Sex differences in vascular transduction of sympathetic nerve activity

Sex-specific responses of central artery stiffness to cold pressor test-mediated sympathetic activation

Considerable attention has been devoted to investigating whether acute activation of the sympathetic nervous system, triggered by daily life stressors, increases central artery stiffness (CAS). Overt sex differences in sympathetic neurovascular transduction lead to distinct cardiovascular responses to sympathoexcitation in men versus women. Our study aimed to determine if the cold pressor test (CPT), chosen to simulate highly painful stressors individuals encounter daily, would induce CAS augmentation in a sex-balanced cohort of young individuals and whether any observed CAS increase would differ between sexes. Twenty-eight participants (14 men, 14 women) provided baseline and CPT measurements of carotid-femoral pulse wave velocity (cf-PWV), mean arterial pressure (MAP), cardiac output (CO), total peripheral resistance (TPR), and heart rate (HR). CPT involved immersing participants' feet in 5 °C ice-water slurry. Data were analyzed using 2-way analysis of variance and post hoc testing, where applicable. CPT significantly increased (p < 0.01) cf-PWV values in our population, with women showing lower collective CAS values compared to men across baseline and CPT conditions. CPT augmented (p < 0.01) MAP, HR, and CO without altering stroke volume (SV), but increased (p = 0.002) TPR only in men. Men collectively exhibited higher (p < 0.01) CO and SV values across baseline and CPT conditions, as well as lower (p < 0.01) TPR values compared to women. These results support the notion that CPT increases CAS in a healthy young population. Despite observed sex differences in peripheral vascular responses to CPT, these variances did not differently impact the central pressor response or central artery stiffening between sexes. Given the well-documented sex differences in cardiovascular risk and incidence, these findings deepen understanding of how cardiocirculatory responses differ between sexes under stress. These findings support sex-specific approaches in cardiovascular medicine and prevention, including tailored coping strategies for managing stress.

The role of peripheral venous distension reflex in regulating hemodynamics: mini review

Significant volume is pooled in veins in humans and the amount is dramatically altered by various physiological stresses and diseases. Several animal and human studies demonstrated that limb venous distension evoked significant increases in blood pressure and sympathetic nerve activity (venous distension reflex, VDR). VDR has attracted much attention because of its potential to explain the still unknown mechanism of autonomic dysfunction in several diseases, which would lead to a new treatment approach. This mini review discusses accumulated evidence of VDR at this point and what should be investigated in the future to apply the current understanding of VDR in clinical practice.

Nadir blood pressure responses to longer consecutive cardiac cycle sequences absent of sympathetic bursts are associated with popliteal endothelial-dependent dilation

PURPOSE

The nadir pressure responses to cardiac cycles absent of muscle sympathetic nerve activity (MSNA) bursts (or non-bursts) are typically reported in studies quantifying sympathetic transduction, but the information gained by studying non-bursts is unclear. We tested the hypothesis that longer sequences of non-bursts (≥8 cardiac cycles) would be associated with a greater nadir diastolic blood pressure (DBP) and that better popliteal artery function would be associated with an augmented reduction in DBP.

METHODS

Resting beat-by-beat DBP (via finger photoplethysmography) and common peroneal nerve MSNA (via microneurography) were recorded in 39 healthy, adults (age 23.4 ± 5.3 years; 19 females). For each cardiac cycle absent of MSNA bursts, the mean nadir DBP (ΔDBP) during the 12 cardiac cycles following were determined, and separate analyses were conducted for ≥8 or < 8 cardiac cycle sequences. Popliteal artery endothelial-dependent (via flow-mediated dilation; FMD) and endothelial-independent vasodilation (via nitroglycerin-mediated dilation; NMD) were determined.

RESULTS

The nadir DBP responses to sequences ≥8 cardiac cycles were larger (-1.40 ± 1.27 mmHg) than sequences <8 (-0.38 ± 0.46 mmHg; p < 0.001). In adjusting for sex and burst frequency (14 ± 8 bursts/min), larger absolute or relative FMD (p < 0.01), but not NMD (p > 0.53) was associated with an augmented nadir DBP. This overall DBP-FMD relationship was similar in sequences ≥8 (p = 0.04-0.05), but not <8 (p > 0.72).

CONCLUSION

The DBP responses to non-bursts, particularly longer sequences, were inversely associated with popliteal endothelial function, but not vascular smooth muscle sensitivity. This study provides insight into the information gained by quantifying the DBP responses to cardiac cycles absent of MSNA.

Sex Differences in Sympathetic Responses to Lower-Body Negative Pressure

INTRODUCTION

Trauma-induced hemorrhage is a leading cause of death in prehospital settings. Experimental data demonstrate that females have a lower tolerance to simulated hemorrhage (i.e., central hypovolemia). However, the mechanism(s) underpinning these responses are unknown. Therefore, this study aimed to compare autonomic cardiovascular responses during central hypovolemia between the sexes. We hypothesized that females would have a lower tolerance and smaller increase in muscle sympathetic nerve activity (MSNA) to simulated hemorrhage.

METHODS

Data from 17 females and 19 males, aged 19-45 yr, were retrospectively analyzed. Participants completed a progressive lower-body negative pressure (LBNP) protocol to presyncope to simulate hemorrhagic tolerance with continuous measures of MSNA and beat-to-beat hemodynamic variables. We compared responses at baseline, at two LBNP stages (-40 and -50 mmHg), and at immediately before presyncope. In addition, we compared responses at relative percentages (33%, 66%, and 100%) of hemorrhagic tolerance, calculated via the cumulative stress index (i.e., the sum of the product of time and pressure at each LBNP stage).

RESULTS

Females had lower tolerance to central hypovolemia (female: 561 ± 309 vs male: 894 ± 304 min·mmHg [time·LBNP]; P = 0.003). At LBNP -40 and -50 mmHg, females had lower diastolic blood pressures (main effect of sex: P = 0.010). For the relative LBNP analysis, females exhibited lower MSNA burst frequency (main effect of sex: P = 0.016) accompanied by a lower total vascular conductance (sex: P = 0.028; main effect of sex).

CONCLUSIONS

Females have a lower tolerance to central hypovolemia, which was accompanied by lower diastolic blood pressure at -40 and -50 mmHg LBNP. Notably, females had attenuated MSNA responses when assessed as relative LBNP tolerance time.

Influence of sex and sedentary conditions on sympathetic burst characteristics in prepubertal, postpubertal, and young adult rats

Recent evidence indicates that sex-based differences in cardiovascular disease (CVD) begin early in life, particularly when associated with risk factors such as a sedentary lifestyle. CVD is associated with elevated sympathetic nerve activity (SNA), quantified as increased SNA burst activity in humans. Whether burst characteristics are influenced by sex or sedentary conditions at younger ages is unknown. The purpose of our study is to compare SNA bursts in active and sedentary female and male rats at ages including prepuberty and young adulthood. We hypothesized that burst characteristics and blood pressure are higher under sedentary conditions and lower in female rats compared with males. We analyzed splanchnic SNA (SpSNA) recordings from Inactin-anesthetized male and female rats at 4-, 8-, and 16-wk of age. Physically active and sedentary rats were each housed in separate, environmentally controlled chambers where physically active rats had free access to an in-cage running wheel. Sympathetic bursts were obtained by rectifying and integrating the raw SpSNA signal. Burst frequency, burst height, and burst width were calculated using the Peak Parameters extension in LabChart. Our results showed that sedentary conditions produced a greater burst width in 8- and 16-wk-old rats compared with 4-wk-old rats in both males and females (P < 0.001 for both). Burst frequency and incidence were both higher in 16-wk-old males compared with 16-wk-old females (P < 0.001 for both). Our results suggest that there are sedentary lifestyle- and sex-related mechanisms that impact sympathetic regulation of blood pressure at ages that range from prepuberty into young adulthood.NEW & NOTEWORTHY The mechanisms of decreased incidence of cardiovascular disease (CVD) in reproductive-age women compared with age-matched men are unknown. The strong association between elevated sympathetic activity and CVD led us to characterize splanchnic sympathetic bursts in female and male rats. Prepubescent males and females exhibited narrower sympathetic bursts, whereas young adult males had higher resting burst frequency compared with age-matched females. Sex-based regulation of sympathetic activity suggests a need for sex-dependent therapeutic strategies to combat CVD.

Postexercise cardiovascular hemodynamics assessment before and after a 30-minute standing still recovery

BACKGROUND

Although postexercise syncope usually occurs shortly after physical exercise conclusion, athletes commonly reveal symptoms of postexercise hypotension several tens of minutes after exercise completion. Currently, no studies have investigated central hemodynamic regulation during posture changes occurring several tens of minutes after exercise compared to immediately after cessation.

METHODS

This study examined changes in mean arterial pressure (MAP), heart rate (HR), systemic vascular conductance (SVC), cardiac output, and stroke volume during two sets of tilt tests performed before vs. after a 30-minute standing still recovery, respectively. Tilt tests were performed after a short-lasting supramaximal test (WNG) and long-lasting maximal incremental test (INC) in 12 young endurance-trained individuals.

RESULTS

The key findings were that, regardless of the exercise type, the 30-minute recovery augmented (P<0.01) the increase in HR and the drop in SVC during the transition from supine to upright, although the MAP drop was similar (P=0.99) after vs. before recovery. INC led to greater increases (P<0.01) in HR and drops (P<0.01) in SVC compared to WNG during postural transitions both before and after the recovery.

CONCLUSIONS

These findings suggest that, in a population that tolerates postexercise hypotension, MAP neural control is more challenged after a 30-minute standing still recovery than before, as evidenced by an augmented vasodilation capacity along with an increased HR buffering response during posture changes. Moreover, our data suggest that effective MAP control is resulting from an equally effective HR buffering response on MAP. Therefore, exercises that induce greater systemic vasodilation lead to greater HR buffering responses.

Age- and sex-related differences in sympathetic vascular transduction and neurohemodynamic balance in humans

Bursts of muscle sympathetic nerve activity (MSNA) and the ensuing vasoconstriction are pivotal determinants of beat-by-beat blood pressure regulation. Although age and sex impact blood pressure regulation, how these factors affect the central and peripheral arcs of the baroreflex remains unclear. In 27 young [25 (SD 3) yr] males (YM; n = 14) and females (YF; n = 13) and 23 older [71 (SD 5) yr] males (OM; n = 11) and females (OF; n = 12), femoral artery blood flow, blood pressure, and MSNA were recorded for 10 min of supine rest. Sympathetic baroreflex sensitivity (i.e., central arc) was quantified as the relationship between diastolic blood pressure and MSNA burst incidence. Signal averaging was used to determine sympathetic vascular transduction into leg vascular conductance (LVC) for 12 cardiac cycles following MSNA bursts (i.e., peripheral arc). Older adults demonstrated attenuated sympathetic transduction into LVC (both P < 0.001) following MSNA bursts, and smaller increases in sympathetic transduction as a function of MSNA burst size and firing pattern compared with young adults (range, P = 0.004-0.032). YM (r2 = 0.36; P = 0.032) and OM (r2 = 0.51; P = 0.014) exhibited an inverse relationship between the central and peripheral arcs of the baroreflex, whereas females did not (YF, r2 = 0.03, P = 0.621; OF, r2 = 0.06, P = 0.445). MSNA burst incidence was inversely related to sympathetic transduction in YM and OF (range, P = 0.03-0.046) but not in YF or OM (range, P = 0.360-0.603). These data indicate that age is associated with attenuated sympathetic vascular transduction, whereas age- and sex-specific changes are present in the relationship between the central and peripheral arcs of the baroreflex regulation of blood pressure.NEW & NOTEWORTHY Sympathetic vascular transduction is attenuated in older compared with young adults, regardless of biological sex. Males, but not females (regardless of age), demonstrate an inverse relationship between central (sympathetic baroreflex sensitivity) and peripheral (sympathetic vascular transduction) components of the baroreflex arc. Young males and older females exhibit an inverse relationship between resting sympathetic outflow and sympathetic vascular transduction. Our results indicate that age and sex exert independent and interactive effects on sympathetic vascular transduction and sympathetic neurohemodynamic balance in humans.

Circadian and sex differences in post-ischemic vasodilation and reactive hyperemia in young individuals and elderly with and without type 2 diabetes

OBJECTIVE

Cardiovascular events show morning preference and sex differences, and are related to aging and type 2 diabetes. We assessed circadian variations and sex differences in vascular conductance (VC) and blood flow (BF) regulations following a brief bout of forearm ischemia.

METHODS

Young healthy individuals (H18-30) and elderly without (H50-80) and with type 2 diabetes (T2DM50-80) of both sexes were included. Forearm VC and BF, and mean arterial pressure (MAP) at baseline and following circulatory reperfusion were measured at 6 a.m. and 9 p.m.

RESULTS

In the morning compared to evening, following reperfusion, the VC and BF increments were similar in H18-30 (p>.71), but lower in H50-80 (p<.001) and T2DM50-80 (p<.01). VC and BF following circulatory reperfusion were higher in men than women in H18-30 (p<.001), but similar between sexes in the older groups (p>.23).

CONCLUSIONS

Forearm vasodilation following reperfusion is attenuated in the morning in the elderly, impairing BF towards an ischemic area. Diabetes does not affect the circadian regulation of VC and BF, but that of MAP. There are sex differences in VC and BF at baseline and after circulatory reperfusion at a young age, being greater in men, which disappear with aging without being affected by diabetes.

Diversity of cells and signals in the cardiovascular system

This white paper is the outcome of the seventh UC Davis Cardiovascular Research Symposium on Systems Approach to Understanding Cardiovascular Disease and Arrhythmia. This biannual meeting aims to bring together leading experts in subfields of cardiovascular biomedicine to focus on topics of importance to the field. The theme of the 2022 Symposium was 'Cell Diversity in the Cardiovascular System, cell-autonomous and cell-cell signalling'. Experts in the field contributed their experimental and mathematical modelling perspectives and discussed emerging questions, controversies, and challenges in examining cell and signal diversity, co-ordination and interrelationships involved in cardiovascular function. This paper originates from the topics of formal presentations and informal discussions from the Symposium, which aimed to develop a holistic view of how the multiple cell types in the cardiovascular system integrate to influence cardiovascular function, disease progression and therapeutic strategies. The first section describes the major cell types (e.g. cardiomyocytes, vascular smooth muscle and endothelial cells, fibroblasts, neurons, immune cells, etc.) and the signals involved in cardiovascular function. The second section emphasizes the complexity at the subcellular, cellular and system levels in the context of cardiovascular development, ageing and disease. Finally, the third section surveys the technological innovations that allow the interrogation of this diversity and advancing our understanding of the integrated cardiovascular function and dysfunction.

Sympathetic transduction to blood pressure during euglycemic-hyperinsulinemia in young healthy adults: role of burst amplitude

Insulin acts centrally to stimulate sympathetic vasoconstrictor outflow to skeletal muscle and peripherally to promote vasodilation. Given these divergent actions, the "net effect" of insulin on the transduction of muscle sympathetic nerve activity (MSNA) into vasoconstriction and thus, blood pressure (BP) remains unclear. We hypothesized that sympathetic transduction to BP would be attenuated during hyperinsulinemia compared with baseline. In 22 young healthy adults, MSNA (microneurography), and beat-to-beat BP (Finometer or arterial catheter) were continuously recorded, and signal-averaging was performed to quantify the mean arterial pressure (MAP) and total vascular conductance (TVC; Modelflow) responses following spontaneous bursts of MSNA at baseline and during a euglycemic-hyperinsulinemic clamp. Hyperinsulinemia significantly increased MSNA burst frequency and mean burst amplitude (baseline: 46 ± 6 au; insulin: 65 ± 16 au, P < 0.001) but did not alter MAP. The peak MAP (baseline: 3.2 ± 1.5 mmHg; insulin: 3.0 ± 1.9 mmHg, P = 0.67) and nadir TVC (P = 0.45) responses following all MSNA bursts were not different between conditions indicating preserved sympathetic transduction. However, when MSNA bursts were segregated into quartiles based on their amplitudes at baseline and compared with similar amplitude bursts during hyperinsulinemia, the peak MAP and TVC responses were blunted (e.g., largest burst quartile: MAP, baseline: Δ4.4 ± 1.7 mmHg; hyperinsulinemia: Δ3.0 ± 0.8 mmHg, P = 0.02). Notably, ∼15% of bursts during hyperinsulinemia exceeded the size of any burst at baseline, yet the MAP/TVC responses to these larger bursts (MAP, Δ4.9 ± 1.4 mmHg) did not differ from the largest baseline bursts (P = 0.47). These findings indicate that increases in MSNA burst amplitude contribute to the overall maintenance of sympathetic transduction during hyperinsulinemia.

Mental stress augments central artery stiffness in young individuals of both sexes

Mental stress is a daily stimulus that can acutely activate the sympathetic nervous system. Whether sympathetic stimulation can augment central artery stiffness (CAS) has not yet been well documented. Moreover, sex differences in sympathetic neurovascular transduction have been reported. We assessed whether mental stress augments CAS in both sexes and whether any CAS increase is blunted in women compared with men. The hf-PWV (heart-femoral pulse wave velocity; index of CAS), MAP (mean arterial pressure), PP (pulse pressure), TPR (total peripheral resistance), and HR (heart rate) were measured in 26 young individuals (13 men, 13 women) at rest and throughout a 10-minute bout of stress induced by mental arithmetic. Data over the mental stress period were compared to the preceding baseline values and between sexes. Mental stress increased hf-PWV, MAP, PP, and HR from baseline throughout the entire stimulation period (p < .005). TPR diminished in the first minute of stimulation (p < .001) in both sexes and increased in the last minutes in women only (p < .005). Hf-PWV was lower in women than men (p < .001) at rest and during mental stress, but the changes from baseline were similar in both sexes. There were sex differences in the PP and TPR changes, which were evident at different times of stimulation. Mental stress increased CAS in both sexes throughout the stimulation period. Although values of CAS were lower in women both at rest and during mental stress, the CAS increase due to mental stress was similar in both sexes.

Advances in pathogenesis and treatment of essential hypertension

Hypertension is a significant risk factor for cardiovascular and cerebrovascular diseases and the leading cause of premature death worldwide. However, the pathogenesis of the hypertension, especially essential hypertension, is complex and requires in-depth studies. Recently, new findings about essential hypertension have emerged, and these may provide important theoretical bases and therapeutic tools to break through the existing bottleneck of essential hypertension. In this review, we demonstrated important advances in the different pathogenesis areas of essential hypertension, and highlighted new treatments proposed in these areas, hoping to provide insight for the prevention and treatment of the essential hypertension.

Circadian and sex differences in carotid-femoral pulse wave velocity in young individuals and elderly with and without type 2 diabetes

The incidence of cardiovascular events is higher in the morning than in the evening and differs between sexes. We tested the hypothesis that aortic stiffness, a compelling cardiovascular risk factor, increases in the morning than in the evening in young, healthy individuals between 18 and 30 years (H18–30) or in older individuals between 50 and 80 years, either healthy (H50–80) or with type 2 diabetes (T2DM50–80). Sex differences were also investigated. Carotid-femoral pulse wave velocity (cf-PWV) recorded via Doppler Ultrasound, blood pressure and heart rate were checked at 6 a.m. and 9 p.m., at rest and during acute sympathetic activation triggered by handgrip exercise. Cf-PWV values were lower in the morning compared to the evening in all groups (p < 0.01) at rest and lower (p = 0.008) in H18–30 but similar (p > 0.267) in the older groups during sympathetic activation. At rest, cf-PWV values were lower in young women compared to young men (p = 0.001); however, this trend was reversed in the older groups (p < 0.04). During sympathetic activation, the cf-PWV was lower in women in H18–30 (p = 0.001), similar between sexes in H50–80 (p = 0.122), and higher in women in T2DM50–80 (p = 0.004). These data do not support the hypothesis that aortic stiffness increases in the morning compared to the evening within any of the considered groups in both rest and sympathetic activation conditions. There are differences between the sexes, which vary according to age and diabetes status. In particular, aortic stiffness is higher in older women than in men with diabetes during acute stress.

Sympathetic-mediated blunting of forearm vasodilation is similar between young men and women

BACKGROUND

The in-vivo regulation of vascular conductance (VC) is a continuous balance between endothelial vasodilation and sympathetic vasoconstriction. Although women may report blunted sympathetic vasoconstriction along with higher endothelial vasodilation than men, it is currently unknown whether the interaction between vasoconstriction and vasodilation leads to different regulation of VC between sexes. This study assessed sex differences in sympathetic-mediated blunting of endothelial vasodilation after a brief period of ischemia and whether any restriction of vasodilation blunts tissue blood flow (BF) and re-oxygenation.

METHODS

13 young women and 12 young men underwent two 5-min forearm circulatory occlusions followed by reperfusion, one in basal conditions and the other during cold pressor test-induced sympathetic activation (SYMP). Brachial artery diameter and BF, mean arterial pressure, total peripheral resistance (TPR), and thenar eminence oxygenation were collected. Percent changes normalized to baseline values of forearm VC, brachial artery BF and flow-mediated dilation (FMD), TPR, and hand oxygenation after circulatory reperfusion were calculated.

RESULTS

TPR increased during SYMP in men (p = 0.019) but not in women (p = 0.967). Women showed a greater brachial artery FMD than men (p = 0.004) at rest, but sex differences disappeared after normalization to shear rate and baseline diameter (p > 0.11). The percent increases from baseline of peak and average forearm VC after circulatory reperfusion did not differ between sexes in basal conditions (p > 0.98) or during SYMP (p > 0.97), and were restrained by SYMP similarly in both sexes (p < 0.003) without impairing the hand re-oxygenation (p > 0.08) or average hyperemic response (p > 0.09).

CONCLUSIONS

Although women may report blunted sympathetic vasoconstriction than men when assessed separately, the similar sympathetic-mediated restriction of vasodilation suggests a similar dynamic regulation of VC between sexes. SYMP-mediated restrictions of the normal forearm vasodilation do not impair the average hyperemic response and hand re-oxygenation in both sexes.

Aging is associated with enhanced central, but impaired peripheral arms of the sympathetic baroreflex arc

We tested the hypothesis that the baroreflex control of action potential (AP) subpopulations would be blunted in older compared to young adults. Integrated muscle sympathetic nerve activity (MSNA) and the underlying sympathetic APs were obtained using microneurography and a continuous wavelet analysis approach, respectively, during 5 minutes of supine rest in 13 older (45-75 years, 6 females) and 14 young (21-30 years, 7 females) adults. Baroreflex threshold relationships were quantified as the slope of the linear regression between MSNA burst probability (%) and diastolic blood pressure (mmHg), or AP cluster firing probability (%) and diastolic blood pressure (mmHg). Integrated MSNA baroreflex threshold gain was greater in older compared to young adults (older: -7.6±3.6 %/mmHg vs. Young: -3.5±1.5 %/mmHg, P<0.001). Similarly, the baroreflex threshold gain of AP clusters was modified by aging (group-by-cluster effect: P<0.001) such that older adults demonstrated greater baroreflex threshold gains of medium-sized AP clusters (e.g., Cluster 4, older: -8.2±3.2 %/mmHg vs. Young: -3.6±1.9 %/mmHg, P=0.003) but not for the smallest- (Cluster 1, older: -1.6±1.9 %/mmHg vs. Young: -1.0±1.7 %/mmHg, P>0.999) and largest-sized (Cluster 10, older: -0.5±0.5 %/mmHg vs. Young: -0.2±0.1 %/mmHg, P=0.819) AP clusters compared to young adults. In contrast, the peak change in mean arterial pressure (MAP) following a spontaneous MSNA burst (i.e., sympathetic transduction) was impaired with aging (older: -0.7±0.3 mmHg vs. Young: 1.8±1.2 mmHg, P<0.001). We conclude that aging is associated with elevated baroreflex control over high-probability AP content of sympathetic bursts that may compensate for impaired sympathetic neurovascular transduction.

Comparison of signal-averaging and regression approaches to analyzing sympathetic transduction

PURPOSE

Spontaneous sympathetic transduction reflects the vascular and/or pressor responses to bursts of muscle sympathetic nerve activity (MSNA). Separately, signal-averaging and regression-based approaches have been implemented to quantify resting sympathetic transduction. It is unknown whether the outcomes of these analytical approaches provide (dis)similar information, which is imperative for between-study comparisons and the amalgamation of results for synthesis of multiple studies (i.e., meta-analyses). We explored the diastolic blood pressure (DBP) responses to spontaneous bursts of MSNA between these two methods of analysis.

METHODS

Resting beat-by-beat DBP (via finger photoplethysmography) and common peroneal nerve MSNA (via microneurography) were recorded in 52 healthy, normotensive adults (age 38 ± 20 years; 19 females). For the signal-averaged method, transduction was quantified as the mean peak increase in DBP (ΔDBP) during the 12 cardiac cycles following each MSNA burst. In addition, DBP was regressed to a moving two-cardiac-cycle window of normalized relative burst height (mmHg/relative %) to provide the regression-based transduction outcome.

RESULTS

The signal-averaged (1.2 ± 0.7 mmHg) and regression-based approaches (0.009 ± 0.016 mmHg/%) were unrelated (ρ = 0.03, p = 0.86). Adding to the discrepancy, only the signal-averaging approach demonstrated a lower transduction in middle-aged-older males versus younger males.

CONCLUSIONS

The decision of which method to use when calculating sympathetic transduction influences study outcomes, with the two most common methods of determining transduction being unrelated. There are challenges of making sweeping conclusions across studies if different analysis strategies are implemented. An understanding of when to use each method is needed to adopt a harmonized approach to quantifying sympathetic transduction.

Heart Rate Recovery, Central Systolic Pressure, and Augmentation Index in Young Healthy Individuals

Purpose

Measuring heart rate recovery (HRR) holds valuable cardiovascular information and requires minimal technical skill and cost. Understanding the associations between HRR and more robust cardiovascular indicators, such as central systolic blood pressure (CSBP), can provide valuable cardiovascular information with less involvement. CSBP is a strong predictor of certain cardiovascular diseases. The study aims to examine the association between measures of HRR and CSBP and the augmentation index (AIx) in a group of young, healthy individuals and based on sex.

Participants and Methods

One-hundred and seven participants (men – 55, women – 52) were measured for HRR at one minute (HRR₁) and two minutes (HRR₂) after maximum oxygen consumption (VO_2max) testing, CSBP, and the AIx at a heart rate of 75 beats∙min⁻¹ (AIx@75).

Results

The Pearson correlation indicated no association between HRR₁, HRR₂, and CSBP in men and women combined: r = 0.06, P = 0.53; r = 0.05, P = 0.59, respectively, or based on sex: men = r = 0.01, P = 0.95; r = 0.04, P = 0.79, respectively, and women = r = −0.05, P = 0.75; r = −0.09, P = 0.52, respectively. However, there were associations between HRR₁ and AIx@75 in men and women combined: r = −0.37, P < 0.001, and based on sex: men = r = −0.31, P = 0.02, and women = r = −0.38, P < 0.01.

Conclusion

Measures of HRR were not associated with CSBP in a combined group of young men and women or based on sex. Most measures of HRR, especially those established by parasympathetic nervous activity, were associated with lower AIx@75. Though measures of HRR might be good indicators of cardiovascular disease, they might not be good indicators of CSBP in young, healthy individuals.

The pathophysiology of vasovagal syncope: Novel insights

The pathophysiology of vasovagal syncope (VVS) is reviewed, focusing on hemodynamic aspects. Much more is known about orthostatic than about emotional VVS, probably because the former can be studied using a tilt table test (TTT). Recent advances made it possible to quantify the relative contributions of the three factors that control blood pressure: heart rate (HR), stroke volume (SV) and total peripheral resistance (TPR). Orthostatic VVS starts with venous pooling, reflected in a decrease of SV. This is followed by cardioinhibition (CI), which is a decrease of HR that accelerates the ongoing decrease of BP, making the start of CI a literal as well as fundamental turning point. The role of hormonal and other humoral factors, respiration and of psychological influences is reviewed in short, leading to the conclusion that a multidisciplinary approach to the study of the pathophysiology of VVS may yield new insights.

The impact of ageing and sex on sympathetic neurocirculatory regulation

The sympathetic nervous system represents a critical mechanism for homoeostatic blood pressure regulation in humans. This review focuses on age-related alterations in neurocirculatory regulation in men and women by highlighting human studies that examined the relationship between muscle sympathetic nerve activity (MSNA) acquired by microneurography and circulatory variables (e.g., blood pressure, vascular resistance). We frame this review with epidemiological evidence highlighting sex-specific patterns in age-related blood pressure increases in developed nations. Indeed, young women exhibit lower blood pressure than men, but women demonstrate larger blood pressure increases with age, such that by about age 60 years, blood pressure is greater in women. Sympathetic neurocirculatory mechanisms contribute to sex differences in blood pressure rises with age. Muscle sympathetic nerve activity increases with age in both sexes, but women demonstrate greater age-related increases. The circulatory adjustments imposed by MSNA - referred to as neurovascular transduction or autonomic (sympathetic) support of blood pressure - differ in men and women. For example, whereas young men demonstrate a positive relationship between resting MSNA and vascular resistance, this relationship is absent in young women due to beta-2 adrenergic vasodilation, which offsets alpha-adrenergic vasoconstriction. However, post-menopausal women demonstrate a positive relationship between MSNA and vascular resistance due to a decline in beta-2 adrenergic vasodilatory mechanisms. Emerging data suggest that greater aerobic fitness appears to modulate neurocirculatory regulation, at least in young, healthy men and women. This review also highlights recent advances in microneurographic recordings of sympathetic action potential discharge, which may nuance our understanding of age-related alterations in sympathetic neurocirculatory regulation in humans.

Signal-averaged resting sympathetic transduction of blood pressure: is it time to account for prevailing muscle sympathetic burst frequency?

Calculating the blood pressure (BP) response to a burst of muscle sympathetic nerve activity (MSNA), termed sympathetic transduction, may be influenced by an individual's resting burst frequency. We examined the relationships between sympathetic transduction and MSNA in 107 healthy males and females and developed a normalized sympathetic transduction metric to incorporate resting MSNA. Burst-triggered signal-averaging was used to calculate the peak diastolic BP response following each MSNA burst (sympathetic transduction of BP) and following incorporation of MSNA burst cluster patterns and amplitudes (sympathetic transduction slope). MSNA burst frequency was negatively correlated with sympathetic transduction of BP (r=-0.42; P<0.01) and the sympathetic transduction slope (r=-0.66; P<0.01), independent of sex. MSNA burst amplitude was unrelated to sympathetic transduction of BP in males (r=0.04; P=0.78), but positively correlated in females (r=0.44; P<0.01) and with the sympathetic transduction slope in all participants (r=0.42; P<0.01). To control for MSNA, the linear regression slope of the log-log relationship between sympathetic transduction and MSNA burst frequency was used as a correction exponent. In sub-analysis of males (38±10 vs. 14±4bursts/min) and females (28±5 vs. 12±4bursts/min) with high vs. low MSNA, sympathetic transduction of BP and sympathetic transduction slope were lower in participants with high MSNA (all P<0.05). In contrast, normalized sympathetic transduction of BP and normalized sympathetic transduction slope were similar in males and females with high vs. low MSNA (all P>0.22). We propose that incorporating MSNA burst frequency into the calculation of sympathetic transduction will allow comparisons between participants with varying levels of resting MSNA.

Sex-dependent development of enhanced sympathoexcitation in sedentary versus physically active rats

KEY POINTS

The rostral ventrolateral medulla (RVLM) may contribute to sex-based differences in cardiovascular disease (CVD) based on overactivation of the sympathetic nervous system observed in sedentary male rats; however, the added influence of the reproductive cycle in females is currently unknown. To our knowledge this is the first study to demonstrate greater increases in sympathetic nerve activity in response to direct activation of the RVLM in female versus male rats prior to the onset of the reproductive cycle, which persisted after the onset of the reproductive cycle. Lower resting blood pressures in females also suggest peripheral adaptations contribute to sex-based differences in CVD. Sedentary versus physically active conditions appear to promote higher resting sympathetic outflow independent of age and sex. Our results demonstrate the importance of examining sedentary conditions in the context of sex differences and the reproductive cycle in contributing to sympathetic overactivity associated with cardiovascular disease.

ABSTRACT

Female reproductive hormones are considered cardioprotective based on higher risks of cardiovascular disease (CVD) in post- versus pre-menopausal women. Similarly, based on epidemiological studies, a sedentary lifestyle is also a major risk factor for CVD. The mechanisms by which sedentary conditions contribute to CVD, and their influences in the presence and absence of female reproductive hormones are unknown. We hypothesized that sexually immature male and female rats would have similar centrally mediated regulation of blood pressure, but upon sexual maturation, female rats would have lower resting blood pressure and centrally-mediated sympathoexcitation compared to age-matched males. We also predicted resting sympathetic activity would increase upon exposure to sedentary versus active conditions (voluntary wheel running) in males but not in females. We recorded splanchnic sympathetic nerve activity (SSNA) and blood pressure in 4-, 8- and 16-week-old male and female rats under Inactin anaesthesia before and during microinjections of glutamate (1-100 mM) into the rostral ventrolateral medulla (RVLM). Four-week-old female rats had lower resting blood pressure and greater sympathoexcitation following activation of the RVLM, as did 8- and 16-week-old female rats, independent of age or activity condition. Sedentary animals had higher baseline SSNA compared to active animals, independent of sex or age. Our results reveal a complex influence of the interactions between the female reproductive cycle and sedentary conditions. They also demonstrate the importance of examining sedentary conditions in the context of sex- and female reproductive cycle-dependent incidences of cardiovascular disease.

Sex Differences in Muscle Metaboreflex Activation following Static Handgrip Exercise

PURPOSE

Larger blood pressure (BP) responses to relative-intensity static exercise in males vs. females is thought to involve altered muscle metaboreflex activation, but whether this is due to an intrinsic sex difference in metabolite production or to differences in muscle strength and absolute load is unknown.

METHODS

Continuous BP and heart rate were recorded in 200 healthy young males and females (females: n = 109) during 2 minutes of static handgrip exercise at 30% of maximal voluntary contraction (MVC), followed by 2 minutes of post-exercise circulatory occlusion (PECO). Muscle sympathetic nerve activity (MSNA) was recorded in a subset of participants (n = 39; female n = 21), permitting calculation of signal-averaged resting sympathetic transduction (MSNA-diastolic BP). Sex differences were examined with and without statistical adjustment for MVC. Multivariate regression analyses were performed to identify predictors of BP responses.

RESULTS

Males had larger systolic BP responses (interactions, P < 0.0001) to static handgrip exercise (24 ± 10 vs. 17 ± 9 mmHg [mean ± SD], P < 0.0001) and PECO (20 ± 11 vs. 16 ± 9 mmHg, P < 0.0001). Adjustment for MVC abolished these sex differences in BP (interactions, P > 0.7). In the subset with MSNA, neither burst frequency or incidence responses to static handgrip exercise or PECO differed between males and females (interactions, P > 0.2). Resting sympathetic transduction was also similar (P = 0.8). Multivariate analysis showed that MVC, the change in MSNA, and sympathetic transduction were predictors of the systolic BP response to static handgrip but only MVC was associated with responses during PECO.

CONCLUSION

Sex differences in absolute contraction load contribute to differences in BP responses during muscle metaboreflex isolation using PECO. These data do not support an intrinsic effect of sex as being responsible for exercise BP differences between males and females.

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.

An open-source program to analyze spontaneous sympathetic neurohemodynamic transduction

The sympathetic nervous system is important for the beat-by-beat regulation of arterial blood pressure and the control of blood flow to various organs. Microneurographic recordings of pulse-synchronous muscle sympathetic nerve activity (MSNA) are used by numerous laboratories worldwide. The transduction of hemodynamic and vascular responses elicited by spontaneous bursts of MSNA provides novel, mechanistic insight into sympathetic neural control of the circulation. Although some of these laboratories have developed in-house software programs to analyze these sympathetic transduction responses, they are not openly available and most require higher level programming skills and/or costly platforms. In the present paper, we present an open-source, Microsoft Excel-based analysis program designed to examine the pressor and/or vascular responses to spontaneous resting bursts of MSNA, including across longer, continuous MSNA burst sequences, as well as following heartbeats not associated with MSNA bursts. An Excel template with embedded formulas is provided. Detailed written and video-recorded instructions are provided to help facilitate the user and promote its implementation among the research community. Open science activities such as the dissemination of analytical programs and instructions may assist other laboratories in their pursuit to answer novel and impactful research questions regarding sympathetic neural control strategies in human health and disease.NEW & NOTEWORTHY The pressor responses to spontaneous bursts of muscle sympathetic nerve activity provide important information regarding sympathetic regulation of the circulation. Many laboratories worldwide quantify sympathetic neurohemodynamic transduction using in-house, customized software requiring high-level programming skills and/or costly computer programs. To overcome these barriers, this study presents a simple, open-source, Microsoft Excel-based analysis program along with video instructions to assist researchers without the necessary resources to quantify sympathetic neurohemodynamic transduction.

Sympathetic transduction in humans: recent advances and methodological considerations

Ever since their origin more than one half-century ago, microneurographic recordings of sympathetic nerve activity have significantly advanced our understanding of the generation and regulation of central sympathetic outflow in human health and disease. For example, it is now appreciated that a myriad of disease states exhibit chronic sympathetic overactivity, a significant predictor of cardiovascular morbidity and mortality. Although microneurographic recordings allow for the direct quantification of sympathetic outflow, they alone do not provide information with respect to the ensuing sympathetically mediated vasoconstriction and blood pressure (BP) response. Therefore, the study of vascular and/or BP responses to sympathetic outflow (i.e., sympathetic transduction) has now emerged as an area of growing interest within the field of neural cardiovascular control in human health and disease. To date, studies have primarily examined sympathetic transduction under two distinct paradigms: when reflexively evoking sympatho-excitation through the induction of a laboratory stressor (i.e., sympathetic transduction during stress) and/or following spontaneous bursts of sympathetic outflow occurring under resting conditions (i.e., sympathetic transduction at rest). The purpose of this brief review is to highlight how our physiological understanding of sympathetic transduction has been advanced by these studies and to evaluate the primary analytical techniques developed to study sympathetic transduction in humans. We also discuss the framework by which the assessment of sympathetic transduction during stress reflects a fundamentally different process relative to sympathetic transduction at rest and why findings from investigations using these different techniques should be interpreted as such and not necessarily be considered one and the same.

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.

Aerobic fitness and sympathetic responses to spontaneous muscle sympathetic nerve activity in young males

PURPOSE

Lower aerobic fitness increases the risk of developing hypertension. Muscle sympathetic nerve activity (MSNA) is important for the beat-by-beat regulation of blood pressure. Whether the cardiovascular consequences of lower aerobic fitness are due to augmented transduction of MSNA into vascular responses is unclear. We tested the hypothesis that aerobic fitness is inversely related to peak increases in total peripheral resistance (TPR) and mean arterial pressure (MAP) in response to spontaneous MSNA bursts in young males.

METHODS

Relative peak oxygen consumption (VO₂peak, indirect calorimetry) was assessed in 18 young males (23 ± 3 years; 41 ± 8 ml/kg/min). MSNA (microneurography), cardiac intervals (electrocardiogram) and arterial pressure (finger photoplethysmography) were recorded continuously during supine rest. Stroke volume and cardiac output (CO) were estimated via the ModelFlow method. TPR was calculated as MAP/CO. Changes in TPR and MAP were tracked for 12 cardiac cycles following heartbeats associated with or without spontaneous bursts of MSNA.

RESULTS

Overall, aerobic fitness was inversely correlated to the peak ΔTPR (0.8 ± 0.7 mmHg/l/min; R = - 0.61, P = 0.007) and ΔMAP (2.3 ± 0.8 mmHg; R = - 0.69, P < 0.001), but not with the peak ΔCO (0.2 ± 0.1 l/min; P = 0.50), MSNA burst frequency (14 ± 5 bursts/min; P = 0.43) or MSNA relative burst amplitude (65 ± 12%; P = 0.13). Heartbeats without an associated burst of MSNA did not increase TPR, MAP or CO.

CONCLUSION

Although unrelated to traditional MSNA characteristics, aerobic fitness was inversely associated with spontaneous sympathetic neurovascular transduction in young males. This may be a potential mechanism by which aerobic fitness modulates the regulation of arterial blood pressure through the sympathetic nervous system.