Should the sympathetic nervous system be a target to improve cardiometabolic risk in obesity?

Blood Pressure Variability and Autonomic Dysfunction

PURPOSE OF REVIEW

This review considers the relationship between abnormal blood pressure (BP) variability and autonomic dysfunction through an attempt to answer questions about its clinical relevance and pertinence to diabetes and cardiovascular autonomic neuropathy (CAN) and which therapeutic measures can lessen its cardiovascular impact.

RECENT FINDINGS

Office, ambulatory, and home BP monitoring identify posture-related, circadian, short-term, and long-term BP variabilities. Abnormal BP variability is a risk marker for organ damage, mortality, and cardiovascular events. Moreover, BP variability changes are common in diabetes and associated with CAN and possibly exacerbated by comorbidities like nephropathy, obstructive sleep apnoea syndrome, and chronic pain. The prognostic role of nondipping and reverse dipping is well documented in diabetes. Some findings suggest the possibility of restoring dipping with the dosage time of antihypertensive agents. Diabetes is a favorable scenario for altered BP variability, which might mediate the harmful effects of CAN. Preliminary data suggest the protective effect of targeting BP variability. However, further longitudinal outcome studies are needed. In the meantime, BP variability measures and practical expedients in antihypertensive treatment should be implemented in diabetes.

Measurement matters: higher waist-to-hip ratio but not body mass index is associated with deficits in executive functions and episodic memory

Background

The current study aimed to reconcile the inconsistent findings between obesity, executive functions, and episodic memory by addressing major limitations of previous studies, including overreliance on body mass index (BMI), small sample sizes, and failure to control for confounds.

Methods

Participants consisted of 3,712 midlife adults from the Cognitive Project of the National Survey of Midlife Development. Executive functions and episodic memory were measured by a battery of cognitive function tests.

Results

We found that higher waist-to-hip ratio was associated with deficits in both executive functions and episodic memory, above and beyond the influence of demographics, comorbid health issues, health behaviors, personality traits, and self-perceived obesity. However, higher BMI was not associated with deficits in executive functions and episodic memory. More importantly, these differential associations were robust and stable across adulthood.

Discussion

Our findings confirm the association between obesity and episodic memory while highlighting the need for better measures of obesity when examining its associations with individual differences in cognitive functions.

The regulation of aldosterone secretion by leptin: implications in obesity-related cardiovascular disease

PURPOSE OF REVIEW

Although it has been known for some time that increases in body mass enhance aldosterone secretion, particularly in women, the origin of this elevation in aldosterone production is not well defined. Adipocyte-derived factors have emerged as potential candidates to increase aldosterone production in obesity.

RECENT FINDINGS

Emerging evidence suggests the presence of a mechanistic link in which the adipocyte-derived hormone leptin stimulates aldosterone production in obesity, thereby creating a positive feedback loop for obesity-associated cardiovascular disease. In addition, recent reports give credence to the concept that this leptin-aldosterone stimulation pathway in obesity is an underlying mechanism for sex-discrepancies in obesity-associated cardiovascular disease.

SUMMARY

Leptin appears as a new direct regulator of adrenal aldosterone production and leptin-mediated aldosterone production is a novel candidate mechanism underlying obesity-associated hypertension, particularly in females.

The combination of obesity and hypertension: a highly unfavorable phenotype requiring attention

PURPOSE OF REVIEW

Body habitus is a key lifestyle characteristic whose current status and future projections are disconcerting. The percentage of the global population who are either overweight or obese has substantially increased, with no indication that any country has a solution to this issue. Hypertension is a key unfavorable health metric that, like obesity, has disastrous health implications if left uncontrolled. Poor lifestyle characteristics and health metrics often cluster together to create complex and difficult to treat phenotypes. Excess body mass is such an example, creating an obesity-hypertension phenotype, which is the focus of this review.

RECENT FINDINGS

An increased risk for hypertension is clearly linked to obesity, indicating that the two conditions are intimately linked. The cascade of obesity-induced pathophysiologic adaptations creates a clear path to hypertension. Adopting a healthy lifestyle is a primary intervention for the prevention as well as treatment of the obesity-hypertension phenotype.

SUMMARY

The obesity-hypertension phenotype is highly prevalent and has disastrous health implications. A primordial prevention strategy, focused on lifelong healthy lifestyle patterns, is the optimal approach for this condition. For those individuals already afflicted by the obesity-hypertension phenotype, interventions must aggressively focus on weight loss and blood pressure control.

No effect of acute normobaric hypoxia on plasma triglyceride levels in fasting healthy men

Circulating fatty acids are a major systemic energy source in the fasting state as well as a determinant of hepatic triglycerides (TG)-rich very-low-density lipoprotein production. Upon acute hypoxia, sympathetic arousal induces adipose tissue lipolysis, resulting in an increase in circulating nonesterified fatty acids (NEFA). Animal studies suggest that TG clearance may also be strongly reduced under hypoxia, though this effect has been shown to be dependent on temperature. Whether the hypoxia-induced rise in blood fatty acid concentrations affects fasting TG levels in humans under thermoneutral conditions remains unknown. TG, NEFA, and glycerol levels were measured in fasted healthy young men (n = 10) exposed for 6 h to either normoxia (ambient air) or acute hypoxia (fraction of inspired oxygen = 0.12) in a randomized, crossover design. Participants were casually clothed and rested in front of a fan in an environmental chamber maintained at 28 °C during each trial. Under hypoxia, a significantly greater increase in NEFA occurred (condition × time interaction, p = 0.049) and glycerol levels tended to be higher (condition × time, p = 0.104), suggesting an increase in adipose tissue lipolysis. However, plasma TG levels did not change over time and did not differ between the normoxia and hypoxia conditions. In conclusion, acute exposure to normobaric hypoxia under thermoneutral condition in healthy men during fasting state increased lipolysis without affecting circulating TG.

Concentrated Ambient PM2.5-Induced Inflammation and Endothelial Dysfunction in a Murine Model of Neural IKK2 Deficiency

BACKGROUND

Exposure to ambient fine particulate matter (PM_2.5) is associated with cardiovascular mortality, but underlying pathophysiologic mechanisms are not fully understood. Hypothalamic inflammation, characterized by the activation of Inhibitor kappaB kinase 2/Nuclear factor kappaB (IKK2/NF-κB) signaling pathway, may play an important role in the pathogenesis of cardiovascular diseases. We recently demonstrated that hypothalamic inflammation is increased in mice exposed to concentrated ambient PM_2.5 (CAP).

OBJECTIVES

In the present study, we used a neuron-specific IKK2 knockout mouse model to examine the role of neural IKK2 expression and hypothalamic inflammation in the pathophysiologic effects of PM_2.5.

METHODS

We assessed inflammatory and vascular responses in Nestin-creIKK2^flox/flox (IKK2^Neu-KO) and littermate Nestin-creIKK2^flox/+ (control) mice after 4 mo of exposure to filtered air (FA) or CAP.

RESULTS

CAP exposure was associated with significantly higher tumor necrosis factor-α (TNFα) and interleukin (IL)-6 mRNA in the hypothalamus of control mice, but not IKK2^Neu-KO mice. In addition, CAP exposure-induced increases in bronchoalveolar lavage fluid (BALF) leukocytes, pulmonary macrophage infiltration and IL-6 expression, plasma TNFα and IL-1β levels, adipose macrophage infiltration and IL-1β expression, and endothelial dysfunction were reduced or absent in IKK2^Neu-KO mice compared with controls.

CONCLUSIONS

Our findings support a role of neural IKK2 in CAP exposure-induced local and systemic pro-inflammatory cytokine expression, pulmonary and adipose inflammation, and endothelial dysfunction, thus providing insight into pathophysiologic mechanisms that may mediate effects of PM_2.5 exposure. https://doi.org/10.1289/EHP2311.

Effects of weight changes in the autonomic nervous system: A systematic review and meta-analysis

BACKGROUND

Obesity has been linked to autonomic dysfunction, which is thought to be one of the main contributors for hypertension, cardiac remodelling and death. Exercise and diet-based weight loss are the mainstay therapy for obesity, but there is a paucity of data regarding the effect of weight changes in autonomic nervous system (ANS) activity.

OBJECTIVE

To describe the impact of weight changes in autonomic nervous system.

METHODS

A systematic literature search of four biomedical databases was performed evaluating effects of weight changes, thorough diet and/or exercise-based interventions, in the following ANS outcomes: heart rate variability, namely low frequency (LF)/high frequency (HF) ratio (LF/HF ratio), normalized units of LF (LFnu) and HF (HFnu), muscle sympathetic nerve activity (MSNA), noradrenaline spillover rate (NA-SR), standard deviation of normal-to-normal intervals (SDNN), root mean square of successive differences (RMSSD), baroreflex sensitivity and pupillometry. Quality appraisal was performed using the GRADE methodology and, where fitting, studies with comparable outcomes were pooled for meta-analysis.

RESULTS

Twenty-seven studies - 7 controlled clinical trials and 20 observational studies - were included. Weight gain was reported in 4 studies and weight loss in all the other studies. Interventions inducing weight changes included: hypocaloric or hypercaloric diets, exercise (strength, endurance or aerobic training) and hypocaloric diet coupled with exercise programs. Most studies which resulted in weight loss reported decreases in LF/HF ratio, LFnu, MSNA burst frequency and incidence, NA-SR, and an increase of baroreflex sensitivity, HF, HFnu and RMSSD, pointing to a parasympathetic nervous system activation. Meta-analysis regarding weight loss interventions showed a significant pooled effect size (95% CI) with a decreased of MSNA burst frequency -5.09 (-8.42, -1.75), MSNA incidence -6.66 (-12.40, -0.62), however this was not significant for SDNN 14.32 (-4.31, 32.96). Weight gain was associated with an increase in LF/HF, LFnu, MSNA burst frequency and incidence. The weight loss effects were potentiated by the association of hypocaloric diet with exercise. Nevertheless, weight changes effects in these outcomes were based in low or very low quality of evidence.

CONCLUSIONS

Diet and exercise based weight loss appears to increase parasympathetic and decrease sympathetic activity, the opposing effects being observed with weight gain. These findings are not uniformly reported in the literature, possibly due to differences in study design, methodology, characteristics of the participants and techniques used to estimate autonomic nervous activity.

Convergence between biological, behavioural and genetic determinants of obesity

Multiple biological, behavioural and genetic determinants or correlates of obesity have been identified to date. Genome-wide association studies (GWAS) have contributed to the identification of more than 100 obesity-associated genetic variants, but their roles in causal processes leading to obesity remain largely unknown. Most variants are likely to have tissue-specific regulatory roles through joint contributions to biological pathways and networks, through changes in gene expression that influence quantitative traits, or through the regulation of the epigenome. The recent availability of large-scale functional genomics resources provides an opportunity to re-examine obesity GWAS data to begin elucidating the function of genetic variants. Interrogation of knockout mouse phenotype resources provides a further avenue to test for evidence of convergence between genetic variation and biological or behavioural determinants of obesity.

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.

Body Composition in Adolescents During Treatment With Selective Serotonin Reuptake Inhibitors

OBJECTIVES

To examine the independent contribution of major depressive disorder (MDD), generalized anxiety disorder (GAD), and selective serotonin reuptake inhibitors (SSRIs) to changes in body composition in older adolescents.

METHODS

Medically healthy 15- to 20-year-olds who were unmedicated or within 1 month of starting an SSRI were prospectively followed. Psychiatric functioning and medication treatment were assessed monthly. Body Mass Index (BMI) was measured every 4 months. Every 8 months, a whole-body dual-energy radiograph absorptiometry scan was obtained to determine lean BMI, fat mass index, and visceral fat mass. Linear mixed effects regression analysis examined associations between MDD, GAD, and SSRI use variables and body composition measures.

RESULTS

Over 1.51 ± 0.76 years of follow-up, 264 participants contributed 805 observations. After adjusting for age, sex, physical activity, dietary intake, and time in the study, MDD severity was inversely associated, prospectively, with BMI, fat mass index, and lean BMI z scores, whereas cumulative SSRI treatment duration and dose were positively associated with these outcomes. GAD severity and diagnosis were not significantly associated with any body composition outcome. Moreover, citalopram and escitalopram were most strongly associated with the increase in all body composition measures, including visceral fat mass, whereas the associations with fluoxetine were somewhat weaker. Sertraline was not different from no SSRI treatment.

CONCLUSIONS

Depression severity was associated with a decrease in measures of body composition in older adolescents over a mean of 1.5 years, whereas SSRI treatment was positively associated with these outcomes, with differential effects across treatment groups.

Insulin resistance: a new consequence of altered carotid body chemoreflex?

Metabolic diseases affect millions of individuals across the world and represent a group of chronic diseases of very high prevalence and relatively low therapeutic success, making them suitable candidates for pathophysiological studies. The sympathetic nervous system (SNS) contributes to the regulation of energy balance and energy expenditure both in physiological and pathological states. For instance, drugs that stimulate sympathetic activity decrease food intake, increase resting metabolic rate and increase the thermogenic response to food, while pharmacological blockade of the SNS has opposite effects. Likewise, dysmetabolic features such as insulin resistance, dyslipidaemia and obesity are characterized by a basal overactivation of the SNS. Recently, a new line of research linking the SNS to metabolic diseases has emerged with the report that the carotid bodies (CBs) are involved in the development of insulin resistance. The CBs are arterial chemoreceptors that classically sense changes in arterial blood O2 , CO2 and pH levels and whose activity is known to be increased in rodent models of insulin resistance. We have shown that selective bilateral resection of the nerve of the CB, the carotid sinus nerve (CSN), totally prevents diet-induced insulin resistance, hyperglycaemia, dyslipidaemia, hypertension and sympathoadrenal overactivity. These results imply that the beneficial effects of CSN resection on insulin action and glucoregulation are modulated by target-related efferent sympathetic nerves through a reflex that is initiated in the CBs. It also highlights modulation of CB activity as a putative future therapeutic intervention for metabolic diseases.

Central Sympathetic Inhibition: a Neglected Approach for Treatment of Cardiac Arrhythmias?

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia. Overactivation of the sympathetic nervous system (SNS) plays an important role in the pathogenesis of comorbidities related to AF such as hypertension, congestive heart failure, obesity, insulin resistance, and obstructive sleep apnea. Methods that reduce sympathetic drive, such as centrally acting sympatho-inhibitory agents, have been shown to reduce the incidence of spontaneous or induced atrial arrhythmias, suggesting that neuromodulation may be helpful in controlling AF. Moxonidine acts centrally to reduce activity of the SNS, and clinical trials indicate that this is associated with a decreased AF burden in hypertensive patients with paroxysmal AF and reduced post-ablation recurrence of AF in patients with hypertension who underwent pulmonary vein isolation (PVI). Furthermore, device-based approaches to reduce sympathetic drive, such as renal denervation, have yielded promising results in the prevention and treatment of cardiac arrhythmias. In light of these recent findings, targeting elevated sympathetic drive with either pharmacological or device-based approaches has become a focus of clinical research. Here, we review the data currently available to explore the potential utility of sympatho-inhibitory therapies in the prevention and treatment of cardiac arrhythmias.

The adipokine chemerin amplifies electrical field-stimulated contraction in the isolated rat superior mesenteric artery

The adipokine chemerin causes arterial contraction and is implicated in blood pressure regulation, especially in obese subjects with elevated levels of circulating chemerin. Because chemerin is expressed in the perivascular adipose tissue (PVAT) that surrounds the sympathetic innervation of the blood vessel, we tested the hypothesis that chemerin (endogenous and exogenous) amplifies the sympathetic nervous system in mediating electrical field-stimulated (EFS) contraction. The superior mesenteric artery, with or without PVAT and with endothelium and sympathetic nerve intact, was mounted into isolated tissue baths and used for isometric contraction and stimulation. Immunohistochemistry validated a robust expression of chemerin in the PVAT surrounding the superior mesenteric artery. EFS (0.3-20 Hz) caused a frequency-dependent contraction in isolated arteries that was reduced by the chemerin receptor ChemR23 antagonist CCX832 alone (100 nM; with, but not without, PVAT), but not by the inactive congener CCX826 (100 nM). Exogenous chemerin-9 (1 μM)-amplified EFS-induced contraction in arteries (with and without PVAT) was blocked by CCX832 and the α-adrenergic receptor antagonist prazosin. CCX832 did not directly inhibit, nor did chemerin directly amplify, norepinephrine-induced contraction. Whole mount immunohistochemical experiments support colocalization of ChemR23 with the sympathetic nerve marker tyrosine hydroxylase in superior mesenteric PVAT and, to a lesser extent, in arteries and veins. These studies support the idea that exogenous chemerin modifies sympathetic nerve-mediated contraction through ChemR23 and that ChemR23 may be endogenously activated. This is significant because of the well-appreciated role of the sympathetic nervous system in blood pressure control.

Physiological and therapeutic effects of carnosine on cardiometabolic risk and disease

Obesity, type 2 diabetes (T2DM) and cardiovascular disease (CVD) are the most common preventable causes of morbidity and mortality worldwide. They represent major public health threat to our society. Increasing prevalence of obesity and T2DM contributes to escalating morbidity and mortality from CVD and stroke. Carnosine (β-alanyl-L-histidine) is a dipeptide with anti-inflammatory, antioxidant, anti-glycation, anti-ischaemic and chelating roles and is available as an over-the-counter food supplement. Animal evidence suggests that carnosine may offer many promising therapeutic benefits for multiple chronic diseases due to these properties. Carnosine, traditionally used in exercise physiology to increase exercise performance, has potential preventative and therapeutic benefits in obesity, insulin resistance, T2DM and diabetic microvascular and macrovascular complications (CVD and stroke) as well as number of neurological and mental health conditions. However, relatively little evidence is available in humans. Thus, future studies should focus on well-designed clinical trials to confirm or refute a potential role of carnosine in the prevention and treatment of chronic diseases in humans, in addition to advancing knowledge from the basic science and animal studies.

Impact of obesity and acquisition protocol on (123)I-metaiodobenzylguanidine indexes of cardiac sympathetic innervation

BACKGROUND

This study was designed to assess the impact of obesity and acquisition protocol on (123)I-metaiodobenzylguanidine (MIBG) indexes of cardiac sympathetic innervation.

METHODS

Forty-five patients with heart failure (HF) (38 men, age 58±15 years) underwent (123)I-MIBG cardiac imaging. Of these patients, 10 were obese [body mass index (BMI) ≥30 kg/m(2)]. Ten-minute planar images of the thorax in anterior view were performed 15 minutes ("early" image) and 3 hours and 50 minutes ("late" image) after tracer administration in both supine- and prone-position. Early and late (123)I-MIBG heart-to-mediastinum (H/M) ratios and washout rate were computed.

RESULTS

In overall study population, early and late (123)I-MIBG H/M ratios and washout rate were comparable between supine- and prone-position acquisitions. Obese patients had a lower early and late (123)I-MIBG H/M ratios both in supine (P<0.01) and prone (P<0.05) positions compared to non-obese subjects.

CONCLUSIONS

Our results indicate that in HF patients, obesity has a significant impact on (123)I-MIBG indexes of cardiac sympathetic innervation. Prone-position did not change early and late (123)I-MIBG H/M ratios and washout rate compared to supine position both in obese and non-obese HF patients.

Increased peripheral vascular disease risk progressively constrains perfusion adaptability in the skeletal muscle microcirculation

To determine the impact of progressive elevations in peripheral vascular disease (PVD) risk on microvascular function, we utilized eight rat models spanning "healthy" to "high PVD risk" and used a multiscale approach to interrogate microvascular function and outcomes: healthy: Sprague-Dawley rats (SDR) and lean Zucker rats (LZR); mild risk: SDR on high-salt diet (HSD) and SDR on high-fructose diet (HFD); moderate risk: reduced renal mass-hypertensive rats (RRM) and spontaneously hypertensive rats (SHR); high risk: obese Zucker rats (OZR) and Dahl salt-sensitive rats (DSS). Vascular reactivity and biochemical analyses demonstrated that even mild elevations in PVD risk severely attenuated nitric oxide (NO) bioavailability and caused progressive shifts in arachidonic acid metabolism, increasing thromboxane A2 levels. With the introduction of hypertension, arteriolar myogenic activation and adrenergic constriction were increased. However, while functional hyperemia and fatigue resistance of in situ skeletal muscle were not impacted with mild or moderate PVD risk, blood oxygen handling suggested an increasingly heterogeneous perfusion within resting and contracting skeletal muscle. Analysis of in situ networks demonstrated an increasingly stable and heterogeneous distribution of perfusion at arteriolar bifurcations with elevated PVD risk, a phenomenon that was manifested first in the distal microcirculation and evolved proximally with increasing risk. The increased perfusion distribution heterogeneity and loss of flexibility throughout the microvascular network, the result of the combined effects on NO bioavailability, arachidonic acid metabolism, myogenic activation, and adrenergic constriction, may represent the most accurate predictor of the skeletal muscle microvasculopathy and poor health outcomes associated with chronic elevations in PVD risk.

Sex, the brain and hypertension: brain oestrogen receptors and high blood pressure risk factors

Hypertension is a major contributor to worldwide morbidity and mortality rates related to cardiovascular disease. There are important sex differences in the onset and rate of hypertension in humans. Compared with age-matched men, premenopausal women are less likely to develop hypertension. However, after age 60, the incidence of hypertension increases in women and even surpasses that seen in older men. It is thought that changes in levels of circulating ovarian hormones as women age may be involved in the increase in hypertension in older women. One of the key mechanisms involved in the development of hypertension in both men and women is an increase in sympathetic nerve activity (SNA). Brain regions important for the regulation of SNA, such as the subfornical organ, the paraventricular nucleus and the rostral ventral lateral medulla, also express specific subtypes of oestrogen receptors. Each of these brain regions has also been implicated in mechanisms underlying risk factors for hypertension such as obesity, stress and inflammation. The present review brings together evidence that links actions of oestrogen at these receptors to modulate some of the common brain mechanisms involved in the ability of hypertensive risk factors to increase SNA and blood pressure. Understanding the mechanisms by which oestrogen acts at key sites in the brain for the regulation of SNA is important for the development of novel, sex-specific therapies for treating hypertension.