The subtype 2 of angiotensin II receptors and pressure-natriuresis in adult rat kidneys

Brain Angiotensinergic Regulation of the Immune System: Implications for Cardiovascular and Neuroendocrine Responses

OBJECTIVE

The Renin-Angiotensin-Aldosterone System (RAAS) plays a major role in the regulation of cardiovascular functions, water and electrolytic balance, and hormonal responses. We perform a review of the literature, aiming at providing the current concepts regarding the angiotensin interaction with the immune system in the brain and the related implications for cardiovascular and neuroendocrine responses.

METHODS

Appropriate keywords and MeSH terms were identified and searched in Pubmed. Finally, references of original articles and reviews were examined.

RESULTS

Angiotensin II (ANG II), beside stimulating aldosterone, vasopressin and CRH-ACTH release, sodium and water retention, thirst, and sympathetic nerve activity, exerts its effects on the immune system via the Angiotensin Type 1 Receptor (AT 1R) that is located in the brain, pituitary, adrenal gland, and kidney. Several actions are triggered by the binding of circulating ANG II to AT 1R into the circumventricular organs that lack the Blood-Brain-Barrier (BBB). Furthermore, the BBB becomes permeable during chronic hypertension thereby ANG II may also access brain nuclei controlling cardiovascular functions. Subfornical organ, organum vasculosum lamina terminalis, area postrema, paraventricular nucleus, septal nuclei, amygdala, nucleus of the solitary tract and retroventral lateral medulla oblongata are the brain structures that mediate the actions of ANG II since they are provided with a high concentration of AT 1R. ANG II induces also T-lymphocyte activation and vascular infiltration of leukocytes and, moreover, oxidative stress stimulating inflammatory responses via inhibition of endothelial progenitor cells and stimulation of inflammatory and microglial cells facilitating the development of hypertension.

CONCLUSION

Besides the well-known mechanisms by which RAAS activation can lead to the development of hypertension, the interactions between ANG II and the immune system at the brain level can play a significant role.

Renal autoregulation in health and disease

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

Dominant factors that govern pressure natriuresis in diuresis and antidiuresis: a mathematical model

We have developed a whole kidney model of the urine concentrating mechanism and renal autoregulation. The model represents the tubuloglomerular feedback (TGF) and myogenic mechanisms, which together affect the resistance of the afferent arteriole and thus glomerular filtration rate. TGF is activated by fluctuations in macula densa [Cl(-)] and the myogefnic mechanism by changes in hydrostatic pressure. The model was used to investigate the relative contributions of medullary blood flow autoregulation and inhibition of transport in the proximal convoluted tubule to pressure natriuresis in both diuresis and antidiuresis. The model predicts that medullary blood flow autoregulation, which only affects the interstitial solute composition in the model, has negligible influence on the rate of NaCl excretion. However, it exerts a significant effect on urine flow, particularly in the antidiuretic kidney. This suggests that interstitial washout has significant implications for the maintenance of hydration status but little direct bearing on salt excretion, and that medullary blood flow may only play a signaling role for stimulating a pressure-natriuresis response. Inhibited reabsorption in the model proximal convoluted tubule is capable of driving pressure natriuresis when the known actions of vasopressin on the collecting duct epithelium are taken into account.

Hormonal regulation of salt and water excretion: a mathematical model of whole kidney function and pressure natriuresis

We present a lumped-nephron model that explicitly represents the main features of the underlying physiology, incorporating the major hormonal regulatory effects on both tubular and vascular function, and that accurately simulates hormonal regulation of renal salt and water excretion. This is the first model to explicitly couple glomerulovascular and medullary dynamics, and it is much more detailed in structure than existing whole organ models and renal portions of multiorgan models. In contrast to previous medullary models, which have only considered the antidiuretic state, our model is able to regulate water and sodium excretion over a variety of experimental conditions in good agreement with data from experimental studies of the rat. Since the properties of the vasculature and epithelia are explicitly represented, they can be altered to simulate pathophysiological conditions and pharmacological interventions. The model serves as an appropriate starting point for simulations of physiological, pathophysiological, and pharmacological renal conditions and for exploring the relationship between the extrarenal environment and renal excretory function in physiological and pathophysiological contexts.

Gender Differences in Pressure-Natriuresis and Renal Autoregulation

Sexual dimorphism in arterial pressure regulation has been observed in humans and animal models. The mechanisms underlying this gender difference are not fully known. Previous studies in rats have shown that females excrete more salt than males at a similar arterial pressure. The renin-angiotensin system is a powerful regulator of arterial pressure and body fluid volume. This study examined the role of the angiotensin type 2 receptor (AT 2 R) in pressure-natriuresis in male and female rats because AT 2 R expression has been reported to be enhanced in females. Renal function was examined at renal perfusion pressures of 120, 100, and 80 mm Hg in vehicle-treated and AT 2 R antagonist-treated (PD123319; 1 mg/kg/h) groups. The pressure-natriuresis relationship was gender-dependent such that it was shifted upward in female vs male rats ( P <0.001). AT 2 R blockade modulated the pressure-natriuresis relationship, shifting the curve downward in male ( P <0.01) and female ( P <0.01) rats to a similar extent. In females, AT 2 R blockade also reduced the lower end of the autoregulatory range of renal blood flow ( P <0.05) and glomerular filtration rate ( P <0.01). Subsequently, the renal blood flow response to graded angiotensin II infusion was also measured with and without AT 2 R blockade. We found that AT 2 R blockade enhanced the renal vasoconstrictor response to angiotensin II in females but not in males ( P <0.05). In conclusion, the AT 2 R modulates pressure-natriuresis, allowing the same level of sodium to be excreted at a lower pressure in both genders. However, a gender-specific role for the AT 2 R in renal autoregulation was evident in females, which may be a direct vascular AT 2 R effect.

Maternal nutrient restriction in sheep: hypertension and decreased nephron number in offspring at 9 months of age

Pregnant ewes were fed either a 50% nutrient-restricted (NR; n= 8) or a control 100% (C; n= 8) diet from day 28 to day 78 of gestation (dGA; term = 150 dGA). Lambs were born naturally, and fed to appetite throughout the study period. At 245 +/- 1 days postnatal age (DPNA), offspring were instrumented for blood pressure measurements, with tissue collection at 270 DPNA. Protein expression was assessed using Western blot, glomerulus number determined via acid maceration and hormone changes by radioimmunoassay (RIA) or enzyme-linked immunosorbent assay (ELISA). NR lambs had higher mean arterial pressure (MAP; 89.0 +/- 6.6 versus 73.4 +/- 1.6 mmHg; P < 0.05), fewer renal glomeruli (57.8 +/- 23.8 versus 64.6 +/- 19.3 x 10(4); P < 0.05), increased expression of angiotensin converting enzyme (ACE) in the renal cortex (942 +/- 130 versus 464 +/- 60 arbitrary pixel units (apu); P < 0.03), and increased angiotensin II receptor AT2 expression in the renal medulla (63.3 +/- 12.1 versus 19.5 +/- 44.2 x 10(4) apu; P < 0.03). All data are presented as mean +/-S.E.M. The present data indicate that global maternal nutrient restriction (50%) during early to mid-gestation impairs renal nephrogenesis, increases MAP, and alters expression of AT2 and ACE without an associated change in birth weight. These data demonstrate the existence of a critical window of fetal susceptibility during early to mid-gestation that alters kidney development and blood pressure regulation in later life.

Angiotensin AT2 receptors: cardiovascular hope or hype?

British Journal of Pharmacology (2003) 140, 809–824. doi:10.1038/sj.bjp.0705448

Disparate roles of AT2 receptors in the renal cortical and medullary circulations of anesthetized rabbits

The contributions of angiotensin II type 1 (AT1) and type 2 (AT2) receptors to the control of regional kidney blood flow were determined in pentobarbital-anesthetized rabbits. Intravenous candesartan (AT1 antagonist; 10 microg/kg plus 10 microg x kg(-1) x h(-1)) reduced mean arterial pressure (12%) and increased total renal blood flow (29%) and cortical laser-Doppler flux (18%) but not medullary laser-Doppler flux. Neither intravenous PD123319 (AT2 antagonist; 1 mg/kg plus 1 mg x kg(-1) x h(-1)) nor saline vehicle significantly affected these variables, and the responses to candesartan plus PD123319 were indistinguishable from those of candesartan alone. In vehicle-treated rabbits, renal-arterial infusions of angiotensin II (1 to 25 ng x kg(-1) x min(-1)) and angiotensin III (5 to 125 ng x kg(-1) x min(-1)) dose-dependently reduced renal blood flow (up to 51%) and cortical laser-Doppler flux (up to 50%) but did not significantly affect medullary laser-Doppler flux or arterial pressure. Angiotensin(1-7) (20 to 500 ng x kg(-1) x min(-1)) had similar effects but of lesser magnitude. CGP42112A (20 to 500 ng x kg(-1) x min(-1)) did not significantly affect these variables. After PD123319 administration, angiotensin II and angiotensin III dose-dependently increased medullary laser-Doppler flux (up to 84%), and reductions in renal blood flow in response to angiotensin II were enhanced. Candesartan abolished renal hemodynamic responses to the angiotensin peptides, even when given in combination with PD123319. We conclude that AT2 receptor activation counteracts AT1-mediated vasoconstriction in the renal cortex but also counteracts AT1-mediated vasodilatation in vascular elements controlling medullary perfusion. These mechanisms might have an important effect on the control of medullary perfusion under conditions of activation of the renin-angiotensin system.

Function of renal angiotensin AT2 receptors is not enhanced in Lyon hypertensive rats

1. Because we previously observed that angiotensin AT2 receptor stimulation decreased pressure-natriuresis, in the present study we examined the possible involvement of these receptors in altered sodium excretion shown by Lyon hypertensive (LH) rats. 2. In 9-week-old male anaesthetized LH rats and normotensive (LL) controls pretreated with an angiotensin-converting enzyme inhibitor (quinapril; 10 mg/kg) and an AT1 receptor antagonist (losartan; 10 mg/kg), angiotensin (Ang) II was infused (30 ng/kg per min) to stimulate AT2 receptors. In other groups of rats, an AT2 receptor antagonist (PD123319; 50 micro g/kg per min) was added before AngII infusion. 3. During AT2 receptor stimulation, LH differed from LL rats by significantly reduced renal blood flow (RBF), glomerular filtration rate and pressure diuresis and natriuresis. The addition of PD123319 did not change total RBF, whereas it did increase pressure diuresis and natriuresis in both strains. However, the effects of PD123319 were less marked in LH rats than in LL rats. 4. These findings confirm that, under the present experimental conditions, AT2 receptors are antinatriuretic and are not of greater functional importance in hypertensive animals of the Lyon strain.

Angiotensin II mediates catecholamine and neuropeptide Y secretion in human adrenal chromaffin cells through the AT1 receptor

The aim of the present work was to study the effect of angiotensin II (Ang II) on catecholamines and neuropeptide Y (NPY) release in primary cultures of human adrenal chromaffin cells. Ang II stimulates norepinephrine (NE), epinephrine (EP) and NPY release from perifused chromaffin cells by 3-, 2- and 12-fold, respectively. The NPY release is more sustained than that of catecholamines. We found that the receptor-AT(2) agonist, T(2)-(Ang II 4-8)(2) has no effect on NE, EP and NPY release from chromaffin cells. We further showed that Ang II increases intracellular Ca(2+) concentration ([Ca(2+)](i)). The selective AT(1)-receptor antagonist Candesartan blocked [Ca(2+)](i) increase by Ang II, while T(2)-(Ang II 4-8)(2) was ineffective. These findings demonstrate that AT(1) stimulation induces catecholamine secretion from human adrenal chromaffin cells probably by raising cytosolic calcium.

Pressure natriuresis in AT(2) receptor-deficient mice with L-NAME hypertension

AT(2) receptor-disrupted (AT(2) -/-) mice provide a unique opportunity to investigate the cardiovascular and BP-related effects of NO depletion. This study compared the pressure-diuresis-natriuresis relationship in (AT(2) -/-) and wild-type (AT(2) +/+) mice after treating the animals with L-NAME (130 mg/kg body wt per day) for 1 wk. L-NAME increased mean arterial pressure (MAP) more in AT(2) -/- than in AT(2) +/+ mice (118 +/- 2 versus 108 +/- 4 mmHg). This difference occurred even though L-NAME-treated AT(2) +/+ mice had a greater sodium excretion than AT(2) -/- mice (10.9 +/- 0.5 versus 8.0 +/- 1.0 micro mol/h). The pressure-natriuresis relationship in conscious AT(2) -/- mice was shifted rightward compared with controls. RBF was decreased in AT(2) -/- compared with AT(2) +/+ mice. L-NAME decreased RBF in these mice further from 4.08 +/- 0.43 to 2.79 +/- 0.15 ml/min per g of kidney wt. GFR was not significantly different between AT(2) +/+ and AT(2) -/- mice (1.09 +/- 0.08 versus 1.21 +/- 0.09 ml/min per g of kidney wt). L-NAME reduced GFR in AT(2) -/- to 0.87 +/- 0.07 ml/min per g of kidney wt. Fractional sodium (FE(Na)) and water (FE(H2O)) curves were shifted more strongly to the right by L-NAME in AT(2) -/- mice than in AT(2) +/+ mice. AT(1) receptor blocker treatment lowered BP in both L-NAME-treated strains to basal values. It is concluded that the AT(1) receptor plays a key role in the impaired renal sodium and water excretion induced by NO synthesis blockade. Changes in RBF, GFR, and tubular sodium and water reabsorption are involved and may be also responsible for the greater BP increase in L-NAME-treated AT(2) -/- mice.

Angiotensin II and renal medullary blood flow in Lyon rats

The present study evaluated the acute effects of ANG II (5-480 ng/kg iv) and phenylephrine (PE; 0.2-146 microg/kg iv) on total renal (RBF) and medullary blood flow (MBF) in anesthetized Lyon hypertensive (LH) and low-blood-pressure (LL) rats. ANG II and PE induced dose-dependent decreases in both RBF and MBF, which were greater in LH than in LL rats. Interestingly, after ANG II, but not after PE, the initial medullary vasoconstriction was followed by a long-lasting and dose-dependent vasodilation that was significantly blunted in LH compared with LL rats. The mechanisms of the MBF effects of ANG II were studied in LL rats only. Blockade of AT(1) receptors with losartan (10 mg/kg) abolished all the effects of ANG II, whereas AT(2) receptor blockade with PD-123319 (50 microg x kg(-1) x min(-1) iv) did not change these effects. Indomethacin (5 mg/kg) decreased by approximately 90% the medullary vasodilation induced by the lowest doses of ANG II (from 15 ng/kg). In contrast, N(G)-nitro-l-arginine methyl ester (10 mg/kg and 0.1 mg. kg(-1). min(-1) iv) and the bradykinin B(2)-receptor antagonist HOE-140 (20 microg/kg and 10 microg x kg(-1) x min(-1) iv) markedly lowered the medullary vasodilation at the highest doses of ANG II only. In conclusion, this study shows that LH rats exhibit an altered MBF response to ANG II compared with LL rats and indicates that the AT(1) receptor-mediated medullary vasodilator response to low doses of ANG II is mainly due to the release of PGs, whereas the dilator response to high doses of ANG II has additional nitric oxide- and kinin-dependent components.

ANG II AT(1) and AT(2) receptors in developing kidney of normal microswine

To identify an appropriate model of human renin-angiotensin system (RAS) involvement in fetal origins of adult disease, we quantitated renal ANG II AT(1) and AT(2) receptors (AT1R and AT2R, respectively) in fetal (90-day gestation, n = 14), neonatal (3-wk, n = 5), and adult (6-mo, n = 8) microswine by autoradiography ((125)I-labeled [Sar(1)Ile(8)]ANG II+cold CGP-42112 for AT1R, (125)I-CGP-42112 for AT2R) and by whole kidney radioligand binding. The developmental pattern of renal AT1R in microswine, like many species, exhibited a 10-fold increase postnatally (P < 0.001), with maximal postnatal density in glomeruli and lower density AT1R in extraglomerular cortical and outer medullary sites. With aging, postnatal AT1R glomerular profiles increased in size (P < 0.001) and fractional area occupied (P < 0.04), with no change in the number per unit area. Cortical levels of AT2R by autoradiography fell with age from congruent with 5,000 fmol/g in fetal kidneys to congruent with 60 and 20% of fetal levels in neonatal and adult cortex, respectively (P < 0.0001). The pattern of AT2R binding in postnatal pig kidney mimicked that described in human and simian, but not rodent, species: dense AT2R confined to discrete cortical structures, including pre- and juxtaglomerular, but not intraglomerular, vasculature. Our results provide a quantitative assessment of ANG II receptors in developing pig kidney and document the concordance of pigs and primates in developmental regulation of renal AT1R and AT2R.

Role of AT1 and AT2 receptor subtypes in salt-sensitive hypertension induced by sensory nerve degeneration

OBJECTIVE

To define the role of the type 1 angiotensin II (AT1) and type II (AT2) receptors in the development of salt-sensitive hypertension induced by sensory nerve degeneration.

DESIGN AND METHODS

Neonatal Wistar rats were given capsaicin 50 mg/kg s.c. on the first and second days of life. After weaning, male rats were divided into six groups and treated for 3 weeks with: control + high sodium diet (4%, CON-HS), capsaicin pretreatment + normal sodium diet (0.5%, CAP-NS), CAP-HS, CAP + HS + candesartan (10 mg/kg per day) (CAP-HS-CAN), CAP + HS + PD 123319 (30 mg/kg per day) (CAP-HS-PD), and capsaicin pretreatment + high sodium diet + candesartan + PD 123319 (CAP-HS-CAN-PD). Mean arterial pressure (MAP) was measured by carotid arterial catheterization. Urinary Na+ concentrations were determined by using a flame atomic absorption spectrophotometer. Levels of calcitonin gene-related peptide (CGRP) in dorsal root ganglia (DRG) and plasma renin activity (PRA) were determined by radioimmunoassay.

RESULTS

CGRP contents in DRG were decreased by capsaicin (P < 0.05). MAP was higher in CAP-HS rats compared with all the other groups (P < 0.05). The 24 h urine and sodium excretion increased when a high salt diet was given, but they were lower in CAP-HS and CAP-HS-CAN than in CON-HS (P < 0.05). PRA was suppressed in CON-HS and CAP-HS compared with CAP-NS, but it was higher in CAP-HS than in CON-HS (P < 0.05).

CONCLUSION

Insufficiently suppressed PRA by high salt intake may contribute to increased salt sensitivity and account for effectiveness of candesartan in lowering blood pressure in this model. Furthermore, PD 123319 attenuates the development of hypertension in salt-loaded rats neonatally treated with capsaicin, indicating that the AT2 receptor contributes to the increase in blood pressure.

Angiotensin II and its receptors in the diabetic kidney

Studies using either angiotensin-converting enzyme inhibitors or type 1 (AT(1)) angiotensin II (ANG II)-receptor blockers indicate that ANG II is a mediator of progressive injury in diabetic nephropathy. However, suppression of the systemic renin-angiotensin system (RAS) generally has been shown in diabetes mellitus. Evidence suggests that intrarenal RASs within glomeruli and proximal tubules may be activated with hyperglycemia, leading to stimulation of local ANG II production, which may exert feedback inhibition of systemic renin release. Once formed, intrarenal ANG II exerts most of its well-described effects through binding to AT(1) receptors that are abundantly present in cells of the glomeruli, tubules, vasculature, and interstitium. Thus, AT(1)-receptor activation increases vascular resistance, reduces renal blood flow, and stimulates production of extracellular matrix in the mesangium and tubulointerstitium. Recent studies suggest that the adult kidney also expresses type 2 (AT(2)) ANG II receptors in glomeruli, tubular segments, and vasculature. AT(2)-receptor activation is associated with increased intrarenal nitric oxide production, stimulation of natriuresis, and inhibition of cell growth and matrix synthesis, effects that oppose those of kidney AT(1) receptors. A number of studies have shown a reduction in kidney AT(1)-receptor expression in diabetic nephropathy, suggesting that the balance between AT(1)- and AT(2)-receptor-mediated cell-signaling events may be a determinant of progression rate in diabetic nephropathy and that unopposed stimulation of AT(2) receptors by ANG II with use of AT(1)-receptor blockers may contribute to the beneficial properties of these agents. Determination of the expression pattern of AT(2) receptors in diabetes and further definition of the role of AT(2) receptors in opposing the detrimental effects of AT(1) receptors may lead to more selective targeting of the RAS in diabetic nephropathy.

Update: role of the angiotensin type-2 (AT(2)) receptor in blood pressure regulation

In the past, virtually all of the physiologic actions of angiotensin II (ANG II) were thought to be mediated by the type-1 ANG II receptor. However, there is now a compelling body of evidence suggesting that the type-2 (AT2) receptor is an important regulator of renal function and blood pressure (BP). The AT2 receptor stimulates a bradykinin (BK)-nitric oxide (NO)-cyclic GMP vasodilator cascade in blood vessels and in the kidney. Recent studies have shown that absence of the AT2 receptor lends to pressor and natriuretic hypersensitivity to ANG II. Furthermore, there is now excellent evidence that the AT2 receptor mediates pressure natriuresis. The AT2 receptor also stimulates the conversion of prostaglandin E2 (PGE2) to PGF2. In addition, it is now apparent that the therapeutic reduction in BP with AT1 receptor blockade (eg, losartan, valsartan, candesartan) is mediated by ANG II stimulation of the AT2 receptor, leading to increased levels of BK, NO, and cGMP. Current evidence predicts that AT2 receptor agonists would be beneficial in the treatment of hypertension.

Role of the angiotensin type 2 receptor in the regulation of blood pressure and renal function

The renin-angiotensin system is a major physiological regulator of body fluid volume, electrolyte balance, and arterial pressure. Virtually all of the biological actions of the principle effector peptide angiotensin II (ANG II) have been attributed to an action at the type 1 (AT(1)) ANG receptor. Until recently, the functional role of the type 2 (AT(2)) receptor, if any, has been unknown, possibly because the AT(2) receptor has a low degree of expression compared with that of the AT(1) receptor. Evidence has now accumulated that the AT(2) receptor opposes functions mediated by the AT(1) receptor. Whereas the AT(1) receptor stimulates cell proliferation, the AT(2) receptor inhibits proliferation and promotes cell differentiation. These differences in growth responses have been ascribed to different cell signaling pathways in which the AT(1) receptor stimulates protein phosphorylation and the AT(2) receptor dephosphorylation. During the past 5 years, studies have demonstrated that the AT(2) receptor is responsible for vasodilation and natriuresis, thus opposing the vasoconstrictor and antinatriuretic effects of ANG II mediated through the AT(1) receptor. Work from our laboratory and others indicates that the AT(2) receptor stimulates vasodilation and natriuresis by an autocrine cascade including bradykinin, nitric oxide, and cyclic GMP. The AT(2) receptor also has been found to control vasodilator prostaglandins, which have a role in blood pressure regulation. The AT(2) receptor appears to play a counterregulatory protective role in the regulation of blood pressure and sodium excretion that opposes the AT(1) receptor.

Inhibition of pressure natriuresis in mice lacking the AT2 receptor

Inhibition of pressure natriuresis in mice lacking the AT2 receptor.

BACKGROUND

Angiotensin II type 2 (AT2) receptor knockout mice have higher blood pressures than wild-type mice; however, the hypertension is imperfectly defined. We tested the hypothesis that renal mechanisms could be contributory.

METHODS

We conducted pressure-natriuresis-diuresis experiments, measured renal cortical and medullary blood flow by laser Doppler methods, and explored cytochrome P450-dependent arachidonic acid metabolism by means of reverse transcription-polymerase chain reaction.

RESULTS

Blood pressure was 15 mm Hg higher in AT2 receptor knockout mice than in controls, and pressure diuresis and natriuresis curves were shifted rightward. At similar renal perfusion pressures (113 to 118 mm Hg), wild-type mice excreted threefold more sodium and water than AT2 receptor knockout mice. Fractional sodium and water excretion curves were shifted rightward in parallel. Renal blood flow ranged between 6.72 and 7.88 mL/min/g kidney wet weight (kwt) in wild-type and between 5.84 and 6.15 mL/min/g kwt in AT2 receptor knockout mice. Renal vascular resistance was increased in AT2A receptor knockout mice. Cortical blood flow readings leveled at 2.5 V in wild-type and 1.5 V in AT2 receptor knockout mice. Medullary blood flow readings ranged between 0.8 and 1.0 V and increased 116% in wild-type mice as renal perfusion pressure was increased. This increase did not occur in AT2 receptor knockout mice. The glomerular filtration rate (GFR) was similar in both groups at approximately 1 mL/min/g kwt. Renal microsomes from AT2 receptor knockout mice had less activity in hydroxylating arachidonic acid to 20-hydroxyeicosatetraenoic acid (20-meter) than controls, whereas renal AT1 receptor gene expression was increased in AT2 receptor knockout mice.

CONCLUSIONS

Hemodynamic and tubular factors modify renal sodium handling in AT2 receptor knockout mice and may cause hypertension. AT2 receptor disruption induces alterations of other regulatory systems, including altered arachidonic acid metabolism, that may contribute to the intrarenal differences observed between AT2 receptor knockout and wild-type mice.

Signal transduction from the angiotensin II AT2 receptor

Recent studies of genetically engineered animals have established a role for the angiotensin II (AT2) receptor in cardiovascular, renal and central functions, as well as in developmental processes. This review summarizes new insights into major AT2 signaling pathways--activation of protein phosphatases, the nitric oxide-cGMP system and phospholipase A2--which have been related to specific cellular responses or functions of this receptor.