Angiotensin II type 1a receptor-deficient mice with hypotension and hyperreninemia

Renal tubulointerstitial damage caused by persistent proteinuria is attenuated in AT1-deficient mice: role of endothelin-1

Using angiotensin II (AngII) type 1A receptor-deficient mice [AT1(-/-)], in which we induced protein overload nephropathy, we explored the potential implication of AngII and endothelin-1 (ET-1) in the tubulointerstitial damage because of persistent proteinuria. At day 7, AT1(-/-) showed marked proteinuria to a similar extent to that of wild-type mice (WT). However, at day14, AT1(-/-) had significantly less proteinuria, renal damage, transforming growth factor-beta, and matrix mRNA expression and mortality. AT1(-/-) also showed a significant diminution in the activation of the transcriptional factors nuclear factor-kappaB and AP-1. Unexpectedly, AT1(-/-) had a higher interstitial infiltration than WT. The administration of the angiotensin-converting enzyme inhibitor quinapril to WT caused a marked improvement in proteinuria and renal lesions, resembling that seen in untreated AT1(-/-). However, the interstitial infiltration persisted in AT1(-/-) when treated with quinapril. Because ET-1 may participate in the recruitment of mononuclear cells, we also studied the implication of this peptide. AT1(-/-) had a significantly higher ET-1 expression in tubular epithelial cells than WT. The administration of the dual ETA/ETB antagonist bosentan to AT1(-/-) considerably reduced the interstitial infiltrates. Bosentan also exerted a beneficial effect on proteinuria, renal lesions, and mortality in WT. These data show that in overload nephropathy, proteinuria and renal lesions are, to a large extent, AngII-dependent. The up-regulation of ET-1 in tubular epithelial cells in AT1(-/-), associated with interstitial infiltrates, suggests that the combination of drugs interfering with both vasopeptides may be of therapeutic interest in renal diseases with severe proteinuria and tubulointerstitial damage.

The angiotensin II type 1 receptor and receptor-associated proteins

The mechanisms of regulation, activation and signal transduction of the angiotensin II (Ang II) type 1 (AT1) receptor have been studied extensively in the decade after its cloning. The AT1 receptor is a major component of the renin-angiotensin system (RAS). It mediates the classical biological actions of Ang II. Among the structures required for regulation and activation of the receptor, its carboxyl-terminal region plays crucial roles in receptor internalization, desensitization and phosphorylation. The mechanisms involved in heterotrimeric G-protein coupling to the receptor, activation of the downstream signaling pathway by G proteins and the Ang II signal transduction pathways leading to specific cellular responses are discussed. In addition, recent work on the identification and characterization of novel proteins associated with carboxyl-terminus of the AT1 receptor is presented. These novel proteins will advance our understanding of how the receptor is internalized and recycled as they provide molecular mechanisms for the activation and regulation of G-protein-coupled receptors.

The action of salt and captopril on blood pressure in mice with genetic hypertension

OBJECTIVE

Does captopril lower blood pressure in genetically hypertensive, normotensive and hypotensive mice under normal and salt-loaded conditions?

DESIGN AND METHODS

Groups of inbred mice that were genetically hypertensive, normotensive or hypotensive were given one of the following treatments: (a) captopril in drinking water for 7 days; controls were given water. (b) 0.85% saline to drink for up to 14 days; controls were given water. (c) Water or saline followed by captopril/water or captopril/saline for 7 days. (d) In hypotensive mice only, 0.85% saline, 0.85% saline plus captopril, water or captopril in water. Systolic blood pressures (SBP) were measured by a computerized tail-cuff sphygmomanometer. Results were compared by analysis of variance (ANOVA).

RESULTS

Captopril lowered SBP in all strains of mice. When saline was given with captopril, the fall in SBP was slower but the final SBP level was similar to that of mice given captopril in water. Hypotensive mice showed a transient rise in SBP on saline, which was abolished by concurrent treatment with captopril.

CONCLUSION

Captopril lowers blood pressure in hypertensive, normotensive and hypotensive mice. Salt-loading retards the captopril-induced fall in SBP, but the final level of SBP achieved is similar to that in mice given captopril with water. The BPL1 strain of mouse was slightly salt-sensitive, and this was abolished by captopril.

An essential role of angiotensin II receptor type 1a in recipient kidney, not in transplanted peripheral blood leukocytes, in progressive immune-mediated renal injury

Despite an intensive effort of elucidating the pathogenic role of angiotensin II (AII) in immune-mediated renal injury, the precise mechanisms are poorly understood. In the present study, we examined the site of AII action, peripheral blood leukocytes or resident renal cells, in immune-mediated renal injury using AII type 1a receptor (AT1a)-deficient homozygous (AT1a -/-) mice and wild-type (AT1a +/+) mice. The AT1a -/- mice showed delayed-type hypersensitivity similar to that of the AT1a +/+ mice, suggesting that the lack of AT1a does not impair a Th1-type cellular immune response of peripheral blood leukocytes involved in immune-mediated renal injury. We then generated the radiation bone marrow chimera mice, WA and AW, which have transplanted peripheral blood leukocytes from the AT1a +/+ and AT1a -/- mice into the AT1a -/- and AT1a +/+ mice, respectively. As controls, WW and AA, the AT1a +/+ and AT1a -/- mice given bone marrow cells from the AT1a +/+ and AT1a -/- mice, respectively, were generated. Seven days after induction of antiglomerular basement membrane nephritis, glomerulosclerosis observed in the WW mice was markedly ameliorated in the WA mice, but not in the AW mice. In addition, the recruitment of monocytes/macrophages and the expressions of monocyte chemoattractant protein-1 and intercellular adhesion molecule-1 in the glomeruli of the AW and WW mice was evident, but such significant phenotypes were not seen in the WA and AA mice, showing a marked amelioration of renal injury dependent on the host AT1a genotype. These results demonstrate an essential role of AT1a in intrinsic renal cells for progressive immune-mediated renal injury and indicate a beneficial effect of blocking the renin-angiotensin system in the treatment of such diseases.

Role of the renin-angiotensin system during alterations of sodium intake in conscious mice

The present studies were performed to quantify circulating components of the renin-angiotensin-aldosterone axis and to determine the functional importance of this system during alterations in sodium intake in conscious mice. Increasing sodium intake from approximately 200 to 1,000 microeq/day significantly decreased plasma renin concentration from 472 +/- 96 to 304 +/- 83 ng ANG I. ml(-1). h(-1) (n = 5) but did not alter plasma renin activity from the low-sodium level of 7.7 +/- 1.1 ng ANG I. ml(-1). h(-1). Despite the elevated plasma renin concentration, plasma ANG II in mice on low-sodium level averaged 14 +/- 3 pg/ml and was significantly suppressed to 6 +/- 1 pg/ml by high-sodium intake (n = 7). Consistent with the modulation of ANG II, plasma aldosterone significantly decreased from 41 +/- 8 to 8 +/- 3 ng/dl when sodium intake was elevated (n = 6). In a final set of experiments, the continuous infusion of ANG II (20 ng. kg(-1). min(-1)) led to a mild salt-sensitive increase in mean arterial pressure from 108 +/- 2 to 131 +/- 2 mmHg as sodium intake was varied from low to high (n = 7). In vehicle-infused mice, mean arterial pressure was unaltered from 109 +/- 2 mmHg when sodium intake was increased (n = 6). These studies indicate that the physiological suppression of circulating ANG II may be required to maintain a constancy of arterial pressure during alterations in sodium intake in normal mice.

Hyperplastic vascular smooth muscle cells of the intrarenal arteries in angiotensin II type 1a receptor null mutant mice

BACKGROUND

Angiotensin II (Ang II), which contracts vascular smooth muscle cells (VSMCs), has been reported to regulate VSMC growth. Recently formed transgenic mice without angiotensinogen or Ang II receptors showed vascular alterations. However, it is still unclear how their VSMCs alter. We explored the role of Ang II via the Ang II type 1a receptor (AT1a) in VSMCs in vivo using AT1a null mutant mice.

METHODS

We analyzed the ultrastructure of the intrarenal arteries in AT1a null mutant mice that were homozygous for a targeted disruption of AT1a receptor gene using light and electron microscopy.

RESULTS

The structural changes of the intrarenal arteries in AT1a null mutant mice showed the wall thickening, which in the interlobar, arcuate, and proximal interlobular arteries consisted of two additional populations of VSMCs, on the luminal and abluminal sides of the media. The luminal overpopulation of smooth muscle cells (SMCs) was arranged in a longitudinal direction separated by increased interposed elastic laminae. The abluminal overpopulation of SMCs ran in circumferential directions separated from the main population. The cytological structure of VSMCs in AT1a null mutant mice was smaller in size, contained more organelles for protein synthesis and secretion than in control mice, and had poorly developed contractile apparatus.

CONCLUSIONS

The lack of AT1a signaling causes structural abnormalities in the renal vascular system and transforms the phenotype of VSMCs into cell proliferation, induces the escape of VSMCs from the circular mechanical integrity, and results in increased synthesis of extracellular matrices.

Vascular smooth muscle cell-directed overexpression of heme oxygenase-1 elevates blood pressure through attenuation of nitric oxide-induced vasodilation in mice

To elucidate pathophysiological roles of heme oxygenase (HO)-1 in regulation of vascular tone in vivo, we have developed and characterized transgenic (Tg) mice that overexpress HO-1 site specifically in vascular smooth muscle cells (VSMCs). The Tg mice were generated by use of human HO-1 cDNA under the control of SM22-alpha promoter. The HO-1 gene overexpression was demonstrated by Northern blot analysis and coincided with increases in the protein expression in VSMCs and total HO activities. Tg mice exhibited a significant increase in arterial pressure at various ages and displayed impaired nitrovasodilatory responses in isolated aortic segments versus nontransgenic littermates while enhancing their nitric oxide (NO) production. The pressure of Tg mice was unchanged by systemic administration of either N(omega)-nitro-L-arginine or SNP. Furthermore, the isolated aorta in these mice exhibited lesser extents of NO-elicited cGMP elevation via soluble guanylate cyclase (sGC), while exhibiting no notable downregulation of sGC expression. Such impairment of the NO-elicited cGMP increase was restored significantly by tin protoporphyrin IX, an HO inhibitor. On the other hand, 3-(5'-hydroxymethyl-2' furyl)-1-benzyl-indazol (YC-1), an NO-independent activator of sGC, increased cGMP and relaxed aortas from Tg mice to levels comparable with those from nontransgenic mice, which indicates that contents of functionally intact sGC are unlikely to differ between the two systems. These findings suggest that site-specific overexpression of HO-1 in VSMCs suppresses vasodilatory response to NO and thereby leads to an elevation of arterial pressure.

Collectrin, a collecting duct-specific transmembrane glycoprotein, is a novel homolog of ACE2 and is developmentally regulated in embryonic kidneys

Collectrin, a novel homolog of angiotensin-converting enzyme-related carboxypeptidase (ACE2), was identified during polymerase chain reaction-based cDNA subtraction and up-regulated in 5/6 ablated kidneys at hypertrophic phase. Collectrin, with 222 amino acids, has an apparent signal peptide and a transmembrane domain; the sequence is conserved in mouse, rat, and human and shares 81.9% identity. Human collectrin has 47.8% identity with non-catalytic extracellular, transmembrane, and cytosolic domains of ACE2; however, unlike ACE and ACE2, collectrin lacks active dipeptidyl carboxypeptidase catalytic domains. The collectrin mRNA transcripts are expressed exclusively in the kidney. In situ hybridization reveals its mRNA expression in renal collecting ducts, and immunohistochemistry shows that it is localized to the luminal surface and cytoplasm of collecting ducts. Immunoprecipitation studies, using [35S]methionine-labeled renal cortical and inner medullar collecting duct cells, i.e. M-1 and mIMCD-3, indicate that the protein size is approximately 32 kDa. During the development of mouse kidney, mRNA signal is detectable at day 13 of gestation, and the protein product is observed in the ureteric bud branches. Its expression is progressively increased during later stages of the gestation extending into the neonatal periods and then is decreased in adult life. Up-regulated expression of collectrin in the hypertrophic kidneys after renal ablation and restricted spatio-temporal expression during development indicates a possible role(s)in the process of progressive renal failure and renal organogenesis.

Enhanced central response to dehydration in mice lacking angiotensin AT(1a) receptors

The objective was to determine the central nervous system (CNS) responses to dehydration (c-Fos and vasopressin mRNA) in mice lacking the ANG AT(1a) receptor [ANG AT(1a) knockout (KO)]. Control and AT(1a) KO mice were dehydrated for 24 or 48 h. Baseline plasma vasopressin (VP) was not different between the groups; however, the response to dehydration was attenuated in AT(1a) KO (24 +/- 11 vs. 10.6 +/- 2.7 pg/ml). Dehydration produced similar increases in plasma osmolality and depletion of posterior pituitary VP content. Neuronal activation was observed as increases in c-Fos protein and VP mRNA. The supraoptic responses were not different between groups. In the paraventricular nucleus (PVN), c-Fos-positive neurons (57.4 +/- 10.7 vs. 98.4 +/- 7.4 c-Fos cells/PVN, control vs. AT(1a) KO) and VP mRNA levels (1.0 +/- 0.1 vs. 1.4 +/- 0.1 microCi, control vs. AT(1a) KO) were increased with greater responses in AT(1a) KO. A comparison of 1- to 2-day water deprivation showed that plasma VP, brain c-Fos, and VP mRNA returned toward control on day 2, although plasma osmolality remained high. Data demonstrate that AT(1a) KO mice show a dichotomous response to dehydration, reduced for plasma VP and enhanced for PVN c-Fos protein and VP mRNA. The results illustrate the importance of ANG AT(1a) receptors in the regulation of osmotic and endocrine balance.

Angiotensin II and calcium channels

Sixty years after its initial discovery, the octapeptide hormone angiotensin II (AngII) has proved to play numerous physiological roles that reach far beyond its initial description as a hypertensive factor. In spite of the host of target tissues that have been identified, only two major receptor subtypes, AT1 and AT2, are currently fully identified. The specificity of the effects of AngII relies upon numerous and complex intracellular signaling pathways that often mobilize calcium ions from intracellular stores or from the extracellular medium. Various types of calcium channels (store- or voltage-operated channels) endowed with distinct functional properties play a crucial role in these processes. The activity of these channels can be modulated by AngII in a positive and/or negative fashion, depending on the cell type under observation. This chapter reviews the main characteristics of AngII receptor subtypes and of the various calcium channels as well as the involvement of the multiple signal transduction mechanisms triggered by the hormone in the cell-specific modulation of the activity of these channels.

Renal interstitial fibrosis is reduced in angiotensin II type 1a receptor-deficient mice

Unilateral ureteral obstruction (UUO) results in tubulointerstitial fibrosis of the affected kidney by stimulating the renin-angiotensin system. This study established a UUO model in angiotensin type 1a receptor (AT1a) deficient (mutant) mice to elucidate the role of angiotensin II through AT1a on the fibrosis of the obstructed kidney (OBK). The relative volume of the tubulointerstitium was measured by an image analyzer; deposition of collagen types III and IV and monocyte/macrophage infiltration were histologically examined using specific antibodies. Also determined were the mRNA levels of transforming growth factor-beta by Northern blot analysis. Nuclear factor-kappaB activity was assessed by gel shift assay. UUO in wild mice resulted in a marked expansion of relative volume of the tubulointerstitium, together with increased deposition of collagen types III and IV and number of infiltrated monocytes/macrophages in the interstitium, relative to sham-operated mice. In comparison, these changes were significantly lower in mutant mice with UUO. The mRNA level of transforming growth factor-beta was significantly higher in the OBK of wild mice with UUO compared with sham-operated mice. In contrast, the increase in mRNA level in the OBK of mutant mice was significantly less than in wild mice. Finally, UUO resulted in activation of nuclear factor-kappaB in wild mice but was inhibited in the OBK of mutant mice. The results provide direct evidence that angiotensin II acting via the AT1a plays a pivotal role in the development of tubulointerstitial fibrosis in UUO.

Mouse models of blood pressure regulation and hypertension

Human essential hypertension is recognized as a multifactorial disease involving many genes, but the causative genes have not yet been identified. For many years hypertension was studied primarily in the rat, but more recently several candidate genes for hypertension have been used to produce transgenic mice for gain of function and gene-targeted mice for loss of function studies. These genetically engineered mouse strains with hypertension or hypotension are providing insights into the mechanisms of blood pressure regulation. However, genetically engineered mice are used to study one gene at a time, and another complementary approach is needed for polygenic inheritance and gene interaction. The phenotype-driven approach to hypertension studies uses the natural variation among inbred strains and crosses to find quantitative trait loci. The four mouse crosses carried out so far have found several quantitative trait loci that are concordant with hypertension loci found in rats and humans.

Knockout of renin-angiotensin system genes: effects on vascular development

Pharmacologic inhibition of the renin-angiotensin system (RAS) is a widely accepted and effective treatment for hypertension. However, in the past several years, much attention has been focused on additional roles of the RAS including the possibility that its end-product, angiotensin II, could elicit end-organ pathologies independent of its effect on blood pressure. The ability to selectively delete genes in mice (by homologous recombination or gene knockouts) has led to new--and sometimes surprising--insights into the roles of the RAS in the developmental modeling and pathologic remodeling of the heart and blood vessels.

Transgenic mouse models of angiotensin receptor subtype function in the cardiovascular system

Angiotensin II mediates is biological actions via different subtypes of G protein-coupled receptors, termed AT(1) and AT(2) receptors. In rodents, two AT(1) receptors have been identified, AT(1A) and AT(1B), whereas in humans a single AT(1) receptor exists. Recently, a number of transgenic animal models have been generated which overexpress or lack functional angiotensin II receptor subtypes. This review focuses on the physiological significance of angiotensin II receptor subtype diversity in the cardiovascular system. In the mouse, AT(1A) receptors are the major regulators of cardiovascular homeostasis by determining vascular tone and natriuresis. In addition, AT(1A) receptors mediate growth-stimulating signals in vascular and cardiac myocytes. AT(1B) receptors participate in blood pressure regulation, and their functions become apparent when the AT(1A) receptor gene is deleted. Deletion of the mouse gene for the AT(2) receptor subtype led to hypersensitivity to pressor and antinatriuretic effects of angiotensin II in vivo, suggesting that the AT(2) receptor subtype counteracts some of the biological effects of AT(1) receptor signalling.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

Importance of quantitative genetic variations in the etiology of hypertension

Recent progress has been remarkable in identifying mutations which cause diseases (mostly uncommon) that are inherited simply. Unfortunately, the common diseases of humankind with a strong genetic component, such as those affecting cardiovascular function, have proved less tractable. Their etiology is complex with substantial environmental components and strong indications that multiple genes are implicated. In this article, we consider the genetic etiology of essential hypertension. After presenting the distribution of blood pressures in the population, we propose the hypothesis that essential hypertension is the consequence of different combinations of genetic variations that are individually of little consequence. The candidate gene approach to finding relevant genes is exemplified by studies that identified potentially causative variations associated with quantitative differences in the expression of the angiotensinogen gene (AGT). Experiments to test causation directly are possible in mice, and we describe their use to establish that blood pressures are indeed altered by genetic changes in AGT expression. Tests of differences in expression of the genes coding for the angiotensin-converting enzyme (ACE) and for the natriuretic peptide receptor A are also considered, and we provide a tabulation of all comparable experiments in mice. Computer simulations are presented that resolve the paradoxical finding that while ACE inhibitors are effective, genetic variations in the expression of the ACE gene do not affect blood pressure. We emphasize the usefulness of studying animals heterozygous for an inactivating mutation and a wild-type allele, and briefly discuss a way of establishing causative links between complex phenotypes and single nucleotide polymorphisms.

LXRalpha functions as a cAMP-responsive transcriptional regulator of gene expression

LXRalpha is a member of a nuclear receptor superfamily that regulates transcription. LXRalpha forms a heterodimer with RXRalpha, another member of this family, to regulate the expression of cholesterol 7alpha-hydroxylase by means of binding to the DR4-type cis-element. Here, we describe a function for LXRalpha as a cAMP-responsive regulator of renin and c-myc gene transcriptions by the interaction with a specific cis-acting DNA element, CNRE (an overlapping cAMP response element and a negative response element). Our previous studies showed that renin gene expression is regulated by cAMP, at least partly, through the CNRE sequence in its 5'-flanking region. This sequence is also found in c-myc and several other genes. Based on our cloning results using the yeast one-hybrid system, we discovered that the mouse homologue of human LXRalpha binds to the CNRE and demonstrated that it binds as a monomer. To define the function of LXRalpha on gene expression, we transfected the renin-producing renal As4.1 cells with LXRalpha expression plasmid. Overexpression of LXRalpha in As4.1 cells confers cAMP inducibility to reporter constructs containing the renin CNRE. After stable transfection of LXRalpha, As4.1 cells show a cAMP-inducible up-regulation of renin mRNA expression. In parallel experiments, we demonstrated that LXRalpha can also bind to the homologous CNRE in the c-myc promoter. cAMP promotes transcription through c-myc/CNRE:LXRalpha interaction in LXRalpha transiently transfected cells and increases c-myc mRNA expression in stably transfected cells. Identification of LXRalpha as a cAMP-responsive nuclear modulator of renin and c-myc expression not only has cardiovascular significance but may have generalized implication in the regulation of gene transcription.

Angiotensin II and the heart : on the intracrine renin-angiotensin system

-The active end product of the renin-angiotensin system, angiotensin II (Ang II), through the activation of specific Ang II receptors, regulates cardiac contractility, cell coupling, and impulse propagation and is involved in cardiac remodeling, growth, and apoptosis. We review these subjects, as well as the second messengers that are involved, and the synthesis of Ang II in the heart under normal and pathological conditions. Finally, we discuss the possibility that there is an intracrine renin-angiotensin system in the heart that plays a role in the control of cell communication and inward Ca(2+) current.

Insights derived from ACE knockout mice

The evaluation of ACE knockout mice has illustrated the tremendous physiologic importance of the RAAS. We have discussed how interruption of this system influences blood pressure, renal function, renal development, serum and urine electrolyte composition, haematocrit and male reproductive capacity. This body of data underlines the modelling of the RAAS as a type of biological machine that is positioned to respond to environmental insult and to maintain a homeostasis of blood pressure, blood volume and electrolyte composition. These data also emphasise Harry Goldblatt's seminal observation that the kidney and the RAAS are intimately linked in the regulation of normal blood pressure.

Understanding hypertension through genetic manipulation in mice

With the advances in mouse molecular genetics and physiology during the last decade, the mouse has become the animal model of choice for studying the genetic basis of many diseases. Terms such as "transgenic" and "knockout" have become part of a colloquial language used in most research laboratories that are investigating human diseases. These terms refer to the two most commonly used methods for analyzing the function of a gene in vivo: overexpression (transgenic mouse) and deletion (knockout mouse). Both methods have proved to be extremely useful in establishing the importance of specific genes in genetic disorders, such as hypertension. The choice of genes being investigated in relationship to hypertension was governed by the knowledge of systems regulating vascular and renal physiology. Thus, it is not surprising that most of the focus was given to the renin-angiotensin system (RAS). Apart from the RAS, other systems known to regulate vascular tone and/or electrolyte and fluid homeostasis have also been analyzed using transgenic and knockout approaches. This review briefly summarizes some of the mouse models relevant to renal mechanisms of hypertension and then discusses the future of genetic manipulation in mice for studying the genetics of hypertension.

Regulation of sodium balance and blood pressure by the AT(1A) receptor for angiotensin II

To examine the role of the angiotensin II (AT)(1A) receptor in the regulation of blood pressure and sodium balance, we measured systolic blood pressure responses in AT(1A) receptor-deficient (Agtr1a-/-) and wild-type (Agtr1a+/+) mice while dietary sodium content was systematically altered. On a 0.4% sodium diet, systolic blood pressures were significantly lower in Agtr1a-/- than in +/+ mice. In Agtr1a+/+ mice, changing dietary sodium content did not affect blood pressure. In contrast, when Agtr1a-/- mice were fed a high-salt diet (6% NaCl), their systolic blood pressures increased significantly from 79+/-4 to 94+/-4 mm Hg (P<0.006). The low blood pressures of Agtr1a-/- mice decreased further while on a low-salt diet from 82+/-3 to 69+/-3 mm Hg (P<0.03). On the high-salt diet, urinary sodium excretion increased to similar levels in Agtr1a+/+ and -/- mice. Although urinary sodium excretion was substantially reduced in both groups during the low-salt diet, cumulative sodium balances became negative in Agtr1a-/- mice despite a 6-fold increase in urinary aldosterone. We infer, therefore, that the reduced blood pressures in Agtr1a-/- mice on a normal diet are caused by depletion of sodium and extracellular volume. Their "sodium sensitivity" suggests a critical role for renal AT(1A) receptors to modulate sodium handling.

Angiotensin II type 1A receptor knockout mice display less left ventricular remodeling and improved survival after myocardial infarction

BACKGROUND

Angiotensin II (Ang II) has been implicated in ventricular remodeling after myocardial infarction (MI), which is an important determinant for prognosis after MI. The aim of this study was to determine whether Ang II type 1A receptor (AT(1A))-mediated Ang II signals are critically involved in the mortality and LV remodeling after MI.

METHODS AND RESULTS

We examined survival, cardiac geometry and function, cardiac fibrosis, and gene expression of AT(1A) knockout (KO) mice and wild-type (WT) mice at 1 and 4 weeks after large MI. The survival rate was higher in KO mice than in WT mice at 4 weeks after MI. All WT survivors showed severe heart failure, detected by marked increases in both RV weight and lung weight. LV remodeling, such as the development of LV dilatation, LV dysfunction, and cardiac fibrosis at the noninfarcted area, were comparable in both kinds of mice at 1 week after MI. At 4 weeks after MI, however, WT mice showed more marked remodeling than KO mice. mRNA levels of AT(1) at the noninfarcted area were increased from 1 to 4 weeks after MI only in WT mice, whereas levels of AT(2) were not changed by MI in either kind of mouse. Accompanied by the development of geometric and structural remodeling, expression of fetal-type genes, collagen, and transforming growth factor-beta(1) genes were upregulated and sustained in the noninfarcted area of WT hearts. In contrast, they were rapidly downregulated to basal levels at 4 weeks after MI in that of KO hearts.

CONCLUSIONS

These results indicate that AT(1A) signals play a pivotal role in the progression of LV remodeling after MI, resulting in overt heart failure.

Renal renin-angiotensin system dysregulation caused by partial bladder outlet obstruction in fetal sheep

BACKGROUND

To determine whether fetal renal obstruction activates the renal renin-angiotensin system (RAS), an important mediator in normal kidney development and obstructive nephropathy, we used a model of fetal partial bladder outlet obstruction (PBOO).

METHODS

Total RNA and protein was extracted from kidney of sheep fetuses with partial bladder outlet obstruction created at 95 days gestation, after 2 (N = 6) and 5 weeks of obstruction (term; N = 6), and from normal fetal sheep at various time points between 60 and 135 days of gestation (total N = 19). Relative levels of mRNA for renin, angiotensinogen, type 1 and 2 angiotensin II (Ang II) receptors (AT-1 and AT-2), and transforming growth factor-beta1 (TGF-beta1) were assessed by semiquantitative reverse transcription-polymerase chain reaction. Expression levels of AT-2 receptor protein were measured by Western blot analysis.

RESULTS

Renin mRNA expression was increased (250%) after two weeks of obstruction. In normal fetuses, AT-1 expression was low at 60 to 75 days of gestation and increased toward the end of gestation, whereas AT-2 expression showed a reversed pattern. At 109 days, PBOO caused an increased expression of AT-2 mRNA compared with normals (400%). Correspondingly, AT-2 receptor protein was more abundant in obstructed kidneys. TGF-beta1 mRNA expression was significantly increased in obstructed kidneys at 109 days gestation.

CONCLUSIONS

These observations confirm the reciprocal developmental regulation of AT-1 and AT-2 receptors' expression, suggesting their functional role in renal development. Partial bladder outlet obstruction produces specific alterations: increased renin expression and altered balance of receptor subtypes, which may induce altered functional and vascular regulation of the obstructed fetal kidney. TGF-beta1, a mediator of Ang II-induced fibrosis, may play a role in inducing and propagating interstitial fibrosis.

Transgenic and knockout models in renin-angiotensin system

The renin-angiotensin system, composed of enzymatic and signal-transduction cascades, plays a key role in the regulation of arterial blood pressure and in the development of certain forms of experimental and human hypertension. The products of this system, angiotensin peptides, exert a wide range of physiologically important effects on many tissues, including those of the cardiovascular systems, through their actions on angiotensin receptors. Molecular genetic and transgenic studies have begun to implicate some of the genes encoding components of the renin-angiotensin system in the development of cardiovascular diseases. Recently, we succeeded in generating transgenic mice with chronic hypertension and with inducible hypertension during pregnancy and in creating mice homozygous for a targeted disruption of the angiotensinogen gene (the only known precursor of angiotensins), resulting in the complete loss of angiotensin signals in vivo. Here, we will review recent advances related to the functional analysis of the renin-angiotensin system, in particular by focusing on transgenic approaches including gene targeting.

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 II type 2 receptor overexpression activates the vascular kinin system and causes vasodilation

Angiotensin II (Ang II) is a potent vasopressor peptide that interacts with 2 major receptor isoforms - AT1 and AT2. Although blood pressure is increased in AT2 knockout mice, the underlying mechanisms remain undefined because of the low levels of expression of AT2 in the vasculature. Here we overexpressed AT2 in vascular smooth muscle (VSM) cells in transgenic (TG) mice. Aortic AT1 was not affected by overexpression of AT2. Chronic infusion of Ang II into AT2-TG mice completely abolished the AT1-mediated pressor effect, which was blocked by inhibitors of bradykinin type 2 receptor (icatibant) and nitric oxide (NO) synthase (L-NAME). Aortic explants from TG mice showed greatly increased cGMP production and diminished Ang II-induced vascular constriction. Removal of endothelium or treatment with icatibant and L-NAME abolished these AT2-mediated effects. AT2 blocked the amiloride-sensitive Na(+)/H(+) exchanger, promoting intracellular acidosis in VSM cells and activating kininogenases. The resulting enhancement of aortic kinin formation in TG mice was not affected by removal of endothelium. Our results suggest that AT2 in aortic VSM cells stimulates the production of bradykinin, which stimulates the NO/cGMP system in a paracrine manner to promote vasodilation. Selective stimulation of AT2 in the presence of AT1 antagonists is predicted to have a beneficial clinical effect in controlling blood pressure.

1166 A/C polymorphism of the angiotensin II type 1 receptor gene and the response to short-term infusion of angiotensin II

BACKGROUND

Previous studies reported an association of the 1166 A/C polymorphism of the angiotensin II (Ang II) type 1 receptor gene with high blood pressure and cardiovascular disease. We tested the hypothesis that this polymorphism affects the blood-pressure, renal hemodynamic, and aldosterone response to infused Ang II.

METHODS AND RESULTS

Young, male, white volunteers (n = 116) with normal (n = 65) or mildly elevated (n = 51) blood pressure on a high salt intake were genotyped for the 1166 A/C polymorphism. Two doses of Ang II (0.5 and 3 ng x kg(-1) x min(-1) over 30 minutes each) increased blood pressure, plasma aldosterone, glomerular filtration rate, and filtration fraction and decreased renal blood flow. The blood-pressure, renal hemodynamic, and aldosterone responses were not significantly different between subjects homozygous for the A allele (n = 56) and heterozygous subjects (n = 47) or subjects homozygous for the C allele (n = 13). Comparison of A allele homozygotes with all C allele carriers pooled (n = 60) or restriction of the analysis to normotensive volunteers also revealed no significant differences between genotypes.

CONCLUSIONS

The 1166 C variant of the Ang II type 1 receptor does not lead to a greater blood-pressure, aldosterone, or renal vascular response to infused Ang II in young, male, white subjects. We conclude that the 1166 A/C polymorphism does not have a major effect on these actions of Ang II.

Gene targeting approaches to analyzing hypertension

Essential hypertension probably results from combinations of small genetic variations that are partly normal variations and may not be appreciably harmful individually. Strategies to identify genes contributing to hypertension are discussed in this review. Gene targeting approaches, especially gene titration, have been used in these studies of hypertension. Gene titration experiments vary the expression of a chosen gene product by generating animals having different numbers of copies of the gene coding for the product. Gene titration is powerful for analyzing quantitative variations seen in common polygenic disorders, such as kidney diseases, diabetes mellitus, and atherosclerosis, as well as hypertension, because it allows tests of causation by determining the effects on a phenotype by changes in expression of the altered gene and because it matches normal quantitative variations more closely than is possible with classic transgenic mice. The use of zero-copy (gene "knockout") animals generated by gene disruption for studies of qualitative gene effects is also discussed. These various gene targeting experiments help identify genes regulating BP, promote a better understanding of the pathophysiology of the condition, and help identify potential targets for therapies.

Angiotensin II is a growth factor in the peri-implantation rat embryo

Angiotensin II (ANG II) is increasingly recognised as a growth factor, both in its own right and through interactions with other growth factors. There is a high density of ANG II receptors in the rat fetus, especially the AT2 receptor, the function of which is still uncertain. We have now studied the effects of ANG II on growth and development in the rat embryo in vitro between d 9.5 and 11.5, and characterised the receptor subtype mediating these effects. Embryos were cultured in whole rat serum, a high molecular weight retenate after ultrafiltration of whole rat serum, retenate with angiotensin II and retenate with ANG II and AT1 or AT2 receptor blockers. Growth and development were scored using conventional methods. Culture in retenate was associated with a marked reduction in growth and development by comparison with whole rat serum. This was partly, and significantly (P < 0.001), reversed by angiotensin II. The optimum concentration of angiotensin II was found to be angiotensin II 10(-11) M, within the physiological range. Angiotensin II had highly significant effects on both somatic (P < 0.001) and yolk sac/allantoic (P < 0.005) development. The latter effects suggest a role for angiotensin II in placentation. The effects of angiotensin II were blocked by PD123319, an AT2 blocker, but not by GR117289, an AT1 blocker. Interestingly, culture in retenate with GR117289 without added angiotensin II was also associated with some increase in growth (P < 0.05). Angiotensin II in low concentrations was measurable in the retenate, presumably arising from the action of endogenous renin on angiotensinogen. We therefore postulate that this effect of GR117289 was due to the action of endogenous angiotensin II on 'uncovered' AT2 receptors. This study has thus demonstrated a direct growth promoting effect of angiotensin II during organogenesis in the whole rat embryo in vitro. This effect is mediated through the AT2 receptors.

Angiotensin receptors: molecular biology and signalling

1. The active peptide hormone angiotensin II (AngII) is formed from its prohormone angiotensinogen by way of inactive angiotensin I. The highly specific protease, renin, responsible for the initiation of this system was elusive and considered unstable. We isolated it in a pure and stable form from the kidney of the pig, human, rat, and land submandibular glands of the mouse. It was shown that there is only one type of renin with highly stringent substrate specificity, except certain strains of the mouse which have two gene products. 2. The well-known diversity of action of AngII can be attributed to the presence of more than two subtypes, AT1 and AT2, as well as multiple signalling pathways for both of them. 3. The first subtype AT1 was shown to mediate most of the traditionally recognized AngII functions such as vasoconstriction, electrolyte homeostasis etc. 4. Although the identification of the signalling modes of the second subtype AT2 still remains elusive, we and others have shown evidence that its action is generally antagonistic to that of AT1. AT2 inhibits AT1 (growth factor-stimulated cell growth), AT2 attenuates the vasoconstriction induced by AT1. Since AT2 seems to mediate nitric oxide formation in the renal cells, it may initiate a natriuretic pathway in contrast to the sodium-retaining action of AT1-mediated AngII action. 5. Newer mechanisms and functions of these and other receptors will be clarified by the combination of molecular, cellular and integrated physiological studies.

Cardiac hypertrophy: sorting out the circuitry

Cardiac hypertrophy is an adaptive response of the heart to a variety of intrinsic and extrinsic stimuli. The hypertrophic response, during which cardiomyocytes increase in size without undergoing cell division, initially serves to compensate for decreased cardiac output; however, prolonged hypertrophy can become detrimental, resulting in dilated cardiomyopathy and heart failure. Cardiac hypertrophy requires coupling of intracellular signal transduction systems with transcription factors that activate and maintain the hypertrophic program. Over the past year, signaling pathways involving G proteins, mitogen-activated protein kinases and calcium-responsive phosphatases have emerged as critical regulators of cardiac hypertrophy.

Angiotensin II plays a pathogenic role in immune-mediated renal injury in mice

Several lines of evidence show the importance of angiotensin II (AII) in renal injuries, especially when hemodynamic abnormalities are involved. To elucidate the role of AII in immune-mediated renal injury, we studied anti-glomerular basement membrane (GBM) nephritis in AII type 1a receptor (AT1a)-deficient homozygous (AT1a-/-) and wild-type (AT1a+/+) mice. A transient activation of the renin-angiotensin system (RAS) was observed in both groups of mice at around day 1. A renal expression of monocyte chemoattractant protein-1 (MCP-1) was transiently induced at six hours in both groups, which was then downregulated at day 1. In the AT1a+/+ mice, after RAS activation, the glomerular expression of MCP-1 was exacerbated at days 7 and 14. Thereafter, severe proteinuria developed, and the renal expressions of transforming growth factor-beta1 (TGF-beta1) and collagen type I increased, resulting in severe glomerulosclerosis and interstitial fibrosis. In contrast, glomerular expression of MCP-1, proteinuria, and tissue damage were markedly ameliorated in the AT1a-/- mice. Because this amelioration is likely due to the lack of AT1a, we can conclude that AII action, mediated by AT1a, plays a pathogenic role in anti-GBM nephritis, in which AII may contribute to the exacerbation of glomerular MCP-1 expression. These results suggest the involvement of AII in immune-mediated renal injuries.

Vascular injury causes neointimal formation in angiotensin II type 1a receptor knockout mice

Many studies using small-animal models suggest that angiotensin II (Ang II) plays an important role in neointimal formation after vascular injury. In the present study, we examined whether Ang II type 1 receptor (AT1)-mediated Ang II signaling is indispensable for the development of injury-induced neointimal formation using AT1a knockout (KO) mice. Reverse transcriptase-polymerase chain reaction analysis revealed that AT1 mRNA was not detectable in both uninjured and injured carotid arteries of KO mice, whereas the AT1 gene was expressed in uninjured carotid arteries of wild-type (WT) mice. At 14 days after injury, AT1 mRNA levels were increased by 1.5-fold in injured arteries of WT mice. Although AT2 mRNA was not detectable in uninjured arteries, expression of AT2 gene was induced in both animal groups at 2 weeks after injury. Vascular injury induced neointimal formation in KO mice as well as in WT mice. There were no significant differences between WT and KO mice in the extent of histological findings such as increased cross-sectional areas of the neointima and the media, the number of proliferating smooth muscle cells, and the amount of collagen and fibronectin. Treatment with subpressor doses of Ang II after injury enhanced the growth of neointima in WT mice but not in KO mice. Moreover, treatment with the selective AT1 antagonist CV-11974 before injury significantly decreased the formation of neointima in only WT mice, whereas treatment with the selective AT2 antagonist PD-123319 before injury had no effects in both animal groups. These results suggest that AT1-mediated Ang II signaling is not essential for the development of neointimal formation, although it may modify it.

Effects of mineralocorticoid receptor gene disruption on the components of the renin-angiotensin system in 8-day-old mice

Targeted disruption of mineralocorticoid receptor (MR) gene results in pseudohypoaldosteronism type I with failure to thrive, severe dehydration, hyperkalemia, hyponatremia, and high plasma levels of renin, angiotensin II, and aldosterone. In this study, mRNA expression of the different components of the renin-angiotensin system (RAS) were evaluated in liver, lung, heart, kidney and adrenal gland to assess their response to a state of extreme sodium depletion. Angiotensinogen, renin, angiotensin-I converting enzyme, and angiotensin II receptor (AT1 and AT2) mRNA expressions were determined by Northern blot and RT-PCR analysis. Furthermore, in situ hybridization and immunohistochemistry allowed us to identify the cell types involved in the variation of the RAS component expression. In the heterozygous mice (MR+/-), compared with wild-type mice (MR+/+), there was no significant variation of any mRNA of the RAS components. In MR knockout mice (MR-/-), compared with wild-type mice, there were significant increases in the expression level of several RAS components. In the liver, angiotensinogen and AT1 receptor mRNA expressions were moderately stimulated. In the kidney, renin mRNA was increased up to 10-fold and in situ hybridization showed a marked recruitment of renin-producing cells; however, the levels of angiotensin-I converting enzyme mRNA and AT1 mRNA were not changed. Interestingly, in adrenal gland, renin expression was also strongly up-regulated in a thickened zona glomerulosa, whereas AT1 mRNA expression remained unchanged. Altogether, these results demonstrate that in the MR knockout mice model, RAS component expressions are differentially altered, renin being the most stimulated component. Angiotensinogen and AT1 in the liver are also increased, but the other elements of the RAS are not affected.

Renin-angiotensin system and fibronectin gene expression in Dahl Iwai salt-sensitive and salt-resistant rats

OBJECTIVE

The tissue renin-angiotensin system and extracellular matrix are involved in the cardiovascular hypertrophy and remodeling induced by hypertension. In this study, we examined the gene expression of the tissue renin-angiotensin system and fibronectin in inbred Dahl Iwai salt-sensitive and salt-resistant rats.

MATERIALS AND METHODS

Eight pairs of 6-week-old male Dahl Iwai salt-sensitive and salt-resistant rats were fed either a low- or high-salt diet (0.3% or 8% NaCl, respectively) for 4 weeks. Activities of the circulating renin-angiotensin system were measured by radioimmunoassay and the gene expression of tissue angiotensinogen, the angiotensin II type 1 receptor (AT1) and fibronectin were analyzed by Northern blot analysis.

RESULTS

Salt loading significantly increased blood pressure and produced cardiovascular hypertrophy and nephrosclerosis in the salt-sensitive rats. Activities of the circulating renin-angiotensin system were lower in salt-sensitive rats than in salt-resistant rats fed the low-salt diet, and salt loading lowered these activities in salt-resistant rats but not in salt-sensitive rats. In salt-resistant rats, salt loading increased renal, cardiac and aortic angiotensinogen, AT1 and fibronectin messenger (m)RNA expression except for aortic fibronectin mRNA expression. In contrast, in the salt-sensitive rats, salt loading stimulated the expression of cardiac fibronectin and aortic angiotensinogen, AT1 and fibronectin mRNAs. Furthermore, the cardiac and aortic fibronectin mRNA levels in salt-sensitive rats were higher than those in salt-resistant rats when both strains were fed the high-salt diet.

CONCLUSIONS

These results demonstrate that the expression of tissue angiotensinogen, AT1 and fibronectin mRNAs is regulated differently in Dahl Iwai salt-sensitive and salt-resistant rats, and indicate that salt-mediated hypertension activates the cardiac fibronectin gene independently of the tissue renin-angiotensin system and stimulates the aortic fibronectin gene with activation of the tissue renin-angiotensin system.

Neuroendocrine effects of dehydration in mice lacking the angiotensin AT1a receptor

Angiotensin (Ang) type 1a (AT1a) receptors are critical in the control of blood pressure and water balance. Experiments were performed to determine the influence of dehydration on brain Ang receptors and plasma vasopressin (VP) in mice lacking this receptor. Control or AT1a knockout (AT1aKO) male mice were give water ad libitum or deprived of water for 48 hours. Animals were anesthetized with halothane, blood samples were collected by heart puncture, and brains were processed for Ang-receptor autoradiography with 125I-sarthran (0.4 nmol/L). Dehydration produced an increase in AT1 receptors in the paraventricular nucleus (PVN) and anterior pituitary (AP) in control mice (PVN: 70+/-16 versus 146+/-10 fmol/mg protein; AP: 41+/-7 versus 86+/-15 fmol/mg protein). No changes were noted in the median preoptic nucleus. The majority of the brain receptors were of the AT1 subtype. There was little or no specific Ang binding in AT1aKO mice and no effect of dehydration. Plasma VP levels were elevated in the halothane-anesthetized animals (>200 pg/mL) with no significant effect of dehydration. A separate experiment was performed with decapitated mice anesthetized with pentobarbital. Dehydration increased plasma VP in control mice, from 3.3+/-0.6 to 13.3+/-4.7 pg/mL, whereas no change was noted in the AT1aKO mice, 5.1+/-0.3 versus 6.1+/-0.7 pg/mL (water versus dehydration). These results demonstrate a differential response to dehydration in mice lacking AT1a receptors. There was no evidence for AT1 receptors of any subtype in the brain regions examined and no effect of dehydration on VP secretion or brain Ang receptors.

Reduced growth, abnormal kidney structure, and type 2 (AT2) angiotensin receptor-mediated blood pressure regulation in mice lacking both AT1A and AT1B receptors for angiotensin II

The classically recognized functions of the renin-angiotensin system are mediated by type 1 (AT1) angiotensin receptors. Whereas man possesses a single AT1 receptor, there are two AT1 receptor isoforms in rodents (AT1A and AT1B) that are products of separate genes (Agtr1a and Agtr1b). We have generated mice lacking AT1B (Agtr1b -/-) and both AT1A and AT1B receptors (Agtr1a -/-Agtr1b -/-). Agtr1b -/- mice are healthy, without an abnormal phenotype. In contrast, Agtr1a -/-Agtr1b -/- mice have diminished growth, vascular thickening within the kidney, and atrophy of the inner renal medulla. This phenotype is virtually identical to that seen in angiotensinogen-deficient (Agt-/-) and angiotensin-converting enzyme-deficient (Ace -/-) mice that are unable to synthesize angiotensin II. Agtr1a -/-Agtr1b -/- mice have no systemic pressor response to infusions of angiotensin II, but they respond normally to another vasoconstrictor, epinephrine. Blood pressure is reduced substantially in the Agtr1a -/- Agtr1b -/- mice and following administration of an angiotensin converting enzyme inhibitor, their blood pressure increases paradoxically. We suggest that this is a result of interruption of AT2-receptor signaling. In summary, our studies suggest that both AT1 receptors promote somatic growth and maintenance of normal kidney structure. The absence of either of the AT1 receptor isoforms alone can be compensated in varying degrees by the other isoform. These studies reaffirm and extend the importance of AT1 receptors to mediate physiological functions of the renin-angiotensin system.

Pathophysiological role of angiotensin II type 2 receptor in cardiovascular and renal diseases

Since the discovery of nonpeptidic ligands, the receptors for angiotensin (Ang) II have been classified into 2 subtypes (Ang II type 1 receptor [AT1-R] and Ang II type 2 receptor [AT2-R]). AT1-R mediates most of the cardiovascular actions of Ang II. AT2-R is expressed at very high levels in the developing fetus. Its expression is very low in the cardiovascular system of the adult. The expression of AT2-R can be modulated by pathological states associated with tissue remodeling or inflammation. In failing hearts or neointima formation after vascular injury, AT2-R is reexpressed in cells proliferating in interstitial regions or neointima and exerts an inhibitory effect on Ang II-induced mitogen signals or synthesis of extracellular matrix proteins, resulting in attenuation of the tissue remodeling. An extreme form of cell growth inhibition ends in programmed cell death, and this process, which is initiated by the withdrawal of growth factors, is also enhanced by AT2-R. Cardiac myocyte- or vascular smooth muscle-specific mice that overexpress AT2-R display an inhibition of Ang II-induced chronotropic or pressor actions, suggesting the role of AT2-R on the activity of cardiac pacemaker cells and the maintenance of vascular resistance. AT2-R also activates the kinin/nitric oxide/cGMP system in the cardiovascular and renal systems, resulting in AT2-R-mediated cardioprotection, vasodilation, and pressure natriuresis. These effects, transmitted by AT2-R, are mainly exerted by stimulation of protein tyrosine or serine/threonine phosphatases in a Gi protein-dependent manner. The expression level of AT2-R is much higher in human hearts than in rodent hearts, and the AT2-R-mediated actions are likely enhanced, especially by clinical application of AT1-R antagonists. Thus, in this review, the regulation of AT2-R expression, its cellular localization, its pathological role in cardiovascular and kidney diseases, and pharmacotherapeutic effects of AT2-R stimulation are discussed.

Angiotensin II signal transduction pathways

It has been 100 years since the discovery of renin by Tigerstedt and Bergman. Since that time, numerous discoveries have advanced our understanding of the renin-angiotensin system, including the observation that angiotensin II is the effector molecule of this system. A remarkable aspect of angiotensin II is the many different physiological responses this simple peptide induces in different cell types. Here, we focus on the signal transduction pathways that are activated as a consequence of angiotensin II binding to the AT1 receptor. Classical signaling pathways such as the activation of heterotrimeric G proteins by the AT1 receptor are discussed. In addition, recent work examining the role of tyrosine phosphorylation in angiotensin II-mediated signal transduction is also examined. Understanding how these distinct signaling pathways transduce signals from the cell surface will advance our understanding of how such a simple molecule elicits such a wide variety of specific cellular responses.

Deferential roles of angiotensin receptor subtypes in adrenocortical function in mice

The functional significance of angiotensin II (Ang II) receptor subtypes in adrenals remains unknown. Ang II receptor type 1a (AT1a) expression was localized by in situ hybridization to the zona glomerulosa and zona fasciculata, while AT1b was localized to the zona glomerulosa. Plasma aldosterone and corticosterone levels were measured after injection with Ang II or the type 2 receptor (AT2) agonist CGP-42112 in wild-type and AT1a deficient mice. Aldosterone and corticosterone levels were lower in AT1a deficient mice. Ang II increased plasma aldosterone levels in AT1a deficient mice, but to a lesser extent in mice pretreated with nonselective AT1a/AT1b antagonist, CV-11974. CGP-42112 did not affect aldosterone levels. Ang II increased corticosterone levels in wild-type mice but not in AT1a deficient mice. Results suggest Ang II stimulates aldosterone secretion via AT1a and AT1b in the zona glomerulosa and corticosterone secretion via AT1a in the zona fasciculata, and provide first evidence for differential roles of AT1a and AT1b in the adrenals.

Chronic sodium balance and blood pressure response to captopril in conscious mice

The influence of chronic administration of the converting enzyme inhibitor captopril on blood pressure and sodium balance was evaluated in conscious Swiss Webster mice. Arterial pressure was measured with chronic indwelling catheters, and sodium balance was determined by infusing sodium intravenously in isotonic saline and collecting urine 24 h/d. Experiments to validate sodium balance measurements in mice demonstrated recovery of 100+/-3% of sodium intake under steady-state conditions (n=20 mice on 70 individual days, sodium intake range 160 to 1000 micromol/d). It was further demonstrated that mean arterial pressure, heart rate, and body weight were unaltered from 115+/-7 mm Hg, 646+/-12 bpm, and 34+/-0.6 g, respectively, as sodium intake was increased stepwise from 150 to 900 micromol NaCl per day. An additional validation group (n=7) demonstrated that daily and cumulative sodium balance can be accurately determined during and after the intravenous administration of an agent known to alter renal sodium handling (furosemide 50 mg. kg-1. d-1). Experiments were then performed to examine the influence of intravenous captopril infusion (40 mg. kg-1. d-1, n=7) in mice in which the daily sodium intake was fixed at approximately 200 micromol/d. This dose of captopril was determined to significantly decrease the pressor response to a 10-ng bolus of angiotensin I (Ang I) from 24+/-5 in the control state to 6+/-2 mm Hg (n=5). After 5 days of infusion of the converting enzyme inhibitor, mean arterial pressure significantly fell from 114+/-3 to 58+/-2 mm Hg, body weight significantly decreased from 36+/-1 to 33+/-1 g, and cumulative sodium balance significantly decreased to -270+/-55 micromol. These parameters returned toward control during 5 postcontrol days. Results of this study demonstrate that accurate sodium balance measurements can be obtained from individual conscious mice over a 5-fold range of sodium intake. The experiments also indicate that converting enzyme inhibition has a potent influence to lower blood pressure in normal mice; the hypotensive response appears to be due in part to increased urinary sodium excretion.

Angiotensin induces the urinary peristaltic machinery during the perinatal period

The embryonic development of mammalian kidneys is completed during the perinatal period with a dramatic increase in urine production, as the burden of eliminating nitrogenous metabolic waste shifts from the placenta to the kidney. This urine is normally removed by peristaltic contraction of the renal pelvis, a smooth muscle structure unique to placental mammals. Mutant mice completely lacking angiotensin type 1 receptor genes do not develop a renal pelvis, resulting in the buildup of urine and progressive kidney damage. In mutants the ureteral smooth muscle layer is hypoplastic and lacks peristaltic movements. We show that angiotensin can induce the ureteral smooth muscles in organ cultures of wild-type, but not mutant, ureteral tissues and that, in wild-type mice, expression of both renal angiotensin and the receptor are transiently upregulated at the renal outlet at birth. These results reveal a new role for angiotensin in the unique cellular adaptations of the mammalian kidney to the physiological stresses of postnatal life.

Mechanism of angiotensin II-mediated regulation of fibronectin gene in rat vascular smooth muscle cells

This study was performed to investigate a mechanism of angiotensin II (Ang II)-mediated activation of the fibronectin (FN) gene in rat vascular smooth muscle cells. Actinomycin D and CV11974 completely inhibited Ang II-mediated increase in FN mRNA levels. Inhibitors of protein kinase C (PKC), protein-tyrosine kinase (PTK), phosphatidylinositol-specific phospholipase C, Ras, phosphatidylinositol 3-kinase, p70 S6 kinase, and Ca2+/calmodulin kinase also decreased Ang II-induced activation of FN mRNA. In contrast, cycloheximide; PD123319; or inhibitors of Gi, protein kinase A, or mitogen-activated protein kinase kinase did not affect the induction. FN promoter contained a putative AP-1 binding site (rFN/AP-1; -463 to -437), and the results of a transient transfection and electrophoretic mobility shift assay showed that Ang II enhanced rFN/AP-1 activity. CV11974 and inhibitors of PKC or PTK suppressed Ang II-mediated increases in rFN/AP-1 activity, although neither PD123319 nor a protein kinase A inhibitor affected the induction. Furthermore, mutation of rFN/AP-1 that disrupted nuclear binding suppressed Ang II-induced transcription in the native FN promoter (-1908 to +136) context. Thus, Ang II activates transcription of the FN gene through the Ang II type 1 receptor in vascular smooth muscle cells, at least in part, via the activation of AP-1 by a signaling mechanism dependent on PKC and PTK.