Central interactions of aldosterone and angiotensin II in aldosterone- and angiotensin II-induced hypertension

Control of sodium appetite by hindbrain aldosterone-sensitive neurons

Mineralocorticoids play a key role in hydromineral balance by regulating sodium retention and potassium wasting. Through favoring sodium, mineralocorticoids can cause hypertension from fluid overload under conditions of hyperaldosteronism, such as aldosterone-secreting tumors. An often-overlooked mechanism by which aldosterone functions to increase sodium is through stimulation of salt appetite. To drive sodium intake, aldosterone targets neurons in the hindbrain which uniquely express 11β-hydroxysteroid dehydrogenase type 2 (HSD2). This enzyme is a necessary precondition for aldosterone-sensing cells as it metabolizes glucocorticoids - preventing their activation of the mineralocorticoid receptor. In this review, we will consider the role of hindbrain HSD2 neurons in regulating sodium appetite by discussing HSD2 expression in the brain, regulation of hindbrain HSD2 neuron activity, and the circuitry mediating the effects of these aldosterone-sensitive neurons. Reducing the activity of hindbrain HSD2 neurons may be a viable strategy to reduce sodium intake and cardiovascular risk, particularly for conditions of hyperaldosteronism.

Brain-Splenic Immune System Interactions in Hypertension: Cellular and Molecular Mechanisms

Hypertension represents a major worldwide cause of death and disability, and it is becoming increasingly clear that available therapies are not sufficient to reduce the risk of major cardiovascular events. Various mechanisms contribute to blood pressure increase: neurohormonal activation, autonomic nervous system imbalance, and immune activation. Of note, the brain is an important regulator of blood pressure levels; it recognizes the peripheral perturbation and organizes a reflex response by modulating immune system and hormonal release to attempt at restoring the homeostasis. The connection between the brain and peripheral organs is mediated by the autonomic nervous system, which also modulates immune and inflammatory responses. Interestingly, an increased autonomic nervous system activity has been correlated with an altered immune response in cardiovascular diseases. The spleen is the largest immune organ exerting a potent influence on the cardiovascular system during disease and is characterized by a dense noradrenergic innervation. Taken together, these aspects led to hypothesize a key role of neuroimmune mechanisms in the onset and progression of hypertension. This review discusses how the nervous and splenic immune systems interact and how the mechanisms underlying the neuroimmune cross talk influence the disease progression.

Unilateral chemical ablation of the adrenal gland lowers blood pressure and alleviates target organ damage in spontaneously hypertensive rats

Adrenal gland hormones play a critical role in the development and maintenance of hypertension. Adrenal ablation has been used to treat primary aldosteronism but not essential hypertension. The present study aimed to investigate the effectiveness and safety of unilateral adrenal gland ablation in treating spontaneously hypertensive rats (SHR). SHR and Wistar-Kyoto rats (WKY) were subjected to unilateral adrenal ablation with injection of anhydrous ethanol or a sham procedure. Blood pressure was monitored by both tail-cuff plethysmography and telemetry until 6 months after the procedure. Adrenal ablation significantly lowered systolic and diastolic blood pressure of the SHR (P < 0.05 or P < 0.01) but not WKY from 4 to 24 weeks after the procedure. Adrenal ablation substantially damaged adrenal cortex and medulla with fibrosis in SHR and WKY rats. The ablation procedure remarkably reduced the levels of renin, angiotensin II, aldosterone, cortisol, noradrenaline, and epinephrine in SHR (all P < 0.05) but not in WKY. Hypokalemia in SHR was significantly improved by adrenal ablation (P < 0.05), while the serum sodium levels were not affected by adrenal ablation in either SHR or WKY rats. Additionally, adrenal ablation improved cardiac, renal, and vascular remodeling and function measured 3 months after the procedure in SHR. In conclusion, the present study shows that ethanol ablation of adrenal gland can effectively lower blood pressure and prevent target organ damage in SHR. These findings suggest that unilateral debulking of the adrenal gland using ethanol ablation is a promising therapeutic strategy for treating hypertension.

METHODS

Spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) were subjected to unilateral adrenal ablation with injection of ethanol or a sham procedure. Blood pressure was monitored for 24 weeks.

RESULTS

Adrenal ablation significantly lowered systolic and diastolic blood pressure of SHR but not WKY from 4 to 24 weeks after the procedure.

CONCLUSION

Unilateral debulking of the adrenal gland using ethanol ablation is a promising therapeutic strategy for treating hypertension.

Association between serum uric acid/high-density lipoprotein cholesterol ratio and hypertension among reproductive-aged women

BACKGROUND

Uric acid/high-density lipoprotein cholesterol ratio (UHR) is a novel index of inflammation and metabolism that has been investigated in various diseases. However, association between UHR and hypertension among reproductive-aged women is unclear.

METHODS

In this cross-sectional study, we investigated the association between serum UHR and hypertension among 5485 women aged 20-44 years based on the National Health and Nutrition Examination Survey (NHANES) database using various methods, including univariate and multivariate logistic regression analysis, stratified analysis, and spline regression. P < 0.05 was considered statistically significant.

RESULTS

There was significant difference in UHR between the women with and without hypertension (P < 0.001). After adjusting for several covariates, UHR was positively correlated with hypertension (OR > 1, P < 0.001). In the subgroup analysis, the positive correlations still remained between UHR and hypertension in women with various age and those with BMI ≥ 30 kg/m2 (P < 0.05) excepted for adjusting for all covariates. We further found an inflection point of the threshold effect for UHR, and the prevalence of hypertension showed different increased trends below and above the threshold.

CONCLUSION

This study indicated a positive association between serum UHR and hypertension among reproductive-aged women, indicating that UHR is a potential clinical marker of hypertension in women.

Inhibition of Microbiota-dependent Trimethylamine N-Oxide Production Ameliorates High Salt Diet-Induced Sympathetic Excitation and Hypertension in Rats by Attenuating Central Neuroinflammation and Oxidative Stress

Excessive dietary salt intake induces neuroinflammation and oxidative stress in the brain, which lead to sympathetic excitation, contributing to hypertension. However, the underlying mechanisms remain elusive. Accumulating evidence reveals that trimethylamine-N-oxide (TMAO), a gut microbiota-derived metabolite, is implicated in the pathogenesis of multiple cardiovascular diseases. The present study sought to determine whether central TMAO is elevated and associated with neuroinflammation and oxidative stress in the brain after long-term high salt (HS) diet intake and, if so, whether inhibition of TMAO generation ameliorates HS-induced sympathetic excitation and hypertension. Sprague-Dawley rats were fed either a HS diet or a normal salt (NS) diet and simultaneously treated with vehicle (VEH) or 1.0% 3,3-Dimethyl-1-butanol (DMB, an inhibitor of trimethylamine formation) for 8 weeks. HS + VEH rats, compared with NS + VEH rats, had elevated TMAO in plasma and cerebrospinal fluid (CSF), increased blood pressure (BP), and increased sympathetic drive as indicated by the BP response to ganglionic blockade and plasma norepinephrine levels. HS-induced these changes were attenuated by DMB, which significantly reduced TMAO in plasma and CSF. Neuroinflammation as assessed by proinflammatory cytokine expression and NF-κB activity and microglial activity, and oxidative stress as measured by NAD(P)H oxidase subunit expression and NAD(P)H activity and reactive oxygen species (ROS) production in the hypothalamic paraventricular nucleus (PVN) were increased in HS + VEH rats but were decreased by DMB. DMB had no effects on above measured parameters in NS rats. The results suggest that long-term HS diet intake causes elevation in TMAO in the circulation and brain, which is associated with increased neuroinflammation and oxidative stress in the PVN, an important cardiovascular regulatory center. Inhibition of TMAO generation ameliorates HS-induced sympathetic excitation and hypertension by reducing neuroinflammation and oxidative stress in the PVN.

Voluntary Exercise Prevents Hypertensive Response Sensitization Induced by Angiotensin II

Exercise training has profound effects on the renin-angiotensin system, inflammatory cytokines and oxidative stress, all of which affect autonomic nervous system activity and regulate blood pressure (BP) in both physiological and pathophysiological states. Using the Induction-Delay-Expression paradigm, our previous studies demonstrated that various challenges (stressors) during Induction resulted in hypertensive response sensitization (HTRS) during Expression. The present study tested whether voluntary exercise would protect against subpressor angiotensin (ANG) II-induced HTRS in rats. Adult male rats were given access to either “blocked” (sedentary rats) or functional running (exercise rats) wheels for 12 weeks, and the Induction-Delay-Expression paradigm was applied for the rats during the last 4 weeks. A subpressor dose of ANG II given during Induction produced an enhanced hypertensive response to a pressor dose of ANG II given during Expression in sedentary rats in comparison to sedentary animals that received saline (vehicle control) during Induction. Voluntary exercise did not attenuate the pressor dose of ANG II-induced hypertension but prevented the expression of HTRS seen in sedentary animals. Moreover, voluntary exercise reduced body weight gain and feed efficiency, abolished the augmented BP reduction after ganglionic blockade, reversed the increased mRNA expression of pro-hypertensive components, and upregulated mRNA expression of antihypertensive components in the lamina terminalis and hypothalamic paraventricular nucleus, two key brain nuclei involved in the control of sympathetic activity and BP regulation. These results indicate that exercise training plays a beneficial role in preventing HTRS and that this is associated with shifting the balance of the brain prohypertensive and antihypertensive pathways in favor of attenuated central activity driving sympathetic outflow and reduced BP.

Extrarenal Effects of Aldosterone on Potassium Homeostasis

The role of aldosterone in regulating K⁺ excretion in the distal nephron is well established in kidney physiology. In addition to effects on the kidney, aldosterone modulates K⁺ and Na⁺ transport in salivary fluid, sweat, airway epithelia, and colonic fluid. More controversial and less well defined is the role of aldosterone in determining the internal distribution of K⁺ across cell membranes in nontransporting epithelia. In vivo studies have been limited by the difficulty in accurately measuring overall K⁺ balance and factoring in both variability and secondary changes in acid-base balance, systemic hemodynamics, and other K⁺-regulatory factors such as hormones and adrenergic activity. Despite these limitations, the aggregate data support a contributory role of aldosterone along with insulin and catecholamines in the normal physiologic regulation of internal K⁺ distribution. The authors speculate differences in tissue sensitivity to aldosterone may also contribute to differential tissue response of cardiac and skeletal muscle to conditions of total body K⁺ depletion.

Role of the central renin‑angiotensin system in hypertension (Review)

Present in more than one billion adults, hypertension is the most significant modifiable risk factor for mortality resulting from cardiovascular disease. Although its pathogenesis is not yet fully understood, the disruption of the renin-angiotensin system (RAS), consisting of the systemic and brain RAS, has been recognized as one of the primary reasons for several types of hypertension. Therefore, acquiring sound knowledge of the basic science of RAS and the under- lying mechanisms of the signaling pathways associated with RAS may facilitate the discovery of novel therapeutic targets with which to promote the management of patients with cardiovascular and kidney disease. In total, 4 types of angiotensin II receptors have been identified (AT1R-AT4R), of which AT1R plays the most important role in vasoconstriction and has been most extensively studied. It has been found in several regions of the brain, and its distribution is highly associated with that of angiotensin-like immunoreactivity in nerve terminals. The effect of AT1R involves the activation of multiple media and signaling pathways, among which the most important signaling pathways are considered to be AT1R/JAK/STAT and Ras/Raf/MAPK pathways. In addition, the regulation of the nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) and cyclic AMP response element-binding (CREB) pathways is also closely related to the effect of ATR1. Their mechanisms of action are related to pro-inflammatory and sympathetic excitatory effects. Central AT1R is involved in almost all types of hypertension, including spontaneous hypertension, salt-sensitive hypertension, obesity-induced hypertension, renovascular hypertension, diabetic hypertension, L-NAME-induced hypertension, stress-induced hypertension, angiotensin II-induced hyper- tension and aldosterone-induced hypertension. There are 2 types of central AT1R blockade, acute blockade and chronic blockade. The latter can be achieved by chemical blockade or genetic engineering. The present review article aimed to high- light the prevalence, functions, interactions and modulation means of central AT-1R in an effort to assist in the treatment of several pathological conditions. The identification of angiotensin-derived peptides and the development of AT-2R agonists may provide a wider perspective on RAS, as well as novel therapeutic strategies.

Role of Mineralocorticoid and Angiotensin Type 1 Receptors in the Paraventricular Nucleus in Angiotensin-Induced Hypertension

The hypothalamic paraventricular nucleus (PVN) is an important site where an interaction between circulating angiotensin (Ang) and mineralocorticoid receptor (MR) activity may modify sympathetic nerve activity (SNA) to influence long-term elevation of blood pressure. We examined in conscious Ang II-treated rabbits, the effects on blood pressure and tonic and reflex renal SNA (RSNA) of microinjecting into the PVN either RU28318 to block MR, losartan to block Ang (AT1) receptors or muscimol to inhibit GABA A receptor agonist actions. Male rabbits received a moderate dose of Ang II (24 ng/kg/min subcutaneously) for 3 months (n = 13) or sham treatment (n = 13). At 3 months, blood pressure increased by +19% in the Ang II group compared to 10% in the sham (P = 0.022) but RSNA was similar. RU28318 lowered blood pressure in both Ang II and shams but had a greater effect on RSNA and heart rate in the Ang II-treated group (P < 0.05). Losartan also lowered RSNA, while muscimol produced sympatho-excitation in both groups. In Ang II-treated rabbits, RU28318 attenuated the blood pressure increase following chemoreceptor stimulation but did not affect responses to air jet stress. In contrast losartan and muscimol reduced blood pressure and RSNA responses to both hypoxia and air jet. While neither RU28318 nor losartan changed the RSNA baroreflex, RU28318 augmented the range of the heart rate baroreflex by 10% in Ang II-treated rabbits. Muscimol, however, augmented the RSNA baroreflex by 11% in sham animals and none of the treatments altered baroreflex sensitivity. In conclusion, 3 months of moderate Ang II treatment promotes activation of reflex RSNA principally via MR activation in the PVN, rather than via activation of AT1 receptors. However, the onset of hypertension is independent of both. Interestingly, the sympatho-excitatory effects of muscimol in both groups suggest that overall, the PVN regulates a tonic sympatho-inhibitory influence on blood pressure control.

Hypertension and Age-Related Cognitive Impairment: Common Risk Factors and a Role for Precision Aging

Purpose of Review

Precision Aging® is a novel concept that we have recently employed to describe how the model of precision medicine can be used to understand and define the multivariate risks that drive age-related cognitive impairment (ARCI). Hypertension and cardiovascular disease are key risk factors for both brain function and cognitive aging. In this review, we will discuss the common mechanisms underlying the risk factors for both hypertension and ARCI and how the convergence of these mechanisms may be amplified in an individual to drive changes in brain health and accelerate cognitive decline.

Recent Findings

Currently, our cognitive health span does not match our life span. Age-related cognitive impairment and preventing and treating ARCI will require an in-depth understanding of the interrelated risk factors, including individual genetic profiles, that affect brain health and brain aging. Hypertension and cardiovascular disease are important risk factors for ARCI. And, many of the risk factors for developing hypertension, such as diabetes, smoking, stress, viral infection, and age, are shared with the development of ARCI. We must first understand the mechanisms common to the converging risk factors in hypertension and ARCI and then design person-specific therapies to optimize individual brain health.

Summary

The understanding of the convergence of shared risk factors between hypertension and ARCI is required to develop individualized interventions to optimize brain health across the life span. We will conclude with a discussion of possible steps that may be taken to decrease ARCI and optimize an individual’s cognitive life span.

ACE2, Much More Than Just a Receptor for SARS-COV-2

The rapidly evolving pandemic of severe acute respiratory syndrome coronavirus (SARS-CoV-2) infection worldwide cost many lives. The angiotensin converting enzyme-2 (ACE-2) has been identified as the receptor for the SARS-CoV-2 viral entry. As such, it is now receiving renewed attention as a potential target for anti-viral therapeutics. We review the physiological functions of ACE2 in the cardiovascular system and the lungs, and how the activation of ACE2/MAS/G protein coupled receptor contributes in reducing acute injury and inhibiting fibrogenesis of the lungs and protecting the cardiovascular system. In this perspective, we predominantly focus on the impact of SARS-CoV-2 infection on ACE2 and dysregulation of the protective effect of ACE2/MAS/G protein pathway vs. the deleterious effect of Renin/Angiotensin/Aldosterone. We discuss the potential effect of invasion of SARS-CoV-2 on the function of ACE2 and the loss of the protective effect of the ACE2/MAS pathway in alveolar epithelial cells and how this may amplify systemic deleterious effect of renin-angiotensin aldosterone system (RAS) in the host. Furthermore, we speculate the potential of exploiting the modulation of ACE2/MAS pathway as a natural protection of lung injury by modulation of ACE2/MAS axis or by developing targeted drugs to inhibit proteases required for viral entry.

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.

MiR155-5p Inhibits Cell Migration and Oxidative Stress in Vascular Smooth Muscle Cells of Spontaneously Hypertensive Rats

Migration of vascular smooth muscle cells (VSMCs) is essential for vascular reconstruction in hypertension and several vascular diseases. Our recent study showed that extracellular vesicles derived from vascular adventitial fibroblasts of normal rats inhibited VSMC proliferation by delivering miR155-5p to VSMCs. It is unknown whether miR155-5p inhibits cell migration and oxidative stress in VSMCs of spontaneously hypertensive rats (SHR) and in angiotensin II (Ang II)-treated VSMCs. The purpose of this study was to determine the role of miR155-5p in VSMC migration and its underlying mechanisms. Primary VSMCs were isolated from the aortic media of Wistar-Kyoto rats (WKY) and SHR. Wound healing assay and Boyden chamber assay were used to evaluate VSMC migration. A miR155-5p mimic inhibited, and a miR155-5p inhibitor promoted the migration of VSMC of SHR but had no significant effect on the migration of VSMC of WKY. The miR155-5p mimic inhibited angiotensin-converting enzyme (ACE) mRNA and protein expression in VSMCs. It also reduced superoxide anion production, NAD(P)H oxidase (NOX) activity, as well as NOX2, interleukin-1β (IL-1β), and tumor necrosis factor α (TNF-α) expression levels in VSMCs of SHR but not in VSMCs of WKY rats. Overexpression of miR155-5p inhibited VSMC migration and superoxide anion and IL-1β production in VSMCs of SHR but had no impact on exogenous Ang II-induced VSMC migration and on superoxide anion and IL-1β production in WKY rats and SHR. These results indicate that miR155-5p inhibits VSMC migration in SHR by suppressing ACE expression and its downstream production of Ang II, superoxide anion, and inflammatory factors. However, miR155-5p had no effects on exogenous Ang II-induced VSMC migration.

ACE2, Much More Than Just a Receptor for SARS-COV-2

The rapidly evolving pandemic of severe acute respiratory syndrome coronavirus (SARS-CoV-2) infection worldwide cost many lives. The angiotensin converting enzyme-2 (ACE-2) has been identified as the receptor for the SARS-CoV-2 viral entry. As such, it is now receiving renewed attention as a potential target for anti-viral therapeutics. We review the physiological functions of ACE2 in the cardiovascular system and the lungs, and how the activation of ACE2/MAS/G protein coupled receptor contributes in reducing acute injury and inhibiting fibrogenesis of the lungs and protecting the cardiovascular system. In this perspective, we predominantly focus on the impact of SARS-CoV-2 infection on ACE2 and dysregulation of the protective effect of ACE2/MAS/G protein pathway vs. the deleterious effect of Renin/Angiotensin/Aldosterone. We discuss the potential effect of invasion of SARS-CoV-2 on the function of ACE2 and the loss of the protective effect of the ACE2/MAS pathway in alveolar epithelial cells and how this may amplify systemic deleterious effect of renin-angiotensin aldosterone system (RAS) in the host. Furthermore, we speculate the potential of exploiting the modulation of ACE2/MAS pathway as a natural protection of lung injury by modulation of ACE2/MAS axis or by developing targeted drugs to inhibit proteases required for viral entry.

Adrenal Tissue-Specific Deletion of TASK Channels Causes Aldosterone-Driven Angiotensin II-Independent Hypertension

The renin-angiotensin system tightly controls aldosterone synthesis. Dysregulation is evident in hypertension (primary aldosteronism), low renin, and resistant hypertension) but also can exist in normotension. Whether chronic, mild aldosterone autonomy can elicit hypertension remains untested. Previously, we reported that global genetic deletion of 2 pore-domain TWIK-relative acid-sensitive potassium channels, TASK-1 and TASK-3, from mice produces striking aldosterone excess, low renin, and hypertension. Here, we deleted TASK-1 and TASK-3 channels selectively from zona glomerulosa cells and generated a model of mild aldosterone autonomy with attendant hypertension that is aldosterone-driven and Ang II (angiotensin II)-independent. This study shows that a zona glomerulosa-specific channel defect can produce mild autonomous hyperaldosteronism sufficient to cause chronic blood pressure elevation.

New Insights on the Role of Sodium in the Physiological Regulation of Blood Pressure and Development of Hypertension

A precise maintenance of sodium and fluid balance is an essential step in the regulation of blood pressure and alterations of this balance may lead to the development of hypertension. In recent years, several new advances were made in our understanding of the interaction between sodium and blood pressure regulation. The first is the discovery made possible with by new technology, such as ²³Na-MRI, that sodium can be stored non-osmotically in tissues including the skin and muscles particularly when subjects are on a high sodium diet or have a reduced renal capacity to excrete sodium. These observations prompted the refinement of the original model of regulation of sodium balance from a two-compartment model comprising the extracellular fluid within the intravascular and interstitial spaces to a three-compartment model that includes the intracellular space of some tissues, most prominently the skin. In this new model, the immune system plays a role, thereby supporting many previous studies indicating that the immune system is a crucial co-contributor to the maintenance of hypertension through pro-hypertensive effects in the kidney, vasculature, and brain. Lastly, there is now evidence that sodium can affect the gut microbiome, and induce pro-inflammatory and immune responses, which might contribute to the development of salt-sensitive hypertension.

Central nervous system neuroplasticity and the sensitization of hypertension

The causes of essential hypertension remain an enigma. Interactions between genetic and external factors are generally recognized to act as aetiological mechanisms that trigger the pathogenesis of high blood pressure. However, the questions of which genes and factors are involved, and when and where such interactions occur, remain unresolved. Emerging evidence indicates that the hypertensive response to pressor stimuli, like many other physiological and behavioural adaptations, can become sensitized to particular stimuli. Studies in animal models show that, similarly to other response systems controlled by the brain, hypertensive response sensitization (HTRS) is mediated by neuroplasticity. The brain circuitry involved in HTRS controls the sympathetic nervous system. This Review outlines evidence supporting the phenomenon of HTRS and describes the range of physiological and psychosocial stressors that can produce a sensitized hypertensive state. Also discussed are the cellular and molecular changes in the brain neural network controlling sympathetic tone involved in long-term storage of information relating to stressors, which could serve to maintain a sensitized state. Finally, this Review concludes with a discussion of why a sensitized hypertensive response might previously have been beneficial and increased biological fitness under some environmental conditions and why today it has become a health-related liability.

Elevated Oxidative Stress and Inflammation in Hypothalamic Paraventricular Nucleus Are Associated With Sympathetic Excitation and Hypertension in Rats Exposed to Chronic Intermittent Hypoxia

Obstructive sleep apnea (OSA), characterized by recurrent collapse of the upper airway during sleep leading to chronic intermittent hypoxia (CIH), is an independent risk factor for hypertension. Sympathetic excitation has been shown to play a major role in the pathogenesis of OSA-associated hypertension. Accumulating evidence indicates that oxidative stress and inflammation in the hypothalamic paraventricular nucleus (PVN), a critical cardiovascular and autonomic center, mediate sympathetic excitation in many cardiovascular diseases. Here we tested the hypothesis that CIH elevates oxidative stress and inflammation in the PVN, which might be associated with sympathetic excitation and increased blood pressure in a rat model of CIH that mimics the oxygen profile in patients with OSA. Sprague-Dawley rats were pretreated with intracerebroventricular (ICV) infusion of vehicle or superoxide scavenger tempol, and then exposed to control or CIH for 7 days. Compared with control+vehicle rats, CIH+vehicle rats exhibited increased blood pressure, and increased sympathetic drive as indicated by the blood pressure response to ganglionic blockade and plasma norepinephrine levels. Pretreatment with ICV tempol prevented CIH-induced increases in blood pressure and sympathetic drive. Molecular studies revealed that expression of NAD(P)H oxidase subunits, production of reactive oxygen species, expression of proinflammatory cytokines and neuronal excitation in the PVN were elevated in CIH+vehicle rats, compared with control+vehicle rats, but were normalized or reduced in CIH rat pretreated with ICV tempol. Notably, CIH+vehicle rats also had increased systemic oxidative stress and inflammation, which were not altered by ICV tempol. The results suggest that CIH induces elevated oxidative stress and inflammation in the PVN, which lead to PVN neuronal excitation and are associated with sympathetic excitation and increased blood pressure. Central oxidative stress and inflammation may be novel targets for the prevention and treatment of hypertension in patients with OSA.

Aldosterone infusion into the 4th ventricle produces sodium appetite with baroreflex attenuation independent of renal or blood pressure changes

Aldosterone infusion into the 4th ventricle (4th V), upstream the nucleus of the solitary tract (NTS), produces strong 0.3 M NaCl intake. In the present study, we investigated whether aldosterone infusion into the 4th V activates HSD2 neurons, changes renal excretion, or alters blood pressure and cardiovascular reflexes. Chronic infusion of aldosterone (100 ng/h) into the 4th V increased daily 0.3 M NaCl intake (up to 44 ± 10, vs. vehicle: 5.6 ± 3.4 ml/24 h) and also c-Fos expression in HSD2 neurons in the NTS and in non-HSD2 neurons in the NTS. Natriuresis, diuresis and positive sodium balance were present in rats that ingested 0.3 M NaCl, however, renal excretion was not modified by 4th V aldosterone in rats that had no access to NaCl. 4th V aldosterone also reduced baroreflex sensitivity (-2.8 ± 0.5, vs. vehicle: -5.1 ± 0.9 bpm/mmHg) in animals that had sodium available, without changing blood pressure. The results suggest that sodium intake induced by aldosterone infused into the 4th V is associated with activation of NTS neurons, among them the HSD2 neurons. Aldosterone infused into the 4th V in association with sodium intake also impairs baroreflex sensitivity, without changing arterial pressure.

Role of Kidneys in Sex Differences in Angiotensin II-Induced Hypertension

The significance of kidneys in regulation of sodium and water balance and hemodynamics has been demonstrated both in patients and animal models. In the present study, we tested our hypothesis that kidneys play an essential role in control of sex differences in angiotensin II (Ang II)-dependent hypertension. Kidney transplantations (KTXs) were performed between male (M) and female (F) C57BL/6 mice (donor→recipient: F→F, M→M, F→M, and M→F). Radiotelemetry transmitters were implanted for measurement of mean arterial pressure during the infusion of Ang II (600 ng·kg^-1·min^-1). Gene expressions and inflammatory responses in the transplanted grafts were assessed. We found that same-sex-KTX mice still exhibited sex differences in Ang II-dependent hypertension (31.3±0.8 mm Hg in M→M versus 12.2±0.6 mm Hg in F→F), which were reduced between males and females when they received kidneys of the opposite sex (32.9±1 mm Hg in M→F versus 22.3±0.7 mm Hg in F→M). The sex differences in gene expressions, including AT₁R (angiotensin II receptor, type 1), AT₁R/AT₂R, ET-1 (endothelin-1), ETA (endothelin receptor type A), NHE3 (sodium-hydrogen exchanger 3), α-ENaC (α-epithelial sodium channel), and γ-ENaC, were unaltered in same-sex KTXs and much lessened in cross-sex KTXs. In addition, the cross-sex KTXs exhibited more robust inflammatory responses reflected by higher expression of IL-6 (interleukin 6), TNFα (tumor necrosis factor α), and KC (keratinocyte-derived chemokine) than same-sex KTX. Our results indicate that kidneys play an essential role in sex differences of Ang II-dependent hypertension. KTX of male kidneys to females augmented the blood pressure response, whereas KTX of female kidneys to males attenuated the blood pressure response. The host's extrarenal systems modulate expressions of many genes and inflammatory response, which may also contribute to the sex differences in blood pressure regulation.

Update on angiotensin II: new endocrine connections between the brain, adrenal glands and the cardiovascular system

In the brain, angiotensinergic pathways play a major role in chronic regulation of cardiovascular and electrolyte homeostasis. Increases in plasma angiotensin II (Ang II), aldosterone, [Na⁺] and cytokines can directly activate these pathways. Chronically, these stimuli also activate a slow neuromodulatory pathway involving local aldosterone, mineralocorticoid receptors (MRs), epithelial sodium channels and endogenous ouabain (EO). This pathway increases AT₁R and NADPH oxidase subunits and maintains/further increases the activity of angiotensinergic pathways. These brain pathways not only increase the setpoint of sympathetic activity per se, but also enhance its effectiveness by increasing plasma EO and EO-dependent reprogramming of arterial and cardiac function. Blockade of any step in this slow pathway or of AT₁R prevents Ang II-, aldosterone- or salt and renal injury-induced forms of hypertension. MR/AT₁R activation in the CNS also contributes to the activation of sympathetic activity, the circulatory and cardiac RAAS and increase in circulating cytokines in HF post MI. Chronic central infusion of an aldosterone synthase inhibitor, MR blocker or AT₁R blocker prevents a major part of the structural remodeling of the heart and the decrease in LV function post MI, indicating that MR activation in the CNS post MI depends on aldosterone, locally produced in the CNS. Thus, Ang II, aldosterone and EO are not simply circulating hormones that act on the CNS but rather they are also paracrine neurohormones, locally produced in the CNS, that exert powerful effects in key CNS pathways involved in the long-term control of sympathetic and neuro-endocrine function and cardiovascular homeostasis.

Mineralocorticoid receptor antagonists attenuate exaggerated exercise pressor reflex responses in hypertensive rats

Exaggerated heart rate (HR) and blood pressure responses to exercise in hypertension are mediated, in part, by overactivity of the exercise pressor reflex (EPR). The mechanisms underlying this EPR dysfunction have not been fully elucidated. Previous studies have shown that stimulation of mineralocorticoid receptors (MRs) with exogenous administration of aldosterone in normal, healthy rats reproduces the EPR overactivity characteristic of hypertensive animals. Conversely, the purpose of this study was to examine whether antagonizing MR with spironolactone (SPIR) or eplerenone (EPL) in decerebrated hypertensive rats ameliorates abnormal EPR function. Changes in mean arterial pressure (MAP) and HR induced by EPR or muscle mechanoreflex (a component of EPR) activation were assessed in normotensive Wistar-Kyoto rats and spontaneously hypertensive rats (SHRs) fed normal chow (NC) or a customized diet containing either SPIR or EPL for 3 wk. SHRs treated with SPIR or EPL had significantly attenuated MAP responses to EPR (NC: 45 ± 7 mmHg, SPIR: 26 ± 4 mmHg, and EPL: 24 ± 5 mmHg, P = 0.02) and mechanoreflex (NC: 34 ± 9 mmHg, SPIR: 17 ± 3 mmHg, and EPL: 15 ± 3 mmHg, P = 0.03) activation. SHRs treated with SPIR or EPL also showed significantly attenuated HR responses to EPR (NC: 17 ± 3 beats/min, SPIR: 9 ± 1 beats/min, and EPL: 9 ± 2 beats/min, P = 0.01) and mechanoreflex (NC: 15 ± 3 beats/min, SPIR: 6 ± 1 beats/min, and EPL: 7 ± 1 beats/min, P = 0.01) activation. Wistar-Kyoto rats treated with SPIR did not demonstrate significant differences in MAP or HR responses to EPR or mechanoreflex activation. The data suggest that antagonizing MRs may be an effective strategy for the treatment of EPR overactivity in hypertension.NEW & NOTEWORTHY Exaggerated cardiovascular responses to exercise in hypertensive patients are linked with overactive exercise pressor reflexes (EPRs). Administration of low-dose mineralocorticoid receptor antagonists (spironolactone or eplerenone) effectively ameliorates abnormal EPR function in hypertension. Effective treatment of EPR overactivity may reduce the cardiovascular risks associated with physical activity in hypertension.

Hypothalamic and inflammatory basis of hypertension

Hypertension is a major health problem with great consequences for public health. Despite its role as the primary cause of significant morbidity and mortality associated with cardiovascular disease, the pathogenesis of essential hypertension remains largely unknown. The central nervous system (CNS) in general, and the hypothalamus in particular, are intricately involved in the development and maintenance of hypertension. Over the last several decades, the understanding of the brain's role in the development of hypertension has dramatically increased. This brief review is to summarize the neural mechanisms of hypertension with a focus on neuroendocrine and neurotransmitter involvement, highlighting recent findings that suggest that hypothalamic inflammation disrupts key signalling pathways to affect the central control of blood pressure, and therefore suggesting future development of interventional strategies that exploit recent findings pertaining to the hypothalamic control of blood pressure as well as the inflammatory-sympathetic mechanisms involved in hypertension.

Mineralocorticoid receptor associates with pro-inflammatory bias in the hippocampus of spontaneously hypertensive rats

Damage observed in the hippocampus of the adult spontaneously hypertensive rat (SHR) resembles the neuropathology of mineralocorticoid-induced hypertension, supporting a similar endocrine dysfunction in both entities. In the present study, we tested the hypothesis that increased expression of the hippocampal mineralocorticoid receptor (MR) in SHR animals is associated with a prevalent expression of pro-inflammatory over anti-inflammatory factors. Accordingly, in the hippocampus, we measured mRNA expression and immunoreactivity of the MR and glucocorticoid receptor (GR) using a quantitative polymerase chain reaction and histochemistry. We also measured serum-glucocorticoid-activated kinase 1 (Sgk1 mRNA), the number and phenotype of Iba1+ microglia, as well as mRNA expression levels of the pro-inflammatory factors cyclo-oxygenase 2 (Cox2), Nlrp3 inflammasome and tumour necrosis factor α (Tnfα). Expression of anti-inflammatory transforming growth factor (Tgf)β mRNA and the NADPH-diaphorase activity of nitric oxide synthase (NOS) were also determined. The results showed that, in the hippocampus of SHR rats, expression of MR and the number of immunoreactive MR/GR co-expressing cells were increased compared to Wistar-Kyoto control animals. Expression of Sgk1, Cox2, Nlrp3 and the number of ramified glia cells positive for Iba1+ were also increased, whereas Tgfβ mRNA expression and the NADPH-diaphorase activity of NOS were decreased. We propose that, in the SHR hippocampus, increased MR expression causes a bias towards a pro-inflammatory phenotype characteristic for hypertensive encephalopathy.

30 YEARS OF THE MINERALOCORTICOID RECEPTOR: Nongenomic effects via the mineralocorticoid receptor

The mineralocorticoid receptor (MR) belongs to the steroid hormone receptor family and classically functions as a ligand-dependent transcription factor. It is involved in water-electrolyte homeostasis and blood pressure regulation but independent from these effects also furthers inflammation, fibrosis, hypertrophy and remodeling in cardiovascular tissues. Next to genomic effects, aldosterone elicits very rapid actions within minutes that do not require transcription or translation and that occur not only in classical MR epithelial target organs like kidney and colon but also in nonepithelial tissues like heart, vasculature and adipose tissue. Most of these effects can be mediated by classical MR and its crosstalk with different signaling cascades. Near the plasma membrane, the MR seems to be associated with caveolin and striatin as well as with receptor tyrosine kinases like EGFR, PDGFR and IGF1R and G protein-coupled receptors like AT1 and GPER1, which then mediate nongenomic aldosterone effects. GPER1 has also been named a putative novel MR. There is a close interaction and functional synergism between the genomic and the nongenomic signaling so that nongenomic signaling can lead to long-term effects and support genomic actions. Therefore, understanding nongenomic aldosterone/MR effects is of potential relevance for modulating genomic aldosterone effects and may provide additional targets for intervention.

Sex differences in vascular physiology and pathophysiology: estrogen and androgen signaling in health and disease

Sex differences between women and men are often overlooked and underappreciated when studying the cardiovascular system. It has been long assumed that men and women are physiologically similar, and this notion has resulted in women being clinically evaluated and treated for cardiovascular pathophysiological complications as men. Currently, there is increased recognition of fundamental sex differences in cardiovascular function, anatomy, cell signaling, and pathophysiology. The National Institutes of Health have enacted guidelines expressly to gain knowledge about ways the sexes differ in both normal function and diseases at the various research levels (molecular, cellular, tissue, and organ system). Greater understanding of these sex differences will be used to steer future directions in the biomedical sciences and translational and clinical research. This review describes sex-based differences in the physiology and pathophysiology of the vasculature, with a special emphasis on sex steroid receptor (estrogen and androgen receptor) signaling and their potential impact on vascular function in health and diseases (e.g., atherosclerosis, hypertension, peripheral artery disease, abdominal aortic aneurysms, cerebral aneurysms, and stroke).

Third-generation Mineralocorticoid Receptor Antagonists: Why Do We Need a Fourth?

The first mineralocorticoid receptor (MR) antagonist, spironolactone, was developed almost 60 years ago to treat primary aldosteronism and pathological edema. Its use waned in part because of its lack of selectivity. Subsequently, knowledge of the scope of MR function was expanded along with clinical evidence of the therapeutic importance of MR antagonists to prevent the ravages of inappropriate MR activation. Forty-two years elapsed between the first and MR-selective second generation of MR antagonists. Fifteen years later, despite serious shortcomings of the existing antagonists, a third-generation antagonist has yet to be marketed. Progress has been slowed by the lack of appreciation of the large variety of cell types that express the MR and its diverse cell-type-specific actions, and also its unique complex interaction actions at the molecular level. New MR antagonists should preferentially target the inflammatory and fibrotic effects of MR and perhaps its excitatory effects on sympathetic nervous system, but not the renal tubular epithelium or neurons of the cortex and hippocampus. This review briefly describes efforts to develop a third-generation MR antagonist and why fourth generation antagonists and selective agonists based on structural determinants of tissue and ligand-specific MR activation should be contemplated.

Intracerebroventricular tempol administration in older rats reduces oxidative stress in the hypothalamus but does not change STAT3 signalling or SIRT1/AMPK pathway

Hypothalamic inflammation and increased oxidative stress are believed to be mechanisms that contribute to obesity. 4-Hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (tempol), a free radical scavenger, has been shown to reduce inflammation and oxidative stress. We hypothesized that brain infusion of tempol would reduce oxidative stress, and thus would reduce food intake and body weight and improve body composition in rats with age-related obesity and known elevated oxidative stress. Furthermore, we predicted an associated increase in markers of leptin signalling, including the silent mating type information regulator 2 homolog 1 (SIRT1)/5'AMP-activated protein kinase (AMPK) pathway and the signal transducer and activator of transcription 3 (STAT3) pathway. For this purpose, osmotic minipumps were placed in the intracerebroventricular region of young (3 months) and aged (23 months) male Fischer 344 x Brown Norway rats for the continuous infusion of tempol or vehicle for 2 weeks. Tempol significantly decreased (p < 0.01) nicotinamide adenine dinucleotide phosphate oxidase activity in the hypothalamus but failed to reduce food intake or weight gain and did not alter body composition. SIRT1 activity and Acetyl p53 were decreased and phosphorylation of AMPK was increased with age, but they were unchanged with tempol. Basal phosphorylation of STAT3 was unchanged with age or tempol. These results indicate that tempol decreases oxidative stress but fails to alter feeding behaviour, body weight, or body composition. Moreover, tempol does not modulate the SIRT1/AMPK/p53 pathway and does not change leptin signalling. Thus, a reduction in hypothalamic oxidative stress is not sufficient to reverse age-related obesity.

Renal denervation enhances GABA-ergic input into the PVN leading to blood pressure lowering in chronic kidney disease

OBJECTIVE

Sympathoexcitation plays an important role in the pathogenesis of hypertension in patients with chronic kidney disease (CKD). The paraventricular nucleus of the hypothalamus (PVN) in the brain controls sympathetic outflow through γ-amino butyric acid (GABA)-ergic mechanisms. Renal denervation (RDN) exerts a long-term antihypertensive effect in hypertension with CKD; however, the effects of RDN on sympathetic nerve activity and GABA-ergic modulation in the PVN are not clear. We aimed to elucidate whether RDN modulates sympathetic outflow through GABA-ergic mechanisms in the PVN in hypertensive mice with CKD.

METHODS AND RESULTS

In 5/6-nephrectomized male Institute of Cancer Research mice (Nx) at 4 weeks after nephrectomy, systolic blood pressure (SBP) was significantly increased, accompanied by sympathoexcitation. The Nx-mice underwent RDN or sham operation, and the mice were divided into three groups (Control, Nx-Sham, and Nx-RDN). At 2 weeks after RDN, SBP was significantly decreased and urinary sodium excretion was increased in Nx-RDN compared with Nx-Sham. Urinary norepinephrine excretion (uNE) levels did not differ significantly between Nx-RDN and Nx-Sham. At 6 weeks after RDN, SBP continued to decrease and uNE levels also decreased in Nx-RDN compared with Nx-Sham. Bicuculline microinjection into the PVN increased mean arterial pressure and lumbar sympathetic nerve activity in all groups. The pressor responses and change in lumbar sympathetic nerve activity were significantly attenuated in Nx-Sham, but were enhanced in Nx-RDN at 6 weeks after RDN.

CONCLUSIONS

The findings from the present study indicate that RDN has a prolonged antihypertensive effect and, at least in the late phase, decreases sympathetic nerve activity in association with enhanced GABA-ergic input into the PVN in mice with CKD.

Vascular and Central Activation of Peroxisome Proliferator-Activated Receptor-β Attenuates Angiotensin II-Induced Hypertension: Role of RGS-5

Activation of peroxisome proliferator-activated receptor-β/δ (PPARβ) lowers blood pressure in genetic and mineralocorticoid-induced hypertension. Regulator of G-protein-coupled receptor signaling 5 (RGS5) protein, which interferes in angiotensin II (AngII) signaling, is a target gene to PPARβ The aim of the present study was to examine whether PPARβ activation in resistance arteries and brain tissues prevents the elevated blood pressure in AngII-induced hypertension and evaluate the role of RGS5 in this effect. C57BL/6J male mice were divided into five groups (control mice, PPARβ agonist [4-[[[2-[3-Fluoro-4-(trifluoromethyl)phenyl]-4-methyl-5-thiazolyl]methyl]thio]-2-methylphenoxy]acetic acid (GW0742)-treated mice AngII-infused mice, GW0742-treated AngII-infused mice, and AngII-infused mice treated with GW0742 plus PPARβ antagonist 3-[[[2-Methoxy-4-(phenylamino)phenyl]amino]sulfonyl]-2-thiophenecarboxylic acid methyl ester (GSK0660)) and were followed for 3 weeks. GW0742 prevented the increase in both arterial blood pressure and plasma noradrenaline levels and the higher reduction of blood pressure after ganglionic blockade, whereas it reduced the mesenteric arterial remodeling and the hyper-responsiveness to vasoconstrictors (AngII and endothelin-1) in AngII-infused mice. These effects were accompanied by an inhibition of NADPH oxidase expression and activity in the brain. Gene expression profiling revealed a marked loss of brainstem and vascular RGS5 in AngII-infused mice, which was restored by GW0742. GW0742-induced effects were abolished by GSK0660. Small interfering RNA targeting RGS5 caused augmented contractile response to AngII in resistance mesenteric arteries and blunted the inhibitory effect of GW0742 on this response. In conclusion, GW0742 exerted antihypertensive effects, restoring sympathetic tone and vascular structure and function in AngII-infused mice by PPARβ activation in brain and vessels inhibiting AngII signaling as a result of RGS5 upregulation.

Mineralocorticoid and angiotensin II type 1 receptors in the subfornical organ mediate angiotensin II - induced hypothalamic reactive oxygen species and hypertension

Activation of angiotensinergic pathways by central aldosterone (Aldo)-mineralocorticoid receptor (MR) pathway plays a critical role in angiotensin II (Ang II)-induced hypertension. The subfornical organ (SFO) contains both MR and angiotensin II type 1 receptors (AT1R) and can relay the signals of circulating Ang II to downstream nuclei such as the paraventricular nucleus (PVN), supraoptic nucleus (SON) and rostral ventrolateral medulla (RVLM). In Wistar rats, subcutaneous (sc) infusion of Ang II at 500ng/min/kg for 1 or 2weeks increased reactive oxygen species (ROS) as measured by dihydroethidium (DHE) staining in a nucleus - specific pattern. Intra-SFO infusion of AAV-MR- or AT1aR-siRNA prevented the Ang II-induced increase in AT1R mRNA expression in the SFO and decreased MR mRNA. Both MR- and AT1aR-siRNA prevented increases in ROS in the PVN and RVLM. MR- but not AT1aR-siRNA in the SFO prevented the Ang II-induced ROS in the SON. Both MR- and AT1aR-siRNA in the SFO prevented most of the Ang II-induced hypertension as assessed by telemetry. These results indicate that Aldo-MR signaling in the SFO is needed for the activation of Ang II-AT1R-ROS signaling from the SFO to the PVN and RVLM. Activation of Aldo-MR signaling from the SFO to the SON may enhance AT1R dependent activation of pre-sympathetic neurons in the PVN.

The roles of sensitization and neuroplasticity in the long-term regulation of blood pressure and hypertension

After decades of investigation, the causes of essential hypertension remain obscure. The contribution of the nervous system has been excluded by some on the basis that baroreceptor mechanisms maintain blood pressure only over the short term. However, this point of view ignores one of the most powerful contributions of the brain in maintaining biological fitness-specifically, the ability to promote adaptation of behavioral and physiological responses to cope with new challenges and maintain this new capacity through processes involving neuroplasticity. We present a body of recent findings demonstrating that prior, short-term challenges can induce persistent changes in the central nervous system to result in an enhanced blood pressure response to hypertension-eliciting stimuli. This sensitized hypertensinogenic state is maintained in the absence of the inducing stimuli, and it is accompanied by sustained upregulation of components of the brain renin-angiotensin-aldosterone system and other molecular changes recognized to be associated with central nervous system neuroplasticity. Although the heritability of hypertension is high, it is becoming increasingly clear that factors beyond just genes contribute to the etiology of this disease. Life experiences and attendant changes in cellular and molecular components in the neural network controlling sympathetic tone can enhance the hypertensive response to recurrent, sustained, or new stressors. Although the epigenetic mechanisms that allow the brain to be reprogrammed in the face of challenges to cardiovascular homeostasis can be adaptive, this capacity can also be maladaptive under conditions present in different evolutionary eras or ontogenetic periods.

Central Renin-Angiotensin System Activation and Inflammation Induced by High-Fat Diet Sensitize Angiotensin II-Elicited Hypertension

Obesity has been shown to promote renin-angiotensin system activity and inflammation in the brain and to be accompanied by increased sympathetic activity and blood pressure. Our previous studies demonstrated that administration of a subpressor dose of angiotensin (Ang) II sensitizes subsequent Ang II-elicited hypertension. The present study tested whether high-fat diet (HFD) feeding also sensitizes the Ang II-elicited hypertensive response and whether HFD-induced sensitization is mediated by an increase in renin-angiotensin system activity and inflammatory mechanisms in the brain. HFD did not increase baseline blood pressure, but enhanced the hypertensive response to Ang II compared with a normal-fat diet. The sensitization produced by the HFD was abolished by concomitant central infusions of either a tumor necrosis factor-α synthesis inhibitor, pentoxifylline, an Ang II type 1 receptor blocker, irbesartan, or an inhibitor of microglial activation, minocycline. Furthermore, central pretreatment with tumor necrosis factor-α mimicked the sensitizing action of a central subpressor dose of Ang II, whereas central pentoxifylline or minocycline abolished this Ang II-induced sensitization. Real-time quantitative reverse transcription-polymerase chain reaction analysis of lamina terminalis tissue indicated that HFD feeding, central tumor necrosis factor-α, or a central subpressor dose of Ang II upregulated mRNA expression of several components of the renin-angiotensin system and proinflammatory cytokines, whereas inhibition of Ang II type 1 receptor and of inflammation reversed these changes. The results suggest that HFD-induced sensitization of Ang II-elicited hypertension is mediated by upregulation of the brain renin-angiotensin system and of central proinflammatory cytokines.

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.

Aldosterone Contributes to Sympathoexcitation in Renovascular Hypertension

BACKGROUND

Although angiotensin II (Ang II) is essential to the development of renovascular hypertension, aldosterone plays a role as well. Recent studies have demonstrated a cross-talk between Ang II type 1 and mineralocorticoid receptors in the brain and kidneys. However, the role of aldosterone in the autonomic and renal dysfunction of renovascular hypertension is not well understood.

AIM

The current study evaluated whether aldosterone contributes to cardiovascular and renal dysfunction in the 2 kidney-1 clip (2K1C) model.

METHODS

Mean arterial pressure (MAP) and baroreceptor reflex for control of the heart rate were evaluated in 2K1C treated or not treated with spironolactone (200mg/kg/day, 7 days). Tonic and reflex control of renal sympathetic nerve activity (rSNA) were assessed in urethane-anaesthetized rats. Plasma renin activity (PRA), kidney renin protein expression, renal injury, and central AT1 receptor protein expression were assessed.

RESULTS

Spiro reduced MAP (198±4 vs. 170±9mm Hg; P < 0.05), normalized rSNA (147±9 vs. 96±10 pps; P < 0.05), and increased renal baroreceptor reflex sensitivity in the 2K1C rats. Spiro reduced α-smooth muscle actin expression in the nonclipped kidney in the 2K1C group (5±0.6 vs. 1.1±0.2%; P < 0.05). There was no change in PRA; however, a decrease in renin protein expression in the nonclipped kidney was found in the 2K1C treated group (217±30 vs. 160±19%; P < 0.05). Spiro treatment decreased AT1 receptor in the central nervous system (CNS) only in 2K1C rats (138±10 vs. 84±12%; P < 0.05).

CONCLUSION

Aldosterone contributes to autonomic dysfunction and intrarenal injury in 2K1C, these effects are mediated by the CNS.

Central nervous system circuits modified in heart failure: pathophysiology and therapeutic implications

The pathophysiology of heart failure (HF) is characterized by an abnormal activation of neurohumoral systems, including the sympathetic nervous and the renin-angiotensin-aldosterone systems, which have long-term deleterious effects on the disease progression. Perpetuation of this neurohumoral activation is partially dependent of central nervous system (CNS) pathways, mainly involving the paraventricular nucleus of the hypothalamus and some regions of the brainstem. Modifications in these integrative CNS circuits result in the attenuation of sympathoinhibitory and exacerbation of sympathoexcitatory pathways. In addition to the regulation of sympathetic outflow, these central pathways coordinate a complex network of agents with an established pathophysiological relevance in HF such as angiotensin, aldosterone, and proinflammatory cytokines. Central pathways could be potential targets in HF therapy since the current mainstay of HF pharmacotherapy aims primarily at antagonizing the peripheral mechanisms. Thus, in the present review, we describe the role of CNS pathways in HF pathophysiology and as potential novel therapeutic targets.

Activation of central PPAR-γ attenuates angiotensin II-induced hypertension

Inflammation and renin-angiotensin system activity in the brain contribute to hypertension through effects on fluid intake, vasopressin release, and sympathetic nerve activity. We recently reported that activation of brain peroxisome proliferator-activated receptor (PPAR)-γ in heart failure rats reduced inflammation and renin-angiotensin system activity in the hypothalamic paraventricular nucleus and ameliorated the peripheral manifestations of heart failure. We hypothesized that the activation of brain PPAR-γ might have beneficial effects in angiotensin II-induced hypertension. Sprague-Dawley rats received a 2-week subcutaneous infusion of angiotensin II (120 ng/kg per minute) combined with a continuous intracerebroventricular infusion of vehicle, the PPAR-γ agonist pioglitazone (3 nmol/h) or the PPAR-γ antagonist GW9662 (7 nmol/h). Angiotensin II+vehicle rats had increased mean blood pressure, increased sympathetic drive as indicated by the mean blood pressure response to ganglionic blockade, and increased water consumption. PPAR-γ mRNA in subfornical organ and hypothalamic paraventricular nucleus was unchanged, but PPAR-γ DNA-binding activity was reduced. mRNA for interleukin-1β, tumor necrosis factor-α, cyclooxygenase-2, and angiotensin II type 1 receptor was augmented in both nuclei, and hypothalamic paraventricular nucleus neuronal activity was increased. The plasma vasopressin response to a 6-hour water restriction also increased. These responses to angiotensin II were exacerbated by GW9662 and ameliorated by pioglitazone, which increased PPAR-γ mRNA and PPAR-γ DNA-binding activity in subfornical organ and hypothalamic paraventricular nucleus. Pioglitazone and GW9662 had no effects on control rats. The results suggest that activating brain PPAR-γ to reduce central inflammation and brain renin-angiotensin system activity may be a useful adjunct in the treatment of angiotensin II-dependent hypertension.

Age-related changes in thirst, salt appetite, and arterial blood pressure in response to aldosterone-dexamethasone combination in rats

This work examined the effects of age on daily water and sodium ingestion and cardiovascular responses to chronic administration of the mineralocorticoid, aldosterone (ALDO) either alone or together with the glucocorticoid, dexamethasone (DEX). Young (4 mo), adult (12 mo), and aged (30 mo) male Brown Norway rats were prepared for continuous telemetry recording of blood pressure (BP) and heart rate (HR). Baseline water and sodium (i.e., 0.3 M NaCl) intake, BP, and HR were established for 10 days. Then ALDO (60 μg/day sc) was infused alone, or together with DEX (2.5 or 20 μg/day sc), for another 10 days. Compared with baseline levels, ALDO stimulated comparable increases in daily saline intake at all ages. ALDO together with the higher dose of DEX (i.e., ALDO/DEX20) increased daily saline intake more than did ALDO, but less so in aged rats. Infusion of ALDO/DEX20 increased mean arterial pressure (MAP), and decreased HR, more than did infusion of ALDO. The changes in MAP in response to both treatments depended on age. For all ages, MAP and saline intake increased simultaneously during ALDO, while MAP always increased before saline intake did during ALDO/DEX20. Contrary to our predictions, MAP did not increase more in old rats in response to either treatment. We speculate that age-related declines in cardiovascular responses to glucocorticoids contributed to the attenuated increases in sodium intake in response to glucocorticoids that were observed in older animals.

Mechanisms of brain renin angiotensin system-induced drinking and blood pressure: importance of the subfornical organ

It is critical for cells to maintain a homeostatic balance of water and electrolytes because disturbances can disrupt cellular function, which can lead to profound effects on the physiology of an organism. Dehydration can be classified as either intra- or extracellular, and different mechanisms have developed to restore homeostasis in response to each. Whereas the renin-angiotensin system (RAS) is important for restoring homeostasis after dehydration, the pathways mediating the responses to intra- and extracellular dehydration may differ. Thirst responses mediated through the angiotensin type 1 receptor (AT1R) and angiotensin type 2 receptors (AT2R) respond to extracellular dehydration and intracellular dehydration, respectively. Intracellular signaling factors, such as protein kinase C (PKC), reactive oxygen species (ROS), and the mitogen-activated protein (MAP) kinase pathway, mediate the effects of central angiotensin II (ANG II). Experimental evidence also demonstrates the importance of the subfornical organ (SFO) in mediating some of the fluid intake effects of central ANG II. The purpose of this review is to highlight the importance of the SFO in mediating fluid intake responses to dehydration and ANG II.

Brain mineralocorticoid receptors in cognition and cardiovascular homeostasis

Mineralocorticoid receptors (MR) mediate diverse functions supporting osmotic and hemodynamic homeostasis, response to injury and inflammation, and neuronal changes required for learning and memory. Inappropriate MR activation in kidneys, heart, vessels, and brain hemodynamic control centers results in cardiovascular and renal pathology and hypertension. MR binds aldosterone, cortisol and corticosterone with similar affinity, while the glucocorticoid receptor (GR) has less affinity for cortisol and corticosterone. As glucocorticoids are more abundant than aldosterone, aldosterone activates MR in cells co-expressing enzymes with 11β-hydroxydehydrogenase activity to inactivate them. MR and GR co-expressed in the same cell interact at the molecular and functional level and these functions may be complementary or opposing depending on the cell type. Thus the balance between MR and GR expression and activation is crucial for normal function. Where 11β-hydroxydehydrogenase 2 (11β-HSD2) that inactivates cortisol and corticosterone in aldosterone target cells of the kidney and nucleus tractus solitarius (NTS) is not expressed, as in most neurons, MR are activated at basal glucocorticoid concentrations, GR at stress concentrations. An exception may be pre-autonomic neurons of the PVN which express MR and 11β-HSD1 in the absence of hexose-6-phosphate dehydrogenase required to generate the requisite cofactor for reductase activity, thus it acts as a dehydrogenase. MR antagonists, valuable adjuncts to the treatment of cardiovascular disease, also inhibit MR in the brain that are crucial for memory formation and exacerbate detrimental effects of excessive GR activation on cognition and mood. 11β-HSD1 inhibitors combat metabolic and cognitive diseases related to glucocorticoid excess, but may exacerbate MR action where 11β-HSD1 acts as a dehydrogenase, while non-selective 11β-HSD1&2 inhibitors cause injurious disruption of MR hemodynamic control. MR functions in the brain are multifaceted and optimal MR:GR activity is crucial. Therefore selectively targeting down-stream effectors of MR specific actions may be a better therapeutic goal.

Sensitization of sodium appetite: evidence for sustained molecular changes in the lamina terminalis

Animals with a history of sodium depletions exhibit increases in salt intake, a phenomenon described as the sensitization of sodium appetite. Using a novel experimental design, the present experiments investigated whether putative molecular markers of neural plasticity and changes in the message for components of the brain renin-angiotensin-aldosterone-system (RAAS) accompany the sensitization of sodium appetite. An initial set of experiments examined whether the glutamatergic N-methyl-d-aspartate receptor antagonist MK-801 would attenuate sodium appetite sensitization and prevent changes in mRNA expression associated with sensitization. Rats with repeated sodium depletions exhibited enhanced sodium appetite and mRNA expression for components of the RAAS in areas along the lamina terminalis (LT), a region of the brain that is important for the regulation of body fluid homeostasis, and these effects were significantly attenuated by MK-801 pretreatment. A second set of experiments investigated whether successive sodium depletions would elevate sodium intake and induce a pattern of fos-B staining consistent with the Δfos-B isoform in areas along the LT. The pattern of fos-B staining in the subfornical organ was consistent with the characteristics of Δfos-B expression. Specifically, fos-B/Δfos-B expression was increased 4 days after the last of a series of sodium depletions, fos-B/Δfos-B expression was nearly absent in control rats, and the quantity of fos-B/Δfos-B staining was directly associated with a history of sodium depletions. These findings demonstrate that the sensitization of sodium appetite is associated with sustained molecular alterations in the LT that are indicative of neural plasticity and upregulation of the central RAAS.

Pressure natriuresis and the renal control of arterial blood pressure

The regulation of extracellular fluid volume by renal sodium excretion lies at the centre of blood pressure homeostasis. Renal perfusion pressure can directly regulate sodium reabsorption in the proximal tubule. This acute pressure natriuresis response is a uniquely powerful means of stabilizing long-term blood pressure around a set point. By logical extension, deviation from the set point can only be sustained if the pressure natriuresis mechanism is impaired, suggesting that hypertension is caused or sustained by a defect in the relationship between renal perfusion pressure and sodium excretion. Here we describe the role of pressure natriuresis in blood pressure control and outline the cascade of biophysical and paracrine events in the renal medulla that integrate the vascular and tubular response to altered perfusion pressure. Pressure natriuresis is impaired in hypertension and mechanistic insight into dysfunction comes from genetic analysis of blood pressure disorders. Transplantation studies in rats show that blood pressure is determined by the genotype of the kidney and Mendelian hypertension indicates that the distal nephron influences the overall natriuretic efficiency. These approaches and the outcomes of genome-wide-association studies broaden our view of blood pressure control, suggesting that renal sympathetic nerve activity and local inflammation can impair pressure natriuresis to cause hypertension. Understanding how these systems interact is necessary to tackle the global burden of hypertension.

The multifaceted mineralocorticoid receptor

The primary adrenal cortical steroid hormones, aldosterone, and the glucocorticoids cortisol and corticosterone, act through the structurally similar mineralocorticoid (MR) and glucocorticoid receptors (GRs). Aldosterone is crucial for fluid, electrolyte, and hemodynamic homeostasis and tissue repair; the significantly more abundant glucocorticoids are indispensable for energy homeostasis, appropriate responses to stress, and limiting inflammation. Steroid receptors initiate gene transcription for proteins that effect their actions as well as rapid non-genomic effects through classical cell signaling pathways. GR and MR are expressed in many tissues types, often in the same cells, where they interact at molecular and functional levels, at times in synergy, others in opposition. Thus the appropriate balance of MR and GR activation is crucial for homeostasis. MR has the same binding affinity for aldosterone, cortisol, and corticosterone. Glucocorticoids activate MR in most tissues at basal levels and GR at stress levels. Inactivation of cortisol and corticosterone by 11β-HSD2 allows aldosterone to activate MR within aldosterone target cells and limits activation of the GR. Under most conditions, 11β-HSD1 acts as a reductase and activates cortisol/corticosterone, amplifying circulating levels. 11β-HSD1 and MR antagonists mitigate inappropriate activation of MR under conditions of oxidative stress that contributes to the pathophysiology of the cardiometabolic syndrome; however, MR antagonists decrease normal MR/GR functional interactions, a particular concern for neurons mediating cognition, memory, and affect.

Knockdown of mineralocorticoid or angiotensin II type 1 receptor gene expression in the paraventricular nucleus prevents angiotensin II hypertension in rats

Circulating Ang II activates an aldosterone-mineralocorticoid receptor (MR) - angiotensin II (Ang II) - angiotensin type 1 receptor (AT1R) pathway in the hypothalamus. To obtain insights into the actual neuronal projections involved, adeno-associated virus carrying small interfering RNA against either AT1aR (AAV-AT1aR-siRNA) or MR (AAV-MR-siRNA) were infused into the paraventricular nucleus (PVN) in Wistar rats. Intra-PVN infusion of AAV-AT1aR-siRNA or AAV-MR-siRNA decreased AT1R or MR expression in the PVN but not in the subfornical organ (SFO) or supraoptic nucleus (SON). Subcutaneous infusion of Ang II at 500 ng kg(-1) min(-1) for 2 weeks increased mean arterial pressure by 60-70 mmHg, and increased AT1R and MR expression in the SFO, SON and PVN. Intra-PVN AT1aR-siRNA prevented the Ang II-induced increase in AT1R but not MR expression in the PVN, and MR-siRNA prevented MR but not AT1R expression in the PVN. The increases in AT1R and MR expression in both the SFO and the SON were not changed by the two AAV-siRNAs. Specific knockdown of AT1R or MR in the PVN by AAV-siRNA each prevented most of the Ang II-induced hypertension. Prevention of the subcutaneous Ang II-induced increase in MR but not the increase in AT1R by knockdown of MR and vice versa suggests an independent regulation of MR and AT1R expression in the PVN. Both AT1R and MR activation in the PVN play a critical role in Ang II-induced hypertension in rats.

Expression of mineralocorticoid and glucocorticoid receptors in preautonomic neurons of the rat paraventricular nucleus

Activation of mineralocorticoid receptors (MR) of the hypothalamic paraventricular nucleus (PVN) increases sympathetic excitation. To determine whether MR and glucocorticoid receptors (GR) are expressed in preautonomic neurons of the PVN and how they relate to endogenous aldosterone levels in healthy rats, retrograde tracer was injected into the intermediolateral cell column at T4 to identify preautonomic neurons in the PVN. Expression of MR, GR, 11-β hydroxysteroid dehydrogenase1 and 2 (11β-HSD1, 2), and hexose-6-phosphate dehydrogenase (H6PD) required for 11β-HSD1 reductase activity was assessed by immunohistochemistry. RT-PCR and Western blot analysis were used to determine MR gene and protein expression. Most preautonomic neurons were in the caudal mediocellular region of PVN, and most expressed MR; none expressed GR. 11β-HSD1, but not 11β-HSD2 nor H6PD immunoreactivity, was detected in the PVN. In rats with chronic low or high sodium intakes, the low-sodium diet was associated with significantly higher plasma aldosterone, MR mRNA and protein expression, and c-Fos immunoreactivity within labeled preautonomic neurons. Plasma corticosterone and sodium and expression of tonicity-responsive enhancer binding protein in the PVN did not differ between groups, suggesting osmotic adaptation to the altered sodium intake. These results suggest that MR within preautonomic neurons in the PVN directly participate in the regulation of sympathetic nervous system drive, and aldosterone may be a relevant ligand for MR in preautonomic neurons of the PVN under physiological conditions. Dehydrogenase activity of 11β-HSD1 occurs in the absence of H6PD, which regenerates NADP(+) from NADPH and may increase MR gene expression under physiological conditions.