Function Of AT2 Receptor Research Articles

Inhibition of NAD(P)H oxidase potentiates AT2 receptor agonist-induced natriuresis in Sprague-Dawley rats

A positive association between renin-angiotensin system, especially AT1 receptor, and oxidative stress in the pathogenesis of hypertension and cardiovascular/renal diseases has been suggested. However, the role of oxidative stress, especially superoxide radicals in renal sodium handling in response to AT1 and AT2 receptors, is not known. Therefore, the present study was designed to investigate the role of NAD(P)H oxidase (NOX), a major superoxide radical producing enzyme, in AT1 and AT2 receptor function on natriuresis/diuresis in Sprague-Dawley rats. The rats under anesthesia were intravenously infused with NOX inhibitor apocynin (3.5 μg·kg(-1)·min(-1)), the AT1 receptor antagonist candesartan (100 μg/kg; bolus), and the AT2 receptor agonist CGP-42112A (1 μg·kg(-1)·min(-1)) alone and in combinations. Candesartan alone significantly increased urinary flow (UF; μl/30 min) by 53 and urinary Na excretion (U(Na)V; μmol/min) by 0.4 over basal. Preinfusion of apocynin had no effect on the net increase in UF or U(Na)V in response to candesartan. On the other hand, apocynin preinfusion caused profound increases in CGP-42112A-induced UF by 72, U(Na)V by 1.14, and fractional excretion of Na by 7.8. Apocynin and CGP-42112A alone did not cause significant increase in UF or U(Na)V over the basal. CGP-42112A infusion in the presence of apocynin increased urinary nitrite/nitrates and cGMP over basal. The infusion of candesartan, apocynin, and CGP-42112A alone or in combinations had no effect on the blood pressure or the glomerular filtration rate, suggesting tubular effects on natriuresis/diuresis. The data suggest that NOX may have an antagonistic role in AT2 receptor-mediated natriuresis/diuresis possibly via neutralizing nitric oxide and thereby influence fluid-Na homeostasis.

The past, present and future of angiotensin II type 2 receptor stimulation

Studying the angiotensin type 2 receptor (AT(2)) has been problematic in the past because a pharmacological tool for direct, specific in vitro and in vivo stimulation of the receptor has been lacking. Consequently, current knowledge about AT(2) receptor signalling and function had to be obtained by indirect approaches, like studying animals or cells with genetically altered AT(2) receptor expression levels, inhibitory experiments using specific AT(2) receptor antagonists, stimulation with angiotensin II under concomitant angiotensin II type 1 receptor blockade or stimulation with the peptide agonist CGP42112A, which has additional AT(2) receptor antagonistic properties. The recently developed non-peptide AT(2) receptor agonist Compound 21 now, for the first time, allows direct, selective and specific AT(2) receptor stimulation in vitro and in vivo. This new tool will certainly revolutionize AT(2) receptor research, enable many new insights into AT(2) receptor function and may also have the potential to become a future medical drug. This article reviews milestone findings about AT(2) receptor functional properties obtained by 'conventional' experimental approaches within the last 20 years. Moreover, it provides an overview of the first results obtained by direct AT(2) receptor stimulation with Compound 21, comprising effects on alkaline secretion, neurite outgrowth, blood pressure and post-infarct cardiac function.

Contractility of Placental Vascular Smooth Muscle Cells in Response to Stimuli Produced by the Placenta: Roles of ACE vs. Non-ACE and AT1 vs. AT2 in Placental Vessel Cells

Our previously published work has shown that non-ACE angiotensin II (Ang II) generating system is dominate in the placenta and may play a critical role in regulation of placental vascular contractile function. In the present study, using a collagen gel contraction assay we further studied contractility of placental vascular smooth muscle cells (VSMCs) in response to factors produced by preeclamptic (PE) placentas. Placental VSMCs/type-1 collagen gels were incubated with PE placental conditioned medium in the presence or absence of inhibitors or receptor blockers. Captopril (an ACE inhibitor), chymostatin (a non-ACE chymase inhibitor), losartan (an AT1 receptor blocker) and PD123,319 (an AT2 receptor blocker) were used to study the specific ACE vs. non-ACE and AT1 vs. AT2 effects on placental VSMC contractility, respectively. Our results showed that chymostatin, but not captopril, and PD123,319, but not losartan, significantly attenuated placental VSMC/collagen gel contraction, p < 0.01, respectively. The inhibitory effects of chymostatin and PD123,319 were dose-dependent. Our results suggest that chymase, a non-ACE Ang II generating enzyme, may contribute significantly to Ang II generated in the placenta vascular tissue and that the AT2 receptor may play an important role in the regulation of Ang II induced contractility of placental VSMCs. These results provide new insights into Ang II generation and Ang II receptor regulation of vessel contractile function in the placental vasculature. These results also suggest the potential role of increased chymase activity and altered AT2 receptor function in placental related pregnancy disorders such as preeclampsia and IUGR.

Angiotensin II Type 2 Receptor Gene Deficiency Attenuates Susceptibility to Tobacco-Specific Nitrosamine-Induced Lung Tumorigenesis: Involvement of Transforming Growth Factor-β-Dependent Cell Growth Attenuation

To clarify an involvement of angiotensin II signaling in lung neoplasia, we have examined the effect of angiotensin II receptor deficiency on 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)-induced lung tumorigenesis. Male angiotensin II type 2 receptor (AT2)-null mice with an SWR/J genetic background and control wild-type mice were treated with NNK (100 mg/kg, i.p.) or saline vehicle. NNK treatment caused the development of lung tumors in all wild-type control mice (100 % tumor prevalence), but only 85% of AT2-null mice developed tumors. The tumor multiplicity in AT2-null mice (1.9 +/- 0.3) was significantly smaller than that in wild-type mice (4.1 +/- 0.9). Primary cultured lung fibroblasts prepared from both AT2-null and wild-type mice markedly increased the colony counts of A549 lung cancer cells in soft agar, but a consistently higher colony count was observed with the wild-type fibroblasts (fold increase in colony number, 5.6 +/- 0.5) than with the AT2-null fibroblasts (3.5 +/- 0.8). The underlying mechanism by which angiotensin II regulates cancer cell growth is due to the regulation of active transforming growth factor-beta (TGF-beta) production. Although the total level of TGF-beta was significantly stimulated when A549 cells were cocultured with either type of fibroblasts, the level of active TGF-beta in the conditioned medium was consistently higher with AT2-null fibroblasts than with wild-type fibroblasts. These results imply that the AT2 receptor negatively regulates the level of active TGF-beta and thus increases NNK-induced lung tumorigenesis. The AT2 receptor function in lung stromal fibroblasts may be a potential modulator of tumor susceptibility in chemical carcinogen-induced lung tumorigenesis.

Vasodilation Mediated by Angiotensin II Type 2 Receptor Is Impaired in Afferent Arterioles of Young Spontaneously Hypertensive Rats

Renal vasoconstrictor action of angiotensin II (Ang II) is exaggerated in the spontaneously hypertensive rat (SHR) before development of hypertension. We have recently demonstrated that in the rabbit afferent arteriole (Af-Art) activation of the AT2 receptor causes vasodilation, which modulates the vasoconstrictor action of Ang II mediated by the AT1 receptor. In this study, we tested the hypothesis that vasoconstrictor action of Ang II is exaggerated in SHR Af-Arts due to an impaired function of the AT2 receptor before development of hypertension. Af-Arts were microdissected from the superficial cortex of 4- to 5-week-old SHR or age-matched Wistar Kyoto rats (WKY), and perfused at 60 mm Hg in vitro. Ang II (10^–11 to 10^–8 M) decreased the luminal diameter of Af-Arts of both strains in a dose-dependent manner. However, the constriction was stronger in SHR; at 10^–10 M, the diameter decreased by 34 ± 4% in SHR (n = 6) compared to 18 ± 3% in WKY (n = 6; p < 0.01). Pretreatment with PD123319 (PD), an AT2 receptor antagonist, significantly augmented Ang II-induced constriction in WKY but not SHR Af-Arts; at 10^–10 M, the diameter now decreased by 41 ± 5 and 37 ± 1% in SHR (n = 6) and WKY (n = 6), respectively. Thus, blockade of the AT2 receptor abolished the difference in Ang II action on Af-Arts between strains. Moreover, with the AT1 receptor blockade Ang II caused dose-dependent dilation of preconstricted Af-Arts only in WKY (27 ± 5% at 10^–8 M, n = 5), and the dilation was abolished by simultaneous treatment with PD. In contrast, no such dilation was observed in SHR Af-Arts. These results suggest that activation of the AT2 receptor modulates AT1 receptor vasoconstriction in WKY Af-Arts, while impaired modulatory function of AT2 receptor may play a role in the exaggerated vasoconstrictor action of Ang II on the Af-Art of prehypertensive SHR.

Expression cloning of type 2 angiotension II receptor reveals a unique class of seven-transmembrane receptors.

Angiotensin II acts on at least two distinct receptor subtypes (AT1 and AT2). Most known effects of angiotensin II in adult tissues are attributable to the AT1 receptor. The function of AT2 receptor is undefined, but its abundant expressions in fetal tissues, immature brain, skin wound, and atretic ovarian follicles suggest a role in growth and development. Previous studies suggested that AT2 receptor may not be G protein-coupled. Here, from a rat fetus expression library, we cloned a cDNA encoding a unique 363-amino acid protein with pharmacological specificity, tissue distribution, and developmental pattern of the AT2 receptor. It is 34% identical in sequence to the AT1 receptor, sharing a seven-transmembrane domain topology. A review of prior data on other receptors suggests that this receptor may belong to a unique class of seven-transmembrane receptors (including somatostatin SSTR1, dopamine D3, and frizzled protein Fz) for which G protein coupling has not been demonstrated. All members of this class exhibit fetal and developmental and/or neuronal-specific expression. A conserved motif in the third intracellular loop, distinguishing this class from "classical" G protein-coupled receptors, may mediate novel intracellular effects.

Expression cloning of type 2 angiotensin II receptor reveals a unique class of seven-transmembrane receptors.

Angiotensin II acts on at least two distinct receptor subtypes (AT1 and AT2). Most known effects of angiotensin II in adult tissues are attributable to the AT1 receptor. The function of AT2 receptor is undefined, but its abundant expressions in fetal tissues, immature brain, skin wound, and atretic ovarian follicles suggest a role in growth and development. Previous studies suggested that AT2 receptor may not be G protein-coupled. Here, from a rat fetus expression library, we cloned a cDNA encoding a unique 363-amino acid protein with pharmacological specificity, tissue distribution, and developmental pattern of the AT2 receptor. It is 34% identical in sequence to the AT1 receptor, sharing a seven-transmembrane domain topology. A review of prior data on other receptors suggests that this receptor may belong to a unique class of seven-transmembrane receptors (including somatostatin SSTR1, dopamine D3, and frizzled protein Fz) for which G protein coupling has not been demonstrated. All members of this class exhibit fetal and developmental and/or neuronal-specific expression. A conserved motif in the third intracellular loop, distinguishing this class from "classical" G protein-coupled receptors, may mediate novel intracellular effects.