Baroreceptor reflex modulation by circulating angiotensin II is mediated by AT1 receptors in the nucleus tractus solitarius

Decreased sympathetic nerve activity in young hypertensive rats reared by normotensive mothers

AIMS

Early postnatal development can be significantly compromised by changes in factors provided by the mother, leading to increased vulnerability to hypertension in her offspring. TGR(mRen-2)27 (TGR) mothers, characterised by an overactivated renin-angiotensin system, exhibit altered ion composition in their breast milk. Therefore, we aimed to analyse the impact of cross-fostering on cardiovascular parameters in hypertensive TGR and normotensive Hannover Sprague-Dawley (HanSD) offspring.

MATERIALS AND METHODS

We measured cardiovascular parameters in 5- to 10-week-old male offspring by telemetry. The expression of proteins related to vascular function was assessed by western blotting in the aortic samples obtained from 6- to 12-week-old male offspring. Plasma renin activity and plasma angiotensin II (Ang II) levels were evaluated by radioimmunoassay (RIA).

KEY FINDINGS

The development of hypertension was in TGR accompanied by increased low-to-high frequency ratio (LF/HF; a marker of sympathovagal balance; 0.51 ± 0.16 in week 10). Furthermore, TGR exhibited increased aortic expression of mineralocorticoid receptor (MR; p < 0.05) and transforming growth factor beta type 1 (TGF-β1; p = 0.002) compared to HanSD offspring. Fostering significantly decreased sympathovagal balance (0.23 ± 0.10 in week 10) and, transiently, plasma Ang II levels and MR expression in TGR offspring reared by HanSD mothers.

SIGNIFICANCE

These findings highlight the importance of understanding the complex interplay between early life experiences, maternal factors, and later cardiovascular function. Understanding the mechanisms behind the observed effects may help to identify potential interventions to prevent the development of hypertension later in life.

Sodium Intake and Disease: Another Relationship to Consider

Sodium (Na+) is crucial for numerous homeostatic processes in the body and, consequentially, its levels are tightly regulated by multiple organ systems. Sodium is acquired from the diet, commonly in the form of NaCl (table salt), and substances that contain sodium taste salty and are innately palatable at concentrations that are advantageous to physiological homeostasis. The importance of sodium homeostasis is reflected by sodium appetite, an "all-hands-on-deck" response involving the brain, multiple peripheral organ systems, and endocrine factors, to increase sodium intake and replenish sodium levels in times of depletion. Visceral sensory information and endocrine signals are integrated by the brain to regulate sodium intake. Dysregulation of the systems involved can lead to sodium overconsumption, which numerous studies have considered causal for the development of diseases, such as hypertension. The purpose here is to consider the inverse-how disease impacts sodium intake, with a focus on stress-related and cardiometabolic diseases. Our proposition is that such diseases contribute to an increase in sodium intake, potentially eliciting a vicious cycle toward disease exacerbation. First, we describe the mechanism(s) that regulate each of these processes independently. Then, we highlight the points of overlap and integration of these processes. We propose that the analogous neural circuitry involved in regulating sodium intake and blood pressure, at least in part, underlies the reciprocal relationship between neural control of these functions. Finally, we conclude with a discussion on how stress-related and cardiometabolic diseases influence these circuitries to alter the consumption of sodium.

Angiotensin II and the Cardiac Parasympathetic Nervous System in Hypertension

The renin-angiotensin-aldosterone system (RAAS) impacts cardiovascular homeostasis via direct actions on peripheral blood vessels and via modulation of the autonomic nervous system. To date, research has primarily focused on the actions of the RAAS on the sympathetic nervous system. Here, we review the critical role of the RAAS on parasympathetic nerve function during normal physiology and its role in cardiovascular disease, focusing on hypertension. Angiotensin (Ang) II receptors are present throughout the parasympathetic nerves and can modulate vagal activity via actions at the level of the nerve endings as well as via the circumventricular organs and as a neuromodulator acting within brain regions. There is tonic inhibition of cardiac vagal tone by endogenous Ang II. We review the actions of Ang II via peripheral nerve endings as well as via central actions on brain regions. We review the evidence that Ang II modulates arterial baroreflex function and examine the pathways via which Ang II can modulate baroreflex control of cardiac vagal drive. Although there is evidence that Ang II can modulate parasympathetic activity and has the potential to contribute to impaired baseline levels and impaired baroreflex control during hypertension, the exact central regions where Ang II acts need further investigation. The beneficial actions of angiotensin receptor blockers in hypertension may be mediated in part via actions on the parasympathetic nervous system. We highlight important unknown questions about the interaction between the RAAS and the parasympathetic nervous system and conclude that this remains an important area where future research is needed.

Increased angiotensin II formation in the brain modulates cardiovascular homeostasis and erythropoiesis

In spite of the fact that the modulatory effects of angiotensin II (Ang II) on the sympathetic nerve activity to targeted organs involved in blood pressure (BP) regulation is well acknowledged, the local production of this peptide in the brain and the consequences of enhanced central Ang II beyond the cardiovascular system are not yet well comprehended. In the present study, we generated and validated a new transgenic mouse line overexpressing the rat full-length angiotensinogen (Agt) protein specifically in the brain (Agt-Tg). Adult Agt-Tg mice presented overall increased gene expression of total Agt in the brain including brainstem and hypothalamus. In addition, the excess of Agt led to abundantly detectable brain Ang II levels as well as increased circulating copeptin levels. Agt-Tg displayed raised BP in acute recordings, while long-term telemetrically measured basal BP was indistinguishable from wild-types. Agt-Tg has altered peripheral renin–angiotensin system and vasomotor sympathetic tone homeostasis because renal gene expression analysis, plasma Ang II measurements and ganglionic blockade experiments revealed suppressed renin expression and reduced Ang II and higher neurogenic pressure response, respectively. Plasma and urine screens revealed apparently normal fluid and electrolyte handling in Agt-Tg. Interestingly, hematological analyses showed increased hematocrit in Agt-Tg caused by enhanced erythropoiesis, which was reverted by submitting the transgenic mice to a long-term peripheral sympathectomy protocol. Collectively, our findings suggest that Agt-Tg is a valuable tool to study not only brain Ang II formation and its modulatory effects on cardiovascular homeostasis but also its role in erythropoiesis control via autonomic modulation.

Impact of antihypertensive treatment on resiliency to clinical pain

OBJECTIVES

Although progress has been made in the regulation of hypertension over the past decades, the USA and some other countries have faced a significant rise in incidence of chronic pain management cases during the same period. Studies of the relationship between pain and blood pressure (BP) regulations propose that these two processes may be interconnected. Studies of effects of antihypertensive treatment on pain in general and its chronification have never been reviewed.

METHODS

A narrative review of respective studies with analysis of credibility of the findings was conducted.

RESULTS

Some studies have suggested that aggressive reduction of high BP may contribute to a return in pain symptoms and may require more aggressive, long-term pain management. Other studies propose that long-term antihypertensive medication could also increase the risk for new cases of chronic pain. Pain initiates a central neuroplastic resetting of the baroreceptor activation accounting for sustained increase of BP with an adaptive 'pain-killing' or maladaptive 'pain-complication' effect associated with pain chronification, and these mechanisms may be moderated by antihypertensive medications. However, different antihypertensive drugs and nondrug treatments may diversely affect pain mechanisms at different stages of treatments.

CONCLUSION

Uncontrollable reduction of high BP in some patients with hypertension could increase the risk for chronic pain incidence and its severity. Practical recommendations in BP control should be reconsidered to take into account patients' chronic pain. Further research is needed of moderation effects of different antihypertensive manipulations on pain to improve pain management in these patients.

GABA is a mediator of brain AT1 and AT2 receptor-mediated blood pressure responses

The nucleus tractus solitarius (NTS), paraventricular nucleus (PVN), and rostral ventrolateral medulla (RVLM) are the most targeted regions of central blood pressure control studies. Glutamate and gamma-aminobutyric acid (GABA) interact within these brain regions to modulate blood pressure. The brain renin-angiotensin system also participates in central blood pressure control. Angiotensin II increases blood pressure through the stimulation of angiotensin II type 1 (AT₁) receptors within the PVN and RVLM and attenuates baroreceptor sensitivity, resulting in elevated blood pressure within the NTS. Angiotensin II type 2 (AT₂) receptors in cardiovascular control centers in the brain also appear to be involved in blood pressure control and counteract AT₁ receptor-mediated effects. The current review is focused on the interaction of GABA with AT₁ and AT₂ receptors in the control of blood pressure within the RVLM, PVN and NTS. Within the NTS, GABA is released from local GABAergic interneurons that are stimulated by local AT₁ receptors and mediates a hypertensive response. In contrast, the local increase in GABA levels observed after AT₂ receptor stimulation within the RVLM, likely from GABAergic nerve endings originating in the caudal ventrolateral medulla, is important in the mediation of the hypotensive response. Preliminary results suggest that the hypertensive response to AT₁ receptor stimulation within the RVLM is associated with a reduction in GABA release. The current experimental evidence therefore indicates that GABA is an important mediator of brainstem responses to AT₁ and AT₂ receptor stimulation and that increased GABA release may play a role in hypertensive and hypotensive responses, depending on the site of action.

Astrocytes Modulate Baroreflex Sensitivity at the Level of the Nucleus of the Solitary Tract

Maintenance of cardiorespiratory homeostasis depends on autonomic reflexes controlled by neuronal circuits of the brainstem. The neurophysiology and neuroanatomy of these reflex pathways are well understood, however, the mechanisms and functional significance of autonomic circuit modulation by glial cells remain largely unknown. In the experiments conducted in male laboratory rats we show that astrocytes of the nucleus of the solitary tract (NTS), the brain area that receives and integrates sensory information from the heart and blood vessels, respond to incoming afferent inputs with [Ca2+]i elevations. Astroglial [Ca2+]i responses are triggered by transmitters released by vagal afferents, glutamate acting at AMPA receptors and 5-HT acting at 5-HT2A receptors. In conscious freely behaving animals blockade of Ca2+-dependent vesicular release mechanisms in NTS astrocytes by virally driven expression of a dominant-negative SNARE protein (dnSNARE) increased baroreflex sensitivity by 70% (p < 0.001). This effect of compromised astroglial function was specific to the NTS as expression of dnSNARE in astrocytes of the ventrolateral brainstem had no effect. ATP is considered the principle gliotransmitter and is released by vesicular mechanisms blocked by dnSNARE expression. Consistent with this hypothesis, in anesthetized rats, pharmacological activation of P2Y1 purinoceptors in the NTS decreased baroreflex gain by 40% (p = 0.031), whereas blockade of P2Y1 receptors increased baroreflex gain by 57% (p = 0.018). These results suggest that glutamate and 5-HT, released by NTS afferent terminals, trigger Ca2+-dependent astroglial release of ATP to modulate baroreflex sensitivity via P2Y1 receptors. These data add to the growing body of evidence supporting an active role of astrocytes in brain information processing.SIGNIFICANCE STATEMENT Cardiorespiratory reflexes maintain autonomic balance and ensure cardiovascular health. Impaired baroreflex may contribute to the development of cardiovascular disease and serves as a robust predictor of cardiovascular and all-cause mortality. The data obtained in this study suggest that astrocytes are integral components of the brainstem mechanisms that process afferent information and modulate baroreflex sensitivity via the release of ATP. Any condition associated with higher levels of "ambient" ATP in the NTS would be expected to decrease baroreflex gain by the mechanism described here. As ATP is the primary signaling molecule of glial cells (astrocytes, microglia), responding to metabolic stress and inflammatory stimuli, our study suggests a plausible mechanism of how the central component of the baroreflex is affected in pathological conditions.

Angiotensin II-mediated nondipping during sleep in healthy humans: effects on baroreflex function at subsequent daytime

Blood pressure dipping at night is mediated by sleep-inherent, active downregulation of sympathetic vascular tone. Concomitantly, activity of the renin-angiotensin system is reduced, which might contribute to the beneficial effect of baroreflex downward resetting on daytime blood pressure homeostasis. To evaluate whether experimental nondipping mediated by angiotensin II during sleep would alter blood pressure and baroreflex function the next day in healthy humans, angiotensin-II or placebo (saline) was infused for a 7-h period at night, preventing blood pressure dipping in 11 sleeping normotensive individuals (5 males, balanced, crossover design). Baroreflex function was assessed about 1 h upon awakening and stop of infusion via microneurographic recordings of muscle sympathetic nerve activity (MSNA), showing that resting MSNA was significantly increased following angiotensin II nondipping compared with placebo ( P = 0.029), whereas blood pressure and heart rate remained unchanged. Baroreflex sensitivity in response to vasoactive drug challenge was preserved, and neuroendocrine markers of fluid balance and electrolytes did not differ between conditions. Ambulatory blood pressure during subsequent daytime was not altered. Data were compared with analog experiments previously performed within the same subjects during awake daytime (ANCOVA). We conclude that angiotensin-II mediated nocturnal nondipping did not induce blood pressure elevation at subsequent daytime in healthy humans but was linked to increased vasoconstrictive sympathetic activity. This is in contrast to a prolonged increase in blood pressure in corresponding daytime experiments of the same individuals. Evidently, sleep strongly preserves normotensive blood pressure homeostasis in healthy humans.

AT1 Receptor Mediated Hypertensive Response to Ang II in the Nucleus Tractus Solitarii of Normotensive Rats Involves NO Dependent Local GABA Release

Aim

It is well-established that angiotensin II exerts a dampening effect on the baroreflex within the nucleus tractus solitarii (NTS), the principal brainstem site for termination of baroreceptor afferents and which is densely populated with gamma-aminobutyric acid (GABA)ergic neurons and nerve terminals. The present study was designed to investigate whether local release of GABA is involved in the effects mediated by local angiotensin II within the NTS.

Methods

In vivo microdialysis was used for measurement of extracellular glutamate and GABA levels and for infusion of angiotensin II within the NTS of conscious normotensive Wistar rats. The mean arterial pressure (MAP) and heart rate response to local infusion of angiotensin II were subsequently monitored with a pressure transducer under anesthesia. The angiotensin II type 1 receptor (AT1R) antagonist, candesartan, was used to assess whether responses were AT1R dependent and the nitric oxide (NO) synthase inhibitor, N(ω)-nitro-L-arginine methyl ester (L-NAME), was used to assess the involvement of NO in the evoked responses by infusion of angiotensin II. The MAP and heart rate responses were monitored with a pressure transducer.

Results

Local infusion into the NTS of angiotensin II induced a significant to ninefold significantly increase in extracellular GABA levels; as well as MAP was increased by 15 mmHg. These responses were both abolished by co-infusion of either, the angiotensin II type 1 receptor antagonist, candesartan, or the NO synthase inhibitor, L-NAME, demonstrating that the effect is not only AT1R dependent but also NO dependent. The pressor response to angiotensin II was reversed by co-infusion with the GABA_A receptor antagonist, bicuculline. Local blockade of NO synthase decreased both, GABA and glutamate concentrations.

Conclusion

Our results suggest that the AT1R mediated hypertensive response to angiotensin II within the NTS in normotensive rats is GABA and NO dependent. Nitric oxide produced within the NTS tonically potentiates local GABA and glutamate release.

Intermittent low-level lead exposure provokes anxiety, hypertension, autonomic dysfunction and neuroinflammation

BACKGROUND

Exposures to lead (Pb) during developmental phases can alter the normal course of development, with lifelong health consequences. Permanent Pb exposure leads to behavioral changes, cognitive impairment, sympathoexcitation, tachycardia, hypertension and autonomic dysfunction. However, the effects of an intermittent lead exposure are not yet studied. This pattern of exposure has been recently increasing due to migrations, implementation of school exchange programs and/or residential changes.

OBJECTIVE

To determine and compare lead effects on mammal's behavior and physiology, using a rat model of intermittent and permanent Pb exposures.

METHODS

Fetuses were intermittently (PbI) or permanently (PbP) exposed to water containing lead acetate (0.2% w/v) throughout life until adulthood (28 weeks of age). A control group (CTL) without any exposure to lead was also used. Anxiety was assessed by elevated plus maze (EPM) and locomotor activity and exploration by open field test (OFT). Blood pressure (BP), electrocardiogram (ECG), heart rate (HR), respiratory frequency (RF), sympathetic and parasympathetic activity and baro- and chemoreceptor reflex profiles were evaluated. Immunohistochemistry protocol for the assessment of neuroinflammation, neuronal loss (NeuN), gliosis and synaptic alterations (Iba-1, GFAP, Syn), were performed at the hippocampus. One-way ANOVA with Tukey's multiple comparison between means were used (significance p < 0.05) for statistical analysis.

RESULTS

The intermittent lead exposure produced a significant increase in diastolic and mean BP values, concomitant with a tendency to sympathetic overactivity (estimated by increased low-frequency power) and without significant changes in systolic BP, HR and RF. A chemoreceptor hypersensitivity and a baroreflex impairment were also observed, however, less pronounced when compared to the permanent exposure. Regarding behavioral changes, both lead exposure profiles showed an anxiety-like behavior without changes in locomotor and exploratory activity. Increase in GFAP and Iba-1 positive cells, without changes in NeuN positive cells were found in both exposed groups. Syn staining suffered a significant decrease in PbI group and a significant increase in PbP group.

CONCLUSION

This study is the first to show that developmental Pb exposure since fetal period can cause lasting impairments in physiological parameters. The intermittent lead exposure causes adverse health effects, i.e, hypertension, increased respiratory frequency and chemoreflex sensitivity, baroreflex impairment, anxiety, decreased synaptic activity, neuroinflammation and reactive gliosis, in some ways similar to a permanent exposure, however some are lower-grade, due to the shorter duration of exposure. This study brings new insights on the environmental factors that influence autonomic and cardiovascular systems during development, which can help in creating public policy strategies to prevent and control the adverse effects of Pb toxicity.

Effect of centrally acting angiotensin converting enzyme inhibitor on the exercise-induced increases in muscle sympathetic nerve activity

KEY POINTS

The arterial baroreflex's operating point pressure is reset upwards and rightwards from rest in direct relation to the increases in dynamic exercise intensity. The intraneural pathways and signalling mechanisms that lead to upwards and rightwards resetting of the operating point pressure, and hence the increases in central sympathetic outflow during exercise, remain to be identified. We tested the hypothesis that the central production of angiotensin II during dynamic exercise mediates the increases in sympathetic outflow and, therefore, the arterial baroreflex operating point pressure resetting during acute and prolonged dynamic exercise. The results identify that perindopril, a centrally acting angiotensin converting enzyme inhibitor, markedly attenuates the central sympathetic outflow during acute and prolonged dynamic exercise.

ABSTRACT

We tested the hypothesis that the signalling mechanisms associated with the dynamic exercise intensity related increases in muscle sympathetic nerve activity (MSNA) and arterial baroreflex resetting during exercise are located within the central nervous system. Participants performed three randomly ordered trials of 70° upright back-supported dynamic leg cycling after ingestion of placebo and two different lipid soluble angiotensin converting enzyme inhibitors (ACEi): perindopril (high lipid solubility), captopril (low lipid solubility). Repeated measurements of whole venous blood (n = 8), MSNA (n = 7) and arterial blood pressures (n = 14) were obtained at rest and during an acute (SS1) and prolonged (SS2) bout of steady state dynamic exercise. Arterial baroreflex function curves were modelled at rest and during exercise. Peripheral venous superoxide concentrations measured by electron spin resonance spectroscopy were elevated during exercise and were not altered by ACEi at rest (P ≥ 0.4) or during exercise (P ≥ 0.3). Baseline MSNA and mean arterial pressure were unchanged at rest (P ≥ 0.1; P ≥ 0.8, respectively). However, during both SS1 and SS2, the centrally acting ACEi perindopril attenuated MSNA compared to captopril and the placebo (P < 0.05). Arterial pressures at the operating point and threshold pressures were decreased with perindopril from baseline to SS1 with no further changes in the operating point pressure during SS2 under all three conditions. These data suggest that centrally acting ACEi is significantly more effective at attenuating the increase in the acute and prolonged exercise-induced increases in MSNA.

Dysregulation of the Renin-Angiotensin System and the Vasopressinergic System Interactions in Cardiovascular Disorders

Purpose of Review

In many instances, the renin-angiotensin system (RAS) and the vasopressinergic system (VPS) are jointly activated by the same stimuli and engaged in the regulation of the same processes.

Recent Findings

Angiotensin II (Ang II) and arginine vasopressin (AVP), which are the main active compounds of the RAS and the VPS, interact at several levels. Firstly, Ang II, acting on AT1 receptors (AT1R), plays a significant role in the release of AVP from vasopressinergic neurons and AVP, stimulating V1a receptors (V1aR), regulates the release of renin in the kidney. Secondly, Ang II and AVP, acting on AT1R and V1aR, respectively, exert vasoconstriction, increase cardiac contractility, stimulate the sympathoadrenal system, and elevate blood pressure. At the same time, they act antagonistically in the regulation of blood pressure by baroreflex. Thirdly, the cooperative action of Ang II acting on AT1R and AVP stimulating both V1aR and V2 receptors in the kidney is necessary for the appropriate regulation of renal blood flow and the efficient resorption of sodium and water. Furthermore, both peptides enhance the release of aldosterone and potentiate its action in the renal tubules.

Summary

In this review, we (1) point attention to the role of the cooperative action of Ang II and AVP for the regulation of blood pressure and the water-electrolyte balance under physiological conditions, (2) present the subcellular mechanisms underlying interactions of these two peptides, and (3) provide evidence that dysregulation of the cooperative action of Ang II and AVP significantly contributes to the development of disturbances in the regulation of blood pressure and the water-electrolyte balance in cardiovascular diseases.

AT1 and AT2 Receptors in the Prelimbic Cortex Modulate the Cardiovascular Response Evoked by Acute Exposure to Restraint Stress in Rats

The prelimbic cortex (PL) is an important structure in the neural pathway integrating stress responses. Brain angiotensin is involved in cardiovascular control and modulation of stress responses. Blockade of angiotensin receptors has been reported to reduce stress responses. Acute restraint stress (ARS) is a stress model, which evokes sustained blood pressure increase, tachycardia, and reduction in tail temperature. We therefore hypothesized that PL locally generated angiotensin and angiotensin receptors modulate stress autonomic responses. To test this hypothesis, we microinjected an angiotensin-converting enzyme (ACE) inhibitor or angiotensin antagonists into the PL, prior to ARS. Male Wistar rats were used; guide cannulas were bilaterally implanted in the PL for microinjection of vehicle or drugs. A polyethylene catheter was introduced into the femoral artery to record cardiovascular parameters. Tail temperature was measured using a thermal camera. ARS was started 10 min after PL treatment with drugs. Pretreatment with ACE inhibitor lisinopril (0.5 nmol/100 nL) reduced the pressor response, but did not affect ARS-evoked tachycardia. At a dose of 1 nmol/100 nL, it reduced both ARS pressor and tachycardic responses. Pretreatment with candesartan, AT1 receptor antagonist reduced ARS-evoked pressor response, but not tachycardia. Pretreatment with PD123177, AT2 receptor antagonist, reduced tachycardia, but did not affect ARS pressor response. No treatment affected ARS fall in tail temperature. Results suggest involvement of PL angiotensin in the mediation of ARS cardiovascular responses, with participation of both AT1 and AT2 receptors. In conclusion, results indicate that PL AT1-receptors modulate the ARS-evoked pressor response, while AT2-receptors modulate the tachycardic component of the autonomic response.

The renin-angiotensin system: a possible new target for depression

Depression remains a debilitating condition with an uncertain aetiology. Recently, attention has been given to the renin-angiotensin system. In the central nervous system, angiotensin II may be important in multiple pathways related to neurodevelopment and regulation of the stress response. Studies of drugs targeting the renin-angiotensin system have yielded promising results. Here, we review the potential beneficial effects of angiotensin blockers in depression and their mechanisms of action. Drugs blocking the angiotensin system have efficacy in several animal models of depression. While no randomised clinical trials were found, case reports and observational studies showed that angiotensin-converting enzyme inhibitors or angiotensin receptor blockers had positive effects on depression, whereas other antihypertensive agents did not. Drugs targeting the renin-angiotensin system act on inflammatory pathways implicated in depression. Both preclinical and clinical data suggest that these drugs possess antidepressant properties. In light of these results, angiotensin system-blocking agents offer new horizons in mood disorder treatment.

Increased Expression of Macrophage Migration Inhibitory Factor in the Nucleus of the Solitary Tract Attenuates Renovascular Hypertension in Rats

BACKGROUND

Macrophage migration inhibitory factor (MIF) is an intracellular inhibitory regulator of the actions of angiotensin II in the central nervous system. Renovascular hypertensive 2-kidney, 1-clip (2K1C) rats have an increased activity of the renin-angiotensin system and a decrease in baroreflex function compared to normotensive (NT) rats. In the present study, we tested the effects of MIF overexpression within the nucleus of the solitary tract (NTS), a key brainstem region for cardiovascular regulation, on the development of hypertension, on baroreflex function, and on water and food intake in 2K1C rats.

METHODS

Holtzman NT rats received a silver clip around the left renal artery to induce 2K1C hypertension. Three weeks later, rats were microinjected in the NTS with AAV2-CBA-MIF, to increase the expression of MIF, or with the control vector AAV2-CBA-enhanced green fluorescent protein. Mean arterial pressure (MAP) and heart rate were recorded by telemetry. Baroreflex function was tested, and water and food intake were also measured.

RESULTS

Increasing MIF expression in the NTS of 2K1C rats attenuated the development of hypertension, reversed the impairment of baroreflex function, and reduced the increase in water intake. In contrast to 2K1C rats, similar increases in MIF expression in the NTS of NT rats produced no changes in baseline MAP, baroreflex function, or water intake.

CONCLUSIONS

These results indicate that an increased expression of MIF within the NTS attenuates the development of hypertension and restores the baroreflex function in 2K1C rats.

Lead toxicity promotes autonomic dysfunction with increased chemoreceptor sensitivity

Mortality and morbidity by toxic metals is an important issue of occupational health. Lead is an ubiquitous heavy metal in our environment despite having no physiological role in biological systems. Being an homeostatic controller is expected that the autonomic nervous system would show a degree of impairment in lead toxicity. In fact, sympathoexcitation associated to high blood pressure and tachypnea has been described together with baroreflex dysfunction. However, the mechanisms underlying the autonomic dysfunction and the interplay between baro- and chemoreflex are not yet fully clarified. The angiotensinogenic PVN-NTS axis (paraventricular nucleus of the hypothalamus - nucleus tractus solitarius axis) is a particularly important neuronal pathway that could be responsible for the autonomic dysfunction and the cardiorespiratory impairment in lead toxicity. Within the current work, we addressed in vivo, baro- and chemoreceptor reflex behaviour, before and after central angiotensin inhibition, in order to better understand the cardiorespiratory autonomic mechanisms underlying the toxic effects of long-term lead exposure. For that, arterial pressure, heart rate, respiratory rate, sympathetic and parasympathetic activity and baro- and chemoreceptor reflex profiles of anaesthetized young adult rats exposed to lead, from foetal period to adulthood, were evaluated. Results showed increased chemosensitivity together with baroreceptor reflex impairment, sympathetic over-excitation, hypertension and tachypnea. Chemosensitivity and sympathetic overexcitation were reversed towards normality values by NTS treatment with A-779, an angiotensin (1-7) antagonist. No parasympathetic changes were observed before and after A-799 treatment. In conclusion, angiotensin (1-7) at NTS level is involved in the autonomic dysfunction observed in lead toxicity. The increased sensitivity of chemoreceptor reflex expresses the clear impairment of autonomic outflow to the cardiovascular and respiratory systems induced by putative persistent, long duration, alert reaction evoked by the long term exposure to lead toxic effects. The present study brings new insights on the central mechanisms implicated in the autonomic dysfunction induced by lead exposure which are relevant for the development of additional therapeutic options to tackle lead toxicity symptoms.

Compromised blood-brain barrier permeability: novel mechanism by which circulating angiotensin II signals to sympathoexcitatory centres during hypertension

Angiotensin II (AngII) is a pivotal peptide implicated in the regulation of blood pressure. In addition to its systemic vascular and renal effects, AngII acts centrally to modulate the activities of neuroendocrine and sympathetic neuronal networks, influencing in turn sympatho-humoral outflows to the circulation. Moreover, a large body of evidence supports AngII signalling dysregulation as a key mechanism contributing to exacerbated sympathoexcitation during hypertension. Due to its hydrophilic actions, circulating AngII does not cross the blood-brain barrier (BBB), signalling to the brain via the circumventricular organs which lack a tight BBB. In this review, we present and discuss recent studies from our laboratory showing that elevated circulating levels of AngII during hypertension result in disruption of the BBB integrity, allowing access of circulating AngII to critical sympathoexcitatory brain centres such as the paraventricular nucleus of the hypothalamus and the rostral ventrolateral medulla. We propose the novel hypothesis that AngII-driven BBB breakdown constitutes a complementary mechanism by which circulating AngII, working in tandem with the central renin-angiotensin system, further exacerbates sympatho-humoral activation during hypertension. These results are discussed within the context of a growing body of evidence in the literature supporting AngII as a pro-inflammatory signal, and brain microglia as key cell targets mediating central AngII actions during hypertension.

Sympathetic overactivity occurs before hypertension in the two-kidney, one-clip model

Our knowledge of mechanisms responsible for both the development and the maintenance of hypertension remains incomplete in the Goldblatt (two-kidney, one-clip; 2K1C) model. We tested the hypothesis that elevated sympathetic nerve activity (SNA) occurs before the onset of hypertension in 2K1C rats, considering the time course of the increase in SNA in relationship to the onset of the hypertension. We used a decorticated in situ working heart-brainstem preparation of three groups of male Wistar rats, namely sham-operated animals (SHAM, n = 7) and animals 3 weeks post-2K1C, of which some were hypertensive (2K1C-H, n = 6) and others normotensive (2K1C-N, n = 9), as determined in vivo a priori. Perfusion pressure was higher in both 2K1C groups (2K1C-H, 76 ± 1 mmHg; 2K1C-N, 74 ± 3 mmHg; versus SHAM, 60 ± 2 mmHg, P < 0.05). The SNA was significantly elevated in both 2K1C groups (2K1C-H, 47.7 ± 6.1 μV; 2K1C-N, 32.8 ± 2.8 μV; versus SHAM, 20.5 ± 2.5 μV, P < 0.05) owing to its increased respiratory modulation; the chemoreflex was augmented and baroreflex depressed. Precollicular transection reduced SNA in all groups (2K1C-H, -32.5 ± 7.5%; 2K1C-NH, -48 ± 6.9%; versus SHAM, -13.2 ± 1%, P < 0.05). Subsequent medullary spinal cord transection abolished SNA in both SHAM and 2K1C-N groups, but decreased it by only 57 ± 5.5% in 2K1C-H preparations. Thus, SNA is raised before the onset of hypertension, by the third week after renal artery clipping, and this originates, in part, from its enhanced respiratory modulation. Spinal circuits contribute to the elevation of SNA in the 2K1C model, but only after hypertension has developed.

Prolonged blood pressure elevation following continuous infusion of angiotensin II-a baroreflex study in healthy humans

ANG II interacts with the sympathetic nervous system at central nervous blood pressure-regulating structures, including the baroreflex. It is unknown whether prolonged BP elevation mediated by high ANG II plasma levels could induce a persistent shift of the central nervous baroreflex setpoint, lasting beyond the short ANG II plasmatic half time of a few seconds, thereby consolidating elevated BP and/or increased SNA in healthy humans. In a blinded crossover design, ANG II or placebo (saline) was infused for a 6-h period in 12 resting normotensive students (6 males, 6 females) raising BP to borderline hypertensive levels. Between 60 and 120 min after the infusion period, muscle sympathetic nerve activity (MSNA) was assessed microneurographically and correlated with oscillometric BP measurements and heart rate at supine rest (baseline) and during pharmacologic baroreceptor challenge. Infusion of ANG II increased BP to borderline-hypertensive levels, as intended, whereas heart rate remained unaltered. At baroreflex assessment (i.e., 60-120 min after end of infusion period), systolic BP was significantly higher compared with placebo (Δ8.4 ± 3.1 mmHg; P < 0.05), whereas diastolic values were nearly equal between conditions. Baseline MSNA was neither decreased nor increased, and baroreflex sensitivity to vasoactive drug challenge was not altered. Our results show that elevation of ANG II plasma levels over 6 h was able to increase systolic, but not diastolic, BP far beyond blood-mediated ANG II effects. MSNA or heart rate did not counter-regulate this BP elevation, indicating that ANG II had sustainably reset the central nervous BP threshold of sympathetic baroreflex function to accept elevated BP input signals without counter-regulatory response.

Sympathetic neural activity to the cardiovascular system: integrator of systemic physiology and interindividual characteristics

The sympathetic nervous system is a ubiquitous, integrating controller of myriad physiological functions. In the present article, we review the physiology of sympathetic neural control of cardiovascular function with a focus on integrative mechanisms in humans. Direct measurement of sympathetic neural activity (SNA) in humans can be accomplished using microneurography, most commonly performed in the peroneal (fibular) nerve. In humans, muscle SNA (MSNA) is composed of vasoconstrictor fibers; its best-recognized characteristic is its participation in transient, moment-to-moment control of arterial blood pressure via the arterial baroreflex. This property of MSNA contributes to its typical "bursting" pattern which is strongly linked to the cardiac cycle. Recent evidence suggests that sympathetic neural mechanisms and the baroreflex have important roles in the long term control of blood pressure as well. One of the striking characteristics of MSNA is its large interindividual variability. However, in young, normotensive humans, higher MSNA is not linked to higher blood pressure due to balancing influences of other cardiovascular variables. In men, an inverse relationship between MSNA and cardiac output is a major factor in this balance, whereas in women, beta-adrenergic vasodilation offsets the vasoconstrictor/pressor effects of higher MSNA. As people get older (and in people with hypertension) higher MSNA is more likely to be linked to higher blood pressure. Skin SNA (SSNA) can also be measured in humans, although interpretation of SSNA signals is complicated by multiple types of neurons involved (vasoconstrictor, vasodilator, sudomotor and pilomotor). In addition to blood pressure regulation, the sympathetic nervous system contributes to cardiovascular regulation during numerous other reflexes, including those involved in exercise, thermoregulation, chemoreflex regulation, and responses to mental stress.

Disruption of cardiovascular circadian rhythms in mice post myocardial infarction: relationship with central angiotensin II receptor expression

Angiotensin II (Ang II) is well known to participate in the abnormal autonomic cardiovascular control that occurs during the development of chronic heart failure (CHF). Disrupted cardiovascular circadian rhythm in CHF is also well accepted; however, the mechanisms underlying and the role of central Ang II type 1 receptors (AT1R) and oxidative stress in mediating such changes are not clear. In a post myocardial infarction (MI) CHF mouse model we investigated the circadian rhythm for mean arterial pressure (MAP), heart rate (HR), and baroreflex sensitivity (BRS) following MI. The cardiovascular parameters represent the middle 6‐h averages during daytime (6:00–18:00) and nighttime (18:00–6:00). HR increased with the severity of CHF reaching its maximum by 12 weeks post‐MI; loss of circadian HR and BRS rhythms were observed as early as 4 weeks post‐MI in conjunction with a significant blunting of the BRS and an upregulation in the AT1R and gp91^phox proteins in the brainstem. Loss of MAP circadian rhythm was observed 8 weeks post‐MI. Circadian AT1R expression was demonstrated in sham animals but was lost 8 weeks following MI. Losartan reduced AT1R expression in daytime (1.18 ± 0.1 vs. 0.85 ± 0.1; P < 0.05) with a trend toward a reduction in the AT1R mRNA expression in the nighttime (1.2 ± 0.1 vs. 1.0 ± 0.1; P > 0.05) but failed to restore circadian variability. The disruption of circadian rhythm for HR, MAP and BRS along with the upregulation of AT1 and gp91^phox suggests a possible role for central oxidative stress as a mediator of circadian cardiovascular parameters in the post‐MI state.

Increases in central angiotenisn II signaling provide a driving force for sympatho‐excitation in heart failure. In this study, we show a loss of circadian variability in angiotensin type 1 receptor expression in the brainstem of mice post myocardial infarction. These changes correlate with a loss of cardiovascular circadian variability. These data suggest that sympatho‐ excitation may be increased in the post‐MI state at times when sympathetic outflow is normally reduced.

Revealing the role of the autonomic nervous system in the development and maintenance of Goldblatt hypertension in rats

Despite extensive use of the renovascular/Goldblatt model of hypertension—2K-1C, and the use of renal denervation to treat drug resistant hypertensive patients, autonomic mechanisms that underpin the maintenance of this hypertension are important yet remain unclear. Our aim was to analyse cardiovascular autonomic function by power spectral density analysis of both arterial pressure and pulse interval measured continuously by radio telemetry for 6 weeks after renal artery clipping. Mean arterial pressure increased from 106 ± 5 to 185 ± 2 mm Hg during 5 weeks post clipping when it stabilized. A tachycardia developed during the 4th week, which plateaued between weeks 5 and 6. The gain of the cardiac vagal baroreflex decreased immediately after clipping and continued to do so until the 5th week when it plateaued (from − 2.4 ± 0.09 to − 0.8 ± 0.04 bpm/mm Hg; P < 0.05). A similar time course of changes in the high frequency power spectral density of the pulse interval was observed (decrease from 13.4 ± 0.6 to 8.3 ± 0.01 ms²; P < 0.05). There was an increase in both the very low frequency and low frequency components of systolic blood pressure that occurred 3 and 4 weeks after clipping, respectively. Thus, we show for the first time the temporal profile of autonomic mechanisms underpinning the initiation, development and maintenance of renovascular hypertension including: an immediate depression of cardiac baroreflex gain followed by a delayed cardiac sympathetic predominance; elevated sympathetic vasomotor drive occurring after the initiation of the hypertension but coinciding during its mid-development and maintenance.

Central losartan attenuates increases in arterial pressure and expression of FosB/ΔFosB along the autonomic axis associated with chronic intermittent hypoxia

Chronic intermittent hypoxia (CIH) increases mean arterial pressure (MAP) and FosB/ΔFosB staining in central autonomic nuclei. To test the role of the brain renin-angiotensin system (RAS) in CIH hypertension, rats were implanted with intracerebroventricular (icv) cannulae delivering losartan (1 μg/h) or vehicle (VEH) via miniosmotic pumps and telemetry devices for arterial pressure recording. A third group was given the same dose of losartan subcutaneously (sc). Two groups of losartan-treated rats served as normoxic controls. Rats were exposed to CIH or normoxia for 7 days and then euthanized for immunohistochemistry. Intracerebroventricular losartan attenuated CIH-induced increases in arterial pressure during CIH exposure (0800-1600 during the light phase) on days 1, 6, and 7 and each day during the normoxic dark phase. FosB/ΔFosB staining in the organum vasculosum of the lamina terminalis (OVLT), median preoptic nucleus (MnPO), paraventricular nucleus of the hypothalamus (PVN), the rostral ventrolateral medulla (RVLM), and the nucleus of the solitary tract (NTS) was decreased in icv losartan-treated rats. Subcutaneous losartan also reduced CIH hypertension during the last 2 days of CIH and produced bradycardia prior to the effect on blood pressure. Following sc losartan, FosB/ΔFosB staining was reduced only in the OVLT, MnPO, PVN, and NTS. These data indicate that the central and peripheral RAS contribute to CIH-induced hypertension and transcriptional activation of autonomic nuclei and that the contribution of the central RAS is greater during the normoxic dark phase of CIH hypertension.

The expression of angiotensin-converting enzyme 2-angiotensin-(1-7)-Mas receptor axis are upregulated after acute cerebral ischemic stroke in rats

There is now unequivocal evidence that the angiotensin-converting enzyme 2(ACE2)-Ang-(1-7)-Mas axis is a key component of the renin-angiotensin system (RAS) cascade, which is closely correlated with ischemic insult occurrence. Our previous studies demonstrated that the Ang-(1-7), was an active member of the brain RAS. However, the ACE2-Ang-(1-7)-Mas axis expression after cerebral ischemic injury are currently unclear. In the present study, we investigated the time course of ACE2-Ang-(1-7) and Mas receptor expression in the acute stage of cerebral ischemic stroke. The content of Ang-(1-7) in ischemic tissues and blood serum was measured by specific EIA kits. Real-time PCR and western blot were used to determine messenger RNA (mRNA) and protein levels of the ACE2 and Mas. The cerebral ischemic lesion resulted in a significant increase of regional cerebral and circulating Ang-(1-7) at 6-48 h compared with sham operation group following focal ischemic stroke (12h: 7.276±0.320 ng/ml vs. 2.466±0.410 ng/ml, serum; 1.024±0.056 ng/mg vs. 0.499±0.032, brain) (P<0.05). Both ACE2 and Mas expression were markedly enhanced compared to the control in the ischemic tissues (P<0.05). Mas immunopositive neurons were also seen stronger expression in the ischemic cortex (19.167±2.858 vs. 7.833±2.483) (P<0.05). The evidence collected in our present study will indicate that, ACE2-Ang-(1-7)-Mas axis are upregulated after acute ischemic stroke and would play a pivotal role in the regulation of acute neuron injury in ischemic cerebrovascular diseases.

Angiotensin-(1-12) in the rostral ventrolateral medullary pressor area of the rat elicits sympathoexcitatory responses

The rostral ventrolateral medullary pressor area (RVLM) is known to be critical in the regulation of cardiovascular function. In this study, it was hypothesized that the RVLM may be one of the sites of cardiovascular actions of a newly discovered angiotensin, angiotensin-(1-12) [Ang-(1-12)]. Experiments were carried out in urethane-anaesthetized, artificially ventilated, adult male Wistar rats. The RVLM was identified by microinjections of L-glutamate (5 mM). The volume of all microinjections into the RVLM was 100 nl. Microinjections of Ang-(1-12) (0.1-1.0 mM) into the RVLM elicited increases in mean arterial pressure and heart rate. Maximal cardiovascular responses were elicited by 0.5 mM Ang-(1-12); this concentration was used in the other experiments described. Microinjections of Ang-(1-12) increased greater splanchnic nerve activity. The tachycardic responses to Ang-(1-12) were not altered by bilateral vagotomy. The cardiovascular responses elicited by Ang-(1-12) were attenuated by microinjections of an angiotensin II type 1 receptor (AT(1)R) antagonist (losartan), but not an AT(2)R antagonist (PD123319), into the RVLM. Combined inhibition of angiotensin-converting enzyme and chymase in the RVLM abolished Ang-(1-12)-induced responses. Angiotensin-(1-12)-immunoreactive cells were present in the RVLM. Angiotensin II type 1 receptors and phenylethanolamine-N-methyl-transferase were present in the RVLM neurons retrogradely labelled by microinjections of Fluoro-Gold into the intermediolateral cell column of the thoracic spinal cord. Angiotensin-(1-12)-containing neurons in the hypothalamic paraventricular nucleus did not project to the RVLM. These results indicated that: (1) microinjections of Ang-(1-12) into the RVLM elicited increases in mean arterial pressure, heart rate and greater splanchnic nerve activity; (2) both angiotensin-converting enzyme and chymase were needed to convert Ang-(1-12) into angiotensin II; and (3) AT(1)Rs, but not AT(2)Rs, in the RVLM mediated the Ang-(1-12)-induced responses.

Angiotensin-converting enzyme inhibition, but not AT(1) receptor blockade, in the solitary tract nucleus improves baroreflex sensitivity in anesthetized transgenic hypertensive (mRen2)27 rats

Transgenic hypertensive (mRen2)27 rats overexpress the murine Ren2 gene and have impaired baroreflex sensitivity (BRS) for control of the heart rate. Removal of endogenous angiotensin (Ang)-(1-7) tone using a receptor blocker does not further lower BRS. Therefore, we assessed whether blockade of Ang II with a receptor antagonist or combined reduction in Ang II and restoration of endogenous Ang-(1-7) levels with Ang-converting enzyme (ACE) inhibition will improve BRS in these animals. Bilateral solitary tract nucleus (nTS) microinjections of the AT(1) receptor blocker, candesartan (CAN, 24 pmol in 120 nl, n=9), or a peptidic ACE inhibitor, bradykinin (BK) potentiating nonapeptide (Pyr-Trp-Pro-Arg-Pro-Gln-Ile-Pro-Pro; BPP9α, 9 nmol in 60 nl, n=12), in anesthetized male (mRen2)27 rats (15-25 weeks of age) show that AT(1) receptor blockade had no significant effect on BRS, whereas microinjection of BPP9α improved BRS over 60-120 min. To determine whether Ang-(1-7) or BK contribute to the increase in BRS, separate experiments using the Ang-(1-7) receptor antagonist D-Ala(7)-Ang-(1-7) or the BK antagonist HOE-140 showed that only the Ang-(1-7) receptor blocker completely reversed the BRS improvement. Thus, acute AT(1) blockade is unable to reverse the effects of long-term Ang II overexpression on BRS, whereas ACE inhibition restores BRS over this same time frame. As the BPP9α potentiation of BK actions is a rapid phenomenon, the likely mechanism for the observed delayed increase in BRS is through ACE inhibition and elevation of endogenous Ang-(1-7).

Cardiovascular actions of angiotensin-(1-12) in the hypothalamic paraventricular nucleus of the rat are mediated via angiotensin II

The role of the hypothalamic paraventricular nucleus (PVN) in cardiovascular regulation is well established. In this study, it was hypothesized that the PVN may be one of the sites of cardiovascular actions of a newly discovered angiotensin, angiotensin-(1-12). Experiments were carried out in urethane-anaesthetized, artificially ventilated, adult male Wistar rats. The PVN was identified by microinjections of NMDA (10 mm). Microinjections (50 nl) of angiotensin-(1-12) (1 mm) into the PVN elicited increases in mean arterial pressure, heart rate and renal sympathetic nerve activity. The tachycardic responses to angiotensin-(1-12) were attenuated by bilateral vagotomy. The cardiovascular responses elicited by angiotensin-(1-12) were attenuated by microinjections of an angiotensin II type 1 receptor (AT(1)R) antagonist (losartan), but not an angiotensin II type 1 receptor (AT(2)R) antagonist (PD123319), into the PVN. Combined inhibition of angiotensin-converting enzyme and chymase in the PVN abolished angiotensin-(1-12)-induced responses. Angiotensin-(1-12)-immunoreactive cells and fibres were more numerous in the middle and caudal regions of the PVN. Angiotensin-(1-12) was present in many, but not all, vasopressinergic PVN cells. This peptide was also present in some non-vasopressinergic PVN cells, but not in oxytocin-containing PVN cells. These results can be summarized as follows: (1) microinjections of angiotensin-(1-12) into the PVN elicited increases in mean arterial pressure, heart rate and renal sympathetic nerve activity; (2) heart rate responses were mediated via both sympathetic and vagus nerves; (3) both angiotensin-converting enzyme and chymase were needed to convert angiotensin-(1-12) to angiotensin II in the PVN; and (4) AT(1)Rs, but not AT(2)Rs, in the PVN mediated angiotensin-(1-12)-induced responses. It was concluded that the cardiovascular actions of angiotensin-(1-12) in the PVN are mediated via its conversion to angiotensin II.

Contributions of vascular inflammation in the brainstem for neurogenic hypertension

Essential hypertension is idiopathic although it is accepted as a complex polygenic trait with underlying genetic components, which remain unknown. Our supposition is that primary hypertension involves activation of the sympathetic nervous system. One pivotal region controlling arterial pressure set point is nucleus tractus solitarii (NTS). We recently identified that pro-inflammatory molecules, such as junctional adhesion molecule-1, were over expressed in endothelial cells of the microvasculature supplying the NTS in an animal model of human hypertension (the spontaneously hypertensive rat: SHR) compared to normotensive Wistar Kyoto (WKY) rats. We have also shown endogenous leukocyte accumulation inside capillaries within the NTS of SHR but not WKY rats. Despite the inflammatory state in the NTS of SHR, transcripts of some inflammatory molecules such as chemokine (C-C motif) ligand 5 (Ccl5), and its receptors, chemokine (C-C motif) receptor 1 and 3 were down-regulated in the NTS of SHR compared to WKY rats. This may be compensatory to avoid further strong inflammatory activity. More importantly, we found that down-regulation of Ccl5 in the NTS of SHR may be pro-hypertensive since microinjection of Ccl5 into the NTS of SHR decreased arterial pressure but was less effective in WKY rats. Leukocyte accumulation of the NTS microvasculature may also induce an increase in vascular resistance and hypoperfusion within the NTS; the latter may trigger release of pro-inflammatory molecules which via paracrine signaling may affect central neural cardiovascular activity conducive to neurogenic hypertension. All told, we suggest that vascular inflammation within the brainstem contributes to neurogenic hypertension by multiple pathways.

Autonomic nervous system adjustment (ANSA) in patients with hypertension treated with enalapril

BACKGROUND AND AIM

Reduced baroreflex sensitivity (BRS), decreased heart rate variability (HRV) and increased blood pressure (BP) variability have serious consequences for target organ damage in patients with hypertension, beside the BP level. The study was aimed to evaluate acute and long-term effects of enalapril, on BRS and HRV in individuals with prehypertension and mild essential hypertension.

METHODOLOGY

We enrolled in the study 85 patients (male 53%, age 42-67) with prehypertension and mild hypertension (systolic blood pressure ≥120 mmHg or diastolic blood pressure ≥80 mmHg). All patients were tested before, 30 minutes after first oral enalapril dose and three weeks after monotherapy with enalapril. The methodology included: beat to beat HRV analysis, BRS measurements with sequence technique, ECG with short term and 24-hour HRV analysis, 24-hour ambulatory blood pressure monitoring with systolic and diastolic BP variability analysis. Patients were divided into groups with sympathetic or vagal predominance based on Autonomic Nervous System Adjustment (ANSA) method.

RESULTS

1) 30 min after initial dose enalapril significantly and effectively reduced systolic BP with shift towards the parasympathetic predominance as reflected in decreased heart rate and low frequency (LF) to high frequency (HF) ratio (LF/HF ratio); 2) chronic enalapril therapy effectively reduced BP, including both systolic BP, diastolic BP and pulls pressure, had positively influence on dipping status, but did not significantly change BP variability; 3) in the whole patient group chronic enalapril therapy did not significantly change HRV, but it significantly decreased LF(nu) in the patient with high basal sympathetic activity, and significantly increased LF(nu) in patients with high basal vagal activity; enalapril also significantly decreased HF (nu) in patients with high basal parasympathetic activity; 4) despite initial, transit changes in maximal and minimal slope of BRS enalapril did not significantly affect sensitivity of spontaneous baroreflex activation.

CONCLUSION

Initial enalapril dose significantly reduced systolic BP and had beneficial effects on autonomic tone and baroreflex sensitivity. Chronic enalapril therapy effectively reduced BP without significant influence on BRS, but had significant autonomic effects on HRV when initial autonomic profile in each patient was analyzed by ANSA. Analyzed by ANSA method chronic enalapril therapy managed to retrieve disturbed sympathovagal balance and established autonomic equilibrium.

Autonomic control of the heart is altered in Sprague-Dawley rats with spontaneous hydronephrosis

The renal medulla plays an important role in cardiovascular regulation, through interactions with the autonomic nervous system. Hydronephrosis is characterized by substantial loss of renal medullary tissue. However, whether alterations in autonomic control of the heart are observed in this condition is unknown. Thus we assessed resting hemodynamics and baroreflex sensitivity (BRS) for control of heart rate in urethane/chloralose-anesthetized Sprague-Dawley rats with normal or hydronephrotic kidneys. While resting arterial pressure was similar, heart rate was higher in rats with hydronephrosis (290 ± 12 normal vs. 344 ± 11 mild/moderate vs. 355 ± 13 beats/min severe; P < 0.05). The evoked BRS to increases, but not decreases, in pressure was lower in hydronephrotic rats (1.06 ± 0.06 normal vs. 0.72 ± 0.10 mild/moderate vs. 0.63 ± 0.07 ms/mmHg severe; P < 0.05). Spectral analysis methods confirmed reduced parasympathetic function in hydronephrosis, with no differences in measures of indirect sympathetic activity among conditions. As a secondary aim, we investigated whether autonomic dysfunction in hydronephrosis is associated with activation of the renin-angiotensin system (RAS). There were no differences in circulating angiotensin peptides among conditions, suggesting that the impaired autonomic function in hydronephrosis is independent of peripheral RAS activation. A possible site for angiotensin II-mediated BRS impairment is the solitary tract nucleus (NTS). In normal and mild/moderate hydronephrotic rats, NTS administration of the angiotensin II type 1 receptor antagonist candesartan significantly improved the BRS, suggesting that angiotensin II provides tonic suppression to the baroreflex. In contrast, angiotensin II blockade produced no significant effect in severe hydronephrosis, indicating that at least within the NTS baroreflex suppression in these animals is independent of angiotensin II.

Angiotensin peptides and central autonomic regulation

Aging, hypertension, and fetal-programmed cardiovascular disease are associated with a functional deficiency of angiotensin (Ang)-(1-7) in the brain dorsomedial medulla. The resulting unrestrained activity of Ang II in brainstem regions negatively impacts resting mean arterial pressure, sympathovagal balance, and baroreflex sensitivity for control of heart rate. The differential effects of Ang II and Ang-(1-7) may be related to the cellular sources of these peptides as well as different precursor pathways. Long-term alterations of the brain renin-angiotensin system may influence signaling pathways including phosphoinositol-3-kinase and mitogen-activated protein kinase and their downstream mediators, and as a consequence may influence metabolic function. Differential regulation of signaling pathways in aging and hypertension by Ang II versus Ang-(1-7) may contribute to the autonomic dysfunction accompanying these states.

Brain angiotensin peptides regulate sympathetic tone and blood pressure

Brain angiotensin II (Ang II) induces tonic sympathoexcitatory effects through AT1 receptor stimulation of glutamatergic neurons and sympathoinhibitory effects via GABAergic neurons in the rostral ventrolateral medulla, the brainstem 'pressor area'. NADPH-derived superoxide production and reactive oxygen species signalling is critical in these actions, and AT2 receptors in the rostral ventrolateral medulla appear to mediate opposing effects on sympathetic outflow. In the hypothalamic paraventricular nucleus, Ang II has AT1 receptor-mediated sympathoexcitatory effects and enhances nitric oxide formation, which in turn inhibits the Ang II effects through a GABAergic mechanism. Ang II also decreases the tonic sympathoinhibitory effect of gamma amino butyric acid within the paraventricular nucleus. Angiotensin III and Angiotensin IV increase blood pressure via brain AT1 receptor stimulation. Angiotensin (1-7) influences cardiovascular function through a specific Mas-receptor. This review examines the evidence that brain angiotensin peptides, glutamate, gamma amino butyric acid and nitric oxide interact within the rostral ventrolateral medulla and paraventricular nucleus to control sympathetic tone and blood pressure.

Cardiovascular responses elicited by a new endogenous angiotensin in the nucleus tractus solitarius of the rat

Cardiovascular effects of angiotensin-(1-12) [ANG-(1-12)] were studied in the medial nucleus of the tractus solitarius (mNTS) in anesthetized, artificially ventilated, adult male Wistar rats. Microinjections (100 nl) of ANG-(1-12) (0.06 mM) into the mNTS elicited maximum decreases in mean arterial pressure (MAP; 34 ± 5.8 mmHg) and heart rate (HR; 39 ± 3.7 beats/min). Bilateral vagotomy abolished ANG-(1-12)-induced bradycardia. Efferent greater splanchnic nerve activity was decreased by microinjections of ANG-(1-12) into the mNTS. Blockade of ANG type 1 receptors (AT(1)Rs; using ZD-7155 or L-158,809), but not ANG type 2 receptors (AT(2)Rs; using PD-123319), significantly attenuated ANG-(1-12)-induced cardiovascular responses. Simultaneous inhibition of both angiotensin-converting enzyme (ACE; using captopril) and chymase (using chymostatin) completely blocked the effects of ANG-(1-12). Microinjections of A-779 [ANG-(1-7) antagonist] did not attenuate ANG-(1-12)-induced responses. Pressure ejection of ANG-(1-12) (0.06 mM, 2 nl) caused excitation of barosensitive mNTS neurons, which was blocked by prior application of the AT(1)R antagonist. ANG-(1-12)-induced excitation of mNTS neurons was also blocked by prior sequential applications of captopril and chymostatin. These results indicate that 1) microinjections of ANG-(1-12) into the mNTS elicited depressor and bradycardic responses by exciting barosensitive mNTS neurons; 2) the decreases in MAP and HR were mediated via sympathetic and vagus nerves, respectively; 3) AT(1)Rs, but not AT(2)Rs, mediated these actions of ANG-(1-12); 4) the responses were mediated via the conversion of ANG-(1-12) to ANG II and both ACE and chymase were involved in this conversion; and 5) ANG-(1-7) was not one of the metabolites of ANG-(1-12) in the mNTS.

Response of cardiac sympathetic nerve activity to intravenous irbesartan in heart failure

To determine the effect of irbesartan treatment on resting levels and arterial baroreflex control of cardiac sympathetic nerve activity (CSNA) in heart failure (HF), we studied conscious normal sheep and sheep with HF induced by rapid ventricular pacing for 8-10 wk (n = 7 per group). In HF, there is a large increase in CSNA that is detrimental to outcome. The causes of this increase in CSNA and the effect of angiotensin receptor blockers on CSNA in HF are unclear. CSNA, arterial blood pressure, heart rate (HR), and arterial baroreflex curves were recorded during a resting period and after 90 min of irbesartan infusion (12 mg.kg(-1).h(-1) iv). This dose of irbesartan abolished the pressor response to intravenous ANG II infusion but caused only a slight decrease in the pressor response to centrally administered ANG II. In HF, there was a large increase in CSNA (from 44 +/- 3 to 87 +/- 3 bursts/100 heartbeats). Irbesartan reduced arterial pressure in the normal and HF groups, but the usual baroreflex-mediated increases in CSNA and HR were prevented. This resulted from a significant leftward shift in the CSNA and HR baroreflex curves in both groups. Irbesartan also decreased the sensitivity of the arterial baroreflex control of CSNA. Short-term treatment with an angiotensin receptor blocker, at a dose that abolished the response to circulating, but not central, ANG II, prevented the reflex increase in CSNA in response to the drug-induced fall in arterial pressure.

IL-6 microinjected in the nucleus tractus solitarii attenuates cardiac baroreceptor reflex function in rats

Recent gene array and molecular studies have suggested that an abnormal gene expression profile of interleukin-6 (IL-6) in the nucleus tractus solitarii (NTS), a pivotal region for regulating arterial pressure, may be related to the development of neurogenic hypertension. However, the precise functional role of IL-6 in the NTS remains unknown. In the present study, we have tested whether IL-6 affects cardiovascular control at the level of the NTS. IL-6 (1, 10, and 100 fmol) was microinjected in the NTS of Wistar rats (280-350 g) under urethane anesthesia. Although the baseline levels of arterial pressure and heart rate did not change following IL-6 injections, the cardiac baroreflex in response to increased arterial pressure was dose-dependently attenuated. In addition, IL-6 (100 fmol) microinjections also attenuated l-glutamate-induced bradycardia at the level of the NTS. Immunohistochemical detection of IL-6 in naïve rats demonstrated that it was predominantly observed in neurons within the brain stem, including the NTS. These findings suggest that IL-6 within the NTS may play an important role for regulating cardiovascular control via modulation of input signals from baroreceptor afferents. Whether the abnormal gene expression of IL-6 in the NTS is associated in a causal way with hypertension remains to be resolved.

High levels of circulating angiotensin II shift the open-loop baroreflex control of splanchnic sympathetic nerve activity, heart rate and arterial pressure in anesthetized rats

Although an acute arterial pressure (AP) elevation induced by intravenous angiotensin II (ANG II) does not inhibit sympathetic nerve activity (SNA) compared to an equivalent AP elevation induced by phenylephrine, there are conflicting reports as to how circulating ANG II affects the baroreflex control of SNA. Because most studies have estimated the baroreflex function under closed-loop conditions, differences in the rate of input pressure change and the magnitude of pulsatility may have biased the estimation results. We examined the effects of intravenous ANG II (10 microg kg(-1) h(-1)) on the open-loop system characteristics of the carotid sinus baroreflex in anesthetized and vagotomized rats. Carotid sinus pressure (CSP) was raised from 60 to 180 mmHg in increments of 20 mmHg every minute, and steady-state responses in systemic AP, splanchnic SNA and heart rate (HR) were analyzed using a four-parameter logistic function. ANG II significantly increased the minimum values of AP (67.6 +/- 4.6 vs. 101.4 +/- 10.9 mmHg, P < 0.01), SNA (33.3 +/- 5.4 vs. 56.5 +/- 11.5%, P < 0.05) and HR (391.1 +/- 13.7 vs. 417.4 +/- 11.5 beats/min, P < 0.01). ANG II, however, did not attenuate the response range for AP (56.2 +/- 7.2 vs. 49.7 +/- 6.2 mmHg), SNA (69.6 +/- 5.7 vs. 78.9 +/- 9.1%) or HR (41.7 +/- 5.1 vs. 51.2 +/- 3.8 beats/min). The maximum gain was not affected for AP (1.57 +/- 0.28 vs. 1.20 +/- 0.25), SNA (1.94 +/- 0.34 vs. 2.04 +/- 0.42%/mmHg) or HR (1.11 +/- 0.12 vs. 1.28 +/- 0.19 beats min(-1) mmHg(-1)). It is concluded that high levels of circulating ANG II did not attenuate the response range of open-loop carotid sinus baroreflex control for AP, SNA or HR in anesthetized and vagotomized rats.

Shift to an involvement of phosphatidylinositol 3-kinase in angiotensin II actions on nucleus tractus solitarii neurons of the spontaneously hypertensive rat

RATIONALE

Central angiotensin (Ang) II inhibits baroreflex and plays an important role in the pathogenesis of hypertension. However, the underlying molecular mechanisms are still not fully understood.

OBJECTIVE

Our objective in the present study was to characterize the signal transduction mechanism of phosphatidylinositol 3-kinase (PI3K) involvement in Ang II-induced stimulation of central neuronal activity in cultured neurons and Ang II-induced inhibition of baroreflex in spontaneously hypertensive rats (SHR) versus WKY rats.

METHODS AND RESULTS

Application of Ang II to neurons produced a 42% greater increase in neuronal firing in cells from the SHR than the WKY rat. Although the Ang II-mediated increase in firing rate was abolished entirely by the protein kinase (PK)C inhibitor GF109230 in the WKY, blockade of both PKC and PI3K activity was necessary in the SHR. This was associated with an increased ability of Ang II to stimulate NADPH oxidase-reactive oxygen species (ROS)-mediated signaling involving phosphorylation of the p47phox subunit of the NADPH oxidase and was dependent on the activation of PI3K in the SHR. Inhibition of PI3K resulted in the reduction of levels of p47phox phosphorylation, NADPH oxidase activity, ROS levels, and ultimately neuronal activity in cells from the SHR but not the WKY rat. In addition, in working heart-brainstem preparations, inhibition of PKC activity in the nucleus of the solitary tract in situ abolished the Ang II-mediated depression of cardiac and sympathetic baroreceptor reflex gain in the WKY. In contrast, PKC inhibition in the nucleus of the solitary tract of SHR only partially reduced the effect of Ang II on the baroreceptor reflex gain.

CONCLUSIONS

These observations demonstrate that PI3K in the cardiovascular brainstem regions of the SHR may be selectively involved in Ang II-mediated signaling that includes a reduction in baroreceptor reflex function, presumably via a NADPH-ROS mediated pathway.

The cardiovascular response of normal rats to dual lesion of the subfornical organ and area postrema at rest and to chronic losartan

The subfornical organ (SFO) and the area postrema (AP), two of the sensory circumventricular organs (CVO), are known to play a role in the chronic central control of blood pressure. In previous studies in which these regions were independently lesioned, the chronic hypotensive effects of the AT(1) receptor blocker losartan (10 mg/kg/day) were attenuated by ~15 mm Hg. In the present study, we sought to investigate the effect of concurrent lesion of both the SFO and the AP on the cardiovascular effects of chronic losartan infusion in order to test the hypothesis that a greater attenuation of the hypotensive effects of losartan would be observed in rats with dual lesions. To do so, arterial pressure and heart rate responses to 10-day infusion of losartan were compared in sham rats and those with dual lesions of the AP and SFO. Two important findings resulted from this study. First, dual lesion rats exhibited a sustained and significant decrease in resting blood pressure (83+/-1 mm Hg vs. 104+/-1 mm Hg, respectively) and heart rate (356+/-3 bpm vs. 398+/-6 bpm, respectively) compared to sham animals. Secondly, rats with concurrent lesion of both the AP and the SFO demonstrated a significantly attenuated response to losartan compared to sham animals but showed no greater attenuation of losartan's chronic hypotensive effects than animals with lesion of either the SFO or the AP (approximately 15 mm Hg). Although these results do not support the stated hypothesis, they do suggest redundancy and compensatory roles of the AP and SFO in basal cardiovascular control.

Angiotensin II enhances GABA(B) receptor-mediated responses and expression in nucleus tractus solitarii of rats

Angiotensin II (ANG II) increases GABA(B) receptor expression in neuronal cultures from the nucleus tractus solitarii (NTS). In the present study, the chronic effects of ANG II on GABA(B) receptor expression and activity were examined in the NTS of Sprague-Dawley rats. Intracerebroventricular infusion of ANG II caused a significant elevation in blood pressure (BP) and an increase in GABA(B) receptor expression in the NTS. Conversely, chronic N(G)-nitro-l-arginine methyl ester (l-NAME) treatment also increased BP, but had no effect on GABA(B) receptor expression in the NTS. Next, we examined the BP response to the GABA(B) receptor agonist baclofen microinjected into the NTS of ANG II- or artificial cerebrospinal fluid (aCSF)-infused rats. NTS microinjection of baclofen increased BP in both groups of rats. However, the pressor response to baclofen was enhanced in ANG II-infused rats compared with aCSF-infused rats. In addition, bilateral microinjection of the GABA(B) receptor antagonist CGP-35348 into the NTS evoked a decrease in BP in both group of rats, and the depressor responses to CGP-35348 were enhanced in the ANG II-infused rats. In contrast, the pressor responses to the GABA(A) receptor agonist muscimol and the depressor responses to the GABA(A) receptor antagonist bicuculline were comparable between aCSF- and ANG II-infused rats. These results indicate that chronic ANG II infusion stimulates GABA(B) receptor expression and augments GABA(B) receptor-mediated responses in the NTS. This effect could contribute to the central nervous system actions of ANG II that result in dampening of baroreflexes and elevation in arterial BP.

Mild chronic hypoxemia modifies expression of brain stem angiotensin peptide receptors and reflex responses in fetal sheep

The effects of chronic mild hypoxemia on the binding of angiotensin receptors in selected brain stem nuclei and reflex responses were studied in fetal sheep. Fetal and maternal catheters were placed at 120 days' gestation, and animals received intratracheal maternal administration of nitrogen (n = 16) or compressed air in controls (n = 19). Nitrogen infusion was adjusted to reduce fetal brachial artery PO(2) by 25% during 5 days. Spontaneous baroreflex sensitivity and spectral analysis of the pulse interval were analyzed during the 5 days hypoxemia period using 90 min of daily recording. Brains of control and hypoxemic animals were collected, and brain stem angiotensin receptor binding was studied by in vitro autoradiography at 130 days of gestation. After 5 days of hypoxemia, some animals in each group were submitted to one complete umbilical cord occlusion during 5 min. [(125)I]sarthran binding showed that chronic mild hypoxemia significantly increases angiotensin type 1 receptor, angiotensin type 2 receptor, and ANG-(1-7) angiotensin receptor binding sites in the nucleus tractus solitarius and dorsal motor nucleus of the vagus (P < 0.05). Hypoxemia induced lower baroreflex sensitivity and a higher low frequency-to-high frequency ratio in the fetus, consistent with a shift from vagal to sympathetic autonomic cardiac regulation. Cord occlusion to elicit a chemoreflex response induced a greater bradycardic response in hypoxemic fetuses (slope of the initial fall in heart rate; 11.3 +/- 1.9 vs. 6.4 +/- 1.2 beats x min(-1) x s(-1), P < 0.05). In summary, chronic mild hypoxemia increased binding of angiotensin receptors in brain stem nuclei, decreased spontaneous baroreflex gain, and increased chemoreflex responses to asphyxia in the fetus. These results suggest hypoxemia-induced alterations in brain stem mechanisms for cardiovascular control.

Neurohormonal regulation of the sympathetic nervous system: new insights into central mechanisms of action

To regulate blood pressure, the brain controls circulating hormones, which influence the brain by binding to brain neurons that lie outside the blood-brain barrier. Recent work has demonstrated that "cardiovascular" hormones are synthesized and released in the brain as neurotransmitters/neuromodulators and can, in some cases, signal through the blood-brain barrier. The renin-angiotensin system is a prototype for these newly appreciated mechanisms. The brain's intrinsic renin-angiotensin system plays an important role in blood pressure control. Angiotensin II in brain neurons affects other neurons both through activation of angiotensin receptors and via generation of nitric oxide and reactive oxygen molecules. Similarly, angiotensin in blood vessels activates endothelial nitric oxide, which can diffuse across the blood-brain barrier and thereby alter neuronal activity in cardiovascular control nuclei. The relative importance of these mechanisms to blood pressure control remains to be fully elucidated.

The enhanced hyperglycemic response to hemorrhage hypotension in obese rats is related to an impaired baroreflex

The aim of the present study was to assess the metabolic adjustments in adult rats with autonomic imbalance induced by hypercaloric diet. Male Wistar rats (4 weeks of age) were fed a chow diet (CD, n = 12) or hypercaloric diet (HD, n = 13) for 19 weeks. Body weight and dietary intake were measured every week and the basal metabolic rate was assessed. After 19 weeks of diet, six animals from each group were anesthetized with a lethal dose of barbital sodium (100 mg/Kg body weight, intraperitoneal; i.p.). Lee index was evaluated and adipose pads weighted. The remaining animals had a silastic cannula placed into the jugular vein for drug administration, blood collecting, and hemorrhage (1.2 mL/100 g bw/2 min). A polyethylene catheter (PE50) was inserted into the abdominal artery through the femoral artery for cardiovascular monitoring. The assessment of autonomic balance was done by evaluation of baroreflex sensitivity (intravenous (IV) injection of phenylephrine and sodium nitroprusside) and hemorrhage (1.2 mL/100 g bw/2 min). As expected, the HD induced obesity; increased weight gain (28%), adipose pads weight, and baroreflex dysfunction. The plasma level of free fatty acids and triacylglycerols were increased in HD rats by about 124% and 424%, respectively, as well as the basal metabolic rate measured at 19th weeks of diet (p < 0.01). We observed that baroreflex sensitivity to phenylephrine was reduced by about 50%, and the hyperglycemic response to hemorrhage hypotension was increased by 128% in HD rats. We found also a negative correlation between the alteration in baroreflex sensitivity and the increase in hyperglycemic response to hemorrhage in the obese rats (r = 0.72, p < 0.01) and a strong positive correlation between the increased Lee index and the hemorrhagic hyperglycemia (r = 0.93, p < 0.01). Our data demonstrate that obesity induced by hypercaloric diet in Wistar rats promotes an autonomic imbalance, which interferes with metabolic responses dependent on baroreflex sensitivity. In addition, we showed the existence of close correlation between the loss of baroreflex sensitivity and the degree of obesity.