Sex and age differences in the association between sympathetic outflow and central elastic artery wall thickness in humans

Abstract

Aging is characterized by increased wall thickness of the central elastic arteries (i.e., aorta and carotid arteries), although the mechanisms involved are unclear. Evidence suggests that age-related increases in muscle sympathetic nerve activity (MSNA) may be a contributing factor. However, studies in humans have been lacking. Therefore, we tested the hypothesis that age-related increases in MSNA would be independently associated with carotid artery intima-media thickness (IMT) but not in young women given the reduced influence of MSNA on the vasculature in this group. In 93 young and middle-age/older (MA/O) adults (19–73 yr, 41 women), we performed assessments of MSNA (microneurography) and common carotid IMT and lumen diameter (ultrasonography). Multiple regression that included MSNA and other cardiovascular disease risk factors indicated that MSNA (P = 0.002) and 24-h systolic blood pressure (BP) (P = 0.024) were independent determinants of carotid IMT-to-lumen ratio (model R² = 0.38, P < 0.001). However, when examining only young women (<45 yr), no correlation was observed between MSNA and carotid IMT-to-lumen ratio (R = −0.01, P = 0.963). MSNA was significantly correlated with IMT-to-lumen ratio while controlling for 24-h systolic BP among young men (R = 0.49, P < 0.001) and MA/O women (R = 0.59, P = 0.022). However, among MA/O men, controlling for 24-h systolic BP attenuated the association between MSNA and carotid IMT-to-lumen ratio (R = 0.50, P = 0.115). Significant age differences in IMT-to-lumen ratio between young and MA/O men (P = 0.047) and young and MA/O women (P = 0.023) were removed when adjusting for MSNA (men: P = 0.970; women: P = 0.152). These findings demonstrate an association between higher sympathetic outflow and carotid artery wall thickness with a particular exception to young women.

NEW & NOTEWORTHY Increased wall thickness of the large elastic arteries serves as a graded marker for cardiovascular disease risk and progression of atherosclerosis. Findings from the present study establish an independent association between higher sympathetic outflow and carotid artery wall thickness in adults with an exception to young women and extend findings from animal models that demonstrate hypertrophy of vascular smooth muscle following chronic sympathetic-adrenergic stimulation.

INTRODUCTION

Aging is characterized by increased intima-media wall thickness (IMT) of the central elastic arteries such as the aorta and carotid arteries. Increased IMT is associated with elevated risk of myocardial infarction and stroke (5, 28) and serves as a graded marker for cardiovascular disease (CVD) risk and progression of atherosclerosis (45). CVD risk factors that are associated with increased carotid IMT include elevated blood pressure (BP) (30), high circulating cholesterol concentrations (34), cigarette smoking (3), diabetes (22), menopause (8), age, and male sex (4). However, increases in IMT occur in healthy aging even in the absence of these CVD risk factors (21). Thus, the mechanisms contributing to the age-related increase in IMT of the central arteries remain unclear.

Advancing age is also characterized by elevated sympathetic nerve activity (17, 26, 44). In the muscular femoral arteries, which share some structural and mechanical properties with the central elastic arteries, Dinenno et al. (7) reported a significant association between muscle sympathetic nerve activity (MSNA) and greater IMT in healthy men. In regard to the central elastic arteries, there is emerging evidence in humans that heightened MSNA influences the mechanical properties of the vessel wall such as the stiffness of the aorta and carotid arteries (13, 25). Moreover, evidence from experimental animal models support increased wall thickening of the central elastic arteries with greater sympathetic activity (2, 29, 42, 43). However, there is a lack of translational evidence in humans. No studies have examined the relation between age-related increases in MSNA and the wall thickness of central elastic arteries such as the carotid arteries.

Therefore, we sought to examine the association between age-related increases in sympathetic outflow and carotid IMT in a cross-sectional analysis of men (n = 52) and women (n = 41) varying in age (19–72 yr). We hypothesized that higher MSNA would be a significant determinant of greater carotid IMT independent of traditional CVD risk factors. Also, given known sex differences in sympathetic regulation of the vasculature in which no relation between MSNA and peripheral resistance is present in young women (10), we further hypothesized that the association between MSNA and carotid IMT would be more prominent among men compared with women. Carotid IMT was examined relative to lumen diameter (i.e., IMT-to-lumen ratio) to account for adaptive changes in IMT secondary to increases in lumen diameter (6) and sex differences in carotid diameter (4, 9). As such, we hypothesized that age-related increases in IMT-to-lumen ratio would be independently associated with MSNA in men but not women.

METHODS

All experimental procedures and protocols conformed to the Declaration of Helsinki and were approved by the Institutional Review Board at the University of Iowa. Each subject received a verbal and written explanation of the study objectives, measurement techniques, and risks and benefits associated with the investigation before providing written informed consent on the initial visit.

Subjects.

Ninety-three participants (41 women and 52 men) who were nonsmokers and free of metabolic or neurological disease were recruited though a University of Iowa mass email. To address the novel hypothesis of this study with an ample number of subjects, participants (n = 88) from a previous study were included in the sample (13), and 8 additional participants were recruited to support sex and age comparisons. Age < 45 yr was considered young as previously defined (13, 16). Participants had no history of CVD or diabetes but a history of CVD risk factors such as hypertension and borderline hypertension were present in 9 women (20%) and 14 men (27%) (Table 1). No participants were smokers (for at least 3 mo previous to study participation used as inclusion criteria), but 7% of men and 12% of women had self-reported history of smoking. Among the 44 women that participated, 13 were postmenopausal (none were receiving hormone replacement therapy, but 2 had history of hormone replacement therapy more than 1 yr before participating in the study), 2 had a hysterectomy, and 11 were on a hormone contraceptive. The phase of the menstrual cycle was not controlled, but carotid IMT is unchanged over the course of the menstrual cycle in healthy women (12). No pregnant women were studied as confirmed by a urine pregnancy test the morning of the initial visit and the morning of the study visit.

Table 1.

Subject characteristics

Variable	Young Men	Young Women	MA/O Men	MA/O Women	P Value
n	40	25	12	16
Age, years	27 ± 1	30 ± 1	60 ± 3*†	56 ± 1*†	<0.001
Age range, years	19–44	20–44	47–73	46–66	–
BMI, kg·m2	28 ± 1	28 ± 1	29 ± 1	29 ± 1	0.411
Glucose, mg/dL	92 ± 1	93 ± 4	97 ± 2	100 ± 9	0.201
Insulin, µIU/mL	12.2 ± 1.7	9.3 ± 1.4	5.9 ± 0.8	10.1 ± 2.2	0.065
Triglycerides, mg/dL	99 ± 8	72 ± 7	82 ± 11	92 ± 13	0.298
LDL, mg/dL	106 ± 4	89 ± 6	106 ± 10	101 ± 7	0.142
HDL, mg/dL	43 ± 2	62 ± 3*	60 ± 2*	59 ± 3*	<0.001
Total cholesterol, mg/dL	173 ± 6	165 ± 6	182 ± 9	178 ± 7	0.426
MSNA
Burst incidence, bursts/100 heartbeats	31 ± 3	24 ± 3	58 ± 7*†	43 ± 4†	<0.001
Burst frequency, bursts/min	19 ± 2	15 ± 2	33 ± 4*†	26 ± 2†	<0.001
Common carotid artery
IMT, mm	0.41 ± 0.02	0.40 ± 0.02	0.52 ± 0.04*†	0.50 ± 0.02*†	0.001
Lumen diameter, mm	6.45 ± 0.10	6.03 ± 0.08*	6.66 ± 0.25†	6.33 ± 0.18	0.006
IMT-to-lumen ratio	0.064 ± 0.003	0.067 ± 0.003	0.079 ± 0.008	0.079 ± 0.004*	0.009
Cardiovascular variables
Resting heart rate, beats/min	63 ± 1	62 ± 2	56 ± 3	61 ± 2	0.168
Resting systolic BP, mmHg	125 ± 2	118 ± 2	132 ± 5	129 ± 5	0.060
Resting diastolic BP, mmHg	70 ± 2	69 ± 2	79 ± 2*†	76 ± 2	0.006
Resting mean BP, mmHg	88 ± 2	85 ± 2	96 ± 3†	94 ± 3†	0.006
24-h systolic BP, mmHg	128 ± 1	122 ± 2	130 ± 4	125 ± 4	0.095
24-h diastolic BP, mmHg	75 ± 1	74 ± 1	80 ± 2	75 ± 3	0.123
24-h mean BP, mmHg	92 ± 1	91 ± 2	96 ± 3	93 ± 3	0.295
History
HTN or borderline HTN, n (%)	11 (28)	3 (12)	3 (25)	6 (38)	0.295
Family HTN, n (%)	17 (43)	17 (68)	6 (50)	12 (75)	0.076
Smoking, n (%)	6 (15)	8 (32)	1 (8)	3 (19)	0.268
Cardiovascular medications
Statin, n (%)	0 (0)	0 (0)	1 (8)	1 (6)	–
Diuretic, n (%)	0 (0)	2 (8)	0 (0)	1 (6)	–
ACE inhibitor, n (%)	2 (5)	0 (0)	0 (0)	1 (6)	–

Values are means ± SE. History indicates self-reported history. ACE, angiotensin-converting enzyme; BMI, body mass index; BP, blood pressure; LDL, low-density lipoprotein; HDL, high-density lipoprotein; HTN, hypertension; IMT, intima-media thickness; MA/O, middle-aged/older; MSNA, muscle sympathetic nerve activity. P values are one-way ANOVA results across 4 groups.

^*P < 0.05 vs. young men,

^†P < 0.05 vs. young women.

Cardiovascular variables.

Heart rate (HR) was determined from lead II of a three-lead ECG, and auscultatory BP at the brachial artery was recorded in triplicate using the Noninvasive Hemodynamics (NIHem) workstation (Cardiovascular Engineering, Norwood, MA) (20).

Carotid IMT and lumen diameter.

High-resolution B-mode ultrasound imaging with a 12-MHz linear transducer (Logiq 7; GE Healthcare) was used to assess common carotid artery IMT and diameter. Ultrasound images were acquired for 30 s at 15 frames per second using Vascular Analysis Tools Analyzer 5.5 (Medical Imaging Applications, Coralville, IA). Analysis was performed on a wall segment 5–10 mm in length devoid of focal plaques and 1–2 cm proximal to the carotid bifurcation. Carotid IMT was defined as the maximal distance between the lumen-intima interface and the media-adventitia interface at the far-wall at end diastole (32). Carotid IMT was normalized to lumen diameter, which was defined as the distance between the near-wall media-adventitia interface and the far-wall lumen-intima interface at end diastole (37), and expressed as IMT-to-lumen ratio (see Fig. 1).

Fig. 1.

Example recordings of resting multiunit muscle sympathetic nerve activity (MSNA) and common carotid intima-media thickness (IMT) in 5 men (A–E) and 5 women (F–J) of varying levels of MSNA. Each upward inflection in the MSNA recordings depicts a synchronized burst of action potentials. Space within the calipers on the carotid far wall indicates IMT, i.e., distance between media-adventitia interface (black arrow) and lumen-intima interface (white arrow).

Muscle sympathetic nerve activity.

Multiunit postganglionic MSNA was recorded using standard microneurographic techniques as previously described (14, 15, 38, 40). A tungsten microelectrode was placed into the peroneal nerve near the left fibular head. Signals were amplified, filtered (bandwidth 0.7–2.0 kHz), rectified, and integrated (0.1-s time constant) to obtain mean voltage neurograms (Nerve Traffic Analyzer; University of Iowa Bioengineering, Iowa City, IA). MSNA was identified by the presence of spontaneous bursts with characteristic pulse synchronicity and by its responsiveness to end-expiratory breath holds but not to arousal or skin stimulation. MSNA data were acquired at a frequency of 1,000 Hz using a Powerlab data acquisition system and LabChart version 8.1.5 (ADInstruments, Colorado Springs, CO). Resting MSNA was calculated as mean values over a 10-min baseline period and quantified as burst frequency (bursts/min) and burst incidence (bursts/100 heartbeats).

Ambulatory 24-h BP monitoring.

Noninvasive 24-h ambulatory BP, which is regarded as the gold standard for the prediction of risk related to BP (31, 39), was obtained using oscillometric SpaceLabs 90207 monitors (SpaceLabs Healthcare, Snoqualmie, WA) (27). Monitors were programmed to obtain BP readings at intervals of 30 min during the day from 0600 to 2200 h and at night every 60 min from 2200 to 0600 h. Daytime (awake) and nocturnal (sleeping) BP was adjusted to the nearest hr based on each participant’s written record of their activities and sleep periods for the 24-h monitoring period. At least 10 daytime readings and 5 nighttime readings and at least 80% successful readings of planned measurements over the 24 h were required (14). Three young women elected to not wear the 24-h BP monitor and were excluded from analyses to leave a total of 41 women.

Experimental protocol.

On the first visit to the laboratory, subjects received verbal explanation of the study and provided written informed consent. Subjects completed a health history survey and were instrumented with a 24-h BP monitor. On the experimental day (within 2 wk of the initial visit), participants were instructed to refrain from medication use and fast overnight before arriving at the laboratory between 0700 and 0900 h. Subjects were also instructed to abstain from caffeinated beverages the morning of the study and strenuous physical activity and alcohol for at least 24 h before experimental sessions. All experiments were performed in a dimly lit room at an ambient temperature of 22°C–24°C. Assessments of carotid IMT and lumen diameter were performed using ultrasonography. Once the MSNA signal was acquired, data were collected for at least a 10-min baseline period.

Statistical analysis.

Bivariate correlations were examined using Pearson correlation coefficient. One-way analysis of variance (ANOVA) was used to examine group differences in continuous variables, and χ² test was used to examine group differences in categorical variables. The Holm-Sidak method was used for multiple comparisons. When normality failed, Kruskal-Wallis one-way ANOVA (ranks) tests were used, and pairwise comparisons were made using Dunn’s Method. Partial correlation was used to test the strength of associations while controlling for sex or ambulatory 24-h systolic BP. Ambulatory 24-h systolic BP was used as a primary variable because it has previously been associated with carotid IMT (35, 36), although ambulatory 24-h diastolic and 24-h mean BP were also examined. Analysis of covariance (ANCOVA) was used to determine differences between young and middle-aged/older (MA/O) in carotid IMT and IMT-to-lumen ratio after adjusting for resting MSNA. Multiple linear regression was used to examine whether MSNA was a significant determinant of carotid IMT beyond other independent variables associated with carotid IMT (age, sex, BMI, ambulatory 24-h systolic BP, total cholesterol/HDL ratio, history of hypertension or borderline hypertension, and family history of hypertension). Data are reported as means ± SE. Statistical significance was set at P < 0.05.

RESULTS

Subject characteristics.

Resting MSNA, common carotid IMT, and cardiovascular variables, including ambulatory 24-h BP are shown in Table 1. MSNA and carotid IMT were greater in MA/O men and women compared with young men and women. Example ultrasound images of the common carotid artery in five men and five women of different ages and their respective recordings of resting MSNA are shown in Fig. 1. As expected, considerably higher MSNA, carotid IMT, and IMT-to-lumen ratio was observed with advancing age in men and women (Fig. 1). Indeed, MSNA, carotid IMT, and IMT-to-lumen ratio were strongly correlated with advancing age (Fig. 2), and when statistically controlling for sex using partial correlation, there was no attenuation of these associations between advancing age and higher MSNA, carotid IMT, and IMT-to-lumen.

Fig. 2.

Bivariate correlations between age and muscle sympathetic nerve activity (MSNA) burst incidence (A), common carotid intima-media thickness (IMT) (B), and IMT relative to lumen diameter (IMT-to-lumen ratio) (C) in men (n = 52, open circles) and women (n = 41, closed circles). Partial correlation was used to control for the influence of sex.

Participants that reported history of hypertension or borderline hypertension (n = 23) (Table 1) had significantly greater carotid IMT (0.49 ± 0.03 vs. 0.42 ± 0.01 mm, P = 0.011) and MSNA burst incidence (42 ± 4 vs. 32 ± 2 bursts/100 heartbeats, P = 0.038) compared with the rest of the cohort. Participants that reported positive family history of hypertension (biological mother, father, or siblings) (n = 52) tended to have greater carotid IMT (0.46 ± 0.02 vs. 0.41 ± 0.02 mm, P = 0.053) and had significantly greater IMT-to-lumen ratio (0.073 ± 0.003 vs. 0.065 ± 0.003, P = 0.018) compared with the rest of the cohort. Participants that reported past history of smoking (n = 19) did not differ from the rest of the cohort in carotid IMT (P = 0.753), IMT-to-lumen ratio (P = 0.900), and MSNA (P = 0.947).

Multiple regression analysis.

Results from multiple linear regression analysis (independent variables: MSNA, age, sex, body mass index, ambulatory 24-h systolic BP, total cholesterol/HDL ratio, history of hypertension or borderline hypertension, and family history of hypertension) showed that MSNA burst incidence (P = 0.042), age (P = 0.002), body mass index (P = 0.020), and ambulatory 24-h systolic BP (P = 0.002) were significant determinants of carotid IMT (model R² = 0.47) (Table 2). Ambulatory 24-h diastolic BP (P = 0.031) and 24-h mean BP (P = 0.004) were also significant determinants of carotid IMT when used in place of 24-h systolic BP. When expressed as burst frequency, MSNA was not a significant determinant of carotid IMT (P = 0.078).

Table 2.

Multiple regression analysis

	Dependent Variables
	IMT		IMT-To-Lumen Ratio
Independent Variables	β	P value	β	P value
MSNA	0.21	0.042	0.35	0.002
Age	0.33	0.002	0.17	0.121
Sex	−0.02	0.807	−0.14	0.177
Body mass index	0.22	0.020	0.13	0.205
History of HTN*	0.02	0.860	0.01	0.932
Family history of HTN	0.05	0.586	0.08	0.411
Ambulatory 24-h SBP	0.30	0.002	0.23	0.024
Total cholesterol/HDL ratio	−0.07	0.464	−0.06	0.583
Model variance (R2):	0.47	<0.001	0.38	<0.001

Men: n = 52, women: n = 41. MSNA expressed as burst incidence (bursts/100 heartbeats). β values are standardized coefficients. HDL, high-density lipoprotein; IMT, intima-media thickness; MSNA, muscle sympathetic nerve activity; NA, muscle sympathetic nerve activity; SBP, systolic blood pressure.

^*Reported history of hypertension (HTN) or borderline HTN.

When examining IMT-to-lumen ratio as the dependent variable, results showed that only MSNA burst incidence (P = 0.002) and ambulatory 24-h systolic BP (P = 0.024) were significant determinants (model R² = 0.38). When expressed as burst frequency, MSNA also was a significant determinant of IMT-to-lumen ratio (P = 0.011). Ambulatory 24-h diastolic BP was not a significant determinant of IMT-to-lumen ratio (P = 0.081), whereas 24-h mean BP was significant (P = 0.022) when used in place of 24-h systolic BP. Together, these findings demonstrate that MSNA is a significant determinant of carotid IMT particularly when normalizing IMT to lumen diameter.

Association between MSNA and carotid IMT in young men and women.

MSNA burst incidence was strongly correlated with carotid IMT (Fig. 3A) and IMT-to-lumen ratio (Fig. 3B) in young men, and these correlations were not attenuated when controlling for ambulatory 24-h systolic BP. These associations were also not attenuated by 24-h diastolic BP (IMT: R = 0.51, P = 0.001; IMT-to-lumen ratio: R = 0.51, P = 0.001) and 24-h mean BP (IMT: R = 0.51, P = 0.001; IMT-to-lumen ratio: R = 0.51, P = 0.001). When expressing MSNA as burst frequency (bursts/min) in young men, similar correlations were observed between MSNA and carotid IMT (R = 0.49, P = 0.001) and IMT-to-lumen ratio (R = 0.48, P = 0.002), and these correlations were unchanged when controlling for 24-h systolic, diastolic, and mean BP (all P < 0.05). However, in young women, there was no relation between MSNA burst incidence and carotid IMT (Fig. 3C) and IMT-to-lumen ratio (Fig. 3D), and these results were similar when controlling for 24-h systolic, diastolic, and mean BP. When expressing MSNA as burst frequency among young women, there was also no relation between MSNA and carotid IMT (R = −0.01, P = 0.963) and IMT-to-lumen ratio (R = −0.01, P = 0.952).

Fig. 3.

Bivariate correlation analysis between resting muscle sympathetic nerve activity (MSNA) burst incidence and common carotid artery intima-media thickness (IMT) and IMT relative to lumen diameter (IMT-to-lumen ratio) in young men (n = 40; A and B) and women (n = 25; C and D). Partial correlation was used to control for ambulatory 24-h systolic blood pressure (SBP).

Given that young men had significantly higher ambulatory 24-h systolic BP compared with young women (P = 0.006) and included more participants with reported history of hypertension or borderline hypertension (11 vs. 3), further analyses were performed while including only young men and women with ambulatory 24-h systolic BP < 130 mmHg (young men: n = 26; young women: n = 20). Similar to results above, MSNA burst incidence was significantly correlated with carotid IMT (R = 0.41, P = 0.032) and IMT-to-lumen ratio (R = 0.46, P = 0.018) in young men, whereas no relation was observed in young women (IMT: R = 0.05, P = 0.832; IMT-to-lumen ratio: R = 0.07, P = 0.784). Moreover, MSNA burst incidence remained significantly correlated with carotid IMT and IMT-to-lumen ratio in young men when controlling for 24-h systolic BP (IMT: R = 0.45, P = 0.023; IMT-to-lumen ratio: R = 0.47, P = 0.019), 24-h diastolic BP (IMT: R = 0.41, P = 0.040; IMT-to-lumen ratio: R = 0.46, P = 0.022), or 24-h mean BP (IMT: R = 0.43, P = 0.031; IMT-to-lumen ratio: R = 0.46, P = 0.019), further suggesting that greater carotid wall thickness is associated with higher MSNA independent of BP in healthy young men but not young women.

Association between MSNA and carotid IMT in MA/O men and women.

In MA/O men, MSNA burst incidence was moderately correlated with carotid IMT (R = 0.56, P = 0.056) (Fig. 4A) and IMT-to-lumen ratio (R = 0.63, P = 0.028) (Fig. 3B), and these correlations were attenuated when controlling for 24-h systolic BP. These associations were also attenuated by 24-h diastolic BP (IMT: R = 0.31, P = 0.351; IMT-to-lumen ratio: R = 0.45, P = 0.171) and 24-h mean BP (IMT: R = 0.23, P = 0.500; IMT-to-lumen ratio: R = 0.40, P = 0.225). Associations using MSNA burst frequency were also attenuated by 24-h systolic, diastolic, and mean BP (all P > 0.05).

Fig. 4.

Bivariate correlation between resting muscle sympathetic nerve activity (MSNA) burst incidence and common carotid intima-media thickness (IMT) and IMT relative to lumen diameter (IMT-to-lumen ratio) in middle-aged/older (MA/O) men (n = 12) (A and B) and MA/O women (n = 16) (C and D). Partial correlation was used to control for ambulatory 24-h systolic blood pressure (SBP).

In MA/O women, there was no relation between MSNA and carotid IMT [MSNA burst incidence: R = −0.05, P = 0.856 (Fig. 4C); MSNA burst frequency: R = 0.15, P = 0.574]. However, there was a moderate association between MSNA and IMT-to-lumen ratio in MA/O women [MSNA as burst incidence: R = 0.43, P = 0.098 (Fig. 4D); MSNA burst frequency: R = 0.57, P = 0.021]. In contrast to MA/O men, the relation between MSNA and IMT-to-lumen ratio in MA/O women was not abolished by 24-h systolic BP [MSNA burst incidence: R = 0.59, P = 0.022 (Fig. 4D); MSNA burst frequency: R = 0.61, P = 0.020]. Moreover, the relation between MSNA and IMT-to-lumen ratio in MA/O women was not abolished by 24-h diastolic BP [MSNA burst incidence: R = 0.48, P = 0.068; MSNA burst frequency: R = 0.56, P = 0.029] or 24-h mean BP (IMT: R = 0.59, P = 0.020; MSNA burst incidence: R = 0.54, P = 0.037).

Carotid IMT and age: influence of increased MSNA.

When compared with young men and women, carotid IMT and IMT-to-lumen ratio were significantly greater among MA/O men (Fig. 5, A–B, left) and women (Fig. 5, C–D, left). However, in men, differences in carotid IMT (Fig. 3A, right) and IMT-to-lumen ratio (Fig. 3B, right) were abolished when adjusting for MSNA using ANCOVA, supporting an association between age-related increases MSNA and carotid wall thickness among men. When adjusting for MSNA using ANCOVA between young and MA/O women, differences in carotid IMT were not attenuated (Fig. 5C, right) whereas differences in IMT-to-lumen ratio were attenuated (Fig. 5D, right), further suggesting an association between MSNA and carotid IMT in women only when normalizing IMT to lumen diameter.

Fig. 5.

Mean summary data of carotid intima-media thickness (IMT) and IMT-to-lumen ratio in young versus middle-age/older (MA/O) men (A and B) and women (C and D). Right side of each panel includes mean summary data adjusted for muscle sympathetic nerve activity (MSNA) burst incidence in young and MA/O using analysis of covariance (ANCOVA). Young men: n = 40; MA/O men: n = 12; young women: n = 25; MA/O women: n = 16. Data are means ± SE.

DISCUSSION

Findings from the present study, which examined MSNA and carotid IMT in a large cohort of men and women, provide two important and novel findings. First, MSNA and 24-h systolic BP were significant determinants of greater carotid IMT among men and women, particularly when normalizing IMT to lumen diameter (i.e., IMT-to-lumen ratio). Second, MSNA and IMT-to-lumen ratio were associated within sex and age groups except for young women. Overall, the age-related increases in MSNA and carotid IMT-to-lumen ratio were associated because the significant age differences in IMT-to-lumen ratio between young and MA/O men and women were removed when adjusting for MSNA. Together, these results demonstrate an independent association between higher sympathetic outflow and carotid artery wall thickness with an exception to young women.

Influence of MSNA on carotid IMT.

Heightened sympathetic activity promotes a trophic effect on the vascular wall as exhibited by α₁-adrenergic receptor-mediated increases in vascular smooth muscle cell (VSMC) growth in the isolated rat aorta (29). Consistent with these findings, VSMC growth can be attenuated by α₁-adrenergic receptor inhibition (42, 43). To date, no studies in humans have examined the relation between sympathetic activity and wall thickness of the central elastic arteries. Dinenno et al. (2000) reported an association between elevated MSNA and greater common femoral IMT in men that was independent of age despite age-related increases in MSNA (7). The results of the present study support and extend these findings by demonstrating an association between MSNA and IMT of the central elastic (carotid) arteries in young and MA/O men. Furthermore, the positive correlation between MSNA and carotid IMT in young men was independent of 24-h systolic BP, whereas this association was attenuated in MA/O men while controlling for 24-h systolic BP. These findings are consistent with large population studies demonstrating 24-h systolic BP as a major risk factor for increased carotid IMT in middle-aged men (35), although this large study also included middle-aged men with history of heart disease and CVD risk factors such as diabetes. Moreover, MSNA was a significant determinant of carotid IMT and IMT-to-lumen ratio independent of traditional CVD risk factors including BMI, sex, cholesterol, ambulatory 24-h systolic BP, history of elevated BP, and family history of elevated BP. It is possible that elevated sympathetic activity contributes to greater central elastic artery IMT by promoting the proliferation of VSMC in the media. It also remains plausible that sympathetic activity facilitates the atherosclerotic process, although carotid ultrasound images were void of any visible focal plaques and there was no relation between carotid IMT and plasma cholesterol and triglyceride concentrations among these participants. However, speculation about potential mechanisms should be met with caution because of the correlative and cross-sectional nature of the study. The statistical approaches used, i.e., multiple regression to identify significant determinants and bivariate regression analysis to examine the strength of the association between MSNA and carotid IMT, do not allow for determining causation or the temporal relation between MSNA and carotid IMT.

MSNA and carotid IMT in aging women.

MA/O women demonstrated an association between MSNA and carotid IMT when normalizing to lumen diameter, whereas there was no such relation observed in the young women. In accordance, the difference between young and MA/O women in carotid IMT was attenuated when adjusting for age-related increases in MSNA using ANCOVA only when normalizing IMT to lumen diameter. The MA/O women in the present study were predominately postmenopausal (81%). Compared with the remaining cohort of women, postmenopausal women had significantly higher MSNA (45 ± 4 vs. 25 ± 3 bursts/100 heartbeats, P < 0.001) and IMT-to-lumen ratio (0.081 ± 0.004 vs. 0.066 ± 0.003 mm, P = 0.006). Indeed, middle-aged women demonstrate accelerated increases in MSNA relative to younger women and their counterparts of the opposite sex (26). Also, the progression rate of carotid IMT accelerates during menopause (8) but is attenuated with estrogen replacement therapy (11, 24), dietary interventions, and aerobic exercise (41). However, some large cross-sectional studies suggest no association between carotid IMT and menopause (1, 23). Nonetheless, findings from the present study support that advancing age appears to be a critical factor in the relation between sympathetic outflow and central elastic artery wall thickness in women.

Strengths and limitations.

There are several strengths of the present study, including 1) direct measures of MSNA using microneurography, 2) a wide age range of human subjects, and 3) thorough statistical adjustment for relevant confounding CVD risk factors. Among these CVD risk factors was ambulatory 24-h BP, which is regarded as the gold standard for the prediction of CVD risk related to BP (31, 39).

There were also several limitations. First, differentiation between the intimal and medial layers of the vascular wall was not possible with ultrasound in the present study. This restriction precludes a determination of the precise structures involved in wall thickening and whether greater IMT was predominantly a reflection of VSMC proliferation in the media. Also, reliable assessments of carotid adventitial thickness could not be performed, and therefore outer cross-sectional diameter was not calculated. This may be important because some evidence suggests that the outer diameter rather than the inner diameter of the common carotid artery increases as wall thickness increases (18, 33). Although no relation was observed between carotid inner (lumen) diameter and MSNA in the present study, this provides rationale for future studies examining the potential influence of higher sympathetic outflow on the outer diameter of central elastic arteries in humans.

The phase of the menstrual cycle was not controlled in our study. The menstrual cycle is relevant because there is an influence of hormonal fluctuations on MSNA during the menstrual cycle (19). However, carotid IMT is unchanged over the course of the menstrual cycle in healthy women (12). Thus, fluctuation of hormones within a menstrual cycle (~1 mo) may alter MSNA but is likely not sufficient to change IMT. It is more likely that a change in average estradiol concentrations over months and years, such as during menopause, would influence carotid IMT, and therefore becomes important in the association between MSNA and carotid IMT.

Conclusion.

In summary, we report an independent association between higher MSNA and wall thickness of the central elastic arteries. Moreover, examinations in women alone revealed that the association between MSNA and carotid IMT-to-lumen ratio was present in MA/O women but not young women. These findings have important implications for age-related increases in central elastic artery IMT and atherosclerosis and should be confirmed by prospective studies with repeated measures of MSNA and central artery IMT in aging men and women.

GRANTS

This work was supported in part by a National Institutes of Health Iowa Cardiovascular Interdisciplinary Research Fellowship grant no. T32HL007121 (to S. W. Holwerda), American Heart Association grant nos. 17POST33440101 (to S. W. Holwerda) and 13SDG143400012 (to G. L. Pierce) and National Institutes of Health grant nos. P01 HL-014388-48 (G. L. Pierce) and CTSA UL1TR002537 (University of Iowa).

DISCLOSURES

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

AUTHOR CONTRIBUTIONS

S.W.H., G.L.P., conceived and designed research; S.W.H., R.E.L., L.E.D., and G.L.P. performed experiments; S.W.H., L.E.D, R.M. analyzed data; S.W.H prepared figures; S.W.H. and G.L.P. interpreted results of experiments; S.W.H drafted manuscript; S.W.H. and G.L.P. edited and revised the manuscript; S.W.H., R.E.L., L.E.D., R.M., and G.L.P. approved the final version of the manuscript.