Abnormal Neurovascular Control during Sympathoexcitation in Obesity

Association between ambient temperature and cause-specific cardiovascular disease admissions in Japan: A nationwide study

BACKGROUND

Substantial evidence suggests that non-optimal temperatures can increase the risk of cardiovascular disease (CVD) mortality and morbidity; however, limited studies have reported inconsistent results for hospital admissions depending on study locations, which also lack national-level investigations on cause-specific CVDs.

METHODS

We performed a two-stage meta-regression analysis to examine the short-term associations between temperature and acute CVD hospital admissions by specific categories [i.e., ischemic heart disease (IHD), heart failure (HF), and stroke] in 47 prefectures of Japan from 2011 to 2018. First, we estimated the prefecture-specific associations using a time-stratified case-crossover design with a distributed lag nonlinear model. We then used a multivariate meta-regression model to obtain national average associations.

RESULTS

During the study period, a total of 4,611,984 CVD admissions were reported. We found cold temperatures significantly increased the risk of total CVD admissions and cause-specific categories. Compared with the minimum hospitalization temperature (MHT) at the 98^th percentile of temperature (29.9 °C), the cumulative relative risks (RRs) for cold (5^th percentile, 1.7 °C) and heat (99^th percentile, 30.5 °C) on total CVD were 1.226 [95% confidence interval (CI): 1.195, 1.258] and 1.000 (95% CI: 0.998, 1.002), respectively. The RR for cold on HF [RR = 1.571 (95% CI: 1.487, 1.660)] was higher than those of IHD [RR = 1.119 (95% CI: 1.040, 1.204)] and stroke [RR = 1.107 (95% CI: 1.062, 1.155)], comparing to their cause-specific MHTs. We also observed that extreme heat increased the risk of HF with RR of 1.030 (95% CI: 1.007, 1.054). Subgroup analysis showed that the age group ≥85 years was more vulnerable to these non-optimal temperature risks.

CONCLUSIONS

This study indicated that cold and heat exposure could increase the risk of hospital admissions for CVD, varying depending on the cause-specific categories, which may provide new evidence to reduce the burden of CVD.

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.

Sex differences in the vascular response to sympathetic activation during acute hypoxaemia

NEW FINDINGS

What is the central question of this study? Sympathetically mediated vasoconstriction is preserved during hypoxaemia in humans, but our understanding of vascular control comes from predominantly male cohorts. We tested the hypothesis that young women attenuate sympathetically mediated vasoconstriction during steady-state hypoxaemia, whereas men do not? What is the main finding and its importance? Sympathetically mediated vasoconstriction is preserved or even enhanced during steady-state hypoxia in young men, and the peripheral vascular response to sympathetic activation during hypoxaemia is attenuated in young women. These data advance our understanding of sex-related differences in hypoxic vascular control.

ABSTRACT

Activation of the sympathetic nervous system causes vasoconstriction and a reduction in peripheral blood flow. Sympathetically mediated vasoconstriction may be attenuated during systemic hypoxia to maintain oxygen delivery; however, in predominantly male participants sympathetically mediated vasoconstriction is preserved or even enhanced during hypoxaemia. Given the potential for sex-specific differences in hypoxic vascular control, prior results are limited in application. We tested the hypothesis that young women attenuate sympathetically mediated vasoconstriction during steady-state hypoxaemia, whereas men do not. Healthy young men (n = 13, 25 ± 4 years) and women (n = 11, 24 ± 4 years) completed two trials consisting of a 2-min cold pressor test (CPT, a well-established sympathoexcitatory stimulus) during baseline normoxia and steady-state hypoxaemia. Beat-to-beat blood pressure (finger photoplethysmography) and forearm blood flow (venous occlusion plethysmography) were measured continuously. Total and forearm vascular conductance (TVC and FVC, respectfully) were calculated. A change (Δ) in TVC and FVC from steady-state during the last 1 min of CPT was calculated and differences between normoxia and systemic hypoxia were assessed. In men, the reduction in TVC during CPT was greater during hypoxia compared to normoxia (ΔTVC, P = 0.02), whereas ΔTVC did not differ between conditions in women (P = 0.49). In men, ΔFVC did not differ between normoxia and hypoxia (P = 0.92). In women, the reduction in FVC during CPT was attenuated during hypoxia (ΔFVC, P < 0.01). We confirm sympathetically mediated vasoconstriction is preserved or enhanced during hypoxaemia in young men, whereas peripheral vascular responsiveness to sympathetic activation during hypoxaemia is attenuated in young women. The results advance our understanding of sex-related differences in hypoxic vascular control.

Sympathetic Neural Overdrive in the Obese and Overweight State

Nerve traffic recordings (muscle sympathetic nerve traffic [MSNA]) have shown that sympathetic activation may occur in obesity. However, the small sample size of the available studies, presence of comorbidities, heterogeneity of the subjects examined represented major weaknesses not allowing to draw definite conclusions. This is the case for the overweight state. The present meta-analysis evaluated 1438 obese or overweight subjects recruited in 45 microneurographic studies. The analysis was primarily based on MSNA quantification in obesity and overweight, excluding as concomitant conditions hypertension, metabolic syndrome, and other comorbidities. Assessment was extended to the relationships of MSNA with other neuroadrenergic markers, such as plasma norepinephrine and heart rate, anthropometric variables, as body mass index, waist-to-hip ratio, presence/absence of obstructive sleep apnea, and metabolic profile. Compared with normoweights MSNA was significantly greater in overweight and more in obese individuals (37.0±4.1 versus 43.2±3.5 and 50.4±5.0 burts/100 heartbeats, P<0.01). This was the case even in the absence of obstructive sleep apnea. MSNA was significantly directly related to body mass index and waist-to-hip ratio ( r=0.41 and r=0.64, P<0.04 and <0.01, respectively), clinic blood pressure ( r=0.68, P<0.01), total cholesterol, LDL (low-density lipoprotein) cholesterol, and triglycerides ( r=0.91, r=0.94, and r=0.80, respectively, P<0.01) but unrelated to plasma insulin, glucose, and homeostatic model assessment for insulin resistance. No significant correlation was found between MSNA, heart rate, and norepinephrine. Thus, obesity and overweight are characterized by sympathetic overactivity which mirrors the severity of the clinical condition and reflects metabolic alterations, with the exclusion of glucose/insulin profile. Neither heart rate nor norepinephrine appear to represent faithful markers of the muscle sympathetic overdrive.

Preserved ability to blunt sympathetically-mediated vasoconstriction in exercising skeletal muscle of young obese humans

Sympathetic vasoconstriction is attenuated in exercising muscles to assist in matching of blood flow with metabolic demand. This "functional sympatholysis" may be impaired in young obese individuals due to greater sympathetic activation and/or reduced local vasodilatory capacity of both small and large arteries, but this remains poorly understood. We tested the hypothesis that functional sympatholysis is impaired in obese individuals compared with normal-weight counterparts. In 36 obese and normal-weight young healthy adults (n = 18/group), we measured forearm blood flow and calculated forearm vascular conductance (FVC) responses to reflex increases in sympathetic nerve activity induced by lower body negative pressure (LBNP) at rest and during rhythmic handgrip exercise at 15% and 30% of the maximal voluntary contraction (MVC). FVC was normalized to lean forearm mass. In normal-weight individuals, LBNP evoked a decrease in FVC (-16.1 ± 5.7%) in the resting forearm, and the reduction in FVC (15%MVC: -8.1 ± 3.3%; 30%MVC: -1.0 ± 4.0%) was blunted during exercise in an intensity-dependent manner (P < 0.05). Similarly, in obese individuals, LBNP evoked a comparable decrease in FVC (-10.9 ± 5.7%) in the resting forearm, with the reduction in FVC (15%MVC: -9.7 ± 3.3%; 30%MVC: -0.3 ± 4.0%) also blunted during exercise in an intensity-dependent manner (P < 0.05). The magnitude of sympatholysis was similar between groups (P > 0.05) and was intensity-dependent (P < 0.05). Our findings suggest that functional sympatholysis is not impaired in young obese individuals without overt cardiovascular diseases.

Cardioautonomic responses to acute ingestion of ice water and its correlation to body mass index

Background A significant relationship has been documented in the literature between the autonomic nervous system imbalance and cardiovascular mortality. In patients with autonomic failure, water ingestion has been shown to increase blood pressure (BP), induce bradycardia, and cause low heart rate variability (HRV). A few studies showed the altered HRV as an acute effect of ice water intake in healthy subjects. None of the studies have shown light on the relationship of BP and HRV to ice water intake in obese and overweight subjects. The present study is aimed to correlate BP and HRV with body mass index (BMI) after ice water ingestion. Methods This cross-sectional study included a total of 60 subjects of both sexes aged between 18 and 24 years old. Subjects were assigned into three groups based on their BMI: normal, overweight, and obese. Before and after ice water ingestion, BP and HRV parameters were recorded and compared between the groups. Statistically data were analyzed by Student's paired t-test and one-way analysis of variance. Results Basal HF was significant (p<0.05) in all three groups after ice water ingestion [F(2, 27), 44.1; p-value, 0.02]. After ice water ingestion, all HRV values were significant (p<0.001) in the three groups. The post-hoc Tukey HSD test demonstrated the less mean score for mean RR interval, standard deviation of all NN interval, standard deviation of differences between adjacent, HF and high for HR, LF, and LHR in overweight and obese subjects. Conclusions Because of the effective buffering system, healthy subjects showed increased HR and unchanged BP. Overweight and obese subjects showed decreased HR and increased BP.

Vascular Response During Mental Stress in Sedentary and Physically Active Patients With Obstructive Sleep Apnea

STUDY OBJECTIVES

To compare vascular function of sedentary (SED) versus physically active (ACTIVE) patients with obstructive sleep apnea (OSA) during rest and mental stress.

METHODS

Patients with untreated OSA without other comorbidities were classified into SED and ACTIVE groups according to the International Physical Activity Questionnaire. Blood pressure (BP), heart rate (HR), forearm blood flow (FBF) (plethysmography), and forearm vascular conductance (FVC = FBF / mean BP × 100) were continuously measured at rest (4 minutes) followed by 3 minutes of mental stress (Stroop Color Word Test).

RESULTS

We studied 40 patients with OSA (men = 24, age = 50 ± 1 years, body mass index = 29.3 ± 0.5 kg/m², apnea-hypopnea index = 39.3 ± 4 events/h). Leisure time physical activity domain in SED (n = 19) and ACTIVE (n = 21) was 20 ± 8 and 239 ± 32 min/wk, (P < .05). Baseline profile and perception of stress were similar in both groups. Baseline FBF (3.5 ± 0.2 mL/min/100 mL versus 2.4 ± 0.14 mL/min/100 mL) and FVC (3.5 ± 0.2 U versus 2.3 ± 0.1 U) were significantly lower in the SED group than in the ACTIVE group, respectively (P < .05). HR and BP increased similarly during mental stress test in both groups. Changes during mental stress in FBF (0.65 ± 0.12 versus 1.04 ± 0.12) and FVC (0.58 ± 0.11 versus 0.99 ± 0.11) were significantly lower in the SED group than in the ACTIVE group, respectively (P < .05). There was a significant correlation between leisure time physical activity and FBF (r = .57, P < .05) and FVC (r = .48, P < .05) during mental stress.

CONCLUSIONS

The vascular response among patients with OSA is influenced by the level of physical activity. A high level of physical activity may partially protect against the cardiovascular dysfunction associated with OSA.

The effect of acute maximal exercise on postexercise hemodynamics and central arterial stiffness in obese and normal-weight individuals

Central arterial stiffness is associated with incident hypertension and negative cardiovascular outcomes. Obese individuals have higher central blood pressure (BP) and central arterial stiffness than their normal‐weight counterparts, but it is unclear whether obesity also affects hemodynamics and central arterial stiffness after maximal exercise. We evaluated central hemodynamics and arterial stiffness during recovery from acute maximal aerobic exercise in obese and normal‐weight individuals. Forty‐six normal‐weight and twenty‐one obese individuals underwent measurements of central BP and central arterial stiffness at rest and 15 and 30 min following acute maximal exercise. Central BP and normalized augmentation index (AIx@75) were derived from radial artery applanation tonometry, and central arterial stiffness was obtained via carotid‐femoral pulse wave velocity (cPWV) and corrected for central mean arterial pressure (cPWV/cMAP). Central arterial stiffness increased in obese individuals but decreased in normal‐weight individuals following acute maximal exercise, after adjusting for fitness. Obese individuals also exhibited an overall higher central BP (P < 0.05), with no exercise effect. The increase in heart rate was greater in obese versus normal‐weight individuals following exercise (P < 0.05), but there was no group differences or exercise effect for AIx@75. In conclusion, obese (but not normal‐weight) individuals increased central arterial stiffness following acute maximal exercise. An assessment of arterial stiffness response to acute exercise may serve a useful detection tool for subclinical vascular dysfunction.

The Role of the Autonomic Nervous System in the Pathophysiology of Obesity

Obesity is reaching epidemic proportions globally and represents a major cause of comorbidities, mostly related to cardiovascular disease. The autonomic nervous system (ANS) dysfunction has a two-way relationship with obesity. Indeed, alterations of the ANS might be involved in the pathogenesis of obesity, acting on different pathways. On the other hand, the excess weight induces ANS dysfunction, which may be involved in the haemodynamic and metabolic alterations that increase the cardiovascular risk of obese individuals, i.e., hypertension, insulin resistance and dyslipidemia. This article will review current evidence about the role of the ANS in short-term and long-term regulation of energy homeostasis. Furthermore, an increased sympathetic activity has been demonstrated in obese patients, particularly in the muscle vasculature and in the kidneys, possibily contributing to increased cardiovascular risk. Selective leptin resistance, obstructive sleep apnea syndrome, hyperinsulinemia and low ghrelin levels are possible mechanisms underlying sympathetic activation in obesity. Weight loss is able to reverse metabolic and autonomic alterations associated with obesity. Given the crucial role of autonomic dysfunction in the pathophysiology of obesity and its cardiovascular complications, vagal nerve modulation and sympathetic inhibition may serve as therapeutic targets in this condition.

Baroreflex dysfunction and augmented sympathetic nerve responses during mental stress in veterans with post-traumatic stress disorder

KEY POINTS

Patients with post-traumatic stress disorder (PTSD) are at a significantly higher risk of developing hypertension and cardiovascular disease. The mechanisms underlying this increased risk are not known. Studies have suggested that PTSD patients have an overactive sympathetic nervous system (SNS) that could contribute to cardiovascular risk; however, sympathetic function has not previously been rigorously evaluated in PTSD patients. Using direct measurements of sympathetic nerve activity and pharmacological manipulation of blood pressure, we show that veterans with PTSD have augmented SNS and haemodynamic reactivity during both combat-related and non-combat related mental stress, impaired sympathetic and cardiovagal baroreflex sensitivity, and increased inflammation. Identifying the mechanisms contributing to increased cardiovascular (CV) risk in PTSD will pave the way for developing interventions to improve sympathetic function and reduce CV risk in these patients.

ABSTRACT

Post-traumatic stress disorder (PTSD) is associated with increased cardiovascular (CV) risk. We tested the hypothesis that PTSD patients have augmented sympathetic nervous system (SNS) and haemodynamic reactivity during mental stress, as well as impaired arterial baroreflex sensitivity (BRS). Fourteen otherwise healthy Veterans with combat-related PTSD were compared with 14 matched Controls without PTSD.  Muscle sympathetic nerve activity (MSNA), continuous blood pressure (BP) and electrocardiography were measured at baseline, as well as during two types of mental stress:  combat-related mental stress using virtual reality combat exposure (VRCE) and non-combat related stress using mental arithmetic (MA). A cold pressor test (CPT) was administered for comparison. BRS was tested using pharmacological manipulation of BP via the Modified Oxford technique at rest and during VRCE. Blood samples were analysed for inflammatory biomarkers. Baseline characteristics, MSNA and haemodynamics were similar between the groups. In PTSD vs. Controls, MSNA (+8.2 ± 1.0 vs. +1.2 ± 1.3 bursts min^-1 , P < 0.001) and heart rate responses (+3.2 ± 1.1 vs. -2.3 ± 1.0 beats min^-1 , P = 0.003) were significantly augmented during VRCE.  Similarly, in PTSD vs. Controls, MSNA (+21.0 ± 2.6 vs. +6.7 ± 1.5 bursts min^-1 , P < 0.001) and diastolic BP responses (+6.3 ± 1.0 vs. +3.5 ± 1.0 mmHg, P = 0.011) were significantly augmented during MA but not during CPT (P = not significant). In the PTSD group, sympathetic BRS (-1.2 ± 0.2 vs. -2.0 ± 0.3 burst incidence mmHg^-1 , P = 0.026) and cardiovagal BRS (9.5 ± 1.4 vs. 23.6 ± 4.3 ms mmHg^-1 , P = 0.008) were significantly blunted at rest. PTSD patients had significantly higher highly sensitive-C-reactive protein levels compared to Controls (2.1 ± 0.4 vs. 1.0 ± 0.3 mg L^-1 , P = 0.047). Augmented SNS and haemodynamic responses to mental stress, blunted BRS and inflammation may contribute to an increased CV risk in PTSD.

Impact of ambient temperature on morbidity and mortality: An overview of reviews

The objectives were (i) to conduct an overview of systematic reviews to summarize evidence from and evaluate the methodological quality of systematic reviews assessing the impact of ambient temperature on morbidity and mortality; and (ii) to reanalyse meta-analyses of cold-induced cardiovascular morbidity in different age groups. The registration number is PROSPERO-CRD42016047179. PubMed, Embase, the Cochrane Library, Web of Science, the Cumulative Index to Nursing and Allied Health Literature (CINAHL), and Global Health were systematically searched to identify systematic reviews. Two reviewers independently selected studies for inclusion, extracted data, and assessed quality. The Assessment of Multiple Systematic Reviews (AMSTAR) checklist was used to assess the methodological quality of included systematic reviews. Estimates of morbidity and mortality risk in association with heat exposure, cold exposure, heatwaves, cold spells and diurnal temperature ranges (DTRs) were the primary outcomes. Twenty-eight systematic reviews were included in the overview of systematic reviews. (i) The median (interquartile range) AMSTAR scores were 7 (1.75) for quantitative reviews and 3.5 (1.75) for qualitative reviews. (ii) Heat exposure was identified to be associated with increased risk of cardiovascular, cerebrovascular and respiratory mortality, but was not found to have an impact on cardiovascular or cerebrovascular morbidity. (iii) Reanalysis of the meta-analyses indicated that cold-induced cardiovascular morbidity increased in youth and middle-age (RR=1.009, 95% CI: 1.004-1.015) as well as the elderly (RR=1.013, 95% CI: 1.007-1.018). (iv) The definitions of temperature exposure adopted by different studies included various temperature indicators and thresholds. In conclusion, heat exposure seemed to have an adverse effect on mortality and cold-induced cardiovascular morbidity increased in the elderly. Developing definitions of temperature exposure at the regional level may contribute to more accurate evaluations of the health effects of temperature.

Rate of rise in diastolic blood pressure influences vascular sympathetic response to mental stress

KEY POINTS

Research indicates that individuals may experience a rise (positive responders) or fall (negative responders) in muscle sympathetic nerve activity (MSNA) during mental stress. In this study, we examined the early blood pressure responses (including the peak, time of peak and rate of rise in blood pressure) to mental stress in positive and negative responders. Negative MSNA responders to mental stress exhibit a more rapid rise in diastolic pressure at the onset of the stressor, suggesting a baroreflex-mediated suppression of MSNA. In positive responders there is a more sluggish rise in blood pressure during mental stress, which appears to be MSNA-driven. This study suggests that whether MSNA has a role in the pressor response is dependent upon the reactivity of blood pressure early in the task.

ABSTRACT

Research indicates that individuals may experience a rise (positive responders) or fall (negative responders) in muscle sympathetic nerve activity (MSNA) during mental stress. The aim was to examine the early blood pressure response to stress in positive and negative responders and thus its influence on the direction of change in MSNA. Blood pressure and MSNA were recorded continuously in 21 healthy young males during 2 min mental stressors (mental arithmetic, Stroop test) and physical stressors (cold pressor, handgrip exercise, post-exercise ischaemia). Participants were classified as negative or positive responders according to the direction of the mean change in MSNA during the stressor tasks. The peak changes, time of peak and rate of changes in blood pressure were compared between groups. During mental arithmetic negative responders experienced a significantly greater rate of rise in diastolic blood pressure in the first minute of the task (1.3 ± 0.5 mmHg s^-1 ) compared with positive responders (0.4 ± 0.1 mmHg s^-1 ; P = 0.03). Similar results were found for the Stroop test. Physical tasks elicited robust parallel increases in blood pressure and MSNA across participants. It is concluded that negative MSNA responders to mental stress exhibit a more rapid rise in diastolic pressure at the onset of the stressor, suggesting a baroreflex-mediated suppression of MSNA. In positive responders there is a more sluggish rise in blood pressure during mental stress, which appears to be MSNA-driven. This study suggests that whether MSNA has a role in the pressor response is dependent upon the reactivity of blood pressure early in the task.

Acute cyclooxygenase inhibition and baroreflex sensitivity in lean and obese adults

PURPOSE

Obese adults exhibit increased levels of inflammation, which may negatively affect blood pressure regulation. Based on existing literature, we hypothesized: (1) baroreflex sensitivity would be lower in obese adults when compared to lean adults; (2) acute ibuprofen (IBU, a cyclooxygenase inhibitor and nonsteroidal antiinflammatory agent) administration would increase baroreflex sensitivity in obese adults, with no effect in lean adults.

METHODS

Seven lean (4 male, 3 female) and six obese (5 M, 1 F) adults completed two visits randomized to control (CON) or IBU (800 mg oral). On each visit, blood pressure (intra-arterial catheter), heart rate (ECG), and muscle sympathetic nerve activity (MSNA, microneurography) were measured continuously. Sympathetic and cardiac baroreflex sensitivities were assessed using the modified Oxford technique.

RESULTS

Measures of systemic inflammation [C-reactive protein (CRP) and interleukin-6 (IL-6)] were higher in obese adults when compared to lean adults and tended to decrease with IBU (IL-6: p < 0.05; CRP: p = 0.14). Cardiac baroreflex sensitivity was lower in obese adults (14 ± 2 vs. 24 ± 2 ms/mmHg, p = 0.02), whereas sympathetic baroreflex sensitivity was higher in obese adults (-3.6 ± 0.5 vs. -2.1 ± 0.5 bursts/100 beats/mmHg, p = 0.03) when compared to lean. There was no effect of IBU on cardiac or sympathetic baroreflex sensitivity in either group (p value range 0.20-0.71).

CONCLUSION

Despite obese individuals exhibiting higher levels of systemic inflammation and lower cardiac baroreflex sensitivity when compared to lean adults, an acute dose of IBU has no effect on cardiac or sympathetic baroreflex sensitivity.

Increased Muscle Sympathetic Nerve Activity and Impaired Executive Performance Capacity in Obstructive Sleep Apnea

STUDY OBJECTIVES

To investigate muscle sympathetic nerve activity (MSNA) response and executive performance during mental stress in obstructive sleep apnea (OSA).

METHODS

Individuals with no other comorbidities (age = 52 ± 1 y, body mass index = 29 ± 0.4, kg/m2) were divided into two groups: (1) control (n = 15) and (2) untreated OSA (n = 20) defined by polysomnography. Mini-Mental State of Examination (MMSE) and Inteligence quocient (IQ) were assessed. Heart rate (HR), blood pressure (BP), and MSNA (microneurography) were measured at baseline and during 3 min of the Stroop Color Word Test (SCWT). Sustained attention and inhibitory control were assessed by the number of correct answers and errors during SCWT.

RESULTS

Control and OSA groups (apnea-hypopnea index, AHI = 8 ± 1 and 47 ± 1 events/h, respectively) were similar in age, MMSE, and IQ. Baseline HR and BP were similar and increased similarly during SCWT in control and OSA groups. In contrast, baseline MSNA was higher in OSA compared to controls. Moreover, MSNA significantly increased in the third minute of SCWT in OSA, but remained unchanged in controls (P < 0.05). The number of correct answers was lower and the number of errors was significantly higher during the second and third minutes of SCWT in the OSA group (P < 0.05). There was a significant correlation (P < 0.01) between the number of errors in the third minute of SCWT with AHI (r = 0.59), arousal index (r = 0.55), and minimum O2 saturation (r = -0.57).

CONCLUSIONS

As compared to controls, MSNA is increased in patients with OSA at rest, and further significant MSNA increments and worse executive performance are seen during mental stress.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrials.gov, registration number: NCT002289625.

Aortic Hemodynamics and Arterial Stiffness Responses to Muscle Metaboreflex Activation With Concurrent Cold Pressor Test

BACKGROUND

Overweight/obese young men have increased sympatho-excitation to cold and pain stress-induced by the cold pressor test (CPT) that may lead to abnormal hemodynamic responses. Concurrent CPT and exercise may augment the sympathetic-induced increases in aortic blood pressure (BP), pressure wave reflection (augmentation index, AIx), and arterial stiffness (pulse wave velocity, PWV). Since obesity is related with hypertension and sympathetic activity, we evaluated the aortic hemodynamic and PWV responses to muscle metaboreflex activation imposed by postexercise muscle ischemia (PEMI) concurrent with CPT in overweight/obese men.

METHODS

In 16 healthy (body mass index >25 and <40kg/m(2)) men (24±2 years), heart rate (HR), brachial BP, aortic BP, augmented pressure (AP), AIx, AIx adjusted to 75 beats/min (AIx@75), reflection time (Tr), first (P1) and second systolic peak (P2, wave reflection magnitude), and PWV (brachial-ankle PWV, baPWV) were evaluated at baseline, during isometric-handgrip exercise (IHG), and PEMI with (PEMI + CPT) and without CPT.

RESULTS

During IHG, brachial BP, aortic BP, AP, AIx, AIx@75, P1, and P2 increased ( P < 0.01) while Tr decreased ( P < 0.05) compared with baseline. During PEMI, all hemodynamic parameters remained elevated ( P < 0.05) and baPWV increased (P < 0.05) while Tr and HR returned to baseline. Compared with PEMI, the increases in HR, brachial BP, aortic BP, AIx@75, P1, P2, and baPWV were greater ( P < 0.05) during PEMI + CPT. During PEMI + CPT, Tr remained lower ( P < 0.05) than baseline.

CONCLUSIONS

Cold exposure with concurrent metaboreflex activation induces a significant increase in aortic hemodynamics and arterial stiffness, which may explain the high risk of adverse cardiovascular events during physiological stress.

CLINICAL TRIALS REGISTRATION

Trial Number NCT02104375.

Cardiovascular response to thermoregulatory challenges

A growing number of extreme climate events are occurring in the setting of ongoing climate change, with an increase in both the intensity and frequency. It has been shown that ambient temperature challenges have a direct and highly varied impact on cardiovascular health. With a rapidly growing amount of literature on this issue, we aim to review the recent publications regarding the impact of cold and heat on human populations with regard to cardiovascular disease (CVD) mortality/morbidity while also examining lag effects, vulnerable subgroups, and relevant mechanisms. Although the relative risk of morbidity/mortality associated with extreme temperature varied greatly across different studies, both cold and hot temperatures were associated with a positive mean excess of cardiovascular deaths or hospital admissions. Cause-specific study of CVD morbidity/mortality indicated that the sensitivity to temperature was disease-specific, with different patterns for acute and chronic ischemic heart disease. Vulnerability to temperature-related mortality was associated with some characteristics of the populations, including sex, age, location, socioeconomic condition, and comorbidities such as cardiac diseases, kidney diseases, diabetes, and hypertension. Temperature-induced damage is thought to be related to enhanced sympathetic reactivity followed by activation of the sympathetic nervous system, renin-angiotensin system, as well as dehydration and a systemic inflammatory response. Future research should focus on multidisciplinary adaptation strategies that incorporate epidemiology, climatology, indoor/building environments, energy usage, labor legislative perfection, and human thermal comfort models. Studies on the underlying mechanism by which temperature challenge induces pathophysiological response and CVD await profound and lasting investigation.

Sympathoneural and adrenomedullary responses to mental stress

This concept-based review provides historical perspectives and updates about sympathetic noradrenergic and sympathetic adrenergic responses to mental stress. The topic of this review has incited perennial debate, because of disagreements over definitions, controversial inferences, and limited availability of relevant measurement tools. The discussion begins appropriately with Cannon's "homeostasis" and his pioneering work in the area. This is followed by mental stress as a scientific idea and the relatively new notions of allostasis and allostatic load. Experimental models of mental stress in rodents and humans are discussed, with particular attention to ethical constraints in humans. Sections follow on sympathoneural responses to mental stress, reactivity of catecholamine systems, clinical pathophysiologic states, and the cardiovascular reactivity hypothesis. Future advancement of the field will require integrative approaches and coordinated efforts between physiologists and psychologists on this interdisciplinary topic.

Augmented Endothelial-Specific L-Arginine Transport Blunts the Contribution of the Sympathetic Nervous System to Obesity Induced Hypertension in Mice

Augmenting endothelial specific transport of the nitric oxide precursor L-arginine via cationic amino acid transporter-1 (CAT1) can prevent obesity related hypertension. We tested the hypotheses that CAT1 overexpression prevents obesity-induced hypertension by buffering the influence of the sympathetic nervous system (SNS) on the maintenance of arterial pressure and by buffering pressor responses to stress. Wild type (WT; n=13) and CAT1 overexpressing mice (CAT+; n=13) were fed a normal or a high fat diet for 20 weeks. Mice fed a high fat diet were returned to the control diet before experiments commenced. Baseline mean arterial pressure (MAP) and effects of restraint-, shaker- and almond feeding-stress and ganglionic blockade (pentolinium; 5 mg/kg; i.p.) on MAP were determined in conscious mice. Fat feeding increased body weight to a similar extent in WT and CAT+ but MAP was greater only in WT compared to appropriate controls (by 29%). The depressor response to pentolinium was 65% greater in obese WT than lean WT (P < 0.001), but was similar in obese and lean CAT+ (P = 0.65). In lean WT and CAT+, pressor responses to shaker and feeding stress, but not restraint stress, were less in the latter genotype compared to the former (P ≤ 0.001). Pressor responses to shaker and feeding stress were less in obese WT than lean WT (P ≤ 0.001), but similar in obese and lean CAT+. The increase in MAP in response to restraint stress was less in obese WT (22 ± 2%), but greater in obese CAT+ (37 ± 2%), when compared to respective lean WT (31 ± 3%) and lean CAT+ controls (27 ± 2%; P ≤ 0.02). We conclude that CAT1 overexpression prevents obesity-induced hypertension by reducing the influence of the SNS on the maintenance of arterial pressure but not by buffering pressor responses to stress.

Central nervous system dysfunction in obesity-induced hypertension

The activation of the sympathetic nervous system is a major mechanism underlying both human and experimental models of obesity-related hypertension. While insulin and the adipokine leptin have long been thought to contribute to obesity-related neurogenic mechanisms, the evidence is now very strong that they play a major role, shown particularly in animal studies using selective receptor antagonists. There is not just maintenance of leptin's sympatho-excitatory actions as previously suggested but considerable amplification particularly in renal sympathetic nervous activity. Importantly, these changes are not dependent on short-term elevation or reduction in plasma leptin or insulin, but require some weeks to develop indicating a slow "neural adaptivity" within hypothalamic signalling. These effects can be carried across generations even when offspring are raised on a normal diet. A better understanding of the underlying mechanism should be a high research priority given the prevalence of obesity not just in the current population but also for future generations.

Metabolic syndrome: a sympathetic disease?

Metabolic syndrome is associated with adverse health outcomes and is a growing problem worldwide. Although efforts to harmonise the definition of metabolic syndrome have helped to better understand the prevalence and the adverse outcomes associated with the disorder on a global scale, the mechanisms underpinning the metabolic changes that define it are incompletely understood. Accumulating evidence from laboratory and human studies suggests that activation of the sympathetic nervous system has an important role in metabolic syndrome. Indeed, treatment strategies commonly recommended for patients with metabolic syndrome, such as diet and exercise to induce weight loss, are associated with sympathetic inhibition. Pharmacological and device-based approaches to target activation of the sympathetic nervous system directly are available and have provided evidence to support the important part played by sympathetic regulation, particularly for blood pressure and glucose control. Preliminary evidence is encouraging, but whether therapeutically targeting sympathetic overactivity could help to prevent metabolic syndrome and attenuate its adverse outcomes remains to be determined.

Acute cyclooxygenase inhibition does not alter muscle sympathetic nerve activity or forearm vasodilator responsiveness in lean and obese adults

Obesity is often characterized by chronic inflammation that may contribute to increased cardiovascular risk via sympathoexcitation and decreased vasodilator responsiveness. We hypothesized that obese individuals would have greater indices of inflammation compared with lean controls, and that cyclooxygenase inhibition using ibuprofen would reduce muscle sympathetic nerve activity (MSNA) and increase forearm blood flow in these subjects. We measured MSNA, inflammatory biomarkers (C‐reactive protein [CRP] and Interleukin‐6 [IL‐6]), and forearm vasodilator responses to brachial artery acetylcholine and sodium nitroprusside in 13 men and women (7 lean; 6 obese) on two separate study days: control (CON) and after 800 mg ibuprofen (IBU). CRP (1.7 ± 0.4 vs. 0.6 ± 0.3 mg/L; P < 0.05) and IL‐6 (4.1 ± 1.5 vs. 1.0 ± 0.1pg/mL; P < 0.05) were higher in the obese group during CON and tended to decrease with IBU (IL‐6: P < 0.05; CRP: P = 0.14). MSNA was not different between groups during CON (26 ± 4 bursts/100 heart beats (lean) versus 26 ± 4 bursts/100 heart beats (obese); P = 0.50) or IBU (25 ± 4 bursts/100 heart beats (lean) versus 30 ± 5 bursts/100 heart beats (obese); P = 0.25), and was not altered by IBU. Forearm vasodilator responses were unaffected by IBU in both groups. In summary, an acute dose of ibuprofen did not alter sympathetic nerve activity or forearm blood flow responses in healthy obese individuals, suggesting that the cyclooxygenase pathway is not a major contributor to these variables in this group.

Obesity is often characterized by chronic inflammation that may contribute to increased cardiovascular risk via sympathoexcitation. However, an acute dose of the cyclooxygenase inhibitor ibuprofen did not alter blood pressure or muscle sympathetic nerve activity in lean and obese humans.

Stress task specific impairments of cardiovascular functioning in obese participants

The role that excess adipose tissue plays in chronic inflammation gives rise to its importance as an independent risk factor in cardiovascular dysfunction. Operationalizing chronic stress as obesity, we sought to explore the relationship between obesity, perceived stress and cardiovascular reactivity and recovery from laboratory stressors. Cardiovascular function was assessed using blood pressure and heart rate. Two stress tasks (mental arithmetic and cold pressor) were employed to examine potential differences between type of stress and cardiovascular response. Body mass index (BMI) was able to predict dysfunction in both cardiovascular reactivity and recovery. Participants with a higher BMI exhibited blunted systolic blood pressure and heart rate reactivity to the mental arithmetic task. In contrast, BMI has an incongruent effect on blood pressure reactivity to the cold pressor task that is dependent on the level of perceived stress. This suggests that in some instances the effect of BMI on cardiovascular response to acute stress may be moderated by perceived stress. Further, we found greater adiposity was related to delayed heart rate recovery following both stress tasks.

Effects of watermelon supplementation on aortic hemodynamic responses to the cold pressor test in obese hypertensive adults

BACKGROUND

Cold-induced increases in aortic blood pressure (BP) may cause adverse cardiac events in hypertensives by increasing ventricular afterload. L-citrulline supplementation reduces BP at baseline and during the cold pressor test (CPT), but the effect on wave reflection (augmentation pressure (AP) and index (AIx)) is controversial. Our aim was to assess the effect of L-citrulline-rich watermelon supplementation on aortic hemodynamic responses to CPT in hypertensive adults.

METHODS

Brachial systolic BP (bSBP) and aortic systolic BP (aSBP), AP, AIx, AIx adjusted to 75 beats/min (AIx75), reflection time (Tr), first (P1) and second systolic peak (P2; wave reflection magnitude), heart rate (HR), and systolic time index (STI; myocardial oxygen demand) at baseline and during CPT and magnitude of the response from baseline to CPT were evaluated in 13 individuals (10 women; 57±1 year; bSBP 151±5 mm Hg). Participants were randomized to a 6-week watermelon or placebo supplementation in a crossover design.

RESULTS

Watermelon reduced (P < 0.05) bSBP, aSBP, P1, and P2 at baseline and CPT compared with placebo; thus, increases from baseline to CPT were unaffected. Watermelon did not affect AP, AIx, AIx75, and STI at baseline but decreased (P < 0.05) AP and STI during CPT and the increases in AP (~5mm Hg) and AIx75 (~7.3%) from baseline to CPT.

CONCLUSIONS

Watermelon supplementation reduced aortic BP and myocardial oxygen demand during CPT and the magnitude of the cold-induced increase in wave reflection in obese adults with hypertension. Watermelon may provide cardioprotection by attenuating cold-induced aortic hemodynamic responses.

CLINICAL TRIALS REGISTRATION

Clinicaltrial.gov register, NCT01185041.

The effects of acute stress exposure on striatal activity during Pavlovian conditioning with monetary gains and losses

Pavlovian conditioning involves the association of an inherently neutral stimulus with an appetitive or aversive outcome, such that the neutral stimulus itself acquires reinforcing properties. Across species, this type of learning has been shown to involve subcortical brain regions such as the striatum and the amygdala. It is less clear, however, how the neural circuitry involved in the acquisition of Pavlovian contingencies in humans, particularly in the striatum, is affected by acute stress. In the current study, we investigate the effect of acute stress exposure on Pavlovian conditioning using monetary reinforcers. Participants underwent a partial reinforcement conditioning procedure in which neutral stimuli were paired with high and low magnitude monetary gains and losses. A between-subjects design was used, such that half of the participants were exposed to cold stress while the remaining participants were exposed to a no stress control procedure. Cortisol measurements and subjective ratings were used as measures of stress. We observed an interaction between stress, valence, and magnitude in the ventral striatum, with the peak in the putamen. More specifically, the stress group exhibited an increased sensitivity to magnitude in the gain domain. This effect was driven by those participants who experienced a larger increase in circulating cortisol levels in response to the stress manipulation. Taken together, these results suggest that acute stress can lead to individual differences in circulating cortisol levels which influence the striatum during Pavlovian conditioning with monetary reinforcers.

Body mass index-modified relationship of chronic mental stress with resting blood pressure during 5 years in Japanese middle-aged male workers

BACKGROUND

Chronic mental stress has been reported to be directly or inversely proportional to blood pressure (BP). To explain this inconsistent relationship, we assumed effect-modification by body mass index (BMI).

METHODS AND RESULTS

We examined 1,673 Japanese male local government employees who were not taking antihypertensive drugs or had no history of cardiovascular disease. BP and BMI were recorded at yearly health checkups. Exposure to mental stress, smoking, drinking, exercising, and salty taste were checked by questionnaire in 1997 and 2002. The main effect and interaction of stress and BMI on the averages and changes of resting systolic and diastolic BPs over the 5 years were assessed by a general linear model by adjusting for confounders. Obesity (BMI ≥25kg/m(2)) was significantly related with higher average systolic and diastolic BPs (P<0.001, P<0.001, respectively), whereas mental stress was not, showing a significantly different relationship dependent on BMI (P for interaction =0.002, 0.004): a significant and directly proportional association with systolic and diastolic BPs (P=0.001, 0.001) in the obese, but borderline significant and inversely proportional association (P=0.07, 0.08) in the lean. Only BMI was significantly related to the degree of BP change.

CONCLUSIONS

Whereas BMI was proportionally associated with BP, BMI was a modifier which, depending on its level, inverted the direction of the association between chronic mental stress and resting BP.

Blunted GABA-mediated inhibition within the dorsomedial hypothalamus potentiates the cardiovascular response to emotional stress in rats fed a high-fat diet

Rats fed a high-fat diet (HFD) present an exaggerated endocrine response to stress conditions, which, like obesity, show a high correlation with cardiovascular diseases. Meanwhile the GABAergic neurotransmission within the dorsomedial hypothalamus (DMH) is involved in the regulation of the physiological responses during emotional stress. Here we evaluated the influence of obesity, induced by a HFD, on the cardiovascular responses induced by air jet stress in rats, and the role of the GABAergic tonus within the DMH in these changes. Our results showed that consumption of a HFD (45% w/w fat) for 9 weeks induced obesity and increases in baseline mean arterial pressure (MAP) and heart rate (HR). Moreover, obesity potentiated stress responsiveness, evidenced by the greater changes in MAP and HR induced by stress in obese rats. The injection of muscimol into the DMH reduced the maximal increases in HR and MAP induced by stress in both groups; however, the reduction in the maximal increases in MAP in the HFD group was less pronounced. Moreover, the injection of muscimol into the DMH of obese rats was less effective in reducing the stress-induced tachycardia, since the HR attained the same levels at the end of the stress paradigm as after the vehicle injection. Injection of bicuculline into DMH induced increases in MAP and HR in both groups. Nevertheless, obesity shortened the tachycardic response to bicuculline injection. These data show that obesity potentiates the cardiovascular response to stress in rats due to an inefficient GABAA-mediated inhibition within the DMH.

Factors associated with blood pressure response to the cold pressor test: the GenSalt Study

BACKGROUND

Blood pressure (BP) response to the cold pressor test (CPT) has been associated with increased risk of cardiovascular disease. We studied risk factors associated with BP response to CPT.

METHODS

We conducted the CPT among 2,682 individuals in rural north China. BP was measured using a standard mercury sphygmomanometer prior to and at 0, 1, 2, and 4 minutes after the participants immersed their right hand in ice water for 1 minute.

RESULTS

Sex, age, and baseline BP levels were significantly associated with BP response to the CPT. For example, maximum systolic BP response (mean ± SD) was greater in women than in men (15.5±10.7 vs. 13.8±10.0mm Hg; P < 0.0001), correspondingly higher with age (12.4±8.7, 13.8±10.0, and 16.4±11.2mm Hg for those aged < 35, 35-44, and ≥ 45 years, respectively; P for trend < 0.0001), and greater with higher BP (13.5±10.0, 14.9±10.2, and 17.4±11.5mm Hg for those with baseline BP < 120/80, 120-139/80-89, and ≥ 140/90mm Hg, respectively; P for trend < 0.0001). In multivariable analyses, we also observed that higher body mass index, physical inactivity, and alcohol consumption were significantly associated with greater BP response to the CPT.

CONCLUSIONS

Our study indicates that females, older age, and elevated baseline BP levels are associated with greater BP response to the CPT. In addition, physical inactivity, higher weight, and alcohol consumption may also be related to BP hyperreactivity to stress.

Augmented limb blood flow during neurovascular stress in physically fit women

The study examined whether cardiorespiratory fitness modifies cardiovascular responses by normotensive men and women during the Stroop color-word interference test. Independent of age and an estimate of body fatness, fitness level was positively related (R²  = .39 and .51) to increases in limb blood flow and vascular conductance, coherent with cardiac-vagal withdrawal and a decrease in heart period, among women but not men. Fitness was unrelated to changes in systolic and diastolic blood pressures and muscle sympathetic nerve activity. The augmented hemodynamic responses among fitter women were not consistent with passive vasodilation via withdrawal of sympathetic neural tone. The results encourage further gender comparisons testing whether fitness augments limb blood flow during mental stress by neurohumoral and flow-mediated vasodilatory mechanisms or by increased cardiac output.

Stress-induced microvascular reactivity in normal-weight and obese individuals

Obesity has been shown to have profound effects on hemodynamics and neurological states in humans. Previous studies have demonstrated that obese individuals are highly susceptible to increases in tension, anxiety, and depression. However, the relationship between mental stressors and vascular fluidity in obese humans is not well understood. Thus, the purpose of this study was to investigate mental-stress-induced microvascular reactivity (excess blood flow (EBF)) in normal-weight and obese individuals. In addition, the relationships between potential vascular response modulators (heart rate (HR) and norepinephrine (NE)) and EBF were examined. Twenty-two male subjects were classified as obese (n = 12) or normal-weight (n = 10), and each subject completed a 20 min bout of acute mental stress. Our analyses demonstrate significant elevations in forearm blood flow (FBF) and EBF immediately after mental stress in both normal-weight and obese groups. HR was only correlated with EBF immediately poststress in the normal-weight group. Furthermore, stress-induced plasma NE was not associated with FBF or EBF in either group, although in the obese group, stress-induced plasma NE was associated with body mass index and percent body fat. These results suggest that microvascular reactivity after mental stress is not directly related to plasma NE in normal-weight or obese individuals. The novel results presented in this study provide a foundation for additional examination of the mechanisms involved in the effects of mental stress on microvascular reactivity.

Hemodynamic reactivity to laboratory stressors in healthy subjects: influence of gender and family history of cardiovascular diseases

Although laboratory stressor tests have been applied as a preliminary protocol in some cardiovascular studies, there is a lack of data comparing the pressor and chronotropic responses among the main stressor tests. Therefore, the aim of this study was to evaluate the variability in hemodynamic responsiveness to the main stressor tests, establish a hyperresponsiveness cutoff criterion and analyze the influence of gender and family history of cardiovascular diseases (CVDs) in healthy subjects. We examined hemodynamic responses to physical (cold pressor and handgrip tests) and mental (Stroop color-word test) stressors in 98 subjects (48 males and 50 females) without CVDs. All stressor tests resulted in increased blood pressure (BP) levels, which were lower and less dispersed in the handgrip test compared to the cold pressor test. Adopting the 75(th) percentile as the cutoff in our data, we classified subjects exhibiting absolute pressor changes equal to or higher than 14, 24 and 36 mmHg in systolic and 9, 13 and 24 mmHg in diastolic BP during the handgrip, Stroop and cold pressor test, respectively, as hyperresponsives. Males exhibited greater (p<0.05) increases in systolic BP in the handgrip (11% vs. 8%) and cold pressor (25% vs. 21%) tests and in diastolic BP in the handgrip (12% vs. 7%) and Stroop (22% vs. 19%) tests than females. A positive association between family history of CVDs and pressor hyperreactivity to stressor tests was observed. We propose using the 75(th) percentile of hemodynamic sample values as a cutoff criterion to classify individuals as pressor or chronotropic hyperreactives. We conclude that hemodynamic responsiveness to stressor tests in healthy subjects is positively influenced by male gender and family history of CVDs.

Abnormal sympathetic reactivity to the cold pressor test in overweight humans

BACKGROUND

Overweight individuals (body mass index (BMI) 25-29.9 kg/m²) are at higher risk for developing cardiovascular disease and hypertension when compared with lean individuals of normal weight (BMI 18.5-24.9 kg/m²). The purpose of this study was to test the hypothesis that exaggerated sympathetic nervous system responses to stressors may be one potential mechanism that predisposes overweight individuals to developing hypertension.

METHODS

We compared heart rate (HR), blood pressure (BP), and muscle sympathetic nerve activity (MSNA) using microneurography, in normotensive overweight individuals compared with age-matched lean controls, at baseline and during two sympathoexcitatory maneuvers: cold pressor test (CPT), and static handgrip exercise (SHG 30%).

RESULTS

During CPT, MSNA increased in both groups, but the magnitude of MSNA response was significantly greater (P = 0.03) in overweight (+18.1 ± 2.8 bursts/min) compared with lean controls (+10.8 ± 1.2 bursts/min). MSNA response to SHG at 30% maximum voluntary contraction (MVC) was similar between the two groups. There were no significant differences in systolic (SBP) or diastolic BP (DBP) responses or HR responses between the two groups during either maneuver.

CONCLUSIONS

Normotensive overweight individuals have an exaggerated MSNA response to the CPT. Augmented sympathetic reactivity to cold stress may contribute to increased risk of hypertension in overweight individuals.

Sympathetic nervous activity, depressive symptoms, and metabolic syndrome in black Africans: the sympathetic activity and ambulatory blood pressure in Africans study

The aim of this study was to examine the association between sympathetic nervous activity, depressive symptoms, and metabolic syndrome in a sample of black and Caucasian Africans. The sample consisted of healthy men and women: 194 blacks (aged, mean ± SD, 44.1 ± 7.9 years) and 206 Caucasians (aged, mean ± SD, 44.7 ± 10.8 years). Salivary 3-methoxy-phenylglycol (MHPG) concentration, the major metabolite of norepinephrine, was measured during the Stroop mental challenge. Depressive symptoms were assessed from the 9-item Patient Health Questionnaire. Metabolic syndrome (defined as central obesity plus any other two risk factors including raised serum triglycerides, reduced serum high-density lipoprotein- cholesterol, raised blood pressure, and raised fasting plasma glucose) was prevalent in 43.0% and 36.4% of blacks and Caucasians, respectively. In blacks there was, on average, a 16.4% increase in salivary MHPG concentration following mental stress, although no significant response was observed in Caucasians. The salivary MHPG response in blacks was associated with risk of metabolic syndrome (odds ratio [OR] = 1.11, 95% CI, 1.00-1.24) after adjusting for age, sex, and baseline salivary MHPG concentration. This association was mainly driven by the central obesity component of the metabolic syndrome. The salivary MHPG response was also related to moderate-severe depressive symptoms (OR = 1.16, 95% CI, 1.04-1.30), and further adjustment for depressive symptoms attenuated the association between salivary MHPG response and metabolic syndrome (OR = 1.07, 95% CI, 0.96-1.20). These data indicate an association between sympathetic activity, depressive symptoms, and metabolic syndrome in a sample of black Africans.

Rapid onset of renal sympathetic nerve activation in rabbits fed a high-fat diet

Hypertension and elevated sympathetic drive result from consumption of a high-calorie diet and deposition of abdominal fat, but the etiology and temporal characteristics are unknown. Rabbits instrumented for telemetric recording of arterial pressure and renal sympathetic nerve activity (RSNA) were fed a high-fat diet for 3 weeks then control diet for 1 week or control diet for 4 weeks. Baroreflexes and responses to air-jet stress and hypoxia were determined weekly. After 1 week of high-fat diet, caloric intake increased by 62%, accompanied by elevated body weight, blood glucose, plasma insulin, and leptin (8%, 14%, 134%, and 252%, respectively). Mean arterial pressure, heart rate, and RSNA also increased after 1 week (6%, 11%, and 57%, respectively). Whereas mean arterial pressure and body weight continued to rise over 3 weeks of high-fat diet, heart rate and RSNA did not change further. The RSNA baroreflex was attenuated from the first week of the diet. Excitatory responses to air-jet stress diminished over 3 weeks of high-fat diet, but responses to hypoxia were invariant. Resumption of a normal diet returned glucose, insulin, leptin, and heart rate to control levels, but body weight, mean arterial pressure, and RSNA remained elevated. In conclusion, elevated sympathetic drive and impaired baroreflex function, which occur within 1 week of consumption of a high-fat, high-calorie diet, appear integral to the rapid development of obesity-related hypertension. Increased plasma leptin and insulin may contribute to the initiation of hypertension but are not required for maintenance of mean arterial pressure, which likely lies in alterations in the response of neurons in the hypothalamus.

Blood Pressure Control at Rest and during Exercise in Obese Children and Adults

The hemodynamic responses to exercise have been studied to a great extent over the past decades, and an exaggerated blood pressure response during an acute exercise bout has been considered as an indicator of cardiovascular risk. Obesity is a major factor influencing the blood pressure response to exercise since evidence indicates that the arterial pressure response to exercise is exacerbated in obese compared with lean adults. Signs of augmented responses (such as an exaggerated blood pressure response) to physical exertion appear early in life (from the prepubertal years) in obese individuals. Understanding the mechanisms that drive the altered hemodynamic responses during exercise in obese individuals and prevent the progression to hypertension is vitally important. This paper focuses on the evidence linking obesity with alterations of the autonomic nervous system and discusses the potential mechanisms and consequences of the altered sympathetic nervous system behavior in obese individuals at rest and during exercise. Furthermore, this paper presents the alterations in the reflex regulatory mechanisms ("exercise pressor reflex" and baroreflex) in obese children and adults and addresses the effects of training on obesity-related disturbances.

Muscle sympathetic nervous activity in depressed patients before and after treatment with sertraline

BACKGROUND

Sympathetic hyperactivity is one of the mechanisms involved in the increased cardiovascular risk associated with depression, and there is evidence that antidepressants decrease sympathetic activity.

OBJECTIVES

We tested the following two hypotheses: patients with major depressive disorder with high scores of depressive symptoms (HMDD) have augmented muscle sympathetic nervous system activity (MSNA) at rest and during mental stress compared with patients with major depressive disorder with low scores of depressive symptoms (LMDD) and controls; sertraline decreases MSNA in depressed patients.

METHODS

Ten HMDD, nine LMDD and 11 body weight-matched controls were studied. MSNA was directly measured from the peroneal nerve using microneurography for 3 min at rest and 4 min during the Stroop color word test. For the LMDD and HMDD groups, the tests were repeated after treatment with sertraline (103.3 +/- 40 mg).

RESULTS

Resting MSNA was significantly higher in the HMDD [29.1 bursts/min (SE 2.9)] compared with LMDD [19.9 (1.6)] and controls [22.2 (2.0)] groups (P = 0.026 and 0.046, respectively). There was a significant positive correlation between resting MSNA and severity of depression. MSNA increased significantly and similarly during stress in all the studied groups. Sertraline significantly decreased resting MSNA in the LMDD group and MSNA during mental stress in LMDD and HMDD groups. Sertraline significantly decreased resting heart rate and heart rate response to mental stress in the HMDD group.

CONCLUSION

Moderate-to-severe depression is associated with increased MSNA. Sertraline treatment reduces MSNA at rest and during mental challenge in depressed patients, which may have prognostic implications in this group.

Stress responses and baroreflex function in coronary disease

Exaggerated pressor responses to mental stress in patients with coronary artery disease (CAD) are associated with increased risk for subsequent cardiovascular events. The integrated baroreflex gain and its mechanical and neural component were estimated and then related to the blood pressure and heart rate responses to simulated real-life stressors: mental arithmetic and public speaking. Eighteen healthy individuals (aged 61 +/- 8 yr) and 29 individuals with documented CAD but no other comorbidities (aged 59 +/- 8 yr) were studied. Heart rate and blood pressures were continuously assessed before, during preparation for, and during performance of a math task and a speech task. The assessment of beat-to-beat carotid diameters during baroreflex engagement was used to estimate the integrated baroreflex gain and its mechanical and neural component. The CAD subjects demonstrated significantly greater increases in heart rate and blood pressures for the performance of the speech task. However, there were no group differences in integrated cardiovagal baroreflex gain or either mechanical or neural baroreflex component. These findings indicate that the augmented pressor responses in CAD do not result from a generalized arterial baroreflex deficit.

Alpha-adrenoceptor-mediated vasoconstriction is not involved in impaired functional vasodilation in the obese Zucker rat

1. Obesity/metabolic syndrome is associated with augmented a-adrenoceptor sensitivity and impaired hyperaemic responses to exercise. Thus, it is possible that this elevated a-adrenoceptor constriction contributes to the blunted hyperaemic response. 2. Male lean and obese Zucker rats were instrumented for acute measurements of blood pressure (BP) and iliac blood flow (BF). Changes in BP and BF were determined in anaesthetized animals in response to intravenous administration of increasing doses of the a(1)-adrenoceptor agonist phenylephrine (PE). Once BF and BP returned to normal, a single bolus of the a-adrenoceptor antagonist phentolamine (0.5 mg) was administered. In separate animals, the spinotrapezius muscle was exteriorized for direct in situ observation of the microcirculation in response to phentolamine and muscle contraction. 3. Administration of PE demonstrated that iliac BF is highly autoregulated in the face of increasing perfusion pressure. Iliac conductance following phentolamine was significantly greater in obese rats. Following phentolamine administration, iliac vascular conductance was significantly greater in obese rats compared with lean animals. However, a-adrenoceptor blockade did not significantly alter arteriolar diameter in the spinotrapezius muscle during muscle contraction in either lean or obese animals. 4. These results suggest a greater contribution of the a-adrenoceptors in basal hindlimb vascular tone in obese rats. Furthermore, an augmented a-adrenoceptor-mediated vasoconstriction may not contribute to the impaired functional dilation in anaesthetized obese rats.

Impact of obesity and insulin resistance on vasomotor tone: nitric oxide and beyond

1. Obesity is rapidly increasing in Western populations, driving a parallel increase in hypertension, diabetes and vascular disease. Prior to the development of overt diabetes or hypertension, obese patients spend years in a state of progressive insulin resistance and metabolic disease. Mounting evidence suggests that this insulin-resistant state has deleterious effects on the control of blood flow, thus placing organ systems at a higher risk for end-organ damage and increasing cardiovascular mortality. 2. The purpose of the present review is to examine the current literature on the effects of obesity and insulin resistance on the acute control of vascular tone. Effects on nitric oxide (NO)-mediated control of vascular tone are particularly examined with regard to proximal causes and distal mechanisms of the impaired NO-mediation of vasodilation. 3. Finally, novel pathways of impaired control of perfusion are summarized from the recent literature to identify new avenues of exploring impaired vascular function in patients with metabolic disease.

Size at birth and autonomic function during psychological stress

Small size at birth is associated with exaggerated blood pressure responses to psychological stressors, which increase the risk of developing sustained hypertension in adult life. Explanatory mechanisms for this association are not well characterized. We investigated the hypothesis that an adverse fetal environment, reflected by small size at birth, persistently alters autonomic nervous system and baroreflex control of cardiovascular function, resulting in exaggerated blood pressure and heart rate responses to stressors. Men and women from an Australian prospective cohort study underwent a series of 3 psychological stressors (Stroop, mirror-tracing, and speech) while their blood pressure was recorded continuously using a Portapres. Indices of autonomic function were derived using spectrum analysis (wavelet packet transform), and baroreflex function was estimated using an adaptive autoregressive model. We found that women who were small at birth demonstrated increased levels of low-frequency blood pressure variability at rest (r=-0.28; P<0.05) and during stress (r=-0.42; P<0.001), reduced levels of high-frequency heart period variability (r=0.22; P<0.05), and reduced baroreflex sensitivity (r=0.34; P<0.01). These findings were not present in the men. This study provides evidence that markers of impaired fetal growth are related to autonomic cardiovascular control involving modulation of both sympathetic and parasympathetic function but in a sex-specific manner. We also provide the first human evidence of a relationship between size at birth and baroreflex function.

Impact of moderate overweight and body composition on postexercise hemodynamic responses in healthy men

Postexercise hypotension (PEH) is a well-established phenomenon that may contribute to the antihypertensive mechanisms of exercise. The purpose of this study was to examine the impact of moderate overweight on postexercise hemodynamic responses in a group of healthy nonobese men (n = 16, aged 20.4+/-1.8 years) with apparently normal cardiovascular function at rest. Forearm blood flow, using strain gauge plethysmography, blood pressure, using a Finapres device, and cardiac output (CO), using impedance cardiography, were measured on a control day and on a separate day following a bout of moderate intensity exercise (20 min at 75% heart rate reserve). Linear regression analysis, adjusted for exercise intensity, revealed that body mass index (BMI) was associated with specific postexercise hemodynamic responses. Higher BMI was associated with greater reductions in CO and stroke volume, but lower reductions in total peripheral resistance. These findings suggest body composition impacts on mechanisms of PEH and should therefore be considered as an important confounding variable in future studies.

Effects of diet and exercise training on neurovascular control during mental stress in obese women

Since neurovascular control is altered in obese subjects, we hypothesized that weight loss by diet (D) or diet plus exercise training (D + ET) would improve neurovascular control during mental stress in obese women. In a study with a dietary reduction of 600 kcal/day with or without exercise training for 4 months, 53 obese women were subdivided in D (N = 22, 33 +/- 1 years, BMI 34 +/- 1 kg/m2), D + ET (N = 22, 33 +/- 1 years, BMI 33 +/- 1 kg/m2), and nonadherent (NA, N = 9, 35 +/- 2 years, BMI 33 +/- 1 kg/m2) groups. Muscle sympathetic nerve activity (MSNA) was measured by microneurography and forearm blood flow by venous occlusion plethysmography. Mental stress was elicited by a 3-min Stroop color word test. Weight loss was similar between D and D + ET groups (87 +/- 2 vs 79 +/- 2 and 85 +/- 2 vs 76 +/- 2 kg, respectively, P < 0.05) with a significant reduction in MSNA during mental stress (58 +/- 2 vs 50 +/- 2, P = 0.0001, and 59 +/- 3 vs 50 +/- 2 bursts/100 beats, P = 0.0001, respectively), although the magnitude of the response was unchanged. Forearm vascular conductance during mental stress was significantly increased only in D + ET (2.74 +/- 0.22 vs 3.52 +/- 0.19 units, P = 0.02). Weight loss reduces MSNA during mental stress in obese women. The increase in forearm vascular conductance after weight loss provides convincing evidence for D + ET interventions as a nonpharmacologic therapy of human obesity.

Diet and exercise training restore blood pressure and vasodilatory responses during physiological maneuvers in obese children

BACKGROUND

The effects of diet and diet plus exercise training on muscle vasodilatation during physiological maneuvers in obese children are unknown. We tested the hypothesis that (1) blood pressure (BP) and forearm vascular conductance (FVC) responses during handgrip exercise and mental stress would be altered in obese children and (2) diet plus exercise training would restore BP and FVC responses during exercise and mental stress in obese children.

METHODS AND RESULTS

Thirty-nine obese children (aged 10+/-0.2 years) were randomly divided into 2 groups: diet plus exercise training (n=21; body mass index [BMI]=28+/-0.5 kg/m2) and diet (n=18; BMI=30+/-0.4 kg/m2). Ten age-matched lean control children (BMI=17+/-0.5 kg/m2) were also studied. Forearm blood flow was measured by venous occlusion plethysmography. BP was monitored noninvasively. Handgrip exercise was performed at 30% maximal voluntary contraction for 3 minutes. Stroop color word test was performed for 4 minutes. Baseline BP was significantly higher and FVC was significantly lower in obese children. During exercise and mental stress, BP responses were significantly higher and FVC responses were significantly lower in obese children. Diet and diet plus exercise training significantly reduced body weight. Diet and diet plus exercise training significantly decreased BP levels during exercise and mental stress. Diet plus exercise training, in contrast to diet alone, significantly increased FVC responses during exercise (3.7+/-0.3 versus 5.6+/-0.4 U; P=0.01) and mental stress (3.5+/-0.5 versus 4.5+/-0.4 U; P=0.02). After diet plus exercise training, BP and FVC responses during exercise and mental stress were similar between obese children and the control group.

CONCLUSIONS

Obesity exacerbates BP responses and impairs FVC responses during exercise and mental stress in children. Diet and exercise training restore BP and FVC responses in obese children.

Reflex vasoconstrictor responses of the healthy human fingertip skin. Normal range, repeatability, and influencing factors

Skin vasoconstrictor reflexes (SVCRs) are mediated by sympathetic vasoconstrictor nerve fibers and used to test sympathetic function of the extremities. Since there is a dearth of published systematic data on inspiratory gasp-induced SVCR, we studied its range, repeatability, and influencing factors in 64 healthy subjects (mean age 41.9 years, range 20-78 years, 32 males). Mean vasoconstriction for the second digit was 54% (+/-19)/54% (+/-19) (right/left) and 69% (+/-16)/68% (+/-15) for the fourth digit. Forth digit vasoconstriction was significantly larger than the second digit (P < 0.01), but second and fourth digit correlation was good (Pearson: 0.521-0.741). Repeatability using single measure intraclass correlation was good with 0.62/0.69 for the second digit and 0.64/0.80 for the fourth digit (right/left). A trend for decreasing SVCR was seen with increasing age and weight, no effect was seen for gender, height, or blood pressure. The SVCR repeatability and range of variation seen in this study compare favorably with tests of heart rate variability, thermal threshold detection, and motor nerve conduction.

Gly16 + Glu27 beta2-adrenoceptor polymorphisms cause increased forearm blood flow responses to mental stress and handgrip in humans

We hypothesized that the muscle vasodilatation during mental stress and exercise would vary among humans who are polymorphic at alleles 16 and 27 of the beta(2)-adrenoceptors. From 216 preselected volunteers, we studied 64 healthy, middle-aged normotensive women selected to represent three genotypes: homozygous for the alleles Arg(16) and Gln(27) (Arg(16)/Gln(27), n = 34), Gly(16) and Gln(27) (Gly(16)/Gln(27), n = 20), and Gly(16) and Glu(27) (Gly(16)/Glu(27), n = 10). Forearm blood flow (plethysmography) and muscle sympathetic nerve activity (microneurography) were recorded during 3-min Stroop color-word test and 3-min handgrip isometric exercise (30% maximal voluntary contraction). Baseline muscle sympathetic nerve activity, forearm vascular conductance, mean blood pressure, and heart rate were not different among groups. During mental stress, the peak forearm vascular conductance responses were greater in Gly(16)/Glu(27) group than in Gly(16)/Gln(27) and Arg(16)/Gln(27) groups (1.79 +/- 0.66 vs. 0.70 +/- 0.11 and 0.58 +/- 0.12 units, P = 0.03). Similar results were found during exercise (0.80 +/- 0.25 vs. 0.28 +/- 0.08 and 0.31 +/- 0.08 units, P = 0.02). Further analysis in a subset of subjects showed that brachial intra-arterial propranolol infusion abolished the difference in vasodilatory response between Gly(16)/Glu(27) (n = 6) and Arg(16)/Gln(27) (n = 7) groups during mental stress (0.33 +/- 0.20 vs. 0.46 +/- 0.21 units, P = 0.50) and exercise (0.08 +/- 0.06 vs. 0.03 +/- 0.03 units, P = 0.21). Plasma epinephrine concentration in Arg(16)/Gln(27) and Gly(16)/Glu(27) groups was similar. In conclusion, women who are homozygous for Gly(16)/Glu(27) of the beta(2)-adrenoceptors have augmented muscle vasodilatory responsiveness to mental stress and exercise.