Baroreceptor reflex function

Gender Difference in Orthostatic Vascular Stiffness Increase in Young Subjects

Background/Objectives: Early detection of increased vascular stiffness in young populations may facilitate the development of more effective strategies for the primary prevention of arterial hypertension and other age-related cardiovascular diseases. To examine gender differences in orthostatic increases in vascular stiffness during the head-up tilt test (HUTT), standardized by hydrostatic column height. Materials and Methods: A total of 133 healthy adults aged 18-20 years (93 females and 40 males) were evaluated. Blood pressure and pulse wave velocity at the brachial-ankle artery site (baPWV) were measured using an ABI system 100 PWV multichannel sphygmomanometer. Orthostatic changes in arterial stiffness were assessed during a head-up tilt test (HUTT) using the Luanda protocol, which standardizes hydrostatic column height. The functional reserve coefficient (FRC) of orthostatic circulatory regulation was introduced as a measure of adaptive capacity: FRC = ΔbaPWV/baPWVb. This coefficient accounts for both structural (baPWVb) and functional (ΔbaPWV = baPWVt - baPWVb) components influencing cardiovascular system adaptation, which exhibit multidirectional changes with age. Results: Baseline baPWV (baPWVb) values in the horizontal position showed no significant differences between genders and were within normal age ranges. However, baPWV values in the upright HUTT position (baPWVt) were significantly higher in men (p = 0.0007). Dynamic biomarkers of vascular reserve, including ΔbaPWV and FRC, were also significantly elevated in men (p = 0.0009 and p = 0.0064, respectively). Conclusions: While baseline baPWVb values were comparable between genders, dynamic biomarkers of vascular reserve, such as ΔbaPWV and FRC, were significantly higher in men. Prospective studies are needed to establish optimal reference values for these dynamic biomarkers, enabling the assessment of individual trends in vascular aging and evaluating the effects of treatment, lifestyle modifications, and other preventive measures on vascular health.

Evaluation of the effect of carotid sinus blockade on hemodynamic stability in carotid surgery: A retrospective study

This study assesses the effect of carotid sinus blockade applied with a local anesthetic on hemodynamic parameters during carotid endarterectomy (CEA) operations performed under general anesthesia. The medical records of patients who underwent CEA under general anesthesia between January 2020 and December 2022, were retrospectively reviewed. It was recorded whether the patients received carotid sinus block with 2 mL of 2% prilocaine. Intraoperative and 48-hour postoperative hemodynamic data were examined in the patients included in the study. A total of 129 patients were evaluated in the study, with 70 patients who received carotid sinus blockade (Group I) and 59 patients who did not receive blockade (Group II) during CEA. The comparison of heart rate variability immediately before clamping, immediately after clamping, and at 5, 10, and 20 minutes post-clamping indicated a significantly greater reduction in Group II compared to Group I (P < .05). In the postoperative period, the total dose of glyceryl trinitrate administered was 40.8 ± 31.9 mg in Group I and 53 ± 17.2 mg in Group II, showing a statistically significant difference (P = .001). Additionally, blood pressure measurements during this period were significantly higher in Group II than in Group I (P < .05). While the application of a local anesthetic during CEA appears to provide better intraoperative heart rate and postoperative blood pressure control, attributing these results solely to local anesthesia may not be entirely accurate. Hemodynamic instability observed during and after CEA is influenced by various factors.

The Effect of Anesthetic Regimen on Bone Cement Implantation Syndrome in Cemented Hemiarthroplasty for Hip Fracture

INTRODUCTION

Bone cement implantation syndrome (BCIS) is characterized by hypoxia, hypotension, and cardiovascular compromise during cementation in arthroplasty cases. This study examines the association between anesthetic regimen and risk of BCIS in cemented hemiarthroplasty for hip fractures. We hypothesized that neuraxial anesthesia would be associated with markedly lower BCIS incidence compared with general anesthesia alone or in combination with regional anesthesia.

METHODS

This retrospective cohort study included patients aged 50 years or older who underwent cemented hemiarthroplasty for hip fractures at a single institution between January 2017 and December 2022. Patient demographics, comorbidities, procedural factors, anesthetic characteristics including anesthetic regimen (general, general plus regional, or neuraxial), cement timing, BCIS development, postoperative complications, and mortality data were extracted. BCIS was identified by changes in postcementation vitals and its severity classified according to previously established criteria. Univariate statistical analyses and multivariate logistic regression were conducted.

RESULTS

Of the 137 included patients, 58 (43%) developed BCIS. No notable differences were observed in demographics, comorbidities, or procedural characteristics between patients who developed BCIS and those who did not. However, anesthetic regimen was markedly correlated with BCIS development, with general anesthesia demonstrating the highest rates (26/43, 55%), followed by general plus regional anesthesia (26/58, 45%) and neuraxial anesthesia (6/32, 19%; P = 0.005). Compared with neuraxial anesthesia, general anesthesia and general plus regional anesthesia conferred 6.8 and 5.5 times greater odds of developing BCIS, respectively. The development of BCIS was associated with significantly higher rates of postoperative hypoxia ( P = 0.04) and unplanned prolonged intubation ( P = 0.04).

CONCLUSION

BCIS was highly prevalent among patients undergoing cemented hemiarthroplasty for hip fractures. The anesthetic regimen was the only variable markedly associated with BCIS development and is a potentially modifiable risk factor.

LEVEL OF EVIDENCE

III.

Optogenetic modulation of cardiac autonomic nervous system

The following is a narrative review of the fundamentals of optogenetics. It focuses on the advantages and constraints of manipulating the autonomic nervous system by modifying the pathophysiological characteristics that arise in different diseases. Although the use of this technique is currently experimental, we will discuss improvements that have been implemented and identify the necessary measures for potential preclinical translation in the control of the cardiac autonomic nervous system.

The Importance of Orthostatic Increase in Pulse Wave Velocity in the Diagnosis of Early Vascular Aging

Background/Objectives: Vascular aging can be assessed by arterial stiffness measured through pulse wave velocity (PWV). Increased PWV predicts arterial hypertension, cardiovascular events and all-cause mortality. Detection of early signs of vascular aging remains an unmet problem. To search for the most sensitive markers for the early increase in vascular stiffness in a healthy population. Methods: One-hundred and twenty healthy subjects were divided in three equal age groups: <30 years, 30-45 years and >45 years. Head-up tilt test (HUTT) protocol was applied, providing a standardized hydrostatic column height. PWV at the brachial-ankle artery site (baPWV) was measured using a multichannel sphygmomanometer ABI System 100 PWV in three positions: in the baseline horizontal (supine) position-baPWVb; during the head tilt-up with an individual angle of inclination-baPWVt; and when returning to supine. Results: The most sensitive marker of early stiffness increase in a healthy population is the relative orthostatic increase in baPWV, ΔbaPWV/baPWVb, where ΔbaPWV = baPWVt - baPWVb. The significance of differences in this parameter between the young and elderly groups reached p = 0.000075 and p = 0.000006, respectively. Conclusions: The proposed index ΔbaPWV/baPWVb can be considered as a promising sensitive early biomarker of vascular aging and as a potential effective indicator in cardiovascular prevention. A longitudinal cohort study is needed to confirm this assumption.

Triggered by your heart: Effects of cardioafferent traffic and stress on automatic responses in a Simon task

Variations in cardioafferent traffic are relayed to the brain via arterial baroreceptors and have been shown to modulate perceptual processing. However, less is known about the cognitive-behavioral consequences of these effects and their role during stress. Here, we investigated in how far automatic responses during the Simon task were modulated by exposure to a laboratory stressor and the different phases of the cardiac cycle. In this study, 30 participants performed three blocks of a combined horizontal and vertical Simon task, which is characterized by either sensorimotor or cognitive response conflicts, respectively. Before each block, subjects were exposed to both the cold pressor test (CPT) and a control condition according to a within-subjects design. Target stimuli were presented during either systole or diastole. Behavioral and EEG-correlates of task processing were assessed along with subjective, cardiovascular, and endocrine measures of stress. The stress induction was successful yielding significant increases in all these measures compared to control. Moreover, we found the expected Simon effects: in incompatible compared to compatible trials performance was decreased and LRP latency as well as anterior N2 area increased. Importantly, accuracy was improved in compatible but reduced in incompatible trials during systole vs. diastole but only in the horizontal Simon condition. Stress dampened N2 area, however, no interactions with cardiac cycle were evident. These results indicate a faciliatory effect of cardioafferent traffic on automated sensorimotor processes.

Detection of Preclinical Orthostatic Disorders in Young African and European Adults Using the Head-Up Tilt Test with a Standardized Hydrostatic Column Height: A Pilot Study

Arterial hypertension (AH) remains the most common disease. One possible way to improve the effectiveness of the primary prevention of AH is to identify and control the preclinical orthostatic disturbances that precede the development of AH. The aim of the study was to determine the feasibility of a new protocol for the head-up tilt test (HUTT) with a standardized hydrostatic column height for the detection of asymptomatic orthostatic circulatory disorders and their racial differences in young African and European adults. Methods. In total, 80 young healthy adults (40 African and 40 European) aged 20–23 years performed the HUTT with a standardized hydrostatic column height of 133 cm. The hemodynamic parameters were recorded using a Task Force Monitor (3040i). The cardio-ankle vascular index (CAVI) was measured using a VaSera VS-2000 volumetric sphygmograph. Results. The baseline and orthostatic hemodynamic changes in both racial groups were within normal limits. Orthostatic circulatory disturbances were not detected in 70% of the European participants and 65% of the African participants; however, preclinical orthostatic hypertension, which precedes AH, was detected using the new HUTT protocol in 32.5% of the African participants and 20% of the European participants. The baseline CAVI was higher in the European group compared to the African group. Conclusion. The results of this study showed the feasibility of the detection of preclinical orthostatic disturbances in young adults and the detection of their racial differences using the HUTT protocol, providing the use of a standard gravity load. Further study on the evolution of preclinical orthostatic disturbances and their relation to increased vascular stiffness is necessary among large samples.

How spaceflight challenges human cardiovascular health

The harsh environmental conditions in space, particularly weightlessness and radiation exposure, can negatively affect cardiovascular function and structure. In the future, preventive cardiology will be crucial in enabling safe space travel. Indeed, future space missions destined to the Moon and from there to Mars will create new challenges to cardiovascular health while limiting medical management. Moreover, commercial spaceflight evolves rapidly such that older persons with cardiovascular risk factors will be exposed to space conditions. This review provides an overview on studies conducted in space and in terrestrial models, particularly head-down bedrest studies. These studies showed that weightlessness elicits a fluid shift towards the head, which likely predisposes to the spaceflight-associated neuro-ocular syndrome, neck vein thrombosis, and orthostatic intolerance after return to Earth. Moreover, cardiovascular unloading produces cardiopulmonary deconditioning which may be associated with cardiac atrophy. In addition to limiting physical performance, the mechanism further worsens orthostatic tolerance after return to Earth. Finally, space conditions may directly affect vascular health, however, the clinical relevance of these findings in terms of morbidity and mortality is unknown. Targeted preventive measures, which are referred to as countermeasures in aerospace medicine, and technologies to identify vascular risks early on will be required to maintain cardiovascular performance and health during future space missions.

Baroreflex Modulation During Acute High-Altitude Exposure in Rats

Baroreflex (BR) control is critically dependent of sympathetic and parasympathetic modulation. It has been documented that during acute hypobaric hypoxia there is a BR control impairment, however, the effect of a natural hypoxic environment on BR function is limited and controversial. Therefore, the aim of this study was to determine the effect of acute High-Altitude exposure on sympathetic/parasympathetic modulation of BR control in normal rats. Male Sprague Dawley rats were randomly allocated into Sea-Level (n = 7) and High-Altitude (n = 5) (3,270 m above sea level) groups. The BR control was studied using phenylephrine (Phe) and sodium nitroprusside (SNP) through sigmoidal analysis. The autonomic control of the heart was estimated using heart rate variability (HRV) analysis in frequency domain. Additionally, to determine the maximum sympathetic and parasympathetic activation of BR, spectral non-stationary method analysis, during Phe (0.05 μg/mL) and SNP administration (0.10 μg/mL) were used. Compared to Sea-Level condition, the High-Altitude group displayed parasympathetic withdrawal (high frequency, 0.6-2.4 Hz) and sympathoexcitation (low frequency, 0.04-0.6 Hz). Regarding to BR modulation, rats showed a significant decrease (p < 0.05) of curvature and parasympathetic bradycardic responses to Phe, without significant differences in sympathetic tachycardic responses to SNP after High-Altitude exposure. In addition, the non-stationary analysis of HRV showed a reduction of parasympathetic activation (Phe) in the High-Altitude group. Our results suggest that acute exposure to High-Altitude produces an autonomic and BR control impairment, characterized by parasympathetic withdrawal after 24 h of high-altitude exposure.

Disentangling sensorimotor and cognitive cardioafferent effects: A cardiac-cycle-time study on spatial stimulus-response compatibility

Cardiac-cycle-time effects are attributed to variations in baroreceptor (BR) activity and have been shown to impinge on subcortical as well as cortical processes. However, cognitive and sensorimotor processes mediating voluntary responses seem to be differentially affected. We sought to disentangle cardiac-cycle-time effects on subcortical and cortical levels as well as sensorimotor and cognitive processes within a spatial stimulus-response-compatibility paradigm employing startling stimuli of different modalities. Air-puffs and white noise-bursts were presented unilaterally during either cardiac systole or diastole while bilateral startle EMG responses were recorded. Modality, laterality and cardiac-cycle-time were randomly varied within-subjects. Cognitive and sensorimotor stimulus-response-compatibility was orthogonally varied between-subjects: Participants (N = 80) responded to the stimuli via left/right button-push made with either the contra- or ipsilateral hand (sensorimotor compatibility) on either the ipsi- or contralateral button (cognitive compatibility). We found that sensorimotor compatible reactions were speeded during systole whereas sensorimotor incompatible ones were prolonged. This effect was independent of cognitive compatibility and restricted to auditory stimuli. Startle was inhibited during systole irrespective of modality or compatibility. Our results demonstrate how differential cardiac-cycle-time effects influence performance in conflict tasks and further suggest that stimulus-response-compatibility paradigms offer a viable method to uncover the complex interactions underlying behavioral BR effects.

Autonomic cardiovascular control in older patients with acute infection and delirium: a pilot study of orthostatic stress responses

BACKGROUND

Alterations in autonomic nervous system (ANS) activity might be involved in the pathophysiology of delirium. The aim was to explore autonomic cardiovascular control in older patients with and without delirium.

METHODS

Fourteen patients (five with delirium) acutely admitted to the geriatric ward with an infection were enrolled in the study. Patients with atrial fibrillation, a pacemaker, or on treatment with beta-blockers, calcium channel blockers or acetylcholinesterase inhibitors were not eligible. Continuous, non-invasive hemodynamic variables were measured during supine rest (5 min) and head-up tilt (HUT) to 15 degrees (10 min). Heart rate (HR), blood pressure (BP) and stroke volume (SV) were recorded beat-to-beat. Cardiac output (CO), total peripheral resistance (TPR), end-diastolic volume (EDV) and heart rate variability (HRV) values were calculated.

RESULTS

Median age was 86 years. HR, BP, SV, CO, TPR and EDV were similar across the two groups at rest, but there was a trend towards a greater increase in systolic BP and HR during HUT in the delirium group. At rest, all HRV indices were higher in the delirium group, but the differences were not statistically significant. During HUT, the delirium group had higher power spectral density (PSD) (representing total variability) (p = 0.06) and a lower low frequency (LF)/high frequency (HF)-ratio (an index of sympathovagal balance) than the control group (p = 0.06). Also, delirious patients had a significantly greater reduction in standard deviation of RR-intervals (SDNN) (representing total variability) from baseline than controls (p = 0.01) during HUT.

CONCLUSIONS

This explorative pilot study on autonomic cardiovascular control in delirium suggests that there may be differences in HRV that should be further investigated in larger samples.

The Effects of High Altitude on Glucose Homeostasis, Metabolic Control, and Other Diabetes-Related Parameters: From Animal Studies to Real Life

Exposure to high altitude activates several complex and adaptive mechanisms aiming to protect human homeostasis from extreme environmental conditions, such as hypoxia and low temperatures. Short-term exposure is followed by transient hyperglycemia, mainly triggered by the activation of the sympathetic system, whereas long-term exposure results in lower plasma glucose concentrations, mediated by improved insulin sensitivity and augmented peripheral glucose disposal. An inverse relationship between altitude, diabetes, and obesity has been well documented. This is the result of genetic and physiological adaptations principally to hypoxia that favorably affect glucose metabolism; however, the contribution of financial, dietary, and other life-style parameters may also be important. According to existing evidence, people with diabetes are capable of undertaking demanding physical challenges even at extreme altitudes. Still, a number of issues should be taken into account, including the increased physical activity leading to changes in insulin demands and resistance, the performance of measurement systems under extreme weather conditions and the potential deterioration of metabolic control during climbing expeditions. The aim of this review is to present available evidence in the field in a comprehensive way, beginning from the physiology of glucose homeostasis adaptation mechanisms to high altitudes and ending to what real life experience has taught us.

Cardiac cycle time effects on selection efficiency in vision

The effect of cardiac cycle time on attentional selection was investigated in an experiment in which participants classified target letters in a visual selection task. Stimulus onsets were aligned to the R wave of the electrocardiogram and stimuli presented either during the ventricular systole or diastole. Selection efficiency was operationalized as difference in target selection performance under conditions of homogeneous and heterogeneous distractors. Differences in performance (i.e., the impact selection difficulty had on the ability to select the target) were attenuated for stimuli presented during the ventricular systole compared to the diastole. Increased baroafferent signal transmission during the systole appears to reduce interference of highly distracting stimuli on visual selection efficiency.

Glucose homeostasis during short-term and prolonged exposure to high altitudes

Most of the literature related to high altitude medicine is devoted to the short-term effects of high-altitude exposure on human physiology. However, long-term effects of living at high altitudes may be more important in relation to human disease because more than 400 million people worldwide reside above 1500 m. Interestingly, individuals living at higher altitudes have a lower fasting glycemia and better glucose tolerance compared with those who live near sea level. There is also emerging evidence of the lower prevalence of both obesity and diabetes at higher altitudes. The mechanisms underlying improved glucose control at higher altitudes remain unclear. In this review, we present the most current evidence about glucose homeostasis in residents living above 1500 m and discuss possible mechanisms that could explain the lower fasting glycemia and lower prevalence of obesity and diabetes in this population. Understanding the mechanisms that regulate and maintain the lower fasting glycemia in individuals who live at higher altitudes could lead to new therapeutics for impaired glucose homeostasis.

Dysautonomic responses during percutaneous carotid intervention: principles of physiology and management

Percutaneous carotid artery stenting (CAS) has emerged as a less invasive alternative to carotid endarterectomy for the treatment of carotid atherosclerotic disease. The main risk of CAS is the occurrence of neuro-vascular complications; however, carotid artery stenting-related dysautonomia (CAS-D) (hypertension, hypotension, and bradycardia) is the most frequently reported problem occurring in the periprocedural period. Alterations in autonomic homeostasis result from baroreceptor stimulation, which occurs particularly at the time of balloon inflation in the region of the carotid sinus. The response can be profound enough to induce asystole or even complete cessation of postganglionic sympathetic nerve activity. Frequency and factors predisposing a patient to CAS-D have been investigated in several studies; however, there are significant discrepancies in results among reports. Lack of consistent findings may arise from using different methods and definitions, as well as other factors discussed in detail in this review. Furthermore, a correlation of CAS-D with short and long-term outcomes has been investigated only in small and mostly retrospective studies, explaining why its prognostic significance remains uncertain. In this manuscript, we have focused on risk factors, pathophysiology and management of periprocedural autonomic dysfunction. As there is no standardized approach to the treatment of CAS-D, we present an algorithm for the periprocedural management of patients undergoing CAS. The proposed algorithm was developed based on our procedural experience as well as data from the available literature. The Yale Algorithm was successfully implemented at our institution and we are currently collecting data for short- and long-term safety. © 2014 Wiley Periodicals, Inc.

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.

Water deprivation increases Fos expression in hypothalamic corticotropin-releasing factor neurons induced by right atrial distension in awake rats

Atrial mechanoreceptors, sensitive to stretch, contribute in regulating heart rate and intravascular volume. The information from those receptors reaches the nucleus tractus solitarius and then the paraventricular nucleus (PVN), known to have a crucial role in the regulation of cardiovascular function. Neurons in the PVN synthesize CRF, AVP, and oxytocin (OT). Stimulation of atrial mechanoreceptors was performed in awake rats implanted with a balloon at the junction of the superior vena cava and right atrium. Plasma ACTH, AVP, and OT concentrations and Fos, CRF, AVP, and OT immunolabeling in the PVN were determined after balloon inflation in hydrated and water-deprived rats. The distension of the balloon increased the plasma ACTH concentrations, which were higher in water-deprived than in hydrated rats ( P < 0.05). In addition, the distension in the water-deprived group decreased plasma AVP concentrations ( P < 0.05), compared with the respective control group. The distension increased the number of Fos- and double-labeled Fos/CRF neurons in the parvocellular PVN, which was higher in the water-deprived than in the hydrated group ( P < 0.01). There was no difference in the Fos expression in magnocellular PVN neurons after distension in hydrated and water-deprived groups, compared with respective controls. In conclusion, parvocellular CRF neurons showed an increase of Fos expression induced by stimulation of right atrial mechanoreceptors, suggesting that CRF participates in the cardiovascular reflex adjustments elicited by volume loading. Activation of CRF neurons in the PVN by cardiovascular reflex is affected by osmotic stimulation.

Evaluation of baroreceptor reflex function in the chronic mild stress rodent model of depression

OBJECTIVE

There is a bidirectional relationship between mood disorders (e.g., depression) and altered cardiovascular regulation (e.g., heart disease); however, the precise causal and/or common mechanisms underlying this association are unclear. In previous studies, we have noted indications of altered sympathetic drive to the heart in rats that exhibit anhedonia, an operational sign of depression induced by subjecting the animals to a series of mild and unpredictable stressors (chronic mild stress (CMS) rodent model of depression). The purpose of the present study was to more fully characterize baroreceptor reflex function in rats with experimentally induced depression.

METHODS

Male, Sprague-Dawley rats were exposed to either 4 weeks of mild, unpredictable stressors (CMS group) or standard housing conditions (control group). Depression-like behavior, resting hemodynamic and cardiac parameters, and baroreceptor reflex function were investigated in all animals after the CMS period.

RESULTS

CMS produced anhedonia, evidenced by reduced sucrose intake and sucrose preference, as well as elevated resting heart rate (HR), slightly elevated blood pressure, and reduced HR variability. These animals also exhibited significantly attenuated sympathoexcitatory responses to hypotension, and an elevation of basal sympathetic nerve activity.

CONCLUSIONS

These findings suggest that CMS is associated with altered sympathoexcitatory responses after baroreceptor unloading and provide further insights into potential common mechanisms underlying the association of depression and altered cardiovascular control.

Theoretical and electrophysiological evidence for axial loading about aortic baroreceptor nerve terminals in rats

Arterial baroreceptors are essential for neurocirculatory control, providing rapid hemodynamic feedback to the central nervous system. The pressure-dependent discharge of carotid and aortic baroreceptor afferents has been extensively studied. A common assumption has been that circumferential deformation of the arterial wall is the predominant mechanical force affecting baroreceptor discharge. However, in vivo the arterial tree is under significant longitudinal tension, leading to the hypothesis that axially directed forces may contribute to baroreceptor function. To test this hypothesis, we utilized a combination of finite element modeling methods and an in vitro rat aortic arch preparation. Model formulation utilized traditional analytic constructs available in the literature followed by refinement of model material parameters through direct comparison of computationally and experimentally generated pressure-diameter curves. The numerical simulations strongly indicated a functional role for axial loading within the region of the baroreceptive nerve terminal. This prediction was confirmed through single-fiber recording of baroreceptor nerve discharge under conditions with and without longitudinal tension in the vessel preparation. The recordings (n = 5) demonstrated that longitudinal tension significantly (P < 0.02) lowered both the pressure threshold (P(th), mmHg) for baroreceptor discharge and sensitivity (S(th), Hz/mmHg). The effect was nearly instantaneous and sustained; i.e., under longitudinal tension average P(th) was 84 +/- 3 mmHg and S(th) was 0.71 +/- 0.15 Hz/mmHg, which immediately increased to a P(th) of 94 +/- 4 mmHg and a S(th) of 1.20 +/- 0.32 Hz/mmHg with loss of axial tension. Possible explanations of how an abrupt change in axial loading could result in a synchronized increase in afferent drive of the baroreceptor reflex, and the potentiating effect this could have on neurogenically mediated orthostatic intolerance are discussed.

Vasopressin V1A receptor enhances baroreflex via the central component of the reflex arc

The neurohypophyseal peptide [Arg(8)]-vasopressin (AVP) exerts its physiological actions via 3 distinct receptor isoforms designated V1A, V1B, and V2. We recently showed that V1A receptor was involved in the baroreflex control of heart rate using V1A receptor knockout mice. The present study was undertaken to further clarify this finding. In conscious mice, resting blood pressure of the knockout group was lower than that of the wild-type group (wild-type, 108+/-2.0 mm Hg; knockout, 98+/-3.8 mm Hg; n=6-7) without notable change in heart rate. Although phenylephrine and nitroprusside-induced changes in blood pressure did not differ in these strains, the subsequent bradycardia and tachycardia were markedly blunted in the knockout mice (mean slopes for baroreflex curve after phenylephrine treatment; wild-type, -5.65+/-0.30 bpm/mm Hg; knockout, -3.97+/-0.52 bpm/mm Hg; those after nitroprusside treatment; wild-type, -0.51+/-0.10 bpm/mm Hg; knockout, -0.18+/-0.05 bpm/mm Hg; n=6-7). Under urethane anesthesia (1.0-1.2 g/kg, i.p.), electrical stimulation of the vagal afferent nerve evoked frequency-dependent hypotension and bradycardia in the wild-type mice. In contrast, in the knockout mice such stimulation induced a pressor, not a depressor, response and diminished bradycardia. Moreover, electrical stimulation-induced hemodynamic changes through the vagal afferent nerve in the wild-type mice were significantly attenuated by pretreatment with intravenously administered V1A receptor antagonist d(CH(2))(5)Tyr(Me)AVP. Electrical stimulation of the vagal efferent nerve-induced hemodynamic changes (depressor and bradycardia) and chronotropic responses to adrenergic and cholinergic stimuli were not different between the 2 strains. These results suggest that the V1A receptor in the central nervous system is involved in the regulation of the heart rate via the baroreflex arc.

Sensory neurons with afferents from hind limbs: enhanced sensitivity in secondary hypertension

Sensory nerve fibers from the dorsal root ganglia (DRG) may contribute to the regulation of peripheral vascular resistance. Axons of DRG neurons of the lower thoracic cord project mainly to resistance vessels in the lower limbs, likely opposing the vasoconstrictor effects of the sympathetic activity. This mechanism might be of importance in hypertension with increased sympathetic activity. We tested the hypothesis that sensory neurons of the DRG in the lower thoracic cord show an altered sensitivity to mechanical stimuli in hypertension. Neurons from DRG (T11 to L1) of rats with hypertension (2 kidney-1 clip hypertensive rats and 5 of 6 nephrectomized rats) were cultured on coverslips. Current time relationships were established with whole-cell patch recordings. Cells were characterized under control conditions and after exposure to hypoosmotic solutions to induce mechanical stress. Neurons with projections to the kidney were studied for comparison. The hypoosmotic extracellular medium induced a significant change in conductance of the cells in all of the groups of rats. In hypertensive rats, responses of cells with hindlimb axons were significantly different from controls: (2 kidney-1 clip hypertensives: delta-351+/-52 pA and 5 of 6 nephrectomized rats: delta-372+/-43 pA versus controls: delta-190+/-25 pA; P<0.05). Responses of DRG cells with renal afferents to mechanical stress were unaffected. Neurons from DRG in the lower thoracic cord with projections to the lower limbs exhibited an increased sensitivity to mechanical stress. We speculate that this observation may indicate an increased activity of these neurons, their axons, and neurotransmitters in the control of resistance vessels in hypertension.

Baroreceptor reflex sensitivity in human neonates: the effect of postmenstrual age

We performed a cross-sectional study in human infants to determine if indices of R-R interval variability, systolic blood pressure (SBP) variability, and baroreceptor reflex sensitivity change with postmenstrual age (PMA: gestational age+postnatal age). The electrocardiogram, arterial SBP and respiration were recorded in clinically stable infants (PMA, 28-42 weeks) in the quiet sleep state in the first days after birth. (Cross-)spectral analyses of R-R interval series and SBP series were performed to calculate the power of low-frequency (LF, indicating baroreceptor reflex activity, 0.04-0.15 Hz) and high-frequency (HF, indicating parasympathetic activity, individualized between the p-10 and p-90 values of respiratory frequency) fluctuations, and transfer function phase and gain. The mean R-R interval, and LF and HF spectral powers of R-R interval series increased with PMA. The mean SBP increased with PMA, but not the LF and HF spectral powers of SBP series. In the LF range, cross-spectral analysis showed high coherence values (>0.5) with a consistent negative phase shift between R-R interval and SBP, indicating a approximately 3 s lag in R-R interval changes in relation to SBP. Baroreceptor reflex sensitivity, calculated from LF transfer gain, increased significantly with PMA, from 5 (preterm) to 15 ms mmHg-1 (term). Baroreceptor reflex sensitivity correlated significantly with the (LF and) HF spectral powers of R-R interval series, but not with the LF and HF spectral powers of SBP series. The principal conclusions are that baroreceptor reflex sensitivity and spectral power in R-R interval series increase in parallel with PMA, suggesting a progressive vagal maturation with PMA.

Modulation of the baroreceptor reflex by alpha 2A-adrenoceptors: a study in alpha 2A knockout mice

1. Our objective was to determine whether alpha(2A)-adrenoceptors modulate the baroreceptor reflex. The efficacy of the reflex was evaluated by measuring the spontaneous blood pressure and heart rate variability at rest and the heart rate responses to evoked changes in blood pressure. Experiments were carried out in conscious, unrestrained, and anaesthetized alpha(2A)-adrenoceptor-deficient (alpha(2A)-KO) mice and WT mice. 2. In conscious alpha(2A)-KO mice, the spontaneous blood pressure variability was greater, and the spontaneous heart rate variability was lower than in conscious WT mice. This was also observed in anaesthetized animals. 3. The reflex bradycardia after intravenous injection of phenylephrine was greatly attenuated in conscious alpha(2A)-KO compared to conscious WT mice; the baroreceptor reflex gain (ratio maximal change in heart rate/maximal change in mean arterial pressure) was decreased by 40%. 4. Similar results were obtained when reflex bradycardia was elicited by intra-arterial volume loading of conscious WT and alpha(2A)-KO mice. The baroreceptor reflex gain upon volume loading was also low in anaesthetized alpha(2A)-KO mice. 5. The reflex tachycardia evoked by intravenous sodium nitroprusside injection was also significantly less in alpha(2A)-KO mice as compared to WT, conscious as well as anaesthetized; the baroreceptor reflex gains were decreased by 50 and 65%, respectively. 6. Direct stimulation of cardiac beta-adrenoceptors by the agonist isoprenaline produced similar cardioacceleration in alpha(2A)-KO and WT animals. 7. Our results show that the baroreceptor reflex function is impaired in mice lacking alpha(2A)-adrenoceptors. We conclude that central alpha(2A)-adrenoceptors facilitate the reflex response to both loading and unloading of the arterial baroreceptors.

Functional magnetic resonance signal changes in neural structures to baroreceptor reflex activation

The sequence of neural responses to exogenous arterial pressure manipulation remains unclear, especially for extramedullary sites. We used functional magnetic resonance imaging procedures to visualize neural responses during pressor (phenylephrine) and depressor (sodium nitroprusside) challenges in seven isoflurane-anesthetized adult cats. Depressor challenges produced signal-intensity declines in multiple cardiovascular-related sites in the medulla, including the nucleus tractus solitarius, and caudal and rostral ventrolateral medulla. Signal decreases also emerged in the cerebellar vermis, inferior olive, dorsolateral pons, and right insula. Rostral sites, such as the amygdala and hypothalamus, increased signal intensity as arterial pressure declined. In contrast, arterial pressure elevation elicited smaller signal increases in medullary regions, the dorsolateral pons, and the right insula and signal declines in regions of the hypothalamus, with no change in deep cerebellar areas. Responses to both pressor and depressor challenges were typically lateralized. In a subset of animals, barodenervation resulted in rises and falls of blood pressure that were comparable to these resulting from the pharmacological challenges but different regional neural responses, indicating that the regional signal intensity responses did not derive from global perfusion effects but from baroreceptor mediation of central mechanisms. The findings demonstrate widespread lateralized distribution of neural sites responsive to blood pressure manipulation. The distribution and time course of neural responses follow patterns associated with early and late compensatory reactions.

Angiotensin potentiates excitatory sensory synaptic transmission to medial solitary tract nucleus neurons

Femtomole doses of angiotensin (ANG) II microinjected into nucleus tractus solitarii (nTS) decrease blood pressure and heart rate, mimicking activation of the baroreflex, whereas higher doses depress this reflex. ANG II might generate cardioinhibitory responses by augmenting cardiovascular afferent synaptic transmission onto nTS neurons. Intracellular recordings were obtained from 99 dorsal medial nTS region neurons in rat medulla horizontal slices to investigate whether ANG II modulated short-latency excitatory postsynaptic potentials (EPSPs) evoked by solitary tract (TS) stimulation. ANG II (200 fmol) increased TS-evoked EPSP amplitudes 20-200% with minimal membrane depolarization in 12 neurons excited by ANG II and glutamate, but not substance P (group A). Blockade of non-N-methyl-d-aspartate receptors eliminated TS-evoked EPSPs and responses to ANG II. ANG II did not alter TS-evoked EPSPs in 14 other neurons depolarized substantially by ANG II and substance P (group B). ANG II appeared to selectively augment presynaptic sensory transmission in one class of nTS neurons but had only postsynaptic effects on another group of cells. Thus ANG II is likely to modulate cardiovascular function by more than one nTS neuronal pathway.

Glutamate receptors in RVLM modulate sympathetic baroreflex in conscious rabbits

In this study, we examined the effect of excitatory amino acid (EAA) receptor blockade in the rostral ventrolateral medulla (RVLM) on the renal sympathetic baroreflex in conscious rabbits. Rabbits were implanted with guide cannulas for bilateral microinjections into the RVLM (+2 to +3 mm from the obex, n = 8) or into the intermediate ventrolateral medulla (IVLM; 0 to +1 mm from the obex, n = 5) and with an electrode for measuring renal sympathetic nerve activity (RSNA). After 7 days of recovery, microinjection of the EAA receptor antagonist kynurenate (10 nmol) into the RVLM did not affect resting RSNA or arterial pressure. Kynurenate decreased the gain of the RSNA baroreflex by 53% but did not change the reflex range. By contrast, injection of kynurenate into the IVLM increased resting arterial pressure and RSNA by 27 mmHg and 88%, respectively, but did not alter the RSNA baroreflex gain or range. Pentobarbital sodium anesthesia attenuated the gain and range of the RSNA baroreflex by 78 and 40%, respectively. Under these conditions, microinjection of kynurenate into the RVLM did not cause any further change in the gain of this reflex. These results suggest that endogenous EAA neurotransmitters in the RVLM are important in modulating the sympathetic baroreflex in conscious rabbits. Anesthesia can mask the functional significance of EAAs in the RVLM in modulating the baroreflexes, which may explain why previous studies in anesthetized animals found no effect of blocking EAA receptors in the RVLM on sympathetic baroreflexes.