Regulation by angiotensin II of its receptors in resistance blood vessels

Molecular Mechanisms in Genetic Aortopathy-Signaling Pathways and Potential Interventions

Thoracic aortic disease affects people of all ages and the majority of those aged <60 years have an underlying genetic cause. There is presently no effective medical therapy for thoracic aneurysm and surgery remains the principal intervention. Unlike abdominal aortic aneurysm, for which the inflammatory/atherosclerotic pathogenesis is well established, the mechanism of thoracic aneurysm is less understood. This paper examines the key cell signaling systems responsible for the growth and development of the aorta, homeostasis of endothelial and vascular smooth muscle cells and interactions between pathways. The evidence supporting a role for individual signaling pathways in pathogenesis of thoracic aortic aneurysm is examined and potential novel therapeutic approaches are reviewed. Several key signaling pathways, notably TGF-β, WNT, NOTCH, PI3K/AKT and ANGII contribute to growth, proliferation, cell phenotype and survival for both vascular smooth muscle and endothelial cells. There is crosstalk between pathways, and between vascular smooth muscle and endothelial cells, with both synergistic and antagonistic interactions. A common feature of the activation of each is response to injury or abnormal cell stress. Considerable experimental evidence supports a contribution of each of these pathways to aneurysm formation. Although human information is less, there is sufficient data to implicate each pathway in the pathogenesis of human thoracic aneurysm. As some pathways i.e., WNT and NOTCH, play key roles in tissue growth and organogenesis in early life, it is possible that dysregulation of these pathways results in an abnormal aortic architecture even in infancy, thereby setting the stage for aneurysm development in later life. Given the fine tuning of these signaling systems, functional polymorphisms in key signaling elements may set up a future risk of thoracic aneurysm. Multiple novel therapeutic agents have been developed, targeting cell signaling pathways, predominantly in cancer medicine. Future investigations addressing cell specific targeting, reduced toxicity and also less intense treatment effects may hold promise for effective new medical treatments of thoracic aortic aneurysm.

Polarized Proteins in Endothelium and Their Contribution to Function

Protein localization in endothelial cells is tightly regulated to create distinct signaling domains within their tight spatial restrictions including luminal membranes, abluminal membranes, and interendothelial junctions, as well as caveolae and calcium signaling domains. Protein localization in endothelial cells is also determined in part by the vascular bed, with differences between arteries and veins and between large and small arteries. Specific protein polarity and localization is essential for endothelial cells in responding to various extracellular stimuli. In this review, we examine protein localization in the endothelium of resistance arteries, with occasional references to other vessels for contrast, and how that polarization contributes to endothelial function and ultimately whole organism physiology. We highlight the protein localization on the luminal surface, discussing important physiological receptors and the glycocalyx. The protein polarization to the abluminal membrane is especially unique in small resistance arteries with the presence of the myoendothelial junction, a signaling microdomain that regulates vasodilation, feedback to smooth muscle cells, and ultimately total peripheral resistance. We also discuss the interendothelial junction, where tight junctions, adherens junctions, and gap junctions all convene and regulate endothelial function. Finally, we address planar cell polarity, or axial polarity, and how this is regulated by mechanosensory signals like blood flow.

Sex and salt intake dependent renin-angiotensin plasticity in the liver of the rat

OBJECTIVE

Epidemiological studies confirm that hypertensive patients respond differently to renin-angiotensin system (RAS) inhibition depending on their gender. The aim of present work is to focus on sex-dependent differences in RAS regulation under conditions of increased salt intake.

METHOD

To investigate RAS, we measured the expression of angiotensinogen (Agt) mRNA, angiotensin receptor type 1 (AT1) mRNA and mitochondria assembly receptor (MasR) in the liver of rats under control conditions and after feeding with a salt diet (2% NaCl). In parallel, vascular endothelial growth factor A (VEGF-A) mRNA was analyzed.

RESULTS

Regression analysis revealed sex-dependent differences in the correlation between mRNA expression of AT1 and that of Agt, MasR and VEGF-A in both groups. There was a significant negative correlation between AT1 and Agt mRNA expression in the male control group, but this correlation disappeared in males exposed to a salt diet. In females, AT1 and Agt expression correlated only in the group exposed to the salt diet. In control males, there was a borderline trend to correlation between AT1 and MasR mRNA expression. The correlation between AT1 and VEGF-A mRNA expression was significant only in the control females, however, after exposure to a salt diet, this correlation diminished.

CONCLUSIONS

We hypothesize that RAS components expression is compensated differently in males and females. The observed loss of compensatory relationships in RAS between AT1 and Agt and AT1 and MasR in male rats under a salt diet can contribute to the differences observed in human with hypertension associated with an unhealthy diet.

Impaired neuronal and vascular responses to angiotensin II in a rabbit congestive heart failure model

Congestive heart failure (CHF) is characterised by activation of the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system (SNS). Both systems are known to interact and to potentiate each other's activities. We recently demonstrated that angiotensin II (Ang II) enhances sympathetic nerve traffic via prejunctionally-located AT1-receptors. At present, little is known about the effects of Ang II at the level of the sympathetic neurones in CHF.

Accordingly, we investigated the effect of Ang II in the presence and absence of the AT1-receptor antagonist, eprosartan, on stimulation-induced nerve traffic in isolated thoracic aorta preparations obtained from rabbits suffering from experimentally-induced CHF. Control-preparations were obtained from age-matched animals. Sympathetic activity was assessed by a [3H]noradrenaline spill-over model. Additionally, Ang II constrictor responses were compared between CHF and control vessels in the presence and absence of eprosartan. Additionally, to study postjunctional facilitation, the effects of Ang II on postsynaptic α-adrenoceptor-mediated responses were studied using noradrenaline.

Stimulation-evoked SNS-neurotransmission was similar in both groups (CHF versus control). Ang II (0.1 nM—0.1 µM) caused a concentration-dependent increase of the stimulation-evoked sympathetic outflow in both groups, with a maximum at 10 nM (control [n=7], FR2/FR12.03±0.11 and CHF-preparations [n=7], FR2/FR11.71±0.07). The enhancement by Ang II was decreased in CHF-preparations compared with controls (p<0.05). Eprosartan concentration-dependently attenuated the Ang II-enhanced (10 nM) sympathetic outflow in both CHF- and control preparations. The sympathoinhibitory potency of eprosartan was similar in both groups (control pIC508.81±0.31; CHF 8.65±0.42).

Ang II (1 nM—0.3 µM) concentration-dependently increased the contractile force in control preparations (Emax21.64±3.86 mN, pD27.63±0.02, n=7). Eprosartan (1 nM—0.1 µM) influenced the Ang IIcontractions via a mixed form of antagonism. In CHF-preparations, Ang II caused impaired vascular contraction. The KCl-induced contraction was decreased in the CHF- compared with control preparations (13.02±0.64 mN versus 30.40±0.89 mN). The relative Ang II contraction (% of KCl) was also decreased (2.3% vs. 58.0%). Concentration-response curves to noradrenaline (%KCl) were similar (control pD26.93±0.05, Emax131.0±2.7; CHF pD27.00±0.05, Emax136.7±2.6) (p>0.05) and were not affected by Ang II.

We conclude that Ang II-enhanced sympathetic neurotransmission is mediated by the prejunctional AT1-receptor in both control and CHF-preparations. The decreased facilitation of SNS effects by Ang II may be explained by down-regulation or desensitisation of the neuronal AT1-receptor. Additionally, the aortic contractile capacity in heart failure rabbits appears to be decreased, probably as a result of heart failure-associated neuroendocrine and functional changes.

High Angiotensin II Responsiveness is Associated with Decreased Endothelium-Dependent Relaxation in Human Arteries

Introduction. Animal studies demonstrated an interaction between angiotensin II (Ang II) responsiveness and endothelium-dependent relaxation (EDR). However, this relation has not been well described in humans. Therefore, we investigated the relation between Ang II responsiveness and EDR in isolated human arteries.

Materials and Methods. Segments of the internal mammary artery (IMA) were harvested from 89 patients undergoing coronary bypass surgery. Rings of these segments were exposed in organ bath experiments to metacholine (ME; 10 nmol/L -0.1 mmol/L) after precontraction with phenylephrine (PE; 10 µmol/L), and secondly to increasing concentrations of Ang II (0.1 nmol/L —1 µmol/L).

Results. Patients with the highest contraction to Ang II showed the lowest ME relaxation (r=0.312; p=0.003). Angiotensin-converting enzyme (ACE)-inhibition significantly increased Ang II sensitivity (p=0.03). This increase was accompanied by a tendency toward decreased EDR (p=0.07). The inverse relation between Ang responsiveness and endotheliumdependent relaxation could not be explained by an increased tissue or serum ACE-inhibition in patients with a higher endothelium-dependent relaxation.

Conclusions. High Ang II responsiveness inversely correlates to EDR in IMA's of patients with established coronary artery disease. Short-term treatment with an ACE-inhibitor increased the response to Ang II, but had an adverse effect on EDR.

International Union of Basic and Clinical Pharmacology. XCIX. Angiotensin Receptors: Interpreters of Pathophysiological Angiotensinergic Stimuli [corrected]

The renin angiotensin system (RAS) produced hormone peptides regulate many vital body functions. Dysfunctional signaling by receptors for RAS peptides leads to pathologic states. Nearly half of humanity today would likely benefit from modern drugs targeting these receptors. The receptors for RAS peptides consist of three G-protein-coupled receptors—the angiotensin II type 1 receptor (AT1 receptor), the angiotensin II type 2 receptor (AT2 receptor), the MAS receptor—and a type II trans-membrane zinc protein—the candidate angiotensin IV receptor (AngIV binding site). The prorenin receptor is a relatively new contender for consideration, but is not included here because the role of prorenin receptor as an independent endocrine mediator is presently unclear. The full spectrum of biologic characteristics of these receptors is still evolving, but there is evidence establishing unique roles of each receptor in cardiovascular, hemodynamic, neurologic, renal, and endothelial functions, as well as in cell proliferation, survival, matrix-cell interaction, and inflammation. Therapeutic agents targeted to these receptors are either in active use in clinical intervention of major common diseases or under evaluation for repurposing in many other disorders. Broad-spectrum influence these receptors produce in complex pathophysiological context in our body highlights their role as precise interpreters of distinctive angiotensinergic peptide cues. This review article summarizes findings published in the last 15 years on the structure, pharmacology, signaling, physiology, and disease states related to angiotensin receptors. We also discuss the challenges the pharmacologist presently faces in formally accepting newer members as established angiotensin receptors and emphasize necessary future developments.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Hemopexin activity is associated with angiotensin II responsiveness in humans

BACKGROUND

Hemopexin, an acute phase protein, can downregulate the angiotensin (ang) II type 1 receptor (AT1-R) in vitro. Whether hemopexin is involved in the responsiveness to ang II in vivo is unknown. Therefore, we tested whether variations in endogenous hemopexin activity are associated with the responsiveness of blood pressure to ang II in healthy volunteers.

METHOD

Healthy men (n = 33, age 26 ± 9) were studied in balance on low sodium (50 mmol Na per 24 h) and high sodium (200 mmol Na per 24 h) diet, respectively. After baseline measurements of blood pressure, ang II was infused at 0.3, 1 and 3 ng/kg per min for 1 h per dose. Hemopexin activity was measured at baseline in EDTA-plasma samples by an amidolytic assay with a chromogenic substrate suitable for hemopexin activity evaluation.

RESULTS

During high sodium the hemopexin activity was lower; 1.6 × 10 (0.6 × 10 - 4.7 × 10) versus 2.8 × 10 (1.1 × 10 - 5.1 × 10) arbitrary units (P < 0.01) and the pressor response to 3 ng ang II/kg per minute larger than during low sodium (17.6 ± 6.5 versus 14.6 ± 6.9 mmHg, P < 0.01). Hemopexin activity negatively correlated with the pressor response to ang II during either type of sodium intake (high sodium: r = 0.42, P < 0.05; low sodium: r = 0.35, P < 0.05).

CONCLUSION

These in-vivo data obtained in healthy individuals support recent in-vitro data showing that active hemopexin downregulates the availability of the AT1-R. Therefore, activated hemopexin might be considered as a factor mediating ang II effects upon blood pressure by modulating AT1-R availability.

The angiotensin II AT1 receptor-associated protein Arap1 is involved in sepsis-induced hypotension

Introduction

Hypotension in septic patients results from hypovolemia, vasodilatation and hyporeactivity to vasoconstrictors, such as angiotensin II. The AT1 receptor-associated protein 1 (Arap1) is expressed in vascular smooth muscle cells and increases the surface expression of the AT1-receptor in vitro. We hypothesized that dysregulation of Arap1 may contribute to vascular hyporeactivity to angiotensin II during endotoxemia.

Methods

Arap1-deficient mice were used to assess the role of Arap1 in sepsis-induced hypotension. The isolated perfused kidney was used as an in vitro model to determine the relevance of Arap1 for vascular resistance and sensitivity to angiotensin II.

Results

During endotoxemia, mean arterial blood pressure (MAP) decreased in both genotypes, with the time course of sepsis-induced hypotension being markedly accelerated in Arap1-/- compared to +/+ mice. However, baseline MAP was similar in Arap1-/- and wildtype mice (102 ± 2 vs.103 ± 2 mmHg; telemetry measurements; n = 10; P = 0.66). Following lipopolysaccharide (LPS) injections (3 mg/kg), Arap1 expression was successively down-regulated in the wildtype mice, reaching levels below 10% of baseline expression. The endotoxemia-related decline in Arap1 expression could be recapitulated in cultured mesangial cells by incubation with pro-inflammatory cytokines, such as tumor necrosis factor α and interferon γ. Plasma renin concentration was increased in Arap1-/- mice compared to wildtype mice (66 ± 6 vs. 41 ± 4 ng AngI/ml/h; n = 23; P = 0.001), presumably contributing to preserved MAP under baseline conditions. The sensitivity of the vasculature to angiotensin II was reduced in Arap1-/- compared to +/+ mice, as determined in the isolated perfused kidney.

Conclusions

Our data suggest that down-regulation of Arap1 expression during sepsis contributes to the development of hypotension by causing reduced vascular sensitivity to angiotensin II.

Angiotensin II-mediated vascular changes in aged offspring rats exposed to perinatal nicotine

This study determined the long-term influence of prenatal nicotine exposure (PN) on blood pressure and vascular functions in the aged offspring rats. PN did not affect body weight and plasma adrenocorticotropic hormone level; however, it significantly reduced plasma angiotensin I and angiotensin II in both sexes. Systolic pressure in the male aged PN offspring was significantly higher. Angiotensin II-increased mean arterial pressure was higher in the aged PN offspring than that in the control regardless of sex. AT1 receptor blocker losartan, not AT2 receptor antagonist PD123319, reduced blood pressure in the aged PN rats more than that in the control. In the aged PN offspring, angiotensin II-increased vessel contraction and intracellular calcium level were higher in small mesenteric arteries. Acetylcholine-mediated vascular relaxation was weaker, and nitric oxide-related endothelial functions were damaged in aortic rings of PN offspring. Thickness of the wall of mesenteric arteries was increased in the male aged PN offspring. Ratio of AT1/AT2 receptors was significantly increased in the vessel of the PN group regardless of sex. These data provide new information on the very long term influence of PN on vascular structures and functions in the aged offspring, demonstrate that the aged PN female rats were not free of vascular risks after menopause, and suggest that multiple pathways may be involved in the detrimental alterations of the cardiovascular system of the PN rats.

How and why chemicals from tobacco smoke can induce a rise in blood pressure

The primary objective of this article is to analyze the role of tobacco smoke compounds able to damage the cardiovascular system and, in particular, to interfere with blood pressure. They are products of tobacco plant leaves, like nicotine, thiocyanate and aromatic amines, and a chemical derived from cigarette combustion, carbon monoxide. Of the other thousands of chemicals, there is no clear evidence of cardiovascular damage. Nicotine and its major metabolite, cotinine, usually increase blood pressure by a direct action and an action stimulating neuro-humoral metabolites of the body as well as sympathetic stimulation. An indirect mechanism of damage exerted by elevated carboxyhemoglobin concentrations is mediated by carbon monoxide, which, mainly induces arterial wall damage and, consequently, late rising in blood pressure by a toxic direct action on endothelial and blood cells. Thiocyanate, in turn, reinforces the hypoxic effects determined by carbon monoxide. Aromatic amines, depending on their chemical structure, may exert toxic effects on the cardiovascular system although they have little effect on blood pressure. A rise in blood pressure determined by smoking compounds is a consequence of both their direct toxicity and the characteristics of their chemical chains that are strongly reactive with a large number of molecules for their spatial shape. In addition, a rise in blood pressure has been documented in individuals smoking a cigarette, acutely and chronically, with irreversible artery wall alterations several years after beginning smoking. Since cigarette smoking has a worldwide diffusion, the evidence of this topic meets the interest of both the scientific community and those individuals aiming to control smoking.

Response to angiotensin-converting enzyme inhibition is selectively blunted by high sodium in angiotensin-converting enzyme DD genotype: evidence for gene-environment interaction in healthy volunteers

BACKGROUND

Renin-angiotensin-aldosterone system blockade is a cornerstone in cardiovascular protection. Angiotensin-converting enzyme (ACE)-DD genotype has been associated with resistance to angiotensin-converting enzyme inhibition (ACEi), but data are conflicting. As sodium intake modifies the effect of ACEi as well as the genotype-phenotype relationship, we hypothesize gene-environment interaction between sodium-status, the response to ACEi, and ACE genotype.

METHOD

Thirty-five male volunteers (26 ± 9 years; II n = 6, ID n = 18, DD n = 11) were studied during placebo and ACEi (double blind, enalapril 20 mg/day) on low [7 days 50 mmol Na/day (low salt)] and high [7 days 200 mmol Na/day (high salt)] sodium, with a washout of 6 weeks in-between. After each period mean arterial pressure (MAP) was measured before and during graded infusion of angiotensin II (Ang II).

RESULTS

During high salt, ACEi reduced MAP in II and ID, but not in DD [II: 88 (78-94) versus 76 (72-88); ID: 87 (84-91) versus 83 (79-87); both P < 0.05 and DD: 86 (82-96) versus 88 (80-90); ns, P < 0.05 between genotypes]. However, during low salt, ACEi reduced MAP in all genotype groups [II: 83 (78-89) versus 77 (72-83); ID: 88 (84-91) versus 82 (78-86); DD: 84 (80-91) versus 81 (75-85); all P < 0.05]. During high salt + ACEi, the Ang II response was blunted in DD, with an 18% rise in MAP during the highest dose versus 22 and 31% in ID and II (P < 0.05). Low salt annihilated these differences.

CONCLUSION

In healthy participants, the MAP response to ACEi is selectively blunted in DD genotype during high salt, accompanied by blunted sensitivity to Ang II. Low salt corrects both abnormalities. Further analysis of this gene-environment interaction in patients may contribute to strategies for improvement of individual treatment efficacy.

Angiotensin-converting enzyme inhibition augments the expression of rat elastase-2, an angiotensin II-forming enzyme

Mounting evidence suggest that tissue levels of angiotensin (ANG) II are maintained in animals submitted to chronic angiotensin-converting enzyme (ACE) inhibitor treatment. We examined the expression levels of transcripts for elastase-2, a chymostatin-sensitive serine protease identified as the alternative pathway for ANG II generation from ANG I in the rat vascular tissue and the relative role of ACE-dependent and -independent pathways in generating ANG II in the rat isolated carotid artery rings of spontaneously hypertensive rats (SHR) and Wistar normotensive rats (WNR) treated with enalapril for 7 days. Enalapril treatment decreased blood pressure of SHR only and resulted in significantly more elastase-2 mRNA expression in carotid artery of both enalapril-treated WNR and SHR. Captopril induced a comparable rightward shift of concentration-response curves to ANG I in vehicle and enalapril-treated rats, although this effect was of lesser magnitude in SHR group. Chymostatin induced a rightward shift of the dose response to ANG I in vehicle-treated and a decrease in maximal effect of 22% in enalapril-treated WNR group. Maximal response induced by ANG I was remarkably reduced by chymostatin in enalapril-treated SHR carotid artery (by 80%) compared with controls (by 23%). Our data show that chronic ACE inhibition was associated with augmented functional role of non-ACE pathway in generating ANG II and increased elastase-2 gene expression, suggesting that this protease may contribute as an alternative pathway for ANG II generation when ACE is inhibited in the rat vascular tissue.

Physiology of Kidney Renin

The protease renin is the key enzyme of the renin-angiotensin-aldosterone cascade, which is relevant under both physiological and pathophysiological settings. The kidney is the only organ capable of releasing enzymatically active renin. Although the characteristic juxtaglomerular position is the best known site of renin generation, renin-producing cells in the kidney can vary in number and localization. (Pro)renin gene transcription in these cells is controlled by a number of transcription factors, among which CREB is the best characterized. Pro-renin is stored in vesicles, activated to renin, and then released upon demand. The release of renin is under the control of the cAMP (stimulatory) and Ca2+(inhibitory) signaling pathways. Meanwhile, a great number of intrarenally generated or systemically acting factors have been identified that control the renin secretion directly at the level of renin-producing cells, by activating either of the signaling pathways mentioned above. The broad spectrum of biological actions of (pro)renin is mediated by receptors for (pro)renin, angiotensin II and angiotensin-( 1 – 7 ).

High intraluminal pressure via H2O2 upregulates arteriolar constrictions to angiotensin II by increasing the functional availability of AT1 receptors

Previously, we found that high intraluminal pressure leads to production of reactive oxygen species (ROS) and also upregulates several components of the renin-angiotensin system in the wall of small arteries. We hypothesized that acute exposure of arterioles to high intraluminal pressure in vitro via increasing ROS production enhances the functional availability of type 1 angiotensin II (Ang II) receptors (AT1 receptors), resulting in sustained constrictions. In arterioles ( approximately 180 mum) isolated from rat skeletal muscle, Ang II elicited dose-dependent constrictions, which decreased significantly by the second application [maximum (max.): from 59% +/- 4% to 26% +/- 5% at 10(-8) M; P < 0.05] in the presence of 80 mmHg of intraluminal pressure. In contrast, if the arterioles were exposed to high intraluminal pressure (160 mmHg for 30 min), Ang II-induced constrictions remained substantial on the second application (max.: 51% +/- 3% at 10(-8) M). In the presence of Tiron and polyethylene glycol (PEG)-catalase, known to reduce the level of superoxide anion and hydrogen peroxide (H(2)O(2)), second applications of Ang II evoked similarly reduced constrictions, even after high-pressure exposure (29% +/- 4% at 10(-8) M). Furthermore, when arterioles were exposed to H(2)O(2) (for 30 min, 10(-7) M, at normal 80 mmHg pressure), Ang II-induced constrictions remained substantial on second applications (59% +/- 5% at 10(-8) M). These findings suggest that high pressure, likely via inducing H(2)O(2) production, increases the functional availability of AT1 receptors and thus enhances Ang II-induced arteriolar constrictions. We propose that in hypertension-regardless of etiology-high intraluminal pressure, via oxidative stress, enhances the functional availability of AT1 receptors augmenting Ang II-induced constrictions.

Mechanisms underlying developmental programming of elevated blood pressure and vascular dysfunction: evidence from human studies and experimental animal models

Cardiovascular-related diseases are the leading cause of death in the world in both men and women. In addition to the environmental and genetic factors, early life conditions are now also considered important contributing elements to these pathologies. The concept of 'fetal' or 'developmental' origins of adult diseases has received increased recognition over the last decade, yet the mechanism by which altered perinatal environment can lead to dysfunction mostly apparent in the adult are incompletely understood. This review will focus on the mechanisms and pathways that epidemiological studies and experimental models have revealed underlying the adult cardiovascular phenotype dictated by the perinatal experience, as well as the probable key causal or triggering elements. Programmed elevated blood pressure in the adult human or animal is characterized by vascular dysfunction and microvascular rarefaction. Developmental mechanisms that have been more extensively studied include glucocorticoid exposure, the role of the kidneys and the renin-angiotensin system. Other pathophysiological pathways have been explored, such as the role of the brain and the sympathetic nervous system, oxidative stress and epigenetic changes. As with many complex diseases, a unifying hypothesis linking the perinatal environment to elevated blood pressure and vascular dysfunction in later life cannot be presumed, and a better understanding of those mechanisms is critical before clinical trials of preventive or 'deprogramming' measures can be designed.

Angiotensin II Signaling in Vascular Physiology and Pathophysiology

Initially recognized as a physiologic regulator of blood pressure and body fluid homeostasis, angiotensin (Ang) II has now been shown in innumerable experiments and clinical studies to contribute to the development and maintenance of cardiovascular disease. Dissection of its signaling mechanisms over the past decades has led to the discovery of several novel concepts, such as tissue-specific metabolism of Ang peptides. Identification and cloning of the various receptors through which Ang II acts on almost all tissues has led to the development of specific pharmacologic inhibitors with proven clinical benefit in patients with cardiovascular disorders. Work on the G-protein-coupled Ang II Type 1 receptor has demonstrated that different receptors interact through oligomerization, compartmentalization, and transactivation, and may explain how Ang II can activate G-protein-independent pathways. Unraveling the downstream effects of Ang II in specific cell types corroborates the importance of the cellular redox state on certain signaling pathways. Finally, the effects of Ang II on cell function and phenotype, such as the expression of inflammatory cytokines and receptors promoting the recruitment of inflammatory cells into vascular tissues, have indicated its role in local inflammation as a general pathogenetic basis of cardiovascular disease. The recognition of Ang II as a contributor to such fundamental pathophysiologic mechanisms, which are believed to be a common pathway for diverse cardiovascular risk factors like hypertension and diabetes, has greatly advanced our knowledge of pathologic signaling in vascular tissues and may help to eventually define novel targets for pharmacologic interventions.

Late ventricular remodeling in non-reperfused acute myocardial infarction in humans is predicted by angiotensin II type 1 receptor density on blood platelets

BACKGROUND

Ventricular remodeling after myocardial infarction (MI) is largely dependent on renin-angiotensin system activity, which is determined by angiotensin II concentration and angiotensin II type 1 receptor (AT(1)R) density in target tissues. We have recently shown that AT(1)R density in the acute phase of MI determines post-MI ventricular remodeling at discharge (8 days). The aim of this study was to test whether this correlation is retained in a longer follow-up (6 months), in the same group of patients.

METHODS

In 48 patients with first acute MI who did not undergo reperfusion therapy, angiotensin AT(1)R density on blood platelets (a presumable marker of cardiovascular AT(1)R density) was assessed 13+/-5 h after the onset of MI, using radioligand binding assay. Echocardiographic indices of left ventricular function and dimensions were used as measures of ventricular remodeling.

RESULTS

6 months after the infarction patients who at baseline had AT(1)R density above median (N=17) as compared to those with AT(1)R density below median (N=20) had higher left ventricular end-systolic volume index (LVESVI, 41.3+/-2.7 vs. 33.2+/-2.3) and lower ejection fraction (LVEF 48.1+/-1.8 vs. 54.7+/-2.0). Moreover LVESVI positively and LVEF negatively correlated with AT(1)R density although the strength of these correlations was weaker than at discharge. Infarct size as reflected by a single troponin T measurement and post-MI therapy did not differ between high- and low-AT(1)R groups: over 85% patients received ACE-inhibitor, beta-blocker and statin.

CONCLUSIONS

High AT(1)R density on blood platelets (a presumable marker of cardiovascular AT(1)R density) drawn in the acute phase of MI predicts poorer left ventricular systolic function in 6-month follow up. This suggests that modern therapy offers suboptimal blockade of renin-angiotensin system activity in the setting of MI.

Challenged sodium balance and expression of angiotensin type 1A receptor mRNA in the hypothalamus of Wistar and Dahl rat strains

The present study investigates the influence of a chronic high Na+ diet (8% Na+) on the expression of the angiotensin type 1A (AT1A) receptor gene in the lamina terminalis and paraventricular nucleus of the hypothalamus (PVH) in normotensive Wistar (W) rats, as well as in Dahl salt-resistant (DR) and Dahl salt-sensitive (DS) rats. Three weeks of 8% Na+ diet led to a higher blood pressure in DS rats compared to DR and W rats. Moreover, the high Na+ diet was correlated with a decreased expression of AT1A receptor mRNA in the median preoptic nucleus (MnPO) and in the PVH of DS rats, compared to DR and W rats. Contrastingly, the AT1A receptor mRNA expression was not altered by the high Na+ diet in the forebrain circumventricular organs of all the rat strains. Interestingly, a furosemide-induced Na+ depletion was correlated with an increased expression of AT1A receptor mRNA in the PVH, MnPO and SFO of both the DS and DR rats. It is concluded that chronic high Na+ diet did differently regulate the expression of AT1A receptor mRNA in two hypothalamic integrative centers for hydromineral and cardiovascular balance (the PVH and MnPO) in DS rats, compared to DR and W rats. However, the AT1A receptor mRNA expression was similarly regulated in DS and DR rats in response to an acute Na+ depletion, suggesting a distinct high Na+ -induced regulation of the AT1A receptor gene in the PVH and MnPO of DS rats.

Exaggerated vasomotor response to ANG II in rats with fetal programming of hypertension associated with exposure to a low-protein diet during gestation

The renin-angiotensin system plays a key role in the initiation and maintenance of elevated blood pressure associated with altered intrauterine milieu. The current studies were undertaken to verify whether vascular response to ANG II is increased in adult offspring of low-protein fed dams (LP) compared with control (CTRL) and if so, to examine underlying mechanism(s). ANG II-induced contraction of carotid rings was increased in LP (Emax, the maximum asymptote of the curve, relative to maximal response to KCl 80 mM: 230 ± 3% LP vs. 201 ± 2% CTRL, P < 0.05). In both groups, contraction to ANG II was mediated solely by AT1R. Responses to thromboxane A2 analog U-46619 and to KCl 80 mM under step increases in tension were similar between groups. Endothelium depletion enhanced contraction to ANG II in both groups, more so in LP. Blockade of endothelin formation had no effect on response to ANG II, and ANG-(1–7) did not elicit vasomotor response in either group. Superoxide dismutase (SOD) analog Tempol normalized LP without modifying CTRL response to ANG II. Basal levels of superoxide (aortic segments, lucigenin-enhanced chemiluminescence and fluorescent dye hydroethidine) were higher in LP. ANG II further increased superoxide production in LP only, and this was inhibited by coincubation with diphenylene iodonium or apocynin (inhibitor of NADPH oxidase complex). AT1R expression in carotid arteries was increased in LP, whereas SOD expression was unchanged. In conclusion, vasoconstriction to ANG II is exaggerated in this model of developmental programming of hypertension, secondary to enhanced vascular production of superoxide anion by NADPH oxidase with concomitant increase of AT1R expression.

Tissue angiotensin-converting enzyme inhibitors: are they more effective than serum angiotensin-converting enzyme inhibitors?

Since their discovery in the 1980s, angiotensin-converting enzyme (ACE) inhibitors have been shown to decrease angiotensin formation, prevent breakdown of bradykinin, and may also act on peptides of the renin-angiotensin system. They are effective in reducing the risk of heart failure, myocardial infarction, and death from cardiovascular causes in patients with left ventricular systolic dysfunction or heart failure, and have been shown to reduce atherosclerotic complications in patients who have vascular disease without heart failure. They may preserve endothelial function and counteract initiation and progression of atherosclerosis. Broadly, ACE inhibitors can be divided into tissue specific or serum ACE inhibitors. Tissue-specific ACE inhibitors as a group are not superior to serum ACE inhibitors in the treatment of coronary artery disease. Pending direct comparator clinical trials between a tissue ACE inhibitor and a plasma ACE inhibitor, both ramipril and perindopril can be recommended for secondary risk prevention, based on the evidence.

Angiotensin II cell signaling: physiological and pathological effects in the cardiovascular system

The renin-angiotensin system is a central component of the physiological and pathological responses of cardiovascular system. Its primary effector hormone, angiotensin II (ANG II), not only mediates immediate physiological effects of vasoconstriction and blood pressure regulation, but is also implicated in inflammation, endothelial dysfunction, atherosclerosis, hypertension, and congestive heart failure. The myriad effects of ANG II depend on time (acute vs. chronic) and on the cells/tissues upon which it acts. In addition to inducing G protein- and non-G protein-related signaling pathways, ANG II, via AT(1) receptors, carries out its functions via MAP kinases (ERK 1/2, JNK, p38MAPK), receptor tyrosine kinases [PDGF, EGFR, insulin receptor], and nonreceptor tyrosine kinases [Src, JAK/STAT, focal adhesion kinase (FAK)]. AT(1)R-mediated NAD(P)H oxidase activation leads to generation of reactive oxygen species, widely implicated in vascular inflammation and fibrosis. ANG II also promotes the association of scaffolding proteins, such as paxillin, talin, and p130Cas, leading to focal adhesion and extracellular matrix formation. These signaling cascades lead to contraction, smooth muscle cell growth, hypertrophy, and cell migration, events that contribute to normal vascular function, and to disease progression. This review focuses on the structure and function of AT(1) receptors and the major signaling mechanisms by which angiotensin influences cardiovascular physiology and pathology.

Angiotensin II AT1 receptor density on blood platelets predicts early left ventricular remodelling in non-reperfused acute myocardial infarction in humans

BACKGROUND

Renin-angiotensin-system activity, a principal factor determining ventricular remodelling after myocardial infarction (MI), is dependent on local angiotensin II concentration and angiotensin AT1 receptor (AT1R) density. The latter is regulated by systemic factors acting independently from angiotensin II concentration.

OBJECTIVE

To test the hypothesis that AT1R density at the onset of MI determines post-MI ventricular remodelling.

METHODS

In 48 patients with first acute MI who did not undergo reperfusion therapy, angiotensin AT1R density on blood platelets (reflecting cardiovascular AT1R density) was assessed 13+/-5 h after the onset of MI, using radioligand binding assay. Left ventricular end-systolic (LVESVI) and end-diastolic volume indices (LVEDVI) and ejection fraction (EF) were assessed by two-dimensional echocardiography as measures of ventricular remodelling.

RESULTS

Predischarge LVESVI and LVEDVI positively and EF negatively correlated with AT1R density. Patients with AT1R density below median had significantly lower LVESVI (33.2+/-2.4 mL/m2), LVEDVI (70.0+/-2.8 mL/m2) and higher EF (52.8+/-2.3%) than patients with AT1R density above median (LVESVI = 44.9+/-2.6, LVEDVI = 81.3+/-3.9 mL/m2 and EF = 44.9+/-2.6%, all p<0.01). In multivariate analysis, only AT1R density and infarct size were independent predictors of early post-MI ventricular dilation.

CONCLUSIONS

High density of AT1R at the onset of MI is a predictor of early left ventricular remodelling.

Vascular response to angiotensin II predicts long-term prognosis in patients undergoing coronary artery bypass grafting

Persistent activation of the renin-angiotensin system leads to downregulation of the angiotensin type-1 receptor, and consequently, to a decreased response to exogenous angiotensin II. In the present study, we investigated the association of angiotensin II responsiveness to clinical outcome after coronary artery bypass grafting (CABG). We studied the responsiveness to exogenous angiotensin II in human thoracic artery preparations of 114 CABG patients. Mean duration of follow-up was 7.3+/-0.1 years, during which 21 patients experienced a cardiovascular event. A diminished response to angiotensin II remained in multivariate Cox regression analysis, after adjustment for sex, age, blood pressure, and number of diseased coronary arteries, the strongest predictor for cardiovascular events (relative risk, 3.37 [95% confidence interval, 1.20 to 9.51]; P=0.022). Furthermore, diminished response to angiotensin II was associated with an increased mean arterial pressure (102.85+/-1.38 versus 97.40+/-1.37; P=0.003) and a nonsignificant increase in angiotensin-converting enzyme activity, suggestive for a persistently activated renin-angiotensin system. In conclusion, these results suggest that in patients undergoing CABG, a diminished vascular responsiveness of the thoracic artery to exogenous angiotensin II is related to an increased risk of future cardiovascular events.

Serum cholesterol levels, blood pressure response to stress and incidence of stable hypertension in young subjects with high normal blood pressure

RATIONALE

Elevated serum cholesterol levels are common in patients with high blood pressure (BP) and could contribute to the progression of the hypertensive disease.

OBJECTIVE

To determine whether serum cholesterol levels affect the BP response to mental stress (MA) and the development of stable hypertension in young subjects with high normal BP.

METHODS

Seventy young (age < 45 years) high normal BP subjects with elevated (> 200 mg/dl, n = 49; HC) or normal (< or = 199 mg/dl, n = 21; NC) serum cholesterol levels, and 20 normotensive normocholesterolaemic (serum cholesterol < 199 mg/dl; C) subjects undergoing standardized mental challenge (mental arithmetic) were followed up for 15 years according to a prospective, longitudinal, cohort study design conducted in an ambulatory setting. The main outcome measure was the evaluation of the 15-year incidence of stable hypertension (diastolic BP > 95 mmHg).

RESULTS

After adjustment for age, resting BP, family history of high BP and body mass index at the study entry, high normal BP subjects with HC showed an enhanced BP reactivity to stress and a higher 15-year incidence of stable hypertension compared to high normal BP and NC subjects [relative risk (RR) = 2.1; 95% confidence interval (CI) = 1.7-5.5, P < 0.001] and controls (RR = 3.1; 95% CI = 1.4-5.3, P < 0.001). In a multivariate analysis of data the presence of high cholesterol levels was an independent predictor for the development of hypertension.

CONCLUSION

These data suggest that subjects with high normal BP and elevated serum cholesterol might have an exaggerated cardiovascular response to stress and have an increased risk for stable hypertension that can be detected at young age.

Transforming Growth Factor-β1 Inhibition of Vascular Smooth Muscle Cell Activation Is Mediated via Smad3

Activation of vascular smooth muscle cells (VSMCs) by proinflammatory cytokines is a key feature of atherosclerotic lesion formation. Transforming growth factor (TGF)-beta1 is a pleiotropic growth factor that can modulate the inflammatory response in diverse cell types including VSMCs. However, the mechanisms by which TGF-beta1 is able to mediate these effects remains incompletely understood. We demonstrate here that the ability of TGF-beta1 to inhibit markers of VSMC activation, inducible nitric-oxide synthase (iNOS) and interleukin (IL)-6, is mediated through its downstream effector Smad3. In reporter gene transfection studies, we found that among a panel of Smads, Smad3 could inhibit iNOS induction in an analogous manner as exogenous TGF-beta1. Adenoviral overexpression of Smad3 potently repressed inducible expression of endogenous iNOS and IL-6. Conversely, TGF-beta1 inhibition of cytokine-mediated induction of iNOS and IL-6 expression was completely blocked in Smad3-deficient VSMCs. Previous studies demonstrate that CCAAT/enhancer-binding protein (C/EBP) and NF-kappaB sites are critical for cytokine induction of both the iNOS and IL-6 promoters. We demonstrate that the inhibitory effect of Smad3 occurs via a novel antagonistic effect of Smad3 on C/EBP DNA-protein binding and activity. Smad3 mediates this effect in part by inhibiting C/EBP-beta and C/EBP-delta through distinct mechanisms. Furthermore, we find that Smad3 prevents the cooperative induction of the iNOS promoter by C/EBP and NF-kappaB. These data demonstrate that Smad3 plays an essential role in mediating TGF-beta1 anti-inflammatory response in VSMCs.

Cross Talk between Angiotensin II and Alpha 1 Adrenergic Receptors in Rabbit Aorta: Role of Endothelium

Interaction between the renin-angiotensin system and the sympathetic nervous system has been proposed to be like a physiological regulation mechanism. The present work was designed to study the cross talk between angiotensin II and adrenergic receptors on the smooth muscle contractile response and the endothelium influence in this phenomenon. Homologous and endothelium independent desensitization of angiotensin II-contractile response was observed. Treatment with noradrenaline between two cumulative doses response curves (CDRC) to angiotensin II caused a rightward shift of the second CDRC in unrubbed arteries and increased the maximal response in rubbed arteries. Prazosin blocked these effects. No homologous desensitization of noradrenaline contractile response was found. Treatment with angiotensin II between two CDRC to noradrenaline caused a loss of affinity in the second CDRC in unrubbed arteries. Losartan was able to avoid this phenomenon. Maximal response was enhanced both in arteries with and without endothelium treated or not with angiotensin II. Results demonstrate homologous and endothelium-independent desensitization of the contractile response to angiotensin II but not to noradrenaline. In addition, heterologous and endothelium-dependent desensitization induced by noradrenaline and angiotensin II on the contractile response to each other was found. Furthermore, results provided the first evidence that there is an endothelium-dependent cross talk between alpha1-adrenergic and angiotensin II receptors in smooth muscle of rabbit aorta.

Ganglionic Action of Angiotensin Contributes to Sympathetic Activity in Renin-Angiotensinogen Transgenic Mice

In addition to central nervous system actions, angiotensin (Ang) II may increase sympathetic nerve activity (SNA) via a direct action on sympathetic ganglia. We hypothesized that sympathetic ganglionic actions of endogenous Ang II contribute to SNA in transgenic mice that overexpress renin and angiotensinogen (R + A + mice). Renal SNA and arterial pressure were recorded in anesthetized R + A + and littermate control mice before and after ganglionic blockade, and after additional blockade of angiotensin type 1 (AT 1 ) receptors with losartan. Ganglionic blockade essentially abolished SNA in control mice, but only reduced SNA to 47±18% of baseline in R + A + mice. The residual SNA remaining after ganglionic blockade in R + A + mice was reduced from 47±18% to 8±6% of baseline by losartan ( P <0.05). The sympathoinhibitory response to losartan was accompanied by an enhanced decrease in arterial pressure in R + A + mice compared with that observed in control mice. AT 1 receptor expression in sympathetic ganglia, as measured by real-time reverse transcription–polymerase chain reaction, was increased ≈3-fold in R + A + versus control mice. The results demonstrate that, as anticipated, essentially all of the renal postganglionic SNA in control mice is driven by preganglionic input. The major new finding is that Ang II–evoked ganglionic activity accounts for ≈40% of total SNA in R + A + mice. The significant contribution of the direct ganglionic action of Ang II in R + A + mice likely reflects both increased levels of Ang II and upregulation of AT 1 receptors in sympathetic ganglia.

Decreased facilitation by angiotensin II of noradrenergic neurotransmission in isolated mesenteric artery of rabbits with chronic heart failure

Both in human and in experimental heart failure (HF), the renin-angiotensin system and the sympathetic nervous system are activated. In a previous study a facilitatory action of angiotensin II (Ang II) was shown in the rabbit mesenteric artery, which was mediated via prejunctionally located Ang II type 1 (AT ) receptors. Very little is known about the effects of Ang II on sympathetic neurotransmission at the peripheral level in congestive heart failure (CFH). Accordingly, in the isolated mesenteric arteries obtained from rabbits with experimentally induced CHF, as well as in age-matched control rabbits, the effect of Ang II on contractions provoked by electrical field stimulation was investigated in the presence and absence of the AT receptor antagonist eprosartan. Additionally, to investigate a possible postjunctional facilitation, the effects of Ang II on alpha-adrenoceptor-mediated responses were studied using noradrenaline (NA). Lastly, the vasoconstrictor effects of Ang II were compared between HF rabbits and controls, by constructing concentration-response curves to Ang II. In control rabbits, Ang II 0.5 n caused an enhancement of stimulation-induced responses by a factor 3.2 +/- 0.5, 2.4 +/- 0.3, and 1.5 +/- 0.08, at 1, 2, and 4 Hz, respectively ( < 0.05 at all frequencies compared with vehicle). In rabbits with HF, the enhancement by Ang II (0.5 n ) amounted to a factor 2.1 +/- 0.2, 1.7 +/- 0.1, and 1.2 +/- 0.04, at 1, 2, and 4 Hz, respectively ( < 0.05 compared with vehicle at all frequencies). Accordingly, the enhancing effect of Ang II was more pronounced in the control group compared with rabbits with HF ( < 0.05 at each frequency). Eprosartan (1 nM -0.1 microM) could inhibit the facilitatory effects of Ang II in arteries from HF as well as from control rabbits. Contractile responses to exogenous NA (3 n -0.1 m ) were the same in HF rabbits and controls, and they were unaltered in the presence of Ang II 0.5 n Ang II (0.1 nM -1 microM) caused a concentration-dependent increase in contractile force, which was the same in HF rabbits and controls. From these findings it can be concluded that in rabbits with CHF as well as in control animals, Ang II facilitates the stimulation-induced vasoconstrictor responses via prejunctionally located AT receptors. The facilitating effect was decreased in vessels obtained from rabbits with CHF, whereas responses to exogenous Ang II were unchanged. These findings may be explained by downregulation or uncoupling of the prejunctional AT receptor.

Agonistic autoantibodies directed against the angiotensin II AT1 receptor in patients with preeclampsia

We showed that sera from patients with preeclampsia contain autoantibodies directed against the angiotensin II AT1 receptor. The antibodies recognize an epitope on the second extracellular loop of the receptor and are immunoglobulins of the IgG3 subclass. The antibodies accelerate the beating rate of neonatal rat cardiomyocytes. The agonistic effect can be blocked with the AT1 receptor blocker losartan and can be neutralized by a peptide corresponding to the AT1 receptor's second extracellular loop. In further studies we shown that the autoantibodies recognize a specific conformation of the AT1 receptor. Cleavage of the external disulfide bond with dithiothreitol caused an inactivation of the receptor when stimulated either with Ang II or the autoantibodies in a system of cultured neonatal rat cardiomyocytes. Long-term stimulation of the AT1 receptor with either agonists down-regulated the AT1 receptor-mediated response to a second Ang II stimulation. These observations show that the agonistic autoantibodies behave pharmacologically in a similar fashion to Ang II. We have found the autoantibodies in all women meeting the clinical criteria of preeclampsia and suggest that they may be important to the pathogenesis of the disease.

Central role of the AT(1)-receptor in atherosclerosis

The renin-angiotensin system plays a major role in the pathogenesis of atherosclerosis. Most known effects of angiotensin II are mediated via activation of the AT(1)-receptor, which is in turn influenced to a great degree by levels of expression of the AT(1)-receptor. AT(1)-receptor activation is not only involved in vasoconstriction, water and salt homoeostasis and control of other neurohumoral systems, but also induces reactive oxygen species production, cellular hypertrophy and hyperplasia and apoptosis. Expression of this G-protein-coupled receptor is regulated by multiple factors. Among other conditions, oestrogen deficiency and hypercholesterolaemia increase AT(1)-receptor expression. Experimental data suggest that this augments the actions of angiotensin II, contributes to endothelial dysfunction, increases vascular production of reactive oxygen species, and via these mechanisms promotes atherosclerosis. Because of this, AT(1)-receptor regulation is likely to be critical in the development and progression of vascular lesions. Interventional studies demonstrated that ACE inhibitors which reduce AT(1)-receptor activation, improve endothelial dysfunction and inhibit onset and progression of atherosclerosis. The more specific AT(1)-receptor antagonists have also been shown to decrease blood pressure, protect renal function and to improve endothelial function. Thus, there is compelling evidence that AT(1)-receptor activation participates in the pathogenesis of atherosclerosis, and more importantly, that treatment regimens aiming at inhibition of AT(1)-receptor activation are promising anti-atherosclerotic therapeutic options.

Comparison of the effects of enalapril and losartan on posttransplantation erythrocytosis in renal transplant recipients: prospective randomized study

BACKGROUND

The aim of this prospective randomized study was to compare the safety and efficacy of enalapril (E) and losartan (L) in the treatment of posttransplantation erythrocytosis and the effect of the ACE genotype on response to therapy.

METHODS

Twenty-seven (24 male and 3 female, mean age 34+/-8 years) renal transplant recipients with erythrocytosis were treated either with E (15 patients) (10 mg/day) or L (12 patients) (50 mg/day) for 8 weeks.

RESULTS

The hemoglobin levels were significantly decreased in the L (17.1+/-0.7 to 15.9+/-1.3 g/dl, P=0.01) and E groups (17.4+/-1.1 to 14.9+/-2.2 g/dl, P=0.001). Among the responders who discontinued treatment, there was a trend for longer time to relapse in the L group (7.38+/-3.75 months; 95% confidence interval: 0.03-14.7) compared with the E group (2.75+/-0.70 (95% confidence interval: 1.37-4.13) (P=0.11). Decrease in hemoglobin was more prominent with E compared with L (-3.26+/-0.65 vs. -1.70+/-0.39 g/dl, P=0.05). Decrease in hemoglobin levels between DD and non-DD genotype groups was similar (-2.0+/-1.5 vs. -1.7+/-2.3 g/dl, P=0.69).

CONCLUSIONS

Enalapril caused a greater decrease but faster relapse in hemoglobin levels compared with losartan in patients with posttransplantation erythrocytosis. The DD type polymorphism had no effect on response.

Central AT1 and AT2 receptors mediate chronic intracerebroventricular angiotensin II-induced drinking in rats fed high sodium chloride diet from weaning

Intracerebroventricular (ICV) angiotensin (AIl) administration stimulates central AII receptors to induce water consumption in rats. The aim of this study was to determine the role of brain AT1 and AT2 receptors in mediating chronic ICV AII-induced drinking in rats raised on normal or high sodium chloride diets from weaning. Rats were weaned at 21 days of age and placed on normal or high sodium chloride diet for 10-12 weeks. At adulthood, the animals were instrumented with brain lateral ventricular cannulas and femoral arterial catheters. Low dose chronic central AII infusion (20 ng min(-1)) significantly (P < 0.05) increased water intake in both groups of rats when compared with their respective controls of 24 h artificial cerebrospinal fluid infusions. In a separate group of high sodium fed rats, coinfusion of AII with the AT1 receptor antagonist, losartan (0.25 microg min(-1)) or the AT2 receptor blocker, PD 123319 (0.50 microg min(-1)) blocked chronic ICV AII-induced drinking. Upon reinfusion of AII water intake increased above control. Following the cessation of AII infusions, water intake returned to values not significantly different from control (P > 0.05). In contrast, in the normal sodium fed rats losartan, but not PD 123319, blocked the AII-mediated water intake. The data demonstrate that in high sodium chloride fed rats AII stimulates both central AT1 and AT2 receptors to induce drinking, while in the normal sodium chloride fed rats the peptide activates the drinking response primarily by stimulation of central AT1 receptors.

Selective down-regulation of AT2 receptors in uterine arteries from pregnant ewes given 24-h intravenous infusions of angiotensin II

Previously, we showed that uterine arteries from late gestation pregnant ewes infused intravenously with angiotensin II (Ang II) for 24 h, displayed heightened responsiveness to Ang II in vitro. Furthermore, we found that a small population of ewes with a "preeclampsia-like" disorder also displayed this. Therefore, we have investigated the density and affinity of Ang II receptor subtypes in the uterine arteries from these groups. Ang II receptor binding was measured using 125I [Sar1Ile8] Ang II. Proportions of AT1 and AT2 receptors were determined by inhibiting 125I [Sar1Ile8] Ang II with losartan (AT1 antagonist) or PD 123319 (AT2 antagonist). Uterine arteries from 24-h Ang II-infused ewes had a lower proportion of AT2 receptors (56.2+/-2.3%) than control (saline-infused) ewes (84.1+/-1.0%; P<0.05). The density of AT2 receptors was reduced (P<0.05) while the density of AT1 receptors was not different. Thus, 24-h infusions of Ang II selectively down-regulated AT2 receptors in the uterine artery, resulting in heightened Ang II reactivity. By contrast, the binding properties of Ang II receptor subtypes in uterine arteries from ewes with the "preeclampsia-like" disorder were not different from control ewes.

Reactive oxygen species-mediated homologous downregulation of angiotensin II type 1 receptor mRNA by angiotensin II

Recent studies suggest a crucial role of reactive oxygen species (ROS) for the signaling of angiotensin (Ang) II through Ang II type 1 receptor (AT(1)-R). However, the role of ROS in the regulation of AT(1)-R expression has not been explored. In this study, we examined the effect of an antioxidant on the homologous downregulation of AT(1)-R by Ang II. Ang II (10(-6) mol/L) decreased AT(1)-R mRNA with a peak suppression at 6 hours of stimulation in rat aortic vascular smooth muscle cells. Preincubation of vascular smooth muscle cells with N:-acetylcysteine (NAC), a potent antioxidant, almost completely inhibited the Ang II-induced downregulation of AT(1)-R mRNA. The effect of NAC was due to stabilization of the AT(1)-R mRNA that was destabilized by Ang II. The Ang II-induced AT(1)-R mRNA downregulation was also blocked by PD98059, an extracellular signal-regulated protein kinase (ERK) kinase inhibitor. Ang II-induced ERK activation was inhibited by NAC as well as by PD98059. Exogenous H(2)O(2) also suppressed AT(1)-R mRNA. These results suggest that the production of ROS and the activation of ERK are critical for the downregulation of AT(1)-R mRNA. The generation of ROS through stimulation of AT(1)-R not only mediates signaling of Ang II but also may play a crucial role in the adaptation process of AT(1)-R to the sustained stimulation of Ang II.

Glial angiotensinogen regulates brain angiotensin II receptors in transgenic rats TGR(ASrAOGEN)

TGR(ASrAOGEN)680, a newly developed transgenic rat line with specific downregulation of astroglial synthesis of angiotensinogen, exhibits decreased brain angiotensinogen content associated with a mild diabetes insipidus and lower blood pressure. Autoradiographic experiments were performed on TGR(ASrAOGEN) (TG) and Sprague-Dawley (SD) control rats to quantify AT(1) and AT(2) receptor-binding sites in different brain nuclei and circumventricular organs. Dose-response curves for drinking response to intracerebroventricular injections of ANG II were compared between SD and TG rats. In most of the regions inside the blood-brain barrier [paraventricular nucleus (PVN), piriform cortex, lateral olfactory tract (LOT), and lateral preoptic area (LPO)], AT(1) receptor binding (sensitive to CV-11974) was significantly higher in TG compared with SD. In contrast, in the circumventricular organs investigated [subfornical organ (SFO) and area postrema], AT(1) receptor binding was significantly lower in TG. AT(2) receptors (binding sensitive to PD-123319) were detected at similar levels in the inferior olive (IO) of both strains. Angiotensin-binding sites sensitive to both CV-11974 and PD-123319 were detected in the LPO of SD rats and specifically upregulated in LOT, IO, and most notably PVN and SFO of TG. The dose-response curve for water intake after intracerebroventricular injections showed a higher sensitivity to ANG II of TG (EC(50) = 3.1 ng) compared with SD (EC(50) = 11.2 ng), strongly suggesting that the upregulation of AT(1) receptors inside the blood-brain barrier of TG rats is functional. Finally, we showed that downregulation of angiotensinogen synthesized by astroglial cells differentially regulates angiotensin receptor subtypes inside the brain and in circumventricular organs.

Angiotensin II type 1 receptor A1166C gene polymorphism is associated with an increased response to angiotensin II in human arteries

An adenine/cytosine (A/C) base substitution at position 1166 in the angiotensin II type 1 receptor (AT(1)R) gene is associated with the incidence of essential hypertension and increased coronary artery vasoconstriction. However, it is still unknown whether this polymorphism is associated with a difference in angiotensin II responsiveness. Therefore, we assessed whether the AT(1)R polymorphism is associated with different responses to angiotensin II in isolated human arteries. Furthermore, we evaluated whether inhibition of the renin-angiotensin system modifies the effect of the AT(1)R polymorphism. One hundred twelve patients who were undergoing coronary artery bypass graft surgery were prospectively randomized to receive an ACE inhibitor or a placebo for 1 week before surgery. Excess segments of the internal mammary artery were exposed to angiotensin II (0.1 nmol/L to 1 micromol/L) and KCl (60 mmol/L) in organ bath experiments. Patients homozygous for the C allele (n=17) had significantly greater angiotensin II responses (percentage of this maximal KCl-induced response) than did patients genotyped with AA+AC (n=95, P<0.05). Although ACE inhibition increased the response to angiotensin II, the difference in the response to angiotensin II, between CC and AA+AC patients remained intact in ACE inhibitor-treated patients. These results indicate increased responses to angiotensin II in patients with the CC genotype. The mechanism is preserved during ACE inhibition, which in itself also increased the response to angiotensin II. This reveals that the A1166C polymorphism may be in linkage disequilibrium with a functional mutation that alters angiotensin II responsiveness, which may explain the described relation between this polymorphism and cardiovascular abnormalities.

AT(1) receptor regulation in salt-sensitive hypertension

The molecular events governing salt-sensitive hypertension are currently unknown. Because the renin-ANG system plays a central role in blood pressure regulation and electrolyte balance, it may be closely involved in the phenomenon of salt sensitivity. Therefore, we examined the effect of a high-salt diet (8%) and a low-salt diet (0.4%) on ANG II-caused vascular constriction and ANG II type 1 (AT(1)) receptor expression in aorta, brain, and kidney of Dahl S (salt-sensitive) and Dahl R (salt-resistant) rats by means of radioligand binding assays and quantitative PCR. NaCl diet at 8% led to a significant increase of blood pressure in Dahl S but not in Dahl R rats. High-sodium intake caused a profound decrease of ANG II-induced aortic vasoconstriction in both Dahl R and Dahl S rats. The underlying mechanism was a downregulation of aortic AT(1) receptor density and AT(1) receptor mRNA. AT(1) receptor mRNA was downregulated to 57.8% in Dahl R and 59.0% in Dahl S rats by an 8% NaCl diet compared with a 0.4% NaCl diet (P < 0.05). There was a similar decrease in aortic AT(1) receptor density. Additionally, AT(1) receptor mRNA was also downregulated in the kidney but upregulated the brain of Dahl R and S rats on a high-salt diet. Thus high NaCl intake causes organ-specific AT(1) receptor regulation in Dahl R and in Dahl S rats despite the differential blood pressure regulation in these animal models in response to a high-salt diet. These findings suggest that the regulation of vascular AT(1) receptors is influenced by numerous factors such as the renin-ANG system and obviously by various other events that are currently only partly understood.

Modulation and function of extrarenal angiotensin receptors

Historically, physiological modulation of the activity of the renin-angiotensin system (RAS) was thought to be mediated only by changes in renin secretion. Hence, altered dietary sodium (Na) intake, changes in renal perfusion pressure, and/or renal adrenoreceptor activity would lead to changes in renin release and plasma angiotensin II (Ang II) concentration, which in turn contribute to regulation of blood pressure and sodium balance. Later, it became apparent that angiotensinogen availability and Ang-converting enzyme activity are also rate-limiting factors that influence the activity of RAS. Finally, over the past few years, evidence has accumulated that indicates the number of Ang II receptors and their subtypes are of great importance in regulating the activity and function of RAS. Cloning of the Ang II receptor genes, development of specific receptor-antagonist ligands, and establishment of genetically mutated animal models have led to greater understanding of the role of Ang II receptors in the regulation of RAS function and activity. This review focuses on the functions and regulation of Ang II receptors in vascular tissues and in the adrenal gland. The authors suggest that identification of control elements for Ang II receptor expression, which are tissue-specific, may provide a basis for future therapeutic manipulation of Ang II receptors in cardiovascular disease states.

Angiotensin AT1 receptor subtype as a cardiac target of aldosterone: role in aldosterone-salt-induced fibrosis

This study tests the hypothesis that aldosterone induces cardiac fibrosis through an increase of cardiac angiotensin II (Ang II) AT1 receptor levels, thereby potentiating the fibrotic effect of Ang II by determining the effects of spironolactone and losartan on cardiac fibrosis, AT1 density, and gene expression in aldosterone-salt-treated rats. Fibrosis was quantified by slot blots of collagen I and III mRNA levels and videomorphometry of Sirius red-stained collagen. AT1 receptor density was determined by (125I-Sar1-Ile8)-Ang II competition binding, and AT1 mRNA levels were analyzed by quantitative reverse transcriptase polymerase chain reaction. One month of aldosterone-salt treatment induced a decrease in plasma Ang II and an increase in blood pressure, left ventricular hypertrophy, and ventricular fibrosis. Spironolactone (20 mg/kg per day) and losartan spironolactone (10 mg/kg per day) had no effect on the first 3 parameters. Losartan was as effective as spironolactone in preventing ventricular collagen mRNA increase and fibrosis. Ventricular density of AT1 receptors increased 2-fold and was accompanied by a 3-fold increase in the corresponding mRNA in aldosterone-salt compared with sham-operated rats. Both spironolactone and losartan prevented the elevation of ventricular AT1 density and that of right ventricular AT1 mRNA levels. These results demonstrate that the mechanism by which aldosterone-salt induces cardiac fibrosis involves Ang II acting through AT1 receptors. They also suggest that the cardiac AT1 receptor is a target for aldosterone.