Characterization of angiotensin II receptor subtypes in rat liver

Alternative Renin-Angiotensin System

The renin-angiotensin system is the most important peptide hormone system in the regulation of cardiovascular homeostasis. Its classical arm consists of the enzymes, renin, and angiotensin-converting enzyme, generating angiotensin II from angiotensinogen, which activates its AT1 receptor, thereby increasing blood pressure, retaining salt and water, and inducing cardiovascular hypertrophy and fibrosis. However, angiotensin II can also activate a second receptor, the AT2 receptor. Moreover, the removal of the C-terminal phenylalanine from angiotensin II by ACE2 (angiotensin-converting enzyme 2) yields angiotensin-(1-7), and this peptide interacts with its receptor Mas. When the aminoterminal Asp of angiotensin-(1-7) is decarboxylated, alamandine is generated, which activates the Mas-related G-protein-coupled receptor D, MrgD (Mas-related G-protein-coupled receptor type D). Since Mas, MrgD, and the AT2 receptor have opposing effects to the classical AT1 receptor, they and the enzymes and peptides activating them are called the alternative or protective arm of the renin-angiotensin system. This review will cover the historical aspects and the current standing of this recent addition to the biology of the renin-angiotensin system.

Ascorbic acid decreases the binding affinity of the AT1 receptor for angiotensin II

BACKGROUND

Ascorbic acid is an essential vitamin and a powerful antioxidant. Many studies have highlighted the benefits of ascorbic acid for chronic cardiovascular diseases such as hypertension in which angiotensin II (Ang II) plays an significant role. We therefore hypothesized that ascorbic acid could modify the pharmacological properties of the AT(1) receptor for Ang II.

METHODS

Binding studies and Ca(2+) mobilization studies were performed with HEK293 cells stably expressing the AT(1) receptor for Ang II. Smooth muscle contraction studies were performed with rabbit aorta strips that endogenously express the AT(1) receptor.

RESULTS

Scatchard analysis revealed that ascorbic acid decreased the binding affinity of the AT(1) receptor without modifying its maximal binding capacity. Ascorbic acid did not modify the binding affinity of the AT(2) receptor for Ang II or of the UT receptor for urotensin II. In single-cell Ca(2+) imaging assays, ascorbic acid reduced the frequency of intracellular Ca(2+) oscillations induced by a low dose of Ang II. In functional assays, ascorbic acid significantly diminished the contraction of rabbit aorta pre-contracted with Ang II but not those pre-contracted with urotensin II.

CONCLUSIONS

Ascorbic acid decreases the binding affinity of the AT(1) receptor. These results offer a mechanistic explanation for the reported blood pressure lowering effect of ascorbic acid.

Enzymatic pathways of the brain renin-angiotensin system: unsolved problems and continuing challenges

The brain renin-angiotensin system continues to be enigmatic more than 40 years after the brain was first recognized to be a site of action of angiotensin II. This review focuses on the enzymatic pathways for the formation and degradation of the growing number of active angiotensins in the brain. A brief description and nomenclature of the peptidases involved in the processing of angiotensin peptides in the brain is given. Of primary interest is the array of enzymes that degrade radiolabeled angiotensins in receptor binding assays. This poses major challenges to studies of brain angiotensin receptors and it is debatable whether an accurate determination of brain angiotensin receptor binding kinetics has yet been made. The quandary facing the investigator of brain angiotensin receptors is the need to protect the radioligand from metabolic alteration while maintaining the characteristics of the receptors in situ. It is the tenet of this review that we have yet to fully understand the binding characteristics of brain angiotensin receptors and the extent of their distribution in the brain because of our inability to fully protect the angiotensins from metabolic alteration until equilibrium binding conditions can be attained.

A non-radioactive method for angiotensin II receptor binding studies using the rat liver

INTRODUCTION

A new non-radioactive method based on competitive ELISA has been developed for binding studies on angiotensin II (Ang II) receptors.

METHOD

Rat liver membrane was used as the source of angiotensin receptors and FITC-angiotensin II (FITC-Ang II) was used as the labeled ligand with an affinity similar to unlabeled Ang II. The effects of different concentrations of Ang II, losartan, CGP-42112A and saralasin were studied on FITC-Ang II binding.

RESULTS

The Ki values for Ang II, losartan and CGP-42112A were calculated as 0.52+/-0.22 nM, 6+/-3 nM and 0.15+/-0.07 nM, respectively. Saralasin inhibited the binding of labeled ligand biphasically, revealing two different populations of Ang receptor with different affinities for saralasin. About 74% of the binding sites were more sensitive to saralasin with a Ki value of 0.32+/-0.04 nM while saralasin showed a Ki value of 2.7+/-0.8 nM for the remaining binding sites.

DISCUSSION

The competitive ELISA method developed in this work yields Ki values for angiotensin antagonists similar to those obtained by others using radiolabeled ligands. The simplicity of this method makes it a suitable alternative to radioligand studies for routine analysis of interaction of drugs with angiotensin receptors.

Immunohistochemical colocalization of type II angiotensin receptors with somatostatin in rat pancreas

Earlier studies indicate that binding sites of type II angiotensin (AT2) receptors are detected all over the pancreas, as well as in the pancreatic exocrine cell line AR4-2J. However, lack of corresponding functional AT2 receptor responses can be detected in the exocrine pancreas. The aim of present study is to determine the protein expression of AT2 receptors in the pancreas by probing with an AT2 receptor-specific antibody, and to examine the role of AT2 receptors in the regulation of pancreatic endocrine hormone release. In Western protein analysis of adult rat tissues, expression of AT2 receptor-immunoreactive bands of 56, 68, and 78 kDa was detected in the adrenal, kidney, liver, salivary glands, and pancreas. In adult rat pancreas, strong immunoreactivity was detected on cells that were located at the outer region of Langerhans islets. Immunohistochemical studies indicated that AT2 receptors colocalized with somatostatin-producing cells in the endocrine pancreas. Consistent with the findings in adult pancreas, abundant expression of AT2 receptors was also detected in immortalized rat pancreatic endocrinal cells lines RIN-m and RIN-14B. To examine the role of AT2 receptors on somatostatin secretion in the pancreas, angiotensin-stimulated somatostatin release from pancreatic RIN-14B cells was studied by an enzyme immunoassay in the absence or presence of various subtype-selective angiotensin analogues. There was a basal release of somatostatin from RIN-14B cells at a rate of 8.72 +/- 4.21 ng/10(6) cells (n = 7). Angiotensin II (1 nM-10 microM) stimulated a biphasic somatostatin release in a dose-dependent manner with an apparent EC50 value of 49.3 +/- 25.9 nM (n = 5), and reached maximal release at 1 microM angiotensin II (982 +/- 147.34% over basal secretion; n = 5). Moreover, the AT2 receptor-selective angiotensin analogue, CGP42112, was 1000 times more potent than the AT1 receptor-selective angiotensin analogue, losartan, in inhibiting angiotensin II-stimulated somatostatin release. These results suggest that angiotensin may modulate pancreatic hormone release via regulation of somatostatin secretion.

Plasma lipoprotein concentrations in ethnic populations

There is a complex interplay between genetic and environmental factors that influences the expression of plasma lipoprotein levels. It is therefore not surprising that differences in lipid levels have been reported between ethnic groups. There are conflicting data on racial and ethnic variations in lipids, and also limited data on the relationship between lipoprotein levels and coronary heart disease risk in specific populations. This review summarizes available data on ethnic variations in plasma lipoproteins and the potential impact on coronary morbidity and mortality.

Agonistic autoantibodies directed against the angiotensin II AT1 receptor in patients with preeclampsia

We showed that sera from patients with preeclampsia contain autoantibodies directed against the angiotensin II AT1 receptor. The antibodies recognize an epitope on the second extracellular loop of the receptor and are immunoglobulins of the IgG3 subclass. The antibodies accelerate the beating rate of neonatal rat cardiomyocytes. The agonistic effect can be blocked with the AT1 receptor blocker losartan and can be neutralized by a peptide corresponding to the AT1 receptor's second extracellular loop. In further studies we shown that the autoantibodies recognize a specific conformation of the AT1 receptor. Cleavage of the external disulfide bond with dithiothreitol caused an inactivation of the receptor when stimulated either with Ang II or the autoantibodies in a system of cultured neonatal rat cardiomyocytes. Long-term stimulation of the AT1 receptor with either agonists down-regulated the AT1 receptor-mediated response to a second Ang II stimulation. These observations show that the agonistic autoantibodies behave pharmacologically in a similar fashion to Ang II. We have found the autoantibodies in all women meeting the clinical criteria of preeclampsia and suggest that they may be important to the pathogenesis of the disease.

Direct evidence for an angiotensin AT1 receptor type in rat vas deferens

Physiological experiments suggest that the angiotensin AT1 receptor type predominates in rat vas deferens. Membrane binding experiments, using 125I-[Sarl,Ile8]angiotensin II, confirm the presence of angiotensin AT1 receptors and the absence of angiotensin AT2 receptors in this tissue. Angiotensin II and the angiotensin AT1 receptor-specific antagonist, losartan, bind to rat vas deferens membranes with comparable affinity, with KD equal to 22.7 and 34.1 nM, respectively. The affinities of angiotensin AT2 receptor-specific ligands are 3 orders of magnitude lower. According to the numbers of binding sites and Western blotting of membrane proteins, the concentration of angiotensin AT1 receptors in the rat vas deferens is rather low. The fact that similar numbers of binding sites were obtained from binding data for angiotensin II and losartan further supports the hypothesis of exclusive existence of angiotensin AT1 receptor type in rat vas deferens.

Characterization of the functional angiotensin II-receptor complex isoform in rat liver plasma membrane

In rat liver plasma membrane a single angiotensin II (Ang II) binding site (Kd of 3.71 +/- 0.33 nM and Bmax of 1143.7 +/- 83.9 fmol/mg protein) was identified using radioligand binding assay. Pharmacologically, this receptor match with the AT1 receptor subtypes in term of affinity for the selective antagonist Losartan, and probably with the AT1A receptor form in term of insensitivity for the antagonist PD123319. Nevertheless, using polyacrylamide gel isoelectric focusing, two 125I-Ang II binding sites migrating to pI 6.8 and 6.5 were found in these membrane preparations. Monophasic displacement of 125I-Ang II bound to isoform migrating at pI 6.8 clearly indicate that this isoform represents a functional Ang II-receptor complex. In contrast, the high concentrations of agonist and peptidic derivates of Ang necessary to displace 125I-Ang II bound to isoform migrating at pI 6.5 indicate that this atypical 125I-Ang II binding site represents a biologically nonfunctional Ang II binding molecule, presumably a nonspecific 125I-Ang II binding site.

Angiotensin II receptors in endothelial cells

1. Angiotensin II (Ang II), the main effector of the renin-angiotensin system, exerts its vasoconstrictory and trophic actions on smooth muscle cells via AT1 receptors. However, Ang II does not act only on smooth muscle cells, as Ang II receptors are also present in endothelial cells. 2. The receptor type on these cells differs depending on the origin of the endothelium and the species. The rat endothelial receptors are mostly of the AT1 type, but AT2 receptors have also been found. The pharmacological characteristics of the AT1 receptors on endothelial cells are similar to those of other cell types. 3. Ang II stimulates phospholipase C and phospholipase A2 activation via the AT1 receptor in endothelial cells. Ang II also stimulates the tyrosine phosphorylation of several proteins in these cells. 4. Some studies suggest that the AT1 receptor mediates the release of vasodilator molecules by endothelial cells and could modulate Ang II effect on smooth muscle cells. Ang II may also inhibit endothelial cell growth via the AT2 receptor. Finally, endothelial Ang II receptors may be implicated in the regulation of fibrinolysis.

Glucose and the insulin-releasing drug tolbutamide attenuate the effects of morphine and angiotensin on alcohol consumption

Animals studies have shown that insulin injections reduce alcohol intake, implicating glucoregulatory processes in alcohol consumption. Angiotensin (ANG) II reduces alcohol intake and promotes glycogen breakdown in the liver but no studies have assessed the role of glucoregulatory processes in ANG II's effect. Similarly, glucose injections attenuate the analgesic and cognitive effects of opiates, yet no studies have assessed the effect of glucose on the well-documented ability of opiates to enhance alcohol consumption. The present experiments further examine the role of glucoregulatory processes in alcohol intake by assessing the effect of glucose injections on morphine-enhanced alcohol consumption and by evaluating the effect of the insulin-releasing drug, tolbutamide, on ANG II-reduced alcohol consumption. Adult male Wistar rats acquired alcohol drinking using the limited access procedure that offers daily 40-min access to both 6% w/v alcohol and water and ensures reliable alcohol drinking in bouts large enough to produce pharmacologically relevant intakes. Experiment 1: after intake stabilized, four groups of rats were first pretreated with vehicle injections and in the next phase, three of the four groups received either 50, 100, or 200 mg/kg glucose intraperitoneally (i.p.) prior to access to alcohol. Neither the vehicle injections nor any of the glucose doses had an effect on alcohol intake. In the final phase all groups continued to receive their respective glucose doses or vehicle but were now also treated with 5 mg/kg morphine sulphate i.p. prior to alcohol access. Morphine stimulated alcohol intake to a similar degree in all groups except the 200 mg/kg group, which showed a significant attenuation in morphine-enhanced alcohol intake. Experiment 2: after intake stabilized, different groups of rats were pretreated with vehicle injections and in the next phase received either 5, 25, 50, or 100 mg/kg tolbutamide or vehicle subcutaneously (s.c.) prior to alcohol access. The vehicle injections did not alter alcohol intake, and only the 100 mg/kg dose of tolbutamide produced a reduction in alcohol intake. In the final phase the groups continued to receive their respective doses of tolbutamide or vehicle but were also treated with 400 micrograms/kg ANG II s.c. immediately prior to alcohol access. ANG II reduced alcohol intake a similar extent in the groups pretreated with 5-50 mg/kg tolbutamide. However, the 100 mg/kg dose of tolbutamide significantly attenuated ANG II's ability to reduce alcohol intake. These results demonstrate that manipulations that engage glucoregulatory processes can influence the mechanism(s) by which morphine and angiotensin respectively increase and decrease alcohol drinking.

AT1 angiotensin II receptor subtype in the human larynx and squamous laryngeal carcinoma

The presence of the angiotensin type 1 receptor (AT1 Ang II-R) was investigated in normal and diseased human larynx using a specific monoclonal antibody (6313/G2). When tissue AT1 content was studied by SDS electrophoresis with immunoblotting, the receptor was detected in 10/10 laryngeal tumours, and in 7/10 samples of normal tissue from the same patients. Two immunostaining bands, approximately 75 kDa, were present in all cases. Immunocytochemistry performed on sections of 45 formalin-fixed, paraffin-embedded laryngeal tissue samples showed that the receptor was expressed in normal respiratory epithelium only in a perinuclear pattern, above the nucleus toward the cell apex. In addition, the antigen was invariably present in skeletal muscle cells and in the columnar duct epithelium of minor salivary glands. The secretory cells were negative, but the antibody stained the adjacent myoepithelial cell layer. As expected, smooth muscle cells of the vessel walls also expressed Ang II-R. In metaplastic epithelium deriving from respiratory epithelium, the receptors were distributed diffusely throughout the cytoplasm of basal and parabasal cells. In dysplastic epithelium, cells of all layers were strongly positive. Finally, squamous cell tumours showed varying numbers of immunoreactive cells, which stained in a diffuse cytoplasmic and membranous pattern. Computer-assisted image analysis of the stained sections showed that the positivity for Ang II-R dramatically increased in dysplastic and well-differentiated cancer cells (3- and 5.5-fold higher than in normal epithelium, respectively), but there was less in poorly and very poorly differentiated cancer. Receptor abundance was not correlated with tumour size nor lymph node involvement. These results suggest a possible role of Ang II in the growth or function of normal and neoplastic larynx tissue, which is especially significant in early neoplastic change.

Effect of sympathoadrenal blockade on the hyperglycemic action of angiotensin II

The present experiments were designed to investigate the influence of the sympathoadrenal system on the hyperglycemic action of angiotensin II in freely moving rats divided into four experimental groups: (1) sham-operated animals submitted to intravenous administration of angiotensin II (1.9 nmol) which caused a rapid increase in plasma glucose reaching the highest values at 5 min after the injection (26.5% of the initial values; P < 0.01); (2) Sham-operated animals submitted to blockade of sympathetic noradrenergic pathways by treatment with guanethidine (10 mg/100 g body weight), which greatly decreased the baseline value of plasma glucose (85 +/- 5.5 mg% vs 136 +/- 5.1 mg% P < 0.01), and increased the hyperglycemic response to angiotensin II by 96% (P < 0.01); (3) Animals submitted to adrenodemedullation that did not alter the hyperglycemic response to angiotensin II; (4) Adrenodemedullated animals submitted to sympathetic blockade by guanethidine treatment which caused a 40.5% decrease in the hyperglycemic response to angiotensin II (P < 0.05). These data indicate that angiotensin II has a direct hyperglycemic effect in addition to its action on sympathetic nervous system activation and adrenomedullary secretion.

Angiotensin II-elicited signal transduction via AT1 receptors in endothelial cells

1. Angiotensin II (AII) actions are mediated by two distinct types of receptors: AT1, which includes two subtypes, AT1A and AT1B, and AT2. AII produces vasoconstriction on the vascular wall acting directly on smooth muscle cells via AT1 receptors. AII receptors have recently been demonstrated on endothelial cells. But the pharmacological characteristics of these receptors and the intracellular signal pathways coupled to them remain unclear. 2. The aim of this work was to characterize the AII receptor subtypes in rat aortic endothelial cells (RAEC) in primary culture and to evaluate the signal pathways coupled to these receptors by measuring the activation of phospholipase C (PLC) and phospholipase A2 (PLA2). 3. Labelled AII bound to RAEC in a specific, saturable manner. Scatchard analysis showed a Kd of 1.87 +/- 0.49 nM and a Bmax of 50.2 +/- 10.9 x 10(3) sites per cell. AII was displaced by the AT1-specific antagonist, DuP753 with a Ki of 17.37 +/- 1.49 nM, but not by the AT2 receptor analogues CGP42771B or PD123177. These data were confirmed by the finding of AT1 mRNA in endothelial cells. Analysis of RNA expression by RT-PCR showed the presence of both subtypes, AT1A and AT1B in endothelial cells, whereas smooth muscle cells express only AT1A. 4. The activation of PLC and PLA2 in response to AII was evaluated by measuring inositol phosphate production and arachidonic acid release, respectively. Both were enhanced by AII in a dose-dependent manner, and inhibited by DuP753, but not by PD123177. 5. We conclude that AT1 receptors are expressed by endothelial cells in primary culture and that phospholipase C and phospholipase A2 activated via this receptor.

A functional comparison of the rat type-1 angiotensin II receptors (AT1AR and AT1BR)

To evaluate and functionally compare the rat AT1A and AT1B receptor subtypes, stable Chinese hamster ovary (CHO) cell lines expressing either recombinant receptor in approximately equal numbers were generated. Radioligand binding data suggests that the recombinant AT1A receptor is pharmacologically similar to the recombinant AT1B receptor. Functional studies indicate that both receptor subtypes can independently activate the phospholipase C/IP3 and the dihydropyridine-sensitive voltage-dependent Ca2+ channel signal transduction pathways with equal efficiency, but are unable to modulate cAMP accumulation under our experimental conditions. Furthermore, both receptors can be directly involved in the cellular growth properties of AII. Slot-blot experiments clearly demonstrate that these receptors are expressed in a tissue-specific manner. A sequence comparison of the 5' flanking regions of these two genes shows that they have very little sequence homology (approximately 36%), suggesting that although the AT1A and AT1B receptors appear to be pharmacologically and functionally similar, the control of their expression seems to be governed by distinct transcription factors.

Effect of [1-Sar,8-Thr]-angiotensin II on the hyperglycemic response to hemorrhage in adrenodemedullated and guanethidine-treated rats

The present experiments were designed to further investigate the action of an angiotensin II antagonist on the hyperglycemic response to hemorrhage (1.2 ml/100 g b.wt./2 min). The animals were divided into 3 experimental groups; (1) sham-operated animals submitted to intravenous administration of [1-Sar,8-Thr]-angiotensin II (sarthran), an antagonist of angiotensin II (750 ng/100 g b.wt. as a bolus plus an infusion of 25 ng/100 g b.wt./min over 30 min), which greatly attenuated (51.8% lower than controls; P < 0.01) the hyperglycemic response to hemorrhage; (2) animals submitted to adrenodemedullation which decreased the hyperglycemic response to hemorrhage by 64% (P < 0.01). However, sarthran infusion into adrenodemedullated rats caused a 38.5% further decrease in hyperglycemic response to hemorrhage (P < 0.01); and (3) intact animals submitted to blockade of sympathetic noradrenergic pathways by treatment with guanethidine (10 mg/100 g b.wt.), which greatly decreased the baseline value of plasma glucose (64.1 +/- 3.5 mg% vs. 125.3 +/- 4.5 mg%, P < 0.01), and reduced the hyperglycemic response to hemorrhage by 34% (P < 0.01). Sarthran infusion into guanethidine-treated rats caused a further 34% decrease in hyperglycemic response to hemorrhage (P < 0.01). These data indicate that angiotensin II has a direct hyperglycemic effect in addition to its action on sympathetic nervous system activation and adrenomedullary secretion.

Identification of atypical (non-AT1, non-AT2) angiotensin binding sites with high affinity for angiotensin I on IEC-18 rat intestinal epithelial cells

Specific high-affinity (Kd = 3.4 nM) binding sites for 125I-labelled angiotensin I ([125I]Ang I) were identified on an epithelial cell line (IEC-18) derived from the rat small intestine. The sites, which also have high affinity for Ang II, are insensitive to both AT1- and AT2-specific angiotensin receptor antagonists. The rank order of potency with which various angiotensin peptides inhibited [125I]Ang I binding to the cells (Ang I > or = Ang II > Ang(1-7) > [Sar1,Ile8]-Ang II > Ang(3-8) > Ang III) also distinguishes these sites from AT1 and AT2 angiotensin receptors.

Disulfide bridges in extracellular domains of angiotensin II receptor type IA

Angiotensin II receptor type IA (AT1A) has a cysteine (Cys) residue in each of four extracellular domains, and these Cys residues are believed to form two disulfide bridges. However, the question as to which pairs of Cys residues form disulfide bridges have not been experimentally determined. We constructed four mutants of rat AT1A, in which extracellular Cys residues were individually replaced by glycine (mutant C-1, C-2, C-3 and C-4). Further, we constructed two double mutants, in which two extracellular Cys residues were simultaneously substituted for by glycine. The binding affinity for angiotensin II in a double mutant C-1 + 4 (Cys18,274Gly) was similar to that in individually substituted mutants (C-1, C-2, C-3 and C-4) whereas the ligand binding of a double mutant C-2 + 4 (Cys101,274Gly) was completely abolished. The bindings of the non-peptide AT1A antagonist [125I]EXP-985 to mutants C-1, C-4 and C-1 + 4 were only slightly reduced whereas in mutant C-2, C-3 and C-2 + 4 the specific binding for [125I]EXP-985 was completely abolished. These results suggest that disulfide bridges in AT1A are formed between Cys18 and Cys274, and between Cys101 and Cys180, and the latter disulfide bond is essential for the binding of the non-peptidic antagonists [125I]EXP-985 or losartan.

Hyperglycemic action of angiotensin II in freely moving rats

Angiotensin II has been implicated in the regulation of liver glycogen phosphorylase. Although it has been suggested that angiotensin II can raise blood glucose levels during hemorrhage, experimental data have not been presented. In the present study, the effect of angiotensin II on blood glucose levels was studied in freely moving rats, divided in three experimental groups: 1) intravenous administration of angiotensin II (0.48, 1.9, or 4.8 nmol) caused a dose-dependence response; 2) intracerebroventricular administration of angiotensin II (1.9 or 4.8 nmol) did not cause any significant change in glycemia compared with saline-treated controls; 3) intravenous administration of [Sar1,Thr8]angiotensin II, an antagonist of angiotensin II (750 ng/100 g b. wt. as a bolus plus a continuous injection of 25 ng/100 g b. wt./min over 30 min), greatly attenuated (39.2% lower than controls; p < 0.01) the hyperglycemic response to hemorrhage (1.2 ml/100 g b.wt.). These data indicate an in vivo involvement of angiotensin II in blood glucose regulation.

Brain angiotensin receptor subtypes in the control of physiological and behavioral responses

This review summarizes emerging evidence that supports the notion of a separate brain renin-angiotensin system (RAS) complete with the necessary precursors and enzymes for the formation and degradation of biologically active forms of angiotensins, and several binding subtypes that may mediate their diverse functions. Of these subtypes the most is known about the AT1 site which preferentially binds angiotensin II (AII) and angiotensin III (AIII). The AT1 site appears to mediate the classic angiotensin responses concerned with body water balance and the maintenance of blood pressure. Less is known about the AT2 site which also binds AII and AIII and may play a role in vascular growth. Recently, an AT3 site was discovered in cultured neoblastoma cells, and an AT4 site which preferentially binds AII(3-8), a fragment of AII now referred to as angiotensin IV (AIV). The AT4 site has been implicated in memory acquisition and retrieval, and the regulation of blood flow. In addition to the more well-studied functions of the brain RAS, we review additional less well investigated responses including regulation of cellular function, the modulation of sensory and motor systems, long term potentiation, and stress related mechanisms. Although the receptor subtypes responsible for mediating these physiologies and behaviors have not been definitively identified research efforts are ongoing. We also suggest potential contributions by the RAS to clinically relevant syndromes such as dysfunctions in the regulation of blood flow and ischemia, changes in cognitive affect and memory in clinical depressed and Alzheimer's patients, and angiotensin's contribution to alcohol consumption.

Regulation of cell proliferation and growth by angiotensin II

The peptide hormone angiotensin II (AngII) has clearly defined physiologic roles as a regulator of vasomotor tone and fluid homeostasis. In addition AngII has trophic or mitogenic effects on a variety of target tissues, including vascular smooth muscle and adrenal cells. More recent data indicate that AngII exhibits many characteristics of the 'classical' peptide growth factors such as EGF/TGF alpha, PDGF and IGF-1. These include the capacity for local generation ('autocrine or paracrine' action) and the ability to stimulate tyrosine phosphorylation, to activate MAP kinases and to increase expression of nuclear proto-oncogenes. The type 1 AngII receptor, which is responsible for all known physiologic actions of AngII, has been cloned. Activation of this receptor leads to elevated phosphoinositide hydrolysis, mobilization of intracellular Ca2+ and diacylglycerol, and activation of Ca2+/calmodulin and Ca2+/phospholipid-dependent Ser/Thr kinases, as well as Ca2+ regulated tyrosine kinases. The existence of other AngII receptor subtypes has been postulated, but the function(s) of these sites remains unclear. In vascular smooth muscle, AngII can promote cellular hypertrophy and/or hyperplasia, depending in part on the patterns of induction of secondary factors that are known to stimulate (PDGF, IGF-1, basic FGF) or inhibit (TGF-beta) mitosis. Together, these findings have suggested that AngII plays important roles in both the normal development and pathophysiology of vascular, cardiac, renal and central nervous system tissues.

The biology of angiotensin II receptors

Angiotensin II is very important in the regulation of blood pressure. This small peptide binds to cell surface receptors, initiating a wide diversity of physiologic responses. There are two major subtypes of angiotensin II receptors referred to as AT1 and AT2. In this article we describe the cloning and the biochemical characterization of the AT1 receptor. Antibodies against this receptor have been used to define its tissue distribution. The AT1 receptor is a member of the seven transmembrane spanning class of receptors. It initiates a complex series of signaling events, including activation of membrane phospholipases and intracellular kinases. In the human a single AT1 receptor protein mediates virtually all the effects of angiotensin II, suggesting that tissue specificity of angiotensin II must be due to organ-specific intracellular signaling.

Binding of [3H]angiotensin II and [3H]DuP 753 (Losartan) to rat liver homogenates reveals multiple sites. Relationship to AT1a- and AT1b-type angiotensin receptors and novel nonangiotensin binding sites

The binding characteristics of radiolabeled angiotensin II and the nonpeptidergic angiotensin AT1 receptor antagonist, DuP 753 (Losartan), were studied in rat liver homogenates. Competition experiments with human angiotensin I, II, and III and with the angiotensin antagonists, CGP 42114A, saralasin, DuP 753, and PD123177, confirmed that the [3H]angiotensin II binding was to an AT1-type receptor. Computer analysis of the competition studies using the human angiotensins demonstrated that the data could be best fitted to a model that considers interaction at two sites. Angiotensin II, angiotensin III, and an angiotensin II analogue, [Sar1]angiotensin II, were calculated to have approximately one hundredfold selectivity at each of the two binding sites, but angiotensin I and the antagonists did not show a difference in affinity between the two sites. The addition of 120 mM NaCl and the nonhydrolyzable analogue of GTP, GppNHp (100 microM) to the buffer resulted in a reduction in [3H]angiotensin II binding at both sites. Thus, we suggest that the two sites may represent distinct angiotensin AT1-type receptors, possibly AT1a and AT1b subtypes. The addition of dithiothreitol (DTT) reduced [3H]angiotensin II binding, confirming the binding to AT1-type receptors. Binding studies using the selective AT1 angiotensin II receptor antagonist, [3H]DuP 753, were also performed on the rat liver homogenates. Saturation studies using both angiotensin II and DuP 753 to define nonspecific binding showed that [3H]DuP 753 bound to at least two types of site, one smaller population of receptors that was sensitive to both angiotensin II and DuP 753 and a second site that was sensitive to DuP 753 only.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II(3-8) (ANG IV) hippocampal binding: potential role in the facilitation of memory

The present research characterizes a newly discovered ANG II(3-8) (ANG IV) binding site localized in structures associated with memory function (hippocampus, neocortex, cerebellum), as well as other brain stem structures (thalamus, inferior olivary nucleus). This site is not the AT1 or AT2 site that binds angiotensins II (ANG II) and III (ANG III) nor does it bind the nonpeptide AT1 or AT2 receptor antagonists DuP753 and PD123177, respectively. The intracerebroventricular (ICV) infusion of ANG IV was ineffective at inducing drinking in rats as compared with equivalent doses of ANG II and III. Although not as effective as ANG II or ANG III, ICV infusion of ANG IV did provoke a pressor response at the highest dose (100 pmol/min), which appeared to be mediated by ANG II (AT1)-type receptors and not the specific AIV binding site described here. By contrast, the ICV infusion of ANG IV resulted in greater effects upon retention and retrieval of a passive avoidance task as compared with ANG II. Specifically, ANG II was not different from the ICV infusion of artificial cerebrospinal fluid, while ANG IV improved retention and retrieval of this task.

Comparison of the binding and functional actions of angiotensin agonists in clone 9 cells: additional evidence for angiotensin II receptor heterogeneity

The effects of the angiotensin-II (AII) agonists and antagonists on both 125I-SARILE binding and phosphoinositol (PI) accumulation in clone 9 cells were examined. Clone 9 cells, which are derived from rat liver, have been shown to respond to AII agonists with an increase in PI accumulation which is inhibitable by Sar1,Ile8-AII (SARILE) and DUP-753 but not PD-123319, suggesting that they possess the AT1 subtype of AII receptor. The present results confirmed these properties. The order of potency of AII agonists was AII > AIII > AI. Clone 9 cells also possessed binding sites for 125I-SARILE. The majority of these were AT1 type receptors, although a small number of AT2 receptors may also have been present. The order of potency of AII agonists in inhibiting 125I-SARILE binding was AII >> AIII = AI. The difference in rank order of potency between the functional and binding assay was due to AIII being much less potent in the binding assay than the functional assay. Since the potency of AIII relative to AII was lower than that at either AT1 or AT2 subtypes of AII receptor, these data suggest that an additional subtype, with selectively low affinity for AIII, exists.

Angiotensin-II-induced expression of proto-oncogene (c-fos, jun-B and c-jun) mRNA in bovine adrenocortical fasciculata cells (BAC) is mediated by AT-1 receptors

We have shown previously that angiotensin-II (A-II) controls proto-oncogene (c-fos, jun-B and c-jun) mRNA accumulation in bovine adrenal fasciculata cells (BAC). Since BAC contain both subtypes (AT-1 and AT-2) of the A-II receptor, we have investigated which subtype was involved in the effect of A-II on proto-oncogene mRNA by using a selective antagonist for AT-1 (DUP 753) and for AT-2 (CGP 42112A). DUP 753, but not CGP 42112A, inhibited the stimulatory effect of A-II on proto-oncogene mRNA, with ID50s of 4 x 10(-7) M, 7 x 10(-7) M and 2 x 10(-6) M for c-fos, jun-B and c-jun, respectively. Neither of the two antagonists by themselves had a direct effect on proto-oncogene mRNA. As the A-II AT-1 receptors are coupled to the phospholipase C system in BAC, we have investigated whether the A-II effects on the proto-oncogenes were mediated by protein kinase C (PKC) or by Ca2+ calmodulin. First, activation of PKC by the phorbol ester, PMA, increased the level of three proto-oncogene mRNAs, whereas calcium ionophore had no effect. Second, staurosporine, a specific inhibitor of PKC, reduced the stimulatory action of A-II on proto-oncogene mRNA by 80-90%, whereas trifluoroperazine, an inhibitor of calmodulin, had no significant effect. These results demonstrate that the effects of A-II on proto-oncogene mRNA are mediated by AT1 receptor subtypes, mainly through activation of the PKC pathway.

Characterization of the angiotensin II receptor expressed by the human hepatoma cell line, PLC-PRF-5

Radioligand binding studies were undertaken to establish the expression of angiotensin II (AII) receptors on the human hepatoma cell line, PLC-PRF-5. Cell membranes were shown to express a large number of AII receptors with high and low affinity binding sites having Bmax values of 1269 +/- 365 and 4190 +/- 1055 fmol/mg protein and affinities (Kd) of 2.0 +/- 0.3 nM and 8.7 +/- 0.4 nM, respectively. In intact cells a single class of AII binding site was seen with an affinity (Kd) of 6.7 +/- 1 nM and a Bmax value of 315 +/- 32 fmol/mg. In both membranes and intact cells AII, AIII and the selective angiotensin AT1 receptor antagonist, DuP 753, all had a high affinity for the receptor (Ki values in the nanomolar range), but the selective angiotensin AT2 ligands, PD 123177 and p-aminophenylalanine6 AII, had low affinity (Ki values in the micromolar range). These results indicate that the PLC-PRF-5 cells express the angiotensin AT1 receptor subtype. This was further supported by the demonstration of the sensitivity of the receptor to dithiothreitol (DTT). Pretreatment of membranes with DTT reduced [3H]AII binding in a concentration-dependent manner with an IC50 of 4.2 +/- 0.9 mM. The coupling of the AT1 receptor to signal transduction pathways was investigated. In intact cells AII (100 nM) evoked an increase in intracellular calcium ([Ca2+]i). This increase in [Ca2+]i was unaffected by PD 123177 (100 microM) but was abolished by DuP 753 (100 microM). Furthermore, AII (100 nM) did not inhibit forskolin (0.1-10 microM) stimulated cyclic AMP formation.(ABSTRACT TRUNCATED AT 250 WORDS)

Reciprocal feedback regulation of kidney angiotensinogen and renin mRNA expressions by angiotensin II

The present study asks whether angiotensin II (ANG II), a potent inhibitor of renal renin synthesis and release, regulates renal angiotensinogen synthesis. ANG II (or vehicle) was intravenously infused into male Sprague-Dawley rats for 3 days (vehicle or 100, 300, and 1,000 ng.kg-1 x min-1, n = 8/group), significantly increasing mean plasma ANG II concentrations and raising mean arterial blood pressure (MAP). ANG II dose dependently suppressed plasma renin concentration, kidney renin concentration, and renal renin mRNA levels. In contrast, ANG II infusion increased renal angiotensinogen mRNA levels stepwise to 122, 136 (P < 0.05), and 150% (P < 0.05) of control and also increased both liver mRNA levels (P < 0.05) and plasma angiotensinogen concentration (P < 0.05). Three days of angiotensin-converting enzyme inhibition (10 mg.kg-1 x day-1 quinapril in drinking water, n = 8) significantly decreased MAP (P < 0.05) and increased both mean plasma renin concentration (P < 0.05) and renal renin mRNA levels (P < 0.005). Plasma ANG II concentration tended to decrease (not significant), and neither renal nor hepatic angiotensinogen mRNA levels displayed significant difference. However, when data from ANG II-infused and quinapril-treated rats were analyzed together, correlation between plasma ANG II concentrations and renal angiotensinogen mRNA levels was highly significant (P < 0.005, r = 0.585). Thus plasma ANG II upregulates renal angiotensinogen gene expression and downregulates renal renin gene expression, a reciprocal feedback regulation that may have important physiological consequences.

Could the pharmacological differences observed between angiotensin II antagonists and inhibitors of angiotensin converting enzyme be clinically beneficial?

Over the past several years, angiotensin I converting enzyme (ACE) inhibitors, compounds that block the formation of angiotensin II (ANG II), have become widely used in the treatment of cardiovascular disease. Recently, a new class of orally active, non-peptide inhibitors of the renin-angiotensin system, the ANG II receptor antagonists have also become available. Since both classes of compounds block the renin-angiotensin system, although at different sites, it remains to be determined whether blockade of ANG II receptors will have any specific advantage over inhibition of ACE. The following review assesses the actions of ANG II antagonists and suggests ways in which blockade of ANG II receptors may differ both pharmacologically and clinically from inhibition of ACE.

Characterization of biochemical responses of angiotensin II (AT2) binding sites in the rat pheochromocytoma PC12W cells

Rat pheochromocytoma PC12W cell membranes have previously been shown to exclusively contain the AT2 receptor subtype. The present study extended these binding data and explored the functional expression of these binding sites. Our binding competition studies show a potency series of Ang II = Ang III greater than saralasin greater than Ang I = PD123177 much greater than Ang II(1-7) much much greater than losartan. PD123177 (1 microM) completely eliminated [125I]Ang II binding to PC12W cells. Competitive displacement of [125I]Ang II with Ang II shows a dissociation equilibrium constant (Kd) of 1.79 nM and a binding site maximum (Bmax) of 3.97 fmol/mg protein. Investigating several Ang II signal transduction pathways on these cells, we found that Ang II (10(-8) to 10(-6) M) does not affect basal cAMP, cGMP, arachidonic acid release, prostacyclin release, intracellular Ca2+ mobilization or thymidine incorporation in the PC12W cells. Nerve growth factor, cAMP, 5-fluorouridine deoxyriboside modulation of the number of AT2 receptor sites in PC12W cells failed to unmask any Ang II effects on basal cAMP, cGMP and intracellular Ca2+ mobilization. In conclusion, the present study confirms the exclusive presence of AT2 binding sites in the PC12W cells. However, these binding sites are not functionally coupled to common signal transduction pathways.

Regulatory role of brain angiotensins in the control of physiological and behavioral responses

Considerable evidence now indicates that a separate and distinct renin-angiotensin system (RAS) is present within the brain. The necessary precursors and enzymes required for the formation and degradation of the biologically active forms of angiotensins have been identified in brain tissues as have angiotensin binding sites. Although this brain RAS appears to be regulated independently from the peripheral RAS, circulating angiotensins do exert a portion of their actions via stimulation of brain angiotensin receptors located in circumventricular organs. These circumventricular organs are located in the proximity of brain ventricles, are richly vascularized and possess a reduced blood-brain barrier thus permitting accessibility by peptides. In this way the brain RAS interacts with other neurotransmitter and neuromodulator systems and contributes to the regulation of blood pressure, body fluid homeostasis, cyclicity of reproductive hormones and sexual behavior, and perhaps plays a role in other functions such as memory acquisition and recall, sensory acuity including pain perception and exploratory behavior. An overactive brain RAS has been identified as one of the factors contributing to the pathogenesis and maintenance of hypertension in the spontaneously hypertensive rat (SHR) model of human essential hypertension. Oral treatment with angiotensin-converting enzyme inhibitors, which interfere with the formation of angiotensin II, prevents the development of hypertension in young SHR by acting, at least in part, upon the brain RAS. Delivery of converting enzyme inhibitors or specific angiotensin receptor antagonists into the brain significantly reduces blood pressure in adult SHR. Thus, if the SHR is an appropriate model of human essential hypertension (there is controversy concerning its usefulness), the potential contribution of the brain RAS to this dysfunction must be considered during the development of future antihypertensive compounds.

The rat angiotensin II AT1A receptor couples with three different signal transduction pathways

To examine whether the subpopulation of the rat type 1 angiotensin II (AII) receptor (AT1A) couples with a single or multiple signal transduction pathways, we constructed Chinese hamster ovary (CHO) cell lines producing the recombinant receptor. The expressed AT1A receptor exhibits typical pharmacological characteristics of the AT1 receptor, known to mediate the main physiological function of AII. Addition of AII to the CHO cells induced a rapid, transient increase in intracellular free Ca2+ concentrations ([Ca2+]i) followed by a lower, sustained phase. Nicardipine, a blocker of voltage-dependent L-type Ca2+ channels, attenuated the transient [Ca2+]i response and abolished the sustained phase. The transient phase was also reduced dose-dependently by the phospholipase C inhibitor neomycin. Furthermore, AII inhibited forskolin-evoked cAMP accumulation. These data suggest, although another subpopulation named AT1B is present, that the rat AT1A receptor can independently couple with all three signal transduction pathways known to be induced by AII: i.e., i) activation of phospholipase C resulting in InsP3 generation with a subsequent release of intracellularly stored Ca2+, ii) activation of dihydropyridine-sensitive voltage-dependent Ca2+ channels, and iii) inhibition of adenylate cyclase activity.

Molecular characterization of angiotensin II type II receptors in rat pheochromocytoma cells

The binding sites and biochemical effects of angiotensin (A) II were investigated in rat pheochromocytoma (PC12W) cells. Sarcosine1, [125I]-tyrosine4, isoleucine8-AII ([125I]-SI-AII) bound to a saturable population of sites on membranes with an equilibrium dissociation constant (Kd) of 0.4 nM and a binding site maximum of 254 fmol/mg protein. Competitive displacement of [125I]-SI-AII by agonists and antagonists elucidated a rank order of potency of AIII greater than or equal to AII greater than PD 123177 greater than AI greater than [des-Phe]AII [AII(1-7)] much greater than DuP 753. The stable guanine nucleotide analog 5'-guanylyl imidodiphosphate did not alter the binding affinity or slope of the inhibition curves for AI, AII, AIII, or AII(1-7). Treatment of PC12W cells with AII or AIII did not affect the free intracellular calcium concentration, phosphoinositide metabolism, arachidonate release, cyclic GMP, or cyclic AMP concentrations. [125I]-AII binding sites remained on the cell surface and were not internalized after 2 h at 37 degrees C. Angiotensin II did not stimulate tyrosine, serine, or threonine phosphorylation. Northern analysis of PC12W mRNA with an AT1 receptor gene probe failed to produce an RNA:DNA hybrid at low stringency. These data indicate that PC12W cells express a homogeneous population of AT2 binding sites which differ significantly from AT1 receptors in signal transduction and molecular structure. AT2 sites may act via potentially novel, biochemical pathways or, alternatively, be vestigial receptors.

Pharmacological characterization of angiotensin-induced depolarizations of rat superior cervical ganglion in vitro

1. The depolarizing responses to angiotensin II and angiotensin III of the rat superior cervical ganglion have been characterized in vitro, by the use of peptidase inhibitors, peptide and non-peptide antagonists and dithiothreitol (DTT). 2. Angiotensin II and III depolarized the ganglion in a concentration-related manner. Angiotensin II was approximately 30 fold more potent than angiotensin III. 3. The endopeptidase inhibitor, bacitracin, increased the potency of angiotensin II and III by approximately 4 and 20 fold respectively. The aminopeptidase inhibitor, amastatin, further increased the potency of angiotensin III (but not angiotensin II) by approximately 4 fold. In the presence of bacitracin and amastatin, angiotensin II and III were equipotent. 4. The peptide antagonist [Ile7]angiotensin III (0.01-0.3 microM) produced a non-parallel rightward displacement of the angiotensin II concentration-response curve, with a suppression of the maximum response. The potency of [Ile7]angiotensin III was increased by bacitracin and amastatin. 5. The AT1-selective non-peptide antagonist losartan (DuP 753; 0.03 and 0.1 microM) produced a parallel rightward displacement of the angiotensin II concentration-response curve, with an apparent pKB of 8.3 +/- 0.1. A higher concentration of losartan (0.3 microM) depressed the maximum agonist response by 32 +/- 6.5%, possibly reflecting non-competitive behaviour of the antagonist. The potency of losartan was not influenced by bacitracin. 6. The AT2-selective non-peptide antagonist, PD123177 (3 microM) failed to antagonize the angiotensin II-induced depolarizations. 7. DTT (1 mM) produced a 22% reduction of the maximum response to angiotensin II.8. We conclude that the angiotensin II-induced depolarizations of the rat superior cervical ganglion are mediated by angiotensin II receptors of the AT1 subclass. The ability of peptidase inhibitors to modify the potency of peptide agonists and antagonists highlights the difficulties associated with the use of peptide agents to characterize angiotensin II receptors in this preparation.

Pharmacological characterization of angiotensin II binding sites in the canine pancreas

High affinity 125I-angiotensin II (Ang II) binding sites were characterized in the canine pancreas. Total binding increased with protein concentration and equilibrium was reached within 60-90 min at 22 degrees C. Specific binding was saturable and averaged 70% of total. Scatchard analysis of binding yielded a KD of 0.48 +/- 0.18 nM with a Bmax of 32.8 +/- 6.5 fmol/mg protein (mean +/- SEM, n = 6). The addition of the reducing agent dithiothreitol increased specific binding two-fold. The rank order of displacement of 125I-Ang II binding by native angiotensin peptides was Ang II greater than or equal to Ang III greater than AngI greater than Ang(1-7) much greater than Ang(1-6). The use of the specific Ang II antagonists CGP 42112A, PD 123177, and DuP 753 revealed that the pancreas expresses two receptor subtypes. The majority of Ang II binding sites in the pancreas could be classified as type 2 (AT2), although type 1 (AT1) sites were also detected. In vitro autoradiography revealed binding sites localized over islet cells, acinar and duct cells, as well as the pancreatic vasculature. In addition, the autoradiographic studies confirmed the predominance of the AT2 receptor subtype throughout the pancreas.

Mas oncogene receptor coupling and peptide specificity in Balb 3T3 and vascular smooth muscle cells

The mas oncogene receptor has been reported to confer angiotensin (Ang) responsiveness in NG115-401L neuronal cell line. To test if mas oncogene encodes an Ang receptor in peripheral tissue, Balb 3T3 and rat aortic vascular smooth muscle cells (VSMC) were cotransfected with a plasmid containing the mas oncogene (pSM422) and a plasmid expressing a selectable marker (pRSV-Neo). Transfected cells (Balb/mas and VSMC/mas) expressed the appropriate 2.4 Kb mas transcript, which was not present in parental cells. Both Balb/mas and VSMC/mas cells acquired Ang II and Ang III responsiveness as documented by Ang-stimulated increased [Ca2+]i. The ED50 for these peptides were relatively high (4 - 6 x 10(-5) M). Ang III was approximately two times more potent than Ang II in stimulating 45Ca efflux from Balb/mas cells, and its effect was not blocked by Sar1, Ile8-Ang II. In contrast, substance P and a substance P analogue ([D-Arg1, D-Pro2, D-Trp7,9, Leu11] substance P) behaved as agonists, resulting in the stimulation of 45Ca efflux and [Ca2+]i in Balb/mas cells without affecting control cells. The rank order potency for stimulating 45Ca efflux in Balb/mas cells was substance P analogue much greater than Ang III, substance P greater than Ang II. In summary, the authors show that although Ang III can stimulate biochemical events in mas transfected cells, which are known to be essential for Ang receptor signal transduction in other cell types, ie, [Ca2+]i and pHi transients, as well as inositol triphosphate formation, it did that at supraphysiological concentrations of the peptide.

Ontogeny of angiotensinogen mRNA and angiotensin II receptors in rat brain and liver

The renin-angiotensin-system (RAS) is active in fetal and neonatal life. This study was undertaken to examine the ontogenic regulation of angiotensinogen (AT) gene expression and angiotensin II (A II) receptors in liver and brain. AT gene expression was studied in fetal, neonatal, adult and aged rats, using slot blot hybridization to quantify AT mRNA levels. During fetal life (gestational days 15-20), AT mRNA was more abundant in brain than in liver. Soon after birth, brain AT mRNA levels increased to a concentration 3 fold above fetal levels. In contrast, liver AT mRNA abundance increased 30-fold within 12 h of birth. Aging (3-20 months) resulted in a gradual decrease in AT mRNA in both the brain and liver. Liver A II receptors in the neonate were 2-fold higher than in the fetus, but returned to fetal levels by 8 weeks of age. In the brain, A II receptor abundance increased to a level 75% above fetal levels in 7 days old animals, but returned to fetal levels by 14 days of age. These studies suggest than in the fetus, the liver is not the primary source of AT but that unknown factors at parturition result in a dramatic increase in liver AT mRNA. In contrast, the more modest increases in brain AT mRNA parallel the gradual maturation of the CNS. In both tissues, further aging resulted in a gradual decrease in AT mRNA, reflecting either increased sensitivity to feedback downregulation by A II or age related increases in other extrahepatic sites of AT synthesis. Age related changes were also found in the A II receptor in both the liver and brain.

Amphibian myocardial angiotensin II receptors are distinct from mammalian AT1 and AT2 receptor subtypes

High-affinity receptors for angiotensin II were identified on Xenopus laevis cardiac membranes and characterized by binding-inhibition studies with peptide and non-peptide AII antagonists. Scatchard analysis of the binding data identified a high-affinity site with Kd1 = 1.6 nM and Bmax1 = 3.7 pmol/mg protein and a low-affinity site with Kd2 = 22 nM and Bmax 2 = 9.5 pmol/mg protein. Treatment with dithiothreitol reduced the number of binding sites by greater than 70%. The rank order of potency for ALL analogs was (agent, IC50) [Sar1,Ile8]AII, 0.91 nM greater than AII, 2.0 nM greater than AI, 5.3 nM greater than [Sar1, Ala8]AII, 19 nM much greater than CGP42112A, 1.2 microM much much greater than DuP 753 approximately PD-123177, greater than 100 microM. The relative potencies of these compounds differ markedly from their activities on the two known mammalian AII receptor subtypes, AT1 and AT2. These results indicate that amphibian AII receptors are pharmacologically distinct from both the AT1 and AT2 receptors characterized in mammalian tissues.

Angiotensin II AT2 receptors do not interact with guanine nucleotide binding proteins

We have studied the effect of GTP gamma S on the affinity and binding kinetics of angiotensin II in plasma membrane particulate prepared from tissues expressing either only AT1 (human renal artery smooth muscle cells), only AT2 (human myometrium and bovine cerebellar cortex) or both angiotensin II receptor subtypes (rat adrenal glomerulosa). We also examined the ability of angiotensin II to stimulate GTP gamma[35S] incorporation in these membrane preparations. In contrast to its effects on angiotensin II binding to the AT1 receptor, GTP gamma S does not affect binding parameters to the AT2 receptor. Moreover, in tissues expressing solely AT2 receptors, angiotensin II was unable to induce GTP gamma[35S] incorporation. These findings indicate that AT2 receptors do not interact with G-proteins and that angiotensin II must therefore mediate some of its effects through G-protein-independent mechanisms.

Sulfhydryl reducing agents distinguish two subtypes of angiotensin II receptors in the rat brain

Two angiotensin II receptor subtypes were distinguished in the rat brain using in vitro receptor autoradiography based on the differential effects of sulfhydryl reducing agents on 125I-sarcosine1,isoleucine8 angiotensin II binding in various brain nuclei. At several nuclei, e.g. the hypothalamus, circumventricular organs and the dorsal medulla, 125I-sarcosine1,isoleucine8 angiotensin II binding was strongly inhibited by 30 mM beta-mercaptoethanol or 5 mM dithiothreitol, whereas at other nuclei, e.g. the lateral septum, colliculi, locus coeruleus and medial amygdala, sulfhydryl reducing agents had either little effect on radioligand binding or enhanced the binding. The distribution of the sulfhydryl reducing agent inactivated subtype corresponds exactly with the distribution of DuP 753 sensitive (designated as AII alpha) 125I-sarcosine1,isoleucine8 angiotensin II binding sites25. The subtype not inhibited by sulfhydryl reducing agents corresponds with the DuP 753 insensitive (designated as AII beta) sites in the brain25. The sulfhydryl reducing agent effect on brain angiotensin II receptor subtypes is similar to that seen in angiotensin II receptor subtypes in peripheral tissues. These observations indicate that many previous studies of brain angiotensin II receptor binding that included 5 mM dithiothreitol in the assay medium overlooked the sulfhydryl reducing agent inactivated (AII alpha) receptor subtype.

Angiotensin II receptors in the rat lung are of the AII-1 subtype

In the rat lung [3H]angiotensin II ([3H]AII) recognizes a single class of binding sites with an equilibrium constant (KD) of 2.8 +/- 0.9 nM. The corresponding receptor density (Bmax) was 704 +/- 60 fmol/mg protein. Equilibrium of [3H]AII binding was reached after 60 min. The dissociation of the radioligand was strongly influenced by mono- and divalent cations and the non-hydrolyzable GTP analog 5'-guanylyl-imido-diphosphate (Gpp[NH]p), indicating the interaction with a G protein. Specific binding of [3H]AII to rat lung preparations was inhibited by different agonists and antagonists and sensitive to DuP 753 (2-n-butyl-4-chloro-5-hydroxymethyl-1-[(2'-(1H-tetrazol-5-yl)bi phe nyl-4-yl) methyl]imidazol, potassium salt) but insensitive to PD 123177 (1-(4-amino-3-methyl-benzyl)-5-diphenylacetyl-4,5,6,7-tetrahydro-i midazo [4,5-c]pyridine-6-carboxylic acid) two non-peptide AII antagonists selective for either the AII-1 or AII-2 subtype. Furthermore, specific [3H]AII binding was inhibited by dithiothreitol. It is concluded that in the rat lung [3H]AII binds to receptors of the AII-1 subtype which are coupled to a G protein. These receptors may be involved in the regulation of pulmonary perfusion and resistance.