Stimulation of angiotensin type 1A receptors on catecholaminergic cells contributes to angiotensin-dependent hypertension

Milestone Papers on Signal Transduction Mechanisms of Hypertension and Its Complications

To celebrate 100 years of American Heart Association-supported cardiovascular disease research, this review article highlights milestone papers that have significantly contributed to the current understanding of the signaling mechanisms driving hypertension and associated cardiovascular disorders. This article also includes a few of the future research directions arising from these critical findings. To accomplish this important mission, 4 principal investigators gathered their efforts to cover distinct yet intricately related areas of signaling mechanisms pertaining to the pathogenesis of hypertension. The renin-angiotensin system, canonical and novel contractile and vasodilatory pathways in the resistance vasculature, vascular smooth muscle regulation by membrane channels, and noncanonical regulation of blood pressure and vascular function will be described and discussed as major subjects.

Recent Advances in Understanding the Molecular Pathophysiology of Angiotensin II Receptors: Lessons From Cell-Selective Receptor Deletion in Mice

The renin-angiotensin system (RAS) is an essential hormonal system involved in water and sodium reabsorption, renal blood flow regulation, and arterial constriction. Systemic stimulation of the RAS with infusion of the main peptide angiotensin II (Ang II) in animals as well as pathological elevation of renin (ie, renovascular hypertension) to increase circulatory Ang II in humans ultimately lead to hypertension and end organ damage. In addition to hypertension, accumulating evidence supports that the Ang II type 1 receptor exerts a critical role in cardiovascular and kidney diseases independent of blood pressure elevation. In the past 2 decades, the identification of an increased number of peptides and receptors has facilitated the concept that the RAS has detrimental and beneficial effects on the cardiovascular system depending on which RAS components are activated. For example, angiotensin 1-7 and Ang II type 2 receptors act as a counter-regulatory system against the classical RAS by mediating vasodilation. Although the RAS as an endocrine system for regulation of blood pressure is well established, there remain many unanswered questions and controversial findings regarding blood pressure regulation and pathophysiological regulation of cardiovascular diseases at the tissue level. This review article includes the latest knowledge gleaned from cell type-selective gene deleted mice regarding cell type-specific roles of Ang II receptors and their significance in health and diseases are discussed. In particular, we focus on the roles of these receptors expressed in vascular, cardiac, and kidney epithelial cells.

Genetic Deletion of AT1a Receptor or Na+/H+ Exchanger 3 Selectively in the Proximal Tubules of the Kidney Attenuates Two-Kidney, One-Clip Goldblatt Hypertension in Mice

The roles of angiotensin II (Ang II) AT1 (AT1a) receptors and its downstream target Na+/H+ exchanger 3 (NHE3) in the proximal tubules in the development of two-kidney, 1-clip (2K1C) Goldblatt hypertension have not been investigated previously. The present study tested the hypothesis that deletion of the AT1a receptor or NHE3 selectively in the proximal tubules of the kidney attenuates the development of 2K1C hypertension using novel mouse models with proximal tubule-specific deletion of AT1a receptors or NHE3. 2K1C Goldblatt hypertension was induced by placing a silver clip (0.12 mm) on the left renal artery for 4 weeks in adult male wild-type (WT), global Agtr1a−/−, proximal tubule (PT)-specific PT-Agtr1a−/− or PT-Nhe3−/− mice, respectively. As expected, telemetry blood pressure increased in a time-dependent manner in WT mice, reaching a maximal response by Week 3 (p < 0.01). 2K1C hypertension in WT mice was associated with increases in renin expression in the clipped kidney and decreases in the nonclipped kidney (p < 0.05). Plasma and kidney Ang II were significantly increased in WT mice with 2K1C hypertension (p < 0.05). Tubulointerstitial fibrotic responses were significantly increased in the clipped kidney (p < 0.01). Whole-body deletion of AT1a receptors completely blocked the development of 2K1C hypertension in Agtr1a−/− mice (p < 0.01 vs. WT). Likewise, proximal tubule-specific deletion of Agtr1a in PT-Agtr1a−/− mice or NHE3 in PT-Nhe3−/− mice also blocked the development of 2K1C hypertension (p < 0.01 vs. WT). Taken together, the present study provides new evidence for a critical role of proximal tubule Ang II/AT1 (AT1a)/NHE3 axis in the development of 2K1C Goldblatt hypertension.

Rodent models of hypertension

Elevated blood pressure (BP), or hypertension, is the main risk factor for cardiovascular disease. As a multifactorial and systemic disease that involves multiple organs and systems, hypertension remains a challenging disease to study. Models of hypertension are invaluable to support the discovery of the specific genetic, cellular and molecular mechanisms underlying essential hypertension, as well as to test new possible treatments to lower BP. Rodent models have proven to be an invaluable tool for advancing the field. In this review, we discuss the strengths and weaknesses of rodent models of hypertension through a systems approach. We highlight the ways how target organs and systems including the kidneys, vasculature, the sympathetic nervous system (SNS), immune system and the gut microbiota influence BP in each rodent model. We also discuss often overlooked hypertensive conditions such as pulmonary hypertension and hypertensive-pregnancy disorders, providing an important resource for researchers.

Advances in use of mouse models to study the renin-angiotensin system

The renin-angiotensin system (RAS) is a highly complex hormonal cascade that spans multiple organs and cell types to regulate solute and fluid balance along with cardiovascular function. Much of our current understanding of the functions of the RAS has emerged from a series of key studies in genetically-modified animals. Here, we review key findings from ground-breaking transgenic models, spanning decades of research into the RAS, with a focus on their use in studying blood pressure. We review the physiological importance of this regulatory system as evident through the examination of mouse models for several major RAS components: angiotensinogen, renin, ACE, ACE2, and the type 1 A angiotensin receptor. Both whole-animal and cell-specific knockout models have permitted critical RAS functions to be defined and demonstrate how redundancy and multiplicity within the RAS allow for compensatory adjustments to maintain homeostasis. Moreover, these models present exciting opportunities for continued discovery surrounding the role of the RAS in disease pathogenesis and treatment for cardiovascular disease and beyond.

Sympathetic Activation in Hypertension: Importance of the Central Nervous System

The sympathetic nervous system plays a critical role in the pathogenesis of hypertension. The central nervous system (CNS) organizes the sympathetic outflow and various inputs from the periphery. The brain renin-angiotensin system has been studied in various regions involved in controlling sympathetic outflow. Recent progress in cardiovascular research, particularly in vascular biology and neuroscience, as well as in traditional physiological approaches, has advanced the field of the neural control of hypertension in which the CNS plays a vital role. Cardiovascular research relating to hypertension has focused on the roles of nitric oxide, oxidative stress, inflammation, and immunity, and the network among various organs, including the heart, kidney, spleen, gut, and vasculature. The CNS mechanisms are similarly networked with these factors and are widely studied in neuroscience. In this review, I describe the development of the conceptual flow of this network in the field of hypertension on the basis of several important original research articles and discuss potential future breakthroughs leading to clinical precision medicine.

Neurocardiac regulation: from cardiac mechanisms to novel therapeutic approaches

Cardiac sympathetic overactivity is a well-established contributor to the progression of neurogenic hypertension and heart failure, yet the underlying pathophysiology remains unclear. Recent studies have highlighted the importance of acutely regulated cyclic nucleotides and their effectors in the control of intracellular calcium and exocytosis. Emerging evidence now suggests that a significant component of sympathetic overactivity and enhanced transmission may arise from impaired cyclic nucleotide signalling, resulting from compromised phosphodiesterase activity, as well as alterations in receptor-coupled G-protein activation. In this review, we address some of the key cellular and molecular pathways that contribute to sympathetic overactivity in hypertension and discuss their potential for therapeutic targeting.

Gut microbiota and neuroinflammation in pathogenesis of hypertension: A potential role for hydrogen sulfide

Inflammation and gut dysbiosis are hallmarks of hypertension (HTN). Hydrogen sulfide (H2S) is an important freely diffusing molecule that modulates the function of neural, cardiovascular and immune systems, and circulating levels of H2S are reduced in animals and humans with HTN. While most research to date has focused on H₂S produced endogenously by the host, H2S is also produced by the gut bacteria and may affect the host homeostasis. Here, we review an association between neuroinflammation and gut dysbiosis in HTN, with special emphasis on a potential role of H2S in this interplay.

Angiotensin peptide synthesis and cyclic nucleotide modulation in sympathetic stellate ganglia

Chronically elevated angiotensin II is a widely-established contributor to hypertension and heart failure via its action on the kidneys and vasculature. It also augments the activity of peripheral sympathetic nerves through activation of presynaptic angiotensin II receptors, thus contributing to sympathetic over-activity. Although some cells can synthesise angiotensin II locally, it is not known if this machinery is present in neurons closely coupled to the heart. Using a combination of RNA sequencing and quantitative real-time polymerase chain reaction, we demonstrate evidence for a renin-angiotensin synthesis pathway within human and rat sympathetic stellate ganglia, where significant alterations were observed in the spontaneously hypertensive rat stellate ganglia compared with Wistar stellates. We also used Förster Resonance Energy Transfer to demonstrate that administration of angiotensin II and angiotensin 1-7 peptides significantly elevate cyclic guanosine monophosphate in the rat stellate ganglia. Whether the release of angiotensin peptides from the sympathetic stellate ganglia alters neurotransmission and/or exacerbates cardiac dysfunction in states associated with sympathetic over activity remains to be established.

Angiotensin II Signal Transduction: An Update on Mechanisms of Physiology and Pathophysiology

The renin-angiotensin-aldosterone system plays crucial roles in cardiovascular physiology and pathophysiology. However, many of the signaling mechanisms have been unclear. The angiotensin II (ANG II) type 1 receptor (AT₁R) is believed to mediate most functions of ANG II in the system. AT₁R utilizes various signal transduction cascades causing hypertension, cardiovascular remodeling, and end organ damage. Moreover, functional cross-talk between AT₁R signaling pathways and other signaling pathways have been recognized. Accumulating evidence reveals the complexity of ANG II signal transduction in pathophysiology of the vasculature, heart, kidney, and brain, as well as several pathophysiological features, including inflammation, metabolic dysfunction, and aging. In this review, we provide a comprehensive update of the ANG II receptor signaling events and their functional significances for potential translation into therapeutic strategies. AT₁R remains central to the system in mediating physiological and pathophysiological functions of ANG II, and participation of specific signaling pathways becomes much clearer. There are still certain limitations and many controversies, and several noteworthy new concepts require further support. However, it is expected that rigorous translational research of the ANG II signaling pathways including those in large animals and humans will contribute to establishing effective new therapies against various diseases.

Renin-Angiotensin System Responds to Prolonged Hypotensive Effect Induced by Mandibular Extension in Spontaneously Hypertensive Rats

There is an ongoing interest in the renin-angiotensin system (RAS) contribution either to pathological mechanisms leading to hypertension (mainly regarding the ACE/AngII/AT1R axis), or, to RAS protective and pro-regenerative actions, primarily ascribed to the mediation of the AT2R and the MAS1 receptor. In the present study, we evaluated the modulation of gene expression and protein levels of “deleterious” (ACE/AngII/AT1R) and “protective” [ACE/AngII/AT2R and ACE2/Ang(1-7)/MAS1 arms] RAS components in parietal and frontal areas of cerebral cortex of spontaneously hypertensive rats (SHRs), after two periods of mandibular extensions (MEs). Blood pressure, BP and heart rate, HR were also measured. While no significant changes in BP and HR were present in the sham operated (SO) group, in rats after two MEs (2-ME rats), BP displayed a marked decrease (p < 0.001) at ME2, and remained then stably low for the subsequent observation period. In gene expression analysis, in SHRs undergoing two MEs, either in parietal or frontal cortex, we did not observe any significant variation of AT2R and ACE2 with respect to SO rats. In contrast, we observed a decrease in Mas1 gene expression in parietal area (p < 0.01) and an increase in frontal region (p < 0.01). AT1R and ACE gene expression was significantly higher in 2-ME rats than SO in parietal cortex (p < 0.05) but no difference was observed in the frontal area. Concerning protein levels, in parietal area, AT1R and AT2R did not change whereas MAS1 significantly decreased in 2-ME rats (p < 0.05). In frontal area, both AT1R and AT2R significantly decreased in 2-ME rats (p < 0.05), whereas MAS1 did not significantly change. Gene expression analysis in normotensive (NT) rats revealed the non-detectability of AT1R in both parietal and frontal zone. In parietal area, AT2R (p < 0.0001) and Mas1 (p < 0.01) were significantly decreased in 2-ME NT rats, when compared to SO, and ACE and ACE2 resulted not detectable whereas there was some expression of these genes after 2-ME procedure. In conclusion, our data in rat models indicated that a 2-ME procedure induced a hypotensive response and that a modulation of gene expression and protein levels of RAS components occurred in different cerebral cortex areas.

Systemic angiotensin II does not increase cardiac sympathetic nerve activity in normal conscious sheep

While it is well established that centrally injected angiotensin II (Ang II) has potent actions on sympathetic nervous activity (SNA), it is less clear whether peripheral Ang II can immediately stimulate SNA. In particular, the contribution of cardiac sympathetic nerve activity (CSNA) to the acute pressor response is unknown. We therefore examined the effect of incremental doses of intravenous Ang II (3, 6, 12, 24, and 48 ng/kg/min each for 30 min) on CSNA in eight conscious sheep. Ang II infusions progressively increased plasma Ang II up to 50 pmol/l above control levels in dose-dependent fashion (P<0.001). This was associated with the expected increases in mean arterial pressure (MAP) above control levels from <10 mmHg at lower doses up to 23 mmHg at the highest dose (P<0.001). Heart rate and cardiac output fell progressively with each incremental Ang II infusion achieving significance at higher doses (P<0.001). There was no significant change in plasma catecholamines. At no dose did Ang II increase any of the CSNA parameters measured. Rather, CSNA burst frequency (P<0.001), burst incidence, (P=0.002), and burst area (P=0.004) progressively decreased achieving significance during the three highest doses. In conclusion, Ang II infused at physiologically relevant doses increased MAP in association with a reciprocal decrease in CSNA presumably via baroreceptor-mediated pathways. The present study provides no evidence that even low-dose systemic Ang II stimulates sympathetic traffic directed to the heart, in normal conscious sheep.

Renal sympathetic denervation lowers arterial pressure in canines with obesity-induced hypertension by regulating GAD65 and AT1R expression in rostral ventrolateral medulla

To explore the roles of glutamate acid decarboxylase 65 (GAD65) and angiotensin II type 1 receptor (AT₁R) in the action of renal sympathetic denervation (RSD) on obesity-induced hypertension in canines. Thirty-two beagles were randomly divided into a hypertensive model (n = 22) and control (n = 10) groups. A hypertensive canine model was established by feeding a high-fat diet. Twenty hypertensive beagles were randomized equally to a sham-surgery and RSD-treated group receiving catheter-based radiofrequency RSD. Compared with the control group, the sham-surgery group exhibited significant increases in blood pressure, serum angiotensin II level, rostral ventrolateral medulla (RVLM) glutamate level, and AT₁R mRNA and protein expression and decreases in γ-amino acid butyric acid (γ-GABA) level and GAD65 mRNA and protein expression in the RVLM (all P < 0.05). Treatment with RSD significantly attenuated the above abnormal alterations (all P < 0.05). Linear correlation analysis revealed that angiotensin II level was positively correlated with glutamate level (r = 0.804) and inversely correlated with γ-GABA level (r = -0.765). GAD65 protein expression was positively correlated with γ-GABA level (r = 0.782). Catheter-based radiofrequency RSD can decrease blood pressure in obesity-induced hypertensive canines. The antihypertensive mechanism might be linked to upregulation of GAD65 and downregulation of AT₁R in the RVLM.

The vasoprotective axes of the renin-angiotensin system: Physiological relevance and therapeutic implications in cardiovascular, hypertensive and kidney diseases

The renin-angiotensin system (RAS) is undisputedly one of the most prominent endocrine (tissue-to-tissue), paracrine (cell-to-cell) and intracrine (intracellular/nuclear) vasoactive systems in the physiological regulation of neural, cardiovascular, blood pressure, and kidney function. The importance of the RAS in the development and pathogenesis of cardiovascular, hypertensive and kidney diseases has now been firmly established in clinical trials and practice using renin inhibitors, angiotensin-converting enzyme (ACE) inhibitors, type 1 (AT₁) angiotensin II (ANG II) receptor blockers (ARBs), or aldosterone receptor antagonists as major therapeutic drugs. The major mechanisms of actions for these RAS inhibitors or receptor blockers are mediated primarily by blocking the detrimental effects of the classic angiotensinogen/renin/ACE/ANG II/AT₁/aldosterone axis. However, the RAS has expanded from this classic axis to include several other complex biochemical and physiological axes, which are derived from the metabolism of this classic axis. Currently, at least five axes of the RAS have been described, with each having its key substrate, enzyme, effector peptide, receptor, and/or downstream signaling pathways. These include the classic angiotensinogen/renin/ACE/ANG II/AT₁ receptor, the ANG II/APA/ANG III/AT₂/NO/cGMP, the ANG I/ANG II/ACE2/ANG (1-7)/Mas receptor, the prorenin/renin/prorenin receptor (PRR or Atp6ap2)/MAP kinases ERK1/2/V-ATPase, and the ANG III/APN/ANG IV/IRAP/AT₄ receptor axes. Since the roles and therapeutic implications of the classic angiotensinogen/renin/ACE/ANG II/AT₁ receptor axis have been extensively reviewed, this article will focus primarily on reviewing the roles and therapeutic implications of the vasoprotective axes of the RAS in cardiovascular, hypertensive and kidney diseases.

Redistribution of NMDA Receptors in Estrogen-Receptor-β-Containing Paraventricular Hypothalamic Neurons following Slow-Pressor Angiotensin II Hypertension in Female Mice with Accelerated Ovarian Failure

Hypertension in male and aging female rodents is associated with glutamate-dependent plasticity in the hypothalamus, but existing models have failed to capture distinct transitional menopausal phases that could have a significant impact on the synaptic plasticity and emergent hypertension. In rodents, accelerated ovarian failure (AOF) induced by systemic injection of 4-vinylcyclohexane diepoxide mimics the estrogen fluctuations seen in human menopause including the perimenopause transition (peri-AOF) and postmenopause (post-AOF). Thus, we used the mouse AOF model to determine the impact of slow-pressor angiotensin II (AngII) administration on blood pressure and on the subcellular distribution of obligatory N-methyl-D-aspartate (NMDA) receptor GluN1 subunits in the paraventricular hypothalamic nucleus (PVN), a key estrogen-responsive cardiovascular regulatory area. Estrogen-sensitive neuronal profiles were identified in mice expressing enhanced green fluorescent protein under the promoter for estrogen receptor (ER) β, a major ER in the PVN. Slow-pressor AngII increased arterial blood pressure in mice at peri- and post-AOF time points. In control oil-injected (nonhypertensive) mice, AngII decreased the total number of GluN1 in ERβ-containing PVN dendrites. In contrast, AngII resulted in a reapportionment of GluN1 from the cytoplasm to the plasma membrane of ERβ-containing PVN dendrites in peri-AOF mice. Moreover, in post-AOF mice, AngII increased total GluN1, dendritic size and radical production in ERβ-containing neurons. These results indicate that unique patterns of hypothalamic glutamate receptor plasticity and dendritic structure accompany the elevated blood pressure in peri- and post-AOF time points. Our findings suggest the possibility that distinct neurobiological processes are associated with the increased blood pressure during perimenopausal and postmenopausal periods.

Selective C1 Lesioning Slightly Decreases Angiotensin II Type I Receptor Expression in the Rat Rostral Ventrolateral Medulla (RVLM)

Cardiovascular homeostasis is regulated in large part by the rostral ventrolateral medulla (RVLM) in mammals. Projections from the RVLM to the intermediolateral column of the thoracolumbar spinal cord innervate preganglionic neurons of the sympathetic nervous system causing elevation of blood pressure and heart rate. A large proportion, but not all, of the neurons in the RVLM contain the enzymes necessary for the production of epinephrine and are identified as the C1 cell group. Angiotensin II (Ang II) activates the RVLM acting upon AT1 receptors. To assess the proportion of AT1 receptors that are located on C1 neurons in the rat RVLM this study employed an antibody to dopamine-beta-hydroxylase conjugated to saporin, to selectively destroy C1 neurons in the RVLM. Expression of tyrosine hydroxylase immunoreactive neurons in the RVLM was reduced by 57 % in the toxin injected RVLM compared to the contralateral RVLM. In contrast, densitometric analysis of autoradiographic images of (125)I-sarcosine(1), isoleucine(8) Ang II binding to AT1 receptors of the injected side RVLM revealed a small (10 %) reduction in AT1-receptor expression compared to the contralateral RVLM. These results suggest that the majority of AT1 receptors in the rat RVLM are located on non-C1 neurons or glia.

Female protection from slow-pressor effects of angiotensin II involves prevention of ROS production independent of NMDA receptor trafficking in hypothalamic neurons expressing angiotensin 1A receptors

Renin–angiotensin system overactivity, upregulation of postsynaptic NMDA receptor function, and increased reactive oxygen species (ROS) production in the hypothalamic paraventricular nucleus (PVN) are hallmarks of angiotensin II (AngII)-induced hypertension, which is far more common in young males than in young females. We hypothesize that the sex differences in hypertension are related to differential AngII-induced changes in postsynaptic trafficking of the essential NMDA receptor GluN1 subunit and ROS production in PVN cells expressing angiotensin Type 1a receptor (AT1aR). We tested this hypothesis using slow-pressor (14-day) infusion of AngII (600 ng/kg/min) in mice, which elicits hypertension in males but not in young females. Two-month-old male and female transgenic mice expressing enhanced green fluorescent protein (EGFP) in AT1aR-containing cells were used. In males, but not in females, AngII increased blood pressure and ROS production in AT1aR–EGFP PVN cells at baseline and following NMDA treatment. Electron microscopy showed that AngII increased cytoplasmic and total GluN1–silver-intensified immunogold (SIG) densities and induced a trend toward an increase in near plasmalemmal GluN1–SIG density in AT1aR–EGFP dendrites of males and females. Moreover, AngII decreased dendritic area and diameter in males, but increased dendritic area of small (<1 µm) dendrites and decreased diameter of large (>1 µm) dendrites in females. Fluorescence microscopy revealed that AT1aR and estrogen receptor β do not colocalize, suggesting that if estrogen is involved, its effect is indirect. These data suggest that the sexual dimorphism in AngII-induced hypertension is associated with sex differences in ROS production in AT1aR-containing PVN cells but not with postsynaptic NMDA receptor trafficking.

Rapid metabolism of exogenous angiotensin II by catecholaminergic neuronal cells in culture media

Angiotensin II (AngII) acts on central neurons to increase neuronal firing and induce sympathoexcitation, which contribute to the pathogenesis of cardiovascular diseases including hypertension and heart failure. Numerous studies have examined the precise AngII-induced intraneuronal signaling mechanism in an attempt to identify new therapeutic targets for these diseases. Considering the technical challenges in studying specific intraneuronal signaling pathways in vivo, especially in the cardiovascular control brain regions, most studies have relied on neuronal cell culture models. However, there are numerous limitations in using cell culture models to study AngII intraneuronal signaling, including the lack of evidence indicating the stability of AngII in culture media. Herein, we tested the hypothesis that exogenous AngII is rapidly metabolized in neuronal cell culture media. Using liquid chromatography-tandem mass spectrometry, we measured levels of AngII and its metabolites, Ang III, Ang IV, and Ang-1-7, in neuronal cell culture media after administration of exogenous AngII (100 nmol/L) to a neuronal cell culture model (CATH.a neurons). AngII levels rapidly declined in the media, returning to near baseline levels within 3 h of administration. Additionally, levels of Ang III and Ang-1-7 acutely increased, while levels of Ang IV remained unchanged. Replenishing the media with exogenous AngII every 3 h for 24 h resulted in a consistent and significant increase in AngII levels for the duration of the treatment period. These data indicate that AngII is rapidly metabolized in neuronal cell culture media, and replenishing the media at least every 3 h is needed to sustain chronically elevated levels.

Combined suppression of the intrarenal and circulating vasoconstrictor renin-ACE-ANG II axis and augmentation of the vasodilator ACE2-ANG 1-7-Mas axis attenuates the systemic hypertension in Ren-2 transgenic rats exposed to chronic hypoxia

The aim of the present study was to test the hypothesis that chronic hypoxia would aggravate hypertension in Ren-2 transgenic rats (TGR), a well-defined monogenetic model of hypertension with increased activity of endogenous renin-angiotensin system (RAS). Systolic blood pressure (SBP) in conscious rats and mean arterial pressure (MAP) in anesthetized TGR and normotensive Hannover Sprague-Dawley (HanSD) rats were determined under normoxia that was either continuous or interrupted by two weeks´ hypoxia. Expression, activities and concentrations of individual components of RAS were studied in plasma and kidney of TGR and HanSD rats under normoxic conditions and after exposure to chronic hypoxia. In HanSD rats two weeks´ exposure to chronic hypoxia did not alter SBP and MAP. Surprisingly, in TGR it decreased markedly SBP and MAP; this was associated with substantial reduction in plasma and kidney renin activities and also of angiotensin II (ANG II) levels, without altering angiotensin-converting enzyme (ACE) activities. Simultaneously, in TGR the exposure to hypoxia increased kidney ACE type 2 (ACE2) activity and angiotensin 1-7 (ANG 1-7) concentrations as compared with TGR under continuous normoxia. Based on these results, we propose that suppression of the hypertensiogenic ACE-ANG II axis in the circulation and kidney tissue, combined with augmentation of the intrarenal vasodilator ACE2-ANG 1-7 axis, is the main mechanism responsible for the blood pressure-lowering effects of chronic hypoxia in TGR.