Long-term variability and reproducibility of resting human muscle nerve sympathetic activity at rest, as reassessed after a decade

Inter-individual variability in muscle sympathetic nerve activity at rest and during exercise: Disconnection with blood pressure

Microneurographic recordings of muscle sympathetic nerve activity (MSNA) have provided fundamental insight into sympathetic discharge patterns at rest and during exercise in health and disease. A key feature of MSNA recordings at rest is a large inter-individual variability, even among healthy adults. The physiological consequences of inter-individual variability in MSNA are commonly discussed as being associated with the regulation of resting blood pressure. However, available evidence from large cross-sectional analyses demonstrate a near absence of an association between resting MSNA and blood pressure. Less appreciated, MSNA also exhibits inter-individual variability in response to stress, such as exercise. Again, the consequences of variability in MSNA are unclear and can be dissociated from the blood pressure response, particularly at low-to-moderate intensity muscle contractions for short durations (≤2 min). In this brief review, we summarize several examples of how inter-individual variability in MSNA is unrelated to blood pressure control at rest and during exercise and discuss potential mechanisms responsible for this observation, and key methodological considerations for future study design and interpretation. Additionally, we highlight several unanswered questions that could pave the way for future investigations in the field.

Binge Alcohol Consumption Elevates Sympathetic Transduction to Blood Pressure: A Randomized Controlled Trial

BACKGROUND

Alcohol consumption is associated with cardiovascular disease, and the sympathetic nervous system is a suspected mediator. The present study investigated sympathetic transduction of muscle sympathetic nerve activity to blood pressure at rest and in response to cold pressor test following evening binge alcohol or fluid control, with the hypothesis that sympathetic transduction would be elevated the morning after binge alcohol consumption.

METHODS

Using a randomized, fluid-controlled (FC) crossover design, 26 healthy adults (12 male, 14 female, 25±6 years, 27±4 kg/m2) received an evening binge alcohol dose and a FC. All participants underwent next-morning autonomic-cardiovascular testing consisting of muscle sympathetic nerve activity, beat-to-beat blood pressure, and heart rate during a 10-minute rest period and a 2-minute cold pressor test. Sympathetic transduction was assessed at rest and during the cold pressor test in both experimental conditions.

RESULTS

Evening alcohol increased heart rate (FC: 60±9 versus alcohol: 64±9 bpm; P=0.010) but did not alter resting mean arterial pressure (FC: 80±6 versus alcohol: 80±7 mm Hg; P=0.857) or muscle sympathetic nerve activity (FC: 18±9 versus alcohol: 20±8 bursts/min; P=0.283). Sympathetic transduction to mean arterial pressure (time×condition; P=0.003), diastolic blood pressure (time×condition; P=0.010), and total vascular conductance (time×condition; P=0.004) was augmented after alcohol at rest. Sympathetic transduction during the cold pressor test was also elevated after evening binge alcohol consumption (P=0.002).

CONCLUSIONS

These findings suggest that evening binge alcohol consumption leads to augmented morning-after sympathetic transduction of muscle sympathetic nerve activity to blood pressure, highlighting a new mechanism whereby chronic or excessive alcohol consumption contributes to cardiovascular disease progression via altered end-organ responsiveness to sympathetic neural outflow.

REGISTRATION

URL: https://clinicaltrials.gov/study/NCT03567434; Unique identifier: NCT03567434.

Sex differences in sympathetic transduction in black and white adults: implications for racial disparities in hypertension and cardiovascular disease risk

The prevalence of hypertension in non-Hispanic black (BL) individuals is the greatest of any racial/ethnic group. Whereas women generally display lower rates of hypertension than men of the same background, BL women display a similar if not greater burden of hypertension compared with BL men. The risk for cardiovascular disease and related events is also highest in BL individuals. Given the importance of the sympathetic nervous system for the regulation of the cardiovascular system, a growing body of literature has investigated sympathetic function in BL and non-Hispanic white (WH) individuals. Here, we are focused on emerging evidence indicating that sympathetic function may be altered in BL individuals, with particular emphasis on the process by which bursts of muscle sympathetic nerve activity (MSNA) are transduced into vasoconstriction and increases in blood pressure (sympathetic vascular transduction). To synthesize this growing body of literature we discuss sex and race differences in 1) sympathetic outflow, 2) sympathetic vascular transduction, and 3) adrenergic receptor sensitivity. Sex differences are discussed foremost, to set the stage for new data indicating a sex dimorphism in sympathetic regulation in BL individuals. Specifically, we highlight evidence for a potential neurogenic phenotype including greater adiposity-independent sympathetic outflow and enhanced sympathetic vascular transduction in BL men that is not observed in BL women. The implications of these findings for the greater hypertension and cardiovascular disease risk in BL adults are discussed along with areas that require further investigation.

Actigraphy-based sleep and muscle sympathetic nerve activity in humans

Short and insufficient sleep are prevalent and associated with cardiovascular disease, with the sympathetic nervous system as a suspected mediator. The purpose of the present study was to investigate the association between objective, actigraphy-based total sleep time (TST), sleep efficiency (SE), and cardiovascular and sympathetic regulation in healthy adults. We hypothesized that short TST and low SE would be associated with elevated resting blood pressure, heart rate (HR), and muscle sympathetic nerve activity (MSNA). Participants included 94 individuals [46 males, 48 females, age: 30 ± 15 yr, body mass index (BMI): 26 ± 4 kg/m2]. All participants underwent at least 7 days of at-home, wristwatch actigraphy monitoring (avg: 10 ± 3 days). Seated blood pressures were assessed using brachial blood pressure measurements, followed by a 10-minute supine autonomic testing session consisting of continuous HR (electrocardiogram), beat-by-beat blood pressure (finger plethysmograph), and MSNA (microneurography) monitoring. Partial correlations were used to determine the relationship between sleep and cardiovascular parameters while accounting for the influence of age, sex, and BMI. TST was not associated with MAP (R = -0.105, P = 0.321), HR (R = 0.093, P = 0.383), or MSNA burst frequency (BF; R = -0.168, P = 0.112) and burst incidence (BI; R = -0.162, P = 0.124). Similarly, SE was not associated with MAP (R = -0.088, P = 0.408), HR (R = -0.118, P = 0.263), MSNA BF (R = 0.038, P = 0.723), or MSNA BI (R = 0.079, P = 0.459). In contrast to recent preliminary findings, our results do not support a significant association between actigraphy-based sleep duration or efficiency and measures of resting blood pressure, heart rate, and MSNA.NEW & NOTEWORTHY The present study investigated the independent association between actigraphy-based sleep duration, efficiency, and measures of blood pressure, heart rate, and muscle sympathetic nerve activity (MSNA) in adult males and females. Contrary to our hypothesis, the findings do not support an independent association between habitual sleep and cardiovascular or sympathetic neural activity. However, these findings do not preclude a potential association between these parameters in populations with sleep disorders and/or cardiovascular disease.

Acute sympathetic activation blunts the hyperemic and vasodilatory response to passive leg movement

Heightened muscle sympathetic nerve activity (MSNA) contributes to impaired vasodilatory capacity and vascular dysfunction associated with aging and cardiovascular disease. The contribution of elevated MSNA to the vasodilatory response during passive leg movement (PLM) has not been adequately addressed. This study sought to test the hypothesis that elevated MSNA diminishes the vasodilatory response to PLM in healthy young males (n = 11, 25 ± 2 year). Post exercise circulatory occlusion (PECO) following 2 min of isometric handgrip (HG) exercise performed at 25% (ExPECO 25%) and 40% (ExPECO 40%) of maximum voluntary contraction was used to incrementally engage the metaboreceptors and augment MSNA. Control trials were performed without PECO (ExCON 25% and ExCON 40%) to account for changes due to HG exercise. PLM was performed 2 min after the cessation of exercise and central and peripheral hemodynamics were assessed. MSNA was directly recorded by microneurography in the peroneal nerve (n = 8). Measures of MSNA (i.e., burst incidences) increased during ExPECO 25% (+ 15 ± 5 burst/100 bpm) and ExPECO 40% (+ 22 ± 4 burst/100 bpm) and returned to pre-HG levels during ExCON trials. Vasodilation, assessed by the change in leg vascular conductance during PLM, was reduced by 16% and 44% during ExPECO 25% and ExPECO 40%, respectively. These findings indicate that elevated MSNA attenuates the vasodilatory response to PLM and that the magnitude of reduction in vasodilation during PLM is graded in relation to the degree of sympathoexcitation.

Differential control of sympathetic outflow to muscle and skin during physical and cognitive stressors

PURPOSE

Sympathetic nerve activity towards muscle (MSNA) and skin (SSNA) regulates various physiological parameters. MSNA primarily functions in blood pressure and flow, while SSNA operates in thermoregulation. Physical and cognitive stressors have been shown to have effects on both types of sympathetic activity, but there are inconsistencies as to what these effects are. This article aims to address the discrepancies in the literature and compare MSNA and SSNA responses.

METHODS

Microelectrode recordings were taken from the common peroneal nerve in 29 participants: MSNA (n = 21), SSNA (n = 16) and both MSNA and SSNA (n = 8). Participants were subjected to four different 2-min stressors: two physical (isometric handgrip task, cold pressor test) and two cognitive (mental arithmetic task, Stroop colour-word conflict test), the latter of which saw participants separated into responders and non-responders to the stressors. It was hypothesised that the physical stressors would have a greater effect on MSNA than SSNA, while the cognitive stressors would operate conversely.

RESULTS

Peristimulus time histogram (PSTH) analysis showed the mental arithmetic task to significantly increase both MSNA and SSNA; the isometric handgrip task and cold pressor test to increase MSNA, but not SSNA; and Stroop test to have no significant effects on changing MSNA or SSNA from baseline. Additionally, stress responses did not differ between MSNA and SSNA in participants who had both sets of data recorded.

CONCLUSIONS

This study has provided evidence to support the literature which claims cognitive stressors increase sympathetic activity, and provides much needed SSNA data in response to stressors.

Study Protocol for the Pleiotropic Effects of Sodium-Glucose Cotransporter 2 Inhibitor on Organ-Specific Sympathetic Nerve Activity and Insulin Sensitivity in Participants with Type 2 Diabetes

INTRODUCTION

Hyperinsulinemia and hyperglycemia are associated with exaggerated systemic sympathetic nerve activity (SNA) in patients with type 2 diabetes. Sodium-glucose cotransporter 2 (SGLT2) inhibitors lower insulin levels, whereas sulfonylureas increase insulin levels. We will test whether these two classes of antidiabetic agents have different effects on SNA.

METHODS

The present study is an ongoing, 24-week, one-center (only Kanazawa University Hospital), open-label, randomized, parallel trial (jRCTs 041200035). Participants with type 2 diabetes with multiple atherosclerosis risk factors are randomly assigned in a 1:1 manner to receive 2.5 mg luseogliflozin or 0.5 mg glimepiride once daily. The sample size was calculated to be 14 in each group, with a significance level of 0.05 and a power of 0.80. The design required 40 evaluable study participants. Our primary endpoint will be the change in muscle SNA (MSNA). The secondary endpoints included organ-specific insulin sensitivity measured by a hyperinsulinemic-euglycemic clamp study using an artificial pancreas combined with a stable isotope-labeled glucose infusion, bioelectrical impedance analysis, and organ-specific (cardiac, renal, and hepatic) 123I-meta-iodobenzylguanidine (MIBG) innervation imaging.

PLANNED OUTCOMES

Study recruitment started in April 2020 and will end in June 2024, with 40 participants randomized into the two groups. The treatment follow-up of the participants is currently ongoing and is due to finish by March 2025.

TRIAL REGISTRATION

The study protocol has been approved by the Certified Review Board, Kanazawa University, Ishikawa, Japan, in accordance with the guidelines stipulated in the Declaration of Helsinki (CRB4180005, 2019-001). This trial is registered with the Japan Registry of Clinical Trials, jRCTs 041200035.

Randomized Trial Comparing SGLT2 Inhibition and Hydrochlorothiazide on Sympathetic Traffic in Type 2 Diabetes

Introduction

Reductions in sympathetic nervous system activity may contribute to beneficial effects of sodium glucose cotransporter 2 (SGLT2) inhibition on cardiovascular outcomes. Therefore, we tested the hypothesis that SGLT2 inhibition with empagliflozin (Empa) lowers muscle sympathetic nerve activity (MSNA) in patients with type 2 diabetes mellitus (T2DM) compared with hydrochlorothiazide (HCT) to discern SGLT2-specific actions from responses to increased natriuresis.

Methods

We randomized patients with T2DM on metformin monotherapy to either 25 mg/d Empa (n = 20) or 25 mg/d HCT (n = 21) for 6 weeks in a parallel, double-blind fashion. We assessed MSNA by peroneal microneurography, blood pressure, cardiovascular and metabolic biomarkers at baseline and at the end of treatment.

Results

Both drugs elicited volume depletion, as indicated by increased thoracic impedance. Compared with HCT, Empa caused 1.23 kg more body weight loss (P = 0.011) and improved glycemic control. Seated systolic blood pressure decreased with both treatments (P < 0.002). MSNA did not change significantly with either treatment; however, MSNA changes were negatively correlated with changes in body weight on Empa (P = 0.042) and on HCT(P = 0.001). The relationship was shifted to lower MSNA on Empa compared with HCT (P = 0.002).

Conclusion

Increased renal sodium excretion eliciting body weight loss may promote sympathetic activation. However, sympathetic excitation in the face of increased sodium loss may be attenuated by SGLT2 inhibitor-specific actions.

Associations between locus coeruleus MRI contrast and physiological responses to acute stress in younger and older adults

Acute stress activates the brain's locus coeruleus (LC)-noradrenaline system. Recent studies indicate that a magnetic resonance imaging (MRI)-based measure of LC structure is associated with better cognitive outcomes in later life. Yet despite the LC's documented role in promoting physiological arousal during acute stress, no studies have examined whether MRI-assessed LC structure is related to arousal responses to acute stress. In this study, 102 younger and 51 older adults completed an acute stress induction task while we assessed multiple measures of physiological arousal (heart rate, breathing rate, systolic and diastolic blood pressure, sympathetic tone, and heart rate variability, HRV). We used turbo spin echo MRI scans to quantify LC MRI contrast as a measure of LC structure. We applied univariate and multivariate approaches to assess how LC MRI contrast was associated with arousal at rest and during acute stress reactivity and recovery. In older participants, having higher caudal LC MRI contrast was associated with greater stress-related increases in systolic blood pressure and decreases in HRV, as well as lower HRV during recovery from acute stress. These results suggest that having higher caudal LC MRI contrast in older adulthood is associated with more pronounced physiological responses to acute stress. Further work is needed to confirm these patterns in larger samples of older adults.

Longitudinal observations of sympathetic neural activity and hemodynamics during 6 months recovery from SARS-CoV-2 infection

Cross-sectional data indicate that acute SARS-CoV-2 infection increases resting muscle sympathetic nerve activity (MSNA) and alters hemodynamic responses to orthostasis in young adults. However, the longitudinal impact of contracting SARS-CoV-2 on autonomic function remains unclear. The aim of this study was to longitudinally track MSNA, sympathetic transduction to blood pressure (BP), and hemodynamics over 6 months following SARS-CoV-2 infection. Young adults positive with SARS-CoV-2 reported to the laboratory three times over 6 months (V1:41 ± 17, V2:108 ± 21, V3:173 ± 16 days post-infection). MSNA, systolic (SBP) and diastolic (DBP) blood pressure, and heart rate (HR) were measured at rest, during a cold pressor test (CPT), and at 30° head-up tilt (HUT). Basal SBP (p = 0.019) and DBP (p < 0.001) decreased throughout the 6 months, whereas basal MSNA and HR were not different. Basal sympathetic transduction to BP and estimates of baroreflex sensitivity did not change over time. SBP and DBP were lower during CPT (SBP: p = 0.016, DBP: p = 0.007) and HUT at V3 compared with V1 (SBP: p = 0.041, DBP: p = 0.017), with largely no changes in MSNA. There was a trend toward a visit-by-time interaction for burst incidence (p = 0.055) during HUT, wherein at baseline immediately prior to tilting, burst incidence was lower at V3 compared with V1 (p = 0.014), but there were no differences between visits in the 30 HUT position. These results support impairments to cardiovascular health, and potentially autonomic function, which may improve over time. However, the improvements in BP over 6 months recovery from mild SARS-CoV-2 infection are likely not a direct result of changes in sympathetic activity.

Effect of pulmonary vein isolation on the relationship between left atrial reverse remodeling and sympathetic nerve activity in patients with atrial fibrillation

PURPOSE

Catheter ablation (CA) to isolate the pulmonary vein, which is an established treatment for atrial fibrillation (AF), is associated with left atrium reverse remodeling (LARR). The intrinsic cardiac autonomic nervous system includes the ganglion plexi adjacent to the pulmonary vein in the left atrium (LA). However, little is known about the effect of CA on the relationship between LARR and sympathetic nerve activity in patients with AF.

METHODS

This study enrolled 22 AF patients with a normal left ventricular ejection fraction (LVEF) aged 64.6 ± 12.9 years who were scheduled for CA. Sympathetic nerve activity was evaluated by direct recording of muscle sympathetic nerve activity (MSNA) before and 12 weeks after CA. Blood pressure, heart rate (HR), HR variability, and echocardiography were also measured.

RESULTS

The heart rate increased significantly after CA (63 ± 10.9 vs. 70.6 ± 7.7 beats/min, p < 0.01), but blood pressure did not change. A high frequency (HF) and low frequency (LF) of HR variability decreased significantly after ablation, but no significant change in LF/HF was observed. CA significantly decreased MSNA (38.9 ± 9.9 vs. 28 ± 9.1 bursts/min, p < 0.01). Moreover, regression analysis revealed a positive correlation between the percentage change in MSNA and the LA volume index (r = 0.442, p < 0.05).

CONCLUSIONS

Our results show that CA for AF reduced MSNA and the decrease was associated with the LA volume index in AF patients with a normal LVEF. These findings suggest that LARR induced by CA for AF decrease sympathetic nerve activity.

Aortic haemodynamics: the effects of habitual endurance exercise, age and muscle sympathetic vasomotor outflow in healthy men

PURPOSE

We determined the effect of habitual endurance exercise and age on aortic pulse wave velocity (aPWV), augmentation pressure (AP) and systolic blood pressure (aSBP), with statistical adjustments of aPWV and AP for heart rate and aortic mean arterial pressure, when appropriate. Furthermore, we assessed whether muscle sympathetic nerve activity (MSNA) correlates with AP in young and middle-aged men.

METHODS

Aortic PWV, AP, aortic blood pressure (applanation tonometry; SphygmoCor) and MSNA (peroneal microneurography) were recorded in 46 normotensive men who were either young or middle-aged and endurance-trained runners or recreationally active nonrunners (10 nonrunners and 13 runners within each age-group). Between-group differences and relationships between variables were assessed via ANOVA/ANCOVA and Pearson product-moment correlation coefficients, respectively.

RESULTS

Adjusted aPWV and adjusted AP were similar between runners and nonrunners in both age groups (all, P > 0.05), but higher with age (all, P < 0.001), with a greater effect size for the age-related difference in AP in runners (Hedges' g, 3.6 vs 2.6). aSBP was lower in young (P = 0.009; g = 2.6), but not middle-aged (P = 0.341; g = 1.1), runners compared to nonrunners. MSNA burst frequency did not correlate with AP in either age group (young: r = 0.00, P = 0.994; middle-aged: r = - 0.11, P = 0.604).

CONCLUSION

There is an age-dependent effect of habitual exercise on aortic haemodynamics, with lower aSBP in young runners compared to nonrunners only. Statistical adjustment of aPWV and AP markedly influenced the outcomes of this study, highlighting the importance of performing these analyses. Further, peripheral sympathetic vasomotor outflow and AP were not correlated in young or middle-aged normotensive men.

Sympathetic Neural Control in Humans with Anxiety-Related Disorders

Numerous conceptual models are used to describe the dynamic responsiveness of physiological systems to environmental pressures, originating with Claude Bernard's milieu intérieur and extending to more recent models such as allostasis. The impact of stress and anxiety upon these regulatory processes has both basic science and clinical relevance, extending from the pioneering work of Hans Selye who advanced the concept that stress can significantly impact physiological health and function. Of particular interest within the current article, anxiety is independently associated with cardiovascular risk, yet mechanisms underlying these associations remain equivocal. This link between anxiety and cardiovascular risk is relevant given the high prevalence of anxiety in the general population, as well as its early age of onset. Chronically anxious populations, such as those with anxiety disorders (i.e., generalized anxiety disorder, panic disorder, specific phobias, etc.) offer a human model that interrogates the deleterious effects that chronic stress and allostatic load can have on the nervous system and cardiovascular function. Further, while many of these disorders do not appear to exhibit baseline alterations in sympathetic neural activity, reactivity to mental stress offers insights into applicable, real-world scenarios in which heightened sympathetic reactivity may predispose those individuals to elevated cardiovascular risk. This article also assesses behavioral and lifestyle modifications that have been shown to concurrently improve anxiety symptoms, as well as sympathetic control. Lastly, future directions of research will be discussed, with a focus on better integration of psychological factors within physiological studies examining anxiety and neural cardiovascular health. © 2022 American Physiological Society. Compr Physiol 12:1-33, 2022.

Endovascular baroreflex amplification and the effect on sympathetic nerve activity in patients with resistant hypertension: A proof-of-principle study

BACKGROUND

First in human studies suggest that endovascular baroreflex amplification (EVBA) lowers blood pressure (BP). To explore potential mechanisms for BP reduction, this study examines the effects of EVBA on muscle sympathetic nerve activity (MSNA) and baroreceptor sensitivity (BRS).

METHODS

In a single-center sub-study of the CALM-DIEM study (Controlling And Lowering blood pressure with the MobiusHD-Defining Efficacy Markers), 14 patients with resistant hypertension were treated with EVBA. Microneurography and non-invasive continuous BP measurements were performed at baseline and three months after MobiusHD implantation. The primary outcome was change in MSNA. Secondary outcomes were change in baroreflex sensitivity (BRS), cardiovascular responses to a sympathetic stimulus, BP, heart rate (HR) and heart rate variability (HRV).

RESULTS

The primary endpoint was obtained in 10 of 14 patients enrolled in the sub-study. MSNA burst frequency and burst incidence decreased in 6 of 10 patients: mean change -4.1 bursts/min (95% confidence interval -12.2 to 4.0) and -3.8 bursts/100 heartbeats (-15.2 to 7.7). MSNA spike frequency and spike count decreased in 8 of 10 patients: mean change -2.8 spikes/sec (-7.3 to 1.8) and -3.0 spikes/heartbeat (-6.1 to 0.1). Change in MSNA and BP were not correlated. Office BP decreased by -14/-6 mmHg (-27 to -2/-15 to 3). We observed a trend towards decreased HR (-5 bpm, -10 to 1) and increased total power HRV (623 msec2, 78 to 1168). In contrast, BRS and cardiovascular responses remained unchanged after EVBA.

CONCLUSIONS

In this proof-of-principle study, EVBA did not significantly decrease MSNA in patients with resistant hypertension. EVBA did not impair baroreflex function.

TRIAL REGISTRATION

Clinical trial registration at NCT02827032.

Different Responses of Muscle Sympathetic Nerve Activity to Dapagliflozin Between Patients With Type 2 Diabetes With and Without Heart Failure

Background Sodium-glucose cotransporter 2 inhibitors improve cardiovascular outcomes in patients with diabetes with and without heart failure (HF). However, their influence on sympathetic nerve activity (SNA) remains unclear. The purpose of this study was to evaluate the effect of sodium-glucose cotransporter 2 inhibitors on SNA and compare the responses of SNA to sodium-glucose cotransporter 2 inhibitors in patients with type 2 diabetes with and without HF. Methods and Results Eighteen patients with type 2 diabetes, 10 with HF (65.4±3.68 years) and 8 without HF (63.3±3.62 years), were included. Muscle SNA (MSNA), heart rate, and blood pressure were recorded before and 12 weeks after administration of dapagliflozin (5 mg/day). Sympathetic and cardiovagal baroreflex sensitivity were simultaneously calculated. Brain natriuretic peptide level increased significantly at baseline in patients with HF than those without HF, while MSNA, blood pressure, and hemoglobin A1c did not differ between the 2 groups. Fasting blood glucose and homeostatic model assessment of insulin resistance did not change in either group after administering dapagliflozin. MSNA decreased significantly in both groups. However, the reduction in MSNA was significantly higher in patients with HF than patients with non-HF (-20.2±3.46 versus -9.38±3.65 bursts/100 heartbeats; P=0.049), which was concordant with the decrease in brain natriuretic peptide. Conclusions Dapagliflozin significantly decreased MSNA in patients with type 2 diabetes regardless of its blood glucose-lowering effect. Moreover, the reduction in MSNA was more prominent in patients with HF than in patients with non-HF. These results indicate that the cardioprotective effects of sodium-glucose cotransporter 2 inhibitors may, in part, be attributed to improved SNA.

Effects of inspiratory muscle training combined with aerobic exercise training on neurovascular control in chronic heart failure patients

Aims

We tested the hypothesis that the effects of combined inspiratory muscle training and aerobic exercise training (IMT + AET) on muscle sympathetic nerve activity (MSNA) and forearm blood flow in patients with heart failure with reduced ejection fraction are more pronounced than the effects of AET alone.

Methods and results

Patients aged 30–70 years, New York Heart Association Functional Class II‐III, and left ventricular ejection fraction ≤40% were randomly assigned to four groups: IMT (n = 11), AET (n = 12), IMT + AET (n = 9), and non‐training (NT; n = 10). MSNA was recorded using microneurography. Forearm blood flow was measured by venous occlusion plethysmography and inspiratory muscle strength by maximal inspiratory pressure. IMT consisted of 30 min sessions, five times a week, for 4 months. Moderate AET consisted of 60 min sessions, three times a week for 4 months. AET (−10 ± 2 bursts/min, P = 0.03) and IMT + AET (−13 ± 4 bursts/min, P = 0.007) reduced MSNA. These responses in MSNA were not different between AET and IMT + AET groups. IMT (0.22 ± 0.08 mL/min/100 mL, P = 0.03), AET (0.27 ± 0.09 mL/min/100 mL, P = 0.01), and IMT + AET (0.35 ± 0.12 mL/min/100 mL, P = 0.008) increased forearm blood flow. No differences were found between groups. AET (3 ± 1 mL/kg/min, P = 0.006) and IMT + AET (4 ± 1 mL/kg/min, P = 0.001) increased peak oxygen consumption. These responses were similar between these groups. IMT (20 ± 3 cmH₂O, P = 0.005) and IMT + AET (18 ± 3 cmH₂O, P = 0.01) increased maximal inspiratory pressure. No significant changes were observed in the NT group.

Conclusions

IMT + AET causes no additive effects on neurovascular control in patients with heart failure with reduced ejection fraction compared with AET alone. These findings may be, in part, because few patients had inspiratory muscle weakness.

Exaggerated Exercise Blood Pressure as a Marker of Baroreflex Dysfunction in Normotensive Metabolic Syndrome Patients

Introduction

Exaggerated blood pressure response to exercise (EEBP = SBP ≥ 190 mmHg for women and ≥210 mmHg for men) during cardiopulmonary exercise test (CPET) is a predictor of cardiovascular risk. Sympathetic hyperactivation and decreased baroreflex sensitivity (BRS) seem to be involved in the progression of metabolic syndrome (MetS) to cardiovascular disease.

Objective

To test the hypotheses: (1) MetS patients within normal clinical blood pressure (BP) may present EEBP response to maximal exercise and (2) increased muscle sympathetic nerve activity (MSNA) and reduced BRS are associated with this impairment.

Methods

We selected MetS (ATP III) patients with normal BP (MetS_NT, n = 27, 59.3% males, 46.1 ± 7.2 years) and a control group without MetS (C, n = 19, 48.4 ± 7.4 years). We evaluated BRS for increases (BRS+) and decreases (BRS−) in spontaneous BP and HR fluctuations, MSNA (microneurography), BP from ambulatory blood pressure monitoring (ABPM), and auscultatory BP during CPET.

Results

Normotensive MetS (MetS_NT) had higher body mass index and impairment in all MetS risk factors when compared to the C group. MetS_NT had higher peak systolic BP (SBP) (195 ± 17 vs. 177 ± 24 mmHg, P = 0.007) and diastolic BP (91 ± 11 vs. 79 ± 10 mmHg, P = 0.001) during CPET than C. Additionally, we found that MetS patients with normal BP had lower spontaneous BRS− (9.6 ± 3.3 vs. 12.2 ± 4.9 ms/mmHg, P = 0.044) and higher levels of MSNA (29 ± 6 vs. 18 ± 4 bursts/min, P < 0.001) compared to C. Interestingly, 10 out of 27 MetS_NT (37%) showed EEBP (MetS_NT+), whereas 2 out of 19 C (10.5%) presented (P = 0.044). The subgroup of MetS_NT with EEBP (MetS_NT+, n = 10) had similar MSNA (P = 0.437), but lower BRS+ (P = 0.039) and BRS− (P = 0.039) compared with the subgroup without EEBP (MetS_NT−, n = 17). Either office BP or BP from ABPM was similar between subgroups MetS_NT+ and MetS_NT−, regardless of EEBP response. In the MetS_NT+ subgroup, there was an association of peak SBP with BRS− (R = −0.70; P = 0.02), triglycerides with peak SBP during CPET (R = 0.66; P = 0.039), and of triglycerides with BRS− (R = 0.71; P = 0.022).

Conclusion

Normotensive MetS patients already presented higher peak systolic and diastolic BP during maximal exercise, in addition to sympathetic hyperactivation and decreased baroreflex sensitivity. The EEBP in MetS_NT with apparent well-controlled BP may indicate a potential depressed neural baroreflex function, predisposing these patients to increased cardiovascular risk.

Aerobic fitness is inversely associated with neurohemodynamic transduction and blood pressure variability in older adults

Higher aerobic fitness is independently associated with better cardiovascular health in older adults. The transduction of muscle sympathetic nerve activity (MSNA) into mean arterial pressure (MAP) responses provides important insight regarding beat-by-beat neural circulatory control. Aerobic fitness is negatively associated with peak MAP responses to spontaneous MSNA in young males. Whether this relationship exists in older adults is known. We tested the hypothesis that aerobic fitness was inversely related to sympathetic neurohemodynamic transduction and blood pressure variability (BPV) in older adults. Relative peak oxygen consumption (V̇O₂peak, indirect calorimetry) was assessed in 22 older adults (13 males, 65 ± 5 years, 36.3 ± 11.5 ml/kg/min). Peroneal MSNA (microneurography) and arterial pressure (finger photoplethysmography) were recorded during ≥ 10-min of rest. BPV was assessed using the average real variability index. MAP was tracked for 12 cardiac cycles following heartbeats associated with MSNA bursts (i.e., peak ΔMAP). Peak ΔMAP responses (0.9 ± 0.6 mmHg) were negatively associated (all, P < 0.04) with resting burst frequency (30 ± 11 bursts/min; R = -0.47) and burst incidence (54 ± 22 bursts/100 heartbeats; R = -0.51), but positively associated with BPV (ρ = 0.47). V̇O₂peak was inversely related to the pressor responses to spontaneous bursts (R = -0.47, P = 0.03) and BPV (ρ = -0.54, P = 0.01), positively related to burst incidence (R = 0.42, P = 0.05), but unrelated to MSNA burst frequency (P = 0.20). The V̇O₂peak-BPV relationship remained after controlling for burst frequency, peak ΔMAP, age, and sex. Lower V̇O₂peak was associated with augmented neurohemodynamic transduction and BPV in older adults. These negative hemodynamic outcomes highlight the importance of higher aerobic fitness with ageing for optimal cardiovascular health.

Acute Exposure to Diesel Exhaust Increases Muscle Sympathetic Nerve Activity in Humans

Background

Diesel exhaust (DE) emissions are a major contributor to ambient air pollution and are strongly associated with cardiovascular morbidity and mortality. Exposure to traffic‐related particulate matter is linked with acute adverse cardiovascular events; however, the mechanisms are not fully understood. We examined the role of the autonomic nervous system during exposure to DE that has previously only been indirectly investigated.

Methods and Results

Using microneurography, we measured muscle sympathetic nerve activity (MSNA) directly in the peroneal nerve of 16 healthy individuals. MSNA, heart rate, and respiration were recorded while subjects rested breathing filtered air, filtered air with an exposure mask, and standardized diluted DE (300 µg/m³) through the exposure mask. Heart rate variability was assessed from an ECG. DE inhalation rapidly causes an increase in number of MSNA bursts as well as the size of bursts within 10 minutes, peaking by 30 minutes (P<0.001), compared with baseline filtered air with an exposure mask. No significant changes occurred in heart rate variability indices during DE exposure; however, MSNA frequency correlated negatively with total power (r²=0.294, P=0.03) and low frequency (r²=0.258, P=0.045). Heart rate correlated positively with MSNA frequency (r²=0.268, P=0.04) and the change in percentage of larger bursts (burst amplitude, height >50% of the maximum burst) from filtered air with an exposure mask (r²=0.368, P=0.013).

Conclusions

Our study provides direct evidence for the rapid modulation of the autonomic nervous system after exposure to DE, with an increase in MSNA. The quick increase in sympathetic outflow may explain the strong epidemiological data associating traffic‐related particulate matter to acute adverse cardiovascular events such as myocardial infarction.

Registration

URL: https://www.clinicaltrials.gov; Unique identifier: NCT02892279.

Recording and quantifying sympathetic outflow to muscle and skin in humans: methods, caveats and challenges

The development of microneurography, in which the electrical activity of axons can be recorded via an intrafascicular microelectrode inserted through the skin into a peripheral nerve in awake human participants, has contributed a great deal to our understanding of sensorimotor control and the control of sympathetic outflow to muscle and skin. This review summarises the different approaches to recording muscle sympathetic nerve activity (MSNA) and skin sympathetic nerve activity (SSNA), together with discussion on the issues that determine the quality of a recording. Various analytical approaches are also described, with a primary emphasis on those developed by the author, aimed at maximizing the information content from recordings of postganglionic sympathetic nerve activity in awake humans.

Intranasal oxytocin has sympathoexcitatory effects on vascular tone in healthy males

Oxytocin appears to be involved in the neuroendocrine regulation of sympathetic blood pressure (BP) homeostasis. In animals, intracerebral administration of oxytocin induces BP-relevant sympathetic activation. In humans, central nervous effects of oxytocin on BP regulation remain unclear. Intranasal administration supposedly delivers oligopeptides such as oxytocin directly to the brain. We investigated the effects of intranasal oxytocin on sympathetic vascular baroreflex function in humans using microneurographic techniques. In a balanced, double-blind crossover design, oxytocin or placebo was administered intranasally to 12 lean, healthy males (age 25 ± 4 yr). Muscle sympathetic nerve activity (MSNA) was assessed microneurographically before (presubstance), 30-45 min (postsubstance I), and 105-120 min (postsubstance II) after oxytocin administration. Baroreflex was challenged via graded infusions of vasoactive drugs, and correlation of BP with MSNA and heart rate (HR) defined baroreflex function. Experiments were conducted in the afternoon after a 5-h fasting period. After oxytocin, resting MSNA (burst rate and total activity) showed significant net increases from pre to postsubstance II compared with placebo [Δincrease = +4.3 ± 1.2 (oxytocin) vs. +2.2 ± 1.4 bursts/min (placebo), ANOVA; P < 0.05; total activity = 184 ± 11.5% (oxytocin) vs. 121 ± 14.3% (placebo), ANOVA; P = 0.01). This was combined with a small but significant net increase in resting diastolic BP, whereas systolic and mean arterial BP or HR as well as baroreflex sensitivity at vasoactive drug challenge were not altered. Intranasally administered oxytocin induced vasoconstrictory sympathoactivation in healthy male humans. The concomitant increase of diastolic BP was most likely attributable to increased vascular tone. This suggests oxytocin-mediated upward resetting of the vascular baroreflex set point at centers superordinate to the mere baroreflex-feedback loop.

Action potential subpopulations within human muscle sympathetic nerve activity: Discharge properties and governing mechanisms

Sympathetic emissions directed towards the skeletal muscle circulation - muscle sympathetic nerve activity (MSNA) - represent a key mechanism for maintaining homeostasis and supporting human survival during physiological stress. Pulse-rhythmic bursts formed by the synchronous discharge of differently-sized sympathetic action potentials (APs) represent the primary characteristic of MSNA. Of the APs firing under baseline conditions (reflecting low-threshold c-fibre activity), a range of subpopulations exists, of which three general categories can be discussed based on their peak-to-peak amplitude in the filtered raw neurogram - small, medium, and large. These subpopulations express nonuniform discharge, recruitment, and synchronization patterns. The subpopulation of medium APs fires synchronously in most bursts, while the subpopulations of small and large APs fire less often. However, 30% of total AP discharge occurs asynchronously between sympathetic bursts, a pattern expressed most often by small APs. In response to physiological stress (e.g., baroreflex unloading), the subpopulation of medium APs exhibits the largest increase in firing probability and a subpopulation of previously-silent larger and faster-conducting APs (reflecting high-threshold c-fibre activity) becomes recruited. Heterogeneous discharge, synchronization, and recruitment thresholds among AP subpopulations stem from differential regulation within the sympathetic organization including the arterial baroreflex and paravertebral ganglia. Indeed, the arterial baroreflex strongly regulates medium APs at baseline and enhances its control over this subpopulation during periods of baroreflex unloading. Conversely, small and large APs express weak baroreflex control. Trimethaphan infusion has revealed that ganglionic processes including nicotinic and non-nicotinic mechanisms may contribute to heterogenous firing behaviours among low-threshold AP subpopulations. This review highlights recent work revealing new insight to the discharge properties expressed by, and mechanisms governing, AP subpopulations within human MSNA.

Integrative cardiovascular control in women: Regulation of blood pressure, body temperature, and cerebrovascular responsiveness

Over the past several decades, it has become increasingly clear that women have distinct cardiovascular profiles compared to men. In this review, our goal is to provide an overview of the literature regarding the influences of female sex and reproductive hormones (primarily estradiol) on mechanisms of cardiovascular control relevant to regulation of blood pressure, body temperature, and cerebral blood flow. Young women tend to have lower resting blood pressure compared with men. This sex difference is reversed at menopause, when women develop higher sympathetic nerve activity and the risk of systemic hypertension increases sharply as postmenopausal women age. Vascular responses to thermal stress, including cutaneous vasodilation and vasoconstriction, are also affected by reproductive hormones in women, where estradiol appears to promote vasodilation and heat dissipation. The influence of reproductive hormones on cerebral blood flow and sex differences in the ability of the cerebral vasculature to increase its blood flow (cerebrovascular reactivity) are relatively new areas of investigation. Sex and hormonal influences on integrative blood flow regulation have further implications during challenges to physiological homeostasis, including exercise. We propose that increasing awareness of these sex-specific mechanisms is important for optimizing health care and promotion of wellness in women across the life span.

Reproducibility of the neurocardiovascular responses to common laboratory-based sympathoexcitatory stimuli in young adults

The magnitude of blood pressure (BP) and muscle sympathetic nerve activity (MSNA) responses to laboratory stressors is commonly used to compare neurocardiovascular responsiveness between groups and conditions. However, no studies have rigorously examined the reproducibility of BP and MSNA responsiveness. Here, we assess the within-visit reproducibility of BP (finger photoplethysmography) and MSNA (microneurography) responses to isometric handgrip (HG) and postexercise ischemia (PEI) in young healthy adults (n = 30). In a subset (n = 21), we also examined the between-visit reproducibility of responsiveness to HG, PEI, and the cold pressor test (CPT). Intraclass correlation coefficients (ICCs) were used as a primary reproducibility measure (e.g., ICC >0.75 is considered very good). Within a visit, the increase in mean arterial pressure during HG [ICC = 0.85 (0.69-0.93); P < 0.001] and PEI [ICC = 0.85 (0.69-0.93); P < 0.001] demonstrated very good reproducibility. Furthermore, the between-visit reproducibility of the pressor response to HG [ICC = 0.85 (0.62-0.94); P < 0.001], PEI [ICC = 0.84 (CI = 0.58-0.94); P < 0.001], and the CPT [ICC = 0.89 (0.72-0.95) P < 0.001]) were also very good. However, there was greater variability in both the within- [HG: ICC = 0.58 (-0.22-0.85), P = 0.001; PEI: ICC = 0.33 (-0.24-0.69), P = 0.042] and between-visit reproducibility of MSNA responsiveness [HG: ICC = 0.87 (0.53-0.96), P = 0.001; PEI: ICC = 0.24 (-0.62-0.78), P = 0.27; CPT: ICC = 0.77 (0.29-0.93), P = 0.007]. The magnitude of the BP response to several standard laboratory stimuli was very good, whereas the variability of the MSNA response to these perturbations was generally less consistent, particularly during PEI. These data provide novel insight for both study design and data interpretation when comparing neurocardiovascular responsiveness between different conditions, groups, or studies, as well as before and after interventions/treatments.NEW & NOTEWORTHY The magnitude of the increases in blood pressure and muscle sympathetic nerve activity in response to sympathoexcitatory stimuli such as static handgrip, postexercise ischemia, and the cold pressor test are commonly used to assess neurocardiovascular responsiveness. However, limited studies have comprehensively examined the reproducibility of these responses. We demonstrate that the reproducibility of the pressor response to these perturbations was very good within an individual, whereas the reproducibility of the MSNA response was less consistent.

Effect of exercise training on cardiovascular autonomic and muscular function in subclinical Chagas cardiomyopathy: a randomized controlled trial

PURPOSE

Patients with chronic chagasic cardiomyopathy with preserved ventricular function present with autonomic imbalance. This study evaluated the effects of exercise training (ET) in restoring peripheral and cardiac autonomic control and skeletal muscle phenotype in patients with subclinical chronic chagasic cardiomyopathy.

METHODS

This controlled trial (NCT02295215) included 24 chronic chagasic cardiomyopathy patients who were randomized www.random.org/lists/ into two groups: those who underwent exercise training (n = 12) and those who continued their usual activities (n = 12). Eight patients completed the exercise training protocol, and 10 patients were clinically followed up for 4 months. Muscular sympathetic nerve activity was measured by microneurography and muscle blood flow (MBF) using venous occlusion plethysmography. The low-frequency component of heart rate variability in normalized units (LFnuHR) reflects sympathetic activity in the heart, and the low-frequency component of systolic blood pressure variability in normalized units reflects sympathetic activity in the vessels. The infusion of vasoactive drugs (phenylephrine and sodium nitroprusside) was used to evaluate cardiac baroreflex sensitivity, and a vastus lateralis muscle biopsy was performed to evaluate atrogin-1 and MuRF-1 gene expression.

RESULTS

The baroreflex sensitivity for increases (p = 0.002) and decreases (p = 0.02) in systolic blood pressure increased in the ET group. Muscle blood flow also increased only in the ET group (p = 0.004). Only the ET group had reduced resting muscular sympathetic nerve activity levels (p = 0.008) and sympathetic activity in the heart (LFnu; p = 0.004) and vessels (p = 0.04) after 4 months. Regarding skeletal muscle, after 4 months, participants in the exercise training group presented with lower atrogin-1 gene expression than participants who continued their activities as usual (p = 0.001). The reduction in muscular sympathetic nerve activity was positively associated with reduced atrogin-1 (r = 0.86; p = 0.02) and MuRF-1 gene expression (r = 0.64; p = 0.06); it was negatively associated with improved baroreflex sensitivity both for increases (r = -0.72; p = 0.020) and decreases (r = -0.82; p = 0.001) in blood pressure.

CONCLUSIONS

ET improved cardiac and peripheral autonomic function in patients with subclinical chagasic cardiomyopathy. ET reduced MSNA and sympathetic activity in the heart and vessels and increased cardiac parasympathetic tone and baroreflex sensitivity. Regarding peripheral muscle, after 4 months, patients who underwent exercise training had an increased cross-sectional area of type I fibers and oxidative metabolism of muscle fibers, and decreased atrogin-1 gene expression, compared to participants who continued their activities as usual. In addition, the reduction in MSNA was associated with improved cardiac baroreflex sensitivity, reduced sympathetic cardiovascular tone, and reduced atrogin-1 and MuRF-1 gene expression.

TRIAL REGISTRATION

ID: NCT02295215. Registered in June 2013.

Impact of sex and age on metabolism, sympathetic activity, and hypertension

In this brief review, we summarize the current knowledge on the complex interplay between metabolism, sympathetic activity and hypertension with a focus on sex differences and changes with age in humans. Evidence suggests that in premenopausal women, sex hormones, particularly estrogen exerts a profound cardioprotective effect which may be associated with favorable metabolic profiles, as well as lower sympathetic activity and blood pressure at rest and any given physiological and environmental stimuli compared with men of a similar age. Along this line, premenopausal women seem to be generally protected from obesity-induced metabolic and cardiovascular complications. However, postmenopausal estrogen deprivation during midlife and older age has a detrimental impact on metabolism, may lead to adipose tissue redistribution from the subcutaneous to abdominal area, and augments sympathetic activity. All these changes could contribute significantly to the higher prevalence of hypertension and greater cardiometabolic risk in older women than older men. It is proposed that obesity-related hypertension has a neurogenic component which is characterized by sympathetic overactivity, but the impact of sex and age remains largely unknown. Understanding sex and age-specific differences in obesity and sympathetic neural control of blood pressure is important in the prevention and/or risk reduction of cardiometabolic disorders for both men and women.

Whole Milk and Full-Fat Dairy Products and Hypertensive Risks

Lifestyle modifications in the form of diet and exercise are generally a first-line approach to reduce hypertensive risk and overall cardiovascular disease (CVD) risk. Accumulating research evidence has revealed that consumption of non- and low-fat dairy products incorporated into the routine diet is an effective means to reduce elevated blood pressure and improve vascular functions. However, the idea of incorporating whole-fat or full-fat dairy products in the normal routine diet as a strategy to reduce CVD risk has been met with controversy. The aim of this review is to review both sides of the argument surrounding saturated fat intake and CVD risk from the standpoint of dairy intake. Throughout the review, we examined observational studies on relationships between CVD risk and dairy consumption, dietary intervention studies using non-fat and whole-fat dairy, and mechanistic studies investigating physiological mechanisms of saturated fat intake that may help to explain increases in cardiovascular disease risk. Currently available data have demonstrated that whole-fat dairy is unlikely to augment hypertensive risk when added to the normal routine diet but may negatively impact CVD risk. In conclusion, whole-fat dairy may not be a recommended alternative to non- or low-fat dairy products as a means to reduce hypertensive or overall CVD risk.

Impact of age and sex on neural cardiovascular responsiveness to cold pressor test in humans

Prior longitudinal work suggests that blood pressure (BP) reactivity to the cold pressor test (CPT) helps predict hypertension; yet the impact of age and sex on hemodynamic and neural responsiveness to CPT remains equivocal. Forty-three young (21 ± 1yr, means ± SE) men (YM, n = 20) and women (YW, n = 23) and 16 older (60 ± 1yr) men (OM, n = 9) and women (OW, n = 7) participated in an experimental visit where continuous BP (finger plethysmography) and muscle sympathetic nerve activity (MSNA; microneurography) were recorded during a 3- to 5-min baseline and 2-min CPT. Baseline mean arterial pressure (MAP) was greater in OM than in YM (92 ± 4 vs. 77 ± 1 mmHg, P < 0.01), but similar in women (P = 0.12). Baseline MSNA incidence was greater in OM [69 ± 6 bursts/100 heartbeats (hb)] than in OW (44 ± 7 bursts/100 hb, P = 0.02) and lower in young adults (YM: 17 ± 3 vs. YW: 16 ± 2 bursts/100 hb, P < 0.01), but similar across the sexes (P = 0.83). However, when exposed to the CPT, MSNA increased more rapidly in OW (Δ43 ± 6 bursts/100 hb; group × time, P = 0.01) compared with OM (Δ15 ± 3 bursts/100 hb) but was not different between YW (Δ30 ± 3 bursts/100 hb) and YM (Δ33 ± 4 bursts/100 hb, P = 1.0). There were no differences in MAP with CPT between groups (group × time, P = 0.33). These findings suggest that OW demonstrate a more rapid initial rise in MSNA responsiveness to a CPT compared with OM. This greater sympathetic reactivity in OW may be a contributing mechanism to the increased hypertension risk in postmenopausal women.

Effect of Salt Supplementation on Sympathetic Activity and Endothelial Function in Salt-Sensitive Type 2 Diabetes

CONTEXT

Lower sodium intake is paradoxically associated with higher mortality in type 2 diabetes (T2D).

OBJECTIVE

To determine whether sympathetic nervous system (SNS) activation and endothelial dysfunction contribute to these observations, we examined the effect of salt supplementation on these systems in people with T2D with habitual low sodium. We hypothesized that salt supplementation would lower SNS activity and improve endothelial function compared to placebo.

DESIGN

We conducted a randomized, double-blinded, placebo-controlled crossover trial.

SETTING

The study took place in a tertiary referral diabetes outpatient clinic.

PARTICIPANTS

Twenty-two people with T2D with habitual low sodium intake (24-hour urine sodium <150 mmol/24h) were included.

INTERVENTION

Salt supplementation (100 mmol NaCl/24h) or placebo for 3 weeks was administered.

MAIN OUTCOME MEASURES

The primary outcome of SNS activity and endothelial function was assessed as follows: Microneurography assessed muscle sympathetic nerve activity (MSNA), pulse amplitude tonometry assessed endothelial function via reactive hyperemic index (RHI), and arterial stiffness was assessed via augmentation index (AI). Secondary outcomes included cardiac baroreflex, serum aldosterone, ambulatory blood pressure monitoring (ABPM), heart rate variability (HRV), and salt sensitivity.

RESULTS

Compared to placebo, salt supplementation increased MSNA (burst frequency P = .047, burst incidence P = .016); however, RHI (P = .24), AI (P = .201), ABPM (systolic P = .09, diastolic P = .14), and HRV were unaffected. Salt supplementation improved baroreflex (slope P = .026) and lowered aldosterone (P = .004), and in salt-resistant individuals there was a trend toward improved RHI (P = .07).

CONCLUSIONS

In people with T2D and low habitual sodium intake, salt supplementation increased SNS activity without altering endothelial function or blood pressure but improved baroreflex function, a predictor of cardiac mortality. Salt-resistant individuals trended toward improved endothelial function with salt supplementation.

Sympathetic activity in obesity: a brief review of methods and supportive data

The increase in the prevalence of obesity and the concomitant rise in obesity-related illness have led to substantial pressure on health care systems throughout the world. While the combination of reduced exercise, increased sedentary time, poor diet, and genetic predisposition is undoubtedly pivotal in generating obesity and increasing disease risk, a large body of work indicates that the sympathetic nervous system (SNS) contributes to obesity-related disease development and progression. In obesity, sympathetic nervous activity is regionalized, with activity in some outflows being particularly sensitive to the obese state, whereas other outflows, or responses to stimuli, may be blunted, thereby making the assessment of sympathetic nervous activation in the clinical setting difficult. Isotope dilution methods and direct nerve recording techniques have been developed and utilized in clinical research, demonstrating that in obesity there is preferential activation of the muscle vasoconstrictor and renal sympathetic outflows. With weight loss, sympathetic activity is reduced. Importantly, sympathetic nervous activity is associated with end-organ dysfunction and changes in sympathetic activation that accompany weight loss are often reflected in an improvement of end-organ function. Whether targeting the SNS directly improves obesity-related illness remains unknown, but merits further attention.

Assessment of sympathetic neural activity in chronic insomnia: evidence for elevated cardiovascular risk

Study Objectives

Chronic insomnia affects up to 15 per cent of adults. Recent cross-sectional and prospective epidemiological studies report an association between insomnia and hypertension, including incident hypertension, yet mechanisms underlying the association remain unknown. We hypothesized that participants with chronic insomnia would have elevated sympathetic neural outflow, blunted baroreflex sensitivity, and augmented sympathetic neural and cardiovascular reactivity to stress when compared with good-sleeper controls.

Methods

Twelve participants with chronic insomnia (11 women, 1 man) and 12 controls (8 women, 4 men) underwent one night of laboratory polysomnography, two weeks of at-home wrist actigraphy, and one night of controlled laboratory sleep prior to a comprehensive morning autonomic function test. The autonomic function test consisted of simultaneous recordings of muscle sympathetic nerve activity (MSNA; microneurography), beat-to-beat blood pressure (finger plethysmography), and heart rate (electrocardiogram) during a 10 min supine baseline and a 2 min cold pressor test.

Results

Baseline blood pressure, heart rate, and MSNA were not different between groups, but sympathetic baroreflex sensitivity was significantly blunted in participants with insomnia (-2.1 ± 1.0 vs. -4.3 ± 1.3 bursts/100 heartbeats/mm Hg; p < 0.001). During the cold pressor test, systolic arterial pressure reactivity (Δ21 ± 11 vs. Δ14 ± 8 mm Hg; time × group = 0.04) and total MSNA reactivity (Δ127%, 54%-208% vs. Δ52%, 30%-81%; time × group = 0.02) were augmented in chronic insomnia.

Conclusions

Participants with chronic insomnia demonstrated impaired sympathetic baroreflex function and augmented neural cardiovascular responsiveness to stress, when compared with controls. These findings support growing evidence of cardiovascular risk and physiological hyperarousal in chronic insomnia.

Clinical Trial Registration

NCT02048878. https://clinicaltrials.gov/ct2/show/NCT02048878.

Relation between resting sympathetic outflow and vasoconstrictor responses to sympathetic nerve bursts: sex differences in healthy young adults

Previous studies have demonstrated an inverse relation between resting muscle sympathetic nerve activity (MSNA) and vasoconstrictor responsiveness (i.e., sympathetic transduction), such that those with high resting MSNA have low vascular responsiveness, and vice versa. The purpose of this investigation was to determine whether biological sex influences the balance between resting MSNA and beat-to-beat sympathetic transduction. We measured blood pressure (BP) and MSNA during supine rest in 54 healthy young adults (27 females: 23 ± 4 yr, 107 ± 8/63 ± 8 mmHg; 27 males: 25 ± 3 yr, 115 ± 11/64 ± 7 mmHg; means ± SD). We quantified beat-to-beat fluctuations in mean arterial pressure (MAP, mmHg) and limb vascular conductance (LVC, %) for 10 cardiac cycles after each MSNA burst using signal averaging, an index of sympathetic vascular transduction. In females, there was no correlation between resting MSNA (burst incidence; burst/100 heartbeats) and peak ΔMAP (r = -0.10, P = 0.62) or peak ΔLVC (r = -0.12, P = 0.63). In males, MSNA was related to peak ΔMAP (r = -0.50, P = 0.01) and peak ΔLVC (r = 0.49, P = 0.03); those with higher resting MSNA had blunted increases in MAP and reductions in LVC in response to a burst of MSNA. In a sub-analysis, we performed a median split between high- versus low-MSNA status on ΔMAP and ΔLVC within each sex and found that only males demonstrated a significant difference in ΔMAP and ΔLVC between high- versus low-MSNA groups. These findings support an inverse relation between resting MSNA and sympathetic vascular transduction in males only and advance our understanding on the influence of biological sex on sympathetic nervous system-mediated alterations in beat-to-beat BP regulation.

Microneurography and sympathetic nerve activity: a decade-by-decade journey across 50 years

The technique of microneurography has advanced the field of neuroscience for the past 50 years. While there have been a number of reviews on microneurography, this paper takes an objective approach to exploring the impact of microneurography studies. Briefly, Web of Science (Thomson Reuters) was used to identify the highest citation articles over the past 50 years, and key findings are presented in a decade-by-decade highlight. This includes the establishment of microneurography in the 1960s, the acceleration of the technique by Gunnar Wallin in the 1970s, the international collaborations of the 1980s and 1990s, and finally the highest impact studies from 2000 to present. This journey through 50 years of microneurographic research related to peripheral sympathetic nerve activity includes a historical context for several of the laboratory interventions commonly used today (e.g., cold pressor test, mental stress, lower body negative pressure, isometric handgrip, etc.) and how these interventions and experimental approaches have advanced our knowledge of cardiovascular, cardiometabolic, and other human diseases and conditions.

Microneurography: how it started and how it works

In the first section, this historical review describes endeavors to develop the method for recording normal nerve impulse traffic in humans, designated microneurography. The method was developed at the Department of Clinical Neurophysiology of the Academic Hospital in Uppsala, Sweden. Microneurography involves the impalement of a peripheral nerve with a tungsten needle electrode. Electrode position is adjusted by hand until the activity of interest is discriminated. Nothing similar had previously been tried in animal preparations, and thus the large number of preceding studies that recorded afferent activity in other mammals did not offer pertinent methodological guidance. For 2 years, the two scientists involved in the research impaled their own nerves with electrodes to test various kinds of needles and explore different neural systems, all the while carefully watching for signs of nerve damage. Temporary paresthesiae were common, whereas enduring sequelae never followed. Single-unit impulse trains could be discriminated, even those originating from unmyelinated fibers. An explanation for the discrimination of unitary impulses using a coarse electrode is inferred based on the electrical characteristics of the electrode placed in the flesh and the impulse shapes, as discussed in the second section of this paper. Microneurography and the microstimulation of single afferents, combined with psychophysical methods and behavioral tests, have generated new knowledge particularly regarding four neural systems, namely the proprioceptive system, the cutaneous mechanoreceptive system, the cutaneous nociceptive system, and the sympathetic efferent system to skin structures and muscular blood vessels. Examples of achievements based on microneurography are presented in the final section.

The Stability and Repeatability of Spontaneous Sympathetic Baroreflex Sensitivity in Healthy Young Individuals

Spontaneous sympathetic baroreflex sensitivity (BRS) is a valuable tool for assessing how well the baroreflex buffers beat-to-beat changes in blood pressure. However, there has yet to be a study involving appropriate statistical tests to examine the stability of sympathetic BRS within an experimental session and the repeatability between separate sessions. The aim of this study was to use intra-class correlations, ordinary least products regression, and Bland–Altman analyses to examine the stability and repeatability of spontaneous sympathetic BRS assessment. In addition, the influence of recording duration on values of BRS was assessed. In eighty-four healthy young individuals (49 males, 35 females), continuous measurements of blood pressure, heart rate and muscle sympathetic nerve activity (MSNA) were recorded for 10 min. In a subgroup of 13 participants (11 male, 2 female) the measurements were repeated on a separate day. Sympathetic BRS was quantified using MSNA burst incidence (BRS_inc) and total MSNA (BRS_total) for the first 5-min period, the second 5-min period, and a 2-min segment taken from the second 5-min period. Intra-class correlation coefficients indicated moderate stability in sympathetic BRS_inc and BRS_total between the first and second 5-min periods in males (BRS_incr = 0.63, BRS_totalr = 0.78) and females (BRS_incr = 0.61, BRS_totalr = 0.47) with no proportional bias, but with fixed bias for BRS_inc in females. When comparing the first 5-min with the 2-min period (n = 76), the intra-class correlation coefficient indicated poor to moderate repeatability in sympathetic BRS_inc and BRS_total for males (BRS_incr = -0.01, BRS_totalr = 0.70) and females (BRS_incr = 0.46, BRS_totalr = 0.39). However, Bland–Altman analysis revealed a fixed bias for BRS_total in males and proportional bias for BRS_total in females, with lower BRS values for 5-min recordings. In the subgroup, intra-class correlations indicated moderate repeatability for measures of BRS_inc (9 male, 2 female, r = 0.63) and BRS_total (6 male, 2 female, r = 0.68) assessed using 5-min periods recorded on separate days. However, Bland–Altman analysis indicated proportional bias for BRS_inc and fixed bias for BRS_total. In conclusion, measures of spontaneous sympathetic BRS are moderately stable and repeatable within and between testing sessions in healthy young adults, provided that the same length of recording is used when making comparisons.

Differential effects of lipophilic and hydrophilic statins on muscle sympathetic nerve activity in heart failure with preserved left ventricular ejection fraction

Augmented sympathetic nerve activity is associated with heart failure with preserved left ventricular ejection fraction (HFpEF). Lipophilic statins reduce sympathetic nerve activity in patients with heart failure with reduced left ventricular ejection fraction. However, little is known about whether all types of statins, regardless of solubility, reduce sympathetic nerve activity in HFpEF. We evaluated the effect of atorvastatin, a lipophilic statin, and rosuvastatin, a hydrophilic statin, on muscle sympathetic nerve activity (MSNA) in HFpEF patients. This study was conducted as a prospective, randomized, open-label, crossover trial. Ten HFpEF patients with untreated hyperlipidemia participated in this study. Subjects were assigned to either the atorvastatin (lipophilic) or the rosuvastatin (hydrophilic) group with each drug administered for 8 weeks. Atorvastatin and rosuvastatin treatment resulted in a similar reduction in low-density lipoprotein cholesterol (LDL-C) levels. There was no difference in the effect of either treatment on blood pressure, heart rate, or left ventricular function. Atorvastatin significantly decreased MSNA frequency compared with baseline (31.5 ± 6.3 vs. 47.5 ± 10.7 bursts/min, p < 0.01), but rosuvastatin had no effect on MSNA (40.9 ± 7.3 bursts/min). MSNA was significantly lower in the atorvastatin group than rosuvastatin group (p < 0.05). However, the reduction in MSNA seen in either group did not correlate with the reduction in LDL-C. No significant differences were observed in either the baroreflex control of heart rate or MSNA between the two groups. These results suggest that lipophilic statins have a favorable effect on sympathetic nerve activity beyond lowering LDL-C in HFpEF, but hydrophilic statins do not.

Fifty years of microneurography: learning the language of the peripheral sympathetic nervous system in humans

As a primary component of homeostasis, the sympathetic nervous system enables rapid adjustments to stress through its ability to communicate messages among organs and cause targeted and graded end organ responses. Key in this communication model is the pattern of neural signals emanating from the central to peripheral components of the sympathetic nervous system. But what is the communication strategy employed in peripheral sympathetic nerve activity (SNA)? Can we develop and interpret the system of coding in SNA that improves our understanding of the neural control of the circulation? In 1968, Hagbarth and Vallbo (Hagbarth KE, Vallbo AB. Acta Physiol Scand 74: 96-108, 1968) reported the first use of microneurographic methods to record sympathetic discharges in peripheral nerves of conscious humans, allowing quantification of SNA at rest and sympathetic responsiveness to physiological stressors in health and disease. This technique also has enabled a growing investigation into the coding patterns within, and cardiovascular outcomes associated with, postganglionic SNA. This review outlines how results obtained by microneurographic means have improved our understanding of SNA outflow patterns at the action potential level, focusing on SNA directed toward skeletal muscle in conscious humans.

Physiological and pathophysiological firing properties of single postganglionic sympathetic neurons in humans

It has long been known from microneurographic recordings in human subjects that the activity of postganglionic sympathetic axons occurs as spontaneous bursts, with muscle sympathetic nerve activity (MSNA) exhibiting strong cardiac rhythmicity via the baroreflex and skin sympathetic nerve activity showing much weaker cardiac modulation. Here we review the firing properties of single sympathetic neurons, obtained using highly selective microelectrodes. Individual vasoconstrictor neurons supplying muscle or skin, or sudomotor neurons supplying sweat glands, always discharge with a low firing probability (~30%) and at very low frequencies (~0.5 Hz). Moreover, they usually fire only once per cardiac interval but can fire greater than four times within a burst. Modeling has shown that this pattern can best be explained by individual neurons being driven by, on average, two preganglionic inputs. Unitary recordings of muscle vasoconstrictor neurons have been made in several pathophysiological states, including heart failure, hypertension, obstructive sleep apnea, bronchiectasis, chronic obstructive pulmonary disease, depression, and panic disorder. The augmented MSNA in each of these diseases features an increase in firing probability and discharge frequency of individual muscle vasoconstrictor neurons above that seen in healthy subjects, yet firing rates rarely exceed 1 Hz. However, unlike patients with heart failure, all patients with respiratory disease or panic disorder, and patients with hyperhidrosis, exhibited an increase in multiple within-burst firing, which emphasizes the different modes by which the sympathetic nervous system grades its output in pathophysiological states of high sympathetic nerve activity.

The role of increased glucose on neurovascular dysfunction in patients with the metabolic syndrome

Metabolic syndrome (MetS) causes autonomic alteration and vascular dysfunction. The authors investigated whether impaired fasting glucose (IFG) is the main cause of vascular dysfunction via elevated sympathetic tone in nondiabetic patients with MetS. Pulse wave velocity, muscle sympathetic nerve activity (MSNA), and forearm vascular resistance was measured in patients with MetS divided according to fasting glucose levels: (1) MetS+IFG (blood glucose ≥100 mg/dL) and (2) MetS-IFG (<100 mg/dL) compared with healthy controls. Patients with MetS+IFG had higher pulse wave velocity than patients with MetS-IFG and controls (median 8.0 [interquartile range, 7.2-8.6], 7.3 [interquartile range, 6.9-7.9], and 6.9 [interquartile range, 6.6-7.2] m/s, P=.001). Patients with MetS+IFG had higher MSNA than patients with MetS-IFG and controls, and patients with MetS-IFG had higher MSNA than controls (31±1, 26±1, and 19±1 bursts per minute; P<.001). Patients with MetS+IFG were similar to patients with MetS-IFG but had higher forearm vascular resistance than controls (P=.008). IFG was the only predictor variable of MSNA. MSNA was associated with pulse wave velocity (R=.39, P=.002) and forearm vascular resistance (R=.30, P=.034). In patients with MetS, increased plasma glucose levels leads to an adrenergic burden that can explain vascular dysfunction.

Cardiac and peripheral autonomic control in restrictive cardiomyopathy

AIMS

Autonomic dysfunction determines the advance of dilated cardiomyopathy (DCM) and is related to poor outcomes. However, this autonomic imbalance is unknown in patients with restrictive cardiomyopathy (RCM) even though they have similar symptoms and poor quality of life as DCM patients have. The aim of this study was to evaluate if autonomic and neurovascular controls were altered in RCM patients.

METHODS AND RESULTS

Fifteen RCM patients, 10 DCM patients, and 10 healthy subjects were evaluated. Heart rate and blood pressure (BP) were recorded. Peripheral sympathetic activity [muscle sympathetic nerve activity (MSNA)] by microneurography and cardiac sympathetic activity by power spectrum analysis of heart rate variability. Spontaneous baroreflex sensitivity (BRS) was evaluated by the sequence method and forearm blood flow by venous occlusion plethysmography. Both cardiomyopathy groups had higher MSNA frequency (P < 0.001) and MSNA incidence (P < 0.001), higher cardiac sympathovagal balance (P < 0.02), reduced BRS for increase (P = 0.002) and for decrease in BP (P = 0.002), and lower forearm blood flow (P < 0.001) compared with healthy subjects. We found an inverse correlation between BRS for increase and decrease in BP and peripheral sympathetic activity (r = -0.609, P = 0.001 and r = -0.648, P < 0.001, respectively) and between BRS for increase and decrease in BP and cardiac sympathetic activity (r = -0.503, P = 0.03 and r = -0.487, P = 0.04, respectively).

CONCLUSIONS

The RCM patients had cardiac and peripheral autonomic dysfunctions associated with peripheral vasoconstriction. Nonetheless, the presence of normal ejection fraction underestimates the evolution of the disease and makes clinical treatment difficult. These alterations could lead to a similar cardiovascular risk as that observed in DCM patients.

Endovascular Renal Denervation in End-Stage Kidney Disease Patients: Cardiovascular Protection-A Proof-of-Concept Study

Introduction

Sympathetic neural activation is markedly increased in end-stage kidney disease (ESKD). Catheter-based renal denervation (RDN) reduces sympathetic overactivity and blood pressure in resistant hypertension. We investigated the effect of RDN on sympathetic neural activation and left ventricular mass in patients with ESKD.

Methods

Nine ESKD (6 hemodialysis and 3 peritoneal dialysis) patients with dialysis vintage of ≥11 months were treated with RDN (EnligHTN system). Data were obtained on a nondialysis day; at baseline, 1, 3, and 12 months post-RDN.

Results

At baseline sympathetic neural activation measured by muscle sympathetic nervous activity (MSNA) and plasma norepinephrine concentrations were markedly elevated. Left ventricular hypertrophy (LVH) was evident in 8 of the 9 patients. At 12 months post-RDN, blind analysis revealed that MSNA_frequency (–12.2 bursts/min¹, 95% CI [–13.6, –10.7]) and LV mass (–27 g/m², 95% CI [–47, –8]) were reduced. Mean ambulatory BP (systolic: –24 mm Hg, 95% CI [–42, –5] and diastolic: –13 mm Hg, 95% CI [–22, –4]) was also reduced at 12 months. Office BP was reduced as early as 1 month (systolic: –25 mm Hg, 95% CI [–45, –5] and diastolic: –13 mm Hg, 95% CI [–24, –1]). Both ambulatory and office BP had clinically significant reductions in at least 50% of patients out to 12 months.

Discussion

Catheter-based RDN significantly reduced MSNA and LV mass as well as systemic BP in this group of patients with ESKD.

Sympathetic Nerve Activity in Monozygotic Twins

Microneurographic recordings of human muscle sympathetic nerve activity responses to sudden sensory stimuli (ie, arousal) have revealed 2 intraindividually reproducible response profiles in healthy young males that predict different neural and blood pressure responses to more sustained stress. Approximately 50% of subjects inhibit muscle sympathetic nerve activity during arousal, whereas the remaining 50% do not, and the latter group displays a markedly greater blood pressure increase in response to arousal, as well as during and after 3 minutes of mental arithmetic. Studying a group of monozygotic twins (10 pairs, 2 excluded from analysis), the aim of the present study was to evaluate the degree of genetic determination of these sympathetic response profiles. Muscle sympathetic burst incidence at rest was similar in twins, with a within-pair burst incidence ratio of 0.87±0.02 (SEM) compared with 0.73±0.07 found in unrelated pairs ( P =0.002), confirming a previous study from our laboratory. In contrast, the sympathetic responses to arousal showed large twin within-pair variance (arousal inhibition ratio 0.56±0.11), which did not significantly differ ( P =0.939) from the variance in pairs of unrelated subjects (0.46±0.11). The finding that human muscle sympathetic nerve responses to arousal are less determined by genotype than the resting level of corresponding sympathetic nerve activity suggests that the arousal response pattern is more prone to be altered by environmental factors. This raises the possibility that these intraindividually reproducible sympathetic neural response profiles can be modified in a positive direction from a cardiovascular risk perspective.

Neck Circumference Is Associated with Muscle Sympathetic Nerve Activity in Overweight and Obese Men but Not Women

Background: Neck circumference (NC) is a predictor of cardiometabolic risk. The objective of this study was to explore the relationship of NC to muscle sympathetic nerve activity (MSNA) within an overweight and obese population.

Methods: The study design was a retrospective cross-sectional analysis. Un-medicated persons (72 men, 53 postmenopausal women) aged 56 ± 1 years (mean ± SEM) with body mass index (BMI) 32.8 ± 0.4 kg/m², were studied. NC was measured together with traditional anthropometric measures, supine blood pressure, fasting blood lipids, insulin, and glucose. Insulin sensitivity was assessed by homeostasis model (HOMA-IR) and Matsuda Insulin Sensitivity Index (ISI) derived from 75-g oral glucose tolerance test. Resting multiunit MSNA was recorded by microneurography in the peroneal nerve and expressed as burst frequency and burst incidence.

Results: Men within the highest tertile of NC had significantly higher fasting and post-glucose plasma insulin levels (insulin AUC₀₋₁₂₀), HOMA-IR, non-esterified fatty acids, MSNA (45 ± 2 vs. 36 ± 2 bursts per min; 69 ± 3 vs. 58 ± 3 bursts per 100 hb) and heart rate, and lower Matsuda ISI compared to men in the lowest tertile (P all <0.05). In stepwise regression analyses, NC alone explained 12%, and together with insulin AUC₀₋₁₂₀ it accounted for 22%, of the variance in MSNA in men. In women, NC was associated with anthropometric measures but not with MSNA or metabolic indices.

Conclusions: Among overweight and obese men, NC was independently associated with elevated MSNA and hyperinsulinemia, and thus may be relevant to cardiometabolic risk prediction. The biological basis of gender differences merits further elucidation.

Recording sympathetic nerve activity in conscious humans and other mammals: guidelines and the road to standardization

Over the past several decades, studies of the sympathetic nervous system in humans, sheep, rabbits, rats, and mice have substantially increased mechanistic understanding of cardiovascular function and dysfunction. Recently, interest in sympathetic neural mechanisms contributing to blood pressure control has grown, in part because of the development of devices or surgical procedures that treat hypertension by manipulating sympathetic outflow. Studies in animal models have provided important insights into physiological and pathophysiological mechanisms that are not accessible in human studies. Across species and among laboratories, various approaches have been developed to record, quantify, analyze, and interpret sympathetic nerve activity (SNA). In general, SNA demonstrates "bursting" behavior, where groups of action potentials are synchronized and linked to the cardiac cycle via the arterial baroreflex. In humans, it is common to quantify SNA as bursts per minute or bursts per 100 heart beats. This type of quantification can be done in other species but is only commonly reported in sheep, which have heart rates similar to humans. In rabbits, rats, and mice, SNA is often recorded relative to a maximal level elicited in the laboratory to control for differences in electrode position among animals or on different study days. SNA in humans can also be presented as total activity, where normalization to the largest burst is a common approach. The goal of the present paper is to put together a summary of "best practices" in several of the most common experimental models and to discuss opportunities and challenges relative to the optimal measurement of SNA across species.Listen to this article's corresponding podcast at https://ajpheart.podbean.com/e/guidelines-for-measuring-sympathetic-nerve-activity/.

Muscle sympathetic nerve activity response to heat stress is attenuated in chronic heart failure patients

Heat stress evokes significant increases in muscle sympathetic nerve activity (MSNA) in healthy individuals. The MSNA response to heat stress in chronic heart failure (CHF) is unknown. We hypothesized that the MSNA response to heat stress is attenuated in CHF. Passive whole body heating was applied with water-perfused suits in 13 patients (61 ± 2 yr) with stable class II-III CHF, 12 age-matched (62 ± 2 yr) healthy subjects, and 14 young (24 ± 1 yr) healthy subjects. Mild heating (i.e., increases in skin temperature ΔTsk ~2-4°C, internal temperature ΔTcore <0.3°C) significantly decreased MSNA in CHF patients; however, it did not significantly alter the MSNA in the age-matched and young healthy subjects. Heat stress (i.e., ΔTsk ~4°C and ΔTcore ~0.6°C) raised MSNA in the age-matched (32.9 ± 3.2 to 45.6 ± 4.2 bursts/min; P < 0.001) and young (14.3 ± 1.7 to 26.3 ± 2.4 bursts/min; P < 0.001) controls, but not in CHF (46.2 ± 5.3 to 50.5 ± 5.3 bursts/min; P = 0.06). The MSNA increase by the heat stress in CHF (Δ4.2 ± 2.0 bursts/min) was significantly less than those seen in the age-matched (Δ12.8 ± 1.7 bursts/min, P < 0.05) and young (Δ12.0 ± 2.7 bursts/min, P < 0.05) control groups. These data suggest that the MSNA response to heat stress is attenuated in CHF patients. We speculate that the attenuated MSNA response to heat stress may contribute to impaired cardiovascular adjustments in CHF in a hot environment.