Sex differences in sympathetic activity and pulse wave velocity in adults with chronic kidney disease

Abstract

Chronic kidney disease (CKD) is characterized by sympathetic nervous system (SNS) overactivity that contributes to increased vascular stiffness and cardiovascular risk. Although it is well established that SNS activity and vascular stiffness are substantially elevated in CKD, whether sex differences in autonomic and vascular function exist in CKD remains unknown. We tested the hypothesis that compared with females, males with CKD have higher baseline sympathetic activity that is related to increased arterial stiffness. One hundred twenty-nine participants (96 males and 33 females) with CKD stages III and IV were recruited and enrolled. During two separate study visits, vascular stiffness was assessed by measuring carotid-to-femoral pulse wave velocity (cfPWV), and resting muscle sympathetic nerve activity (MSNA) was measured by microneurography. Males with CKD had higher resting MSNA compared with females with CKD (68 ± 16 vs. 55 ± 14 bursts/100 heart beats, P = 0.005), whereas there was no difference in cfPWV between the groups (P = 0.248). Resting MSNA was not associated with cfPWV in both males and females. In conclusion, males with CKD have higher resting sympathetic activity compared with females with CKD. However, there was no difference in vascular stiffness between the sexes. There was no correlation between resting MSNA and cfPWV, suggesting that non-neural mechanisms may play a greater role in the progression of vascular stiffness in CKD, particularly in females.

NEW & NOTEWORTHY Males with chronic kidney disease (CKD) have higher resting muscle sympathetic nerve activity (MSNA) compared with females. There was no correlation between MSNA and carotid-to-femoral pulse wave velocity (cfPWV), suggesting that non-neural mechanisms may play a greater role in the progression of vascular stiffness in CKD. Sex differences in SNS activity may play a mechanistic role in observations from epidemiological studies suggesting greater cardiovascular risk in males compared with females with CKD.

INTRODUCTION

Chronic kidney disease (CKD) is characterized by sympathetic nervous system (SNS) overactivity (1) that contributes to increased vascular stiffness (2) and cardiovascular (CV) risk. Several causal factors are responsible for chronic sympathetic overactivation in CKD including activation of the renin-angiotensin aldosterone system (RAS), reduced nitric oxide bioavailability, oxidative stress, and neural signals originating from the diseased kidneys that increase sympathetic outflow (3). SNS overactivity contributes to both blood pressure (BP)-dependent and BP-independent effects that accelerate end-organ damage and cardiovascular mortality risk. Importantly, overactivation of the SNS induces adverse vascular remodeling independent of blood pressure. Evidence from animal models (4) and humans (5) has shown that SNS overactivity leads to structural and functional changes in the vasculature that result in elevated large-artery stiffness, a known independent predictor of cardiovascular events and all-cause mortality in both the general population (6, 7) and in CKD (8, 9).

Although it is well established that SNS activity and vascular stiffness are substantially elevated in CKD (10), prior studies have primarily been performed in men. Less is known about SNS regulation in women with CKD, and whether sex differences in neural and vascular function exist within CKD remains unknown. Importantly, the epidemiology of relative CV risk between genders with aging differs in CKD compared with that of the general population. In healthy individuals, younger females are relatively protected from cardiovascular disease compared with their male counterparts and have substantially lower SNS activity (11). However, CV disease risk markedly increases in older females in the general population, with similar degrees of sympathetic overactivity (12), vascular stiffness (13), and cardiovascular risk (14) compared with older age-matched males. In contrast, in the CKD population, evidence shows that cardiovascular mortality risk continues to remain higher in older males compared with age-matched females with CKD (15). Therefore, since CKD is primarily a disease of aging, sex differences in SNS activity and vascular function may persist in CKD compared with the general population. We tested the hypothesis that compared with females, males with CKD have higher baseline sympathetic activity that is related to increased vascular stiffness.

METHODS

Study Population

One hundred twenty-nine participants with CKD stages III and IV were recruited and enrolled from outpatient clinics at the Atlanta Veterans Affairs (VA) Healthcare System. All participants with CKD had a confirmed diagnosis of either stage III [estimated glomerular filtration rate (eGFR) of 30–59 mL·min⁻¹·1.73 m²] or stage IV (eGFR of 15–30 mL·min⁻¹·1.73 m²) CKD (16). The eGFR was calculated using the 2021 CKD-EPI creatinine race-free equation (17). Participants with CKD had a stable antihypertensive medication regimen before enrollment. Exclusion criteria included severe CKD (eGFR < 15 mL·min⁻¹·1.73 m²); heart failure; coronary, cerebral, aortic, or peripheral arterial disease; liver dysfunction; severe anemia with hemoglobin level <10 g/dL; any serious systemic disease that might influence survival; current treatment with clonidine; clinic blood pressure >160/90 mmHg or <110/60 mmHg; changes in medications or surgery within the past 3 mo; alcohol or substance use disorder.

Study Design

After written informed consent was obtained, office BP, demographics, anthropometric measurements, and basic metabolic panel were obtained during a screening visit. During two separate study visits, carotid-to-femoral pulse wave velocity (cfPWV) and resting muscle sympathetic nervous activity (MSNA) by microneurography were obtained. All measurements were obtained in a quiet, temperate (21°C) environment between 8:00 and 10:00 am after abstaining from food, caffeine, smoking, and alcohol for at least 12 h, and exercising for at least 24 h. All participants reported having taken the prescribed medications as normally directed. This study was approved by the Emory University Institutional Review Board and the Atlanta Veterans Affairs Healthcare System Research and Development Committee.

Measurements and Procedures

Office blood pressure.

Office BP was measured after 5 min of rest in a seated position with the dominant arm supported at heart level using an appropriately sized cuff. All BP measurements were taken by a single study coordinator with an automated device (Omron, HEM-907XL; Omron Healthcare, Kyoto, Japan) using the oscillometric method. Each data point of BP was obtained as an average of three consecutive BP measurements separated by 5 min. Mean arterial blood pressure (MAP) was calculated as two-third diastolic BP (DBP) + one-third systolic BP (SBP).

Central blood pressure and vascular stiffness.

Pulse wave analysis and vascular stiffness were measured using the SphygmoCor XCEL device (AtCor Medical, Sydney, Australia). To assess central PWV, carotid pulse waves were measured by applanation tonometry, and femoral pulse waves were simultaneously obtained by placing a partially inflated cuff over the femoral artery at the leg midway between the hip and the knee. The cfPWV was determined by calculating the ratio of the corrected distance between the pulse-measuring sites to the time delay between the carotid and femoral pulse waves. The distance was measured between (a) the suprasternal notch to the carotid site, (b) the femoral artery at the inguinal ligament to the proximal edge of the thigh cuff, and (c) the suprasternal notch to the proximal edge of the thigh cuff. Distances a and b were subtracted from distance c and used in the calculation of cfPWV. Aortic waveform assessments used the automatic recording of standard oscillometric brachial BP, followed by partial cuff inflation to subdiastolic pressure levels to capture a brachial artery waveform. The pulse waveform was calibrated to brachial BP, and a validated generalized transfer function was applied to generate the aortic pressure waveform. (18). Participants were positioned supine on the bed and an appropriately sized cuff was placed on the participants’ right upper arm. BP was measured using standard oscillometric brachial BP, immediately followed by reinflation of the cuff to a subdiastolic pressure level, which was held for a period of 5 s while volumetric (cuff displacement) waveforms were recorded. These waveforms were then calibrated with the SBP and DBP measured with the brachial cuff before a generalized transfer function was applied to estimate a central BP waveform. For all measurements, participants were instructed to keep their arms relaxed at their sides and to refrain from any movement during the inflation and waveform measurement periods (19). Three measurements were recorded, and the two closest to each other were averaged and used in the analyses.

Muscle sympathetic nerve activity.

Multiunit postganglionic MSNA was recorded directly from the peroneal nerve by microneurography. Participants were placed in a supine position, and the leg was positioned for microneurography. A tungsten microelectrode (tip diameter, 5–15 μm) (Bioengineering, University of Iowa) was inserted into the nerve, and a reference microelectrode was inserted subcutaneously 1–2 cm from the recording electrode. The signals were amplified (total gain: 50,000–100,000), filtered (700–2,000 Hz), rectified, and integrated (time constant, 0.1 s) to obtain a mean voltage display of sympathetic nerve activity (Nerve Traffic Analyzer, model 662 C-4; University of Iowa, Bioengineering) that was recorded by the LabChart 7 Program (PowerLab 16sp, ADInstruments). A continuous electrocardiogram (ECG) was recorded simultaneously with the neurogram using a BioAmp system. Beat-to-beat arterial BP was measured concomitantly using a noninvasive monitoring device that detects digital blood flow via finger cuffs and translates blood flow oscillations into continuous pulse pressure waveforms and beat-to-beat values of BP (Finometer, Finapres). Absolute values of BP were internally calibrated using a concomitant upper arm BP reading and were calibrated at the start and every 30 min throughout the study. The tungsten microelectrode was manipulated to obtain a satisfactory nerve recording. After 10 min of rest, baseline BP, heart rate (HR), respiratory rate, and MSNA were recorded continuously for 10 min.

Data Analysis

Muscle sympathetic nerve activity.

MSNA and ECG data were exported from the LabChart data acquisition system to WinCPRS (Absolute Aliens, Turku, Finland) for analysis. R-waves were detected and marked from the continuous ECG recording. MSNA bursts were automatically detected by the program using the following criteria: 3:1 burst-to-noise ratio within a 0.5-s search window, with an average latency in burst occurrence of 1.2–1.4 s from the previous R-wave. After automatic detection, the ECG and MSNA neurograms were visually inspected for accuracy of detection. MSNA was expressed as burst incidence (bursts/100 heart beats) and burst frequency (bursts/min).

Statistics.

Data were expressed as percentages for categorical variables and means ± SD for continuous variables. For the normal distribution analysis, the quantitative data were evaluated using the Smirnorff–Kolmogorov test. A t test was used to compare differences between groups for parametric variables and Mann–Whitney U test for nonparametric variables. Pearson’s correlation was used to analyze the association between continuous variables, cfPWV, and MSNA. A stepwise multiple linear regression model was constructed where the outcome was cfPWV. The best-fitting and simplest model was selected. Data were analyzed statistically using commercial software (SPSS 28.0, IBM SPSS Statistics). P values <0.05 were considered statistically significant for all analyses.

RESULTS

Participant Characteristics

One hundred twenty-nine participants with CKD were included: 33 self-identified as female and 96 self-identified as male. The mean age was similar between males (64 ± 8 yr) and females (65 ± 8 yr) and the majority of participants in both groups were Black (>80%). There was no difference in body mass index (BMI) and comorbid conditions, including smoking status, diabetes, or hypertension, between groups. Males had a higher percentage of treatment with α-blockers, but otherwise, all other classes of antihypertensive medication use including β-blockers and angiotensin-converting enzyme (ACE) inhibitors were similar between the groups. Resting office SBP and heart rate were similar between groups, whereas resting office DBP and MAP were higher in males. Creatinine values were higher in males; however, eGFR was similar between groups. All other laboratory values were also similar between groups (Table 1).

Table 1.

Demographics and baseline characteristics according to sex

	Female (n = 33)	Male (n = 96)	P Value
Age, yr	65 ± 8	64 ± 8	0.266
Race (Black), %	81.3	81.8	0.942
Height, m	1.64 ± 0.08	1.79 ± 0.07	<0.001
Body mass, kg	87 ± 13	98 ± 18	0.001
BMI, kg/m2	32.7 ± 5.6	30.7 ± 5.4	0.074
Current smoker, %	9.1	11.5	0.496
Diabetes, %	45.5	32.3	0.173
Hypertension, %	100	100
RAS inhibitors, %	60.6	57.3	0.739
CCB, %	51.5	60.4	0.371
Thiazide diuretics, %	24.2	27.1	0.749
Loop diuretics (%)	18.2	8.3	0.109
β-Blockers, %	33.3	43.8	0.294
α-Blockers, %	0	17.7	0.004
Office blood pressure
Systolic blood pressure	124 ± 17	130 ± 14	0.056
Diastolic blood pressure	71 ± 10	77 ± 11	0.012
Mean arterial pressure	89 ± 11	95 ± 10	0.005
Heart rate	71 ± 10	68 ± 12	0.103
Laboratory values
Glucose, mg/dL	122 ± 46	111 ± 48	0.201
BUN, mg/dL	25 ± 10	24 ± 11	0.485
Creatinine, mg/dL	1.68 ± 0.54	1.94 ± 0.51	0.002
eGFR, mL·min−1·1.73 m2	37 ± 12	40 ± 9	0.082

All blood pressure values are expressed in mmHg. BMI, body mass index; BUN, blood urea nitrogen; CCB, calcium channel blockers; eGFR, estimated glomerular filtration rate; RAS inhibitors, renin-angiotensin system inhibitors. Significant P values < 0.05 are in bold.

Sympathetic Activity

Resting MSNA was higher in males versus females calculated both as burst frequency (42 ± 10 vs. 36 ± 9 bursts/min, P = 0.047) and burst incidence (68 ± 16 vs. 55 ± 14 bursts/100 heart beats, P = 0.005) (Fig. 1). We next examined the relationship between resting MSNA and several potential predictors of SNS activity within the male and female cohorts: age, BMI, and blood pressure. In both males and females, resting MSNA was not associated with age (Fig. 2, A and C). Resting MSNA was positively associated with BMI in males (Fig. 2D) but not in females (P = 0.308). In terms of the association with office blood pressure, baseline resting MSNA was related to blood pressure in males but not in females. Resting MSNA was positively correlated with office DBP and MAP (but not SBP) in males (Fig. 3), whereas there was no association between office BP and resting MSNA in females.

Figure 1.

Mean muscle sympathetic nerve activity (MSNA) at rest in females (n = 33) vs. males (n = 96) with chronic kidney disease (CKD) expressed as burst frequency (A) and burst incidence (B). Independent t tests were used to compare group means. *P < 0.05.

Figure 2.

Linear regression model showing the relationship between resting muscle sympathetic nerve activity (MSNA) with age and body mass index (BMI) in females (○, n = 33) and males (●, n = 96) with chronic kidney disease (CKD).

Figure 3.

Correlation between office systolic (SBP), diastolic (DBP), and mean (MAP) blood pressures with resting muscle sympathetic nerve activity (MSNA) in females (○, n = 33) and males (●, n = 96) with chronic kidney disease (CKD).

Central Blood Pressure and Vascular Stiffness

Similar to the office BP, central DBP and MAP were higher in males compared with females, whereas pulse pressure was similar between the groups (Table 2). The augmentation index corrected for heart rate was higher in females compared with males. However, vascular stiffness quantified as cfPWV was not different between male and female groups (8.9 ± 1.6 vs. 8.6 ± 1.7, P = 0.248). cfPWV was not associated with age in both males and females. cfPWV was positively associated with BMI in females (β = 0.113; P = 0.033) but not in males (P = 0.840). A multiple regression model including all variables that were associated with cfPWV was built and included age, BMI, CKD stage, resting MSNA burst incidence, and office SBP. In the unadjusted and adjusted models, there was no association between cfPWV and MSNA in males, whereas there was a trend toward an inverse relationship between cfPWV and MSNA in females (P = 0.068 in unadjusted analysis, P = 0.106 in adjusted analysis).

Table 2.

Pulse wave analysis and pulse wave velocity according to sex

	Female (n = 33)	Male (n = 96)	P Value
Central systolic BP	116 ± 16	123 ± 14	0.094
Central diastolic BP	72 ± 11	81 ± 11	0.002
Mean arterial pressure	87 ± 12	95 ± 11	0.006
Pulse pressure	44 ± 10	41 ± 9	0.272
AI, %	29 ± 11	26 ± 9	0.109
AI@75, %	24 ± 10	20 ± 10	0.033
cfPWV, m/s	8.9 ± 1.6	8.6 ± 1.7	0.248

All blood pressure values are expressed in mmHg. AI, augmentation index; AI@75, augmentation index corrected for heart rate of 75 beats/min; BP, blood pressure; cfPWV, carotid-to-femoral pulse wave velocity. Significant P values < 0.05 are in bold.

DISCUSSION

The present study demonstrates sex differences in resting MSNA in a large cohort of adults with CKD. Although it is well established that SNS activity is chronically elevated in patients with impaired kidney function, we show for the first time that males with CKD have higher resting MSNA compared with females with CKD. Although males had higher baseline sympathetic activity than females, there were no differences in vascular stiffness quantified by cfPWV between the sexes. There was no association between MSNA and cfPWV in males, whereas females had a trend toward an inverse correlation between MSNA and cfPWV in both unadjusted and adjusted models.

The overactivation of the sympathetic nervous system significantly contributes to cardiovascular disease risk and the progression of end-organ damage (20, 21). Multiple studies have shown that resting MSNA is chronically elevated in patients with CKD compared with the general population, and MSNA progressively increases with worsening renal function (22–24). In our study, we showed that females with CKD have significantly lower resting MSNA than males of similar age, race, BMI, comorbidities, and baseline characteristics. In older, healthy people, the evidence with respect to sex differences in MSNA is controversial. Some investigators report a tendency for higher MSNA in older females compared with males that did not reach statistical significance (25, 26), whereas other studies have shown that older females had lower MSNA compared with matched males (27) even before the development of hypertension (28). The current findings that demonstrate higher resting MSNA in males versus females with CKD may contribute to persistently greater cardiovascular risk in males with CKD progression (29). In addition, the risk of kidney dysfunction increases with advancing age, and it is generally accepted that SNS activity also increases progressively with aging (30). Prior studies have shown that White, healthy, and nonobese individuals have a positive correlation between age and resting MSNA both in females and males (25, 27). However, in our participants, comprised primarily of Black and obese individuals with CKD, there was no relationship between age and MSNA in both males and females. These findings suggest that the protective effect of younger age on sympathetic function may not be present in the setting of decreased kidney function in both males and females.

The current cohort of participants with CKD comprises primarily Black obese and overweight older adults. Prior studies have shown a clear association between sympathetic overactivity and a wide variety of measures of obesity (31, 32). Interestingly, in our participants, the association between MSNA and BMI was only observed in males with CKD. These findings are in line with prior studies suggesting sex differences in obesity-induced SNS activation demonstrating that the association between obesity and MSNA present in males may not be present in females (33). Tank et al. (34) showed that in healthy White individuals, resting MSNA was positively correlated to BMI in men but not in women. Similarly, in a study of 167 normotensive participants, BMI correlated with MSNA in normotensive males, hypertensive males, and hypertensive females, but not in normotensive females (35). Race differences have also been described in the relationship between resting MSNA and obesity. In contrast to our current findings, a prior study showed that in Black individuals, unlike White individuals, MSNA was associated with BMI in females, but not in males (36). However, the current study cohort comprises an older patient population with CKD and other comorbidities including obesity, whereas previous studies primarily included middle-aged, normal-weight participants. Whether a direct relationship between obesity and MSNA is present in older people has not been directly investigated.

We also observed sex differences in the relationship between resting MSNA and blood pressure in this CKD cohort. Although resting MSNA was positively associated with DBP and MAP in males with CKD, MSNA did not correlate with blood pressure in females with CKD. Our prior study demonstrated increased α1-adrenergic receptor sensitivity in a cohort of primarily male patients with CKD (37), and another study demonstrated decreasing β1-adrenergic receptor sensitivity, with increasing α1-adrenergic receptor sensitivity in Black men, and those with BMI ≥30 kg/m² (38); these studies suggest that Black males may have greater SNS-mediated increases in blood pressure. In contrast, prior studies have suggested that neural mechanisms may play less of a role in blood pressure regulation in obese Black women. In a study by Marinos et al. (39), removing the effect of sympathetic activity on blood pressure via ganglionic blockade in hypertensive obese Black women did not affect blood pressure, indicating that obesity-induced hypertension in Black women may be less dependent on sympathetic regulation.

CKD is characterized by both SNS overactivation and increased vascular stiffness, which are independent risk factors for cardiovascular events in CKD (3, 22, 40–42) and progressively worsen with decreasing kidney function (43). SNS overactivity is known to contribute mechanistically to the development of arterial stiffness, as heightened SNS activity can lead to both structural and functional changes in the vasculature that lead to increased stiffness (44). Multiple studies, although not all (45, 46), have shown a positive relationship between MSNA and cfPWV, particularly in younger and healthy individuals (47–50). However, in the current CKD cohort, although we observed higher MSNA levels in males versus females with CKD, we did not observe a sex difference in vascular stiffness quantified by cfPWV, although the augmentation index, an indirect marker of vascular stiffness, was lower in males compared with females. However, the sex difference in augmentation index could be explained by the shorter height in females (51). In multivariate-adjusted analyses, we observed no relationship between MSNA and cfPWV among males. Interestingly, we observed a trend toward an inverse relationship between MSNA and cfPWV in females (i.e., higher stiffness was associated with lower MSNA) that did not reach statistical significance. Interestingly, a prior study (52) reported a similar inverse relationship in young women between MSNA and the central augmentation index that may also reflect vascular stiffness, although not interchangeable with PWV. Although speculative, non-neural mechanisms of vascular stiffness may have led to increased blood pressure and baroreflex-mediated suppression of SNS activity in females, sex differences in baroreflex function in CKD are unknown. Multiple pathogenic mechanisms besides SNS overactivity contribute to the progression of vascular stiffness in CKD including inflammation, oxidative stress, disorders of calcium and phosphorus metabolism, accumulation of uremic vascular toxins, and RAS activation (9, 53). Thus, non-neural mechanisms may play a greater role in the progression of vascular stiffness in CKD, particularly in females.

We recognize several limitations in the current study. First, although we have demonstrated sex differences in resting SNS activity in a large cohort of patients with CKD, we do not address the mechanisms underlying these observed differences. Although we have identified differing risk factors associated with MSNA in males versus females, causal relationships cannot be inferred due to the cross-sectional design of the study. Second, we did not collect hormone levels, including estrogen and testosterone levels in both male and female participants. However, the mean age of female participants was 64 yr, and the majority of participants were likely postmenopausal although this was not ascertained. Third, although the male and female groups were well matched for age, BMI, and comorbid conditions including diabetes and hypertension, more male participants were being treated with α-blockers compared with the female participants. However, even after excluding males who were treated with α-blockers, we observed a higher resting MSNA in males compared with females with CKD. Fourth, the majority of participants were older Black individuals, and therefore these results may not be generalizable to other racial groups or younger individuals. Finally, there may be limitations in central BP measurements derived via the XCEL system that do not meet the Artery Society criteria for noninvasive measurement of aortic blood pressure (19).

In summary, the current findings shed new light on sex differences in neural and vascular function in CKD, a population at substantially higher risk of cardiovascular mortality. Future research should investigate the mechanisms underlying sex differences in neurocirculatory control in CKD to develop novel therapies to improve clinical outcomes in this high-risk patient population.

DATA AVAILABILITY

Data will be made available upon reasonable request.

GRANTS

This work was supported by National Institutes of Health Grants R01HL135183, R33AT010457, and KL2TR002381.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

J.P. conceived and designed research; M.G.Z., J.J., D.R.D., and J.P. performed experiments; M.G.Z. analyzed data; M.G.Z., J.J., and J.P. interpreted results of experiments; M.G.Z. prepared figures; M.G.Z. drafted manuscript; M.G.Z., J.J., and J.P. edited and revised manuscript; M.G.Z., J.J., D.R.D., and J.P. approved final version of manuscript.