demonstrated that changes in resting skeletal muscle vascular tone and BP are tightly coupled with spontaneous MSNA bursts. Furthermore, we provide clear evidence that the magnitude of sympathetically mediated changes in skeletal muscle blood flow depends on the quantity of sympathetic outflow (burst pattern and size), but perhaps even more so on how effectively the vasculature responds to this neural stimulation. Important for the context of health, individuals in our study who exhibited robust skeletal muscle vascular responses to neural stimulation also efficiently generated robust pressor responses (r = 0.753, P = 0.007). In this light, the ability to defend against acute changes in BP requires multiple facets that are somewhat independent of arterial baroreflex alterations. Indeed, we speculate that impairments in sympathetic vascular transduction will impair BP control in the face of normal central processing and function of the arterial baroreflex because of insufficient end organ responses, although central compensation by the arterial baroreflex is plausible. Nevertheless, the approach of spike-triggered averaging is well suited to study sympathetic regulation and BP control on a beat-to-beat timescale and may be able to determine the causal elements that contribute to impairments in BP control. Given the inherent between-subject variability in resting MSNA (11), this methodology may be helpful in identifying differential strategies by which subjects with high or low MSNA support BP via sympathetic vasculature transduction. Therefore, future studies are warranted to investigate populations having sympathetic overactivity compared with young healthy volunteers to provide mechanistic insight into sympathetic control and the regulation of BP.

In summary, spontaneous oscillations of individual MSNA bursts are associated with robust and dynamic beat-by-beat changes in skeletal muscle vascular conductance. This influence appeared to be effectively modified by variations in the amplitude of individual bursts, the number of consecutive bursts, and the total size of burst clusters (combination of pattern and size). Furthermore, changes in LVC, TVC, and MAP occur in parallel following spontaneous MSNA bursts, indicating that beat-by-beat sympathetic vascular transduction in skeletal muscle plays a key role in the regulation of systemic hemodynamics under resting conditions.

GRANTS

This work was supported by National Heart, Lung, and Blood Institute Grants RO1-HL-093167 (to P. J. Fadel) and P01-HL-095486 (to M. J. Davis) and American Heart Association Grant AHA11POST5080002 (to J. Padilla).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the author(s).

AUTHOR CONTRIBUTIONS

S.T.F., J.P., L.C.V., and P.J.F. conception and design of research; S.T.F., J.P., and P.J.F. performed experiments; S.T.F. analyzed data; S.T.F., L.C.V., M.J.D., and P.J.F. interpreted results of experiments; S.T.F. prepared figures; S.T.F. drafted manuscript; S.T.F., J.P., L.C.V., M.J.D., and P.J.F. edited and revised manuscript; S.T.F., J.P., L.C.V., M.J.D., and P.J.F. approved final version of manuscript.