Mechanisms determining sensitivity of baroreceptor afferents in health and disease

Independent factors influencing changes in baroreceptor sensitivity after carotid endarterectomy

OBJECTIVE

Carotid endarterectomy (CEA) is a well-established standard therapy for patients with symptomatic or asymptomatic high-grade carotid stenosis. The aim of carotid endarterectomy is to decrease the risk of stroke and avoid relevant functional loss. However, carotid endarterectomy is known to be associated with haemodynamic dysregulation. In this study we compared eversion CEA (E-CEA) and conventional CEA (C-CEA) regarding postoperative blood pressure values as well as preoperative and postoperative baroreceptor sensitivity in the first 7 days after surgery. The aim was to find possible factors influencing changes in baroreceptor sensitivity.

METHODS

Patients (111 patients were enrolled, of which 50 patients received C-CEA and 61 patients E-CEA) were prospectively enrolled in this study. For the measurement of baroreceptor sensitivity, a non-invasive Finometer measuring device from Finapres Medical System B.V. (Amsterdam, The Netherlands) was used. Measurements were performed one day before surgery (PRE), directly after surgery (F1), on day 1 (F2), day 2 (F3) and on day 7 (F4) postoperatively.

RESULTS

Postoperative blood pressure values were significantly higher in the E-CEA group on the day of surgery (F1) (p<0.001) and on day 1 (F2) (p<0.001). From day 2 (F3, F4) postoperatively, no significant difference was found between the two groups. The invasive blood pressure measurement in the postoperative recovery room showed significantly higher systolic blood pressure values in the E-CEA group (p=0.001). The need of acute antihypertensive therapy was significantly higher in the recovery room in the E-CEA group (p=0.020). With regard to changes in baroreceptor sensitivity, significantly lower baroreceptor sensitivity (BRS) values were recorded in the E-CEA group at 1 day (F2) postoperatively (p=0.005). The regression analysis showed that the applied surgical technique and the patients age were significant factors influencing changes in baroreceptor sensitivity.

CONCLUSIONS

In this study we could confirm higher blood pressure levels after E-CEA in the first two days after surgery. Additionally, we identified two factors possibly influencing baroreceptor sensitivity: surgical technique and age. Based on the data obtained in this study, haemodynamic dysregulation after CEA (E-CEA, C-CEA) is temporary and short-term. Already after the second postoperative day there was no significant difference between the E-CEA and E-CEA groups, this effect remained stable after 7 days.

Sex differences in Black Veterans with PTSD: women versus men have higher sympathetic activity, inflammation, and blunted cardiovagal baroreflex sensitivity

PURPOSE

Post-traumatic stress disorder (PTSD) is associated with greater risk of incident hypertension and cardiovascular disease (CVD). Inflammation and autonomic derangements are suggested as contributing mechanisms. Women and Black adults have higher CVD risk associated with stress; however, whether there is a sex difference in autonomic and inflammatory mechanisms among Black individuals with PTSD is not known. We hypothesized that Black women with PTSD have higher inflammation, sympathetic nervous system (SNS) activity and impaired baroreflex sensitivity (BRS).

METHODS

In 42 Black Veterans with PTSD (Women, N = 18 and Men, N = 24), we measured inflammatory biomarkers, continuous blood pressure (BP), heart rate (HR) and muscle sympathetic nerve activity (MSNA) at rest and during arterial BRS testing via the modified Oxford technique.

RESULTS

Groups were matched for age and body mass index (BMI). Resting BP was similar between groups, but HR was higher (76 ± 12 vs. 68 ± 9 beats/min, p = 0.021) in women compared to men. Although women had lower PTSD symptoms severity (57 ± 17 vs. 68 ± 12 a.u.), resting MSNA (27 ± 13 vs. 16 ± 5 bursts/min, p = 0.003) was higher in women compared to men, respectively. Likewise, cardiovagal BRS was blunted (p = 0.002) in women (7.6 ± 4.3 ms/mmHg) compared to men (15.5 ± 8.4 ms/mmHg) while sympathetic BRS was not different between groups (p = 0.381). Black women also had higher (p = 0.020) plasma levels of interleukin-2 (IL-2).

CONCLUSION

Black women with PTSD have higher resting HR and MSNA, greater impairment of cardiovagal BRS and possibly higher inflammation. These findings suggest a higher burden of autonomic and inflammatory derangements in Black women compared to Black men with PTSD.

Autonomic control of ventricular function in health and disease: current state of the art

PURPOSE

Cardiac autonomic dysfunction is one of the main pillars of cardiovascular pathophysiology. The purpose of this review is to provide an overview of the current state of the art on the pathological remodeling that occurs within the autonomic nervous system with cardiac injury and available neuromodulatory therapies for autonomic dysfunction in heart failure.

METHODS

Data from peer-reviewed publications on autonomic function in health and after cardiac injury are reviewed. The role of and evidence behind various neuromodulatory therapies both in preclinical investigation and in-use in clinical practice are summarized.

RESULTS

A harmonic interplay between the heart and the autonomic nervous system exists at multiple levels of the neuraxis. This interplay becomes disrupted in the setting of cardiovascular disease, resulting in pathological changes at multiple levels, from subcellular cardiac signaling of neurotransmitters to extra-cardiac, extra-thoracic remodeling. The subsequent detrimental cycle of sympathovagal imbalance, characterized by sympathoexcitation and parasympathetic withdrawal, predisposes to ventricular arrhythmias, progression of heart failure, and cardiac mortality. Knowledge on the etiology and pathophysiology of this condition has increased exponentially over the past few decades, resulting in a number of different neuromodulatory approaches. However, significant knowledge gaps in both sympathetic and parasympathetic interactions and causal factors that mediate progressive sympathoexcitation and parasympathetic dysfunction remain.

CONCLUSIONS

Although our understanding of autonomic imbalance in cardiovascular diseases has significantly increased, specific, pivotal mediators of this imbalance and the recognition and implementation of available autonomic parameters and neuromodulatory therapies are still lagging.

Age-associated reductions in cardiovagal baroreflex sensitivity are exaggerated in middle-aged and older men with low testosterone

Aging is associated with reductions in cardiovagal baroreflex sensitivity (cBRS), which increases cardiovascular disease risk. Preclinical data indicate that low testosterone reduces cBRS. We determined whether low testosterone is associated with reduced cBRS in healthy men.

METHODS

Twenty-six men categorized as young (N=6; age=31±4 years; testosterone=535±60 ng/dL), middle-aged/older with normal (N=10; aged 56±3 years; testosterone=493±85 ng/dL), or low (N=10; age=57±6 years; testosterone=262±31 ng/dL) testosterone underwent recordings of beat-by-beat blood pressure and R-R interval during rest and two Valsalva maneuvers, and measures of carotid artery compliance. IL-6, CRP, oxidized LDL cholesterol and TAS were measured.

RESULTS

Middle-aged/older men had lower cBRS compared to young men (17.0±6.5 ms/mmHg; p=0.028); middle-age/older men with low testosterone had lower cBRS (5.5±3.2 ms/mmHg; p=0.039) compared to age-matched men with normal testosterone (10.7±4.0 ms/mmHg). No differences existed among groups during Phase II of the Valsalva maneuver; middle-aged/older men with low testosterone had reduced cBRS (4.7±2.6 ms/mmHg) compared to both young (12.8±2.8ms/mmHg; p<0.001) and middle-aged/older men with normal testosterone (8.6±4.4ms/mmHg; p=0.046) during Phase IV of the Valsalva maneuver. There were no differences in oxidized LDL, (p=0.882) or TAS across groups (p=0.633). IL-6 was significantly higher in middle-aged/older men with low testosterone compared to the other groups (p<0.05 for all) and inversely correlated with cBRS (r=-0.594, p=0.007). Middle-aged/older men had reduced carotid artery compliance compared to young, regardless of testosterone status (p<0.001).

CONCLUSIONS

These observations indicate that low testosterone in middle-aged/older men may contribute to a reduction in cBRS; increased inflammation may also contribute to a reduction in cBRS.

Blood Pressure Regulation by the Carotid Sinus Nerve: Clinical Implications for Carotid Body Neuromodulation

Chronic carotid sinus nerve (CSN) electrical modulation through kilohertz frequency alternating current improves metabolic control in rat models of type 2 diabetes, underpinning the potential of bioelectronic modulation of the CSN as a therapeutic modality for metabolic diseases in humans. The CSN carries sensory information from the carotid bodies, peripheral chemoreceptor organs that respond to changes in blood biochemical modifications such as hypoxia, hypercapnia, acidosis, and hyperinsulinemia. In addition, the CSN also delivers information from carotid sinus baroreceptors—mechanoreceptor sensory neurons directly involved in the control of blood pressure—to the central nervous system. The interaction between these powerful reflex systems—chemoreflex and baroreflex—whose sensory receptors are in anatomical proximity, may be regarded as a drawback to the development of selective bioelectronic tools to modulate the CSN. Herein we aimed to disclose CSN influence on cardiovascular regulation, particularly under hypoxic conditions, and we tested the hypothesis that neuromodulation of the CSN, either by electrical stimuli or surgical means, does not significantly impact blood pressure. Experiments were performed in Wistar rats aged 10–12 weeks. No significant effects of acute hypoxia were observed in systolic or diastolic blood pressure or heart rate although there was a significant activation of the cardiac sympathetic nervous system. We conclude that chemoreceptor activation by hypoxia leads to an expected increase in sympathetic activity accompanied by compensatory regional mechanisms that assure blood flow to regional beds and maintenance of hemodynamic homeostasis. Upon surgical denervation or electrical block of the CSN, the increase in cardiac sympathetic nervous system activity in response to hypoxia was lost, and there were no significant changes in blood pressure in comparison to control animals. We conclude that the responses to hypoxia and vasomotor control short-term regulation of blood pressure are dissociated in terms of hypoxic response but integrated to generate an effector response to a given change in arterial pressure.

The influence of external stressors on physiological testing: Implication for return-to-play protocols

External stressors such as alcohol, caffeine, and vigorous exercise are known to alter cellular homeostasis, affecting the autonomic nervous system (ANS) and overall physiological function. However, little direct evidence exists quantifying the impact of these external stressors on physiological testing. We assessed the impact of the above-listed stressors on spontaneous baroreflex sensitivity (BRS), heart rate variability (HRV), heart rate asymmetry (HRA), and systolic blood pressure variability (BPV). Seventeen male university varsity American-style football athletes completed two identical assessments on separate days, once presenting with one or more stressors (recent intake of caffeine, alcohol, or exercise participation; contraindicated assessment) and another with no stressors present (repeat assessment). Both assessments were conducted within one week and at the same time of day. The testing protocol consisted of 5-min of rest followed by 5-min of a squat-stand maneuver (0.05 Hz). Continuous beat-to-beat blood pressure and electrocardiogram measurements were collected and allowed for calculations of BRS, HRV, HRA, and BPV. Significant decreases (p < 0.05) in HRV and HRA metrics (SDNN, SD2, SDNNd, SDNNa, SD2a, SD2d), HRV total power, and BRS-up sequence were found during the contraindicated assessment in comparison to the repeat assessment. When assessing those with exercise as their only stressor, high-frequency HRV and BRS-pooled were significantly decreased and increased, respectively, during the contraindicated assessment. Pre-season physiological baseline testing in sport is becoming increasingly prevalent and thus must consider external stressors to ascertain accurate and reliable data. This data confirms the need for stringent and standardized guidelines for pre-participation baseline physiological testing.

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External stressors (exercise participation, caffeine consumption, and alcohol consumption) decrease heart rate variability and the asymmetrical contribution of heart rate accelerations and decelerations (SDNN, SD2, Total Power, SDNNd, SDNNa, SD2d, and SD2a).

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External stressors decrease spontaneous baroreflex sensitivity up-sequence, but no significant changes were found regarding systolic blood pressure variability.

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The establishment of standardized pre-participation guidelines controlling for external stressors would increase the validity and reliability of physiological testing, improving the clinical utility of such data.

Neck Chamber Technique Revisited: Low-Noise Device Delivering Negative and Positive Pressure and Enabling Concomitant Carotid Artery Imaging With Ultrasonography

Background and Objectives: Recently, novel noiseless device for the assessment of baroreceptor function with the neck suction (NS) has been presented. In this study, we present another in-house approach to the variable-pressure neck chamber method. Our device offers further critical improvements. First, it enables delivery of negative (NS) as well as positive pressure (neck pressurizing, NP) in a noiseless manner. Second, we used small, 3D-printed cups positioned over the carotid sinuses instead of cumbersome neck collar to improve subject comfort and to test feasibility of tracking the pressure-induced changes in carotid artery with ultrasonography. Methods: Five healthy, non-smoking, normal-weight subjects aged 29 ± 3 years (mean ± SD) volunteered for the study. Heart rate (HR, bpm) and mean arterial pressure (MAP, mmHg) responses to short, 7-s long episodes of NS and NP were recorded. Each trial consisted of 12 episodes of variable-pressure: six episodes of NS (suction ranging between -10 and -80 mmHg) and six episodes of NP (pressure ranging between + 10 and + 80 mmHg). Carotid artery sonography was performed during the NS and NP in four subjects, on another occasion. Results: The variable-pressure episodes resulted consistently in the expected pattern of hemodynamic alterations: HR and MAP increases or decreases following the NP and NS, respectively, as evidenced by the coefficient of determination (R²) of ≥0.78 for the carotid-HR response curve (for all five participants) and the carotid-MAP response curve (for four out of five participants; the curve cannot be calculated for one subject). We found a linear, dose-dependent relation between the applied pressure and the systolic-diastolic difference in carotid artery diameter. Conclusion: The novel device enables noiseless stimulation and unloading of the carotid baroreceptors with the negative and positive pressure, respectively, applied on the subject's neck via small, asymmetric and one-side flattened, 3D-printed cups. The unique design of the cups enables concomitant visualizing of the carotid artery during the NS or NP administration, and thereby direct monitoring of the intensity of mechanical stimulus targeting the carotid baroreceptors.

Use of Mayer wave activity to demonstrate aberrant cardiovascular autonomic control following sports concussion injury

Dysregulation of cardiovascular autonomic control is gaining recognition as a prevailing consequence of concussion injury. Characterizing the presence of autonomic dysfunction in concussed persons is inconsistent and conventional metrics of autonomic function cannot differentiate the presence/absence of injury. Mayer wave (MW) activity originates through baroreflex adjustments to blood pressure (BP) oscillations that appear in the low‐frequency (LF: 0.04–0.15 Hz) band of the BP and heart rate (HR) power spectrum after a fast Fourier transform. We prospectively explored MW activity (∼0.1 Hz) in 19 concussed and 19 noninjured athletes for 5 min while seated at rest within 48 h and 1 week of injury. MW activity was derived from the LF band of continuous digital electrocardiogram and beat‐to‐beat BP signals (LFHR, LF‐SBP, MWHR, and MW‐SBP, respectively); a proportion between MWBP and MWHR was computed (cMW). At 48 h, the concussion group had a significantly lower MWBP and cMW than controls; these differences were gone by 1 week. MWHR, LFHR, and LF‐SBP were not different between groups at either visit. Attenuated sympathetic vasomotor tone was present and the central autonomic mechanisms regulating MW activity to the heart and peripheral vasculature became transiently discordant early after concussion with apparent resolution by 1 week.

Exaggerated Exercise Blood Pressure as a Marker of Baroreflex Dysfunction in Normotensive Metabolic Syndrome Patients

Introduction

Exaggerated blood pressure response to exercise (EEBP = SBP ≥ 190 mmHg for women and ≥210 mmHg for men) during cardiopulmonary exercise test (CPET) is a predictor of cardiovascular risk. Sympathetic hyperactivation and decreased baroreflex sensitivity (BRS) seem to be involved in the progression of metabolic syndrome (MetS) to cardiovascular disease.

Objective

To test the hypotheses: (1) MetS patients within normal clinical blood pressure (BP) may present EEBP response to maximal exercise and (2) increased muscle sympathetic nerve activity (MSNA) and reduced BRS are associated with this impairment.

Methods

We selected MetS (ATP III) patients with normal BP (MetS_NT, n = 27, 59.3% males, 46.1 ± 7.2 years) and a control group without MetS (C, n = 19, 48.4 ± 7.4 years). We evaluated BRS for increases (BRS+) and decreases (BRS−) in spontaneous BP and HR fluctuations, MSNA (microneurography), BP from ambulatory blood pressure monitoring (ABPM), and auscultatory BP during CPET.

Results

Normotensive MetS (MetS_NT) had higher body mass index and impairment in all MetS risk factors when compared to the C group. MetS_NT had higher peak systolic BP (SBP) (195 ± 17 vs. 177 ± 24 mmHg, P = 0.007) and diastolic BP (91 ± 11 vs. 79 ± 10 mmHg, P = 0.001) during CPET than C. Additionally, we found that MetS patients with normal BP had lower spontaneous BRS− (9.6 ± 3.3 vs. 12.2 ± 4.9 ms/mmHg, P = 0.044) and higher levels of MSNA (29 ± 6 vs. 18 ± 4 bursts/min, P < 0.001) compared to C. Interestingly, 10 out of 27 MetS_NT (37%) showed EEBP (MetS_NT+), whereas 2 out of 19 C (10.5%) presented (P = 0.044). The subgroup of MetS_NT with EEBP (MetS_NT+, n = 10) had similar MSNA (P = 0.437), but lower BRS+ (P = 0.039) and BRS− (P = 0.039) compared with the subgroup without EEBP (MetS_NT−, n = 17). Either office BP or BP from ABPM was similar between subgroups MetS_NT+ and MetS_NT−, regardless of EEBP response. In the MetS_NT+ subgroup, there was an association of peak SBP with BRS− (R = −0.70; P = 0.02), triglycerides with peak SBP during CPET (R = 0.66; P = 0.039), and of triglycerides with BRS− (R = 0.71; P = 0.022).

Conclusion

Normotensive MetS patients already presented higher peak systolic and diastolic BP during maximal exercise, in addition to sympathetic hyperactivation and decreased baroreflex sensitivity. The EEBP in MetS_NT with apparent well-controlled BP may indicate a potential depressed neural baroreflex function, predisposing these patients to increased cardiovascular risk.

Sensory characteristics according to chronic diseases and chronic pain in adults: cross-sectional study

Aim: To investigate somatosensory, gustative and olfactory characteristics of subjects according to their chronic diseases and the presence of chronic pain complaints. Materials & methods: A total of 254 chronic pain patients and 52 healthy subjects were evaluated with a clinical and sensory systematized evaluation. Statistical analysis consisted of Fisher's exact, Student's t-tests, Pearson's co-efficient and multivariate nonlinear/logistic regressions. Results: Patients had more chronic diseases (p < 0.001) than healthy subjects. Chronic pain was associated with vibratory hypoesthesia (p = 0.047) and sour hypergeusia (p = 0.001) and several chronic diseases correlated with sensory features. Hyposmia was strongly associated with chronic pain symptoms, chronic diseases and cardiovascular disease. Conclusion: The sensory findings observed suggest the need for further investigation about the overlap between the olfactory function, pain chronification, chronic diseases and cognitive impairment in these patients.

The autonomic nervous system in septic shock and its role as a future therapeutic target: a narrative review

The autonomic nervous system (ANS) regulates the cardiovascular system. A growing body of experimental and clinical evidence confirms significant dysfunction of this regulation during sepsis and septic shock. Clinical guidelines do not currently include any evaluation of ANS function during the resuscitation phase of septic shock despite the fact that the severity and persistence of ANS dysfunction are correlated with worse clinical outcomes. In the critical care setting, the clinical use of ANS-related hemodynamic indices is currently limited to preliminary investigations trying to predict and anticipate imminent clinical deterioration. In this review, we discuss the evidence supporting the concept that, in septic shock, restoration of ANS-mediated control of the cardiovascular system or alleviation of the clinical consequences induced by its dysfunction (e.g., excessive tachycardia, etc.), may be an important therapeutic goal, in combination with traditional resuscitation targets. Recent studies, which have used standard and advanced monitoring methods and mathematical models to investigate the ANS-mediated mechanisms of physiological regulation, have shown the feasibility and importance of monitoring ANS hemodynamic indices at the bedside, based on the acquisition of simple signals, such as heart rate and arterial blood pressure fluctuations. During the early phase of septic shock, experimental and/or clinical studies have shown the efficacy of negative-chronotropic agents (i.e., beta-blockers or ivabradine) in controlling persistent tachycardia despite adequate resuscitation. Central α-2 agonists have been shown to prevent peripheral adrenergic receptor desensitization by reducing catecholamine exposure. Whether these new therapeutic approaches can safely improve clinical outcomes remains to be confirmed in larger clinical trials. New technological solutions are now available to non-invasively modulate ANS outflow, such as transcutaneous vagal stimulation, with initial pre-clinical studies showing promising results and paving the way for ANS modulation to be considered as a new potential therapeutic target in patients with septic shock.

Sugar-sweetened soft drink consumption acutely decreases spontaneous baroreflex sensitivity and heart rate variability

In healthy humans, fructose-sweetened water consumption increases blood pressure variability (BPV) and decreases spontaneous cardiovagal baroreflex sensitivity (cBRS) and heart rate variability (HRV). However, whether consuming commercially available soft drinks containing high levels of fructose elicits similar responses is unknown. We hypothesized that high-fructose corn syrup (HFCS)-sweetened soft drink consumption increases BPV and decreases cBRS and HRV to a greater extent compared with artificially sweetened (diet) and sucrose-sweetened (sucrose) soft drinks and water. Twelve subjects completed four randomized, double-blinded trials in which they drank 500 mL of water or commercially available soft drinks matched for taste and caffeine content. We continuously measured beat-to-beat blood pressure (photoplethysmography) and R-R interval (ECG) before and 30 min after drink consumption during supine rest for 5 min during spontaneous and paced breathing. BPV was evaluated using standard deviation (SD), average real variability (ARV), and successive variation (SV) methods for systolic and diastolic blood pressure. cBRS was assessed using the sequence method. HRV was evaluated using the root mean square of successive differences (RMSSD) in R-R interval. There were no differences between conditions in the magnitude of change from baseline in SD, ARV, and SV (P ≥ 0.07). There were greater reductions in cBRS during spontaneous breathing in the HFCS (-3 ± 5 ms/mmHg) and sucrose (-3 ± 5 ms/mmHg) trials compared with the water trial (+1 ± 5 ms/mmHg, P < 0.03). During paced breathing, HFCS evoked greater reductions in RMSSD compared with water (-26 ± 34 vs. +2 ± 26 ms, P < 0.01). These findings suggest that sugar-sweetened soft drink consumption alters cBRS and HRV but not BPV.

Sex differences in post-traumatic stress disorder risk: autonomic control and inflammation

AIM

Over 7 million U.S. adults and about 20% of the military population have post-traumatic stress disorder (PTSD), a debilitating condition that is independently linked to a significantly greater risk of developing cardiovascular disease (CVD). Women have twice the probability of developing PTSD after experiencing a traumatic event compared to men. Existing literatures have reported higher inflammation and autonomic dysfunction including impaired baroreflex sensitivity, increased sympathetic reactivity and decreased parasympathetic activity in PTSD. However, most of these findings stem from studies conducted predominantly in males.

METHODS

We attempt in this narrative review to summarize the mixed literature available on sex differences in autonomic dysfunction and inflammation in PTSD, at rest and in response to stress in PTSD.

RESULTS

This review reveals that there is a paucity of research exploring autonomic function in females with PTSD. Recent studies have included female participants without probing for sex differences. A small number of studies have been conducted exclusively in women. Available data suggest that sympathetic nervous system output tends to be heightened, while parasympathetic activity and arterial baroreflex sensitivity appear more blunted in females with PTSD. Although few studies have investigated sex differences in inflammation in PTSD, data within females suggest chronic increases in inflammation with PTSD. This autonomic dysregulation and inflammation have also been described in males with PTSD.

CONCLUSION

In sum, given the inherent biological differences in CVD clinical presentation and characteristics between men and women, human and animal studies aiming at elucidating sex differences in the pathophysiology of PTSD are needed.

Association Between Heart Rate Variability and Decompression-Induced Physiological Stress

The purpose of this study was to analyze the correlation between decompression-related physiological stress markers, given by inflammatory processes and immune system activation and changes in Heart Rate Variability, evaluating whether Heart Rate Variability can be used to estimate the physiological stress caused by the exposure to hyperbaric environments and subsequent decompression. A total of 28 volunteers participated in the experimental protocol. Electrocardiograms were performed; blood samples were obtained for the quantification of red cells, hemoglobin, hematocrit, neutrophils, lymphocytes, platelets, aspartate transaminase (AST), alanine aminotransferase (ALT), and for immunophenotyping and microparticles (MP) research through Flow Cytometry, before and after each experimental protocol from each volunteer. Also, myeloperoxidase (MPO) expression and microparticles (MPs) deriving from platelets, neutrophils and endothelial cells were quantified. Negative associations between the standard deviation of normal-to-normal intervals (SDNN) in the time domain, the High Frequency in the frequency domain and the total number of circulating microparticles was observed (p-value = 0.03 and p-value = 0.02, respectively). The pre and post exposure ratio of variation in the number of circulating microparticles was negatively correlated with SDNN (p-value = 0.01). Additionally, a model based on the utilization of Radial Basis Function Neural Networks (RBF-NN) was created and was able to predict the SDNN ratio of variation based on the variation of specific inflammatory markers (RMSE = 0.06).

Endovascular baroreflex amplification for resistant hypertension: what you need to know

Endovascular baroreflex amplification is an alternative treatment strategy for patients with resistant hypertension. In endovascular baroreflex, the carotid baroreflex is activated by a MobiusHD^® device (MD) which has been implanted in the internal carotid artery. This review will discuss the MD technology and mechanism of action and promising results in the first-in-human prospective study involving the use of the MD in patients with resistant hypertension.

Symptom severity impacts sympathetic dysregulation and inflammation in post-traumatic stress disorder (PTSD)

Post-traumatic stress disorder (PTSD) is associated with a greater risk of incident hypertension and cardiovascular disease. Inflammation, impaired baroreflex sensitivity (BRS) decreased parasympathetic nervous system (PNS) and overactive sympathetic nervous system (SNS) activity are suggested as contributing mechanisms. Increasing severity of PTSD symptoms has been linked to greater cardiovascular risk; however, the impact of PTSD symptom severity on inflammation and autonomic control of blood pressure has not yet been explored. We hypothesized that increasing PTSD symptom severity is linked to higher inflammation, greater SNS activity, lower PNS reactivity and impaired BRS. Seventy Veterans participated in this study: 28 with severe PTSD ((Clinical Administered PTSD Scale (CAPS) > 60; S-PTSD), 16 with moderate PTSD (CAPS ≥ 45 ≤ 60; M-PTSD) and 26 Controls (CAPS < 45; NO-PTSD). We recorded continuous blood pressure (BP), heart rate (HR) via EKG, heart rate variability (HRV) markers reflecting PNS and muscle sympathetic nerve activity (MSNA) at rest, during arterial baroreflex sensitivity (BRS) testing via the modified Oxford technique, and during 3 min of mental stress via mental arithmetic. Blood samples were analyzed for 12 biomarkers of systemic and vascular inflammation. While BP was comparable between severity groups, HR tended to be higher (p = 0.055) in S-PTSD (76 ± 2 beats/min) than in Controls (67 ± 2 beats/min) but comparable to M-PTSD (70 ± 3 beats/min). There were no differences in resting HRV and MSNA between groups; however, cardiovagal BRS was blunted (p = 0.021) in S-PTSD (10 ± 1 ms/mmHg) compared to controls (16 ± 3 ms/mmHg) but comparable to M-PTSD (12 ± 2 ms/mmHg). Veterans in the S-PTSD group had a higher (p < 0.001) combined inflammatory score compared to both M-PTSD and NO-PTSD. Likewise, while mental stress induced similar SNS and cardiovascular responses between the groups, there was a greater reduction in HRV in S-PTSD compared to both M-PTSD and NO-PTSD. In summary, individuals with severe PTSD symptoms have higher inflammation, greater impairment of BRS, a trend towards higher resting HR and exaggerated PNS withdrawal at the onset of mental stress that may contribute to cardiovascular risk in severe PTSD.

Neuroinflammation in the NTS is associated with changes in cardiovascular reflexes during systemic inflammation

BACKGROUND

Lipopolysaccharide (LPS)-induced systemic inflammation (SI) is associated with neuroinflammation in the brain, hypotension, tachycardia, and multiple organs dysfunctions. Considering that during SI these important cardiovascular and inflammatory changes take place, we measured the sensitivity of the cardiovascular reflexes baroreflex, chemoreflex, and Bezold-Jarisch that are key regulators of hemodynamic function. We also evaluated neuroinflammation in the nucleus tractus solitarius (NTS), the first synaptic station that integrates peripheral signals arising from the cardiovascular and inflammatory status.

METHODS

We combined cardiovascular recordings, immunofluorescence, and assays of inflammatory markers in male Wistar rats that receive iv administration of LPS (1.5 or 2.5 mg kg-1) to investigate putative interactions of the neuroinflammation in the NTS and in the anteroventral preoptic region of the hypothalamus (AVPO) with the short-term regulation of blood pressure and heart rate.

RESULTS

LPS induced hypotension, tachycardia, autonomic disbalance, hypothermia followed by fever, and reduction in spontaneous baroreflex gain. On the other hand, during SI, the bradycardic component of Bezold-Jarisch and chemoreflex activation was increased. These changes were associated with a higher number of activated microglia and interleukin (IL)-1β levels in the NTS.

CONCLUSIONS

The present data are consistent with the notion that during SI and neuroinflammation in the NTS, rats have a reduced baroreflex gain, combined with an enhancement of the bradycardic component of Bezold-Jarisch and chemoreflex despite the important cardiovascular impairments (hypotension and tachycardia). These changes in the cardiac component of Bezold-Jarisch and chemoreflex may be beneficial during SI and indicate that the improvement of theses reflexes responsiveness though specific nerve stimulations may be useful in the management of sepsis.

GABAA receptor, KATP channel and L-type Ca2+ channel is associated with facilitation effect of H2S on the baroreceptor reflex in spontaneous hypertensive rats

BACKGROUND

We aimed to investigate whether the facilitating effect of H₂S on the baroreceptor reflex is associated with the GABA_A receptor, K_ATP channel and L-type Ca²⁺ channel pathway.

METHODS

Spontaneously hypertensive rats (SHRs) and Wistar Kyoto (WKY) rats were used to investigate the facilitating effect of H₂S on the baroreceptor reflex by perfusing the isolated carotid sinus. The mechanism by which H₂S facilitated the baroreceptor reflex was determined by using Bay K8644 (an agonist of calcium channels), glibenclamide (Gli, a K_ATP channel blocker), and picrotoxin (PIC, a blocker of γ-aminobutyric acid [GABA]_A receptor).

RESULTS

As compared with WKY rats, SHRs showed impaired baroreceptor reflex sensitivity, as demonstrated by a right and upward shift of the functional curve for the intrasinus pressure-arterial blood pressure relation. H₂S perfusion (25, 50, or 100 μmol/L) dose-dependently ameliorated the impaired sensitivity of the baroreceptor reflex. Bay K8644 (500 nmol/L), Gli (20 μmol/L) and PIC (50 μmol/L) all prevented H₂S ameliorating the impaired baroreceptor reflex.

CONCLUSIONS

H₂S facilitating the baroreceptor reflex might be associated with activating the GABA_A receptor, opening the K_ATP channel, and closing the L-type Ca²⁺ channel. These areas should provide new targets for preventing and treating hypertension.

Sympathoexcitation and impaired arterial baroreflex sensitivity are linked to vascular inflammation in individuals with elevated resting blood pressure

Elevated Resting Blood Pressure (ERBP) in the prehypertensive range is associated with increased risk of hypertension and cardiovascular disease, the mechanisms of which remain unclear. Prior studies have suggested that ERBP may be associated with overactivation and dysregulation of the sympathetic nervous system (SNS). We hypothesized that compared to normotensives (≤120/80 mmHg), ERBP (120/80-139/89 mmHg) has higher SNS activity, impaired arterial baroreflex sensitivity (BRS), and increased vascular inflammation. Twenty-nine participants were studied: 16 otherwise healthy individuals with ERBP (blood pressure (BP) 130 ± 2/85 ± 2 mmHg) and 13 matched normotensive controls (mean BP 114 ± 2/73 ± 2 mmHg). We measured muscle sympathetic nerve activity (MSNA), beat-to-beat BP, and continuous electrocardiogram at rest and during arterial BRS testing via the modified Oxford technique. Blood was analyzed for the following biomarkers of vascular inflammation: lipoprotein-associated phospholipase A2 (Lp-PLA2), E-selectin, and intercellular adhesion molecule 1 (ICAM-1). Resting MSNA burst frequency (22 ± 2 vs. 16 ± 2 bursts/min, P = 0.036) and burst incidence (36 ± 3 vs. 25 ± 3 bursts/100 heart beats, P = 0.025) were higher in ERBP compared to controls. Cardiovagal BRS was blunted in ERBP compared to controls (13 ± 2 vs. 20 ± 3 msec/mmHg, P = 0.032), while there was no difference in sympathetic BRS between groups. Lp-PLA2 (169 ± 8 vs. 142 ± 9 nmol/min/mL, P = 0.020) and E-selectin (6.89 ± 0.6 vs. 4.45 ± 0.51 ng/mL, P = 0.004) were higher in ERBP versus controls. E-selectin (r = 0.501, P = 0.011) and ICAM-1 (r = 0.481, P = 0.015) were positively correlated with MSNA, while E-selectin was negatively correlated with cardiovagal BRS (r = -0.427, P = 0.030). These findings demonstrate that individuals with ERBP have SNS overactivity and impaired arterial BRS that are linked to biomarkers of vascular inflammation.

Lower cardiovagal tone and baroreflex sensitivity associated with hepatic insulin resistance and promote cardiovascular disorders in Tibetan minipigs induced by a high fat and high cholesterol diet

AIMS

A long-term high-fat/cholesterol (HFC) diet leads to hepatic insulin resistance (IR), which is associated with autonomic dysfunction and cardiovascular diseases risk increasing. However, whether this occurs in Tibetan minipigs remains unknown. We tested that a long-term HFC diet caused hepatic IR and promote cardiovascular disorders in Tibetan minipigs, and are associated with the reduction of cardiovagal tone and baroreflex sensitivity (BRS).

METHODS

Male Tibetan minipigs were fed either a standard diet or a HFC diet, and were euthanized at 12 weeks. Thereafter, the minipigs were tested for biochemical blood indices, glucose tolerance, blood pressure, heart rate variability (HRV), BRS, and insulin receptor substrate (IRS)-associated gene and protein expression levels, as well as cardiac function.

RESULTS

HFC-fed minipigs developed IR by increasing body weight, total cholesterol, fasting blood glucose and insulin levels, and nonesterified fatty acid (NEFA) and high sensitive C-reactive protein (hs-CRP) levels, glucose intolerance. Increased adipose cell size, hepatic fat deposition, malondialdehyde (MDA) content and NEFA level, down-regulation of IRS1, IRS2, PI3K, Akt, p-Akt, Glut2 and PGC1ɑ expression concomitant with up-regulation of mTOR, GSK3β, TNF-ɑ, FOXO1, p-mTOR and p-p70S6K expression in the liver tissue, as well as hypertension and left ventricular diastolic dysfunction were observed in HFC-fed minipigs. HRV parameters and BRS values were further significantly reduced. Furthermore, multiple linear regression analysis showed that the development of hepatic IR toward cardiovascular disease was associated with low HFnu, RMSSD, BRS and LV -dp/dtmax, high NEFA, high hepatic TG content.

CONCLUSION

These data suggest that HFC-fed Tibetan minipigs develop hepatic IR and promote cardiovascular disorders, and are associated with lower cardiovagal tone and BRS.

Baroreceptor Sensitivity Predicts Functional Outcome and Complications after Acute Ischemic Stroke

Autonomic dysfunctions including impaired baroreflex sensitivity (BRS) can develop after acute ischemic stroke (AIS) and may predispose patients to subsequent cardiovascular adverse events and serve as potential indicators of long-term mortality. This study aimed to determine the potential short-term prognostic significance of BRS after AIS. All patients admitted to Kaohsiung Veterans General Hospital within 72 h after onset of first-ever AIS between April 2008 and December 2012 were enrolled. Autonomic evaluation with continuous 10-minute monitoring of beat-to-beat hemodynamic and intracranial parameters was performed within 1 week after stroke by using the Task Force Monitor and transcranial Doppler. The 176 enrolled AIS patients were divided into high-BRS and low-BRS groups. All but two enrolled patients (who died within 3 months after stroke) attended scheduled follow-ups. The high-BRS group had significantly lower National Institutes of Health Stroke Scale (NIHSS) scores at 1 and 2 weeks after stroke and at discharge; lower modified Rankin scale (mRS) scores 1, 3, 6, and 12 months after stroke; and lower rates of complications and stroke recurrence compared to the low-BRS group. This study provides novel evidence of the utility of BRS to independently predict outcomes after AIS. Furthermore, modifying BRS may hold potential in future applications as a novel therapeutic strategy for acute stroke.

Active information sampling varies across the cardiac cycle

Perception and cognition oscillate with fluctuating bodily states. For example, visual processing has been shown to change with alternating cardiac phases. Here, we study the heartbeat's role for active information sampling-testing whether humans implicitly act upon their environment so that relevant signals appear during preferred cardiac phases. During the encoding period of a visual memory experiment, participants clicked through a set of emotional pictures to memorize them for a later recognition test. By self-paced key press, they actively prompted the onset of short (100 ms) presented pictures. Simultaneously recorded electrocardiograms allowed us to analyze the self-initiated picture onsets relative to the heartbeat. We find that self-initiated picture onsets vary across the cardiac cycle, showing an increase during cardiac systole, while memory performance was not affected by the heartbeat. We conclude that active information sampling integrates heart-related signals, thereby extending previous findings on the association between body-brain interactions and behavior.

Cellular and Molecular Mechanisms Underlying Arterial Baroreceptor Remodeling in Cardiovascular Diseases and Diabetes

Clinical trials and animal experimental studies have demonstrated an association of arterial baroreflex impairment with the prognosis and mortality of cardiovascular diseases and diabetes. As a primary part of the arterial baroreflex arc, the pressure sensitivity of arterial baroreceptors is blunted and involved in arterial baroreflex dysfunction in cardiovascular diseases and diabetes. Changes in the arterial vascular walls, mechanosensitive ion channels, and voltage-gated ion channels contribute to the attenuation of arterial baroreceptor sensitivity. Some endogenous substances (such as angiotensin II and superoxide anion) can modulate these morphological and functional alterations through intracellular signaling pathways in impaired arterial baroreceptors. Arterial baroreceptors can be considered as a potential therapeutic target to improve the prognosis of patients with cardiovascular diseases and diabetes.

Endovascular Baroreflex Amplification for Resistant Hypertension

Purpose of review

Most hypertension devices have been designed to interrupt or modify the sympathetic nervous system, which seems to be unbalanced in hypertension. Carotid baroreceptors play a pivotal role in maintaining adrenergic balance via a direct feedback interface and would be an exceptional target for intervention. The purpose of this review is to define the role of the baroreceptor in hypertension, to examine device-based therapies targeting the baroreflex and to explore future promises of endovascular baroreflex amplification (EBA).

Recent findings

In the last two decades, two therapeutic strategies targeting the carotid baroreceptor have evolved: baroreflex activation therapy (BAT) and EBA. Both therapies enhance baroreceptor activity, either directly by electrical stimulation or indirectly by changing the geometric shape of the carotid sinus and increasing pulsatile wall strain.

Summary

By showing a significant, sympathetic inhibition-mediated effect on blood pressure, BAT has laid the foundation for baroreflex-targeting therapies for resistant hypertension. EBA is a less invasive therapy with promising first-in-man study results. Ongoing randomized sham-controlled trials are needed to better understand efficacy, durability, and long-term safety and define phenotypes that may most benefit from this treatment.

Electronic supplementary material

The online version of this article (10.1007/s11906-018-0840-8) contains supplementary material, which is available to authorized users.

Possible Breathing Influences on the Control of Arterial Pressure After Sino-aortic Denervation in Rats

PURPOSE OF REVIEW

Surgical removal of the baroreceptor afferents [sino-aortic denervation (SAD)] leads to a lack of inhibitory feedback to sympathetic outflow, which in turn is expected to result in a large increase in mean arterial pressure (MAP). However, few days after surgery, the sympathetic nerve activity (SNA) and MAP of SAD rats return to a range similar to that observed in control rats. In this review, we present experimental evidence suggesting that breathing contributes to control of SNA and MAP following SAD.The purpose of this review was to discuss studies exploring SNA and MAP regulation in SAD rats, highlighting the possible role of breathing in the neural mechanisms of this modulation of SNA.

RECENT FINDINGS

Recent studies show that baroreceptor afferent stimulation or removal (SAD) results in changes in the respiratory pattern. Changes in the neural respiratory network and in the respiratory pattern must be considered among mechanisms involved in the modulation of the MAP after SAD.

Cardiac and peripheral autonomic control in restrictive cardiomyopathy

AIMS

Autonomic dysfunction determines the advance of dilated cardiomyopathy (DCM) and is related to poor outcomes. However, this autonomic imbalance is unknown in patients with restrictive cardiomyopathy (RCM) even though they have similar symptoms and poor quality of life as DCM patients have. The aim of this study was to evaluate if autonomic and neurovascular controls were altered in RCM patients.

METHODS AND RESULTS

Fifteen RCM patients, 10 DCM patients, and 10 healthy subjects were evaluated. Heart rate and blood pressure (BP) were recorded. Peripheral sympathetic activity [muscle sympathetic nerve activity (MSNA)] by microneurography and cardiac sympathetic activity by power spectrum analysis of heart rate variability. Spontaneous baroreflex sensitivity (BRS) was evaluated by the sequence method and forearm blood flow by venous occlusion plethysmography. Both cardiomyopathy groups had higher MSNA frequency (P < 0.001) and MSNA incidence (P < 0.001), higher cardiac sympathovagal balance (P < 0.02), reduced BRS for increase (P = 0.002) and for decrease in BP (P = 0.002), and lower forearm blood flow (P < 0.001) compared with healthy subjects. We found an inverse correlation between BRS for increase and decrease in BP and peripheral sympathetic activity (r = -0.609, P = 0.001 and r = -0.648, P < 0.001, respectively) and between BRS for increase and decrease in BP and cardiac sympathetic activity (r = -0.503, P = 0.03 and r = -0.487, P = 0.04, respectively).

CONCLUSIONS

The RCM patients had cardiac and peripheral autonomic dysfunctions associated with peripheral vasoconstriction. Nonetheless, the presence of normal ejection fraction underestimates the evolution of the disease and makes clinical treatment difficult. These alterations could lead to a similar cardiovascular risk as that observed in DCM patients.

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.

Endogenous Hydrogen Sulfide Enhances Carotid Sinus Baroreceptor Sensitivity by Activating the Transient Receptor Potential Cation Channel Subfamily V Member 1 (TRPV1) Channel

Background

We aimed to investigate the regulatory effects of hydrogen sulfide (H₂S) on carotid sinus baroreceptor sensitivity and its mechanisms.

Methods and Results

Male Wistar‐Kyoto rats and spontaneously hypertensive rats (SHRs) were used in the experiment and were given an H₂S donor or a cystathionine‐β‐synthase inhibitor, hydroxylamine, for 8 weeks. Systolic blood pressure and the cystathionine‐β‐synthase/H₂S pathway in carotid sinus were detected. Carotid sinus baroreceptor sensitivity and the functional curve of the carotid baroreceptor were analyzed using the isolated carotid sinus perfusion technique. Effects of H₂S on transient receptor potential cation channel subfamily V member 1 (TRPV1) expression and S‐sulfhydration were detected. In SHRs, systolic blood pressure was markedly increased, but the cystathionine‐β‐synthase/H₂S pathway in the carotid sinus was downregulated in comparison to that of Wistar‐Kyoto rats. Carotid sinus baroreceptor sensitivity in SHRs was reduced, demonstrated by the right and upward shift of the functional curve of the carotid baroreceptor. Meanwhile, the downregulation of TRPV1 protein was demonstrated in the carotid sinus; however, H₂S reduced systolic blood pressure but enhanced carotid sinus baroreceptor sensitivity in SHRs, along with TRPV1 upregulation in the carotid sinus. In contrast, hydroxylamine significantly increased the systolic blood pressure of Wistar‐Kyoto rats, along with decreased carotid sinus baroreceptor sensitivity and reduced TRPV1 protein expression in the carotid sinus. Furthermore, H₂S‐induced enhancement of carotid sinus baroreceptor sensitivity of SHRs could be amplified by capsaicin but reduced by capsazepine. Moreover, H₂S facilitated S‐sulfhydration of TRPV1 protein in the carotid sinus of SHRs and Wistar‐Kyoto rats.

Conclusions

H₂S regulated blood pressure via an increase in TRPV1 protein expression and its activity to enhance carotid sinus baroreceptor sensitivity.

Neurocardiology: Structure-Based Function

Cardiac control is mediated via a series of reflex control networks involving somata in the (i) intrinsic cardiac ganglia (heart), (ii) intrathoracic extracardiac ganglia (stellate, middle cervical), (iii) superior cervical ganglia, (iv) spinal cord, (v) brainstem, and (vi) higher centers. Each of these processing centers contains afferent, efferent, and local circuit neurons, which interact locally and in an interdependent fashion with the other levels to coordinate regional cardiac electrical and mechanical indices on a beat-to-beat basis. This control system is optimized to respond to normal physiological stressors (standing, exercise, and temperature); however, it can be catastrophically disrupted by pathological events such as myocardial ischemia. In fact, it is now recognized that autonomic dysregulation is central to the evolution of heart failure and arrhythmias. Autonomic regulation therapy is an emerging modality in the management of acute and chronic cardiac pathologies. Neuromodulation-based approaches that target select nexus points of this hierarchy for cardiac control offer unique opportunities to positively affect therapeutic outcomes via improved efficacy of cardiovascular reflex control. As such, understanding the anatomical and physiological basis for such control is necessary to implement effectively novel neuromodulation therapies. © 2016 American Physiological Society. Compr Physiol 6:1635-1653, 2016.

Translational neurocardiology: preclinical models and cardioneural integrative aspects

Neuronal elements distributed throughout the cardiac nervous system, from the level of the insular cortex to the intrinsic cardiac nervous system, are in constant communication with one another to ensure that cardiac output matches the dynamic process of regional blood flow demand. Neural elements in their various 'levels' become differentially recruited in the transduction of sensory inputs arising from the heart, major vessels, other visceral organs and somatic structures to optimize neuronal coordination of regional cardiac function. This White Paper will review the relevant aspects of the structural and functional organization for autonomic control of the heart in normal conditions, how these systems remodel/adapt during cardiac disease, and finally how such knowledge can be leveraged in the evolving realm of autonomic regulation therapy for cardiac therapeutics.

Neural Control of Vascular Function in Skeletal Muscle

The sympathetic nervous system represents a fundamental homeostatic system that exerts considerable control over blood pressure and the distribution of blood flow. This process has been referred to as neurovascular control. Overall, the concept of neurovascular control includes the following elements: efferent postganglionic sympathetic nerve activity, neurotransmitter release, and the end organ response. Each of these elements reflects multiple levels of control that, in turn, affect complex patterns of change in vascular contractile state. Primarily, this review discusses several of these control layers that combine to produce the integrative physiology of reflex vascular control observed in skeletal muscle. Beginning with three reflexes that provide somewhat dissimilar vascular patterns of response despite similar changes in efferent sympathetic nerve activity, namely, the baroreflex, chemoreflex, and muscle metaboreflex, the article discusses the anatomical and physiological bases of postganglionic sympathetic discharge patterns and recruitment, neurotransmitter release and management, and details of regional variations of receptor density and responses within the microvascular bed. Challenges are addressed regarding the fundamentals of measurement and how conclusions from one response or vascular segment should not be used as an indication of neurovascular control as a generalized physiological dogma. Whereas the bulk of the article focuses on the vasoconstrictor function of sympathetic neurovascular integration, attention is also given to the issues of sympathetic vasodilation as well as the impact of chronic changes in sympathetic activation and innervation on vascular health. © 2016 American Physiological Society.

Decreased baroreflex sensitivity is linked to sympathovagal imbalance, body fat mass and altered cardiometabolic profile in pre-obesity and obesity

OBJECTIVE

Though decreased baroreflex sensitivity (BRS), the predictor of cardiac morbidities and mortality has been reported in obesity, the mechanisms and metabolic biomarkers influencing BRS have not been studied. We aimed to assess the difference in cardiovascular (CV) risk profile in pre-obesity and obesity, and the contribution of body composition and cardiometabolic factors to CV risks in these two conditions.

METHODS

Obesity indices, body composition, blood pressure variability and autonomic function test parameters were recorded in 223 subjects divided into controls (n=72), pre-obese (n=77) and obese (n=74) groups. Insulin resistance (HOMA-IR), atherogenic index (AI), leptin, adiponectin, inflammatory and oxidative stress parameters were measured. Association and independent contribution of altered cardiometabolic parameters with BRS were performed by Pearson's correlation and multiple regression analysis, respectively.

RESULTS

BRS was significantly decreased in pre-obese and obese group compared to controls. Sympathovagal imbalance (SVI) in the form of increased sympathetic and decreased parasympathetic cardiac drives was observed in pre-obesity and obesity. There was significant difference in general markers of obesity (body mass index, and waist-to-hip ratio), between pre-obese and obese group, however no such difference was observed in body composition and cardiometabolic parameters between the two groups. AI, high sensitive C-reactive protein (hs-CRP) and ratio of basal metabolism to body fat (BM/BF) in pre-obese group, and AI, HOMA-IR, leptin, adiponectin, ratio of basal metabolism to body weight (BM/BW), BM/BF, inflammatory and oxidative stress markers in obese group had independent contribution to BRS. Among these metabolic biomarkers, BRS had maximum association with leptin (β=0.532, p=0.000) in the obese group and hs-CRP (β=0.445, p=0.022) in the pre-obese group.

CONCLUSIONS

The present study demonstrates decreased BRS, an important marker of increased CV risk in pre-obesity and obesity. The intensity of cardiometabolic derangements and CV risk was comparable between pre-obese and obese subjects. BM/BF ratio appears to be a better marker of metabolic activity in pre-obesity and obesity. SVI and increased basal metabolism appear to be the physiological link between metabolic derangements and CV risks in both pre-obesity and obesity.

Mechanotransduction of trigeminal ganglion neurons innervating inner walls of rat anterior eye chambers

To address mechanoreceptive roles of trigeminal ganglion (TG) nerve endings in the inner walls of rat anterior eye chambers, we investigated the mechanotransduction process and mechanosensitive (MS) channel on somata of TG neurons innervating this area in vitro. Rat TG neurons innervating inner walls of anterior chambers were labeled by anterior chamber injection of 1,1'-dilinoleyl-3,3,3',3'-tetramethylindocarbocyanine, 4-chlorobenzenesulfonate (FAST DiI). The neuronal cell bodies were voltage clamped using a whole cell patch-clamp technique, while it was deformed by ejection of bath solution to verify mechanotransduction. Immunofluorescence staining was performed on sections of TG ganglia to determine the specific MS channel proteins. Mechanical stimuli induced MS currents in 55 out of 96 FAST DiI-labeled TG neurons. The MS currents exhibited mechanical intensity-dependent and clamp voltage-dependent characteristics. Mechanical stimulation further enhanced the membrane potential and increased the frequency of action potentials. Transient receptor potential ankyrin 1 (TRPA1), TRP vanilloid 4 (TRPV4), acid-sensing ion channel (ASIC) 2 and ASIC3 channel proteins were expressed in FAST DiI-labeled TG neurons. The inhibitory effect of HC-030031, a specific inhibitor of TRPA1, on MS currents demonstrated that TRPA1 was an essential MS channel protein. Taken together, our results show that mechanical stimuli induce MS currents via MS channels such as TRPA1 to trigger mechanotransduction in TG neurons innervating inner walls of anterior chambers. Our results indicate the existence of mechanoreceptive TG nerve endings in inner walls of anterior chambers. Whether the mechanoreceptive TG nerve endings play a role in intraocular pressure sensation warrants further investigation.

Sensory nerves in lung and airways

Sensory nerves innervating the lung and airways play an important role in regulating various cardiopulmonary functions and maintaining homeostasis under both healthy and disease conditions. Their activities conducted by both vagal and sympathetic afferents are also responsible for eliciting important defense reflexes that protect the lung and body from potential health-hazardous effects of airborne particulates and chemical irritants. This article reviews the morphology, transduction properties, reflex functions, and respiratory sensations of these receptors, focusing primarily on recent findings derived from using new technologies such as neural immunochemistry, isolated airway-nerve preparation, cultured airway neurons, patch-clamp electrophysiology, transgenic mice, and other cellular and molecular approaches. Studies of the signal transduction of mechanosensitive afferents have revealed a new concept of sensory unit and cellular mechanism of activation, and identified additional types of sensory receptors in the lung. Chemosensitive properties of these lung afferents are further characterized by the expression of specific ligand-gated ion channels on nerve terminals, ganglion origin, and responses to the action of various inflammatory cells, mediators, and cytokines during acute and chronic airway inflammation and injuries. Increasing interest and extensive investigations have been focused on uncovering the mechanisms underlying hypersensitivity of these airway afferents, and their role in the manifestation of various symptoms under pathophysiological conditions. Several important and challenging questions regarding these sensory nerves are discussed. Searching for these answers will be a critical step in developing the translational research and effective treatments of airway diseases.

The repeated sit-to-stand maneuver is a superior method for cardiac baroreflex assessment: a comparison with the modified Oxford method and Valsalva maneuver

Baroreflex assessment has diagnostic and prognostic utility in the clinical and research environments, and there is a need for a reliable, simple, noninvasive method of assessment. The repeated sit-to-stand method induces oscillatory changes in blood pressure (BP) at a desired frequency and is suitable for assessing dynamic baroreflex sensitivity (BRS). However, little is known about the reliability of this method and its ability to discern fundamental properties of the baroreflex. In this study we sought to: 1) evaluate the reliability of the sit-to-stand method for assessing BRS and compare its performance against two established methods (Oxford method and Valsalva maneuver), and 2) examine whether the frequency of the sit-to-stand method influences hysteresis. Sixteen healthy participants underwent three trials of each method. For the sit-to-stand method, which was performed at 0.1 and 0.05 Hz, BRS was quantified as an integrated response (BRSINT) and in response to falling and rising BP (BRSDOWN and BRSUP, respectively). Test retest reliability was assessed using the intraclass correlation coefficient (ICC). Irrespective of frequency, the ICC for BRSINT during the sit-to-stand method was ≥0.88. The ICC for a rising BP evoked by phenylephrine (PEGAIN) in the Oxford method was 0.78 and ≤0.5 for the remaining measures. During the sit-to-stand method, hysteresis was apparent in all participants at 0.1 Hz but was absent at 0.05 Hz. These findings indicate the sit-to-stand method is a statistically reliable BRS assessment tool and suitable for the examination of baroreflex hysteresis. Using this approach we showed that baroreflex hysteresis is a frequency-dependent phenomenon.

Mindfulness meditation lowers muscle sympathetic nerve activity and blood pressure in African-American males with chronic kidney disease

Mindfulness meditation (MM) is a stress-reduction technique that may have real biological effects on hemodynamics but has never previously been tested in chronic kidney disease (CKD) patients. In addition, the mechanisms underlying the potential blood pressure (BP)-lowering effects of MM are unknown. We sought to determine whether MM acutely lowers BP in CKD patients, and whether these hemodynamic changes are mediated by a reduction in sympathetic nerve activity. In 15 hypertensive African-American (AA) males with CKD, we conducted a randomized, crossover study in which participants underwent 14 min of MM or 14 min of BP education (control intervention) during two separate random-order study visits. Muscle sympathetic nerve activity (MSNA), beat-to-beat arterial BP, heart rate (HR), and respiratory rate (RR) were continuously measured at baseline and during each intervention. A subset had a third study visit to undergo controlled breathing (CB) to determine whether a reduction in RR alone was sufficient in exacting hemodynamic changes. We observed a significantly greater reduction in systolic BP, diastolic BP, mean arterial pressure, and HR, as well as a significantly greater reduction in MSNA, during MM compared with the control intervention. Participants had a significantly lower RR during MM; however, in contrast to MM, CB alone did not reduce BP, HR, or MSNA. MM acutely lowers BP and HR in AA males with hypertensive CKD, and these hemodynamic effects may be mediated by a reduction in sympathetic nerve activity. RR is significantly lower during MM, but CB alone without concomitant meditation does not acutely alter hemodynamics or sympathetic activity in CKD.

The link between stress disorders and autonomic dysfunction in muscular dystrophy

Muscular dystrophy is a progressive disease of muscle weakness, muscle atrophy and cardiac dysfunction. Patients afflicted with muscular dystrophy exhibit autonomic dysfunction along with cognitive impairment, severe depression, sadness, and anxiety. Although the psychological aspects of cardiovascular disorders and stress disorders are well known, the physiological mechanism underlying this relationship is not well understood, particularly in muscular dystrophy. Therefore, the goal of this perspective is to highlight the importance of autonomic dysfunction and psychological stress disorders in the pathogenesis of muscular dystrophy. This article will for the first time—(i) outline autonomic mechanisms that are common to both psychological stress and cardiovascular disorders in muscular dystrophy; (ii) propose therapies that would improve behavioral and autonomic functions in muscular dystrophy.

Infusion of Escherichia coli lipopolysaccharide toxin in rats produces an early and severe impairment of baroreflex function in absence of blood pressure changes

The assessment of baroreflex function since the first appearance of endotoxemia is important because the arterial baroreflex should exert a protective role during sepsis. Nevertheless, contrasting results were previously reported. This could be due to the hemodynamic instability characterizing this condition that may per se interfere with reflex cardiovascular adjustments. The aim of our study was therefore to study the baroreflex function (a) since the very beginning of infusion of Escherichia coli lipopolysaccharide (LPS) toxin and (b) in absence of the unloading effect produced by a decrease in blood pressure. Lipopolysaccharide was infused in 10 rats for 20 min at the infusion rate of 0.05 mg · kg · min. Blood pressure was continuously measured before, during, and after infusion, and the baroreflex function was evaluated analyzing spontaneous fluctuations of systolic blood pressure and pulse interval by the sequence and transfer-function techniques. Plasma concentrations of inflammatory (interleukin 6, tumor necrosis factor α) and anti-inflammatory (interleukin 10) cytokines were measured in other eight rats, similarly instrumented, four of which receiving the same LPS infusion. We found that blood pressure levels did not change with the infusion of LPS, whereas inflammatory cytokines increased significantly. The baroreflex sensitivity was significantly reduced 10 min after the beginning of LPS infusion, reached values about half those at baseline within 15 min after the start of infusion, and remained significantly low after the end of infusion. In conclusion, we documented that septic shock inducing LPS infusion is responsible for a very rapid impairment of the baroreflex function, independent from the level of blood pressure.

Sex differences in cardiac autonomic regulation and in repolarisation electrocardiography

The review summarises the present knowledge on the sex differences in cardiac autonomic regulations and in related aspects of electrocardiography with particular attention to myocardial repolarisation. Although some of the sex differences are far from fully established, multitude of observations show consistent differences between women and men. Despite more pronounced parasympathetic cardiac regulation, women have higher resting heart rate and lower baroreflex sensitivity. Of the electrocardiographic phenomena, women have longer QT interval duration, repolarisation sequence more synchronised with the inverse of the depolarisation sequence, and likely increased regional heterogeneity of myocardial repolarisation. Studies investigating the relationship of these sex disparities to hormonal differences led frequently to conflicting results. Although sex hormones seem to play a key role by influencing both autonomic tone and electrophysiological properties at the cellular level, neither the truly relevant hormones nor their detailed actions are known. Physiologic usefulness of the described sex differences is also unknown. The review suggests that new studies are needed to advance the understanding of the physiologic mechanisms responsible for these inequalities between women and men and provides key methodological suggestions that need to be followed in future research.

Cardiac-autonomic imbalance and baroreflex dysfunction in the renovascular Angiotensin-dependent hypertensive mouse

Mouse models provide powerful tools for studying the mechanisms underlying the dysfunction of the autonomic reflex control of cardiovascular function and those involved in cardiovascular diseases. The established murine model of two-kidney, one-clip (2K1C) angiotensin II-dependent hypertension represents a useful tool for studying the neural control of cardiovascular function. In this paper, we discuss the main contributions from our laboratory and others regarding cardiac-autonomic imbalance and baroreflex dysfunction. We show recent data from the angiotensin-dependent hypertensive mouse demonstrating DNA damage and oxidative stress using the comet assay and flow cytometry, respectively. Finally, we highlight the relationships between angiotensin and peripheral and central nervous system areas of cardiovascular control and oxidative stress in the 2K1C hypertensive mouse.