In vivo occupancy of angiotensin II subtype 1 receptors in rat renal medullary interstitial cells

Genetic Deletion of AT1a Receptor or Na+/H+ Exchanger 3 Selectively in the Proximal Tubules of the Kidney Attenuates Two-Kidney, One-Clip Goldblatt Hypertension in Mice

The roles of angiotensin II (Ang II) AT1 (AT1a) receptors and its downstream target Na+/H+ exchanger 3 (NHE3) in the proximal tubules in the development of two-kidney, 1-clip (2K1C) Goldblatt hypertension have not been investigated previously. The present study tested the hypothesis that deletion of the AT1a receptor or NHE3 selectively in the proximal tubules of the kidney attenuates the development of 2K1C hypertension using novel mouse models with proximal tubule-specific deletion of AT1a receptors or NHE3. 2K1C Goldblatt hypertension was induced by placing a silver clip (0.12 mm) on the left renal artery for 4 weeks in adult male wild-type (WT), global Agtr1a−/−, proximal tubule (PT)-specific PT-Agtr1a−/− or PT-Nhe3−/− mice, respectively. As expected, telemetry blood pressure increased in a time-dependent manner in WT mice, reaching a maximal response by Week 3 (p < 0.01). 2K1C hypertension in WT mice was associated with increases in renin expression in the clipped kidney and decreases in the nonclipped kidney (p < 0.05). Plasma and kidney Ang II were significantly increased in WT mice with 2K1C hypertension (p < 0.05). Tubulointerstitial fibrotic responses were significantly increased in the clipped kidney (p < 0.01). Whole-body deletion of AT1a receptors completely blocked the development of 2K1C hypertension in Agtr1a−/− mice (p < 0.01 vs. WT). Likewise, proximal tubule-specific deletion of Agtr1a in PT-Agtr1a−/− mice or NHE3 in PT-Nhe3−/− mice also blocked the development of 2K1C hypertension (p < 0.01 vs. WT). Taken together, the present study provides new evidence for a critical role of proximal tubule Ang II/AT1 (AT1a)/NHE3 axis in the development of 2K1C Goldblatt hypertension.

Kidney Angiotensin in Cardiovascular Disease: Formation and Drug Targeting

The concept of local formation of angiotensin II in the kidney has changed over the last 10-15 years. Local synthesis of angiotensinogen in the proximal tubule has been proposed, combined with prorenin synthesis in the collecting duct. Binding of prorenin via the so-called (pro)renin receptor has been introduced, as well as megalin-mediated uptake of filtered plasma-derived renin-angiotensin system (RAS) components. Moreover, angiotensin metabolites other than angiotensin II [notably angiotensin-(1-7)] exist, and angiotensins exert their effects via three different receptors, of which angiotensin II type 2 and Mas receptors are considered renoprotective, possibly in a sex-specific manner, whereas angiotensin II type 1 (AT1) receptors are believed to be deleterious. Additionally, internalized angiotensin II may stimulate intracellular receptors. Angiotensin-converting enzyme 2 (ACE2) not only generates angiotensin-(1-7) but also acts as coronavirus receptor. Multiple, if not all, cardiovascular diseases involve the kidney RAS, with renal AT1 receptors often being claimed to exert a crucial role. Urinary RAS component levels, depending on filtration, reabsorption, and local release, are believed to reflect renal RAS activity. Finally, both existing drugs (RAS inhibitors, cyclooxygenase inhibitors) and novel drugs (angiotensin receptor/neprilysin inhibitors, sodium-glucose cotransporter-2 inhibitors, soluble ACE2) affect renal angiotensin formation, thereby displaying cardiovascular efficacy. Particular in the case of the latter three, an important question is to what degree they induce renoprotection (e.g., in a renal RAS-dependent manner). This review provides a unifying view, explaining not only how kidney angiotensin formation occurs and how it is affected by drugs but also why drugs are renoprotective when altering the renal RAS. SIGNIFICANCE STATEMENT: Angiotensin formation in the kidney is widely accepted but little understood, and multiple, often contrasting concepts have been put forward over the last two decades. This paper offers a unifying view, simultaneously explaining how existing and novel drugs exert renoprotection by interfering with kidney angiotensin formation.

Intratubular and intracellular renin-angiotensin system in the kidney: a unifying perspective in blood pressure control

The renin-angiotensin system (RAS) is widely recognized as one of the most important vasoactive hormonal systems in the physiological regulation of blood pressure and the development of hypertension. This recognition is derived from, and supported by, extensive molecular, cellular, genetic, and pharmacological studies on the circulating (tissue-to-tissue), paracrine (cell-to-cell), and intracrine (intracellular, mitochondrial, nuclear) RAS during last several decades. Now, it is widely accepted that circulating and local RAS may act independently or interactively, to regulate sympathetic activity, systemic and renal hemodynamics, body salt and fluid balance, and blood pressure homeostasis. However, there remains continuous debate with respect to the specific sources of intratubular and intracellular RAS in the kidney and other tissues, the relative contributions of the circulating RAS to intratubular and intracellular RAS, and the roles of intratubular compared with intracellular RAS to the normal control of blood pressure or the development of angiotensin II (ANG II)-dependent hypertension. Based on a lecture given at the recent XI International Symposium on Vasoactive Peptides held in Horizonte, Brazil, this article reviews recent studies using mouse models with global, kidney- or proximal tubule-specific overexpression (knockin) or deletion (knockout) of components of the RAS or its receptors. Although much knowledge has been gained from cell- and tissue-specific transgenic or knockout models, a unifying and integrative approach is now required to better understand how the circulating and local intratubular/intracellular RAS act independently, or with other vasoactive systems, to regulate blood pressure, cardiovascular and kidney function.

The vasoprotective axes of the renin-angiotensin system: Physiological relevance and therapeutic implications in cardiovascular, hypertensive and kidney diseases

The renin-angiotensin system (RAS) is undisputedly one of the most prominent endocrine (tissue-to-tissue), paracrine (cell-to-cell) and intracrine (intracellular/nuclear) vasoactive systems in the physiological regulation of neural, cardiovascular, blood pressure, and kidney function. The importance of the RAS in the development and pathogenesis of cardiovascular, hypertensive and kidney diseases has now been firmly established in clinical trials and practice using renin inhibitors, angiotensin-converting enzyme (ACE) inhibitors, type 1 (AT₁) angiotensin II (ANG II) receptor blockers (ARBs), or aldosterone receptor antagonists as major therapeutic drugs. The major mechanisms of actions for these RAS inhibitors or receptor blockers are mediated primarily by blocking the detrimental effects of the classic angiotensinogen/renin/ACE/ANG II/AT₁/aldosterone axis. However, the RAS has expanded from this classic axis to include several other complex biochemical and physiological axes, which are derived from the metabolism of this classic axis. Currently, at least five axes of the RAS have been described, with each having its key substrate, enzyme, effector peptide, receptor, and/or downstream signaling pathways. These include the classic angiotensinogen/renin/ACE/ANG II/AT₁ receptor, the ANG II/APA/ANG III/AT₂/NO/cGMP, the ANG I/ANG II/ACE2/ANG (1-7)/Mas receptor, the prorenin/renin/prorenin receptor (PRR or Atp6ap2)/MAP kinases ERK1/2/V-ATPase, and the ANG III/APN/ANG IV/IRAP/AT₄ receptor axes. Since the roles and therapeutic implications of the classic angiotensinogen/renin/ACE/ANG II/AT₁ receptor axis have been extensively reviewed, this article will focus primarily on reviewing the roles and therapeutic implications of the vasoprotective axes of the RAS in cardiovascular, hypertensive and kidney diseases.

Role of caveolin 1 in AT1a receptor-mediated uptake of angiotensin II in the proximal tubule of the kidney

Caveolin 1 (CAV-1) functions not only as a constitutive scaffolding protein of caveolae but also as a vesicular transporter and signaling regulator. In the present study, we tested the hypothesis that CAV-1 knockout (CAV-1 KO) inhibits ANG II type 1 [AT1 (AT1a)] receptor-mediated uptake of ANG II in the proximal tubule and attenuates blood pressure responses in ANG II-induced hypertension. To determine the role of CAV-1 in mediating the uptake of FITC-labeled ANG II, wild-type (WT) mouse proximal convoluted tubule cells were transfected with CAV-1 small interfering (si)RNA for 48 h before AT1 receptor-mediated uptake of FITC-labeled ANG II was studied. CAV-1 siRNA knocked down CAV-1 expression by >90% (P < 0.01) and inhibited FITC-labeled ANG II uptake by >50% (P < 0.01). Moreover, CAV-1 siRNA attenuated ANG II-induced activation of MAPK ERK1/2 and Na(+)/H(+) exchanger 3 expression, respectively (P < 0.01). To determine whether CAV-1 regulates ANG II uptake in the proximal tubule, Alexa 488-labeled ANG II was infused into anesthetized WT and CAV-1 KO mice for 60 min (20 ng/min iv). Imaging analysis revealed that Alexa 488-labeled ANG II uptake was decreased by >50% in CAV-1 KO mice (P < 0.01). Furthermore, Val(5)-ANG II was infused into WT and CAV-1 KO mice for 2 wk (1.5 mg·kg(-1)·day(-1) ip). Basal systolic pressure was higher, whereas blood pressure and renal excretory and signaling responses to ANG II were attenuated, in CAV-1 KO mice (P < 0.01). We concluded that CAV-1 plays an important role in AT1 receptor-mediated uptake of ANG II in the proximal tubule and modulates blood pressure and renal responses to ANG II.

New frontiers in the intrarenal Renin-Angiotensin system: a critical review of classical and new paradigms

The renin-angiotensin system (RAS) is well-recognized as one of the oldest and most important regulators of arterial blood pressure, cardiovascular, and renal function. New frontiers have recently emerged in the RAS research well beyond its classic paradigm as a potent vasoconstrictor, an aldosterone release stimulator, or a sodium-retaining hormone. First, two new members of the RAS have been uncovered, which include the renin/(Pro)renin receptor (PRR) and angiotensin-converting enzyme 2 (ACE2). Recent studies suggest that prorenin may act on the PRR independent of the classical ACE/ANG II/AT1 receptor axis, whereas ACE2 may degrade ANG II to generate ANG (1-7), which activates the Mas receptor. Second, there is increasing evidence that ANG II may function as an intracellular peptide to activate intracellular and/or nuclear receptors. Third, currently there is a debate on the relative contribution of systemic versus intrarenal RAS to the physiological regulation of blood pressure and the development of hypertension. The objectives of this article are to review and discuss the new insights and perspectives derived from recent studies using novel transgenic mice that either overexpress or are deficient of one key enzyme, ANG peptide, or receptor of the RAS. This information may help us better understand how ANG II acts, both independently or through interactions with other members of the system, to regulate the kidney function and blood pressure in health and disease.

Novel signaling mechanisms of intracellular angiotensin II-induced NHE3 expression and activation in mouse proximal tubule cells

Expression of a cytosolic cyan fluorescent fusion protein of angiotensin II (ECFP/ANG II) in proximal tubules increases blood pressure in rodents. To determine cellular signaling pathways responsible for this response, we expressed ECFP/ANG II in transport-competent mouse proximal convoluted tubule cells (mPCT) from wild-type (WT) and type 1a ANG II receptor-deficient (AT(1a)-KO) mice and measured its effects on intracellular ANG II levels, surrogates of Na/H exchanger 3 (NHE3)-dependent Na(+) absorption, as well as MAP kinases and NF-κB signaling. In WT mPCT cells, ECFP/ANG II expression doubled ANG II levels, increased NHE3 expression and membrane phospho-NHE3 proteins threefold and intracellular Na(+) concentration by 65%. These responses were associated with threefold increases in phospho-ERK 1/2 and phospho-p38 MAPK, fivefold increases in p65 subunit of NF-κB, and threefold increases in phospho-IKKα/β (Ser 176/180) proteins. These signaling responses to ECFP/ANG II were inhibited by losartan (AT(1) blocker), PD123319 (AT(2) blocker), U0126 (MEK1/MEK2 inhibitor), and RO 106-9920 (NF-κB inhibitor). In mPCT cells of AT(1a)-KO mice, ECFP/ANG II also increased the levels of NHE3, p-ERK1/2, and p65 proteins above their controls, but considerably less so than in WT cells. In WT mice, selective expression of ECFP/ANG II in vivo in proximal tubules significantly increased blood pressure and indices of sodium reabsorption, in particular levels of phosphorylated NHE3 protein in the membrane fraction and proton gradient-stimulated (22)Na(+) uptake by proximal tubules. We conclude that intracellular ANG II may induce NHE3 expression and activation in mPCTs via AT(1a)- and AT(2) receptor-mediated activation of MAP kinases ERK 1/2 and NF-κB signaling pathways.

Intrarenal mouse renin-angiotensin system during ANG II-induced hypertension and ACE inhibition

Angiotensin-converting enzyme (ACE) inhibition (ACEi) ameliorates the development of hypertension and the intrarenal ANG II augmentation in ANG II-infused mice. To determine if these effects are associated with changes in the mouse intrarenal renin-angiotensin system, the expression of angiotensinogen (AGT), renin, ACE, angiotensin type 1 receptor (AT(1)R) mRNA (by quanitative RT-PCR) and protein [by Western blot (WB) and/or immunohistochemistry (IHC)] were analyzed. C57BL/6J male mice (9-12 wk old) were distributed as controls (n = 10), ANG II infused (ANG II = 8, 400 ng x kg(-1) x min(-1) for 12 days), ACEi only (ACEi = 10, lisinopril, 100 mg/l), and ANG II infused + ACEi (ANG II + ACEi = 11). When compared with controls (1.00), AGT protein (by WB) was increased by ANG II (1.29 +/- 0.13, P < 0.05), and this was not prevented by ACEi (ACEi + ANG II, 1.31 +/- 0.14, P < 0.05). ACE protein (by WB) was increased by ANG II (1.21 +/- 0.08, P < 0.05), and it was reduced by ACEi alone (0.88 +/- 0.07, P < 0.05) or in combination with ANG II (0.80 +/- 0.07, P < 0.05). AT(1)R protein (by WB) was increased by ANG II (1.27 +/- 0.06, P < 0.05) and ACEi (1.17 +/- 0.06, P < 0.05) but not ANG II + ACEi [1.15 +/- 0.06, not significant (NS)]. Tubular renin protein (semiquantified by IHC) was increased by ANG II (1.49 +/- 0.23, P < 0.05) and ACEi (1.57 +/- 0.15, P < 0.05), but not ANG II + ACEi (1.10 +/- 0.15, NS). No significant changes were observed in AGT, ACE, or AT(1)R mRNA. In summary, reduced responses of intrarenal tubular renin, ACE, and the AT(1)R protein to the stimulatory effects of chronic ANG II infusions, in the presence of ACEi, are associated with the effects of this treatment to ameliorate augmentations in blood pressure and intrarenal ANG II content during ANG II-induced hypertension.

In vivo regulation of AT1a receptor-mediated intracellular uptake of [125I]Val5-ANG II in the kidneys and adrenals of AT1a receptor-deficient mice

Using type 1a angiotensin receptor (AT1a) receptor-deficient (Agtr1a-/-) mice and in vivo autoradiography, we tested the hypothesis that intracellular uptake of ANG II in the kidney and adrenal glands is primarily mediated by AT1a receptors and that the response is regulated by prevailing endogenous ANG II. After pretreatment of wild-type (Agtr1a+/+) and Agtr1a-/- mice (n = 6-9 each group) with or without captopril (25 mg.kg(-1).day(-1)) or losartan (10 mg.kg(-1).day(-1)) for 2 wk, [125I]Val5-ANG II was infused for 60 min. Intracellular uptake of [125I]Val5-ANG II was determined by quantitative in vivo autoradiography after washout of circulating [125I]Val5-ANG II. Basal intracellular ANG II levels were 65% lower in the kidney (P < 0.001), but plasma ANG II levels were threefold higher, in Agtr1a-/- than wild-type mice (P < 0.01). Although plasma [125I]Val5-ANG II levels were similar, urinary excretion of [125I]Val5-ANG II was fourfold higher in Agtr1a-/- mice (P < 0.001). By contrast, intracellular [125I]Val5-ANG II levels were approximately 80% lower in the kidney and adrenal glands of Agtr1a-/- mice (P < 0.01). Captopril decreased endogenous plasma and renal ANG II levels (P < 0.01) but increased intracellular uptake of [125I]Val5-ANG II in the kidney and adrenal glands of wild-type and Agtr1a-/- mice (P < 0.01). Losartan largely blocked renal and adrenal uptake of [125I]Val5-ANG II in wild-type and Agtr1a-/- mice. Thus 80% of intracellular ANG II uptake in the kidney and adrenal glands is mediated by AT1a receptors, whereas AT1b receptor- and other non-receptor-mediated mechanisms account for 20% of the response. Our results suggest that AT1a receptor-mediated uptake of extracellular ANG II may play a physiological role in the kidney and adrenal glands.

Angiotensin II production and distribution in the kidney--II. Model-based analysis of experimental data

Information on the regional concentrations of angiotensin (Ang) II and its type-1 and -2 receptors (AT(1)R, AT(2)R) in the kidney is still incomplete. Published data on the levels of arterially delivered Ang I and II (Ang Ia, Ang IIa) and intrarenally produced Ang I and II (Ang Ii, Ang IIi) in the renal vein and in whole tissue were analyzed by using a kinetic model of Ang production and distribution in the glomerular and peritubular cortical tissue regions (Glom, Pt). (1) 90% of Ang II is cell-associated, due to its binding to AT(1)R and AT(2)R; (2) most Ang II in the renal cortex is Ang IIi; (3) Ang IIa is mainly localized in Glom; (4) Ang Ii rather than Ang Ia is a substrate of renal angiotensin-converting enzyme; (5) Ang IIi is localized in Pt and its concentration in interstitial fluid is 5-15 times the Ang II concentration in arterial plasma; and (6) in Glom the interstitial concentration of cell surface-bound AT(1)R is above 200K(d), and in Pt the AT(1)R and AT(2)R concentrations are above 10K(d). In conclusion, endocrine Ang II mainly acts in Glom, whereas Pt is exposed to paracrine Ang II generated by the conversion of intrarenally produced Ang I. High AT(1)R concentrations in Glom and Pt favor diffusion-limited binding, so that the apparent binding rate constant at sites closest to the source of Ang II delivery is greatly increased. Results may explain why the kidney is responsive to low levels of endocrine Ang II, despite its high content of paracrine Ang II.

Angiotensin II production and distribution in the kidney: I. A kinetic model

Information on the levels of angiotensin II (Ang II) and its receptors in the various renal tissue compartments is still incomplete. A model is presented describing the kinetics of Ang II production, distribution, and disposal in the renal cortex. Basic features are: (1) the model is designed to derive, from Ang II measurements in blood and in whole tissue, estimates of the local densities of the Ang II type 1 (AT(1)) and type 2 (AT(2)) receptors, and to calculate the concentrations of endocrine and paracrine Ang II they actually 'see'; (2) glomerular and peritubular tissue are conceived as separate regions (glomerular region (Glom), peritubular region (Pt)); (3) in Glom and in Pt, Ang II is homogeneously distributed in capillary blood and in interstitial fluid; (4) the model allows for local Ang II concentration gradients between interstitium and blood; (5) Ang II from the circulation diffuses into the interstitium of Glom after convective transcapillary transport; (6) Ang II produced in tubules or Pt enters the microcirculation through diffusive overflow from interstitium; (7) the presence of cell-surface-bound Ang II depends on the reaction with AT(1) and AT(2) receptors, and the presence of intracellular Ang II depends on the internalization of Ang II - AT(1) receptor complex; and (8) the model provides for glomerular filtration, vasopeptidase-mediated degradation, and intracellular degradation as mechanisms of elimination. This model can serve as a framework for detailed quantitative studies of the renin-angiotensin system in the kidney.

Intrarenal AT(1) receptor and ACE binding in ANG II-induced hypertensive rats

The intrarenal expression of angiotensin II (ANG II) type 1 (AT(1)) receptors and angiotensin-converting enzyme (ACE) was determined in ANG II-induced hypertensive rats (80 ng/min; 2 wk). Systolic blood pressure averaged 184 +/- 3 and 125 +/- 1 mmHg in ANG II-infused compared with Sham rats on day 12. Total kidney AT(1) receptor protein levels were not altered significantly. AT(1) receptor binding mapped by quantitative in vitro autoradiography was significantly decreased in glomeruli (172 +/- 25 vs. 275 +/- 34 disintegrations. min(-1). mm(-2)) and the inner stripe of the outer medulla (121 +/- 17 vs. 178 +/- 19 disintegrations. min(-1). mm(-2)), but not proximal convoluted tubules (48 +/- 9 vs. 58 +/- 6 disintegrations. min(-1). mm(-2)) of ANG II-infused compared with Sham rats. Proximal tubule ACE binding was significantly augmented (132 +/- 4 vs. 97 +/- 3 disintegrations. min(-1). mm(-2)) in ANG II-infused rats. In summary, during ANG II-induced hypertension, glomeruli and inner stripe of the outer medulla have reduced AT(1) receptor binding. Proximal convoluted tubules exhibit maintained AT(1) receptor density and increased ACE binding, which together with the elevated ANG II levels suggest that ANG II exerts a sustained influence on tubular reabsorption and consequently contributes to the development and maintenance of ANG II-dependent hypertension.

Intrarenal angiotensin II: interstitial and cellular levels and site of production

BACKGROUND

Both local production and angiotensin II subtype 1 (AT1) receptor-mediated uptake from the circulation contribute to the high levels of angiotensin (Ang) II in the kidney. It is largely unknown where Ang II is produced in the kidney and how much of it originates from the circulation.

METHODS

The concentrations of endogenous and 125I-labeled Ang I and II were measured in renal tissue and in blood from pigs receiving systemic infusions of 125I-Ang I. Pigs were either untreated or treated with the angiotensin converting enzyme (ACE) inhibitor captopril or the AT1 receptor antagonist eprosartan.

RESULTS

125I-Ang I was undetectable in renal tissue but the steady-state concentrations of 125I-Ang II in cortical and medullary tissue were four and two times the concentration in arterial blood plasma, respectively. The tissue concentrations of endogenous Ang II were 100 and 60 times higher than in arterial plasma. Eprosartan reduced 125I-Ang II accumulation by 90%, but did not lower tissue Ang II. Captopril did not alter either 125I-Ang II accumulation or tissue Ang II.

CONCLUSIONS

The bulk of Ang II in the kidney is cell-associated. The high tissue/blood concentration ratio of endogenous Ang II may depend on the same mechanism as demonstrated for 125I-Ang II, that is, AT1 receptor-mediated binding to cells and endocytosis. If so, the results indicate that most renal AT1 receptors are exposed to locally generated Ang II rather than Ang II from the circulation. We propose the existence of a low-Ang II vascular system-related interstitial compartment that is separate from tubular fluid, where, according to micropuncture studies, Ang II levels might be high.

Angiotensin II binding to renomedullary interstitial cells is regulated by osmolality

Angiotensin II (Ang II) AT(1A) receptors are localized to renomedullary interstitial cells (RMIC) in the inner stripe of the outer medulla but not in the inner medulla. Thus, there seems to be a correlation between decreases in AT(1A) receptor binding to RMIC and increases in interstitial osmolality, suggesting that osmolality is important in determining Ang II binding to RMIC. Cultured RMIC were incubated in media of differing osmolalities (330, 630, 930, and 1230 mOsm/kg H(2)O). (125)I-[Sar(1), Ile(8)] Ang II binding to AT(1A) receptors on RMIC grown in hyperosmolal media (930 mOsm/kg H(2)O) was reduced compared with isoosmolal (330 mOsm/kg H(2)O) media and was progressively reduced with further increases of osmolality. Similar studies were performed using bradykinin (BK) as a control peptide. Binding of the BK receptor ligand (125)I-[HPP-Hoe 140] to B(2) receptors was not affected by varying osmolality of the media. Reverse transcriptase-PCR demonstrated the presence of the mRNA expression for both AT(1A) and B(2) receptors at each osmolality. The conclusion is that osmolality modulates Ang II binding to RMIC; in these cells, this phenomenon is restricted to Ang II as BK binding is not affected. Osmolality-induced changes in Ang II binding may modulate the actions of this peptide on RMIC and provide an important mechanism by which these cells modulate renal medullary function.

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.

An intact renin-angiotensin system is a prerequisite for normal renal development

All components of the renin-angiotensin system (RAS) are highly expressed in the developing kidney in a pattern that suggests a role for angiotensin II in renal development In support of this notion, pharmacological interruption of angiotensin II type-1 (AT1) receptor-mediated effects in animals with an ongoing nephrogenesis produces specific renal abnormalities characterized by papillary atrophy, abnormal wall thickening of intrarenal arterioles, tubular atrophy associated with expansion of the interstitium, and a marked impairment in urinary concentrating ability. Similar changes in renal morphology and function also develop in mice with targeted inactivation of the genes that encode angiotensinogen, angiotensin converting enzyme, or both AT1 receptor isoforms simultaneously. Taken together, these results clearly indicate that an intact signalling through AT1 receptors is a prerequisite for normal renal development In a recent study, an increased incidence of congenital anomalies of the kidney and urinary tract was detected in mice deficient in the angiotensin II type-2 receptor, suggesting that this receptor subtype is also involved in the development of the genitourinary tract The present report mainly reviews the renal abnormalities that have been induced by blocking the RAS pharmacologically or by gene targeting in experimental animal models. In addition, pathogenetic mechanisms and clinical implications are discussed.

Roles of AT1 and AT2 receptors in the hypertensive Ren-2 gene transgenic rat kidney

Adult Ren-2 gene transgenic rats, TGR(mRen-2)27, exhibit elevated circulating and kidney angiotensin II (Ang II) levels in the presence of severe hypertension. The aim of this study was to examine whether AT1 and AT2 receptors in the kidney and renal hemodynamic and tubular responses to blockade of these receptors were altered in the Ren-2 gene transgenic rats during the maintenance phase of hypertension. Renal AT1 and AT2 receptors were mapped by in vitro autoradiography (n=8), and the effects of blockade of these receptors on mean arterial pressure (MAP), heart rate (HR), and renal cortical (CBF) and medullary blood flows (MBF) were studied in anaesthetized, adult age-matched male homozygous TGR rats (n=12) and Sprague-Dawley (SD) rats (n=7). TGR rats showed higher basal MAP (P<0.001), heart and kidney weight (P<0.001), plasma renin activity (P<0.05) and plasma Ang II level (P<0.05), and CBF (P<0.05) and MBF (P<0.05) than SD rats. AT1 receptor binding was significantly increased in the glomeruli, proximal tubules, and the inner stripe of the outer medulla of TGR rats (P<0.01), while the AT2 receptor binding was low at all renal sites of TGR and SD rats. Immunohistochemistry revealed that this increased AT1 receptor labeling occurred mainly in vascular smooth muscle layer of intrarenal blood vessels including afferent and efferent arterioles, juxtaglomerular apparatus, glomerular mesangial cells, proximal tubular cells, and renomedullary interstitial cells (RMICs) in the transgenic rats. Blockade of AT1 receptors with losartan in TGR rats markedly reduced MAP to the normotensive level (P<0.001) without altering HR. Both CBF (P<0.005) and MBF (P<0.05) were significantly increased by losartan in the transgenic rats. By contrast, losartan only caused a smaller decrease in MAP and an increase in renal CBF in SD rats (P<0.05). PD 123319 was without any renal effect in both SD and TGR rats. These findings suggest that markedly increased AT1 receptors in renal vasculature, glomerular mesangial cells, and RMICs in the presence of fulminant hypertension and elevated circulating and tissue Ang II levels may play an important role in the maintenance of hypertension in the Ren-2 gene transgenic rats.

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.

Localization and interactions of vasoactive peptide receptors in renomedullary interstitial cells of the kidney

Vasoactive peptides regulate renal medullary microcirculation and tubular function, but the localization of their receptors and mechanisms of actions are currently unknown. Using electron microscopic autoradiography, we have mapped the receptors for angiotensin II (Ang II [AT1 and AT2]), endothelin (ET(A) and ET(B)), and bradykinin (B2) in the rat renal medulla. Although these peptide receptors show distinct vascular and tubular distributions, they overlap strikingly in renomedullary interstitial cells (RMICs) of the inner stripe and the papilla. Using reverse transcription-polymerase chain reaction (RT-PCR) and Southern analysis, mRNAs for AT1A, ET(A), and B2 receptors were detected in cultured adult RMICs. Ang II increases intracellular inositol 1,4,5-triphosphate (IP3) and [Ca2+]i and stimulates [3H]thymidine incorporation and extracellular matrix (ECM) synthesis via AT1A receptors. Endothelin and bradykinin also stimulate cell proliferation and ECM synthesis in RMICs through ET(A) and B2 receptors, respectively, but the actions of endothelin are modulated by concurrent nitric oxide production. By contrast, AT2 receptor mRNA was detected only in embryonic RMICs, in which Ang II inhibits cell proliferation through this receptor. These results suggest that multiple vasoactive peptides may interact with RMICs to exert endocrine and/or paracrine influences on renal medullary microcirculation and tubular function.

Effects of prostaglandins and nitric oxide on the renal effects of angiotensin II in the anaesthetized rat

1. The potential influences of nitric oxide (NO) and prostaglandins on the renal effects of angiotensin II (Ang II) have been investigated in the captopril-treated anaesthetized rat by examining the effect of indomethacin or the NO synthase inhibitor, N(omega)-nitro-L-arginine methyl ester (L-NAME), on the renal responses obtained during infusion of Ang II directly into the renal circulation. 2. Intrarenal artery (i.r.a.) infusion of Ang II (1-30 ng kg(-1) min(-1)) elicited a dose-dependent decrease in renal vascular conductance (RVC; -38+/-3% at 30 ng kg(-1) min(-1); P < 0.01) and increase in filtration fraction (FF; +49+/-8%; P < 0.05) in the absence of any change in carotid mean arterial blood pressure (MBP). Urine output (Uv), absolute (UNaV) and fractional sodium excretion (FENa), and glomerular filtration rate (GFR) were unchanged during infusion of Ang II 1-30 ng kg(-1) min(-1) (+6+/-17%, +11+/-17%, +22+/-23%, and -5+/-9%, respectively, at 30 ng kg(-1) min(-1)). At higher doses, Ang II (100 and 300 ng kg(-1) min(-1)) induced further decreases in RVC, but with associated increases in MBP, Uv and UNaV. 3. Pretreatment with indomethacin (10 mg kg(-1) i.v.) had no significant effect on basal renal function, or on the Ang II-induced reduction in RVC (-25+/-7% vs -38+/-3% at Ang II 30 ng kg(-1) min(-1)). In the presence of indomethacin, Ang II tended to cause a dose-dependent decrease in GFR (-38+/-10% at 30 ng kg(-1) min(-1)); however, this effect was not statistically significant (P=0.078) when evaluated over the dose range of 1-30 ng kg(-1) min(-1), and was not accompanied by any significant changes in Uv, UNaV or FENa (-21+/-12%, -18+/-16% and +36+/-38%, respectively). 4. Pretreatment with L-NAME (10 microg kg(-1) min(-1) i.v.) tended to reduce basal RVC (control -11.8+/-1.4, +L-NAME -7.9+/-1.8 ml min(-1) mmHg(-1) x 10(-2)), and significantly increased basal FF (control +15.9+/-0.8, +L-NAME +31.0+/-3.7%). In the presence of L-NAME, renal vasoconstrictor responses to Ang II were not significantly modified (-38+/-3% vs -35+/-13% at 30 ng kg(-1) min(-1)), but Ang II now induced dose-dependent decreases in GFR, Uv and UNaV (-51+/-11%, -41+/-14% and -31+/-17%, respectively, at an infusion rate of Ang II, 30 ng kg(-1) min(-1)). When evaluated over the range of 1-30 ng kg(-1) min(-1), the effect of Ang II on GFR and Uv were statistically significant (P < 0.05), but on UNaV did not quite achieve statistical significance (P=0.066). However, there was no associated change in FENa observed, suggesting a non-tubular site of interaction between Ang II and NO. 5. In contrast to its effects after pretreatment with L-NAME alone, Ang II (1-30 ng kg(-1) min(-1)) failed to reduce renal vascular conductance in rats pretreated with the combination of L-NAME and the selective angiotensin AT1 receptor antagonist, GR117289 (1 mg kg(-1) i.v.). This suggests that the renal vascular effects of Ang II are mediated through AT1 receptors. Over the same dose range, Ang II also failed to significantly reduce GFR or Uv. 6. In conclusion, the renal haemodynamic effects of Ang II in the rat kidney appear to be modulated by cyclooxygenase-derived prostaglandins and NO. The precise site(s) of such an interaction cannot be determined from the present data, but the data suggest complex interactions at the level of the glomerulus.

Angiotensin II induces TNF production by the thick ascending limb: functional implications

The effects of angiotensin II (ANG II) on tumor necrosis factor-alpha (TNF) production were determined in freshly isolated tubules from the medullary thick ascending limb (MTAL). ANG II (10(-9) M) increased the accumulation of TNF mRNA associated with enhanced production of TNF by approximately five- to sixfold. ANG II also increased prostaglandin E2 (PGE2) production by the MTAL in a dose-dependent manner and exerted biphasic differential effects on 86Rb uptake, depending on the exposure time of the tubules to the peptide and the doses used. Low-dose ANG II (10(-11) M) increased 86Rb uptake by MTAL tubules after a "short-term" (15 min) challenge, whereas uptake was inhibited after a "long-term" (3 h) incubation period. High-dose ANG II (10(-6) M) inhibited MTAL 86Rb uptake, irrespective of incubation time. Uptake of 86Rb was inhibited by approximately 60% in MTAL tubules that were challenged for 3 h with ANG II. The inhibitory action of ANG II was prevented by eliminating the participation of either TNF with antisera to the cytokine or PGE2 by inhibition of cyclooxygenase with indomethacin. We conclude that ANG II regulates TNF production in the MTAL, an interaction that affects 86Rb uptake via an eicosanoid-dependent mechanism in this nephron segment.

Modulation of renal glomerular angiotensin II receptors by ace inhibition and AT1 receptor antagonism

Angiotensin-converting enzyme inhibitors (ACE-I) and specific nonpeptide angiotensin II (ANG II) receptor antagonists have been used extensively to treat a variety of cardiovascular disorders in experimental animals and humans. Despite their widespread use, only a limited amount of data has been published regarding the effect that renin-angiotensin system (RAS) blockade may have on ANG II receptors, and very often this information is contradictory. The present study was designed to investigate whether changes in plasma ANG II levels induced by RAS blockade could alter glomerular ANG II receptor characteristics. Captopril was employed as an ACE-I with losartan and TCV-116, two AT1 receptor antagonists of different chemical structure. Two experimental protocols were established. Protocol 1 contained 3 experimental groups: controls (Sprague-Dawley rats, 250-300 g BW), and animals treated with either captopril (0.5 g/l via drinking water) or losartan (10 mg/kg BW p.o.). In protocol 2, the animals were treated as in protocol 1 except that losartan was replaced by TCV-116 (1 mg/kg BW p.o.). At the end of treatment (3 days), all groups were killed by decapitation, blood was collected for plasma renin activity (PRA) measurement, and hearts and kidneys were excised. ANG II receptors were assessed by radioligand binding assays on membrane preparations of purified glomeruli, by displacement of 125I-[Sar1, Ile8]-ANG II with specific nonpeptide antagonists of AT1 (losartan) and AT2 (PD 123319) receptor subtypes. RAS blockade by either ACE-I or AT1 antagonists increased PRA. The binding assays showed that renal glomeruli from treated rats and controls expressed a single population (AT1) of ANG II receptors. The density of glomerular AT1 receptors was not modulated by captopril, but was significantly lower in animals treated with either losartan (Bmax: 854 +/- 169 vs. 379 +/- 79 fmol/mg protein and Kd: 59 +/- 6 vs. 45 +/- 6 nM for controls and losartan, respectively) or TCV-116 (480 +/- 72 vs. 188 +/- 16 fmol/mg protein and Kd: 45 +/- 9 vs. 37 +/- 18 nM for controls and TCV-116, respectively) than in their controls. No changes in receptor affinity (Kd) were detected. Previous membrane "acid-wash" did not modify the results. We conclude that short-term RAS blockade by AT1 antagonists, but not by ACE-I, induces true downregulation of renal glomerular ANG II receptors. No AT2 receptor subtype was detected.