Influence of age and sex on the pressor response following a spontaneous burst of muscle sympathetic nerve activity

Aging in females has minimal effect on changes in celiac artery blood flow during dynamic light-intensity exercise

Blood flow to the active muscles and arterial blood pressure (ABP) increase during dynamic exercise, whereas blood flow to inactive organs (e.g., splanchnic organs and inactive limbs) declines. Aging leads to exaggerated ABP responses to exercise in females, but whether this is related to greater splanchnic vasoconstriction is unknown. This study sought to clarify the effect of aging in females on celiac artery blood flow during dynamic light-intensity exercise. Twelve healthy young females (YF: 20 ± 2 yr, mean ± SD) and 12 healthy older females (OF: 71 ± 4 yr) performed dynamic knee-extension and knee-flexion exercises at 30% of heart rate reserve for 4 min. The absolute changes from baseline (Δ) for mean arterial blood pressure (MAP), celiac artery mean blood flow (celMBF), and celiac vascular conductance (celVC) during exercise were calculated. ABP was measured using an automated sphygmomanometer, and celMBF was recorded by Doppler ultrasonography. The increase in MAP during exercise was greater in OF than in YF (YF: +14 ± 7 mmHg, OF: +24 ± 13 mmHg, P = 0.028). The celMBF decreased during exercise in both groups, but there was no significant difference in the response between YF and OF (YF: -93.0 ± 66.1 mL/min, OF: -89.6 ± 64.0 mL/min, P = 0.951). The celVC also decreased during exercise and remained lower than baseline during exercise. However, the response was not different between YF and OF (YF: -1.8 ± 1.0 mL/min/mmHg, OF: -1.5 ± 0.6 mL/min/mmHg, P = 0.517). These results demonstrate that aging in females has minimal influence on splanchnic artery hemodynamic responses during dynamic light-intensity exercise, suggesting that exaggerated ABP responses during exercise in OF are not due to greater splanchnic vasoconstriction.NEW & NOTEWORTHY During exercise, the splanchnic arteries vasoconstrict, contributing to blood flow redistribution and the blood pressure response. Blood pressure responses to exercise are exaggerated with aging in females; however, the physiological mechanism responsible has not been clarified. We show that celiac artery blood flow changes during light-intensity dynamic exercise do not differ with age in females. This indicates the exaggerated blood pressure to exercise with aging is likely not due to a difference in splanchnic vasoconstriction.

Intra- and interday reliability of sympathetic transduction to blood pressure in young, healthy adults

Microneurographic recordings of muscle sympathetic nerve activity (MSNA) and the succeeding changes in beat-to-beat blood pressure (i.e., sympathetic transduction) provide important insights into the neural control of the circulation in humans. Despite its widespread use, the reliability of this technique remains unknown. Herein, we assessed the intra- and interday test-retest reliability of signal-averaging sympathetic transduction to blood pressure. Data were analyzed from 15 (9 M/6 F) young, healthy participants who completed two baseline recordings of fibular nerve MSNA separated by 60 min (intraday). The interday reliability was obtained in a subset of participants (n = 13, 9 M/4 F) who completed a follow-up MSNA study. Signal-averaging sympathetic transduction was quantified as peak change in diastolic (DBP) and mean arterial pressure (MAP) following a burst of MSNA. Analyses were also computed considering different MSNA burst sizes (quartiles of normalized MSNA) and burst patterns (singlets, couplets, triplets, and quadruplets+), as well as nonburst responses. Intraclass-correlation coefficients (ICCs) were used as the main reliability measure. Peak changes in MAP [intraday: ICC = 0.76 (0.30-0.92), P = 0.006; interday: ICC = 0.91 (0.63-0.97), P < 0.001] demonstrated very good to excellent reliability. Sympathetic transduction of MSNA burst size displayed moderate to very good reliability, though the reliability of MSNA burst pattern was poor to very good. Nonburst responses revealed poor intraday [ICC = 0.37 (-1.05 to 0.80), P = 0.21], but very good interday [ICC = 0.76 (0.18-0.93), P = 0.01] reliability. Intraday reliability measures were consistently lower than interday reliability. Similar results were obtained using DBP. Collectively, these findings provide evidence that the burst-triggering signal-averaging technique is a reliable measure of sympathetic transduction to blood pressure in young, healthy adults.NEW & NOTEWORTHY We found that signal-averaging sympathetic transduction to blood pressure displayed very good to excellent intra- and interday test-retest reliability in healthy, young adults. Reliability analyses according to muscle sympathetic burst size, burst pattern, and nonburst response were less consistent. Results were similar when using diastolic or mean arterial pressure in the transduction calculation. These findings suggest that the signal-averaging technique can be used with confidence to investigate sympathetic transduction to blood pressure in humans across time.

Sympathetic determinants of resting blood pressure in males and females

Discharge of postganglionic muscle sympathetic nerve activity (MSNA) is related poorly to blood pressure (BP) in adults. Whether neural measurements beyond the prevailing level of MSNA can account for interindividual differences in BP remains unclear. The current study sought to evaluate the relative contributions of sympathetic-BP transduction and sympathetic baroreflex gain on resting BP in young adults. Data were analyzed from 191 (77 females) young adults (18-39 years) who underwent continuous measurement of beat-to-beat BP (finger photoplethysmography), heart rate (electrocardiography), and fibular nerve MSNA (microneurography). Linear regression analyses were computed to determine associations between sympathetic-BP transduction (signal-averaging) or sympathetic baroreflex gain (threshold technique) and resting BP, before and after controlling for age, body mass index, and MSNA burst frequency. K-mean clustering was used to explore sympathetic phenotypes of BP control and consequential influence on resting BP. Sympathetic-BP transduction was unrelated to BP in males or females (both R2 < 0.01; P > 0.67). Sympathetic baroreflex gain was positively associated with BP in males (R2 = 0.09, P < 0.01), but not in females (R2 < 0.01; P = 0.80), before and after controlling for age, body mass index, and MSNA burst frequency. K-means clustering identified a subset of participants with average resting MSNA, yet lower sympathetic-BP transduction and lower sympathetic baroreflex gain. This distinct subgroup presented with elevated BP in males (P < 0.02), but not in females (P = 0.10). Sympathetic-BP transduction is unrelated to resting BP, while the association between sympathetic baroreflex gain and resting BP in males reveals important sex differences in the sympathetic determination of resting BP.NEW & NOTEWORTHY In a sample of 191 normotensive young adults, we confirm that resting muscle sympathetic nerve activity is a poor predictor of resting blood pressure and now demonstrate that sympathetic baroreflex gain is associated with resting blood pressure in males but not females. In contrast, signal-averaged measures of sympathetic-blood pressure transduction are unrelated to resting blood pressure. These findings highlight sex differences in the neural regulation of blood pressure.

Sympathetic transduction of cardiac sympathetic nerve activity in healthy, conscious sheep

Sympathetic transduction is the study of how impulses of sympathetic nerve activity (SNA) affect end-organ function. Recently, the transduction of resting bursts of muscle SNA (MSNA) has been investigated and shown to have a role in the maintenance of blood pressure through changes in vascular tone in humans. In the present study, we investigate whether directly recorded resting cardiac SNA (CSNA) regulates heart rate (HR), coronary blood flow (CoBF), coronary vascular conductance (CVC), cardiac output (CO) and mean arterial pressure. Instrumentation was undertaken to record CSNA and relevant vascular variables in conscious sheep. Recordings were performed at baseline, as well as after the infusion of a β-adrenoceptor blocker (propranolol) to determine the role of β-adrenergic signalling in sympathetic transduction in the heart. The results show that after every burst of CSNA, there was a significant effect of time on HR (n = 10, ∆: +2.1 ± 1.4 beats min-1 , P = 0.002) and CO (n = 8, ∆: +100 ± 150 mL min-1 , P = 0.002) was elevated, followed by an increase in CoBF (n = 9, ∆: +0.76 mL min-1 , P = 0.001) and CVC (n = 8, ∆: +0.0038 mL min-1  mmHg-1 , P = 0.0028). The changes in HR were graded depending on the size and pattern of CSNA bursts. The HR response was significantly attenuated after the infusion of propranolol. Our study is the first to explore resting sympathetic transduction in the heart, suggesting that CSNA can dynamically change HR mediated by an action on β-adrenoceptors. KEY POINTS: Sympathetic transduction is the study of how impulses of sympathetic nerve activity (SNA) affect end-organ function. Previous studies have examined sympathetic transduction primarily in the skeletal muscle and shown that bursts of muscle SNA alter blood flow to skeletal muscle and mean arterial pressure, although this has not been examined in the heart. We investigated sympathetic transduction in the heart and show that, in the conscious condition, the size of bursts of SNA to the heart can result in incremental increases in heart rate and coronary blood flow mediated by β-adrenoceptors. The pattern of bursts of SNA to the heart also resulted in incremental increases in heart rate mediated by β-adrenoceptors. This is the first study to explore the transduction of bursts of SNA to the heart.

Aging and sympathetic transduction to blood pressure in humans: methodological and physiological considerations

Recent reports suggest that quantification of signal-averaged sympathetic transduction is influenced by resting muscle sympathetic nerve activity (MSNA) and burst occurrence relative to the average mean arterial pressure (MAP). Herein, we asked how these findings may influence age-related reductions in sympathetic transduction. Beat-to-beat blood pressure and MSNA were recorded during 5 min of rest in 27 younger (13 females: age, 25 ± 5 yr; BMI, 25 ± 4 kg/m2) and 26 older (15 females: age, 59 ± 5 yr; BMI, 26 ± 4 kg/m2) healthy adults. All MSNA bursts were signal averaged together. Beat-to-beat MAP values were then split into low (T1), middle (T2), and high (T3) tertiles, and signal-averaged transduction was calculated within each tertile. Resting MSNA was higher in older adults and MAP was similar between groups. Older adults exhibited blunted overall MAP transduction (younger, Δ1.5 ± 0.6 vs. older, Δ0.9 ± 0.7 mmHg; P = 0.005), which was irrespective of relation to prevailing MAP. A greater proportion of bursts occurred above the average MAP in older adults (P < 0.001), and a larger proportion of these bursts were associated with depressor responses (P = 0.005). Nonetheless, assessment of bursts above the average MAP associated with pressor responses revealed similar age-associated reductions in transduction (younger, Δ2.6 ± 1.6 vs. older, Δ1.7 ± 0.8 mmHg; P = 0.016). These findings indicate an age-related increase in burst occurrence above the average resting MAP, which alone does not explain blunted transduction, thereby supporting the physiological underpinnings of age-related decrements in sympathetic transduction to blood pressure.NEW & NOTEWORTHY The current study demonstrated that aging is associated with a greater prevalence of sympathetic bursts occurring above the average blood pressure, which offers both methodologically and physiologically relevant information regarding aging and sympathetic control of blood pressure. These data support age-related reductions in sympathetic transduction via a reduced pressor response to sympathetic bursts irrespective of the prevailing absolute blood pressure value, along with increases in sympathetic outflow necessary to maintain blood pressure.

Sympathetic transduction at rest and during cold pressor test in young healthy non-Hispanic Black and White women

Non-Hispanic Black (BL) individuals have the highest prevalence of hypertension and cardiovascular disease (CVD) compared with all other racial/ethnic groups. Previous work focused on racial disparities in sympathetic control and blood pressure (BP) regulation between young BL and White (WH) adults, have mainly included men. Herein, we hypothesized that BL women would exhibit augmented resting sympathetic vascular transduction and greater sympathetic and BP reactivity to cold pressor test (CPT) compared with WH women. Twenty-eight young healthy women (BL: n = 14, 22 [Formula: see text] 4 yr; WH: n = 14, 22 [Formula: see text] 4 yr) participated. Beat-to-beat BP (Finometer), common femoral artery blood flow (duplex Doppler ultrasound), and muscle sympathetic nerve activity (MSNA; microneurography) were continuously recorded. In a subset (BL n = 10, WH n = 11), MSNA and BP were recorded at rest and during a 2-min CPT. Resting sympathetic vascular transduction was quantified as changes in leg vascular conductance (LVC) and mean arterial pressure (MAP) following spontaneous bursts of MSNA using signal averaging. Sympathetic and BP reactivity were quantified as changes in MSNA and MAP during the last minute of CPT. There were no differences in nadir LVC following resting MSNA bursts between BL (-8.70 ± 3.43%) and WH women (-7.30 ± 3.74%; P = 0.394). Likewise, peak increases in MAP following MSNA bursts were not different between groups (BL: +2.80 ± 1.42 mmHg; vs. WH: +2.99 ± 1.15 mmHg; P = 0.683). During CPT, increases in MSNA and MAP were also not different between BL and WH women, with similar transduction estimates between groups (ΔMAP/ΔMSNA; P = 0.182). These findings indicate that young, healthy BL women do not exhibit exaggerated sympathetic transduction or augmented sympathetic and BP reactivity during CPT.NEW & NOTEWORTHY This study was the first to comprehensively investigate sympathetic vascular transduction and sympathetic and BP reactivity during a cold pressor test in young, healthy BL women. We demonstrated that young BL women do not exhibit exaggerated resting sympathetic vascular transduction and do not have augmented sympathetic or BP reactivity during cold stress compared with their WH counterparts. Collectively, these findings suggest that alterations in sympathetic transduction and reactivity are not apparent in young, healthy BL women.

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.

Aortic stiffness contributes to greater pressor responses during static hand grip exercise in healthy young and middle-aged normotensive men

Central arterial stiffness can influence exercise blood pressure (BP) by increasing the rise in arterial pressure per unit increase in aortic inflow. Whether central arterial stiffness influences the pressor response to isometric handgrip exercise (HG) and post-exercise muscle ischemia (PEMI), two common laboratory tests to study sympathetic control of BP, is unknown. We studied 46 healthy non-hypertensive males (23 young and 23 middle-aged) during HG (which increases in cardiac output [Q̇c]) and isolated metaboreflex activation PEMI (no change or decreases in Q̇c). Aortic stiffness (aortic pulse wave velocity [aPWV]; applanation tonometry via SphygmoCor) was measured during supine rest and was correlated to the pressor responses to HG and PEMI. BP (photoplethysmography) and muscle sympathetic nerve activity (MSNA) were continuously recorded at rest, during HG to fatigue (35 % maximal voluntary contraction) and 2-min of PEMI. aPWV was higher in middle-aged compared to young males (7.1 ± 0.9 vs 5.4 ± 0.7 m/s, P < 0.001). Middle-aged males also exhibited greater increases in systolic pressure (∆30 ± 11 vs 10 ± 8 mmHg) and MSNA (∆2313 ± 2006 vs 1387 ± 1482 %/min) compared to young males during HG (both, P < 0.03); with no difference in the Q̇c response (P = 0.090). Responses to PEMI were not different between groups. Sympathetic transduction during these stressors (MSNA-diastolic pressure slope) was not different between groups (P > 0.341). Middle-aged males displayed a greater increase in SBP per unit change of Q̇c during HG (∆SBP/∆Q̇c; 21 ± 18 vs 6 ± 10 mmHg/L/min, P = 0.004), with a strong and moderate relationship between the change in systolic (r = 0.53, P < 0.001) and diastolic pressure (r = 0.34, P = 0.023) and resting aPWV, respectively; with no correlation during PEMI. Central arterial stiffness can modulate pressor responses during stimuli associated with increases in cardiac output and sympathoexcitation in healthy males.

Greater resting muscle sympathetic nerve activity reduces cold pressor autonomic reactivity in older women, but not older men

Previous work demonstrates augmented muscle sympathetic nerve activity (MSNA) responses to the cold pressor test (CPT) in older women. Given its interindividual variability, however, the influence of baseline MSNA on CPT reactivity in older adults remains unknown. Sixty volunteers (60-83y; 30 women) completed testing where MSNA (microneurography), blood pressure (BP), and heart rate (HR) were recorded during baseline and a 2-min CPT (~4°C). Participant data were terciled by baseline MSNA (n=10/group); comparisons were made between the high baseline men (HM) and women (HW), and low baseline men (LM) and women (LW). By design, HM and HW, vs. LM and LW, had greater baseline MSNA burst frequency (37±5 and 38±3 vs. 9±4 and 15±5 bursts/min) and burst incidence (59±14 and 60±8 vs. 16±10 and 23±7 bursts/100hbs; both P<0.001). However, baseline BP and HR were not different between the groups (all P>0.05). During the CPT, there were no differences in the increase in BP and HR (all P>0.05). Conversely, ΔMSNA burst frequency was lower in HW vs. LW (8±9 vs. 22±12 bursts/min; P=0.012) yet was similar in HM vs. LM (17±12 vs. 19±10 bursts/min, P=0.994). Further, ΔMSNA burst incidence was lower in HW vs. LW (9±13 vs. 28±16 bursts/100hbs; P=0.020), with no differences between HM vs. LM (21±17 vs. 31±17 bursts/100hbs; P=0.455). Our findings suggest that heightened baseline activity in older women attenuates the typical CPT-mediated increase in MSNA without changing cardiovascular reactivity. While the underlying mechanisms remain unknown, altered sympathetic recruitment or neurovascular transduction may contribute to these disparate responses.

Concomitantly higher resting arterial blood pressure and transduction of sympathetic neural activity in human obesity without hypertension

OBJECTIVE

Central (abdominal) obesity is associated with elevated adrenergic activity and arterial blood pressure (BP). Therefore, we tested the hypothesis that transduction of spontaneous muscle sympathetic nerve activity (MSNA) to BP, that is, sympathetic transduction, is augmented in abdominal obesity (increased waist circumference) and positively related to prevailing BP.

METHODS

Young/middle-aged obese (32 ± 7 years; BMI: 36 ± 5 kg/m2, n = 14) and nonobese (29 ± 10 years; BMI: 23 ± 4 kg/m2, n = 14) without hypertension (24-h ambulatory average BP < 130/80 mmHg) were included. MSNA (microneurography) and beat-to-beat BP (finger cuff) were measured continuously and the increase in mean arterial pressure (MAP) during 15 cardiac cycles following MSNA bursts of different patterns (single, multiples) and amplitude (quartiles) was signal-averaged over a 10 min baseline period.

RESULTS

MSNA burst frequency was not significantly higher in obese vs. nonobese (21 ± 3 vs. 17 ± 3 bursts/min, P = 0.34). However, resting supine BP was significantly higher in obese compared with nonobese (systolic: 127 ± 3 vs. 114 ± 3; diastolic: 76 ± 2 vs. 64 ± 1 mmHg, both P < 0.01). Importantly, obese showed greater increases in MAP following multiple MSNA bursts (P = 0.02) and MSNA bursts of higher amplitude (P = 0.02), but not single MSNA bursts (P = 0.24), compared with nonobese when adjusting for MSNA burst frequency. The increase in MAP following higher amplitude bursts among all participants was associated with higher resting supine systolic (R = 0.48; P = 0.01) and diastolic (R = 0.48; P = 0.01) BP when controlling for MSNA burst frequency, but not when also controlling for waist circumference (P > 0.05). In contrast, sympathetic transduction was not correlated with 24-h ambulatory average BP.

CONCLUSION

Sympathetic transduction to BP is augmented in abdominal obesity and positively related to higher resting supine BP but not 24-h ambulatory average BP.

Acute hypoxia elicits lasting reductions in the sympathetic action potential transduction of arterial blood pressure in males

Post-hypoxia sympathoexcitation does not elicit corresponding changes in vascular tone, suggesting diminished sympathetic signalling. Blunted sympathetic transduction following acute hypoxia, however, has not been confirmed and the effects of hypoxia on the sympathetic transduction of mean arterial pressure (MAP) as a function of action potential (AP) activity is unknown. We hypothesized that MAP changes would be blunted during acute hypoxia but restored in recovery and asynchronous APs would elicit smaller MAP changes than synchronous APs. Seven healthy males (age: 24 (3) years; BMI: 25 (3) kg/m² ) underwent 20 min isocapnic hypoxia (P_ET O₂ : 47 (2) mmHg) and 30 min recovery. Multi-unit microneurography (muscle sympathetic nerve activity; MSNA) and continuous wavelet transform with matched mother wavelet was used to detect sympathetic APs during baseline, hypoxia, early (first 7 min) and late (last 7 min) recovery. AP groups were classified as synchronous APs, asynchronous APs (occurring outside an MSNA burst) and no AP activity. Sympathetic transduction of MAP was quantified using signal-averaging, with ΔMAP tracked following AP group cardiac cycles. Following synchronous APs, ΔMAP was reduced in hypoxia (+1.8 (0.9) mmHg) and early recovery (+1.5 (0.7) mmHg) compared with baseline (+3.1 (2.2) mmHg). AP group-by-condition interactions show that at rest asynchronous APs attenuate MAP reductions compared with no AP activity (-0.4 (1.1) vs. -2.2 (1.2) mmHg, respectively), with no difference between AP groups in hypoxia, early or late recovery. Sympathetic transduction of MAP is blunted in hypoxia and early recovery. At rest, asynchronous sympathetic APs contribute to neural regulation of MAP by attenuating nadir pressure responses. KEY POINTS: Acute isocapnic hypoxia elicits lasting sympathoexcitation that does not correspond to parallel changes in vascular tone, suggesting blunted sympathetic transduction. Signal-averaging techniques track the magnitude and temporal cardiovascular responses following integrated muscle sympathetic nerve activity (MSNA) burst and non-burst cardiac cycles. However, this does not fully characterize the effects of sympathetic action potential (AP) activity on blood pressure control. We show that hypoxia blunts the sympathetic transduction of mean arterial pressure (MAP) following synchronous APs that form integrated MSNA bursts and that sympathetic transduction of MAP remains attenuated into early recovery. At rest, asynchronous APs attenuate the reduction in MAP compared with cardiac cycles following no AP activity, thus asynchronous sympathetic APs appear to contribute to the neural regulation of blood pressure. The results advance our understanding of sympathetic transduction of arterial pressure during and following exposure to acute isocapnic hypoxia in humans.

Brain and cardiovascular-related changes are associated with aging, hypertension, and atrial fibrillation

PURPOSE

The neural pathways in which the brain regulates the cardiovascular system is via sympathetic and parasympathetic control of the heart and sympathetic control of the systemic vasculature. Various cortical and sub-cortical sites are involved, but how these critical brain regions for cardiovascular control are altered in healthy aging and other risk conditions that may contribute to cardiovascular disease is uncertain.

METHODS

Here we review the functional and structural brain changes in healthy aging, hypertension, and atrial fibrillation - noting their potential influence on the autonomic nervous system and hence on cardiovascular control.

RESULTS

Evidence suggests that aging, hypertension, and atrial fibrillation are each associated with functional and structural changes in specific areas of the central nervous system involved in autonomic control. Increased muscle sympathetic nerve activity (MSNA) and significant alterations in the brain regions involved in the default mode network are commonly reported in aging, hypertension, and atrial fibrillation.

CONCLUSIONS

Further studies using functional and structural magnetic resonance imaging (MRI) coupled with autonomic nerve activity in healthy aging, hypertension, and atrial fibrillation promise to reveal the underlying brain circuitry modulating the abnormal sympathetic nerve activity in these conditions. This understanding will guide future therapies to rectify dysregulation of autonomic and cardiovascular control by the brain.

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.

Sex-differences in the sympathetic neurocirculatory responses to chemoreflex activation

Abstract

The purpose of this study was to determine whether there are sex differences in the cardiorespiratory and sympathetic neurocirculatory responses to central, peripheral, and combined central and peripheral chemoreflex activation. Ten women (29 ± 6 years, 22.8 ± 2.4 kg/m²: mean ± SD) and 10 men (30 ± 7 years, 24.8 ± 3.2 kg/m²) undertook randomized 5 min breathing trials of: room air (eucapnia), isocapnic hypoxia (10% oxygen (O₂); peripheral chemoreflex activation), hypercapnic hyperoxia (7% carbon dioxide (CO₂), 50% O₂; central chemoreflex activation) and hypercapnic hypoxia (7% CO₂, 10% O₂; central and peripheral chemoreflex activation). Control trials of isocapnic hyperoxia (peripheral chemoreflex inhibition) and hypocapnic hyperoxia (central and peripheral chemoreflex inhibition) were also included. Muscle sympathetic nerve activity (MSNA; microneurography), mean arterial pressure (MAP; finger photoplethysmography) and minute ventilation (V˙_E; pneumotachometer) were measured. Total MSNA (P = 1.000 and P = 0.616), MAP (P = 0.265) and V˙_E (P = 0.587 and P = 0.472) were not different in men and women during eucapnia and during isocapnic hypoxia. Women exhibited attenuated increases in V˙_E during hypercapnic hyperoxia (27.3 ± 6.3 vs. 39.5 ± 7.5 l/min, P < 0.0001) and hypercapnic hypoxia (40.9 ± 9.1 vs. 53.8 ± 13.3 l/min, P < 0.0001) compared with men. However, total MSNA responses were augmented in women (hypercapnic hyperoxia 378 ± 215 vs. 258 ± 107%, P = 0.017; hypercapnic hypoxia 607 ± 290 vs. 362 ± 268%, P < 0.0001). No sex differences in total MSNA, MAP or V˙_E were observed during isocapnic hyperoxia and hypocapnic hyperoxia. Our results indicate that young women have augmented sympathetic responses to central chemoreflex activation, which explains the augmented MSNA response to combined central and peripheral chemoreflex activation.

Key points

Sex differences in the control of breathing have been well studied, but whether there are differences in the sympathetic neurocirculatory responses to chemoreflex activation between healthy women and men is incompletely understood.

We observed that, compared with young men, young women displayed augmented increases in muscle sympathetic nerve activity during both hypercapnic hyperoxia (central chemoreflex activation) and hypercapnic hypoxia (central and peripheral chemoreflex activation) but had attenuated increases in minute ventilation.

In contrast, no sex differences were found in either muscle sympathetic nerve activity or minute ventilation responses to isocapnic hypoxia (peripheral chemoreceptor stimulation).

Young women have blunted ventilator, but augmented sympathetic responses, to central (hypercapnic hyperoxia) and combined central and peripheral chemoreflex activation (hypercapnic hypoxia), compared with young men. The possible causative association between the reduced ventilation and heightened sympathetic responses in young women awaits validation.

Sympathetic transduction of blood pressure during graded lower body negative pressure in young healthy adults

Sympathetic transduction of blood pressure (BP) is correlated negatively with resting muscle sympathetic nerve activity (MSNA) in cross-sectional data, but the acute effects of increasing MSNA are unclear. Sixteen (4 females) healthy adults (26±3 years) underwent continuous measurement of heart rate, BP, and MSNA at rest and during graded lower body negative pressure (LBNP) at -10, -20, and -30mmHg. Sympathetic transduction of BP was quantified in the time (signal averaging) and frequency (MSNA-BP gain) domains. The proportion of MSNA bursts firing within each tertile of BP were calculated. As expected, LBNP increased MSNA burst frequency (P<0.01) and burst amplitude (P<0.02), though the proportions of MSNA bursts firing across each BP tertile remained stable (all P>0.44). The MSNA-diastolic BP low frequency transfer function gain (P=0.25) was unchanged during LBNP; the spectral coherence was increased (P=0.03). Signal-averaged sympathetic transduction of diastolic BP was unchanged (from 2.1±1.0 at rest to 2.4±1.5, 2.2±1.3, and 2.3±1.4mmHg; P=0.43) during LBNP, but diastolic BP responses following non-burst cardiac cycles progressively decreased (from -0.8±0.4 at rest to -1.0±0.6, -1.2±0.6, and -1.6±0.9mmHg; P<0.01). As a result, the difference between MSNA burst and non-bursts diastolic BP responses was increased (from 2.9±1.4 at rest to 3.4±1.9, 3.4±1.9, and 3.9±2.1mmHg; P<0.01). In conclusion, acute increases in MSNA using LBNP did not alter traditional signal-averaged or frequency-domain measures of sympathetic transduction of BP or the proportion of MSNA bursts firing at different BP levels. The factors that determine changes in the firing of MSNA bursts relative to oscillations in BP require further investigation.

Acute changes in forearm vascular compliance during transient sympatho-excitation

The study of vascular regulation often omits important information about the elastic properties of arteries under conditions of pulsatile flow. The purpose of this study was to examine the relationship between muscle sympathetic nerve activity (MSNA), vascular bed compliance, and peripheral blood flow responses in humans. We hypothesized that increases in MSNA would correlate with reductions in vascular compliance, and that changes in compliance would correspond with changes in peripheral blood flow during sympatho-excitation. MSNA (microneurography), blood pressure (Finopres), and brachial artery blood flow (Doppler ultrasound), were monitored in six healthy males at baseline and during the last 15 s of voluntary end-inspiratory, expiratory apneas and 5 min of static handgrip exercise (SHG; 20% maximum voluntary contraction) and 3 min of post-exercise circulatory occlusion (SHG + PECO; measured in the non-exercising arm). A lumped Windkessel model was employed to examine vascular bed compliance. During apnea, indices of MSNA were inversely related with vascular compliance, and reductions in compliance correlated with decreased brachial blood flow rate. During SHG, despite increased MSNA, compliance also increased, but was unrelated to increases in blood flow. Neither during SHG nor PECO did indices of MSNA correlate with forearm vascular compliance nor did vascular compliance correlate with brachial flow. However, during PECO, a linear combination of blood pressure and total MSNA was correlated with vascular compliance. These data indicate the elastic components of the forearm vasculature are regulated by adrenergic and myogenic mechanisms during sympatho-excitation, but in a reflex-dependent manner.

Sex differences in sympathetic neuro-vascular and neuro-hemodynamic relationships during the cold pressor test

Muscle sympathetic nerve activity (MSNA) affects vascular resistance differently in women and men. However, whether this sex difference persists during pronounced increases in MSNA remains unclear. Therefore, the purpose of this study was to examine sex differences in neurovascular transduction during cold pressor test (CPT)-mediated sympatho-excitation. Integrated peroneal MSNA (microneurography) was measured at rest and during a 3-minute CPT in young healthy women (n=11) and men (n=10). Mean arterial pressure (MAP) was measured beat-by-beat (Finometer) and superficial femoral artery blood flow was measured using duplex ultrasound. Femoral vascular resistance (FVR) was quantified as MAP/femoral blood flow (mmHg/mL/min). Baseline MSNA was similar between women and men (14±9 vs 15±9 bursts/100hb, respectively; P=0.83), whereas MAP was lower (86±7 vs 92±4 mmHg; P=0.047), and FVR was greater in women than men (0.54±0.16 vs 0.36±0.15 mmHg/mL/min; P=0.02). CPT-induced increases in MSNA were similar between the sexes (+19±11 vs +26±14 bursts/100hb; P=0.26) while increases in MAP (+7±3 vs +10±3mmHg; P=0.03) and FVR (+3.2±18.6 vs +26.8±12.8%; P<0.01) were smaller in women than men. Within men, CPT- induced increases in MSNA predicted increases in MAP (R²=0.51, P=0.02) and FVR (R²=0.49, P=0.02). However, MSNA did not predict MAP (R²=0.11, P=0.35) or FVR (R²=0.07, P=0.46) in women. Our findings demonstrate that men experience robust CPT-induced MAP responses that are driven by both neuro-vascular (MSNA-FVR) and neuro-hemodynamic (MSNA-MAP) coupling. These relationships were not observed in women, indicating that even during pronounced increases in sympathetic outflow, MSNA is not predictive of vascular nor blood pressure outcomes in young healthy women.

Influence of Age and Estradiol on Sympathetic Nerve Activity Responses to Exercise in Women

INTRODUCTION

Postmenopausal women (PMW) display exaggerated increases in blood pressure (BP) during exercise, yet the mechanism(s) involved remain unclear. Moreover, research on the impact of menopausal changes in estradiol on cardiovascular control during exercise are limited. Herein, we tested the hypothesis that sympathetic responses during exercise are augmented in PMWcompared with young women (YW), and estradiol administration attenuates these responses.

METHODS

Muscle sympathetic nerve activity (MSNA) and mean arterial pressure (MAP) were measured in 13 PMW (58 ± 1 yr) and 17 YW (22 ± 1 yr) during 2 min of isometric handgrip. Separately, MSNA and BP responses were measured during isometric handgrip in six PMW (53 ± 1 yr) before and after 1 month of transdermal estradiol (100 μg·d-1). A period of postexercise ischemia (PEI) to isolate muscle metaboreflex activation followed all handgrip bouts.

RESULTS

Resting MAP was similar between PMW and YW, whereas MSNA was greater in PMW (23 ± 3 vs 8 ± 1 bursts per minute; P < 0.05). During handgrip, the increases in MSNA (PMW Δ16 ± 2 vs YW Δ6 ± 1 bursts per minute; P < 0.05) and MAP (PMW Δ18 ± 2 vs YW Δ12 ± 2 mm Hg; P < 0.05) were greater in PMW and remained augmented during PEI. Estradiol administration decreased resting MAP but not MSNA in PMW. Moreover, MSNA (PMW (-E2) Δ27 ± 8 bursts per minute versus PMW (+E2) Δ12 ± 5 bursts per minute; P < 0.05) and MAP (Δ31 ± 8 mm Hg vs Δ20 ± 6 mm Hg; P < 0.05) responses during handgrip were attenuated in PMW after estradiol administration. Likewise, MAP responses during PEI were lower after estradiol.

CONCLUSIONS

These data suggest that PMW exhibit an exaggerated MSNA and BP response to isometric exercise, due in part to heightened metaboreflex activation. Furthermore, estradiol administration attenuated BP and MSNA responses to exercise in PMW.

When it's time for the sex talk, words matter

In recent years, the traditional, unspoken assumption in published biomedical research studies that the young, healthy (usually white) male is the "default human" has received increasing scrutiny and criticism. The historical underrepresentation of female participants in biomedical research has been increasingly recognized and addressed, including with the current call for papers at the American Journal of Physiology-Heart and Circulatory Physiology. Our goal in the present Perspectives is to discuss the topic of terminology (man/woman vs. male/female) for human research participants when considering sex as a biological variable. This important consideration is consistent with the importance of gender identity and related topics to psychological, emotional, and physical health. Just as pronouns are important, so is appropriate terminology when referring to human research volunteers. Despite some disagreement regarding terminology between our two groups of authors, we provide consensus recommendations. Importantly, we all agree that the most vital aspect of the present discussion is the broader focus on sex as a biological variable and appropriate inclusion of biological sex in in vitro, preclinical, and human research studies.

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.

Differential influences of dietary sodium on blood pressure regulation based on race and sex

There are clear differences between men and women, and differences among races, in the incidence and prevalence of hypertension. Furthermore, there is extensive inter-individual variability among humans in the extent to which sodium ingestion alters blood pressure. Orthostatic intolerance and orthostatic hypotension are more common in women; these are often treated with a high salt diet, which has variable efficacy in increasing blood volume and blood pressure. Conversely, people with certain forms of hypertension are often counseled to decrease their sodium intake. Non-Hispanic Black men and women have higher rates of hypertension compared to non-Hispanic White men and women and other racial/ethnic groups. In aggregate, Black women appear to have better orthostatic tolerance than White women. In the present paper, we summarize and evaluate the current evidence for mechanisms of blood pressure regulation in men and women, as well as differences between Black and White groups, with a focus on cardiovascular responses to salt and differences among these groups. We also provide a brief review of factors that are not traditionally considered to be "biological" - such as socio-economic disparities resulting from historic and contemporary inequity across racial groups. These non-biological factors have direct and substantial influences on cardiovascular mechanisms, as well as implications for the influences of salt and sodium intake on blood pressure and cardiovascular health. We conclude that both biological and socio-economic factors provide critical modulating influences when considering the impacts of sodium on cardiovascular health as functions of race and sex.

An open-source application for the standardized burst identification from the integrated muscle sympathetic neurogram

Muscle sympathetic nerve activity (MSNA) can be acquired from humans using the technique of microneurography. The resulting integrated neurogram displays pulse-synchronous bursts of sympathetic activity, which undergoes processing for standard MSNA metrics including burst frequency, height, area, incidence, total activity, and latency. The procedure for detecting bursts of MSNA and calculating burst metrics is tedious and differs widely among laboratories worldwide. We sought to develop an open-source, cross-platform web application that provides a standardized approach for burst identification and a tool to increase research reproducibility for those measuring MSNA. We compared the performance of this web application against a manual scoring approach under conditions of rest, chemoreflex activation (n = 9, 20-min isocapnic hypoxia), and metaboreflex activation (n = 13, 2-min isometric handgrip exercise and 4-min postexercise circulatory occlusion). The intraclass correlation coefficient (ICC) indicated good to strong agreement between scoring approaches for burst frequency (ICC = 0.92-0.99), incidence (ICC = 0.94-0.99), height (ICC = 0.76-0.88), total activity (ICC = 0.85-0.99), and latency (ICC = 0.97-0.99). Agreement with burst area was poor to moderate (ICC = 0.04-0.67) but changes in burst area were similar with chemoreflex and metaboreflex activation. Scoring using the web application was highly efficient and provided data visualization tools that expedited data processing and the analysis of MSNA. We recommend the open-source web application be adopted by the community for the analysis of MSNA.NEW & NOTEWORTHY The basic analysis of muscle sympathetic nerve activity (MSNA) requires the identification of pulse-synchronous bursts from the integrated neurogram before standard MSNA metrics can be quantified. This process is a time-consuming task requiring an experienced microneurographer to visually identify and manually label bursts. We developed an open-source, cross-platform application permitting a standardized approach for sympathetic burst identification and present the performance of this application against a manual scorer under basal conditions and during sympathoexcitatory stresses.

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.

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.

Forearm vascular resistance responses to the Valsalva maneuver in healthy young and older adults

PURPOSE

Effective end-organ peripheral vascular resistance responses are critical to blood pressure control while upright, and prevention of syncope (fainting). The Valsalva maneuver (VM) induces blood pressure decreases that evoke baroreflex-mediated vasoconstriction. We characterized beat-to-beat forearm vascular resistance (FVR) responses to the VM in healthy adults, evaluated the impact of age and sex on these responses, and investigated their association with orthostatic tolerance (OT; susceptibility to syncope). We hypothesized that individuals with smaller FVR responses would be more susceptible to syncope.

METHODS

Healthy young (N = 36; 19 women; age 25.4 ± 4.6 years) and older (N = 21; 12 women; age 62.4 ± 9.6 years) adults performed a supine 40 mmHg, 20 s VM. Graded 60° head-up-tilt with combined lower body negative pressure continued to presyncope was used to determine OT. Non-invasive beat-to-beat blood pressure and heart rate (finger plethysmography) were recorded continuously. FVR was calculated as mean arterial pressure (MAP) divided by brachial blood flow velocity (Doppler ultrasound) relative to baseline.

RESULTS

The VM produces a distinctive FVR pattern that peaks (+137.1 ± 11.6%) in phase 2B (17.5 ± 0.3 s) as the baroreflex responds to low-pressure perturbations. This response increased with age overall (p < 0.001) and within male (p = 0.030) and female subgroups (p < 0.001). Maximum FVR during the VM was significantly correlated with maximal tilt FVR (r = 0.364; p = 0.0153) and with OT when expressed relative to the MAP decrease in phase 2A (Max FVR (%)/MAP_2A-1; r = 0.337; p = 0.0206).

CONCLUSION

This is the first characterization of FVR responses to the VM. The VM elicits large baroreflex-mediated increases in FVR; small FVR responses to the VM may indicate susceptibility to syncope.

Assessing static and dynamic sympathetic transduction using microneurography

The relationship between sympathetic nerve activity and the vasculature has been of great interest due to its potential role in various cardiovascular-related diseases. This relationship, termed "sympathetic transduction," has been quantified using several different laboratory and analytical techniques. The most common method is to assess the association between relative changes in muscle sympathetic nerve activity, measured via microneurography, and physiological outcomes (e.g., blood pressure, total peripheral resistance, blood flow, etc.) in response to a sympathetic stressor (e.g., exercise, cold stress, orthostatic stress). This approach, however, comes with its own caveats. For instance, elevations in blood pressure and heart rate during a sympathetic stressor can have an independent impact on muscle sympathetic nerve activity. Another assessment of sympathetic transduction was developed by Wallin and Nerhed in 1982, where alterations in blood pressure and heart rate were assessed immediately following bursts of muscle sympathetic nerve activity at rest. This approach has since been characterized and further innovated by others, including the breakdown of consecutive burst sequences (e.g., singlet, doublet, triplet, and quadruplet), and burst height (quartile analysis) on specific vascular outcomes (e.g., blood pressure, blood flow, vascular resistance). The purpose of this review is to provide an overview of the literature that has assessed sympathetic transduction using microneurography and various sympathetic stressors (static sympathetic transduction) and using the same or similar approach established by Wallin and Nerhed at rest (dynamic neurovascular transduction). Herein, we discuss the overlapping literature between these two methodologies and highlight the key physiological questions that remain.

Baroreflex dysfunction in Parkinson's disease: integration of central and peripheral mechanisms

The incidence of Parkinson's disease (PD) is increasing worldwide. Although the PD hallmark is the motor impairments, nonmotor dysfunctions are now becoming more recognized. Recently, studies have suggested that baroreflex dysfunction is one of the underlying mechanisms of cardiovascular dysregulation observed in patients with PD. However, the large body of literature on baroreflex function in PD is unclear. The baroreflex system plays a major role in the autonomic, and ultimately blood pressure and heart rate, adjustments that accompany acute cardiovascular stressors on a daily basis. Therefore, impaired baroreflex function (i.e., decreased sensitivity or gain) can lead to altered neural cardiovascular responses. Since PD affects parasympathetic and sympathetic branches of the autonomic nervous system and both are orchestrated by the baroreflex system, understanding of this crucial mechanism in PD is necessary. In the present review, we summarize the potential altered central and peripheral mechanisms affecting the feedback-controlled loops that comprise the reflex arc in patients with PD. Major factors including arterial stiffness, reduced number of C1 and activation of non-C1 neurons, presence of central α-synuclein aggregation, cardiac sympathetic denervation, attenuated muscle sympathetic nerve activity, and lower norepinephrine release could compromise baroreflex function in PD. Results from patients with PD and from animal models of PD provide the reader with a clearer picture of baroreflex function in this clinical condition. By doing so, our intent is to stimulate future studies to evaluate several unanswered questions in this research area.

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.

Effects of muscle sympathetic burst size and burst pattern on time-to-peak sympathetic transduction

The current study evaluated the influence of resting muscle sympathetic nerve activity (MSNA) burst size and firing pattern on time-to-peak sympathetic transduction in 36 young healthy men and women. Participants underwent a 5-10 min resting baseline with beat-to-beat measures of heart rate, mean arterial pressure (MAP), and MSNA (microneurography). Cardiac output and total vascular conductance were calculated using the Modelflow algorithm. Sympathetic transduction was quantified using the burst-triggered signal averaging technique to examine the changes in MAP, cardiac output, and total vascular conductance for 15 cardiac cycles after each MSNA burst or non-burst. A stepwise increase in the peak MAP (i.e., sympathetic transduction) was observed throughout all quartiles of normalized MSNA burst area (quartile 1 (Q1): 1.7 ± 1.3 mm Hg; Q2: 2.1 ± 1.3 mm Hg; Q3: 2.6 ± 1.4 mm Hg; Q4: 3.5 ± 1.4 mm Hg; P < 0.01). The largest quartile of normalized MSNA burst area demonstrated faster time-to-peak MAP responses (5.7 ± 2.5 s) than both Q1 (10.1 ± 3.9 s, P < 0.01) and Q2 (9.3 ± 4.1 s, P < 0.01), as well as, faster time-to-peak cardiac output and time-to-nadir total vascular conductance compared with Q1 and Q2 (All P < 0.05). Larger clusters of sympathetic bursts (i.e., triplets and ≥ quadruplets) did not have increased time-to-peak transduction compared with singlets and doublet bursts across all MSNA quartiles. These results highlight intraindividual variability in the time-course of sympathetic transduction and reveal an intrinsic property of larger sympathetic bursts to increase time-to-peak sympathetic transduction in humans. Novelty: Muscle sympathetic burst size can modulate time-to-peak sympathetic transduction in young healthy men and women. These observations appear independent of the pattern of sympathetic firing.

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.

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.

Highs and lows of sympathetic neurocardiovascular transduction: influence of altitude acclimatization and adaptation

High-altitude (>2,500 m) exposure results in increased muscle sympathetic nervous activity (MSNA) in acclimatizing lowlanders. However, little is known about how altitude affects MSNA in indigenous high-altitude populations. Additionally, the relationship between MSNA and blood pressure regulation (i.e., neurovascular transduction) at high-altitude is unclear. We sought to determine 1) how high-altitude effects neurocardiovascular transduction and 2) whether differences exist in neurocardiovascular transduction between low- and high-altitude populations. Measurements of MSNA (microneurography), mean arterial blood pressure (MAP; finger photoplethysmography), and heart rate (electrocardiogram) were collected in 1) lowlanders (n = 14) at low (344 m) and high altitude (5,050 m), 2) Sherpa highlanders (n = 8; 5,050 m), and 3) Andean (with and without excessive erythrocytosis) highlanders (n = 15; 4,300 m). Cardiovascular responses to MSNA burst sequences (i.e., singlet, couplet, triplet, and quadruplet) were quantified using custom software (coded in MATLAB, v.2015b). Slopes were generated for each individual based on peak responses and normalized total MSNA. High altitude reduced neurocardiovascular transduction in lowlanders (MAP slope: high altitude, 0.0075 ± 0.0060 vs. low altitude, 0.0134 ± 0.080; P = 0.03). Transduction was elevated in Sherpa (MAP slope, 0.012 ± 0.007) compared with Andeans (0.003 ± 0.002, P = 0.001). MAP transduction was not statistically different between acclimatizing lowlanders and Sherpa (MAP slope, P = 0.08) or Andeans (MAP slope, P = 0.07). When resting MSNA is accounted for (ANCOVA), transduction was inversely related to basal MSNA (bursts/minute) independent of population (RRI, r = 0.578 P < 0.001; MAP, r = -0.627, P < 0.0001). Our results demonstrate that transduction is blunted in individuals with higher basal MSNA, suggesting that blunted neurocardiovascular transduction is a physiological adaptation to elevated MSNA rather than an effect or adaptation specific to chronic hypoxic exposure.NEW & NOTEWORTHY This study has identified that sympathetically mediated blood pressure regulation is reduced following ascent to high-altitude. Additionally, we show that high altitude Andean natives have reduced blood pressure responsiveness to sympathetic nervous activity (SNA) compared with Nepalese Sherpa. However, basal sympathetic activity is inversely related to the magnitude of SNA-mediated fluctuations in blood pressure regardless of population or condition. These data set a foundation to explore more precise mechanisms of blood pressure control under conditions of persistent sympathetic activation and hypoxia.

Sex differences in integrated neurocardiovascular control of blood pressure following acute intermittent hypercapnic hypoxia

Repetitive hypoxic apneas, similar to those observed in sleep apnea, result in resetting of the sympathetic baroreflex to higher blood pressures (BP). This baroreflex resetting is associated with hypertension in preclinical models of sleep apnea (intermittent hypoxia, IH); however, the majority of understanding comes from males. There are data to suggest that female rats exposed to IH do not develop high BP. Clinical data further support sex differences in the development of hypertension in sleep apnea, but mechanistic data are lacking. Here we examined sex-related differences in the effect of IH on sympathetic control of BP in humans. We hypothesized that after acute IH we would observe a rise in muscle sympathetic nerve activity (MSNA) and arterial BP in young men (n = 30) that would be absent in young women (n = 19). BP and MSNA were measured during normoxic rest before and after 30 min of IH. Baroreflex sensitivity (modified Oxford) was evaluated before and after IH. A rise in mean BP following IH was observed in men (+2.0 ± 0.7 mmHg, P = 0.03), whereas no change was observed in women (-2.7 ± 1.2 mmHg, P = 0.11). The elevation in MSNA following IH was not different between groups (4.7 ± 1.1 vs. 3.8 ± 1.2 bursts/min, P = 0.65). Sympathetic baroreflex sensitivity did not change after IH in either group (P > 0.05). Our results support sex-related differences in the effect of IH on neurovascular control of BP and show that any BP-raising effects of IH are absent in young women. These data enhance our understanding of sex-specific mechanisms that may contribute to BP changes in sleep apnea.

Race and sex differences in cardiovascular autonomic regulation

Racial and ethnic differences in cardiovascular morbidity and mortality persist despite advances in risk factor identification and implementation of evidence-based treatment strategies. African American men and women are disproportionately affected by cardiovascular risk factors, particularly hypertension. In this context, previous studies have identified sex and racial differences in autonomic cardiovascular regulation which may contribute to the development of hypertension and its high morbidity burden among African Americans. In this review, we provide a comprehensive evaluation of the potential pathophysiological mechanisms of blood pressure control and their differences based on sex and race. These mechanisms include obesity-induced sympathetic activation, sympatho-vascular transduction, baroreflex sensitivity and adrenoreceptor vascular sensitivity, which have been the subjects of prior investigation in this field. Understanding the racial differences in the pathophysiology of hypertension and its co-morbid conditions would allow us to implement better treatment strategies tailored to African Americans, with the ultimate goal of reducing cardiovascular mortality in this population.

Sex differences in vascular transduction of sympathetic nerve activity

PURPOSE

Sympathetic vasoconstriction plays a major role in the beat-to-beat control of blood pressure. To be effective and thus avoid dangerously high or low blood pressures, this mechanism relies upon transduction of sympathetic nerve activity at the level of the vasculature. However, recent evidence suggests that considerable variability exists in beat-to-beat vascular transduction, particularly between the sexes.

METHODS

We reviewed the methods available for quantifying beat-to-beat transduction of muscle sympathetic nerve activity (MSNA) and explored the recent evidence for sex differences in vascular transduction. We paid specific attention to relationships between vascular transduction and factors such as resting levels of sympathetic nerve activity and baroreflex sensitivity.

RESULTS

There are two dominant methods now available for the quantification of beat-to-beat transduction of muscle sympathetic nerve activity at rest. Whilst there is some evidence to suggest that young females exhibit lower levels of vascular transduction, results vary depending on the method used and the direction of change in MSNA. Evidence suggests that compensatory relationships may exist between key components of neurovascular control, such as vascular transduction and resting levels of MSNA. Also consistent is the presence of such relationships in young males but not young females.

CONCLUSION

The lack of significant relationships in young females may reflect the influence of vasodilator mechanisms that counteract sympathetic vasoconstriction. The assessment of vascular transduction following MSNA bursts and non-bursts in males and females, both young and older, may help to gain a mechanistic understanding of the prevalence of hypotensive and hypertensive disorders across the lifespan.

Influence of Sex and Age on Muscle Sympathetic Nerve Activity of Healthy Normotensive Adults

As with blood pressure, age-related changes in muscle sympathetic nerve activity (MSNA) may differ nonlinearly between sexes. Data acquired from 398 male (age: 39±17; range: 18-78 years [mean±SD]) and 260 female (age: 37±18; range: 18-81 years) normotensive healthy nonmedicated volunteers were analyzed using linear regression models with resting MSNA burst frequency as the outcome and the predictors sex, age, MSNA, blood pressure, and body mass index modelled with natural cubic splines. Age and body mass index contributed 41% and 11%, respectively, of MSNA variance in females and 23% and 1% in males. Overall, changes in MSNA with age were sigmoidal. At age 20, mean MSNA of males and females were similar, then diverged significantly, reaching in women a nadir at age 30. After 30, MSNA increased nonlinearly in both sexes. Both MSNA discharge and blood pressure were lower in females until age 50 (17±9 versus 25±10 bursts·min^-1; P<1×10^-19; 106±11/66±8 versus 116±7/68±9 mm Hg; P<0.01) but converged thereafter (38±11 versus 35±12 bursts·min^-1; P=0.17; 119±15/71±13 versus 120±13/72±9 mm Hg; P>0.56). Compared with age 30, MSNA burst frequency at age 70 was 57% higher in males but 3-fold greater in females; corresponding increases in systolic blood pressure were 1 (95% CI, -4 to 5) and 12 (95% CI, 6-16) mm Hg. Except for concordance in females beyond age 40, there was no systematic change with age in any resting MSNA-blood pressure relationship. In normotensive adults, MSNA increases after age 30, with ascendance steeper in women.

Sex differences in dynamic blood pressure regulation: beat-by-beat responses to muscle sympathetic nerve activity

It has been suggested that sex differences in acute blood pressure fluctuations occur during the periods of time between bursts of muscle sympathetic nerve activity. Therefore, we tested the hypothesis that men experience more dynamic changes in mean arterial pressure (Finometer MIDI) than women during acute sympathoinhibition (i.e., slow breathing) in which bursts of sympathetic activity occur more infrequently than at rest. We tested healthy women (n = 9) and men (n = 9) of similar age (22 ± 2 vs. 23 ± 3 yr, P = 0.6). Custom software was used to calculate beat-by-beat changes in blood pressure following sympathetic burst and nonburst sequences (recorded using microneurography) during 10 min of supine rest and a 15-min bout of slow breathing. During slow breathing following nonburst sequences, women demonstrated smaller overall reductions in mean arterial pressure compared with men over the subsequent 15 cardiac cycles (P < 0.01). In addition, following a burst of sympathetic activity, women experienced greater overall increases in mean arterial pressure compared with men over the following 15 cardiac cycles (P < 0.01). Despite these differences, the peak and nadir changes in arterial pressure following burst and nonburst sequences were not different between the sexes (P = 0.45 and P = 0.48, burst and nonburst sequences, respectively). As such, these data suggest that women respond to a burst of sympathetic activity with more sustained increases in blood pressure than men, coupled with improved maintenance of blood pressure during acute periods of sympathetic quiescence. In other words, these findings suggest that men rely more on frequent bursts of sympathetic activity to acutely regulate arterial pressure than women.NEW & NOTEWORTHY We demonstrate that during acute sympathoinhibition, women demonstrate more sustained increases in blood pressure following sympathetic bursts of activity than men. Likewise, during prolonged sympathetic quiescence, blood pressure is less labile in women than men. This suggests that lower overall blood pressure in young women may not be mediated by smaller beat-by-beat changes in blood pressure in response to sympathetic outflow but may instead be mediated by a lower frequency of sympathetic bursts.

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.

Broader adaptive range of sympathetic burst size in response to blood pressure change in older women with greater arterial stiffness

KEY POINTS

In this study, we focused on muscle sympathetic nerve activity (MSNA) burst size and occurrence separately as subcomponents of the sympathetic baroreflex in older adults, and we found that the distribution (variation) of burst size against burst occurrence was greater in women than men. Older women had greater carotid artery stiffness compared with older men, while blood pressure (BP) distribution (variation) was comparable between sexes. Sympathetic baroreflex sensitivity assessed with burst incidence was less sensitive as the carotid artery became stiffer in older men and women, while that assessed with burst area was more sensitive as the carotid artery became stiffer in older women but not in older men. These results help us understand the mechanisms underlying the compensation for the impaired response of MSNA burst occurrence in older women with greater carotid artery stiffness to regulate BP similar to that in older men.

ABSTRACT

There are sex differences in arterial stiffness and neural control of blood pressure (BP) among older adults. We examined whether the sympathetic response to BP is greater in older women than men in burst size but not burst occurrence. Burst occurrence and size were assessed with burst interval and area of muscle sympathetic nerve activity, respectively, and the distributions of these indices were evaluated by range during supine rest in 61 healthy older subjects (30 men (69 ± 6 years) and 31 women (68 ± 6 years); means ± SD). Also, we analysed sympathetic baroreflex sensitivity (BRS) with burst occurrence and area simultaneously. Carotid β-stiffness was measured with B-mode ultrasonic image and carotid BP. The range of burst interval was smaller in older women than men (P = 0.002), while there was no difference in the range of burst area. Carotid β-stiffness was greater in older women than men (6.7 ± 2.7 vs. 5.1 ± 2.7, P = 0.027). Sympathetic BRS assessed with burst incidence was lower in older women than men (-2.3 ± 1.4 vs. -3.3 ± 1.4 bursts·100 beats^-1  mmHg^-1 , P = 0.007), while this sex difference was observed when assessed with burst area after adjusting for carotid β-stiffness (-116.1 ± 135.0 vs. -185.9 ± 148.2 a.u. burst^-1  mmHg^-1 , P = 0.040), but not before. Sympathetic BRS assessed with burst area was negatively (more sensitive) correlated with carotid β-stiffness in older women (r = -0.53, P = 0.002) but not men. These data suggest that the response of burst size within each burst is augmented for the baroreflex BP control despite the impaired response of burst occurrence in older women with greater carotid stiffness.

Altered cardiorespiratory regulation during exercise in patients with Parkinson's disease: A challenging non-motor feature

The incidence of Parkinson’s disease is increasing worldwide. The motor dysfunctions are the hallmark of the disease, but patients also experience non-motor impairments, and over 40% of the patients experience coexistent abnormalities, such as orthostatic hypotension. Exercise training has been suggested as a coping resource to alleviate Parkinson’s disease symptoms and delay disease progression. However, the body of knowledge is showing that the cardiovascular response to exercise in patients with Parkinson’s disease is altered. Adequate cardiovascular and hemodynamic adjustments to exercise are necessary to meet the metabolic demands of working skeletal muscle properly. Therefore, since Parkinson’s disease affects parasympathetic and sympathetic branches of the autonomic nervous system and the latter are crucial in ensuring these adjustments are adequately made, the understanding of these responses during exercise in this population is necessary. Several neural control mechanisms are responsible for the autonomic changes in the cardiovascular and hemodynamic systems seen during exercise. In this sense, the purpose of the present work is to review the current knowledge regarding the cardiovascular responses to dynamic and isometric/resistance exercise as well as the mechanisms by which the body maintains appropriate perfusion pressure to all organs during exercise in patients with Parkinson’s disease. Results from patients with Parkinson’s disease and animal models of Parkinson’s disease provide the reader with a well-rounded knowledge base. Through this, we will highlight what is known and not known about how the neural control of circulation is responding during exercise and the adaptations that occur when individuals exercise regularly.

Neural control of blood pressure is altered following isolated leg heating in aged humans

There is a sustained reduction in arterial blood pressure that occurs in aged adults following exposure to acute leg heating. We tested the hypothesis that acute leg heating would decrease arterial blood pressure in aged adults secondary to sympathoinhibition. We exposed 13 young and 10 aged adults to 45 min of leg heating. Muscle sympathetic nerve activity (radial nerve) was measured before leg heating (preheat) and 30 min after (recovery) and is expressed as burst frequency. Neurovascular transduction was examined by assessing the slope of the relation between muscle sympathetic nerve activity and leg vascular conductance measured at rest and during isometric handgrip exercise performed to fatigue. Arterial blood pressure was well maintained in young adults (preheat, 86 ± 6 mmHg vs. recovery, 88 ± 7 mmHg; P = 0.4) due to increased sympathetic nerve activity (preheat, 16 ± 7 bursts/min vs. recovery, 22 ± 10 bursts/min; P < 0.01). However, in aged adults, sympathetic nerve activity did not differ from preheat (37 ± 5 bursts/min) to recovery (33 ± 6 bursts/min, P = 0.1), despite a marked reduction in arterial blood pressure (preheat, 101 ± 7 mmHg vs. recovery, 94 ± 6 mmHg; P < 0.01). Neurovascular transduction did not differ from preheat to recovery for either age group (P ≥ 0.1). The reduction in arterial blood pressure that occurs in aged adults following exposure to acute leg heating is mediated, in part, by a sympathoinhibitory effect that alters the compensatory neural response to hypotension.NEW & NOTEWORTHY There is a sustained reduction in arterial blood pressure that occurs in aged adults following exposure to acute leg heating. However, the neurovascular mechanisms mediating this response remain unknown. Our findings demonstrate for the first time that this reduction in arterial blood pressure is mediated, in part, by a sympathoinhibitory effect that alters the compensatory neural response to hypotension in aged adults.

Is the blood pressure of right arm measured with synchronous four-limb method is equal to that with single arm method?

OBJECTIVE

Synchronous four-limb blood pressure (BP) measurement (four-limb method) is widely used for diagnosing peripheral artery diseases in clinical practise, but it is unclear whether the BP of right arm measured with this method is similar to that measured with single arm method.

METHODS

This study included 198 in-patients (aged 18-89, mean age 60.1± 13.9 years old, 109 males). They were divided into four groups on the therapy as renin-angiotensin system inhibitor (46 patients), calcium channel blocker (43 patients), beta receptor blocker (27 patients), and combination (Com, 82 patients) groups. The patients were randomly instructed to follow one of two BP measurement proposals designed to avoid the bias induced by BP measurement order. The result with single arm method was recorded as RA-1 and that with four-limb method as RA-4. The difference between RA-4 and RA-1 was calculated as Dif-RA. The percentage increase (PI) was also calculated on the formula: (RA-4 - RA-1)/RA-1.

RESULTS

The mean Dif-RA on SBP and DBP were 1.9/1.5 mmHg. Although the levels of SBP, DBP and pulse pressure of RA-1 were different, the PI of these parameters were comparable among four treatment groups. Multivariate regression analyses showed that age and SBP of RA-1 were positive independent factors for Dif-RA on SBP.

CONCLUSION

The SBP and DBP of right arm measured with synchronous four-limb method are 1.9/1.5 mmHg higher against the single arm method in treated hypertensives, and the increase extent of BP is not associated with the used drugs.

Elevated Muscle Sympathetic Nerve Activity Contributes to Central Artery Stiffness in Young and Middle-Age/Older Adults

Muscle sympathetic nerve activity (MSNA) influences the mechanical properties (ie, vascular smooth muscle tone and stiffness) of peripheral arteries, but it remains controversial whether MSNA contributes to stiffness of central arteries, such as the aorta and carotids. We examined whether elevated MSNA (age-related) would be independently associated with greater stiffness of central (carotid-femoral pulse wave velocity [PWV]) and peripheral (carotid-brachial PWV) arteries, in addition to lower carotid compliance coefficient, in healthy men and women (n=88, age: 19-73 years, 52% men). We also examined whether acute elevations in MSNA without increases in mean arterial pressure using graded levels of lower body negative pressure would augment central and peripheral artery stiffness in young (n=15, 60% men) and middle-age/older (MA/O, n=14, 43% men) adults. Resting MSNA burst frequency (bursts·min^-1) was significantly correlated with carotid-femoral PWV ( R=0.44, P<0.001), carotid-brachial PWV ( R=0.32, P=0.004), and carotid compliance coefficient ( R=0.28, P=0.01) after controlling for sex, mean arterial pressure, heart rate, and waist-to-hip ratio (central obesity), but these correlations were abolished after further controlling for age (all P>0.05). In young and MA/O adults, MSNA was elevated during lower body negative pressure ( P<0.001) and produced significant increases in carotid-femoral PWV (young: Δ+1.3±0.3 versus MA/O: Δ+1.0±0.3 m·s^-1, P=0.53) and carotid-brachial PWV (young: Δ+0.7±0.3 versus MA/O: Δ+0.7±0.5 m·s^-1, P=0.92), whereas carotid compliance coefficient during lower body negative pressure was significantly reduced in young but not MA/O (young: Δ-0.04±0.01 versus MA/O: Δ0.001±0.008 mm²·mm Hg^-1, P<0.01). Collectively, these data demonstrate the influence of MSNA on central artery stiffness and its potential contribution to age-related increases in stiffness of both peripheral and central arteries.