Angiotensin II receptor subtypes AT1 and AT2 are down-regulated by angiotensin II through AT1 receptor by different mechanisms

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.

Increased Reactive Oxygen Species Generation Contributes to the Atherogenic Activity of the B2 Bradykinin Receptor

Atherosclerosis and ensuing cardiovascular disease are major causes of death with insufficient treatment options. In search for pathomechanisms of atherosclerosis, we investigated the impact of the B2 bradykinin receptor, Bdkrb2, on atherosclerotic lesion formation, because to date it is not clear whether the B2 bradykinin receptor is atheroprotective or atherogenic. As a model of atherosclerosis, we used hypercholesterolemic ApoE-deficient (apolipoprotein E-deficient) mice, which develop atherosclerotic lesions in the aorta with increasing age. The role of Bdkrb2 in atherosclerosis was studied in ApoE-deficient mice, which were either Bdkrb2-deficient, or had moderately increased aortic B2 bradykinin receptor protein levels induced by transgenic BDKRB2 expression under control of the ubiquitous CMV promoter. We found that Bdkrb2 deficiency led to a significantly decreased atherosclerotic plaque area whereas transgenic BDKRB2 expression enhanced atherosclerotic lesion formation in the aorta of ApoE-deficient mice at an age of 8 months. Concomitantly, the aortic content of reactive oxygen species (ROS) was higher in BDKRB2-expressing mice whereas Bdkrb2 deficiency decreased aortic ROS levels of ApoE-deficient mice. In addition, aortic nitrate as a marker of nitric oxide activity and the endothelial nitric oxide synthase (eNOS) co-factor, tetrahydrobiopterin (BH4) were reduced in BDKRB2-expressing ApoE-deficient mice. The decreased aortic BH4 content could be a consequence of increased ROS generation and down-regulated aortic expression of the BH4-synthesizing enzyme, Gch1 (GTP cyclohydrolase 1). In agreement with a causal involvement of decreased BH4 levels in the atherogenic function of BDKRB2, we found that treatment with the BH4 analog, sapropterin, significantly retarded atherosclerotic plaque formation in BDKRB2-expressing ApoE-deficient mice. Together our data show that the B2 bradykinin receptor is atherogenic, and the atherosclerosis-promoting function of BDKRB2 is partially caused by decreased aortic BH4 levels, which could account for eNOS uncoupling and further enhancement of ROS generation.

A Genomic Survey of Angiotensin-Converting Enzymes Provides Novel Insights into Their Molecular Evolution in Vertebrates

Angiotensin-converting enzymes, ACE and ACE2, are two main elements in the renin⁻angiotensin system, with a crucial role in the regulation of blood pressure in vertebrates. Previous studies paid much attention to their physiological functions in model organisms, whereas the studies on other animals and related evolution have been sparse. Our present study performed a comprehensive genomic investigation on ace and ace2 genes in vertebrates. We successfully extracted the nucleotide sequences of ace and ace2 genes from high-quality genome assemblies of 36 representative vertebrates. After construction of their evolutionary tree, we observed that most of the phylogenetic positions are consistent with the species tree; however, certain differences appear in coelacanths and frogs, which may suggest a very slow evolutionary rate in the initial evolution of ace and ace2 in vertebrates. We further compared evolutionary rates within the entire sequences of ace and ace2, and determined that ace2 evolved slightly faster than ace. Meanwhile, we counted that the exon numbers of ace and ace2 in vertebrates are usually 25 and 18 respectively, while certain species may occur exon fusion or disruption to decrease or increase their exon numbers. Interestingly, we found three homologous regions between ace and ace2, suggesting existence of gene duplication during their evolutionary process. In summary, this report provides novel insights into vertebrate ace and ace2 genes through a series of genomic and molecular comparisons.

2-Methoxyestradiol causes matrix metalloproteinase 9-mediated transactivation of epidermal growth factor receptor and angiotensin type 1 receptor downregulation in rat aortic smooth muscle cells

Studies have demonstrated the therapeutic potential of estrogen metabolite 2-methoxyestradiol (2ME2) in several cardiovascular disorders, including hypertension. However, the exact mechanism(s) remains unknown. In this study, primary rat aortic smooth muscle cells (RASMCs) were exposed to 2ME2, and angiotensin type 1 receptor (AT1R) expression, function, and associated signaling pathways were evaluated. In RASMCs, 2ME2 downregulated AT1R expression in a concentration- and time-dependent manner, which was correlated with reduced mRNA expression. The 2ME2 effect was through G protein-coupled receptor 30 (GPR30) that inhibits second messenger cAMP. Moreover, 2ME2 exposure phosphorylated ERK1/2 that was sensitive to MEK inhibitor PD98059. Selective epidermal growth factor receptor (EGFR) inhibitor AG1478 blocked 2ME2-induced EGFR transactivation and attenuated subsequent phosphorylation of ERK1/2 preventing AT1R downregulation. The transactivation was dependent on 2ME2-induced release of matrix metalloproteinase 9 (MMP9) and epidermal growth factor demonstrated by ELISA. Furthermore, transfection with small interfering (si) RNA targeting MMP9 impeded ERK1/2 activation and AT1R downregulation in response to 2ME2 and G1 stimulation. Interestingly, under similar conditions, stimulation of GPR30 with the selective agonist G1 elicited similar signaling pathways and downregulated the AT1R expression that was reversed by GPR30 antagonist G15. Furthermore, 2ME2 and G1 inhibited angiotensin II (ANG II) induced Ca2+ release, a response consistent with AT1R downregulation. Collectively, our study demonstrates for the first time that 2ME2 binding to GPR30 induces MMP9 specific transactivation of EGFR that mediates ERK1/2-dependent downregulation of AT1R in RASMCs. The study provides critical insights into the newly discovered role and signaling pathways of 2ME2 in the regulation of AT1R in vascular cells and its potential to be developed as a therapeutic agent that ameliorates hypertension.

Angiotensin II AT2 Receptors Contribute to Regulate the Sympathoadrenal and Hormonal Reaction to Stress Stimuli

Angiotensin II, through AT1 receptor stimulation, mediates multiple cardiovascular, metabolic, and behavioral functions including the response to stressors. Conversely, the function of Angiotensin II AT2 receptors has not been totally clarified. In adult rodents, AT2 receptor distribution is very limited but it is particularly high in the adrenal medulla. Recent results strongly indicate that AT2 receptors contribute to the regulation of the response to stress stimuli. This occurs in association with AT1 receptors, both receptor types reciprocally influencing their expression and therefore their function. AT2 receptors appear to influence the response to many types of stressors and in all components of the hypothalamic-pituitary-adrenal axis. The molecular mechanisms involved in AT2 receptor activation, the complex interactions with AT1 receptors, and additional factors participating in the control of AT2 receptor regulation and activity in response to stressors are only partially understood. Further research is necessary to close this knowledge gap and to clarify whether AT2 receptor activation may carry the potential of a major translational advance.

Fructose-rich diet induces gender-specific changes in expression of the renin-angiotensin system in rat heart and upregulates the ACE/AT1R axis in the male rat aorta

Introduction:

The cardiovascular renin–angiotensin system (RAS) could be affected by gender and dietary regime. We hypothesized that male rats will be more susceptible to activation of RAS in the heart and aorta, as a response to a fructose-rich diet (FRD).

Materials and methods:

Both male and female Wistar rats were given a 10% (w/v) fructose solution for 9 weeks. We measured the biochemical parameters, blood pressure (BP) and heart rate. We used Western blot and real-time polymerase chain reaction (PCR) to quantify protein and gene expression.

Results:

In the male rats, the FRD elevated BP and expression of cardiac angiotensin-converting enzyme (ACE), while the expression of angiotensin-converting enzyme 2 (ACE2) and angiotensin II Type 2 receptor (AT₂R) were significantly decreased. In female rats, there were no changes in cardiac RAS expression due to FRD. Furthermore, the ACE/AT₁R axis was overexpressed in the FRD male rats’ aortae, while only AT₁R was upregulated in the FRD female rats’ aortae. ACE2 expression remained unchanged in the aortae of both genders receiving the FRD.

Conclusions:

The FRD induced gender-specific changes in the expression of the RAS in the heart and aortae of male rats. Further investigations are required in order to get a comprehensive understanding of the underlying mechanisms of gender-specific fructose-induced cardiovascular pathologies.

Multiscale model of dynamic neuromodulation integrating neuropeptide-induced signaling pathway activity with membrane electrophysiology

We developed a multiscale model to bridge neuropeptide receptor-activated signaling pathway activity with membrane electrophysiology. Typically, the neuromodulation of biochemical signaling and biophysics have been investigated separately in modeling studies. We studied the effects of Angiotensin II (AngII) on neuronal excitability changes mediated by signaling dynamics and downstream phosphorylation of ion channels. Experiments have shown that AngII binding to the AngII receptor type-1 elicits baseline-dependent regulation of cytosolic Ca(2+) signaling. Our model simulations revealed a baseline Ca(2+)-dependent response to AngII receptor type-1 activation by AngII. Consistent with experimental observations, AngII evoked a rise in Ca(2+) when starting at a low baseline Ca(2+) level, and a decrease in Ca(2+) when starting at a higher baseline. Our analysis predicted that the kinetics of Ca(2+) transport into the endoplasmic reticulum play a critical role in shaping the Ca(2+) response. The Ca(2+) baseline also influenced the AngII-induced excitability changes such that lower Ca(2+) levels were associated with a larger firing rate increase. We examined the relative contributions of signaling kinases protein kinase C and Ca(2+)/Calmodulin-dependent protein kinase II to AngII-mediated excitability changes by simulating activity blockade individually and in combination. We found that protein kinase C selectively controlled firing rate adaptation whereas Ca(2+)/Calmodulin-dependent protein kinase II induced a delayed effect on the firing rate increase. We tested whether signaling kinetics were necessary for the dynamic effects of AngII on excitability by simulating three scenarios of AngII-mediated KDR channel phosphorylation: (1), an increased steady state; (2), a step-change increase; and (3), dynamic modulation. Our results revealed that the kinetics emerging from neuromodulatory activation of the signaling network were required to account for the dynamical changes in excitability. In summary, our integrated multiscale model provides, to our knowledge, a new approach for quantitative investigation of neuromodulatory effects on signaling and electrophysiology.

Angiotensin receptor type 1 and endothelin receptor type A on immune cells mediate migration and the expression of IL-8 and CCL18 when stimulated by autoantibodies from systemic sclerosis patients

INTRODUCTION

Agonistic autoantibodies (Aabs) against the angiotensin II receptor type 1 (AT1R) and the endothelin receptor type A (ETAR) have been identified in patients with systemic sclerosis (SSc). In our present study, we examined the expression of the AT1R and the ETAR in human immune cells and the pathological effects mediated through these receptors by their corresponding Aabs.

METHODS

Protein expression of AT1R and ETAR on peripheral blood mononuclear cells (PBMCs) from healthy individuals and SSc patients was analyzed using flow cytometry, and mRNA expression of both receptors in PBMCs from healthy donors was examined by real-time PCR. In addition, PBMCs from healthy donors were stimulated in vitro with affinity-purified immunoglobulin G (IgG) fractions from SSc patients positive for AT1R and ETAR Aabs, as well as with IgG from healthy donors serving as controls. Alterations in cell surface marker expression, cytokine secretion and chemotactic motility were analyzed using flow cytometry, enzyme-linked immunosorbent assays and chemotaxis assays, respectively. The results were correlated with the characteristics and clinical findings of the IgG donors.

RESULTS

Both AT1R and ETAR were expressed on PBMCs in humans. Protein expression of both receptors was decreased in SSc patients compared with that of healthy donors and declined during the course of disease. IgG fractions of SSc patients positive for AT1R and ETAR Aabs induced T-cell migration in an Aab level-dependent manner. Moreover, IgG of SSc patients stimulated PBMCs to produce more interleukin 8 (IL-8) and chemokine (C-C motif) ligand 18 (CCL18) than did the IgG of healthy donors. All effects were significantly reduced by selective AT1R and ETAR antagonists. Statistical analysis revealed an association of SSc-IgG induced high IL-8 concentrations with an early disease stage and of high CCL18 concentrations with lung fibrosis onset and vascular complications in the respective IgG donors.

CONCLUSION

In our present study, we could demonstrate the expression of both AT1R and ETAR on human peripheral T cells, B cells and monocytes. The decreased receptor expression in SSc patients, the inflammatory and profibrotic effects upon Aab stimulation of PBMCs in vitro and the associations with clinical findings suggest a role for Aab-induced activation of immune cells mediated by the AT1R and the ETAR in the pathogenesis or even the onset of the disease.

Fetal betamethasone exposure attenuates angiotensin-(1-7)-Mas receptor expression in the dorsal medulla of adult sheep

Glucocorticoids including betamethasone (BM) are routinely administered to women entering into early preterm labor to facilitate fetal lung development and decrease infant mortality; however, fetal steroid exposure may lead to deleterious long term consequences. In a sheep model of fetal programming, BM-exposed (BMX) offspring exhibit elevated mean arterial pressure (MAP) and decreased baroreflex sensitivity (BRS) for control of heart rate by 0.5-years of age associated with changes in the circulating and renal renin-angiotensin systems (RAS). In the brain solitary tract nucleus, angiotensin (Ang) II actions through the AT1 receptor oppose the beneficial actions of Ang-(1-7) at the Mas receptor for BRS regulation. Therefore, we examined Ang peptides, angiotensinogen (Aogen), and receptor expression in this brain region of exposed and control offspring of 0.5- and 1.8-years of age. Mas protein expression was significantly lower (>40%) in the dorsal medulla of BMX animals at both ages; however, AT1 receptor expression was not changed. BMX offspring exhibited a higher ratio of Ang II to Ang-(1-7) (2.30±0.36 versus 0.99±0.28; p<0.01) and Ang II to Ang I at 0.5-years. Although total Aogen was unchanged, Ang I-intact Aogen was lower in 0.5-year BMX animals (0.78±0.06 vs. 1.94±0.41; p<0.05) suggesting a greater degree of enzymatic processing of the precursor protein in exposed animals. We conclude that in utero BM exposure promotes an imbalance in the central RAS pathways of Ang II and Ang-(1-7) that may contribute to the elevated MAP and lower BRS in this model.

Chronic angiotensin receptor blockade suppresses intracardiac angiotensin II in angiotensin II-infused rats

Accumulation of angiotensin II (Ang II) in tissues is an Ang II-receptor-mediated process. In pigs, acute angiotensin receptor blockade (ARB) reduced the heart-to-plasma ratio of Ang II following acute infusion. However in rats, chronic ARB treatment increased heart Ang II levels, suggesting that a differential response to ARB treatment may exist in the mammalian heart. Furthermore, the changes in heart aldosterone following chronic ARB treatment are not well described. To address the discrepancy in heart Ang II concentrations following ARB treatment, three groups (n = 6) of rats were chronically studied: (1) control; (2) angiotensin II (Ang II; 80 ng/min for 28 d); and (3) angiotensin II + olmesartan (ARB; 10 mg/kg/d for 21 d). Ang II-infusion increased intracardiac Ang II by 40% (53 ± 2 versus 74 ± 6 fmol/g) and intrarenal Ang II over 2-fold (96 ± 6 versus 207 ± 14 fmol/g), and chronic ARB treatment decreased Ang II by 48% in the heart (50 ± 7 fmol/g) and over two-fold in the kidney (92 ± 7 fmol/g), suggesting that accumulation of Ang II in the heart is receptor-mediated as in the kidney. Ang II increased plasma aldosterone 2.5-fold (1.4 ± 0.1 versus 3.5 ± 1.2 nmol/L) and was exacerbated by ARB treatment (5.6 ± 1.0 nmol/L). Intracardiac aldosterone was exacerbated by ARB treatment (control: 2.2 ± 0.3; Ang II: 2.7 ± 1.1; ARB: 7.8 ± 1.7 pmol/g). Suppression of intracardiac Ang II with ARB is consistent with the existing view of Ang II-receptor-mediated uptake by tissues.

Renal proximal tubule angiotensin AT1A receptors regulate blood pressure

All components of the renin angiotensin system necessary for ANG II generation and action have been reported to be present in renal proximal convoluted tubules. Given the close relationship between renal sodium handling and blood pressure regulation, we hypothesized that modulating the action of ANG II specifically in the renal proximal tubules would alter the chronic level of blood pressure. To test this, we used a proximal tubule-specific, androgen-dependent, promoter construct (KAP2) to generate mice with either overexpression of a constitutively active angiotensin type 1A receptor transgene or depletion of endogenous angiotensin type 1A receptors. Androgen administration to female transgenic mice caused a robust induction of the transgene in the kidney and increased baseline blood pressure. In the receptor-depleted mice, androgen administration to females resulted in a Cre recombinase-mediated deletion of angiotensin type 1A receptors in the proximal tubule and reduced blood pressure. In contrast to the changes observed at baseline, there was no difference in the blood pressure response to a pressor dose of ANG II in either experimental model. These data, from two separate mouse models, provide evidence that ANG II signaling via the type 1A receptor in the renal proximal tubule is a regulator of systemic blood pressure under baseline conditions.

Perturbation analysis analyzed--athematical modeling of intact and perturbed gene regulatory circuits for animal development

Gene regulatory networks for animal development are the underlying mechanisms controlling cell fate specification and differentiation. The architecture of gene regulatory circuits determines their information processing properties and their developmental function. It is a major task to derive realistic network models from exceedingly advanced high throughput experimental data. Here we use mathematical modeling to study the dynamics of gene regulatory circuits to advance the ability to infer regulatory connections and logic function from experimental data. This study is guided by experimental methodologies that are commonly used to study gene regulatory networks that control cell fate specification. We study the effect of a perturbation of an input on the level of its downstream genes and compare between the cis-regulatory execution of OR and AND logics. Circuits that initiate gene activation and circuits that lock on the expression of genes are analyzed. The model improves our ability to analyze experimental data and construct from it the network topology. The model also illuminates information processing properties of gene regulatory circuits for animal development.

Cardiac phenotype and angiotensin II levels in AT1a, AT1b, and AT2 receptor single, double, and triple knockouts

AIMS

Our aim was to determine the contribution of the three angiotensin (Ang) II receptor subtypes (AT(1a), AT(1b), AT(2)) to coronary responsiveness, cardiac histopathology, and tissue Ang II levels using mice deficient for one, two, or all three Ang II receptors.

METHODS AND RESULTS

Hearts of knockout mice and their wild-type controls were collected for histochemistry or perfused according to Langendorff, and kidneys were removed to measure tissue Ang II. Ang II dose-dependently decreased coronary flow (CF) and left ventricular systolic pressure (LVSP), and these effects were absent in all genotypes deficient for AT(1a), independently of AT(1b) and AT(2). The deletion of Ang II receptors had an effect neither on the morphology of medium-sized vessels in the heart nor on the development of fibrosis. However, the lack of both AT(1) subtypes was associated with atrophic changes in the myocardium, a reduced CF and a reduced LVSP. AT(1a) deletion alone, independently of the presence or absence of AT(1b) and/or AT(2), reduced renal Ang II by 50% despite a five-fold rise of plasma Ang II. AT(1b) deletion, on top of AT(1a) deletion (but not alone), further decreased tissue Ang II, while increasing plasma Ang II. In mice deficient for all three Ang II receptors, renal Ang II was located only extracellularly.

CONCLUSION

The lack of both AT(1) subtypes led to a baseline reduction of CF and LVSP, and the effects of Ang II on CF and LVSP were found to be exclusively mediated via AT(1a). The lack of AT(1a) or AT(1b) does not influence the development or maintenance of normal cardiac morphology, whereas deficiency for both receptors led to atrophic changes in the heart. Renal Ang II levels largely depend on AT(1) binding of extracellularly generated Ang II, and in the absence of all three Ang II receptors, renal Ang II is only located extracellularly.

Transforming growth factor beta1 mediates apoptotic activity of angiotensin II type I receptor blocker on prostate epithelium in vitro

BACKGROUND

The significant association of benign prostatic hyperplasia (BPH) and hypertension indicates a common pathophysiological factor for both diseases. Hyperactivity of the renin-angiotensin system (RAS) has been reported in BPH. Angiotensin II type I (AT1) receptor is the principal mediator of the RAS, and the antagonist, AT1 receptor blocker (ARB), can induce apoptosis in prostate epithelium cells and increase transforming growth factor beta1 (TGF-beta1) expression. We aimed to investigate the mechanism of inhibition of AT1 receptor in prostate epithelium cells and the role of TGF-beta1.

METHODS

Human prostate epithelium cell lines were treated with different concentrations of ARB (losartan) (0, 0.1, 1, 10, 100, and 1,000 microM) for 24-72 hr. Cell proliferation was analyzed by cell proliferation assay. The location of AT1 receptor was shown by immunocytohistochemistry and immunocytofluorescence study. Analysis of apoptosis was by use of terminal transferase TdT-mediated dUTP-biotin end labeling (TUNEL) and caspase 3/7 activity assay. Mitochondrial outer-membrane permeabilization was measured by JC-1 staining. The level of TGF-beta1 was determined by enzyme-linked immunosorbent assay.

RESULTS

Immunohistochemistry and immunofluorescence analysis showed AT1 receptor expressed in epithelium cells. Compared to control cultures, cultures treated with losartan for 24-72 hr showed a dose-dependent significant decrease in cell number, with apoptosis increased by 65.2%. Decreased cell number was reversed on treatment with anti-TGF-beta1 antibody. TUNEL staining showed increased apoptosis in prostate epithelium cells exposed to losartan. Caspase 3/7 activation was increased and mitochondrial membrane potential was downregulated. Expression of TGF-beta1 in cells treated with losartan was higher than that in untreated cells.

CONCLUSIONS

The apoptotic effect of blockade of AT1 receptor on human prostatic epithelium cells may be mediated through an autocrine the production of TGF-beta1. Furthermore, this finding may have implications for medication options. Inc.

Inhibition of angiotensin-converting enzyme stimulates fracture healing and periosteal callus formation - role of a local renin-angiotensin system

BACKGROUND AND PURPOSE

The renin-angiotensin system (RAS) regulates blood pressure and electrolyte homeostasis. In addition, 'local' tissue-specific RAS have been identified, regulating regeneration, cell growth, apoptosis, inflammation and angiogenesis. Although components of the RAS are expressed in osteoblasts and osteoclasts, a local RAS in bone has not yet been described and there is no information on whether the RAS is involved in fracture healing. Therefore, we studied the expression and function of the key RAS component, angiotensin-converting enzyme (ACE), during fracture healing.

EXPERIMENTAL APPROACH

In a murine femur fracture model, animals were treated with the ACE inhibitor perindopril or vehicle only. Fracture healing was analysed after 2, 5 and 10 weeks using X-ray, micro-CT, histomorphometry, immunohistochemistry, Western blotting and biomechanical testing.

KEY RESULTS

ACE was expressed in osteoblasts and hypertrophic chondrocytes in the periosteal callus during fracture healing, accompanied by expression of the angiotensin type-1 and type-2 receptors. Perindopril treatment reduced blood pressure and bone mineral density in unfractured femora. However, it improved periosteal callus formation, bone bridging of the fracture gap and torsional stiffness. ACE inhibition did not affect cell proliferation, but reduced apoptotic cell death. After 10 week treatment, a smaller callus diameter and bone volume after perindopril treatment indicated an advanced stage of bone remodelling.

CONCLUSIONS

Our study provides evidence for a local RAS in bone that influenced the process of fracture healing. We show for the first time that inhibition of ACE is capable of accelerating bone healing and remodelling.

Stretch reduces nephrin expression via an angiotensin II-AT(1)-dependent mechanism in human podocytes: effect of rosiglitazone

Increased glomerular permeability to proteins is a characteristic feature of diabetic nephropathy (DN). The slit diaphragm is the major restriction site to protein filtration, and the loss of nephrin, a key component of the slit diaphragm, has been demonstrated in both human and experimental DN. Both systemic and glomerular hypertension are believed to be important in the pathogenesis of DN. Human immortalized podocytes were subjected to repeated stretch-relaxation cycles by mechanical deformation with the use of a stress unit (10% elongation, 60 cycles/min) in the presence or absence of candesartan (1 microM), PD-123319 (1 microM), and rosiglitazone (0.1 microM). Nephrin mRNA and protein expression were assessed using quantitative real-time PCR, immunoblotting, and immunofluorescence, and the protein expression of AT(1) receptor and angiotensin II secretion were evaluated. Exposure to stretch induced a significant approximately 50% decrease in both nephrin mRNA and protein expression. This effect was mediated by an angiotensin II-AT(1) mechanism. Indeed, podocyte stretching induced both angiotensin II secretion and AT(1) receptor overexpression, podocyte exposure to angiotensin II reduced nephrin protein expression, and both the AT-1 receptor antagonist candesartan and a specific anti-angiotensin II antibody completely abolished stretch-induced nephrin downregulation. Similar to candesartan, the peroxisome proliferator-activated receptor (PPAR)-gamma agonist, rosiglitazone, also inhibited stretch-induced nephrin downregulation, suggesting interference with stretch-induced activation of the angiotensin II-AT(1) receptor system. Accordingly, rosiglitazone did not alter stretch-induced angiotensin II secretion, but it prevented AT(1) upregulation in response to stretch. These results suggest a role for hemodynamic stress in loss of nephrin expression and allude to a role of PPAR-gamma agonists in the prevention of this loss.

New insights into the importance of aminopeptidase A in hypertension

The renin-angiotensin system (RAS) plays an important role in the maintenance of normal blood pressure and the etiology of hypertension; however, minimal attention has been paid to the degradation of the effector peptide, angiotensin II (AngII). Since aminopeptidase A (APA)-deficient mice develop hypertension APA appears to be an essential enzyme in the control of blood pressure via degradation of AngII. The robust hypertension seen in the spontaneously hypertensive rat (SHR) is due to activation of the RAS, and an accompanying decrease in kidney APA. Changes in APA have also been measured during the activation of the RAS in the Goldblatt hypertension model and Dahl salt-sensitive (DSS) rat. The DSS rat shows an elevation in renal APA activity at the onset of hypertension suggesting a protective role against elevations in circulating AngII, followed by decreased APA activity with advancing hypertension. Changes seen in human maternal serum APA activity during preeclampsia are similar to changes measured in renal APA in the DSS rat model. APA activity is higher than during normal pregnancy at the onset of preeclampsia, and with advancing preeclampsia (severe preeclampsia) declines below that seen during normal pregnancy. Serum APA activity is also increased during hormone replacement therapy (HRT), perhaps in reaction to elevated levels of AngII. Thus, it appears important to consider the relationship among activation of the RAS, circulating levels of AngII, and the availability of APA in hypertensive disorders.

Immunohistochemical localization of angiotensin II receptor types 1 and 2 in the mesenteric artery from spontaneously hypertensive rats

Angiotensin II plays a crucial role in the control of blood pressure, acting at AT1 or AT2 receptors, and can act as a potent vasoconstrictor of the peripheral vasculature inducing hypertrophy, hyperplasia, or both, in resistance arteries. The aim of the present study was to investigate whether the pattern of distribution of angiotensin AT1 and AT2 receptors on mesenteric artery sections differs in spontaneously hypertensive rats (SHR) versus their respective controls (Wistar-Kyoto [WKY] rats). Immunohistochemistry using anti-AT1 or anti-AT2 antibodies was performed on perfused-fixed/paraffin-embedded mesenteric arteries from SHR and WKY rats. 3,3'-Diaminobenzidine tetrahydrochloride (DAB; activated by hydrogen peroxide) staining revealed distinct AT1 and AT2 labeling of all artery layers (adventitia, media and intima) from WKY rats, whereas in SHR an abundant AT1 labeling was found in both intima and adventitia and a sparser labeling in the media. There was a vast reduction of AT2 labeling throughout all layers. These results suggest a crucial role for AT2 receptors in the pathogenesis of hypertension.

Quantifying gene network connectivity in silico: scalability and accuracy of a modular approach

Large, complex data sets that are generated from microarray experiments, create a need for systematic analysis techniques to unravel the underlying connectivity of gene regulatory networks. A modular approach, previously proposed by Kholodenko and co-workers, helps to scale down the network complexity into more computationally manageable entities called modules. A functional module includes a gene's mRNA, promoter and resulting products, thus encompassing a large set of interacting states. The essential elements of this approach are described in detail for a three-gene model network and later extended to a ten-gene model network, demonstrating scalability. The network architecture is identified by analysing in silico steady-state changes in the activities of only the module outputs, communicating intermediates, that result from specific perturbations applied to the network modules one at a time. These steady-state changes form the system response matrix, which is used to compute the network connectivity or network interaction map. By employing a known biochemical network, the accuracy of the modular approach and its sensitivity to key assumptions are evaluated.

Role of tyrosine kinase receptors in angiotensin II AT2 receptor signaling: involvement in neurite outgrowth and in p42/p44mapk activation in NG108-15 cells

NG108-15 cells, which have a rounding-up morphology when cultured in serum-supplemented medium, extend neurites when stimulated for 3 d with angiotensin II (Ang II). The aim of the present study was to investigate whether growth factor receptors are necessary for mediating the effects of Ang II. A 3-d treatment with AG879, an inhibitor of nerve growth factor receptor TrkA, strongly affected neurite outgrowth and phosphorylation of p42/p44(mapk) induced by Ang II. PD168393, an inhibitor of epidermal growth factor (EGF) receptor slightly decreased Ang II-induced neurite outgrowth, whereas AG213, an inhibitor of both platelet-derived growth factor receptor and EGF receptor, stimulated neurite outgrowth and p42/p44(mapk) phosphorylation on its own, without affecting further stimulation with Ang II. Moreover, Ang II induced the phosphorylation of TrkA (maximum at 5 min of incubation in the presence of serum or at 20 min in cells depleted in serum for 2 h) and a rapid increase in Rap1 activity, both effects abolished in cells preincubated with 10 microm AG879. In summary, the present results demonstrate that AT(2) receptor-induced sustained activation of p42/p44(mapk) and corresponding neurite outgrowth are mediated by phosphorylation of the nerve growth factor TrkA receptor. However, the results also point out that the presence of other growth factors, such as EGF or PDFG, may interfere with the effect of Ang II. Altogether, the current findings clearly indicate that the effects of the AT(2) receptor on neurite outgrowth dynamics are modulated by the presence of growth factors in the culture medium.

Increased plasma angiotensin II in postural tachycardia syndrome (POTS) is related to reduced blood flow and blood volume

POTS (postural tachycardia syndrome) is associated with low blood volume and reduced renin and aldosterone; however, the role of Ang (angiotensin) II has not been investigated. Previous studies have suggested that a subset of POTS patients with increased vasoconstriction related to decreased bioavailable NO (nitric oxide) have decreased blood volume. Ang II reduces bioavailable NO and is integral to the renin-Ang system. Thus, in the present study, we investigated the relationship between blood volume, Ang II, renin, aldosterone and peripheral blood flow in POTS patients. POTS was diagnosed by 70 degrees upright tilt, and supine calf blood flow, measured by venous occlusion plethysmography, was used to subgroup POTS patients. A total of 23 POTS patients were partitioned; ten with low blood flow, eight with normal flow and five with high flow. There were ten healthy volunteers. Blood volume was measured by dye dilution. All biochemical measurements were performed whilst supine. Blood volume was decreased in low-flow POTS (2.14 +/- 0.12 litres/m2) compared with controls (2.76 +/- 0.20 litres/m2), but not in the other subgroups. PRA (plasma renin activity) was decreased in low-flow POTS compared with controls (0.49 +/- 0.12 compared with 0.90 +/- 0.18 ng of Ang I.ml(-1).h(-1) respectively), whereas plasma Ang II was increased (89 +/- 20 compared with 32 +/- 4 ng/l), but not in the other subgroups. PRA correlated with aldosterone (r = +0.71) in all subjects. PRA correlated negatively with blood volume (r = -0.72) in normal- and high-flow POTS, but positively (r = +0.65) in low-flow POTS. PRA correlated positively with Ang II (r = +0.76) in normal- and high-flow POTS, but negatively (r = -0.83) in low-flow POTS. Blood volume was negatively correlated with Ang II (r = -0.66) in normal- and high-flow POTS and in five low-flow POTS patients. The remaining five low-flow POTS patients had reduced blood volume and increased Ang II which was not correlated with blood volume. The data suggest that plasma Ang II is increased in low-flow POTS patients with hypovolaemia, which may contribute to local blood flow dysregulation and reduced NO bioavailability.

Dynamic expression of the angiotensin II type 2 receptor and duodenal mucosal alkaline secretion in the Sprague-Dawley rat

Activation of angiotensin II type 2 receptors (AT2R) has been shown to stimulate duodenal mucosal alkaline secretion (DMAS) in Sprague-Dawley rats (S-D). This finding could not be confirmed in another line of S-D, and the present study investigates whether the level of AT2R expression determines the response to the AT2R agonist CGP42112A. DMAS was measured in anaesthetized rats using in situ pH-stat titration. Real-time PCR and Western blot were used to assess AT1R and AT2R RNA and protein expression, respectively. CGP42112A (0.1 microg kg(-1)min(-1) I.V.) elicited a 45% net increase in DMAS in the previous S-D line studied, whereas no change occurred in the new S-D line. Luminal administration of prostaglandin E2 (10(-5) M) increased DMAS similarly in both S-D lines. AT2R protein expression was significantly higher in tissue from the previous line compared to the new line. Individual AT1R to AT2R ratios (RNA and protein) were significantly higher in the new line compared to the previous S-D line. In the new S-D line intravenous infusion of angiotensin II (Ang II; 10 microg kg(-1) h(-1)) over 120 min significantly lowered the duodenal AT1aR to AT2R RNA ratio. Prolonged Ang II infusion over 240 min increased AT2R protein expression and evoked a 42% stimulatory response in DMAS to CGP42112A. The level of local AT2R expression determines the effect of the AT2R agonist CGP42112A on rat duodenal mucosal alkaline secretion. AT2R expression should be confirmed before interpreting the experimental effects of pharmacological interferences with this receptor.

Biphasic Effects of Losartan Potassium on Immobility in Mice

The effects of losartan potassium, an angiotensin AT(1) receptor blocker on immobility in forced swim test have been studied. Effect of losartan potassium, nortriptyline HCl, fluoxetine HCl and reserpine per se and in combination on forced swimming-induced immobility in mice have also been studied. In mice, losartan potassium elicits biphasic responses i.e. positive responses at lower doses (0.1, 1.0 and 5 mg/kg, i.p.) in the forced swim test, a test of potential antidepressant activity and vice versa at higher dose (20 and 100 mg/kg, i.p.). In chronic studies, enhancement in immobility was observed for losartan potassium (3 and 30 mg/kg, p.o., 21 days). In acute combination studies, losartan potassium (1 and 5 mg/kg) significantly reversed the reserpine-induced immobility, but vice versa at 100 mg/kg. Losartan potassium (0.1 and 5 mg/kg) potentiate antidepressant activity of nortriptyline (30 mg/kg, i.p.) in mice, but vice versa at 100 mg/kg. Likewise, Losartan potassium (100 mg/kg), significantly reversed antidepressant activity of fluoxetine HCl, but at 0.1 and 5 mg/kg, failed to modify fluoxetine HCl induced immobility. The obtained biphasic effect of losartan potassium on immobility in mice might be due to inhibitory effect on AT(1) receptor at lower dose and pronounced effect on AT(2) receptor at higher dose (large concentrations of losartan potassium can displace Angiotensin II (Ang II) from its AT(1) receptor to AT(2) receptor.

Maternal nutrient restriction and the fetal left ventricle: decreased angiotensin receptor expression

BACKGROUND

Adequate maternal nutrition during gestation is requisite for fetal nutrition and development. While a large group of epidemiological studies indicate poor fetal nutrition increases heart disease risk and mortality in later life, little work has focused on the effects of impaired maternal nutrition on fetal heart development. We have previously shown that 50% global nutrient restriction from 28-78 days of gestation (early to mid-pregnancy; term = 147 days) in sheep at mid-gestation retards fetal growth while protecting growth of heart and results in hypertensive male offspring at nine months of age. In the present study, we evaluate LV gene transcription using RNA protection assay and real-time reverse transcriptase polymerase chain reaction, and protein expression using western blot, of VEGF and AT1 and AT2 receptors for AngII at mid-gestation in fetuses from pregnant ewes fed either 100% (C) or 50% (NR) diet during early to mid-gestation.

RESULTS

No difference between the NR (n = 6) and C (n = 6) groups was found in gene transcription of the AngII receptors. Immunoreactive AT1 (1918.4 +/- 154.2 vs. 3881.2 +/- 494.9; P < 0.01) and AT2 (1729.9 +/- 293.6 vs. 3043.3 +/- 373.2; P < 0.02) was decreased in the LV of NR fetuses compared to C fetuses. The LV of fetuses exposed to NR had greater transcription of mRNA for VEGF (5.42 +/- 0.85 vs. 3.05 +/- 0.19; P < 0.03) than respective C LV, while no change was observed in immunoreactive VEGF.

CONCLUSION

The present study demonstrates that VEGF, AT1 and AT2 message and protein are not tightly coupled, pointing to post-transcriptional control points in the mid gestation NR fetus. The present data also suggest that the role of VEGF and the renin-angiotensin system receptors during conditions inducing protected cardiac growth is distinct from the role these proteins may play in normal fetal cardiac growth. The present findings may help explain epidemiological studies that indicate fetuses with low birth weight carry an increased risk of mortality from coronary and cardiovascular disease, particularly if these individuals have reduced cardiovascular reserve due to an epigenetic decrease in vascularization.

Modulation of body fluids and angiotensin II receptors in a rat model of intra-uterine growth restriction

We previously reported that sodium restriction during pregnancy reduces plasma volume expansion and promotes intra-uterine growth restriction (IUGR) in rats while it activates the renin-angiotensin-aldosterone system (RAAS). In the present study, we proceeded to determine whether expression of the two angiotensin II (ANGII) receptor subtypes (AT(1) and AT(2)) change in relation to maternal water-electrolyte homeostasis and fetal growth. To this end, pregnant (gestation day 15) and non-pregnant Sprague-Dawley rats were randomly assigned to two groups fed either normal, or Na(+)-restricted diets for 7 days. At the end of the treatment period, plasma aldosterone and renin activity as well as plasma and urine electrolytes were measured. Determinations for AT(1) and AT(2) mRNA and protein were made by RNase protection assay and photoaffinity labelling, respectively, using a number of tissues implicated in volume regulation and fetal growth. In non-pregnant rats, Na(+) restriction decreases Na(+) excretion without altering plasma volume, plasma Na(+) concentration or the expression of AT(1) and AT(2) mRNA or protein in the tissues examined. In normally fed pregnant rats when compared to non-pregnant controls, AT(1) mRNA increases in the hypothalamus as well as pituitary and declines in uterine arteries, while AT(1) protein decreases in the kidney and AT(2) mRNA declines in the adrenal cortex. In pregnant rats, Na(+) restriction induces a decrease in plasma Na(+), an increase in plasma urea, as well as a decline in renal urea and creatinine clearance rates. Protein levels for both AT(1) and AT(2) in the pituitary and AT(2) mRNA in the adrenal cortex are lower in the Na(+)-restricted pregnant group when compared to normally fed pregnant animals. Na(+) restriction also induces a decrease in AT(1) protein in the placenta. In conclusion, these results suggest that pregnancy may increase sensitivity to Na(+) depletion by the tissue-specific modulation of ANGII receptors. Finally, these receptors may be implicated in the IUGR response to low Na(+).

A library of 7TM receptor C-terminal tails. Interactions with the proposed post-endocytic sorting proteins ERM-binding phosphoprotein 50 (EBP50), N-ethylmaleimide-sensitive factor (NSF), sorting nexin 1 (SNX1), and G protein-coupled receptor-associated sorting protein (GASP)

Adaptor and scaffolding proteins determine the cellular targeting, the spatial, and thereby the functional association of G protein-coupled seven-transmembrane receptors with co-receptors, transducers, and downstream effectors and the adaptors determine post-signaling events such as receptor sequestration through interactions, mainly with the C-terminal intracellular tails of the receptors. A library of tails from 59 representative members of the super family of seven-transmembrane receptors was probed as glutathione S-transferase fusion proteins for interactions with four different adaptor proteins previously proposed to be involved in post-endocytotic sorting of receptors. Of the two proteins suggested to target receptors for recycling to the cell membrane, which is the route believed to be taken by a majority of receptors, ERM (ezrin-radixin-moesin)-binding phosphoprotein 50 (EBP50) bound only a single receptor tail, i.e. the beta(2)-adrenergic receptor, whereas N-ethylmaleimide-sensitive factor bound 11 of the tail-fusion proteins. Of the two proteins proposed to target receptors for lysosomal degradation, sorting nexin 1 (SNX1) bound 10 and the C-terminal domain of G protein-coupled receptor-associated sorting protein bound 23 of the 59 tail proteins. Surface plasmon resonance analysis of the binding kinetics of selected hits from the glutathione S-transferase pull-down experiments, i.e. the tails of the virally encoded receptor US28 and the delta-opioid receptor, confirmed the expected nanomolar affinities for interaction with SNX1. Truncations of the NK(1) receptor revealed that an extended binding epitope is responsible for the interaction with both SNX1 and G protein-coupled receptor-associated sorting protein as well as with N-ethylmaleimide-sensitive factor. It is concluded that the tail library provides useful information on the general importance of certain adaptor proteins, for example, in this case, ruling out EBP50 as being a broad spectrum-recycling adaptor.

Control of aldosterone secretion: a model for convergence in cellular signaling pathways

Aldosterone secretion by glomerulosa cells is stimulated by angiotensin II (ANG II), extracellular K(+), corticotrophin, and several paracrine factors. Electrophysiological, fluorimetric, and molecular biological techniques have significantly clarified the molecular action of these stimuli. The steroidogenic effect of corticotrophin is mediated by adenylyl cyclase, whereas potassium activates voltage-operated Ca(2+) channels. ANG II, bound to AT(1) receptors, acts through the inositol 1,4,5-trisphosphate (IP(3))-Ca(2+)/calmodulin system. All three types of IP(3) receptors are coexpressed, rendering a complex control of Ca(2+) release possible. Ca(2+) release is followed by both capacitative and voltage-activated Ca(2+) influx. ANG II inhibits the background K(+) channel TASK and Na(+)-K(+)-ATPase, and the ensuing depolarization activates T-type (Ca(v)3.2) Ca(2+) channels. Activation of protein kinase C by diacylglycerol (DAG) inhibits aldosterone production, whereas the arachidonate released from DAG in ANG II-stimulated cells is converted by lipoxygenase to 12-hydroxyeicosatetraenoic acid, which may also induce Ca(2+) signaling. Feedback effects and cross-talk of signal-transducing pathways sensitize glomerulosa cells to low-intensity stimuli, such as physiological elevations of [K(+)] (< or =1 mM), ANG II, and ACTH. Ca(2+) signaling is also modified by cell swelling, as well as receptor desensitization, resensitization, and downregulation. Long-term regulation of glomerulosa cells involves cell growth and proliferation and induction of steroidogenic enzymes. Ca(2+), receptor, and nonreceptor tyrosine kinases and mitogen-activated kinases participate in these processes. Ca(2+)- and cAMP-dependent phosphorylation induce the transfer of the steroid precursor cholesterol from the cytoplasm to the inner mitochondrial membrane. Ca(2+) signaling, transferred into the mitochondria, stimulates the reduction of pyridine nucleotides.

Endogenous angiotensin and pressure modulate brain angiotensinogen and AT1A mRNA expression

In the coarctation hypertension model, we showed both dissociation of plasma renin activity from cardiovascular-induced effects and the reversal of hypertension-induced responses by losartan. In this study, we investigated the effects of hypertension on the expression of brain renin-angiotensin system components and the simultaneous functional responses and effects of long-term angiotensin II (AT) receptor blockade on these responses. Rats were given vehicle or losartan for 9 days and subjected to subdiaphragmatic aortic constriction or sham surgery after 4 days of treatment. On the fifth postsurgical day, pressure and heart rate were measured in the conscious state; the brain was perfused and removed afterward. Sequential slices of brainstem were hybridized with 35S-oligodeoxynucleotide probes for angiotensinogen, AT1A, and AT1B receptors and processed for autoradiography and densitometry. In vehicle-treated rats, hypertension was accompanied by tachycardia and marked increments in angiotensinogen and AT1A mRNA expression in the cardiovascular system-controlling brainstem areas. In the nucleus tractus solitarii, AT1A density was correlated with both pressure and heart rate values (P<0.01), whereas angiotensinogen levels were correlated with pressure only (P<0.05). Losartan did not change the pressure of hypertensive rats (142+/-4 versus 146+/-2 mm Hg, losartan versus vehicle) and the hypertension-induced angiotensinogen mRNA expression but did block both tachycardic response and hypertension-induced AT1A mRNA expression. Hypertension and losartan did not change AT1B mRNA expression. The hypertension-induced positive feedback on angiotensinogen and AT1A mRNA expression supports the concept of a permissive role for brain angiotensin II in orchestrating circulatory responses during the development of hypertension. These data also explain the efficacy of long-term AT1 receptor blockade to reverse hypertension-induced effects.

Hypersensitivity of the Adrenal Cortex to Trophic and Secretory Effects of Angiotensin II in Lyon Genetically-Hypertensive Rats

In Lyon hypertensive (LH) rats, a model of low-renin genetic hypertension, we investigated adrenal sensitivity to angiotensin II in terms of angiotensin II receptor (AT 1 and AT 2 receptors) regulation, morphological changes, and aldosterone and corticosterone secretion. Twelve-week-old LH rats, compared with normotensive LN and LL rats, were either untreated or treated for 4 weeks with AT 1 receptor antagonist irbesartan (50 mg/kg/d), angiotensin-converting enzyme inhibitor perindopril (3 mg/kg/d), or perindopril (3 mg/kg/d) plus angiotensin II infusion (200 ng/kg/min). At 16 weeks, untreated LH rats had high systolic blood pressure ( P <0.05), low aldosterone ( P <0.05), and increased corticosterone ( P <0.05) plasma levels. AT 1 -receptor binding density in the zona glomerulosa was similar in the three strains. In LH rats, angiotensin II infusion increased the relative adrenal weight from 10.5±0.3 to 16.7±0.7 mg/100g ( P <0.05), whereas this change was very modest in normotensive rats. Zona glomerulosa enlarged and plasma aldosterone increased after angiotensin II infusion in the 3 strains, but more markedly in LH versus normotensive rats (2.4- versus 1.3- and 1.6-fold, respectively; 20- versus 10-fold in normotensive rats, P <0.05). Surprisingly, after angiotensin II infusion, despite the absence of angiotensin II receptors in the three strains, the zona fasciculata-reticularis enlarged 1.5-fold and plasma corticosterone increased 1.7-fold only in LH rats ( P <0.05), suggesting an indirect control of this compartment by angiotensin II. The hypertrophy and hypersecretory activity of both zona glomerulosa and zona fasciculata-reticularis in LH rats in response to angiotensin II point to the adrenal cortex as a pivotal tissue in the pathophysiology of hypertension in LH rats.

Importance of input perturbations and stochastic gene expression in the reverse engineering of genetic regulatory networks: insights from an identifiability analysis of an in silico network

Gene expression profiles are an increasingly common data source that can yield insights into the functions of cells at a system-wide level. The present work considers the limitations in information content of gene expression data for reverse engineering regulatory networks. An in silico genetic regulatory network was constructed for this purpose. Using the in silico network, a formal identifiability analysis was performed that considered the accuracy with which the parameters in the network could be estimated using gene expression data and prior structural knowledge (which transcription factors regulate which genes) as a function of the input perturbation and stochastic gene expression. The analysis yielded experimentally relevant results. It was observed that, in addition to prior structural knowledge, prior knowledge of kinetic parameters, particularly mRNA degradation rate constants, was necessary for the network to be identifiable. Also, with the exception of cases where the noise due to stochastic gene expression was high, complex perturbations were more favorable for identifying the network than simple ones. Although the results may be specific to the network considered, the present study provides a framework for posing similar questions in other systems.

Portal hypertension is associated with increased mRNA levels of vasopressor G-protein-coupled receptors in human hepatic arteries

BACKGROUND

The contractile response of human splanchnic vessels to different vasoconstrictors is attenuated in cirrhosis. Functional studies indicate a cellular signalling defect upstream of the G-protein level. The aim of the present study was to analyze expression and mRNA levels of the following most relevant vasopressor receptors in the smooth musculature of human hepatic arteries: alpha1 adrenoceptor (AR) subtypes a, b and d, angiotensin II type 1 receptor (AT1), arginine vasopressin receptor type 1a (V1a), endothelin receptor type A (ETA) and B (ETB).

MATERIALS AND METHODS

Hepatic arteries were collected from 10 donors (noncirrhotic) and 14 recipients (cirrhotic) at liver transplantations. Real-time-PCR was performed to quantify steady-state levels of receptor mRNAs.

RESULTS

alpha 1aAR mRNA levels showed no significant difference between the cirrhotic arteries and the controls while the mRNA levels of the other vasoactive receptors were significantly higher in the cirrhotic hepatic arteries (alpha 1bAR: 4-fold, P = 0.013; AT1: 16-fold, P = 0.024; V1a: 23-fold, P = 0.001; ETA: 4-fold, P = 0.02; ETB: 8-fold, P = 0.008). No mRNA for the alpha 1dAR was detected either in the donor or recipient hepatic arteries.

CONCLUSION

We conclude that vascular hyporeactivity to the most relevant endogenous vasoconstrictors of cirrhotic hepatic arteries is not caused by a receptor down-regulation at mRNA levels. In contrast they were up-regulated.

Cyclic AMP-independent involvement of Rap1/B-Raf in the angiotensin II AT2 receptor signaling pathway in NG108-15 cells

The angiotensin II (Ang II) type 2 (AT(2)) receptor is an atypical seven-transmembrane domain receptor. Controversy surrounding this receptor concerns both the nature of the second messengers produced as well as its associated signaling mechanisms. Using the neuronal cell line NG108-15, we have reported previously that activation of the AT(2) receptor induced morphological differentiation in a p21(ras)-independent, but p42/p44(mapk)-dependent mechanism. The activation of p42/p44(mapk) was delayed, sustained, and had been shown to be essential for neurite elongation. In the present report, we demonstrate that activation of the AT(2) receptor rapidly, but transiently, activated the Rap1/B-Raf complex of signaling proteins. In RapN17- and Rap1GAP-transfected cells, the effects induced by Ang II were abolished, demonstrating that activation of these proteins was responsible for the observed p42/p44(mapk) phosphorylation and for morphological differentiation. To assess whether cAMP was involved in the activation of Rap1/B-Raf and neuronal differentiation induced by Ang II, NG108-15 cells were treated with stimulators or inhibitors of the cAMP pathway. We found that dibutyryl cAMP and forskolin did not stimulate Rap1 or p42/p44(mapk) activity. Furthermore, adding H-89, an inhibitor of protein kinase A, or Rp-8-Br-cAMP-S, an inactive cAMP analog, failed to impair p42/p44(mapk) activity and neurite outgrowth induced by Ang II. The present observations clearly indicate that cAMP, a well known stimulus of neuronal differentiation, did not participate in the AT(2) receptor signaling pathways in the NG108-15 cells. Therefore, the AT(2) receptor of Ang II activates the signaling modules of Rap1/B-Raf and p42/p44(mapk) via a cAMP-independent pathway to induce morphological differentiation of NG108-15 cells.

AT1 angiotensin II receptor mediates intracellular calcium mobilization in normal and cancerous breast cells in primary culture

Angiotensin II (Ang II) increases intracellular calcium concentration ([Ca2+]i) in both normal and cancerous human breast cells in primary culture. Maximal [Ca2+]i increase is obtained using 100nM Ang II in both cell types; in cancerous breast cells, [Ca2+]i increase (delta[Ca2+]i) is 135+/-10nM, while in normal breast cells it reaches 65+/-5 nM (P<0.0001). In both cell types, Ang II evokes a Ca2+ transient peak mediated by thapsigargin (TG) sensitive stores; neither Ca2+ entry through L-type membrane channels or capacitative Ca2+ entry are involved. Type I Ang II receptor subtype (AT1) mediates Ang II-dependent [Ca2+]i increase, since losartan, an AT1 inhibitor, blunted [Ca2+]i increase induced by Ang II in a dose-dependent manner, while CGP 4221A, an AT2 inhibitor, does not. Phospholipase C (PLC) is involved in this signaling mechanism, as U73122, a PLC inhibitor, decreases Ang II-dependent [Ca2+]i transient peak in a dose-dependent mode.Thus, the present study provides new information about Ca2+ signaling pathways mediated through AT1 in breast cells in which no data were yet available.

Evidence for activation of the renin-angiotensin system in the human prostate: increased angiotensin II and reduced AT(1) receptor expression in benign prostatic hyperplasia

The expression and cellular localization of angiotensin II (Ang II) and AT(1) receptor proteins were examined in the normal human prostate and benign prostatic hyperplasia (BPH) by immunohistochemistry. In the normal prostate, Ang II immunoreactivity was localized to the basal layer of the epithelium and AT(1) receptor immunostaining was found predominantly on stromal smooth muscle and also on vascular smooth muscle of prostatic blood vessels. Ang II immunoreactivity was markedly increased in hyperplastic acini in BPH compared with acini in the normal prostate (normal: 7.4+/-0.2%, n=5 vs. BPH: 22.7+/-1.9%, n=5, p<0.001). However, AT(1) receptor immunoreactivity was significantly decreased in BPH compared with the normal prostate [normal: 16.4+/-2.2%, n=4 vs. BPH: 9.4+/-1.3%, n=5, p<0.05 (p=0.025)]. The present study demonstrates the presence of Ang II peptide in the basal layer of the epithelium and AT(1) receptors on stromal smooth muscle, suggesting that Ang II may mediate paracrine functions on cellular growth and smooth muscle tone in the human prostate. Furthermore, AT(1) receptor down-regulation in BPH may be due to receptor hyperstimulation by increased local levels of Ang II in BPH. These data extend previous findings in support of the novel concept that overactivity of the renin-angiotensin system (RAS) may be involved in the pathophysiology of BPH.

ANG II type 1 receptor downregulation does not require receptor endocytosis or G protein coupling

ANG II type 1 (AT(1)) receptors respond to sustained exposure to ANG II by undergoing downregulation of absolute receptor numbers. It has been assumed previously that downregulation involves endocytosis. The present study hypothesized that AT(1) receptor downregulation occurs independently of receptor endocytosis or G protein coupling. Mutant AT(1) receptors with carboxy-terminal deletions internalized <5% of radioligand compared with 65% for wild-type AT(1) receptors. The truncated AT(1) receptors retained the ability to undergo downregulation. These data suggest the existence of an alternative pathway to AT(1) receptor degradation that does not require endocytosis, per se. Point mutations in either the second transmembrane region or second intracellular loop impaired G protein (G(q)) coupling. These receptors exhibited a biphasic pattern of downregulation. The earliest phase of downregulation (0-2 h) was independent of coupling to G(q), but no additional downregulation was observed after 2 h of ANG II exposure in the receptors with impaired coupling to G(q). These data suggest that coupling to G(q) is required for the later phase (2-24 h) of AT(1) receptor downregulation.

Angiotensin II type 1 receptor and ACTH receptor expression in human adrenocortical neoplasms

OBJECTIVE

Type 1 angiotensin II (Ang II) receptors transduce most of the known actions of Ang II, including steroidogenesis and trophic actions on the adrenal cortex. We investigated the type 1 Ang II receptor expression in adrenocortical tissues to define its regulation in adrenocortical neoplasms and to compare its expression with that of the ACTH receptor (ACTH-R).

PATIENTS AND MEASUREMENTS

Poly A RNA was extracted from tumour tissue and electrophoresed through a 1.0% agarose gel, blotted and hybridized with alpha32P-CTP labelled PCR generated type 1 Ang II receptor cDNA probe. Receptor autoradiography was performed on slices from normal adrenals and tumour tissue by incubation with 125I-Sar1, Ile8-Ang II with and without pretreatment with cold Ang II or with the selective type 1 receptor antagonist losartan.

RESULTS

Ang II type 1 receptor mRNA was high in cortisol producing (CPA; n = 5) and aldosterone producing (APA; n = 4) adenomas (normal adrenals 100 +/- 12% vs. 180 +/- 16% in CPA and 154 +/- 26% in APA, mean +/- SEM), but was low in nonfunctioning adenomas (NFA; n = 2; 2 +/- 1%). ACTH receptor mRNA followed a similar pattern (CPA 178 +/- 17, APA 196 +/- 30, NFA 0%, carcinomas 56 +/- 11%) with a good correlation between Ang II type 1 receptor and ACTH-R mRNA of r = 0.692, P = 0.0019. Receptor autoradiography in normal adrenals demonstrated Ang II type 1 receptors predominantly in the zona glomerulosa. In tumour tissue, mainly type 1 receptor expression was found confirming the Northern blot data.

CONCLUSIONS

Angiotensin II type 1 receptor and ACTH receptor expression seems to be correlated with the functional status of adrenocortical tumours, suggesting regulation by similar factors. The predominant receptor expressed in adrenocortical tumours is the Angiotensin II type 1 receptor whereas type 2 receptor expression is minimal.

Novel nuclear signaling pathway mediates activation of fibroblast growth factor-2 gene by type 1 and type 2 angiotensin II receptors

In bovine adrenal medullary cells synergistically acting type 1 and type 2 angiotensin II (AII) receptors activate the fibroblast growth factor-2 (FGF-2) gene through a unique AII-responsive promoter element. Both the type 1 and type 2 AII receptors and the downstream cyclic adenosine 1',3'-monophosphate- and protein kinase C-dependent signaling pathways activate the FGF-2 promoter through a novel signal-transducing mechanism. This mechanism, which we have named integrative nuclear FGF receptor-1 signaling, involves the nuclear translocation of FGF receptor-1 and its subsequent transactivation of the AII-responsive element in the FGF-2 promoter.

Cardiac angiotensin II receptors are downregulated by chronic angiotensin II infusion in rats

We studied the effect of chronic (7 days) angiotensin II infusion in a subpressor (200 ng/kg per minute) dose on angiotensin II receptors in the left ventricle in rats. Infusion of angiotensin II caused an elevation in systolic blood pressure after 3 days, usually to values of about 150 mmHg, and the increase continued during the drug administration. The number of angiotensin II type 1 and angiotensin II type 2 receptors was significantly decreased by 20-30% in the angiotensin II-infused left ventricular membranes without affecting the affinity. Thus, these data suggest that angiotensin II may regulate the number of its own receptors in rat left ventricles.

Regulation by adrenocorticotropin (ACTH), angiotensin II, transforming growth factor-beta, and insulin-like growth factor I of bovine adrenal cell steroidogenic capacity and expression of ACTH receptor, steroidogenic acute regulatory protein, cytochrome P450c17, and 3beta-hydroxysteroid dehydrogenase

The purpose of this study was to evaluate the time-course effect of a 36-h treatment with ACTH (10(-8) M), transforming growth factor-beta1 (TGFbeta1; 10(-10) M), angiotensin II (AngII; 10 (-7) M), and insulin-like growth factor I (IGF-I; 10(-8) M) on the steroidogenic capacity of bovine adrenocortical cells (BAC) and on messenger RNA (mRNA) levels of ACTH receptor, cytochrome P450c17, 3beta-hydroxysteroid dehydrogenase (3betaHSD), steroidogenic acute regulatory protein (StAR), and StAR protein. ACTH and IGF-I enhanced, in a time-dependent manner, the acute 2-h ACTH-induced cortisol production, whereas TGFbeta 1 and AngII markedly reduced it. ACTH, IGF-I, and AngII increased ACTH receptor mRNA, but the opposite was observed after TGFbeta1 treatment. ACTH and IGF-I increased P450c17 and 3betaHSD mRNAs, whereas AngII and TGFbeta1 had the opposite effects. However, the effects of the four peptides on ACTH-induced cortisol production appeared before any significant alterations of the mRNA levels occurred. The most marked and rapid effect of the four peptides was on StAR mRNA. The stimulatory effect of ACTH was seen within 1.5 h, peaked at 4-6 h, and declined thereafter, but at the end of the 36-h pretreatment, the levels of StAR mRNA and protein were higher than those in control cells. IGF-I also enhanced StAR mRNA levels within 1.5 h, and these levels remained fairly constant. The effects of AngII on StAR mRNA expression were biphasic, with a peak within 1.5-3 h, followed by a rapid decline to almost undetectable levels of both mRNA and protein. TGFbeta1 had no significant effect during the first 3 h, but thereafter StAR mRNA declined, and at the end of the experiment the StAR mRNA and protein were almost undetectable. Similar results were observed when cells were treated with ACTH plus TGFbeta1. A 2-h acute ACTH stimulation at the end of the 36-h pretreatment caused a higher increase in StAR mRNA and protein in ACTH- or IGF-I-pretreated cells than in control cells, which, in turn, had higher levels than cells pretreated with TGFbeta1, ACTH plus TGFbeta1, or AngII. These results and the fact that the stimulatory (IGF-I) or inhibitory (AngII and TGFbeta1) effects on ACTH-induced cortisol production were more pronounced than those on the ability of cells to transform pregnenolone into cortisol strongly suggest that regulation of StAR expression is one of the main factors, but not the only one, involved in the positive (IGF-I) or negative (TGFbeta1 and AngII) regulation of BAC for ACTH steroidogenic responsiveness. A high correlation between steady state mRNA level and acute ACTH-induced cortisol production favors this conclusion.

Involvement of the angiotensin II type 2 receptor in apoptosis during human fetal adrenal gland development

The aim of this study was to establish a link between the highly expressed angiotensin II (Ang II) type 2 receptor (AT2) in human fetal adrenal cells and the proposed apoptotic activity in the center of the gland. There was an important increase in apoptotic DNA fragmentation with age in adrenal glands of fetuses from 15-20 weeks gestation. Adrenal cells showing the characteristic apoptotic internucleosomal DNA fragmentation were localized in the central portion of the fetal zone. In cells cultured for 24 h, Ang II, via the AT2 receptor, induced DNA fragmentation and cleavage of the DNA repair enzyme, poly-(ADP-ribose) polymerase. Furthermore, characteristic membrane blebbing was observed specifically on cells of the fetal zone. Immunofluorescence studies demonstrated that stimulation with Ang II or CGP 42112 (an agonist of the AT2 receptor) strongly modified the actin network, now localized exclusively along the plasma membrane, with a predominance of labeling at the base of the bleb formation. This rearrangement of actin distribution was different in cells from the definitive zone, corroborating the observation that these cells express many more Ang II type 1 receptors (AT1) than AT2 receptors. Taken together, our data indicate that the AT2 receptor is involved in the apoptotic process observed in the human fetal adrenal gland and could participate in the morphological changes occurring after birth, leading to involution of the fetal zone.

Interactions between AT1 and AT2 receptors in uterine arteries from pregnant ewes

This study was performed to investigate the roles of angiotensin receptors (AT1 and AT2) in the contractility of uterine arteries during normal pregnancy and after angiotensin II levels have been elevated. Pregnant ewes were given intravenous infusions of saline for 24 h (control) or angiotensin II (30 ng kg(-1) min(-1)) for 2 or 24 h. The contractile responses of uterine arterial rings to angiotensin II (4 microM) and antagonists were then examined in vitro. Most uterine arteries were relatively insensitive to the vasoconstrictor effects of angiotensin II. In rings from control ewes an angiotensin AT2 antagonist enhanced (P < 0.05) the contractile responses to angiotensin II, suggesting that angiotensin AT2 receptors inhibited the angiotensin AT1 receptor mediated contractions. Uterine arterial rings from ewes given intravenous infusions of angiotensin II displayed greater (P < 0.05) contractile responses to angiotensin II in vitro compared to rings from control ewes. This was in part due to down regulation of angiotensin AT2 receptors. Surprisingly, while performing these experiments a small number of ewes had uterine arteries which were "hyperreactive" to angiotensin II (contractile responses 6-fold greater). These ewes also had abnormal renin angiotensin systems and had some features which are characteristic of those seen in preeclampsia. The "hyperreactivity" of these arteries could only in part be explained by down regulation of angiotensin AT2 receptors. It is concluded that in normal pregnancy angiotensin AT2 receptors play a role in maintaining an adequate uterine blood flow for the fetus. When angiotensin II levels are elevated for a prolonged period this protective effect is lost partly because angiotensin AT1 receptors are down regulated.

Promoter-independent regulation of cell-specific dopamine receptor expression

Here we describe the construction of recombinant adenoviruses expressing dopamine D2 and D4 receptors, and their ability to mediate high levels of heterologous expression in a variety of cell types in vitro and in vivo for at least 7 days post infection. These experiments demonstrated that maximum receptor expression is achieved generally within 24 h and remains constant thereafter. Maximum expression levels were highly variable between cell lines and dependent on infection efficiency and promoter strength. Correction for these two variables revealed differences in relative expression levels between cell lines varying by two orders of magnitude. Our results indicate that in addition to gene transcription, post-transcriptional mechanisms play a dominant role in determining dopamine receptor levels in this system.

Regulation of cell proliferation and angiotensin II type 2 receptors in R3T3 cells

The effects of insulin-like growth factor 1 (IGF-1), basic fibroblast growth factor (bFGF), transforming growth factor beta1 (TGFbeta1), fetal calf serum (FCS) and angiotensin II (AngII) on cell proliferation, (3H-thymidine incorporation and cell number) and AT2 receptor number and mRNA levels in R3T3 cells have been studied. All growth factors as well as FCS markedly increased cell proliferation, whereas AngII increased slightly 3H-thymidine incorporation, but not cell number. TGFbeta1, bFGF and FCS reduced by more than 80% both AT2 receptor number and mRNA, by inhibiting the transcription rate. In contrast, IGF-1 and AngII increased about 4-fold AT2 receptor number, but only IGF-1 increased AT2 mRNA. When added together the effects of IGF-1 and AngII were more than additive on AT2 receptor number, but not on mRNA level. None of the factors studied modified AT2 mRNA half-life. In conclusion, the present results demonstrated that: 1/ cell proliferation is not correlated with AT2 expression; 2/ growth factors regulate, positively or negatively, AT2 transcription rate, whereas AngII regulates the translation rate of AT2 mRNA; 3/ all the effects of AngII on R3T3 are mediated by AT2 receptors since they are mimicked by the AT2 agonist CGP42112 and blocked by the AT2 antagonist PD123177.

The human placental renin-angiotensin system

The human placenta and related tissues are considered to be examples of the recently accepted local renin-angiotensin systems (RAS). The brain is another example of a system that is thought to be regulated independently of the kidney and the role of angiotensin within the CNS as a neural mediator has drawn considerable attention. It has been known for a long time that many of the neuroendocrine mediators and receptors are expressed in the placenta and it has been suggested that there are many parallels between the classical neuroendocrine system and the placental one. The present review summarizes information that components of the RAS are expressed in uteroplacental tissues, are regulated by endogenous substances, and have important biological functions within this reproductive system. A comparison of similarities and differences between the classical and the placental RAS may provide clues to functions in other endocrine and neuroendocrine systems. The major components of the placental RAS that are considered are renin, prorenin, angiotensin I, angiotensin II, angiotensin converting enzyme (ACE), angiotensin receptors, and angiotensinogen (renin substrate). The factors that regulate these components at the cellular and the nuclear level are described. It is concluded that prorenin via angiotensin-dependent and angiotensin-independent mechanisms influences functions within uteroplacental tissues. Some of these actions are direct and others are mediated by the release of different signalling molecules. These features are similar to many neuroendocrine systems and utilize some of the same messengers.

Saralasin-induced inhibition of ovulation in the in vitro perfused rat ovary is not replicated by the angiotensin II type-2 receptor antagonist PD123319

OBJECTIVE

Our aim was to explain the effect of the nonspecific angiotensin II antagonist saralasin and the specific angiotensin II type-2 receptor antagonist PD123319 on ovulation.

STUDY DESIGN

Saralasin, 1 micromol/L (n = 5), and PD123319 10 micromol/L (n = 6), were administered to in vitro perfused rat ovary. Prostaglandin (prostaglandin E2, prostaglandin F2alpha, 6-keto-prostaglandin F1alpha), hydroxy-eicosatetraenoic acid (12-hydroxy-eicosatetraenoic acid, 15-hydroxy-eicosatetraenoic acid), estradiol, and progesterone levels in the perfusate and the ovulation rate were compared (Mann-Whitney U test) with controls.

RESULTS

Saralasin significantly (P < .01) inhibited the ovulation rate (3.0 +/- 1.4) versus control (13.1 +/- 1.0) and reduced prostaglandin E2 (at 3 hours P < .01 and 20 hours P < .05) and 6-keto-prostaglandin F1alpha (at 20 hours P < .05) levels. Saralasin did not alter prostaglandin F2alpha, hydroxy-eicosatetraenoic acids, or steroid levels. PD123319 decreased 15-hydroxy-eicosatetraenoic acid levels at 3 hours (P < .05) but had no effects on other eicosanoids, steroid levels, or the ovulation rate.

CONCLUSION

Angiotensin II plays an important role in ovulation in the rat and is associated with ovarian prostaglandin synthesis. This effect is not selectively regulated via the angiotensin II type-2 receptor.

Differential regulation of angiotensin II receptor subtypes in the adrenal gland: role of aldosterone

It has been shown that aldosterone potentiates the action of angiotensin II (Ang II) in cultured rat vascular smooth muscle cells solely by increasing the number of Ang II receptors. The mechanisms responsible for aldosterone-Ang II interactions in the adrenal gland are unknown. The present study was designed to investigate the effect of aldosterone on expression of Ang II receptor subtypes (AT1 and AT2) in the adrenal gland. Seven-week-old male Wistar rats were treated for 2 weeks with either aldosterone (0.05 microg/h, n=14) or vehicle (n=14) by use of implanted osmotic minipumps. Systolic blood pressure was not altered by aldosterone treatment. Plasma aldosterone levels were higher in aldosterone-treated rats (181+/-53 pg/mL) compared with vehicle-treated rats (33+/-21 pg/mL, P<0.05). Northern blot analysis and radioligand binding assay showed that adrenal AT1 mRNA levels and AT1 receptor density in aldosterone-treated rats were not statistically different from those of vehicle-treated rats. However, immunohistochemical studies showed that the highest adrenal AT1 receptor expression was shifted from the zona glomerulosa to the zona fasciculata after aldosterone treatment. In contrast, adrenal AT2 mRNA and AT2 receptor density in aldosterone-treated rats were decreased by approximately 50% and 40%, respectively, compared with vehicle-treated rats (P<0.05). Aldosterone-induced decrease in adrenal AT2 receptor expression occurred mainly in the medulla. Thus, aldosterone differentially modulates the expression of AT1 and AT2 receptors in the adrenal gland. Although the function of the AT2 receptor in the adrenal gland is largely unknown, our data indicate that aldosterone may modulate the effect of Ang II by altering the location of AT1 receptors and by reducing the number of AT2 receptors in the adrenal gland.

Influence of dietary sodium restriction on angiotensin II receptors in rat adrenals

We studied the distribution of angiotensin II (AII) receptors type 1 (AT1) and type 2 (AT2) and the effects of a low sodium intake on these two subtypes of receptors in male rat adrenals. Binding studies on adrenal slices, on cell membranes and on cell suspensions were performed using [125I]AII and specific analogs for AT1 (Losartan) and AT2 (PD 123319) receptors. The distribution of AT1 was also studied by immunofluorescence. Complementary approaches were necessary to reach our goal. Indeed, by autoradiography on adrenal slices, [125I]AII was shown to bind to the zona glomerulosa (ZG) and to the medulla (M). When coincubated with [125I]AII, PD 123319 displaced [125I]AII from the medulla and from the ZG, indicating the presence of AT2 receptors in both zones. Losartan partially displaced [125I]AII from the ZG, indicating the presence of AT1 receptors in that zone. Furthermore, the labeling intensity of the medulla (AT2 receptors) was much stronger in adrenal sections from rats kept on a low sodium regimen than from controls. Immunofluorescence microscopy revealed that AT1 receptors were located mainly in the ZG of control rats. After sodium restriction, AT1 receptors appeared to be uniformly distributed within an enlarged ZG; furthermore AT1 receptor-positive cells were found to a limited degree in the zona fasciculata and possibly in the zona reticularis, and a greater number of these positive cells appeared in these zones under sodium restriction. Cell suspensions from rats fed a low sodium diet showed a 2.7- and 2.1-fold increase in total AII receptors in adrenal ZG and ZFR + M cells when compared with controls. Based on Losartan displacement, we calculated that [125I]AII bound to AT1 and to AT2 receptors was increased in both ZG and ZFR + M cell preparations under sodium restriction. Results of binding studies on cell membranes were also indicative of an increasing effect of sodium restriction on AT1 and AT2 receptors binding capacity. Furthermore, Northern blotting analysis revealed 3.0- and 2.5-fold increases in the level of AT1 receptor mRNA in the ZG and the ZFR + M of rats fed a low sodium diet as compared with those fed a normal diet. The low sodium intake resulted in a weaker increase (1.5-fold) in the level of AT2 receptor messenger RNA in the ZG, with no changes in the ZFR + M preparations. In conclusion, in this study complementary approaches were needed to determine the localization of AT1 and AT2 receptors in the rat adrenal, and to show the increasing effects of a low sodium regimen on the adrenal level of these receptors. Immunofluorescence studies revealed AT1 receptors mainly in the ZG and also in some cells of the inner adrenal cortex zones; in adrenals of rats kept on a low sodium diet the ZG was markedly enlarged, and an increased number of immunoreactive cells with AT1 receptors were observed throughout that zone; also more immunoreactive cells were present in the inner zones of the adrenal cortex. Furthermore in the adrenals of rats kept on a low sodium diet, we observed: 1) an increased number of AT1 and AT2 receptors in cell suspensions from the ZG, and in cell suspensions of the ZFR + M; 2) an increased level of AT1 and AT2 receptor mRNAs in the ZG; 3) an increased level of AT1 receptor mRNA, with no changes in the AT2 mRNA level in the ZFR + M. These results suggest a role for AT1 as well as for AT2 receptors in controlling adrenal function and differentiation under normal as well as under physiological stimulation of AII production following sodium restriction.

Desensitization of AT1 receptor-mediated cellular responses requires long term receptor down-regulation in bovine adrenal glomerulosa cells

Angiotensin II (Ang II) regulates aldosterone production in bovine adrenal glomerulosa cells by interacting with the AT1 receptor. This receptor is coupled to a G protein that controls the activity of phospholipase C. With a primary culture of bovine adrenal glomerulosa cells, we evaluated the desensitization of cellular responses after pretreatment with Ang II. When cells were pretreated for 30 min with 1 microM Ang II at 37 C, we observed a 48% loss of [125I]Ang II-binding activity. Scatchard analysis revealed that this decreased binding activity corresponded to a 53% loss of the total number of binding sites. This phenomenon was time dependent, with a t(1/2) of 20 min, and a maximal loss of 76% of the total binding sites was observed after 14 h. A time-dependent decrease in AT1 receptor messenger RNA levels was also observed after pretreatment with 1 microM Ang II for 12-24 h. Taken together, these results are interpreted as a down-regulation of the AT1 receptor. Desensitization of phospholipase C activity under similar conditions was, however, a slower process, with a t(1/2) of 9 h and a maximal response reduction of 83% observed after 24 h. Dose-response experiments indicated that maximal phospholipase C desensitization was obtained in the presence of 1 microM Ang II, with an EC50 of 90 nM. The desensitization was of a homologous nature, as a 24-h pretreatment with Ang II did not affect bradykinin-induced inositol phosphate production. A 24-h pretreatment with 1 microM Ang II also significantly desensitized the steroidogenic effect of Ang II and the potentiating effect of Ang II on ACTH-induced cAMP production. Lower concentrations of Ang II (10 nM) did not produce any desensitizing effect on these two parameters. This study provides evidence that glomerulosa cells are functionally resistant to short term desensitization of the AT1 receptor and that long term down-regulation with high concentrations of Ang II is needed to desensitize AT1-mediated cellular responses.