Efferent arterioles exclusively express the subtype 1A angiotensin receptor: functional insights from genetic mouse models

Novel hemodynamic structures in the human glomerulus

To investigate human glomerular structure under conditions of physiological perfusion, we have analyzed fresh and perfusion-fixed normal human glomeruli at physiological hydrostatic and oncotic pressures using serial resin section reconstruction, confocal, multiphoton, and electron microscope imaging. Afferent and efferent arterioles (21.5 ± 1.2 µm and 15.9 ± 1.2 µm diameter), recognized from vascular origins, lead into previously undescribed wider regions (43.2 ± 2.8 µm and 38.4 ± 4.9 µm diameter) we have termed vascular chambers (VCs) embedded in the mesangium of the vascular pole. Afferent VC (AVC) volume was 1.6-fold greater than efferent VC (EVC) volume. From the AVC, long nonbranching high-capacity conduit vessels ( n = 7) (Con; 15.9 ± 0.7 µm diameter) led to the glomerular edge, where branching was more frequent. Conduit vessels have fewer podocytes than filtration capillaries. VCs were confirmed in fixed and unfixed specimens with a layer of banded collagen identified in AVC walls by multiphoton and electron microscopy. Thirteen highly branched efferent first-order vessels (E1; 9.9 ± 0.4 µm diameter) converge on the EVC, draining into the efferent arteriole (15.9 ± 1.2 µm diameter). Banded collagen was scarce around EVCs. This previously undescribed branching topology does not conform to the branching of minimum energy expenditure (Murray's law), suggesting that even distribution of pressure/flow to the filtration capillaries is more important than maintaining the minimum work required for blood flow. We propose that AVCs act as plenum manifolds possibly aided by vortical flow in distributing and balancing blood flow/pressure to conduit vessels supplying glomerular lobules. These major adaptations to glomerular capillary structure could regulate hemodynamic pressure and flow in human glomerular capillaries.

Lack of contribution of nitric oxide synthase to cholinergic vasodilation in murine renal afferent arterioles

We have previously reported significant increases in neuronal nitric oxide synthase (NOS) immunostaining in renal arterioles of angiotensin type 1A receptor (AT1A) knockout mice, and in arterioles and macula densa cells of AT1A/AT1B knockout mice. The contribution of nitric oxide derived from endothelial and macula densa cells in the maintenance of afferent arteriolar tone and acetylcholine-induced vasodilation was functionally determined in kidneys of wild-type, AT1A, and AT1A/AT1B knockout mice. Acetylcholine-induced changes in arteriolar diameters of in vitro blood-perfused juxtamedullary nephrons were measured during control conditions, in the presence of the nonspecific NOS inhibitor, N^ω-nitro-l-arginine methyl ester (NLA), or the highly selective neuronal NOS inhibitor, N⁵-(1-imino-3-butenyl)-l-ornithine (VNIO). Acetylcholine (0.1 mM) produced a significant vasoconstriction in afferent arterioles of AT1A/AT1B mice (-10.9 ± 5.1%) and no changes in afferent arteriolar diameters of AT1A knockout mice. NLA (0.01-1 mM) or VNIO (0.01-1 μM) induced significant dose-dependent vasoconstrictions (-19.8 ± 4.0% 1 mM NLA; -7.8 ± 3.5% 1 μM VNIO) in afferent arterioles of kidneys of wild-type mice. VNIO had no effect on afferent arteriole diameters of AT1A knockout or AT1A/AT1B knockout mice, suggesting nonfunctional neuronal nitric oxide synthase. These data indicate that acetylcholine produces a significant renal afferent arteriole vasodilation independently of nitric oxide synthases in wild-type mice. AT1A receptors are essential for the manifestation of renal afferent arteriole responses to neuronal nitric oxide synthase-mediated nitric oxide release.

PGE2, Kidney Disease, and Cardiovascular Risk: Beyond Hypertension and Diabetes

An important measure of cardiovascular health is obtained by evaluating the global cardiovascular risk, which comprises a number of factors, including hypertension and type 2 diabetes, the leading causes of illness and death in the world, as well as the metabolic syndrome. Altered immunity, inflammation, and oxidative stress underlie many of the changes associated with cardiovascular disease, diabetes, and the metabolic syndrome, and recent efforts have begun to elucidate the contribution of PGE2 in these events. This review summarizes the role of PGE2 in kidney disease outcomes that accelerate cardiovascular disease, highlights the role of cyclooxygenase-2/microsomal PGE synthase 1/PGE2 signaling in hypertension and diabetes, and outlines the contribution of PGE2 to other aspects of the metabolic syndrome, particularly abdominal adiposity, dyslipidemia, and atherogenesis. A clearer understanding of the role of PGE2 could lead to new avenues to improve therapeutic options and disease management strategies.

Intrarenal renin-angiotensin system in regulation of glomerular function

PURPOSE OF REVIEW

The purpose of this review is to provide an update on the current knowledge regarding the role of the intrarenal rennin-angiotensin system (RAS) in the regulation of glomerular function including glomerular dynamics and filtration rate, glomerular permeability and structural alterations during chronic increases in intrarenal angiotensin (Ang) II.

RECENT FINDINGS

Recent studies have continued to delineate the complex interactions among the various RAS components that participate in regulating glomerular function. Although Ang II acting on AT1 receptors remains as the predominant influence on glomerular dynamics, some of these effects are indirectly mediated by Ang II modulating the sensitivity of the macula densa tubuloglomerular feedback mechanism as well as the more recently described feedback mechanism from the connecting tubule. Interestingly, the actions of Ang II on these systems cause opposite effects on glomerular function demonstrating the complexities associated with the influences of Ang II on glomerular function. When chronically elevated, Ang II also stimulates and/or interacts with other factors, including reactive oxygen species, cytokines and growth factors and other hormones or paracrine agents, to elicit structural alterations.

SUMMARY

Recent studies have provided further evidence for the presence of many components of the RAS in glomerular structures, which supports the importance of locally produced angiotensin peptides to regulate glomerular haemodynamics, filtration rate and macromolecular permeability and contribute to fibrosis and glomerular injury when inappropriately augmented.

Chronic kidney disease: targeting prostaglandin E2 receptors

Chronic kidney disease is a leading cause of morbidity and mortality in the world. A better understanding of disease mechanisms has been gained in recent years, but the current management strategies are ineffective at preventing disease progression. A widespread focus of research is placed on elucidating the specific processes implicated to find more effective therapeutic options. PGE2, acting on its four EP receptors, regulates many renal disease processes; thus EP receptors could prove to be important targets for kidney disease intervention strategies. This review summarizes the major pathogenic mechanisms contributing to initiation and progression of chronic kidney disease, emphasizing the role of hyperglycemia, hypertension, inflammation, and oxidative stress. We have long recognized the multifaceted role of PGs in both the initiation and progression of chronic kidney disease, yet studies are only now seriously contemplating specific EP receptors as targets for therapy. Given the plethora of renal complications attributed to PG involvement in the kidney, this review highlights these pathogenic events and emphasizes the PGE2 receptor targets as options available to complement current therapeutic strategies.

Proximal nephron

The kidney plays a fundamental role in maintaining body salt and fluid balance and blood pressure homeostasis through the actions of its proximal and distal tubular segments of nephrons. However, proximal tubules are well recognized to exert a more prominent role than distal counterparts. Proximal tubules are responsible for reabsorbing approximately 65% of filtered load and most, if not all, of filtered amino acids, glucose, solutes, and low molecular weight proteins. Proximal tubules also play a key role in regulating acid-base balance by reabsorbing approximately 80% of filtered bicarbonate. The purpose of this review article is to provide a comprehensive overview of new insights and perspectives into current understanding of proximal tubules of nephrons, with an emphasis on the ultrastructure, molecular biology, cellular and integrative physiology, and the underlying signaling transduction mechanisms. The review is divided into three closely related sections. The first section focuses on the classification of nephrons and recent perspectives on the potential role of nephron numbers in human health and diseases. The second section reviews recent research on the structural and biochemical basis of proximal tubular function. The final section provides a comprehensive overview of new insights and perspectives in the physiological regulation of proximal tubular transport by vasoactive hormones. In the latter section, attention is particularly paid to new insights and perspectives learnt from recent cloning of transporters, development of transgenic animals with knockout or knockin of a particular gene of interest, and mapping of signaling pathways using microarrays and/or physiological proteomic approaches.

The renin-angiotensin-aldosterone system in podocytes

The renin-angiotensin-aldosterone system (RAAS) plays a critical role in kidney function and its inhibition reduces proteinuria and preserves kidney function in patients with chronic kidney disease. Recent studies have shown that podocytes generate many components of the RAAS and they express receptors of RAAS, including angiotensin II, mineralocorticoid, and prorenin receptors. Crucial functions of podocytes, such as contraction, apoptosis, autophagocytosis, and cytoskeletal organization, have been shown to be regulated by the angiotensin II type 1 receptors. An activation of the glomerular RAAS and protection from podocyte injury by RAAS inhibitors have been shown in many glomerular diseases. Exploring the interaction between the local RAAS and the signaling involved in RAAS activation in podocytes will lead to new therapeutic strategies of podocyte protection.

Effect of prenatal exposure to nicotine on kidney glomerular mass and AT1R expression in genetically diverse strains of rats

Prenatal exposure to maternal cigarette smoking in humans or nicotine in experimental animals is associated with elevated blood pressure in the offspring. This effect may be limited to genetically vulnerable individuals and related to alterations in the kidneys. Here we investigated whether prenatal exposure to nicotine (PEN) alters kidney morphology and gene expression, and whether these effects differ between two genetically distant strains, i.e. spontaneously hypertensive (SHR) and Brown Norway (BN) rats. The results showed that, in SHR but not in BN offspring, PEN decreases kidney glomerular mass and increases renal expression of the angiotensin II type 1b receptor gene; the latter is not mediated through changes in DNA methylation of the proximal promoter of this gene. The results also showed that PEN alters expression of multiple genes involved in the kidney nervous system function, with mostly opposite effects being seen in SHR and BN. These results suggest that, in genetically vulnerable individuals, PEN leads to morphological and molecular changes in the kidneys that may contribute to fetal programming of hypertension.

Pharmacological targets in the renal peritubular microenvironment: implications for therapy for sepsis-induced acute kidney injury

One of the most frequent and serious complications to develop in septic patients is acute kidney injury (AKI), a disorder characterized by a rapid failure of the kidneys to adequately filter the blood, regulate ion and water balance, and generate urine. AKI greatly worsens the already poor prognosis of sepsis and increases cost of care. To date, therapies have been mostly supportive; consequently there has been little change in the mortality rates over the last decade. This is due, at least in part, to the delay in establishing clinical evidence of an infection and the associated presence of the systemic inflammatory response syndrome and thus, a delay in initiating therapy. A second reason is a lack of understanding regarding the mechanisms leading to renal injury, which has hindered the development of more targeted therapies. In this review, we summarize recent studies, which have examined the development of renal injury during sepsis and propose how changes in the peritubular capillary microenvironment lead to and then perpetuate microcirculatory failure and tubular epithelial cell injury. We also discuss a number of potential therapeutic targets in the renal peritubular microenvironment, which may prevent or lessen injury and/or promote recovery.

Developmental renin expression in mice with a defective renin-angiotensin system

During nephrogenesis, renin expression shifts from the vessel walls of interlobular arteries to the terminal portions of afferent arterioles in a wavelike pattern. Since the mechanisms responsible for the developmental deactivation of renin expression are as yet unknown, we hypothesized that the developing renin-angiotensin system (RAS) may downregulate itself via negative feedback to prevent overactivity of renin. To test for a possible role of angiotensin II in the developmental deactivation of renin expression, we studied the development of intrarenal renin expression in mice lacking ANG II AT1a, AT1b, or AT2 receptors and in animals with abolished circulating ANG II due to deletion of the gene for angiotensin I-converting enzyme (ACE). The development of intrarenal renin expression was normal in mice lacking ANG II AT1b or AT2 receptors. In animals lacking both ANG II AT1a and AT1b receptors, ACE, or ANG II AT1a receptors, renin expression was normal early and renin disappeared from mature vessels until development of cortical interlobular and afferent arterioles began. The development of cortical vessels in these genotypes was accompanied by a markedly increased number of renin-expressing cells, many of which were ectopically located and attached in a grapelike fashion to the outer vessel perimeter. Although the number of renin-expressing cells declined during final maturation of the kidneys, the atypical distribution pattern of renin cells was maintained. These findings suggest that ANG II does not play a central role in the typical developmental shift in renin expression from the arcuate vessels to the afferent arterioles. During postnatal maturation of mouse kidneys, interruption of the RAS causes severe hyperplasia of renin cells via a mechanism that centrally involves AT1a receptors. However, the distribution pattern of renin cells in adult kidneys with an interrupted RAS does not mimic any normal developmental stage since renin expression is frequently found in cells outside the arteriolar vessel walls in RAS mutants.

Glomerular type 1 angiotensin receptors augment kidney injury and inflammation in murine autoimmune nephritis

Studies in humans and animal models indicate a key contribution of angiotensin II to the pathogenesis of glomerular diseases. To examine the role of type 1 angiotensin (AT1) receptors in glomerular inflammation associated with autoimmune disease, we generated MRL-Faslpr/lpr (lpr) mice lacking the major murine type 1 angiotensin receptor (AT1A); lpr mice develop a generalized autoimmune disease with glomerulonephritis that resembles SLE. Surprisingly, AT1A deficiency was not protective against disease but instead substantially accelerated mortality, proteinuria, and kidney pathology. Increased disease severity was not a direct effect of immune cells, since transplantation of AT1A-deficient bone marrow did not affect survival. Moreover, autoimmune injury in extrarenal tissues, including skin, heart, and joints, was unaffected by AT1A deficiency. In murine systems, there is a second type 1 angiotensin receptor isoform, AT1B, and its expression is especially prominent in the renal glomerulus within podocytes. Further, expression of renin was enhanced in kidneys of AT1A-deficient lpr mice, and they showed evidence of exaggerated AT1B receptor activation, including substantially increased podocyte injury and expression of inflammatory mediators. Administration of losartan, which blocks all type 1 angiotensin receptors, reduced markers of kidney disease, including proteinuria, glomerular pathology, and cytokine mRNA expression. Since AT1A-deficient lpr mice had low blood pressure, these findings suggest that activation of type 1 angiotensin receptors in the glomerulus is sufficient to accelerate renal injury and inflammation in the absence of hypertension.

Up- and down-regulation of angiotensin II AT1-A and AT1-B receptors in afferent and efferent rat kidney arterioles

Introduction. The contractile effect of angiotensin II via AT1 receptors on the kidney arterioles is a crucial element in the kidney microcirculation.Angiotensin II also plays a role as an inhibitor via the AT1 receptors in the renin granulation of the arterioles.We have previously demonstrated that the AT1 receptors are downregulated in the renin-positive smooth muscle cells (SMCs) in contrast to renin-negative SMCs. In this study, we estimated the numbers of the AT1 receptor sub-types separately in the afferent and efferent arterioles and the renin-positive and renin-negative SMCs.

Methods. The immunohistochemical signals of theAT1-A and AT1-B receptors were counted by stereological methods.1

Results. The number of AT1-B receptors in the efferent arterioles (expressed in signals/µm 3; mean (CV): 0.32 (0.33)) was significantly higher (78%; p<0.05) as compared with the number in the afferent arterioles (0.18 (0.11)). No differences were found in the AT1-A receptors. In a number ofAT 1-A receptors, significant differences (p<0.01) were detected between the afferent arteriolar renin-positive SMCs (0.13 (0.36)) and the number in renin-negative SMCs (0.25 (0.34)).The AT1-B receptors did not display any differences.

Conclusions. These results indicate that the AT1 receptor sub-types are regulated independently in the SMCs of the normal kidney arterioles.

Pivotal role of angiotensin II receptor subtype 1A in the development of two-kidney, one-clip hypertension: study in angiotensin II receptor subtype 1A knockout mice

OBJECTIVE

The present study was performed to examine in two-kidney, one-clip (2K1C) Goldblatt hypertensive mice: first, the relative contribution of angiotensin II receptor subtypes 1A (AT(1A)) and 1B (AT(1B)); second, the role of angiotensin II type 2 (AT(2)) receptors in the development of hypertension in wild-type (AT(1A)+/+) and AT(1A) receptor knockout (AT(1A)-/-) mice; and third, the role of increased nitric oxide synthase activity in counteracting the hypertensinogenic action of angiotensin II in this model.

METHODS

AT(1A)+/+ and AT(1A)-/- mice underwent clipping of one renal artery and were infused with either saline vehicle or selective AT(2) receptor agonist CGP-42112A (CGP). Blood pressure was monitored by radiotelemetry. Blood pressure responses to the nitric oxide synthase inhibitor nitro-L-arginine-methyl-ester were evaluated.

RESULTS

AT(1A)+/+ mice responded to clipping by a rise in blood pressure that was not modified by CGP infusion. Clip placement caused a slight increase in blood pressure in AT(1A)-/- mice that remained significantly lower than in AT(1A)+/+ mice. Acute nitric oxide synthase inhibition caused greater increase in blood pressure in 2K1C/AT(1A)+/+ than in AT(1A)+/+ mice.

CONCLUSION

The present data support the critical role of AT(1A) receptors in the development of 2K1C hypertension, whereas AT(1B) receptors play only a minor role in blood pressure regulation in this model of angiotensin II-dependent hypertension. Activation of AT(2) receptors does not play an antagonistic role in the AT(1) receptor-mediated hypertensinogenic actions of angiotensin II in this model. Finally, enhanced nitric oxide synthase activity plays a protective role by counteracting the vasoconstrictor influences of angiotensin II in 2K1C hypertensive mice.

Intact renal afferent arteriolar autoregulatory responsiveness in db/db mice

The db/db mouse is a genetic model of type 2 diabetes that exhibits progressive renal disease. Obesity, hyperglycemia, and albuminuria (822 +/- 365 vs. 28 +/- 8 microg/day) are evident in 18-wk-old db/db compared with db/m (lean littermate control) mice. Our goal was to determine the blood pressure (BP) phenotype of the db/db mouse. Mean arterial BP measured in conscious mice by radiotelemetry was not different between db/db (n = 9) and db/m (n = 12) mice, averaging 113 +/- 3 and 112 +/- 2 mmHg, respectively. The circadian BP profile of db/db mice was shifted to the left and exhibited a significant reduction in amplitude compared with db/m mice. Heart rate (487 +/- 9 vs. 542 +/- 7 beats/min; P < 0.05) and locomotor activity were significantly reduced in db/db compared with db/m mice. We tested the hypothesis that intact afferent arteriole (AA) responsiveness to increases in renal artery pressure (RAP) and angiotensin (ANG) II sensitivity contributes to normal BP in this diabetic model. AA diameters of in vitro blood-perfused juxtamedullary nephrons of db/db mice (15.7 +/- 0.5 microm; n = 38) were significantly larger than those of db/m mice (12.5 +/- 0.4 microm; n = 37). AA responses to increases in RAP and ANG II were not different between kidneys of db/db and db/m mice. Significant AA vasoconstriction to 1 nM ANG II was observed in kidneys of db/db mice (-11 +/- 4%), while 10 nM ANG II decreased AA diameter in both groups [db/db, -20 +/- 4%, (n = 12); db/m, -26 +/- 4% (n = 12)]. In summary, AA responses to increases in renal perfusion pressure and ANG II remain intact in db/db mice. Diabetic renal disease occurs in db/db mice independently of elevated BP.

Augmented renal vascular nNOS and renin protein expression in angiotensin type 1 receptor null mice

The present study was performed to determine the influence of absence of angiotensin type 1A (AT(1A)) and/or AT(1B) receptor feedback regulation of kidney neuronal nitric oxide synthase (nNOS) and renin protein expression. Kidneys were harvested from wild-type (WT), AT(1A)(-/-), AT(1B)(-/-), and AT(1A)(-/-)AT(1B)(-/-) mice and immunostained for nNOS and renin protein localization. AT(1A)(-/-) and AT(1A)(-/-)AT(1B)(-/-) kidneys demonstrated an increase in the percentage of glomeruli with nNOS-positive afferent and interlobular arterioles compared with WT mice. Density of vascular nNOS immunostaining was 20-fold higher in kidneys of AT(1A)(-/-) and AT(1A)(-/-)AT(1B)(-/-) compared with WT mice. Density of macula densa nNOS immunostaining was 7-fold higher in AT(1A)(-/-)AT(1B)(-/-) than in WT mice. Percent of glomeruli positive for juxtaglomerular (JG) cell renin was 3-fold higher, whereas the density of JG cell renin immunostaining was 15-fold higher in kidneys of AT(1A)(-/-) and AT(1A)(-/-)AT(1B)(-/-) compared with WT mice. Kidneys of AT(1A)(-/-) and AT(1A)(-/-)AT(1B)(-/-) mice displayed recruitment of renin protein expression along afferent and interlobular arterioles. Absence of AT(1) receptor signaling resulted in enhanced nNOS protein expression in both microvascular and tubular structures. Enhanced NO generation may contribute to the reduced renal vascular tone and blood pressure observed with blockade of the renin-angiotensin system.

Genetic deletion of AT1a receptors attenuates intracellular accumulation of ANG II in the kidney of AT1a receptor-deficient mice

We and others have previously shown that high levels of ANG II are accumulated in the rat kidney via a type 1 (AT(1)) receptor-mediated mechanism, but it is not known which AT(1) receptor is involved in this process in rodents. We tested the hypothesis that AT(1a) receptor-deficient mice (Agtr1a-/-) are unable to accumulate ANG II intracellularly in the kidney because of the absence of AT(1a) receptor-mediated endocytosis. Adult male wild-type (Agtr1a+/+), heterozygous (Agtr1a+/-), and Agtr1a-/- were treated with vehicle, ANG II (40 ng/min ip via osmotic minipump), or ANG II plus the AT(1) antagonist losartan (10 mg.kg(-1).day(-1) po) for 2 wk. In wild-type mice, ANG II induced hypertension (168 +/- 4 vs. 113 +/- 3 mmHg, P < 0.001), increased kidney-to-body weight ratio (P < 0.01), caused pressure natriuresis (P < 0.05), and elevated plasma and whole kidney ANG II levels (P < 0.001). Concurrent administration of ANG II with losartan attenuated these responses to ANG II. In contrast, Agtr1a-/- mice had lower basal systolic pressures (P < 0.001), smaller kidneys with much fewer AT(1b) receptors (P < 0.001), higher basal 24-h urinary sodium excretion (P < 0.01), as well as basal plasma and whole kidney ANG II levels (P < 0.01). However, intracellular ANG II levels in the kidney were lower in Agtr1a-/- mice. In Agtr1a-/- mice, ANG II slightly increased systolic pressure (P < 0.05) but had no effect on the kidney weight, urinary sodium excretion, and whole kidney ANG II levels. Losartan restored systolic pressure to basal levels and decreased whole kidney ANG II levels by approximately 20% (P < 0.05). These results demonstrate a predominant role of AT(1a) receptors in blood pressure regulation and in the renal responses to long-term ANG II administration, that AT(1b) receptors may play a limited role in blood pressure control and mediating intrarenal ANG II accumulation in the absence of AT(1a) receptors.

Compromised renal microvascular reactivity of angiotensin type 1 double null mice

Angiotensin type 1A (AT(1A)) and 1B (AT(1B)) receptor deletion (AT1DKO) results in renal microvascular disease, tubulointerstitial injury, and reduced blood pressure. To test the hypothesis that renal preglomerular responses to angiotensin (ANG) II are mediated by AT(1A) and AT(1B) receptors, experiments were performed in AT1DKO mice using the in vitro blood perfused juxtamedullary nephron technique. Kidneys were harvested from AT1DKO and wild-type (WT) mice and bathed with ANG II (1-100 nM), norepinephrine (NE; 100-1,000 nM), or acetylcholine (ACh; 10 microM). Baseline diameters of afferent (19.5 +/- 0.7 and 13.9 +/- 0.7 microm, n = 17 and 16) and efferent (15.5 +/- 2.1 and 10.8 +/- 1.0 microm, n = 4 and 7) arterioles of AT1DKO were significantly larger than WT. Afferent and efferent arteriolar responses to ANG II, 100, and 300 nM NE were absent in AT1DKO; although significant constriction to 1 microM NE was observed (-17 +/- 5 and -23 +/- 6%, respectively). Afferent arterioles of WT mice dilated significantly in response to ACh (15.1 +/- 0.6 to 17.0 +/- 1.2 microm, n = 6); however, arterioles from AT1DKO tended to contract (19.9 +/- 1.2 to 17.8 +/- 1.6 microm; n = 6, P = 0.06). In summary, loss of ANG II-induced contraction, reduced vasoconstriction to NE, and endothelial cell dysfunction contribute to the renal vascular phenotype of AT1DKO mice. We conclude that ANG II signaling via the AT(1) receptor plays a pivotal role in basal renal microvascular tone and effectiveness to respond to vasoconstrictor and vasodilator agonists.

The Bradykinin B2 receptor gene is a target of angiotensin II type 1 receptor signaling

Cross-talk between G protein-coupled receptors (GPCR) is known to occur at multiple levels, including receptor heterodimerization and intracellular signaling. This study tested the hypothesis that GPCR cross-talk occurs at the transcriptional level. It was demonstrated that the bradykinin B2 receptor gene (BdkrB2) is a direct transcriptional target of the angiotensin II (AngII) type 1 receptor (AT(1)R) in collecting duct cells. AngII induced BdkrB2 mRNA expression in mouse inner medullary collecting duct cells, and this effect was abrogated by AT(1)R blockade; in contrast, AT(2)R blockade was ineffective. Actinomycin D, an inhibitor of gene transcription, abrogated AngII-stimulated BdkrB2 expression. In addition, AngII produced dosage- and time-dependent increases in B2 receptor protein levels (2.9 +/- 0.4 fold; P < 0.05). AngII stimulated phosphorylation of cAMP response element binding protein (CREB) on Ser-133 and assembly of p-CREB on the BdkrB2 promoter in vivo. Moreover, AngII induced hyperacetylation of BdkrB2 promoter-associated H4 histones, a chromatin modification that is associated with gene activation. Mutations of the CRE abrogated AngII-induced activation of the BdkrB2 promoter. AngII-treated inner medullary collecting duct cells exhibited augmented intracellular calcium signaling in response to bradykinin, confirming the functional relevance of AT(1)-B2 receptor signaling. Finally, studies that were conducted in angiotensin type 1 receptor (Agtr1)-null mice revealed that BdkrB2 mRNA levels were significantly lower in the renal medulla of Agtr1(A)(-/-) and Agtr1(A/B)(-/-) than in Agtr1(+/+) and Agtr1(B)(-/-) mice. It is concluded that BdkrB2 is a downstream target of the AT(1)R-CREB signaling pathway. Transcriptional regulation represents a novel form of cross-talk between GPCR that link the renin-angiotensin and kallikrein-kinin systems.