Sensitization of slow pressor angiotensin II (Ang II)-initiated hypertension: induction of sensitization by prior Ang II treatment

Long-term health outcomes associated with hydration status

Body water balance is determined by fundamental homeostatic mechanisms that maintain stable volume, osmolality and the composition of extracellular and intracellular fluids. Water balance is maintained by multiple mechanisms that continuously match water losses through urine, the skin, the gastrointestinal tract and respiration with water gains achieved through drinking, eating and metabolic water production. Hydration status is determined by the state of the water balance. Underhydration occurs when a decrease in body water availability, due to high losses or low gains, stimulates adaptive responses within the water balance network that are aimed at decreasing losses and increasing gains. This stimulation is also accompanied by cardiovascular adjustments. Epidemiological and experimental studies have linked markers of low fluid intake and underhydration - such as increased plasma concentration of vasopressin and sodium, as well as elevated urine osmolality - with an increased risk of new-onset chronic diseases, accelerated aging and premature mortality, suggesting that persistent activation of adaptive responses may be detrimental to long-term health outcomes. The causative nature of these associations is currently being tested in interventional trials. Understanding of the physiological responses to underhydration may help to identify possible mechanisms that underlie potential adverse, long-term effects of underhydration and inform future research to develop preventative and treatment approaches to the optimization of hydration status.

Single-cell transcriptome profiling reveals enriched memory T-cell subpopulations in hypertension

Introduction: The adaptive immune response mediated by T cells plays a vital role in the initiation and maintenance of blood pressure (BP) elevation. Memory T cells, which are antigen-specific T cells, can respond specifically to repeated hypertensive stimuli. Although the roles of memory T cells in animal models are well studied, their maintenance and functions in hypertensive patients are poorly understood.

Method: Here, we focused on the circulating memory T cells of hypertensive patients. By using single-cell RNA sequencing technology, subsets of memory T cells were identified. Differentially expressed genes (DEGs) and functional pathways were explored for related biological functions in each population of memory T cells.

Result and Discussion: Our study identified four subsets of memory T cells in the blood of hypertensive patients, with CD8 effector memory T (TEM) cells accounting for more cells and demonstrating more biological functions than CD4 TEM cells. CD8 TEM cells were further analyzed using single-cell RNA sequencing technology, and subpopulation 1 was demonstrated to contribute to BP elevation. The key marker genes CKS2, PLIN2, and CNBP were identified and validated by mass-spectrum flow cytometry. Our data suggest that CD8 TEM cells as well as the marker genes could be preventive targets for patients with hypertensive cardiovascular disease.

Sensitization of Hypertension: The Impact of Earlier Life Challenges: Excellence Award for Hypertension Research 2021

Hypertension affects over 1 billion individuals worldwide. Because the cause of hypertension is known only in a small fraction of patients, most individuals with high blood pressure are diagnosed as having essential hypertension. Elevated sympathetic nervous system activity has been identified in a large portion of hypertensive patients. However, the root cause for this sympathetic overdrive is unknown. A more complete understanding of the breadth of the functional capabilities of the sympathetic nervous system may lead to new insights into the cause of essential hypertension. By employing a unique experimental paradigm, we have recently discovered that the neural network controlling sympathetic drive is more reactive after rats are exposed to mild challenges (stressors) and that the hypertensive response can be sensitized (ie, hypertensive response sensitization [HTRS]). We have also found that the induction of HTRS involves plasticity in the neural network controlling sympathetic drive. The induction and maintenance of the latent HTRS state also require the functional integrity of the brain renin-angiotensin-aldosterone system and the presence of several central inflammatory factors. In this review, we will discuss the induction and expression of HTRS in adult animals and in the progeny of mothers with prenatal obesity/overnutrition or with maternal gestational hypertension. Also, interventions that reverse the effects of stressor-induced HTRS will be reviewed. Understanding the mechanisms underlying HTRS and identifying the beneficial effects of maternal or offspring early-life interventions that prevent or reverse the sensitized state can provide insights into therapeutic strategies for interrupting the vicious cycle of transgenerational hypertension.

Predator Scent-Induced Sensitization of Hypertension and Anxiety-like Behaviors

Post-traumatic stress disorder (PTSD), an anxiety-related syndrome, is associated with increased risk for cardiovascular diseases. The present study investigated whether predator scent (PS) stress, a model of PTSD, induces sensitization of hypertension and anxiety-like behaviors and underlying mechanisms related to renin-angiotensin systems (RAS) and inflammation. Coyote urine, as a PS stressor, was used to model PTSD. After PS exposures, separate cohorts of rats were studied for hypertensive response sensitization (HTRS), anxiety-like behaviors, and changes in plasma levels and mRNA expression of several components of the RAS and proinflammatory cytokines (PICs) in the lamina terminalis (LT), paraventricular nucleus (PVN), and amygdala (AMY). Rats exposed to PS as compared to control animals exhibited (1) a significantly greater hypertensive response (i.e., HTRS) when challenged with a slow-pressor dose of angiotensin (ANG) II, (2) significant decrease in locomotor activity and increase in time spent in the closed arms of a plus maze as well as general immobility (i.e., behavioral signs of increased anxiety), (3) upregulated plasma levels of ANG II and interleukin-6, and (4) increased expression of message for components of the RAS and PICs in key brain nuclei. All the PS-induced adverse effects were blocked by pretreatment with either an angiotensin-converting enzyme antagonist or a tumor necrosis factor-α inhibitor. The results suggest that PS, used as an experimental model of PTSD, sensitizes ANG II-induced hypertension and produces behavioral signs of anxiety, probably through upregulation of RAS components and inflammatory markers in plasma and brain areas associated with anxiety and blood pressure control.

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.

Studies of salt and stress sensitivity on arterial pressure in renin-b deficient mice

Excessive sodium intake is known to increase the risk for hypertension, heart disease, and stroke. Individuals who are more susceptible to the effects of high salt are at higher risk for cardiovascular diseases even independent of their blood pressure status. Local activation of the renin-angiotensin system (RAS) in the brain, among other mechanisms, has been hypothesized to play a key role in contributing to salt balance. We have previously shown that deletion of the alternative renin isoform termed renin-b disinhibits the classical renin-a encoding preprorenin in the brain resulting in elevated brain RAS activity. Thus, we hypothesized that renin-b deficiency results in higher susceptibility to salt-induced elevation in blood pressure. Telemetry implanted Ren-b^Null and wildtype littermate mice were first offered a low salt diet for a week and subsequently a high salt diet for another week. A high salt diet induced a mild blood pressure elevation in both Ren-b^Null and wildtype mice, but mice lacking renin-b did not exhibit an exaggerated pressor response. When renin-b deficient mice were exposed to a high salt diet for a longer duration (4 weeks), there was a trend for increased myocardial enlargement in Ren-b^Null mice when compared with control mice, but this did not reach statistical significance. Multiple studies have also demonstrated the association of environmental stress with hypertension. Activation of the RAS in the rostral ventrolateral medulla and the hypothalamus is required for stress-induced hypertension. Thus, we next questioned whether the lack of renin-b would result in exacerbated response to an acute restraint-stress. Wildtype and Ren-b^Null mice equally exhibited elevated blood pressure in response to restraint-stress, which was similar in mice fed either a low or high salt diet. These studies suggest that mechanisms unrelated to salt and acute stress alter the cardiovascular phenotype in mice lacking renin-b.

Caloric Restriction and Cardiovascular Health: the Good, the Bad, and the Renin-Angiotensin System

Much excitement exists over the cardioprotective and life-extending effects of caloric restriction (CR). This review integrates population studies with experimental animal research to address the positive and negative impact of mild and severe CR on cardiovascular physiology and pathophysiology, with a particular focus on the renin-angiotensin system (RAS). We also highlight the gaps in knowledge and areas ripe for future physiological research.

Estrogen Protects Vasomotor Functions in Rats During Catecholamine Stress

The incidence of dysfunctional vasomotor diseases has mostly occurred in postmenopausal women but not in premenopausal women. Hence, this study sought to investigate the impact of estrogen deficiency during catecholamine stress on vasomotor function. Also, attempts were made to utilize estrogen replacement therapy to mitigate the adverse effects (pathological remodeling) of stress on the aortic vessels to preserve vasomotor functions. To do this, female Sprague-Dawley (SD) rats were ovariectomized (OVX) along with sham operations (Sham). Day 14 after OVX operation, 17-estradiol (E₂) was subcutaneously implanted (OVX+E₂). Day 35 after operation, stress was induced by isoproterenol (ISO) subcutaneous injections. Clinically relevant blood pressure indexes (systolic, diastolic, and mean atrial blood pressures) were assessed in the rats. Aortic vascular ring tensions were assessed in vitro to ascertain the impact of E₂ on their vasomotor function. Aortic vascular rings (AVRs) from OVX+ISO exhibited a significant increase in contractility in response to phenylephrine than AVRs isolated from Sham+ISO rats. Also, sera levels of nitric oxide (NO) and endothelin-1 (ET-1) and the expression of p-eNOS/eNOS from vascular tissues were ascertained. We demonstrate that, during stress, E₂ prevented excessive weight gain and OVX rats had higher blood pressures than those in the Sham group. Further, we showed that E₂ decreases ET-1 expressions during stress while upregulating NO expressions via enhancing eNOS activities to facilitate vasomotor functions. Finally, histological assessment revealed the E₂ treatments during stress preserved vasomotor functions by preventing excessive intima-media thickening and collagen depositions in the aortic vascular walls.

Sex-Gender Disparities in Cardiovascular Diseases: The Effects of Estrogen on eNOS, Lipid Profile, and NFATs During Catecholamine Stress

Cardiovascular diseases (CVDs) characterized by sex-gender differences remain a leading cause of death globally. Hence, it is imperative to understand the underlying mechanisms of CVDs pathogenesis and the possible factors influencing the sex-gender disparities in clinical demographics. Attempts to elucidate the underlying mechanisms over the recent decades have suggested the mechanistic roles of estrogen in modulating cardioprotective and immunoregulatory effect as a factor for the observed differences in the incidence of CVDs among premenopausal and post-menopausal women and men. This review from a pathomechanical perspective aims at illustrating the roles of estrogen (E2) in the modulation of stimuli signaling in the heart during chronic catecholamine stress (CCS). The probable mechanism employed by E2 to decrease the incidence of hypertension, coronary heart disease, and pathological cardiac hypertrophy in premenopausal women are discussed. Initially, signaling via estrogen receptors and β-adrenergic receptors (βARs) during physiological state and CCS were summarized. By reconciling the impact of estrogen deficiency and hyperstimulation of βARs, the discussions were centered on their implications in disruption of nitric oxide synthesis, dysregulation of lipid profiles, and upregulation of nuclear factor of activated T cells, which induces the aforementioned CVDs, respectively. Finally, updates on E2 therapies for maintaining cardiac health during menopause and suggestions for the advancement treatments were highlighted.

Cerebrovascular function in hypertension: Does high blood pressure make you old?

The majority of individuals over an age of 60 have hypertension. Elevated blood pressure and older age are associated with very similar changes in brain structure and function. We review the parallel brain changes associated with increasing age and blood pressure. This review focuses on joint associations of aging and elevated blood pressure with neuropsychological function, regional cerebral blood flow responses to cognitive and metabolic challenges, white matter disruptions, grey matter volume, cortical thinning, and neurovascular coupling. Treatment of hypertension ameliorates many of these changes but fails to reverse them. Treatment of hypertension itself appears more successful with better initial brain function. We show evidence that sympathetic and renal influences known to increase blood pressure also impact brain integrity. Possible central mechanisms contributing to the course of hypertension and aging are then suggested. An emphasis is placed on psychologically relevant factors: stress, cardiovascular reactions to stress, and diet/obesity. The contribution of some of these factors to biological aging remains unclear and may provide a starting point for defining the independent and interacting effects of aging and increasing blood pressure on the brain.

Role of the renin-angiotensin system in the development of severe COVID-19 in hypertensive patients

A new form of severe acute respiratory syndrome (SARS) caused by SARS-coronavirus 2 (CoV-2), called COVID-19, has become a global threat in 2020. The mortality rate from COVID-19 is high in hypertensive patients, making this association especially dangerous. There appears to be a consensus, despite the lack of experimental data, that angiotensin II (ANG II) is linked to the pathogenesis of COVID-19. This process may occur due to acquired deficiency of angiotensin-converting enzyme 2 (ACE2), resulting in reduced degradation of ANG II. Furthermore, ANG II has a critical role in the genesis and worsening of hypertension. In this context, the idea that there is a surge in the level of ANG II with COVID-19 infection, causing multiple organ injuries in hypertensive patients becomes attractive. However, the role of other components of the renin angiotensin system (RAS) in this scenario requires elucidation. The identification of other RAS components in COVID-19 hypertension may provide both diagnostic and therapeutic benefits. Here, we summarize the pathophysiologic contributions of different components of RAS in hypertension and their possible correlation with poor outcome observed in hypertensive patients with COVID-19.

Persistent Renin-Angiotensin System Sensitization Months After Body Weight Recovery From Severe Food Restriction in Female Fischer Rats

Background Prior exposure to periods of severe food restriction (sFR) is associated with increased risk of developing hypertension and cardiovascular disease later in life. Methods and Results To investigate the mechanism of these long-term adverse effects of sFR, 4-month-old female Fischer rats were divided in 2 groups and maintained on a normal diet ad libitum (control) or on an sFR diet with 60% reduction in daily food intake for 2 weeks that resulted in a 15% reduction in body weight. After the 2-week sFR period ended, both groups received normal chow ad libitum for 3 months. Within 2 weeks after refeeding was initiated in the sFR group, body weight was restored to control levels; however, plasma angiotensinogen (1.3-fold; P<0.05), Ang-[1-8] (2.0-fold; P<0.05), and angiotensin-converting enzyme activity (1.1-fold; P<0.01) were all elevated 3 months after refeeding. Angiotensin type 1 receptor activity was also increased as evidenced by augmented pressor responses to angiotensin-[1-8] (P<0.01) and depressor responses to the angiotensin type 1 receptor antagonist, losartan (P<0.01) in the sFR group. Conclusions These results indicate that sensitization of the renin-angiotensin system persisted months after the sFR period ended. These findings may have implications for women who voluntarily or involuntarily experience an extended period of sFR and thus may be at increased risk of developing cardiovascular disease through sensitization of the renin-angiotensin system even though their body weight, mean arterial pressure, and heart rate appear normal.

Interactions of the Brain Renin-Angiotensin-System (RAS) and Inflammation in the Sensitization of Hypertension

Mounting evidence indicates that the renin-angiotensin (RAS) and immune systems interact with one another in the central nervous system (CNS) and that they are importantly involved in the pathogenesis of hypertension. Components comprising the classic RAS were first identified in the periphery, and subsequently, similar factors were found to be generated de novo in many different organs including the brain. There is humoral-neural coupling between the systemic and brain RASs, which is important for controlling sympathetic tone and the release of endocrine factors that collectively determine blood pressure (BP). Similar to the interactions between the systemic and brain RASs is the communication between the peripheral and brain immune systems. Systemic inflammation activates the brain’s immune response. Importantly, the RAS and inflammatory factors act synergistically in brain regions involved in the regulation of BP. This review presents evidence of how such interactions between the brain RAS and central immune mechanisms contribute to the pathogenesis of hypertension. Emphasis focuses on the role of these interactions to induce neuroplastic changes in a central neural network resulting in hypertensive response sensitization (HTRS). Neuroplasticity and HTRS can be induced by challenges (stressors) presented earlier in life such as a low-dose of angiotensin II or high fat diet (HFD) feeding in adults. Similarly, the offspring of mothers with gestational hypertension or of mothers ingesting a HFD during pregnancy are reprogrammed and manifest HTRS when exposed to new stressors as adults. Consideration of the actions and interactions of the brain RAS and inflammatory mediators in the context of the induction and expression of HTRS will provide insights into the etiology of high BP that may lead to new strategies for the prevention and treatment of hypertension.

Increased Susceptibility of Mice Lacking Renin-b to Angiotensin II-Induced Organ Damage

Several cardiac and renal diseases are attributed to a dysregulation of the renin-angiotensin system. Renin, the rate-limiting enzyme of the renin-angiotensin system, has 2 isoforms. The classical renin isoform (renin-a) encoding preprorenin is mainly confined to the juxtaglomerular cells and released into the circulation upon stimulation. Alternatively, renin-b is predicted to remain intracellular and is expressed in the brain, heart, and adrenal gland. In the brain, ablation of renin-b (Ren-b^Null mice) results in increased brain renin-angiotensin system activity. However, the consequences of renin-b ablation in tissues outside the brain remain unknown. Therefore, we hypothesized that renin-b protects from hypertensive cardiac and renal end-organ damage in mice. Ren-b^Null mice exhibited normal blood pressure at baseline. Thus, we induced hypertension by using a slow pressor dose of Ang II (angiotensin II). Ang II increased blood pressure in both wild type and Ren-b^Null to the same degree. Although the blood pressure between Ren-b^Null and wild-type mice was elevated equally, 4-week infusion of Ang II resulted in exacerbated cardiac remodeling in Ren-b^Null mice compared with wild type. Ren-b^Null mice also exhibited a modest increase in renal glomerular matrix deposition, elevated plasma aldosterone, and a modestly enhanced dipsogenic response to Ang II. Interestingly, ablation of renin-b strongly suppressed plasma renin, but renal cortical renin mRNA was preserved. Altogether, these data indicate that renin-b might play a protective role in the heart, and thus renin-b could be a potential target to treat hypertensive heart disease.

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.

Sympathetic Nervous System Contributions to Hypertension: Updates and Therapeutic Relevance

The sympathetic nervous system plays a pivotal role in the long-term regulation of arterial blood pressure through the ability of the central nervous system to integrate neurohumoral signals and differentially regulate sympathetic neural input to specific end organs. Part 1 of this review will discuss neural mechanisms of salt-sensitive hypertension, obesity-induced hypertension, and the ability of prior experiences to sensitize autonomic networks. Part 2 of this review focuses on new therapeutic advances to treat resistant hypertension including renal denervation and carotid baroactivation. Both advances lower arterial blood pressure by reducing sympathetic outflow. We discuss potential mechanisms and areas of future investigation to target the sympathetic nervous system.

Amphetamine-induced sensitization of hypertension and lamina terminalis neuroinflammation

Psychomotor stimulants are prescribed for many medical conditions, including obesity, sleep disorders, and attention-deficit/hyperactivity disorder. However, despite their acknowledged therapeutic utility, these stimulants are frequently abused, and their use can have both short- and long-term negative consequences. Although stimulants such as amphetamines acutely elevate blood pressure, it is unclear whether they cause any long-term effects on cardiovascular function after use has been discontinued. Previous work in our laboratory has demonstrated that physiological and psychosocial stressors will produce sensitization of the hypertensive response, a heightened pressor response to a hypertensinogenic stimulus delivered after stressor exposure. Here, we tested whether pretreatment with amphetamine for 1 wk can sensitize the hypertensive response in rats. We found that repeated amphetamine administration induced and maintained sensitization of the pressor response to angiotensin II following a 7-day delay after amphetamine injections were terminated. We also found that amphetamine pretreatment altered mRNA expression for molecular markers associated with neuroinflammation and renin-angiotensin-aldosterone system (RAAS) activation in the lamina terminalis, a brain region implicated in the control of sympathetic nervous system tone and blood pressure. The results indicated amphetamine upregulated mRNA expression underlying neuroinflammation and, to a lesser degree, message for components of the RAAS in the lamina terminalis. However, we found no changes in mRNA expression in the paraventricular nucleus. These results suggest that a history of stimulant use may predispose individuals to developing hypertension by promoting neuroinflammation and upregulating activity of the RAAS in the lamina terminalis.

Stress-Induced Sensitization of Angiotensin II Hypertension Is Reversed by Blockade of Angiotensin-Converting Enzyme or Tumor Necrosis Factor-α

BACKGROUND

Post-traumatic stress disorder (PTSD) is characterized by a disordered stress response and associated with increased cardiovascular disease risk. The present study investigated whether angiotensin (Ang) II-elicited hypertensive response is sensitized in a model of PTSD and whether inhibition of angiotensin-converting enzyme (ACE) or tumor necrosis factor (TNF)-α prior to PTSD blocks this sensitization of Ang II hypertension.

METHODS

The resident-intruder paradigm was used to model PTSD. Each intruder rat (male Sprague-Dawley) was given normal drinking water or was pretreated with either an ACE inhibitor (captopril) or a TNF-α inhibitor (pentoxifylline) in the drinking water for 2 weeks. Subsequently, they were exposed to a different resident (male Long-Evans) for 2 hours on 3 days with each session separated by 1 day and then received a subcutaneous infusion of Ang II for 2 weeks.

RESULTS

The stressed rats had a significantly enhanced hypertensive response to the Ang II infusion (stressed Δ40.2 ± 3.9 mm Hg vs. unstressed Δ20.5 ± 4.5 mm Hg) and an upregulation of mRNA or protein expression of renin-angiotensin system (RAS) and proinflammatory cytokine (PIC) components and of a microglial marker in the lamina terminalis and hypothalamic paraventricular nucleus when compared with unstressed control rats. Both the sensitized hypertensive response and enhanced gene and protein expression were blocked by pretreatment with either ACE (Δ21.3 ± 3.9 mm Hg) or TNF-α inhibitor (Δ21.4 ± 2.6 mm Hg).

CONCLUSIONS

The results indicate that upregulation of the brain RAS and PICs produced by severe stress contributes to traumatic-induced sensitization of hypertensive response to Ang II, and disorders such as PTSD may predispose individuals to development of hypertension.

Superimposed Preeclampsia Exacerbates Postpartum Renal Injury Despite Lack of Long-Term Blood Pressure Difference in the Dahl Salt-Sensitive Rat

Preeclampsia results in increased susceptibility to hypertension and chronic kidney disease postpartum; however, the mechanisms responsible for disease progression in these women remain unknown. The purpose of this study was to test the hypothesis that 2 mechanisms contribute to the link between the maternal syndrome of preeclampsia and the increased postpartum risk of cardiovascular and renal disease: (1) increased T cells in the kidney and (2) a decreased NO:ET-1 (endothelin-1) ratio. Dahl S rats (a previously characterized model of preeclampsia superimposed on chronic hypertension) who experienced 2 pregnancies and virgin littermate controls were studied at 6 months of age. Mean arterial pressure was measured via telemetry, and renal injury was assessed through both histological analysis and measurement of urinary markers including nephrin, podocalyxin, and KIM-1 (kidney injury marker 1). Contributing mechanisms were assessed through flow cytometric analysis of renal T cells, quantification of plasma TNF-α (tumor necrosis factor-α) and IL-10 (interleukin-10), and quantification of urinary concentrations of NO metabolites and ET-1. Although prior pregnancy did not exacerbate the hypertension at 6 months, this group showed greater renal injury compared with virgin littermates. Flow cytometric analyses revealed an increase in renal T cells after pregnancy, and cytokine analysis revealed a systemic proinflammatory shift. Finally, the NO:ET-1 ratio was reduced. These results demonstrate that the link between the maternal syndrome of superimposed preeclampsia and postpartum risk of chronic kidney disease could involve both immune system activation and dysregulation of the NO:ET-1 balance.

Sustained Captopril-Induced Reduction in Blood Pressure Is Associated With Alterations in Gut-Brain Axis in the Spontaneously Hypertensive Rat

Background We have demonstrated that the antihypertensive effect of the angiotensin-converting enzyme inhibitor, captopril ( CAP ), is associated with beneficial effects on gut pathology. Coupled with the evidence that CAP exerts prolonged reduction in blood pressure ( BP ) after discontinuation of treatment, we investigate whether persistent beneficial actions of CAP are linked to alterations of gut microbiota and improvement of hypertension-induced gut pathology. Methods and Results Spontaneously hypertensive rats ( SHR ) and Wistar Kyoto rats were treated with CAP (250 mg/kg/day) for 4 weeks followed by withdrawal for 16 weeks. Gut microbiota, gut pathology, BP, and brain neuronal activity were assessed. CAP resulted in a ≈60 mm Hg decrease in systolic BP after 3 weeks of treatment in SHR , and the decrease remained significant at least 5 weeks after CAP withdrawal. In contrast, CAP caused modest decrease in systolic BP in Wistar Kyoto. 16S rRNA gene-sequencing-based gut microbial analyses in SHR showed sustained alteration of gut microbiota and increase in Allobaculum after CAP withdrawal. Phylogenetic investigation of communities by reconstruction of unobserved states analysis revealed significant increase in bacterial sporulation upon CAP treatment in SHR . These were associated with persistent improvement in gut pathology and permeability. Furthermore, manganese-enhanced magnetic resonance imaging showed significantly decreased neuronal activity in the posterior pituitary of SHR 4 weeks after withdrawal. Conclusions Decreased BP , altered gut microbiota, improved gut pathology and permeability, and dampened posterior pituitary neuronal activity were maintained after CAP withdrawal in the SHR . They suggest that CAP influences the brain-gut axis to maintain the sustained antihypertensive effect of CAP after withdrawal.

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.

AT1a influences GABAA-mediated inhibition through regulation of KCC2 expression

The median preoptic nucleus (MnPO) is an integrative site involved in body fluid homeostasis, cardiovascular control, thermoregulation, and sleep homeostasis. Angiotensin II (ANG II), a neuropeptide shown to have excitatory effects on MnPO neurons, is of particular interest with regard to its role in body fluid homeostasis and cardiovascular control. The present study investigated the role of angiotensin type 1a (AT1a) receptor activation on neuronal excitability in the MnPO. Male Sprague-Dawley rats were infused with an adeno-associated virus with an shRNA against the AT1a receptor or a scrambled control. In vitro loose-patch voltage-clamp recordings of spontaneous action potential activity were made from labeled MnPO neurons in response to brief focal application of ANG II or the GABAA receptor agonist muscimol. Additionally, tissue punches from MnPO were taken to asses mRNA and protein expression. AT1a receptor knockdown neurons were insensitive to ANG II and showed a marked reduction in GABAA-mediated inhibition. The reduction in GABAA-mediated inhibition was not associated with reductions in mRNA or protein expression of GABAA β-subunits. Knockdown of the AT1a receptor was associated with a reduction in the potassium-chloride cotransporter KCC2 mRNA as well as a reduction in pS940 KCC2 protein. The impaired GABAA-mediated inhibition in AT1a knockdown neurons was recovered by bath application of phospholipase C and protein kinase C activators. The following study indicates that AT1a receptor activation mediates the excitability of MnPO neurons, in part, through the regulation of KCC2. The regulation of KCC2 influences the intracellular [Cl-] and the subsequent efficacy of GABAA-mediated currents.

A proposed mechanism for the Berecek phenomenon with implications for cardiovascular reprogramming

Berecek et al reported in the 1990s that when spontaneously hypertensive rat (SHR) mating pairs were treated with captopril and the resulting pups were continued on the drug for 2 months followed by drug discontinuation, the pups did not develop full blown hypertension, and the cardiovascular structural changes associated with hypertension in SHR were mitigated. The offspring of the pups also displayed diminished hypertension and structural changes, suggesting that the drug therapy produced a heritable amelioration of the SHR phenotype. This observation is reviewed. The link between cellular renin angiotensin systems and epigenetic histone modification is explored, and a mechanism responsible for the observation is proposed. In any case, the observations of Berecek are sufficiently intriguing and biologically important to merit re-exploration and definitive explanation. Equally important is determining the role of renin angiotensin systems in epigenetic modification.

Maternal high-fat diet acts on the brain to induce baroreflex dysfunction and sensitization of angiotensin II-induced hypertension in adult offspring

Accumulating evidence indicates that maternal high-fat diet (HFD) is associated with metabolic syndrome and cardiovascular disease in adult offspring. The present study tested the hypothesis that maternal HFD modulates the brain renin-angiotensin system (RAS), oxidative stress, and proinflammatory cytokines that alter angiotensin II (ANG II) and TNF-α actions and sensitize the ANG II-elicited hypertensive response in adult offspring. All offspring were cross fostered by dams on the same or opposite diet to yield the following four groups: offspring from normal-fat control diet-fed dams suckled by control diet-fed dams (OCC group) or by HFD-fed dams (OCH group) and offspring from HFD-fed dams fed a HFD suckled by control diet-fed dams (OHC group) or by HFD-fed dams (OHH group). RT-PCR analyses of the lamina terminalis and paraventricular nucleus indicated upregulation of mRNA expression of several RAS components, NADPH oxidase, and proinflammatory cytokines in 10-wk-old male offspring of dams fed a HFD during either pregnancy, lactation, or both (OHC, OCH, and OHH groups). These offspring also showed decreased cardiac baroreflex sensitivity and increased pressor responses to intracerebroventricular microinjection of either ANG II or TNF-α. Furthermore, chronic systemic infusion of ANG II resulted in enhanced upregulation of mRNA expression of RAS components, NADPH oxidase, and proinflammatory cytokines in the lamina terminalis and paraventricular nucleus and an augmented hypertensive response in the OHC, OCH, and OHH groups compared with the OCC group. The results suggest that maternal HFD blunts cardiac baroreflex function and enhances pressor responses to ANG II or proinflammatory cytokines through upregulation of the brain RAS, oxidative stress, and inflammation. NEW & NOTEWORTHY The results of our study indicate that a maternal high-fat diet during either pregnancy or lactation is sufficient for perinatal programming of sensitization for hypertension, which is associated with hyperreactivity of central cardiovascular nuclei that, in all likelihood, involves elevated expression of the renin-angiotensin system, NADPH oxidase, and proinflammatory cytokines. The present study demonstrates, for the first time, the central mechanism underlying maternal high-fat diet sensitization of the hypertensive response in adult offspring.

Differential effects of long-term slow-pressor and subpressor angiotensin II on contractile and relaxant reactivity of resistance versus conductance arteries

Vascular reactivity was evaluated in three separate arteries isolated from rats after angiotensin II (Ang II) was infused chronically in two separate experiments, one using a 14-day high, slow-pressor dose known to produce hypertension and the other using a 7-day low, subpressor but hypertensive-sensitizing dose. There were three new findings. First, there was no evidence of altered vascular reactivity in resistance arteries that might otherwise explain the hypertension due to the high Ang II or the hypertensive-sensitizing effect of the low Ang II dose. Second, the high Ang II dose exerted a novel differential effect on arterial contractile responsiveness to the sympathetic neurotransmitter, norepinephrine, depending on the level of sympathetic innervation. It clearly enhanced that responsiveness in the sparsely innervated aorta but not in small mesenteric resistance arteries or the proximal (conductance) portion of the caudal artery, both of which are densely innervated. This suggests that the increased expression of alpha adrenergic receptors after long-term exposure to Ang II as previously reported for aortic smooth muscle, is prevented in densely innervated arteries, likely due to long-term Ang II-mediated increase in sympathetic neural traffic to those vessels. Third, the same high dose of Ang II impaired aortic relaxation in response to the nitric oxide (NO) donor nitroprusside without impairing aortic endothelium-dependent relaxation. NO is the main relaxing substance released by aortic endothelium. Accordingly, it is possible that this dose of Ang II is also associated with enhanced release of and/or enhanced smooth muscle responsiveness to other endothelial relaxing substances in a compensatory capacity.

Enhanced angiotensin II induced sodium appetite in renovascular hypertensive rats

Renovascular hypertensive 2-kidney, 1-clip (2K1C) rats have an increased activity of the renin-angiotensin system and an initial transitory increase in daily water and NaCl intake. However, the dipsogenic and natriorexigenic responses to angiotensin II (ANG II) have not been tested yet in 2K1C rats. Therefore, in the present study, we evaluated water and 0.3 M NaCl intake induced by water deprivation (WD)-partial rehydration (PR) or intracerebroventricular (icv) ANG II in 2K1C rats. In addition, the cardiovascular changes to these treatments were also evaluated. Male Holtzman rats received a silver clip around the left renal artery to induce 2K1C renovascular hypertension. In the 5th week, a group of animals received a guide cannula in the lateral ventricle for icv injections. Daily water intake increased from the 3rd week after surgery and remained elevated until the 6th week (last recording week), whereas daily 0.3 M NaCl intake transiently increased from the 2nd to the 5th week after surgery. On the 6th week, in spite of comparable daily 0.3 M NaCl intake between 2K1C and sham rats, WD-PR and icv ANG II induced an increased 0.3 M NaCl intake in 2K1C rats. Water intake induced by WD-PR, not by icv ANG II, also increased in 2K1C rats. The increase in arterial pressure to WD-PR or icv ANG II was similar in sham and 2K1C rats. Therefore, these results suggest that 2K1C rats are more responsive to the natriorexigenic effects of ANG II, whereas other responses to ANG II are not modified.

Integrative Physiological Aspects of Brain RAS in Hypertension

PURPOSE OF REVIEW

The renin-angiotensin system (RAS) plays an important role in modulating cardiovascular function and fluid homeostasis. While the systemic actions of the RAS are widely accepted, the role of the RAS in the brain, its regulation of cardiovascular function, and sympathetic outflow remain controversial. In this report, we discuss the current understanding of central RAS on blood pressure (BP) regulation, in light of recent literature and new experimental techniques.

RECENT FINDINGS

Studies using neuronal or glial-specifc mouse models have allowed for greater understanding into the site-specific expression and role centrally expressed RAS proteins have on BP regulation. While all components of the RAS have been identified in cardiovascular regulatory regions of the brain, their actions may be site specific. In a number of animal models of hypertension, reduction in Ang II-mediated signaling, or upregulation of the central ACE2/Ang 1-7 pathway, has been shown to reduce BP, via a reduction in sympathetic signaling and increase parasympathetic tone, respectively. Emerging evidence also suggests that, in part, the female protective phenotype against hypertension may be due to inceased ACE2 activity within cardiovascular regulatory regions of the brain, potentially mediated by estrogen. Increasing evidence suggests the importance of a central renin-angiotensin pathway, although its localization and the mechanisms involved in its expression and regulation still need to be clarified and more precisely defined. All reported studies/experiments with human or animal subjects performed by the authors have been previously published and complied with all applicable ethical standards (including the Helsinki declaration and its amendments, institutional/national research committee standards, and international/national/institutional guidelines).

Sex differences in maternal gestational hypertension-induced sensitization of angiotensin II hypertension in rat offspring: the protective effect of estrogen

Recent studies demonstrate that maternal hypertension during pregnancy sensitizes an angiotensin (ANG) II-induced increase in blood pressure (BP) in adult male offspring that was associated with upregulation of mRNA expression of several renin-angiotensin-aldosterone system (RAAS) components and NADPH oxidase in the lamina terminalis (LT) and paraventricular nucleus (PVN). The purpose of the present study was to test whether there are sex differences in the maternal hypertension-induced sensitization of ANG II hypertension, and whether sex hormones are involved in the sensitization process. Male offspring of hypertensive dams showed an enhanced hypertensive response to systemic ANG II when compared with male offspring of normotensive dams and to female offspring of either normotensive or hypertensive dams. Castration did not alter the hypertensive response to ANG II in male offspring. Intact female offspring had no upregulation of RAAS components and NADPH oxidase in the LT and PVN, whereas ovariectomy (OVX) upregulated mRNA expression of several RAAS components and NADPH oxidase in these nuclei and induced a greater increase in the pressor response to ANG II in female offspring of hypertensive dams compared with female offspring of normotensive dams. This enhanced increase in BP was partially attenuated by 17β-estradiol replacement in the OVX offspring of hypertensive dams. The results suggest that maternal hypertension induces a sex-specific sensitization of ANG II-induced hypertension and mRNA expression of brain RAAS and NADPH oxidase in offspring. Female offspring are protected from maternal hypertension-induced sensitization of ANG II hypertension, and female sex hormones are partially responsible for this protective effect.

Recent Advances in Neurogenic Hypertension: Dietary Salt, Obesity, and Inflammation

Neurally-mediated hypertension results from a dysregulation of sympathetic and/or neuroendocrine mechanisms to increase ABP. Multiple factors may exert multiple central effects to alter neural circuits and produce unique sympathetic signatures and elevate ABP. In this brief review, we have discussed novel observations regarding three contributing factors: dietary salt intake, obesity, and inflammation. However, the interaction among these and other factors is likely much more complex; recent studies suggest a prior exposure to one stimulus may sensitize the response to a subsequent hypertensive stimulus. Insight into the central mechanisms by which these factors selectively alter SNA or cooperatively interact to impact hypertension may represent a platform for novel therapeutic treatment strategies.

Maternal Gestational Hypertension-Induced Sensitization of Angiotensin II Hypertension Is Reversed by Renal Denervation or Angiotensin-Converting Enzyme Inhibition in Rat Offspring

Numerous findings demonstrate that there is a strong association between maternal health during pregnancy and cardiovascular disease in adult offspring. The purpose of the present study was to test whether maternal gestational hypertension modulates brain renin-angiotensin-aldosterone system (RAAS) and proinflammatory cytokines that sensitizes angiotensin II-elicited hypertensive response in adult offspring. In addition, the role of renal nerves and the RAAS in the sensitization process was investigated. Reverse transcription polymerase chain reaction analyses of structures of the lamina terminalis and paraventricular nucleus indicated upregulation of mRNA expression of several RAAS components and proinflammatory cytokines in 10-week-old male offspring of hypertensive dams. Most of these increases were significantly inhibited by either renal denervation performed at 8 weeks of age or treatment with an angiotensin-converting enzyme inhibitor, captopril, in drinking water starting at weaning. When tested beginning at 10 weeks of age, a pressor dose of angiotensin II resulted in enhanced upregulation of mRNA expression of RAAS components and proinflammatory cytokines in the lamina terminalis and paraventricular nucleus and an augmented pressor response in male offspring of hypertensive dams. The augmented blood pressure change and most of the increases in gene expression in the offspring were abolished by either renal denervation or captopril. The results suggest that maternal hypertension during pregnancy enhances pressor responses to angiotensin II through overactivity of renal nerves and the RAAS in male offspring and that upregulation of the brain RAAS and proinflammatory cytokines in these offspring may contribute to maternal gestational hypertension-induced sensitization of the hypertensive response to angiotensin II.

Prophylactic effects of alkaloids from Ba lotus seeds on L-NNA-induced hypertension in mice

Alkaloids from Ba lotus seeds (ABLS) are a kind of important functional compounds in lotus seeds. The present study was designed to determine its hypertension prophylactic effects in the L-NNA-induced mouse hypertension model. The mice were treated with ABLS, the serum and tissues levels of NO, MDA, ET-1, VEGF, and CGRP were determined using the experimental kits, the mRNA levels of various genes in the heart muscle and blood vessel tissues were further determined by RT-PCR assay. ABLS could reduce the systolic blood pressure (SBP), mean blood pressure (MBP), and diastolic blood pressure (DBP), compared to that of the model control group. After ABLS treatment, the NO (nitric oxide) contents in serum, heart, liver, kidney and stomach of the mice were higher than that of the control mice, but the MDA (malonaldehyde) contents were lower than that of the control mice. The serum levels of ET-1 (endothelin-1), VEGF (vascular endothelial growth factor) were decreased after ABLS treatment, but CGRP (calcium gene related peptide) level was increased. The ABLS treated mice had higher mRNA expressions of HO-1, nNOS, and eNOS and lower expressions of ADM, RAMP2, IL-1β, TNF-α, and iNOS than the control mice. Higher concentration of ABLS had greater prophylactic effects, which were close to that of the hypertension drug captopril. These results indicated the hypertension prophylactic effects of ABLS could be further explored as novel medicine or functional food in the future.

Angiotensin II AT1 receptors mediate neuronal sensitization and sustained blood pressure response induced by a single injection of amphetamine

A single exposure to amphetamine induces neurochemical sensitization in striatal areas. The neuropeptide angiotensin II, through AT₁ receptors (AT₁-R) activation, is involved in these responses. However, amphetamine-induced alterations can be extended to extra-striatal areas involved in blood pressure control and their physiological outcomes. Our aim for the present study was to analyze the possible role for AT₁-R in these events using a two-injection protocol and to further characterize the proposed AT₁-R antagonism protocol. Central effect of orally administered AT₁-R blocker (Candesartan, 3mg/kg p.o.×5days) in male Wistar rats was analyzed by spontaneous activity of neurons within locus coeruleus. In another group of animals pretreated with the AT₁-R blocker or vehicle, sensitization was achieved by a single administration of amphetamine (5mg/kg i.p. - day 6) followed by a 3-week period off drug. On day 27, after receiving an amphetamine challenge (0.5mg/kg i.p.), we evaluated: (1) the sensitized c-Fos expression in locus coeruleus (LC), nucleus of the solitary tract (NTS), caudal ventrolateral medulla (A1) and central amygdala (CeAmy); and (2) the blood pressure response. AT₁-R blockade decreased LC neurons' spontaneous firing rate. Moreover, sensitized c-Fos immunoreactivity in TH+neurons was found in LC and NTS; and both responses were blunted by the AT₁-R blocker pretreatment. Meanwhile, no differences were found neither in CeAmy nor A1. Sensitized blood pressure response was observed as sustained changes in mean arterial pressure and was effectively prevented by AT₁-R blockade. Our results extend AT₁-R role in amphetamine-induced sensitization over noradrenergic nuclei and their cardiovascular output.

Possible role for brain prostanoid pathways in the development of angiotensin II-salt hypertension in rats

Prostanoids generated by the cyclooxygenase (COX) pathway appear to contribute to the neurogenic hypertension (HTN) in rats. The first goal of this study was to establish the time frame during which prostanoids participate in ANG II-salt HTN. We induced HTN using ANG II (150 ng·kg(-1)·min(-1) sc) infusion for 14 days in rats on a high-salt (2% NaCl) diet. When ketoprofen pretreatment was combined with treatment during the first 7 days of ANG II infusion, development of HTN and increased neurogenic pressor activity (indexed by the depressor response to ganglion blockade) were significantly attenuated for the entire ANG II infusion period. This suggests that prostanoid generation caused by administration of ANG II and salt leads to an increase in neurogenic pressor activity and blood pressure (BP) via a mechanism that persists without the need for continuing prostanoid input. The second goal of this study was to determine whether prostanoid products specifically in the brain contribute to HTN development. Expression of prostanoid pathway genes was measured in brain regions known to affect neurogenic BP regulation. ANG II-treated rats exhibited changes in gene expression of phospholipase A2 (upregulated in organum vasculosum of the lamina terminalis, paraventricular nucleus, nucleus of the solitary tract, and middle cerebral artery) and lipocalin-type prostaglandin D synthase (upregulated in the organum vasculosum of the lamina terminalis). On the basis of our results, we propose that activation of the brain prostanoid synthesis pathway both upstream and downstream from COX at early stages plays an important role in the development of the neurogenic component of ANG II-salt HTN.

Leptin Mediates High-Fat Diet Sensitization of Angiotensin II-Elicited Hypertension by Upregulating the Brain Renin-Angiotensin System and Inflammation

Obesity is characterized by increased circulating levels of the adipocyte-derived hormone leptin, which can increase sympathetic nerve activity and raise blood pressure. A previous study revealed that rats fed a high-fat diet (HFD) have an enhanced hypertensive response to subsequent angiotensin II administration that is mediated at least, in part, by increased activity of brain renin-angiotensin system and proinflammatory cytokines. This study tested whether leptin mediates this HFD-induced sensitization of angiotensin II-elicited hypertension by interacting with brain renin-angiotensin system and proinflammatory cytokine mechanisms. Rats fed an HFD for 3 weeks had significant increases in white adipose tissue mass, plasma leptin levels, and mRNA expression of leptin and its receptors in the lamina terminalis and hypothalamic paraventricular nucleus. Central infusion of a leptin receptor antagonist during HFD feeding abolished HFD sensitization of angiotensin II-elicited hypertension. Furthermore, central infusion of leptin mimicked the sensitizing action of HFD. Concomitant central infusions of the angiotensin II type 1 receptor antagonist irbesartan, the tumor necrosis factor-α synthesis inhibitor pentoxifylline, or the inhibitor of microglial activation minocycline prevented the sensitization produced by central infusion of leptin. RT-PCR analysis indicated that either HFD or leptin administration upregulated mRNA expression of several components of the renin-angiotensin system and proinflammatory cytokines in the lamina terminalis and paraventricular nucleus. The leptin antagonist and the inhibitors of angiotensin II type 1 receptor, tumor necrosis factor-α synthesis, and microglial activation all reversed the expression of these genes. The results suggest that HFD-induced sensitization of angiotensin II-elicited hypertension is mediated by leptin through upregulation of central renin-angiotensin system and proinflammatory cytokines.

Hypothalamic signaling mechanisms in hypertension

The etiology of hypertension, a critical public health issue affecting one in three US adults, involves the integration of the actions of multiple organ systems, including the central nervous system. Increased activation of the central nervous system, driving enhanced sympathetic outflow and increased blood pressure, has emerged as a major contributor to the pathogenesis of hypertension. The hypothalamus is a key brain site acting to integrate central and peripheral inputs to ultimately impact blood pressure in multiple disease states that evoke hypertension. This review highlights recent advances that have identified novel signal transduction mechanisms within multiple hypothalamic nuclei (e.g., paraventricular nucleus, arcuate nucleus) acting to drive the pathophysiology of hypertension in neurogenic hypertension, angiotensin II hypertension, salt-sensitive hypertension, chronic intermittent hypoxia, and obesity-induced hypertension. Increased understanding of hypothalamic activity in hypertension has the potential to identify novel targets for future therapeutic interventions designed to treat hypertension.

Angiotensin II (de)sensitization: Fluid intake studies with implications for cardiovascular control

Cardiovascular disease is the leading cause of death worldwide and hypertension is the most common risk factor for death. Although many anti-hypertensive pharmacotherapies are approved for use in the United States, rates of hypertension have increased over the past decade. This review article summarizes a presentation given at the 2015 meeting of the Society for the Study of Ingestive Behavior. The presentation described work performed in our laboratory that uses angiotensin II-induced drinking as a model system to study behavioral and cardiovascular effects of the renin-angiotensin system, a key component of blood pressure regulation, and a common target of anti-hypertensives. Angiotensin II (AngII) is a potent dipsogen, but the drinking response shows a rapid desensitization after repeated injections of AngII. This desensitization appears to be dependent upon the timing of the injections, requires activation of the AngII type 1 (AT1) receptor, requires activation of mitogen-activated protein (MAP) kinase family members, and involves the anteroventral third ventricle (AV3V) region as a critical site of action. Moreover, the response does not appear to be the result of a more general suppression of behavior, a sensitized pressor response to AngII, or an aversive state generated by the treatment. More recent studies suggest that the treatment regimen used to produce desensitization in our laboratory also prevents the sensitization that occurs after daily bolus injections of AngII. Our hope is that these findings can be used to support future basic research on the topic that could lead to new developments in treatments for hypertension.

Water deprivation-partial rehydration induces sensitization of sodium appetite and alteration of hypothalamic transcripts

iSodium intake occurs either as a spontaneous or induced behavior, which is enhanced, i.e., sensitized, by repeated episodes of water deprivation followed by subsequent partial rehydration (WD-PR). In the present work, we examined whether repeated WD-PR alters hypothalamic transcripts related to the brain renin-angiotensin system (RAS) and apelin system in male normotensive Holtzman rats (HTZ). We also examined whether the sodium intake of a strain with genetically inherited high expression of the brain RAS, the spontaneously hypertensive rat (SHR), responds differently than HTZ to repeated WD-PR. We found that repeated WD-PR, besides enhancing spontaneous and induced 0.3 M NaCl intake, increased the hypothalamic expression of angiotensinogen, aminopeptidase N, and apelin receptor transcripts (43%, 60%, and 159%, respectively) in HTZ at the end of the third WD-PR. Repeated WD-PR did not change the daily spontaneous 0.3 M NaCl intake and barely changed the need-induced 0.3 M NaCl intake of SHR. The same treatment consistently enhanced spontaneous daily 0.3 M NaCl intake in the normotensive Wistar-Kyoto rats. The results show that repeated WD-PR produces alterations in hypothalamic transcripts and also sensitizes sodium appetite in HTZ. They suggest an association between the components of hypothalamic RAS and the apelin system, with neural and behavioral plasticity produced by repeated episodes of WD-PR in a normotensive strain. The results also indicate that the inherited hyperactive brain RAS is not a guarantee for sensitization of sodium intake in the male adult SHR exposed to repeated WD-PR.

Protective effects of diltiazem against vascular endothelial cell injury induced by angiotensin-II and hypoxia

To provide pharmacological data for future clinical studies, this study investigated the protective effects of diltiazem on vascular endothelial cell (VEC) injury induced by angiotensin-II (AngII), hypoxia, and a combination of both treatments. The concentration of intracellular free calcium and the mitochondrial membrane potential in VEC were assessed as indicators of cell injury. An in vivo hypoxic animal model was used to test the protective effect of diltiazem on vascular endothelial tissues. Our study showed that AngII and hypoxia decreased the mitochondrial membrane potential in VEC, which was significantly inhibited by diltiazem. Diltiazem protected against VEC injury induced by the increased concentration of intracellular free calcium, which was associated with AngII and hypoxia. Diltiazem reduced the apoptosis of rat VEC under a sustained hypoxic condition. In addition, it reduced AngII and endothelin I levels in rat vascular endothelial tissues. Our study confirmed that AngII and hypoxia induced VEC injury by regulating the levels of mitochondrial membrane potential and intracellular free calcium. Diltiazem, a calcium channel blocker, protected VEC from AngII- and hypoxia-induced injury.

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.

Memories that last in hypertension

In recent years, it has become clear that the immune system contributes to the genesis of hypertension. Hypertensive stimuli, such as angiotensin II, DOCA-salt, and norepinephrine, cause T cells and monocytes/macrophages to accumulate in the kidney and vasculature. These cells release inflammatory cytokines, such as IL-6, interferon-γ, and IL-17, that promote renal and vascular dysfunction. These cytokines also promote angiotensinogen production in the proximal tubule and Na(+) retention in the distal nephron and contribute to renal fibrosis and glomerular damage. For several years, we have observed accumulation of memory T cells in the kidney and vasculature. Given the propensity for memory cells to produce cytokines such as interferon-γ and IL-17, interventions to prevent the formation or renal accumulation of specific memory T cell subsets could prevent end-organ damage and blood pressure elevation in response to hypertensive stimuli.

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.

Actions of circulating angiotensin II and aldosterone in the brain contributing to hypertension

In the past 1-2 decades, it has become apparent that the brain renin-angiotensin-aldosterone system (RAAS) plays a crucial role in the regulation of blood pressure (BP) by the circulating RAAS. In the brain, angiotensinergic sympatho-excitatory pathways do not contribute to acute, second-to-second regulation but play a major role in the more chronic regulation of the setpoint for sympathetic tone and BP. Increases in plasma angiotensin II (Ang II) or aldosterone and in cerebrospinal fluid [Na(+)] can directly activate these pathways and chronically further activate/maintain enhanced activity by a slow neuromodulatory pathway involving local aldosterone, mineralocorticoid receptors (MRs), epithelial sodium channels, and endogenous ouabain. Blockade of any step in this slow pathway prevents Ang II-, aldosterone-, or salt and renal injury-induced forms of hypertension. It appears that the renal and arterial actions of circulating aldosterone and Ang II act as amplifiers but are not sufficient to cause chronic hypertension if their central actions are prevented, except perhaps at high concentrations. From a clinical perspective, oral treatment with an angiotensin type 1 (AT1)-receptor blocker at high doses can cause central AT1-receptor blockade and, in humans, lower sympathetic nerve activity. Low doses of the MR blocker spironolactone appear sufficient to cause central MR blockade and a decrease in sympathetic nerve activity. Integrating the brain actions of the circulating RAAS with its direct renal and arterial actions provides a better framework to understand the role of the circulating RAAS in the pathophysiology of hypertension and heart failure and to direct therapeutic strategies.