Chronic baroreflex activation restores spontaneous baroreflex control and variability of heart rate in obesity-induced hypertension

Mechanisms and treatment of obesity-related hypertension-Part 1: Mechanisms

The prevalence of obesity has tripled over the past five decades. Obesity, especially visceral obesity, is closely related to hypertension, increasing the risk of primary (essential) hypertension by 65%-75%. Hypertension is a major risk factor for cardiovascular disease, the leading cause of death worldwide, and its prevalence is rapidly increasing following the pandemic rise in obesity. Although the causal relationship between obesity and high blood pressure (BP) is well established, the detailed mechanisms for such association are still under research. For more than 30 years sympathetic nervous system (SNS) and kidney sodium reabsorption activation, secondary to insulin resistance and compensatory hyperinsulinemia, have been considered as primary mediators of elevated BP in obesity. However, experimental and clinical data show that severe insulin resistance and hyperinsulinemia can occur in the absence of elevated BP, challenging the causal relationship between insulin resistance and hyperinsulinemia as the key factor linking obesity to hypertension. The purpose of Part 1 of this review is to summarize the available data on recently emerging mechanisms believed to contribute to obesity-related hypertension through increased sodium reabsorption and volume expansion, such as: physical compression of the kidney by perirenal/intrarenal fat and overactivation of the systemic/renal SNS and the renin-angiotensin-aldosterone system. The role of hyperleptinemia, impaired chemoreceptor and baroreceptor reflexes, and increased perivascular fat is also discussed. Specifically targeting these mechanisms may pave the way for a new therapeutic intervention in the treatment of obesity-related hypertension in the context of 'precision medicine' principles, which will be discussed in Part 2.

Interplay between baroreflex sensitivity, obesity and related cardiometabolic risk factors (Review)

The baroreflex represents a rapid negative feedback system implicated in blood pressure regulation, which aims to prevent blood pressure variations by regulating peripheral vascular tone and cardiac output. The aim of the present review was to highlight the association between baroreflex sensitivity (BRS) and obesity, including factors associated with obesity, such as metabolic syndrome, hypertension, cardiovascular disease and diabetes. For the present review, a literature search was conducted using the PubMed database until August 21, 2021. The searched terms included ‘baroreflex’, and other terms such as ‘sensitivity’, ‘obesity’, ‘metabolic syndrome’, ‘hypertension’, ‘diabetes’, ‘gender’, ‘aging’, ‘children’, ‘adolescents’, ‘physical activity’, ‘bariatric surgery’, ‘autonomous nervous system’ and ‘cardiometabolic risk factors’. Obesity and its related metabolic disorders can influence baroreflex functionality and decrease BRS, mostly by potentiating sympathetic nervous system activity. Obesity induces inflammation, which can increase sympathetic system activity and lead to a higher incidence of cardiovascular events. Obesity also represents an important risk factor for hypertension through numerous mechanisms; in this setting, dysfunctional baroreceptors are not able to protect against constantly elevated blood pressure. Furthermore, diabetes mellitus and oxidative stress result in deterioration of BRS, whereas aging is also generally related to reduced cardiovagal BRS. Differences in BRS have also been observed between men and women, and overall cardiovagal BRS in healthy women is less intense compared with that in men. BRS appears lower in children with obesity compared with that in children of a healthy weight. Notably, physical exercise can increase BRS in both hypertensive and normotensive subjects, and BRS can also be significantly improved following bariatric surgery and weight loss. In conclusion, obesity and its related metabolic disorders may influence baroreflex functionality and decrease BRS, and baroreceptors cannot protect against the constantly elevated blood pressure in obesity. However, following bariatric surgery and weight loss, BRS can be significantly improved. The present review summarizes the role of obesity and related metabolic risk factors in BRS, providing details on possible mechanisms and shedding light on their interplay leading to autonomic neuropathy.

Benefits of pharmacological and electrical cholinergic stimulation in hypertension and heart failure

Systemic arterial hypertension and heart failure are cardiovascular diseases that affect millions of individuals worldwide. They are characterized by a change in the autonomic nervous system balance, highlighted by an increase in sympathetic activity associated with a decrease in parasympathetic activity. Most therapeutic approaches seek to treat these diseases by medications that attenuate sympathetic activity. However, there is a growing number of studies demonstrating that the improvement of parasympathetic function, by means of pharmacological or electrical stimulation, can be an effective tool for the treatment of these cardiovascular diseases. Therefore, this review aims to describe the advances reported by experimental and clinical studies that addressed the potential of cholinergic stimulation to prevent autonomic and cardiovascular imbalance in hypertension and heart failure. Overall, the published data reviewed demonstrate that the use of central or peripheral acetylcholinesterase inhibitors is efficient to improve the autonomic imbalance and hemodynamic changes observed in heart failure and hypertension. Of note, the baroreflex and the vagus nerve activation have been shown to be safe and effective approaches to be used as an alternative treatment for these cardiovascular diseases. In conclusion, pharmacological and electrical stimulation of the parasympathetic nervous system has the potential to be used as a therapeutic tool for the treatment of hypertension and heart failure, deserving to be more explored in the clinical setting.

Effects of carotid baroreceptor stimulation on aortic remodeling in obese rats

BACKGROUND AND AIM

Our previous study found carotid baroreceptor stimulation (CBS) reduces body weight and white adipose tissue (WAT) weight, restores abnormal secretion of adipocytokines and inflammation factors, decreases systolic blood pressure (SBP) by inhibiting activation of sympathetic nervous system (SNS) and renin-angiotensin system (RAS) in obese rats. In this study, we explore effects of CBS on aortic remodeling in obese rats.

METHODS AND RESULTS

Rats were fed high-fat diet (HFD) for 16 weeks to induce obesity and underwent either CBS device implantation and stimulation or sham operation at 8 weeks. BP and body weight were measured weekly. RAS activity of WAT, histological, biochemical and functional profiles of aortas were detected after 16 weeks. CBS effectively decreased BP in obese rats, downregulated mRNA expression of angiotensinogen (AGT) and renin in WAT, concentrations of AGT, renin, angiotensin II (Ang II), protein levels of Ang II receptor 1 (AT1R) and Ang II receptor 2 (AT2R) in WAT were declined. CBS inhibited reactive oxygen species (ROS) generation, inflammatory response and endoplasmic reticulum (ER) stress in aortas of obese rats, restrained vascular wall thickening and vascular smooth muscle cells (VSMCs) phenotypic switching, increased nitric oxide (NO) synthesis, promoted endothelium-dependent vasodilatation by decreasing protein expression of AT1R and leptin receptor (LepR), increasing protein expression of adiponectin receptor 1 (AdipoR1) in aortic VSMCs.

CONCLUSION

CBS reduced BP and reversed aortic remodeling in obese rats, the underlying mechanism might be related to the suppressed SNS activity, restored adipocytokine secretion and restrained RAS activity of WAT.

Endothelin B receptors impair baroreflex function and increase blood pressure variability during high salt diet

Baroreflex function is an integral component maintaining consistent blood pressure. Hypertension is often associated with baroreflex dysfunction, and environmental risk factors such as high salt diet exacerbate hypertension in subjects with baroreflex dysfunction. However, the interactions between high salt diet, baroreflex dysfunction, and hypertension are incompletely understood. The endothelin system is another potent mediator of blood pressure control especially in response to a high salt diet. We hypothesized that the endothelin B (ETB) receptor activation on adrenergic nerves decreases baroreflex sensitivity. We utilized male ETB receptor deficient (ETB-def) rats that express functional ETB receptors only on adrenergic nerves and transgenic (TG) controls to evaluate baroreflex function during normal (0.49% NaCl) and high (4.0% NaCl) salt diets. In conscious rats equipped with telemetry, ETB-def rats had an increased lability of systolic blood pressure (SBP) compared to TG controls as indicated by higher standard deviation (SD) of SBP under both normal (10.2 ± 0.6 vs. 12.4 ± 0.9 mmHg, respectively, p = 0.0001) and high (11.7 ± 0.6 vs. 16.1 ± 1.0 mmHg, p = 0.0001) salt diets. In anesthetized preparations, ETB-def rats displayed reduced heart rate (p genotype = 0.0167) and renal sympathetic nerve (p genotype = 0.0022) baroreflex sensitivity. We then gave male Sprague-Dawley rats the selective ETB receptor antagonist, A-192621 (10 mg/kg/day), to block ETB receptors. Following ETB receptor antagonism, even though SBP increased (131 ± 7 before vs. 152 ± 8 mmHg after, p < 0.0001), the lability (standard deviation) of SBP decreased (9.3 ± 2.0 vs. 7.1 ± 1.1 mmHg, p = 0.0155). These data support our hypothesis that ETB receptors on adrenergic nerves contribute to baroreflex dysfunction.

Obesity, kidney dysfunction, and inflammation: interactions in hypertension

Obesity contributes 65-75% of the risk for human primary (essential) hypertension (HT) which is a major driver of cardiovascular and kidney diseases. Kidney dysfunction, associated with increased renal sodium reabsorption and compensatory glomerular hyperfiltration, plays a key role in initiating obesity-HT and target organ injury. Mediators of kidney dysfunction and increased blood pressure include (i) elevated renal sympathetic nerve activity (RSNA); (ii) increased antinatriuretic hormones such as angiotensin II and aldosterone; (iii) relative deficiency of natriuretic hormones; (iv) renal compression by fat in and around the kidneys; and (v) activation of innate and adaptive immune cells that invade tissues throughout the body, producing inflammatory cytokines/chemokines that contribute to vascular and target organ injury, and exacerbate HT. These neurohormonal, renal, and inflammatory mechanisms of obesity-HT are interdependent. For example, excess adiposity increases the adipocyte-derived cytokine leptin which increases RSNA by stimulating the central nervous system proopiomelanocortin-melanocortin 4 receptor pathway. Excess visceral, perirenal and renal sinus fat compress the kidneys which, along with increased RSNA, contribute to renin-angiotensin-aldosterone system activation, although obesity may also activate mineralocorticoid receptors independent of aldosterone. Prolonged obesity, HT, metabolic abnormalities, and inflammation cause progressive renal injury, making HT more resistant to therapy and often requiring multiple antihypertensive drugs and concurrent treatment of dyslipidaemia, insulin resistance, diabetes, and inflammation. More effective anti-obesity drugs are needed to prevent the cascade of cardiorenal, metabolic, and immune disorders that threaten to overwhelm health care systems as obesity prevalence continues to increase.

Screening of Pathophysiological Mechanisms and Psychological and Occupational Factors, Determining Risk of Cardiovascular Diseases in High-Risk Professions

Cardiovascular diseases (CVD) are among the most significant and common chronic non-communicable diseases in Bulgaria. Screening, identification and registration of pathophysiological mechanisms and psychological and occupational factors related to CVD should be implemented in the diagnostic work-up. Job analysis contributes to the implementation of screening and identification of the most significant psychological and professional factors related to the work activity.

Aim: The aim of our study is to examine the pathophysiological mechanisms of autonomic cardiovascular control and psychological and occupational factors determining the risk of CVD.

Materials and methods: The following groups of individuals were examined: 83 physicians, 54 nurses, 23 employees and 40 militaries. The autonomic cardiovascular control was studied through the time-and frequency-domain based measures of the Analysis of Heart Rate Variability (HRV).

Results: The results of the ANOVA test showed that there was a statistically significant difference in the time-domain and frequency-domain based HRV measures between the compared groups: X [F (4; 225) = 4.455, p = 0.001]; SDNN [F (4; 225) = 3.876, p = 0.005]; SDNN5 [F (4; 225) = 3.083, p = 0.01]; pNN50 [F (4; 225) = 8.303, p < 0.001]; rMSSD [F (4; 225) = 8.711, p < 0.001]; SDSD [F (4; 225) = 8.323, p < 0.001; VLF [F (4; 225) = 1.204, p = 0.31]; LF [F (4; 225) = 3.874, p = 0.005]; HF [F (4; 225) = 7.790, p < 0.001] and LF/HF [F (4; 225) = 8.601, p < 0.001]. The results of our study revealed that occupational stress and cognitive workload in physicians, nurses, and employees induced a process of reciprocally combined inhibition of the parasympathetic branch of the autonomic nervous system (ANS) (SDNN, SDNN5, pNN50, rMSSD, and HF) and activation of the sympathetic branch of ANS (LF), while in military personnel they cause a process of co-activation of the parasympathetic and sympathetic branches of the ANS (rMSSD, SDNN, SDNN5, pNN50 and LF/HF).

Conclusion: Stress-induced dysfunctional autonomic cardiovascular regulation, patho-physiological control mechanisms, psychological and occupational factors determine the risk of CVD. Their identification could contribute to the diagnostics and prevention of CVD.

Weight-gain induced changes in renal perfusion assessed by contrast-enhanced ultrasound precede increases in urinary protein excretion suggestive of glomerular and tubular injury and normalize after weight-loss in dogs

Early detection of obesity-related glomerulopathy in humans is challenging as it might not be detected by routine biomarkers of kidney function. This study's aim was to use novel kidney biomarkers and contrast-enhanced ultrasound (CEUS) to evaluate the effect of obesity development and weight-loss on kidney function, perfusion, and injury in dogs. Sixteen healthy lean adult beagles were assigned randomly but age-matched to a control group (CG) (n = 8) fed to maintain a lean body weight (BW) for 83 weeks; or to a weight-change group (WCG) (n = 8) fed the same diet to induce obesity (week 0-47), to maintain stable obese weight (week 47-56) and to lose BW (week 56-83). At 8 time points, values of systolic blood pressure (sBP); serum creatinine (sCr); blood urea nitrogen (BUN); serum cystatin C (sCysC); urine protein-to-creatinine ratio (UPC); and urinary biomarkers of glomerular and tubular injury were measured. Glomerular filtration rate (GFR) and renal perfusion using CEUS were assayed (except for week 68). For CEUS, intensity- and time-related parameters representing blood volume and velocity were derived from imaging data, respectively. At 12-22% weight-gain, cortical time-to-peak, representing blood velocity, was shorter in the WCG vs. the CG. After 37% weight-gain, sCysC, UPC, glomerular and tubular biomarkers of injury, urinary immunoglobulin G and urinary neutrophil gelatinase-associated lipocalin, respectively, were higher in the WCG. sBP, sCr, BUN and GFR were not significantly different. After 23% weight-loss, all alterations were attenuated. Early weight-gain in dogs induced renal perfusion changes measured with CEUS, without hyperfiltration, preceding increased urinary protein excretion with potential glomerular and tubular injury. The combined use of routine biomarkers of kidney function, CEUS and site-specific urinary biomarkers might be valuable in assessing kidney health of individuals at risk for obesity-related glomerulopathy in a non-invasive manner.

Smart Baroreceptor Activation Therapy Strikingly Attenuates Blood Pressure Variability in Hypertensive Rats With Impaired Baroreceptor

Increased blood pressure (BP) variability (BPV) is an independent risk factor of cardiovascular events among hypertensive patients. The arterial baroreceptor reflex is a powerful regulator of BP and attenuates BPV via a sympathetic negative feedback control. Conventional baroreceptor activation therapy (cBAT) electrically stimulates the carotid baroreceptors with constant stimulation parameters. While cBAT lowers BP, it does not mount a pressure feedback mechanism. We hypothesized that baroreceptor activation therapy with a pressure feedback system (smart BAT [sBAT]) is able to reduce BPV as well as lower BP. We developed sBAT that electrically stimulated baroreceptors at a frequency proportional to the difference between instantaneous BP and a preset reference pressure, and compared its performance with cBAT. In 14-week-old spontaneously hypertensive rats (n=6), we implanted BP telemeter and created impaired arterial baroreceptors by modified sino-aortic denervation. One week after surgical preparation, we administered sBAT, cBAT or no stimulation (sham) for 15 minutes and compared BP and BPV under freely moving condition. Both cBAT and sBAT significantly lowered mean BP (sham, 141.3±12.8; cBAT, 114.3±11.4; and sBAT, 112.0±7.3 mm Hg). Conventional BAT did not affect BPV at all, while sBAT significantly reduced BPV (sham, 15.4±2.6; cBAT, 16.0±5.2; and sBAT, 9.7±3.3 mm Hg). sBAT also prevented transient excessive BP rise and fall. In conclusion, sBAT was capable of reducing BP and attenuating BPV in hypertensive rats with impaired baroreceptor. sBAT is a novel treatment option for hypertensive patients with increased BPV.

Obesity, kidney dysfunction and hypertension: mechanistic links

Excessive adiposity raises blood pressure and accounts for 65-75% of primary hypertension, which is a major driver of cardiovascular and kidney diseases. In obesity, abnormal kidney function and associated increases in tubular sodium reabsorption initiate hypertension, which is often mild before the development of target organ injury. Factors that contribute to increased sodium reabsorption in obesity include kidney compression by visceral, perirenal and renal sinus fat; increased renal sympathetic nerve activity (RSNA); increased levels of anti-natriuretic hormones, such as angiotensin II and aldosterone; and adipokines, particularly leptin. The renal and neurohormonal pathways of obesity and hypertension are intertwined. For example, leptin increases RSNA by stimulating the central nervous system proopiomelanocortin-melanocortin 4 receptor pathway, and kidney compression and RSNA contribute to renin-angiotensin-aldosterone system activation. Glucocorticoids and/or oxidative stress may also contribute to mineralocorticoid receptor activation in obesity. Prolonged obesity and progressive renal injury often lead to the development of treatment-resistant hypertension. Patient management therefore often requires multiple antihypertensive drugs and concurrent treatment of dyslipidaemia, insulin resistance, diabetes and inflammation. If more effective strategies for the prevention and control of obesity are not developed, cardiorenal, metabolic and other obesity-associated diseases could overwhelm health-care systems in the future.

Device-Based Neuromodulation for Resistant Hypertension Therapy

Despite availability of effective drugs for hypertension therapy, significant numbers of hypertensive patients fail to achieve recommended blood pressure levels on ≥3 antihypertensive drugs of different classes. These individuals have a high prevalence of adverse cardiovascular events and are defined as having resistant hypertension (RHT) although nonadherence to prescribed antihypertensive medications is common in patients with apparent RHT. Furthermore, apparent and true RHT often display increased sympathetic activity. Based on these findings, technology was developed to treat RHT by suppressing sympathetic activity with electrical stimulation of the carotid baroreflex and catheter-based renal denervation (RDN). Over the last 15 years, experimental and clinical studies have provided better understanding of the physiological mechanisms that account for blood pressure lowering with baroreflex activation and RDN and, in so doing, have provided insight into which patients in this heterogeneous hypertensive population are most likely to respond favorably to these device-based therapies. Experimental studies have also played a role in modifying device technology after early clinical trials failed to meet key endpoints for safety and efficacy. At the same time, these studies have exposed potential differences between baroreflex activation and RDN and common challenges that will likely impact antihypertensive treatment and clinical outcomes in patients with RHT. In this review, we emphasize physiological studies that provide mechanistic insights into blood pressure lowering with baroreflex activation and RDN in the context of progression of clinical studies, which are now at a critical point in determining their fate in RHT management.

Upregulation of Nav1.6 expression in the rostral ventrolateral medulla of stress-induced hypertensive rats

The rostral ventrolateral medulla (RVLM) plays a key role in mediating the development of stress-induced hypertension (SIH) by excitation and/or inhibition of sympathetic preganglionic neurons. The voltage-gated sodium channel Nav1.6 has been found to contribute to neuronal hyperexcitability. To examine the expression of Nav1.6 in the RVLM during SIH, a rat model was established by administering electric foot-shocks and noises. We found that Nav1.6 protein expression in the RVLM of SIH rats was higher than that of control rats, peaking at the tenth day of stress. Furthermore, we observed changes in blood pressure correlating with days of stress, with systolic blood pressure (SBP) found to reach a similarly timed peak at the tenth day of stress. Percentages of cells exhibiting colocalization of Nav1.6 with NeuN, a molecular marker of neurons, indicated a strong correlation between upregulation of Nav1.6 expression in NeuN-positive cells and SBP. The level of RSNA was significantly increased after 10 days of stress induction than control group. Compared with the SIHR, knockdown of Nav1.6 in RVLM of the SIHR decreased the level of SBP, heart rate (HR) and renal sympathetic nerve activity (RSNA). These results suggest that upregulated Nav1.6 expression within neurons in the RVLM of SIH rats may contribute to overactivation of the sympathetic system in response to SIH development.

Role of the heart in blood pressure lowering during chronic baroreflex activation: insight from an in silico analysis

Electrical stimulation of the baroreflex chronically suppresses sympathetic activity and arterial pressure and is currently being evaluated for the treatment of resistant hypertension. The antihypertensive effects of baroreflex activation are often attributed to renal sympathoinhibition. However, baroreflex activation also decreases heart rate, and robust blood pressure lowering occurs even after renal denervation. Because controlling renal sympathetic nerve activity (RSNA) and cardiac autonomic activity cannot be achieved experimentally, we used an established mathematical model of human physiology (HumMod) to provide mechanistic insights into their relative and combined contributions to the cardiovascular responses during baroreflex activation. Three-week responses to baroreflex activation closely mimicked experimental observations in dogs including decreases in blood pressure, heart rate, and plasma norepinephrine and increases in plasma atrial natriuretic peptide (ANP), providing validation of the model. Simulations showed that baroreflex-induced alterations in cardiac sympathetic and parasympathetic activity lead to a sustained depression of cardiac function and increased secretion of ANP. Increased ANP and suppression of RSNA both enhanced renal excretory function and accounted for most of the chronic blood pressure lowering during baroreflex activation. However, when suppression of RSNA was blocked, the blood pressure response to baroreflex activation was not appreciably impaired due to inordinate fluid accumulation and further increases in atrial pressure and ANP secretion. These simulations provide a mechanistic understanding of experimental and clinical observations showing that baroreflex activation effectively lowers blood pressure in subjects with previous renal denervation. NEW & NOTEWORTHY Both experimental and clinical studies have shown that the presence of renal nerves is not an obligate requirement for sustained reductions in blood pressure during chronic electrical stimulation of the carotid baroreflex. Simulations using HumMod, a mathematical model of integrative human physiology, indicated that both increased secretion of atrial natriuretic peptide and suppressed renal sympathetic nerve activity play key roles in mediating long-term reductions in blood pressure during chronic baroreflex activation.

Ultra-short heart rate variability recording reliability: The effect of controlled paced breathing

BACKGROUND

Recent studies have reported that Heart Rate Variability (HRV) indices remain reliable even during recordings shorter than 5 min, suggesting the ultra-short recording method as a valuable tool for autonomic assessment. However, the minimum time-epoch to obtain a reliable record for all HRV domains (time, frequency, and Poincare geometric measures), as well as the effect of respiratory rate on the reliability of these indices remains unknown.

METHODS

Twenty volunteers had their HRV recorded in a seated position during spontaneous and controlled respiratory rhythms. HRV intervals with 1, 2, and 3 min were correlated with the gold standard period (6-min duration) and the mean values of all indices were compared in the two respiratory rhythm conditions.

RESULTS

rMSSD and SD1 were more reliable for recordings with ultra-short duration at all time intervals (r values from 0.764 to 0.950, p < 0.05) for spontaneous breathing condition, whereas the other indices require longer recording time to obtain reliable values. The controlled breathing rhythm evokes stronger r values for time domain indices (r values from 0.83 to 0.99, p < 0.05 for rMSSD), but impairs the mean values replicability of domains across most time intervals. Although the use of standardized breathing increases the correlations coefficients, all HRV indices showed an increase in mean values (t values from 3.79 to 14.94, p < 0.001) except the RR and HF that presented a decrease (t = 4.14 and 5.96, p < 0.0001).

CONCLUSION

Our results indicate that proper ultra-short-term recording method can provide a quick and reliable source of cardiac autonomic nervous system assessment.

Reduced Renal Mass, Salt-Sensitive Hypertension Is Resistant to Renal Denervation

Aim: Activation of the sympathetic nervous system is common in resistant hypertension (RHT) and also in chronic kidney disease (CKD), a prevalent condition among resistant hypertensives. However, renal nerve ablation lowers blood pressure (BP) only in some patients with RHT. The influence of loss of nephrons per se on the antihypertensive response to renal denervation (RDNx) is unclear and was the focus of this study.

Methods: Systemic hemodynamics and sympathetically mediated low frequency oscillations of systolic BP were determined continuously from telemetrically acquired BP recordings in rats before and after surgical excision of ∼80% of renal mass and subsequent RDNx.

Results: After reduction of renal mass, rats fed a high salt (HS) diet showed sustained increases in mean arterial pressure (108 ± 3 mmHg to 128 ± 2 mmHg) and suppression of estimated sympathetic activity (∼15%), responses that did not occur with HS before renal ablation. After denervation of the remnant kidney, arterial pressure fell (to 104 ± 4 mmHg), estimated sympathetic activity and heart rate (HR) increased concomitantly, but these changes gradually returned to pre-denervation levels over 2 weeks of follow up. Subsequently, sympathoinhibition with clonidine did not alter arterial pressure while significantly suppressing estimated sympathetic activity and HR.

Conclusion: These results indicate that RDNx does not chronically lower arterial pressure in this model of salt-sensitive hypertension associated with substantial nephron loss, but without ischemia and increased sympathetic activity, thus providing further insight into conditions likely to impact the antihypertensive response to renal-specific sympathoinhibition in subjects with CKD.

Aortic depressor nerve stimulation does not impede the dynamic characteristics of the carotid sinus baroreflex in normotensive or spontaneously hypertensive rats

Recent clinical trials in patients with drug-resistant hypertension indicate that electrical activation of the carotid sinus baroreflex can reduce arterial pressure (AP) for more than a year. To examine whether the electrical stimulation from one baroreflex system impedes normal short-term AP regulation via another unstimulated baroreflex system, we electrically stimulated the left aortic depressor nerve (ADN) while estimating the dynamic characteristics of the carotid sinus baroreflex in anesthetized normotensive Wistar-Kyoto (WKY; n = 8) rats and spontaneously hypertensive rats (SHR; n = 7). Isolated carotid sinus regions were perturbed for 20 min using a Gaussian white noise signal with a mean of 120 mmHg for WKY and 160 mmHg for SHR. Tonic ADN stimulation (2 Hz, 10 V, and 0.1-ms pulse width) decreased mean sympathetic nerve activity (73.4 ± 14.0 vs. 51.6 ± 11.3 arbitrary units in WKY, P = 0.012; and 248.7 ± 33.9 vs. 181.1 ± 16.6 arbitrary units in SHR, P = 0.018) and mean AP (90.8 ± 6.6 vs. 81.2 ± 5.4 mmHg in WKY, P = 0.004; and 128.6 ± 9.8 vs. 114.7 ± 10.3 mmHg in SHR, P = 0.009). The slope of dynamic gain in the neural arc transfer function from carotid sinus pressure to sympathetic nerve activity was not different between trials with and without the ADN stimulation (12.55 ± 0.93 vs. 13.03 ± 1.28 dB/decade in WKY, P = 0.542; and 17.37 ± 1.01 vs. 17.47 ± 1.64 dB/decade in SHR, P = 0.946). These results indicate that the tonic ADN stimulation does not significantly modify the dynamic characteristics of the carotid sinus baroreflex.

Maternal separation diminishes α-adrenergic receptor density and function in renal vasculature from male Wistar-Kyoto rats

Adult rats exposed to maternal separation (MatSep) are normotensive but display lower glomerular filtration rate and increased renal neuroadrenergic drive. The aim of this study was to determine the renal α-adrenergic receptor density and the renal vascular responsiveness to adrenergic stimulation in male rats exposed to MatSep. In addition, baroreflex sensitivity was assessed to determine a component of neural control of the vasculature. Using tissue collected from 4-mo-old MatSep and control rats, α₁-adrenergic receptors (α₁-ARs) were measured in renal cortex and isolated renal vasculature using receptor binding assay, and the α-AR subtype gene expression was determined by RT-PCR. Renal cortical α₁-AR density was similar between MatSep and control tissues (B_max = 44 ± 1 vs. 42 ± 2 fmol/mg protein, respectively); however, MatSep reduced α₁-AR density in renal vasculature (B_max = 47 ± 4 vs. 62 ± 4 fmol/mg protein, P < 0.05, respectively). In a separate group of rats, the pressor, bradycardic, and renal vascular constrictor responses to acute norepinephrine injection (NE, 0.03-0.25 μg/μl) were determined under anesthesia. Attenuated NE-induced renal vasoconstriction was observed in rats exposed to MatSep compared with control (P < 0.05). A third group of rats was infused at steady state with the α₁ agonist phenylephrine (10 μg/min iv) and vasodilator sodium nitroprusside (5 μg/min iv). The difference between the change in heart rate/mean arterial pressure slopes was indicative of reduced baroreflex sensitivity in MatSep vs. control rats (-0.45 ± 0.04 vs. -0.95 ± 0.07 beats·min^-1·mmHg^-1, P < 0.05). These data support the notion that reduced α-adrenergic receptor expression and function in the renal vasculature could develop secondary to MatSep-induced overactivation of the renal neuroadrenergic tone.

Baroreflex deficiency aggravates atherosclerosis via α7 nicotinic acetylcholine receptor in mice

OBJECTIVE

Inflammation and oxidative stress play a key role in the initiation, propagation, and development of atherosclerosis. Arterial baroreflex (ABR) dysfunction induced by sinoaortic denervation (SAD) promoted the development of atherosclerosis in ApoE^-/- mice. The present work was designed to examine whether ABR deficiency affected inflammation and oxidative stress via α7 nicotinic acetylcholine receptor (α7nAChR) leading to the aggravation of atherosclerosis in mice.

METHODS AND RESULTS

ApoE^-/- mice were fed with a high-cholesterol diet for 6weeks and half of the mice received sinoaortic denervation that destroyed ABR. We studied the expression of vesicular acetylcholine transporter (VAChT), α7nAChR and levels of inflammatory response and oxidative stress. The results showed that baroreflex dysfunction could promote atherosclerosis, meanwhile, decrease the expression of VAChT and α7nAChR and significantly increase the levels of oxidative stress and inflammation in SAD mice. After treated with PNU-282987 (a selective α7nAChR agonist, 0.53mg/kg/day) for 6weeks in SAD and Sham mice, we found that PNU-282987 could attenuate atherosclerosis and significantly decreased oxidative stress and inflammation after SAD. In addition, α7nAChR^+/+ and α7nAChR^-/- mice fed with a high-cholesterol diet for 8weeks were co-treated with ketanserin (0.6mg/kg/day), a drug that can enhance baroreflex sensitivity (BRS). Ketanserin could alleviate atherosclerosis and markedly decrease oxidative stress and inflammation in α7nAChR^+/+ mice. But there were no effects in α7nAChR knockout mice.

CONCLUSIONS

Our results demonstrate that ABR dysfunction aggravates atherosclerosis in mice via the vagus-ACh-α7nAChR-inflammation and oxidative stress pathway.

Chronic Interactions Between Carotid Baroreceptors and Chemoreceptors in Obesity Hypertension

Carotid bodies play a critical role in protecting against hypoxemia, and their activation increases sympathetic activity, arterial pressure, and ventilation, responses opposed by acute stimulation of the baroreflex. Although chemoreceptor hypersensitivity is associated with sympathetically mediated hypertension, the mechanisms involved and their significance in the pathogenesis of hypertension remain unclear. We investigated the chronic interactions of these reflexes in dogs with sympathetically mediated, obesity-induced hypertension based on the hypothesis that hypoxemia and tonic activation of carotid chemoreceptors may be associated with obesity. After 5 weeks on a high-fat diet, the animals experienced a 35% to 40% weight gain and increases in arterial pressure from 106±3 to 123±3 mm Hg and respiratory rate from 8±1 to 12±1 breaths/min along with hypoxemia (arterial partial pressure of oxygen=81±3 mm Hg) but eucapnia. During 7 days of carotid baroreflex activation by electric stimulation of the carotid sinus, tachypnea was attenuated, and hypertension was abolished before these variables returned to prestimulation values during a recovery period. After subsequent denervation of the carotid sinus region, respiratory rate decreased transiently in association with further sustained reductions in arterial partial pressure of oxygen (to 65±2 mm Hg) and substantial hypercapnia. Moreover, the severity of hypertension was attenuated from 125±2 to 116±3 mm Hg (45%-50% reduction). These findings suggest that hypoxemia may account for sustained stimulation of peripheral chemoreceptors in obesity and that this activation leads to compensatory increases in ventilation and central sympathetic outflow that contributes to neurogenically mediated hypertension. Furthermore, the excitatory effects of chemoreceptor hyperactivity are abolished by chronic activation of the carotid baroreflex.

Electrical carotid sinus stimulation: chances and challenges in the management of treatment resistant arterial hypertension

Treatment resistant arterial hypertension is associated with excess cardiovascular morbidity and mortality. Electrical carotid sinus stimulators engaging baroreflex afferent activity have been developed for such patients. Indeed, baroreflex mechanisms contribute to long-term blood pressure control by governing efferent sympathetic and parasympathetic activity. The first-generation carotid sinus stimulator applying bilateral bipolar stimulation reduced blood pressure in a controlled clinical trial but nevertheless failed to meet the primary efficacy endpoint. The second-generation device utilizes smaller unilateral unipolar electrodes, thus decreasing invasiveness of the implantation while saving battery. An uncontrolled clinical study suggested improvement in blood pressure with the second-generation device. We hope that these findings as well as preliminary observations suggesting cardiovascular and renal organ protection with electrical carotid sinus stimulation will be confirmed in properly controlled clinical trials. Meanwhile, we should find ways to better identify patients who are most likely to benefit from electrical carotid sinus stimulation.

Xyloketal B attenuates atherosclerotic plaque formation and endothelial dysfunction in apolipoprotein e deficient mice

Our previous studies demonstrated that xyloketal B, a novel marine compound with a unique chemical structure, has strong antioxidant actions and can protect against endothelial injury in different cell types cultured in vitro and model organisms in vivo. The oxidative endothelial dysfunction and decrease in nitric oxide (NO) bioavailability are critical for the development of atherosclerotic lesion. We thus examined whether xyloketal B had an influence on the atherosclerotic plaque area in apolipoprotein E-deficient (apoE-/-) mice fed a high-fat diet and investigated the underlying mechanisms. We found in our present study that the administration of xyloketal B dose-dependently decreased the atherosclerotic plaque area both in the aortic sinus and throughout the aorta in apoE-/- mice fed a high-fat diet. In addition, xyloketal B markedly reduced the levels of vascular oxidative stress, as well as improving the impaired endothelium integrity and NO-dependent aortic vasorelaxation in atherosclerotic mice. Moreover, xyloketal B significantly changed the phosphorylation levels of endothelial nitric oxide synthase (eNOS) and Akt without altering the expression of total eNOS and Akt in cultured human umbilical vein endothelial cells (HUVECs). Here, it increased eNOS phosphorylation at the positive regulatory site of Ser-1177, while inhibiting phosphorylation at the negative regulatory site of Thr-495. Taken together, these findings indicate that xyloketal B has dramatic anti-atherosclerotic effects in vivo, which is partly due to its antioxidant features and/or improvement of endothelial function.

The baroreflex as a long-term controller of arterial pressure

Because of resetting, a role for baroreflexes in long-term control of arterial pressure has been commonly dismissed in the past. However, in recent years, this perspective has changed. Novel approaches for determining chronic neurohormonal and cardiovascular responses to natural variations in baroreceptor activity and to electrical stimulation of the carotid baroreflex indicate incomplete resetting and sustained responses that lead to long-term alterations in sympathetic activity and arterial pressure.

Exposure to a high fat diet during the perinatal period alters vagal motoneurone excitability, even in the absence of obesity

KEY POINTS

Obesity is recognized as being multifactorial in origin, involving both genetic and environmental factors. The perinatal period is known to be critically important in the development of neural circuits responsible for energy homeostasis and the integration of autonomic reflexes. Diet-induced obesity alters the biophysical, pharmacological and morphological properties of vagal neurocircuits regulating upper gastrointestinal tract functions, including satiety. Less information is available, however, regarding the effects of a high fat diet (HFD) itself on the properties of vagal neurocircuits. The present study was designed to test the hypothesis that exposure to a HFD during the perinatal period alters the electrophysiological, pharmacological and morphological properties of vagal efferent motoneurones innervating the stomach. Our data indicate that perinatal HFD decreases the excitability of gastric-projecting dorsal motor nucleus neurones and dysregulates neurotransmitter release from synaptic inputs and that these alterations occur prior to the development of obesity. These findings represent the first direct evidence that exposure to a HFD modulates the processing of central vagal neurocircuits even in the absence of obesity. The perinatal period is critically important to the development of autonomic neural circuits responsible for energy homeostasis. Vagal neurocircuits are vital to the regulation of upper gastrointestinal functions, including satiety. Diet-induced obesity modulates the excitability and responsiveness of both peripheral vagal afferents and central vagal efferents but less information is available regarding the effects of diet per se on vagal neurocircuit functions. The aims of this study were to investigate whether perinatal exposure to a high fat diet (HFD) dysregulated dorsal motor nucleus of the vagus (DMV) neurones, prior to the development of obesity. Whole cell patch clamp recordings were made from gastric-projecting DMV neurones in thin brainstem slices from rats that were exposed to either a control diet or HFD from pregnancy day 13. Our data demonstrate that following perinatal HFD: (i) DMV neurones had decreased excitability and input resistance with a reduced ability to fire action potentials; (ii) the proportion of DMV neurones excited by cholecystokinin (CCK) was unaltered but the proportion of neurones in which CCK increased excitatory glutamatergic synaptic inputs was reduced; (iii) the tonic activation of presynaptic group II metabotropic glutamate receptors on inhibitory nerve terminals was attenuated, allowing modulation of GABAergic synaptic transmission; and (iv) the size and dendritic arborization of gastric-projecting DMV neurones was increased. These results suggest that perinatal HFD exposure compromises the excitability and responsiveness of gastric-projecting DMV neurones, even in the absence of obesity, suggesting that attenuation of vago-vagal reflex signalling may precede the development of obesity.

Baroreflex activation: from mechanisms to therapy for cardiovascular disease

Recent technical advances have led to the development of a medical device that can reliably activate the carotid baroreflex with an acceptable degree of safety. Because activation of the sympathetic nervous system plays an important role in the pathogenesis of hypertension and heart failure, the unique ability of this device to chronically suppress central sympathetic outflow in a controlled manner suggests potential value in the treatment of these conditions. This notion is supported by both clinical and experimental animal studies, and the major aim of this article is to elucidate the physiological mechanisms that account for the favorable effects of baroreflex activation therapy in patients with resistant hypertension and heart failure. Illumination of the neurohormonal, renal, and cardiac actions of baroreflex activation is likely to provide the means for better identification of those patients that are most likely to respond favorably to this device-based therapy.

The baroreceptor as a therapeutic target for heart failure

Sympathoactivation is a prominent feature of heart failure (HF). Its role in cardiac remodeling and arrhythmogenesis is well-recognized today, although incomplete understanding of autonomic mechanisms was a barrier to development of contemporary medical therapies. Despite widespread availability of drugs and devices, mortality and morbidity in HF remain unacceptably high. Recognition of an additional phenotype, HF with preserved ejection fraction (EF), poses additional challenges. New treatment options are required. Electrical modulation of the central nervous system with baroreflex activation therapy offers a new approach. Activation of this afferent pathway induces the central nervous system to rebalance autonomic modulation of the cardiovascular system. Results in animal models of HF demonstrating increased survival and beneficial cardiac remodeling recently led to a clinical feasibility study in HF with reduced EF wherein the clinical course of patients dramatically improved. Results in resistant hypertension patients further suggest potential for benefit in HF with preserved EF.

Heart rate and heart rate variability in resistant versus controlled hypertension and in true versus white-coat resistance

Sympathetic nervous system has an important role in resistant hypertension. Heart rate (HR) is a marker of sympathetic activity, but its association with resistant hypertension has not been assessed. We aimed to evaluate differences in HR values and variability between resistant and controlled patients and between true and white-coat resistant hypertensives (RHs). We compared office and ambulatory HR, nocturnal dip and s.d. in 14,627 RHs versus 11,951 controlled patients (on ⩽ 3 drugs) and in 8730 true (24 h blood pressure (BP) ⩾ 130 and/or 80 mm Hg) versus 4825 white-coat (24-h BP < 130/80 mm Hg) RHs. After adjusting for age, gender, body mass index, diabetes status and beta blocker use, HR values and variability were significantly elevated in resistant versus controlled patients and in true versus white-coat RHs. In logistic regression models, after adjustment for confounders, office HR (odds ratio for each increase in tertile: 1.337; 95% confidence interval: 1.287-1.388; P < 0.001), nocturnal dip (0.958; 0.918-0.999; P = 0.035) and night time s.d. (1.115; 1.057-1.177; P = 0.013) were all significantly associated with the presence of resistant hypertension. Moreover, night time HR (1.160; 1.065-1.265; P < 0.001), nocturnal dip (0.876; 0.830-0.925; P < 0.001) and 24-h s.d. (1.148; 1.092-1.207; P < 0.001) were all significantly associated with true resistant hypertension. In conclusion, both increased HR and variability are associated with resistant hypertension and with true resistance. These suggest the involvement of the sympathetic nervous system in the development of resistance to antihypertensive treatment.