Regulation of angiotensin II receptor subtypes by dexamethasone in rat mesangial cells

Dexamethasone-dependent modulation of cyclic GMP synthesis in podocytes

Podocytes may be direct target for glucocorticoid therapy in glomerular proteinuric disease. Permeability of podocytes largely depends on their capacity to migrate which involves the contractile apparatus in their foot processes. In this study, we examined the effect of synthetic glucocorticoid dexamethasone (DEX) on the ability of podocytes to produce cyclic guanosine monophosphate (cGMP) in the presence of vasoactive factors, atrial natriuretic peptide (ANP), nitric oxide (NO), and angiotensin II (Ang II). We investigated also the effects of cGMP and DEX on podocyte motility. Primary rat podocytes and immortalized mouse podocytes were pretreated with 1 µM DEX for 4 or 24 h. Glomerular hypertension was mimicked by subjecting the cells to mechanical stress. Total and subcellular cGMP levels were determined in podocytes incubated with 0.1 µM ANP, 1 µM S-nitroso-N-acetyl penicillamine (SNAP), and 1 µM Ang II. Cell motility was estimated by a wound-healing assay. The ANP-dependent production of cGMP increased after 4 h exposition to DEX, but was attenuated after 24 h. Adversely, a 24-h pretreatment with DEX augmented the NO-dependent cGMP synthesis. Ang II suppressed the ANP-dependent cGMP production and the effect was enhanced by DEX in mechanical stress conditions. Mechanical stress reduced total cGMP production in the presence of all stimulators, whereas extracellular to total cGMP ratio increased. 8-Br cGMP enhanced podocyte migration which was accompanied by F-actin disassembly. In the presence of DEX these effects were prevented. We conclude that DEX modulates the production of cGMP in podocytes stimulated with vasoactive factors such as Ang II, ANP, and NO, and the effect is time-dependent. cGMP increases podocyte motility, which is prevented by DEX. This mechanism may account for the antiproteinuric effect of glucocorticoids.

Altered urinary excretion of aquaporin 2 in IgA nephropathy

OBJECTIVE

The intrarenal renin-angiotensin system (RAS) activation plays a pivotal role in immunoglobulin A nephropathy (IgAN) pathogenesis, which is still largely undefined. Recently, vasopressin (AVP) has been advocated to contribute to the genesis and progression of chronic kidney diseases (CKD) directly, and indirectly, via RAS activation. Our aim is to explore the intrarenal activity of AVP, its relationship with RAS activity, as well as its modulation by therapies in IgAN.

DESIGN

In this observational study, we measured plasma copeptin, a surrogate marker of AVP, the urine excretion of aquaporin 2 (AQP2), a protein reflecting renal AVP action, and angiotensinogen (AGT), a parameter of renal RAS activation, and their relationship with renal function in 44 IgAN patients at the time of renal biopsy, without any drug therapy, and after 6-month treatment with ACEi or steroid+ACEi. Twenty-one patients with other CKD and 40 healthy subjects were recruited as controls.

METHODS

ELISAs were used to measure all variables of interest.

RESULTS

At baseline, IgAN patients showed higher urinary levels of AQP2, compared with controls and patients with other CKD. Urinary AQP2 and AGT levels strongly correlated with the presence of arterial hypertension. Steroids+ACEi caused the decrease of all the variables examined. The fall of urinary AQP2 and AGT following drug treatments was associated with the decrease of daily proteinuria.

CONCLUSION

Our findings would support the involvement of AVP-AQP2 axis, interacting with the RAS, in the progression of IgAN and candidate AQP2 as a possible novel marker of the disease.

Characterization of the angiotensin (AT1b) receptor promoter and its regulation by glucocorticoids

Angiotensin II acts through two pharmacologically distinct receptors known as AT1 and AT2. Duplication of the AT1 receptor in rodents into At1a and b subtypes allows tissue-specific expression of the AT1b in adrenal and pituitary tissue. Adrenal expression of this receptor is increased in the offspring of rat mothers exposed to a low-protein diet and this is associated with the undermethylation of its promoter. This phenomenon is blocked by the inhibition of maternal glucocorticoid synthesis by metyrapone. We have mapped the transcriptional start site of the promoter and demonstrated that a 1.2 kbp fragment upsteam of this site is effective in driving luciferase expression in mouse Y1 cells. A combination of bioinformatic analysis, electrophoretic mobility shift analysis (EMSA), and mutagenesis studies demonstrates: i) the presence of a putative TATA box and CAAT box; ii) the presence of three Sp1 response elements, capable of binding SP1; mutation of any pair of these sites effectively disables this promoter; iii) the presence of four potential glucocorticoid response elements which each bind glucocorticoid receptor in EMSA, although only two confer dexamethasone inhibition on the promoter; iv) the presence of two AP1 sites. Mutagenesis of the distal AP1 site greatly diminishes promoter function but this is also associated with the loss of dexamethasone inhibition. These studies will facilitate an understanding of the mechanisms by which fetal programming leads to long term alterations in gene expression and the development of adult disease.

Effects of dexamethasone exposure on rat metanephric development: in vitro and in vivo studies

Maternal administration of dexamethasone (DEX) for 48 h early in rat kidney development results in offspring with a reduced nephron endowment. However, the mechanism through which DEX inhibits nephrogenesis is unknown. In this study, we hypothesized that DEX may indirectly inhibit nephrogenesis by inhibiting ureteric branching morphogenesis. Whole metanephroi from embryonic day 14.5 (E14.5) rat embryos were cultured in the presence of DEX. DEX (10(-5) M) exposure for 2 days significantly inhibited ureteric branching compared with metanephroi grown in control media or DEX (10(-7) M). Culturing metanephroi for a further 3 days (in control media only) reduced total glomerular number in metanephroi previously exposed to DEX (10(-5) M) or (10(-7) M) compared with control cultures. Expression of genes known to regulate ureteric branching morphogenesis was determined by real-time PCR in metanephroi after 2 days in culture. DEX exposure in vitro decreased expression of glial cell line-derived neurotrophic factor (GDNF) and increased expression of bone morphogenetic protein-4 (BMP-4) and transforming growth factor-beta1 (TGF-beta1). Similar gene expression changes were found in E16.5 metanephroi in which the dam had been exposed to 2 days of DEX (0.2 mg.kg(-1).day(-1)) at E14.5/15.5 in vivo. However, in kidneys collected at E20.5 after in vivo exposure for 2 days, GDNF expression was increased and BMP-4 and TGF-beta1 expression decreased suggesting a biphasic response in gene expression to DEX exposure. These results show for the first time that inhibition of ureteric branching morphogenesis may be a key mechanism through which DEX exposure results in a reduced nephron endowment.

Angiotensin II mediates LDL-induced superoxide generation in mesangial cells

Lipid abnormalities and activation of the local renin-angiotensin system (RAS) may be involved in the pathogenesis of chronic glomerular disease. This study investigated whether low-density lipoprotein (LDL) activates local RAS in cultured human mesangial cells (HMC) and, at the same time, whether ANG II mediates LDL-induced mesangial cell proliferation, hypertrophy, and superoxide (O2-) generation. Quiescent HMC were exposed to 50 to 200 microg/ml of LDL or 10-7 to 10-10 M ANG II for 0.5 to 24 h in the presence or absence of 10-6 M losartan, an ANG II type I (AT1) receptor antagonist, or 10-5 M diphehylendieodonium (DPI) or 10-4 M apocynin, inhibitors of nicotinamide adenine dinucleotide phosphate oxidase. LDL induced an up to threefold increase in the ANG II levels in the culture medium of HMC. LDL upregulated AT1 receptor and angiotensinogen mRNA expression in HMC. LDL incubated with HMC increased O2- production by up to 3.3 times compared with the level of control cells. The LDL-induced, increased O2- generation was suppressed by losartan, DPI, or apocynin. LDL significantly increased mesangial [3H]thymidine or [3H]leucine incorporation, whereas these processes were abrogated by losartan. In conclusion, LDL increases ANG II production by mesangial cells, which in turn results in increased O2- production, and cell proliferation and hypertrophy, these effects of ANG II being mediated by the AT1 receptor.

Angiotensin II regulates 11beta-hydroxysteroid dehydrogenase type 2 via AT2 receptors

BACKGROUND

In preeclampsia, cortisol degradation by the enzyme 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2) is compromised, which enhances intracellular cortisol availability. This leads to vasoconstriction and renal sodium retention with volume expansion, thus increasing blood pressure. An augmented availability of angiotensin II (Ang II) predisposes to preeclampsia. Some effects of Ang II are mediated by the mitogen-activated protein kinase (MAPK) cascade, which also regulates 11beta-HSD2 activity. Therefore, we hypothesized that Ang II regulates 11beta-HSD2.

METHODS

The human choriocarcinoma cell line JEG-3, which expresses the 11beta-HSD2 isoenzyme, was used. 3H-cortisol/cortisone conversion assays and mRNA analyses by reverse transcription-polymerase chain reaction (RT-PCR) were performed. Cells were stimulated with Ang II and the effect was modulated by Ang II type 1 (AT1) and AT2 receptor blockers DUP753 or L-158809 and PD-123319, respectively. In order to elucidate the signaling cascade, the MAPK kinase inhibitors PD-098059 and U-0126 were probed. The impact of a modulated 11beta-HSD2 activity was assessed by determining the effect of cortisol on AT1 receptor mRNA.

RESULTS

Ang II reduced mRNA and activity of 11beta-HSD2 mainly by a post-transcriptional mechanism. This Ang II effect was abrogated by AT2, but not by AT1 receptor blockade. Mitogen-activated protein (MAP) kinase kinase (MAPKK) inhibitors reversed the Ang II effect. Dexamethasone augmented the mRNA expression of AT1 receptors. Cortisol enhanced AT1 receptor mRNA expression when the 11beta-HSD2 activity was reduced either by Ang II or by glycyrrhetinic acid, an 11beta-HSD2 inhibitor.

CONCLUSION

Ang II decreases the activity of 11beta-HSD2 by an AT2 receptor- and MAPK-dependent mechanism. The decreased activity of 11beta-HSD2 increases the intracellular availability of cortisol, which might be relevant for the pathogenesis of hypertension and preeclampsia.

Angiotensin II activation of the JAK/STAT pathway in mesangial cells is altered by high glucose

BACKGROUND

Both high glucose (HG) and angiotensin II (Ang II) causes glomerular mesangial cell (GMC) growth and increased synthesis of matrix proteins like collagen IV contributing to diabetic nephropathy. We have recently found that exposure of vascular smooth muscle cells to HG augments the Ang II activation of the growth promoting JAK/STAT pathway. We hypothesized that Ang II activation of the JAK/STAT pathway is altered by HG in GMC, and that this pathway might be linked to the Ang II-induced growth and overproduction of collagen IV in GMC in HG conditions.

METHODS

GMC were cultured under normal glucose (NG; 5.5 mmol/L) and HG (25 mmol/L) for 48 hours and stimulated with Ang II (0.1 micromol/L) for various times. GMC lysate was then immunoprecipitated and/or immunoblotted with SHP-1, SHP-2 and phosphospecific JAK2 and STAT antibodies. The HG and Ang II induced growth and collagen IV synthesis studies were performed in GMC transfected with JAK2 antisense or JAK2 sense. GMC growth was monitored via [3H]-thymidine incorporation, and collagen IV synthesis via ELISA.

RESULTS

We found that Ang II-induced JAK2, STAT1, STAT3, STAT5A/B and SHP-2 phosphorylations were enhanced by HG, whereas that of SHP-1 was reduced. Ang II-induced growth and collagen IV synthesis also were increased under HG conditions. Transfection of GMC with JAK2 antisense oligonucleotides blocked the Ang II-induced growth and collagen IV synthesis in both NG and HG conditions.

CONCLUSION

These results provide evidence that activation of the JAK/STAT pathway by HG or/and Ang II may be of importance in the increased GMC cell growth and collagen IV synthesis that is seen in diabetic nephropathy.

Desensitization of type 1 angiotensin II receptor subtypes in the rat kidney

Differences involving serine residues in the sequence of the carboxyl-terminal tail of type 1 angiotensin II (Ang II) receptor subtypes AT(1A) and AT(1B) suggest differences in desensitization ability. We examined the Ang II-induced homologous desensitization patterns of both receptor subtypes in freshly isolated renal structures: glomerulus (Glom), afferent arteriole, and cortical thick ascending limb (CTAL), whose content in each subtype mRNA is different, by measuring variations in intracellular calcium concentration. A preexposure to a maximal dose of Ang II, followed by a second application of the same concentration, induced: 1) a complete desensitization in Glom, where AT(1A) and AT(1B) mRNAs were expressed in similar proportions, and 2) no or partial desensitization in afferent arteriole and CTAL, where AT(1A) mRNA was predominant. In the absence of nephron structure containing only AT(1B) mRNA, we studied rat anterior pituitary cells that exhibit high content in this subtype and observed that desensitization was not complete. In Glom, CTAL, and pituitary cells, desensitization proceeded in a dose-dependent manner. In Glom and CTAL, desensitization occurred via a PKC-independent mechanism. These results suggest that desensitization does not depend on the nature of Ang II receptor subtype but either on the proportion of each subtype in a given cell and/or on cell specific type. This could allow adaptive biological responses to Ang II appropriate to the specific function of a given cell type.

Angiotensin II receptor subtypes determine induced NO production in rat glomerular mesangial cells

Angiotensin II (ANG II) and nitric oxide (NO) have contrasting vascular effects, yet both sustain inflammatory responses. We investigated the impact of ANG II on lipopolysaccharide (LPS)/interferon-gamma (IFN)-induced NO production in cultured rat mesangial cells (MCs). LPS/IFN-induced nitrite production, the inducible form of nitric oxide synthase (NOS-2) mRNA, and protein expression were dose dependently inhibited by ANG II on coincubation, which was abolished on ANG II type 2 (AT(2)) receptor blockade by PD-123319. Homology-based RT-PCR verified the presence of AT(1A), AT(1B), and AT(2) receptors. To shift the AT receptor expression toward the type 1 receptor, two sets of experiments were performed: LPS/IFN preincubation for 24 h was followed by 8-h coincubation with ANG II; or during 24-h coincubation of LPS/IFN and ANG II, dexamethasone was added for the last 6-h period. Both led to an amplified overall expression of NOS-2 protein and NO production that was inhibitable by actinomycin D in the first setup. Induced NO production was enhanced via the AT(1) receptor; however, it was diminished via the AT(2) receptor. In conclusion, induced NO production is negatively controlled by the AT(2), whereas AT(1) receptor stimulation enhanced NO synthesis in MCs. The overall NO availability depended on the onset of the inflammatory stimuli with respect to ANG II exposure and the available AT receptors.

Renomedullary interstitial cells: a target for endocrine and paracrine actions of vasoactive peptides in the renal medulla

1. The renal medulla plays an important role in regulating body sodium and fluid balance and blood pressure homeostasis through its unique structural relationships and interactions between renomedullary interstitial cells (RMIC), renal tubules and medullary vasculature. 2. Several endocrine and/or paracrine factors, including angiotensin (Ang)II, endothelin (ET), bradykinin (BK), atrial natriuretic peptide (ANP) and vasopressin (AVP), are implicated in the regulation of renal medullary function and blood pressure by acting on RMIC, tubules and medullary blood vessels. 3. Renomedullary interstitial cells express multiple vasoactive peptide receptors (AT1, ETA, ETB, BK B2, NPRA and NPRB and V1a) in culture and in tissue. 4. In cultured RMIC, AngII, ET, BK, ANP and AVP act on their respective receptors to induce various cellular responses, including contraction, prostaglandin synthesis, cell proliferation and/or extracellular matrix synthesis. 5. Infusion of vasoactive peptides or their antagonists systemically or directly into the medullary interstitium modulates medullary blood flow, sodium excretion and urine osmolarity. 6. Overall, expression of multiple vasoactive peptide receptors in RMIC, which respond to various vasoactive peptides and paracrine factors in vitro and in vivo, supports the hypothesis that RMIC may be an important paracrine target of various vasoactive peptides in the regulation of renal medullary function and long-term blood pressure homeostasis.

The tissue renin-angiotensin system in rats with fructose-induced hypertension: overexpression of type 1 angiotensin II receptor in adipose tissue

OBJECTIVE

Fructose feeding induces hypertension, insulin-resistance and hypertriglyceridemia in Sprague-Dawley rats. The mechanisms of fructose-induced hypertension are as yet unknown. Here we investigate the effects of fructose feeding and of varying salt intake on blood pressure, glucose tolerance, plasma renin activity, and tissue angiotensinogen, renin, and AT1 receptor mRNA levels in this model of hypertension.

DESIGN AND METHODS

To investigate the role of the renin-angiotensin system in fructose-induced hypertension we measured angiotensinogen, renin and angiotensin II type 1 (AT1) receptor mRNA levels in tissues of Sprague-Dawley rats that were fed either standard rat chow or a diet containing 66% fructose.

RESULTS

Blood pressure (P < 0.05) and triglyceride (P < 0.01) levels were significantly greater in the fructose-fed animals. Plasma glucose and insulin responses to an oral glucose load were significantly greater (P< 0.05) in fructose-fed than control rats. Angiotensinogen mRNA levels in liver and fat, and renin mRNA levels in kidney did not differ between fructose-fed and control animals. Levels of AT1 receptor mRNA were significantly greater in the fat obtained from fructose-fed rats than in that from control rats (P< 0.05), but this was not so in the kidney. To determine whether fructose-induced hypertension is dependent on dietary salt content, rats were fed standard rat chow and a fructose-enriched diet with low and high sodium chloride concentrations. Blood pressure increased significantly (P< 0.05) only in the fructose-fed rats receiving the high-salt diet Similarly, increased AT1 receptor mRNA levels were observed only in the fructose-fed rats that were maintained on the high-salt diet

CONCLUSIONS

Fructose feeding induces hypertension in normal- or high-salt fed animals and it is associated with an increased expression of the AT1 receptor in adipose tissue. These findings suggest that AT1 receptors might play a role in the pathophysiology of metabolic and hemodynamic abnormalities induced by fructose feeding.

Effects of angiotensin II and antagonists on AT(1) receptor expression in mesangial cells

Rat mesangial cells were exposed to angiotensin II, angiotensin AT(1) receptor antagonists such as losartan, EXP 3174 and candesartan, or dexamethasone for increasing periods (1-24 h). Angiotensin AT(1A) and AT(1B) receptor mRNA were measured by reverse transcription-polymerase chain reaction (RT-PCR). Angiotensin II, losartan and EXP 3174 did not modify significantly angiotensin AT(1A) and AT(1B) receptor mRNA. Candesartan increased angiotensin AT(1B) receptor mRNA and, to a lesser extent, angiotensin AT(1A) receptor mRNA. In contrast, dexamethasone decreased mainly angiotensin AT(1B) receptor mRNA. As shown by Western blot analysis, exposure of mesangial cells to angiotensin II, losartan or EXP 3174 did not produce any change in angiotensin AT(1) receptor protein, whereas dexamethasone and candesartan exerted inhibitory effects. In conclusion, the angiotensin AT(1B) receptor subtype, the most abundantly distributed in rat mesangial cells, is inhibited by glucocorticoids. The effect of candesartan is more complex with a slight stimulation of angiotensin AT(1B) mRNA and a marked inhibition of angiotensin AT(1) receptor protein. In contrast, angiotensin II and the other angiotensin AT(1) receptor antagonists studied are inactive on angiotensin AT(1) mRNA and protein.

Aminopeptidase A activity and angiotensin III effects on [Ca2+]i along the rat nephron

BACKGROUND

This study examined the specific effects of angiotensin III (Ang III) along the nephron.

METHODS

We examined the distribution of aminopeptidase A (APA) activity by using a specific APA inhibitor and by immunostaining with an antirat kidney APA antibody, the Ang III-induced variations of [Ca2+]i by using fura-2 and the characterization of the receptor subtype involved in the response to Ang III in cortical thick ascending limb (CTAL).

RESULTS

APA activity was found all along the nephron but was higher in the cortex than in the medulla. This was confirmed by immunostaining. Increases in [Ca2+]i elicited by 10(-7) mol/liter Ang III were observed all along the nephron. The characterization of the receptor subtype involved in the [Ca2+]i response to Ang III in CTAL indicated that EC50 values for Ang III and Ang II were similar (13.5 and 10.3 nmol/liter, respectively), and Ang III-induced responses were totally abolished by AT1 receptor but not by AT2 receptor antagonists. There was a cross-desensitization of [Ca2+]i responses to 10(-7) mol/liter Ang III and Ang II, and the [Ca2+]i responses to 10(-7) mol/liter Ang II and Ang III were not additive.

CONCLUSION

These results show that in CTAL, the [Ca2+]i responses to Ang II and Ang III occur through the same AT1a receptor because this subtype is predominant in this segment. Taken together, these data suggest that APA could be a key enzyme to generate Ang III from Ang II in the kidney.

Regulation of angiotensin II receptors and PKC isoforms by glucose in rat mesangial cells

It has been shown that glomerular angiotensin II (ANG II) receptors are downregulated and protein kinase C (PKC) is activated under diabetic conditions. We, therefore, investigated ANG II receptor and PKC isoform regulation in glomerular mesangial cells (MCs) under normal and elevated glucose concentrations. MCs were isolated from collagenase-treated rat glomeruli and cultured in medium containing normal or high glucose concentrations (5.5 and 25.0 mM, respectively). Competitive binding experiments were performed using the ANG II antagonists losartan and PD-123319, and PKC analysis was conducted by Western blotting. Competitive binding studies showed that the AT1 receptor was the only ANG II receptor detected on MCs grown to either subconfluence or confluence under either glucose concentration. AT1 receptor density was significantly downregulated in cells grown to confluence in high-glucose medium. Furthermore, elevated glucose concentration enhanced the presence of all MC PKC isoforms. In addition, PKCbeta, PKCgamma and PKCepsilon were translocated only in cells cultured in elevated glucose concentrations following 1-min stimulation by ANG II, whereas PKCalpha, PKCtheta, and PKClambda were translocated by ANG II only in cells grown in normal glucose. Moreover, no changes in the translocation of PKCdelta, PKCiota, PKCzeta, and PKCmu were detected in response to ANG II stimulation under euglycemic conditions. We conclude that MCs grown in high glucose concentration show altered ANG II receptor regulation as well as PKC isoform translocation compared with cells grown in normal glucose concentration.

Mapping tissue angiotensin-converting enzyme and angiotensin AT1, AT2 and AT4 receptors

BACKGROUND

The renin-angiotensin system (RAS) functions as both a circulating endocrine system and a tissue paracrine/autocrine system. As a circulating peptide, angiotensin II (Ang II) plays a prominent role in blood-pressure control and body fluid and electrolyte balance by acting on the AT1 receptor in the brain and peripheral tissues. As a paracrine/autocrine peptide, locally formed Ang II also plays additional roles in tissues involving the regulation of regional haemodynamics, cell growth and remodelling, and neurotransmitter release. Evidence is emerging that Ang II is not the only active peptide of the RAS, and other Ang II fragments may also have important biological activities.

OBJECTIVES

To provide a morphological basis for understanding novel actions of angiotensin-converting enzyme (ACE), Ang II and related peptides in tissues, this article will review the localization of ACE and AT1, AT2 and AT4 receptors in the central nervous system, blood vessels and kidney.

RESULTS AND CONCLUSION

Autoradiographic mapping of the major components of the RAS has proved a valuable strategy to reveal, or suggest, cellular sites of novel actions for Ang II and related peptides in tissues. First, colocalization of ACE and AT1 receptors in the substantia nigra, the caudate nucleus and putamen of human and rat brain, which contain the dopamine-synthesizing neurons, suggests that the central RAS may be important in modulating central dopamine release. Secondly, the distribution of AT4 receptors with a striking association with cholinergic neurons, motor and sensory nuclei in the brain reveals that Ang IV may modulate central motor and sensory activities and memory. Thirdly, the occurrence of high levels of ACE and AT1 and/or AT2 receptors in the adventitia of blood vessels suggests important paracrine roles of the vascular RAS. Finally, the identification of abundant AT1 receptor and elucidation of its roles in the renomedullary interstitial cells of the kidney may provide a new impetus to study further the role of Ang II in the regulation of renal medullary function and blood pressure. Overall, circulating and locally produced Ang II and related peptides may exert a remarkable range of actions in the brain, kidney and cardiovascular system through multiple angiotensin receptors.

Effects of ACTH on adrenal angiotensin II receptor subtype expression in vivo

Control of adrenal aldosterone secretion is an important endocrine mechanism mediated by angiotensin II (Ang. II). Recently three subtypes of angiotensin receptors have been demonstrated in the rat adrenal. Our aim was to examine if these receptors are affected by hormone treatment in vivo. Treatment of rats with ACTH resulted in a decrease in AT2 receptor binding from 766 +/- 95 to 310 +/- 51 fmol/mg protein (P < 0.05), without significant changes in AT1 receptors. AT2 receptor mRNA was also decreased (18 +/- 2%, of control, P < 0.05; 25+/-7% of control, P < 0.05) after both 2 and 7 day treatment with ACTH. Changes in AT1a receptor mRNA or total AT1 mRNA did not reach statistical significance. Moreover, treatment with dexamethasone or aldosterone did not affect AT1a, AT1b, or AT2 mRNA. These results demonstrate that ACTH treatment results in subtype-specific changes in adrenal AT receptor gene expression in vivo.

Expression of angiotensin type-1 (AT1) and type-2 (AT2) receptor mRNAs in the adult rat brain: a functional neuroanatomical review

The discovery that all components of the renin-angiotensin system (RAS) are present in the central nervous system led investigators to postulate the existence of a local brain RAS. Supporting this, angiotensin immunoreactive neurons have been visualized in the brain. Two major pathways were described: a forebrain pathway which connects circumventricular organs to the median preoptic nucleus, paraventricular nucleus, and supraoptic nucleus, and a second pathway connecting the hypothalamus to the medulla oblongata. Blood-brain barrier deficient circumventricular organs are rich in angiotensin II receptors. By activating these receptors, circulating angiotensin II may act on central cardiovascular centers via angiotensinergic neurons, providing a link between peripheral and central angiotensin II systems. Among the effector peptides of the brain RAS, angiotensin II and angiotensin III have the same affinity for the two pharmacologically well-defined receptors: type 1 (AT1) and type 2 (AT2). When injected in the brain, these peptides increase blood pressure, water intake, and anterior and posterior pituitary hormone release and may modify memory and learning. The cloning of AT1 and AT2 receptor cDNAs has revealed that these receptors belong to the seven transmembrane domain receptor family. In rodents, two AT1 receptor subtypes, AT1A and AT1B, have been isolated. Using specific riboprobes for in situ hybridization histochemistry, recent studies mapped the distribution of AT1A, AT1B, and AT2 receptor mRNAs in the adult rat and found a predominant expression of AT1A and AT2 mRNA in the brain and of AT1B in the pituitary. Very limited overlap was found between the brain expression of AT1A and AT2 mRNAs. In several functional entities of the brain, such as the preoptic region, the hypothalamus, the olivocerebellary system, and the brainstem baroreflex arc, the colocalization of receptor mRNA, binding sites, and angiotensin immunoreactive nerve terminals suggests local synthesis and expression of angiotensin II receptors. In other areas, such as the bed nucleus of the stria terminalis, the median eminence, or certain parts of the nucleus of the solitary tract, angiotensin II receptors are likely of extrinsic origin. The neuronal expression of AT1A and AT2 receptors was demonstrated in the subfornical organ, the hypothalamus, and the lateral septum. By using double label in situ hybridization, AT1A receptor expression was localized in corticotropin releasing hormone but not in vasopressin containing neurons in the hypothalamus. The information is discussed together with functional data concerning the role of brain angiotensins, in an attempt to provide a better understanding of the physiological and functional roles of each receptor subtype.

Evidence for angiotensin IV receptors in human collecting duct cells

Because angiotensin II (Ang II) has been found at high concentrations in the proximal tubule fluid and because tubular brush border membranes exhibit a marked capacity for degrading Ang II, we thought it of interest to examine the binding sites for Ang II (3-8) (referred to as Ang IV), a metabolite of Ang II, downstream in the nephron. We studied the binding of [125I]-Ang IV and also of [125I]-Sar1, Ala8, Ang II to SV-40 transformed human collecting duct cell (HCD) membranes. No specific binding site for [125I]-Sar1, Ala8, Ang II and no Ang II-dependent cytosolic calcium response could be observed. Moreover, no signal for the human type I Ang II receptor (hAT1) mRNA was present in HCD cells. In contrast, [125I]-Ang IV bound specifically to HCD cell membranes. Mean Kd and Bmax values derived from saturation binding studies were 5.6 +/- 2.0 nM and 1007.6 +/- 140.2 fmol/mg protein, respectively. The rank order of affinity for competitive Ang II-related peptides was: Ang IV > Ang III > Ang II > Ang II (4-8) > Ang II (1-7). [125I]-Ang IV binding was not modified by nonpeptide AT1 (losartan) or AT2 (PD123177) antagonists. GTP gamma S and dithiotreitol did not affect [125I]-Ang IV binding. Ang IV stimulated cAMP production by intact HCD cells in the presence of forskolin but did not modify cGMP production or cytosolic calcium concentration. Taken together, these results indicate that HCD cells represent a target site for Ang IV but do not possess Ang II receptors.