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

125I-Angiotensin 1-7 binds to a different site than angiotensin 1-7 in tissue membrane preparations

PURPOSE

To study the receptor for Angiotensin (Ang) 1-7 using a radioligand (¹²⁵I-Ang 1-7)-binding assay. For more than a decade, Mas has been viewed as the receptor for Ang 1-7; however, Ang 1-7 binding has not been pharmacologically characterized in tissue membrane preparations.

METHODS

Radioligand-binding assays were carried out using tissue membrane preparations using radioiodinated Angiotensin 1-7 (¹²⁵I-Ang 1-7) to characterize its binding site. Non-radioactive ¹²⁷I-Ang 1-7 was used to test if the addition of an iodine to the tyrosine⁴ moiety of Ang 1-7 changes the ability of Ang 1-7 to competitively inhibit ¹²⁵I-Ang 1-7 binding.

RESULTS

¹²⁵I-Ang 1-7 binds saturably, with moderately high affinity (10-20 nM) to a binding site in rat liver membranes that is displaceable by ¹²⁷I-Ang 1-7 at nanomolar concentrations (IC₅₀ = 62 nM) while Ang 1-7 displaces at micromolar concentrations (IC₅₀ = 80 µM) at ~22 °C. This binding was also displaceable by inhibitors of metalloproteases at room temperature. This suggests that ¹²⁵I-Ang 1-7 binds to MMPs and/or ADAMs as well as other liver membrane elements at ~ 22 °C. However, when ¹²⁵I-Ang 1-7-binding assays were run at 0-4 °C, the same MMP inhibitors did not effectively compete for ¹²⁵I-Ang 1-7.

CONCLUSIONS

The addition of an iodine molecule to the tyrosine in position 4 of Ang 1-7 drastically changes the binding characteristics of this peptide making it unsuitable for characterization of Ang 1-7 receptors.

The Other Angiotensin II Receptor: AT2R as a Therapeutic Target

The active hormone of the renin-angiotensin system (RAS), angiotensin II (Ang II), is involved in several human diseases, driving the development and clinical use of several therapeutic drugs, mostly angiotensin I converting enzyme (ACE) inhibitors and angiotensin receptor type I (AT₁R) antagonists. However, angiotensin peptides can also bind to receptors different from AT₁R, in particular, angiotensin receptor type II (AT₂R), resulting in biological and physiological effects different, and sometimes antagonistic, of their binding to AT₁R. In the present Perspective, the components of the RAS and the therapeutic tools developed to control it will be reviewed. In particular, the characteristics of AT₂R and tools to modulate its functions will be discussed. Agonists or antagonists to AT₂R are potential therapeutics in cardiovascular diseases, for agonists, and in the control of pain, for antagonists, respectively. However, controlling their binding properties and their targeting to the target tissues must be optimized.

International Union of Basic and Clinical Pharmacology. XCIX. Angiotensin Receptors: Interpreters of Pathophysiological Angiotensinergic Stimuli [corrected]

The renin angiotensin system (RAS) produced hormone peptides regulate many vital body functions. Dysfunctional signaling by receptors for RAS peptides leads to pathologic states. Nearly half of humanity today would likely benefit from modern drugs targeting these receptors. The receptors for RAS peptides consist of three G-protein-coupled receptors—the angiotensin II type 1 receptor (AT1 receptor), the angiotensin II type 2 receptor (AT2 receptor), the MAS receptor—and a type II trans-membrane zinc protein—the candidate angiotensin IV receptor (AngIV binding site). The prorenin receptor is a relatively new contender for consideration, but is not included here because the role of prorenin receptor as an independent endocrine mediator is presently unclear. The full spectrum of biologic characteristics of these receptors is still evolving, but there is evidence establishing unique roles of each receptor in cardiovascular, hemodynamic, neurologic, renal, and endothelial functions, as well as in cell proliferation, survival, matrix-cell interaction, and inflammation. Therapeutic agents targeted to these receptors are either in active use in clinical intervention of major common diseases or under evaluation for repurposing in many other disorders. Broad-spectrum influence these receptors produce in complex pathophysiological context in our body highlights their role as precise interpreters of distinctive angiotensinergic peptide cues. This review article summarizes findings published in the last 15 years on the structure, pharmacology, signaling, physiology, and disease states related to angiotensin receptors. We also discuss the challenges the pharmacologist presently faces in formally accepting newer members as established angiotensin receptors and emphasize necessary future developments.

The Angiotensin II Type 2 Receptor in Brain Functions: An Update

Angiotensin II (Ang II) is the main active product of the renin-angiotensin system (RAS), mediating its action via two major receptors, namely, the Ang II type 1 (AT₁) receptor and the type 2 (AT₂) receptor. Recent results also implicate several other members of the renin-angiotensin system in various aspects of brain functions. The first aim of this paper is to summarize the current state of knowledge regarding the properties and signaling of the AT₂ receptor, its expression in the brain, and its well-established effects. Secondly, we will highlight the potential role of the AT₂ receptor in cognitive function, neurological disorders and in the regulation of appetite and the possible link with development of metabolic disorders. The potential utility of novel nonpeptide selective AT₂ receptor ligands in clarifying potential roles of this receptor in physiology will also be discussed. If confirmed, these new pharmacological tools should help to improve impaired cognitive performance, not only through its action on brain microcirculation and inflammation, but also through more specific effects on neurons. However, the overall physiological relevance of the AT₂ receptor in the brain must also consider the Ang IV/AT₄ receptor.

Angiotensin modulation of rostral ventrolateral medulla (RVLM) in cardiovascular regulation

The rostral ventrolateral medulla (RVLM) and the presympathetic bulbospinal neurons in this region play a critical role in cardiovascular regulation. However, there is ambiguity regarding the precise anatomical coordinates of the RVLM and much still needs to be learned regarding the regulation and neurochemistry of this region. This brief review discusses some of these issues and focuses on the role of angiotensin-mediated signaling in the RVLM in blood pressure regulation.

Human brain contains a novel non-AT1, non-AT2 binding site for active angiotensin peptides

AIMS

To determine whether the novel non-AT1, non-AT2 binding site for angiotensins recently discovered in rodent brains occurs in the human brain.

MAIN METHODS

Radioligand binding assays of (125)I-sarcosine(1), isoleucine(8) angiotensin II binding were carried out in homogenates of the rostral pole of the temporal cortex of human brains containing 0.3 mM parachloromercuribenzoate (PCMB), 10 microM losartan to saturate AT1 receptors, 10 microM PD123319 to saturate AT2 receptors, with or without 10 microM angiotensin II to define specific binding. Competition binding assays employed a variety of angiotensin peptides, specific angiotensin receptor antagonists, several neuropeptides and an endopeptidase inhibitor to determine pharmacological specificity for this binding site.

KEY FINDINGS

The novel non-AT1, non-AT2 binding site was present in similar amounts in female and male brains: Bmax 1.77+/-0.16 and 1.52+/-0.17 fmol/mg initial wet weight in female and male brains, respectively. The K(D) values, 1.79+/-0.09 nM for females, and 1.53+/-0.06 nM for males were also similar. The binding site shows pharmacological specificity similar to that in rodent brains: sarcosine(1), isoleucine(8) angiotensin II>angiotensin III>angiotensin II>angiotensin I'angiotensin IV>angiotensin 1-7. Shorter angiotensin fragments and non-angiotensin peptides showed low affinity for this binding site.

SIGNIFICANCE

The presence in human brain of this novel non-AT1, non-AT2 binding site supports the concept that this binding site is an important component of the brain angiotensin system. The functional significance of this binding site, either as a novel angiotensin receptor or a highly specific angiotensinase remains to be determined.

Brain angiotensin receptors and binding proteins

This review addresses classical and novel aspects of the brain angiotensin system. The brain contains both the AT1 and AT2 angiotensin II (Ang II) receptor subtypes which are well-characterized guanine nucleotide binding protein (G protein)-coupled receptors (GPCRs). Like other GPCRs, novel signal transduction pathways and protein interactions are being described for Ang II receptors. For brain AT1 receptors, there is a controversy regarding the identity of the active angiotensin peptide in the brain which is addressed in this review. This review also summarizes a recent discovery of a novel, membrane-bound, non-AT1, non-AT2 binding site for angiotensin peptides that appears to be brain-specific. This binding site is unmasked by a limited concentration range of the organometallic sulfhydryl-reactive agent p-chloromercuribenzoic acid (PCMB) suggesting that functional expression of this binding site may depend on the redox state of the milieu of the brain. While this binding site has similarities to a previously described soluble angiotensin-binding protein found in liver that is unmasked by PCMB, it has many different characteristics. The possible functional significance of this novel non-AT1, non-AT2 binding site for angiotensin peptides as a mediator of non-traditional actions of Ang II in the brain, e.g., stimulation of dopamine release from the striatum, as a peptidase, or as a clearance receptor, and the importance of the state of the internal environment of the brain to its function is reviewed.

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.

Central pressor actions of aminopeptidase-resistant angiotensin II analogs: challenging the angiotensin III hypothesis

Intracerebroventricular administration of angiotensins causes pronounced pressor and dipsogenic responses. The suggestion that angiotensin III rather than angiotensin II is the active peptide in the brain spawned what we call The Angiotensin III.

HYPOTHESIS

To test this hypothesis, 5 angiotensin II analogs containing zero or one position substitutions conferring resistance to aminopeptidases were administered intracerebroventricularly to determine their pressor and dipsogenic efficacies. Two aminopeptidase-resistant analogs caused significantly greater pressor responses than angiotensin II, whereas 3 analogs caused pressor responses similar to angiotensin II. Latency to cause a pressor response for 4 of the 5 aminopeptidase-resistant angiotensin II analogs was the same as for angiotensin II. There was no detectable formation of (125)I-angiotensin III from 1 of the intracerebroventricularly administered analogs, (125)I- N-Methyl-l-Asp(1)-angiotensin II, indicating its aminopeptidase resistance. Latency to drink also did not differ between the angiotensins. After the initial dipsogenic response, water was removed until 25 minutes after angiotensin administration to avoid interfering with the pressor response. The dipsogenic stimulus was sustained 25 minutes after intracerebroventricular injection of angiotensin II and its aminopeptidase-resistant analogs. Comparison of angiotensin III and angiotensin II showed equivalent pressor responses with similar latencies and durations. The latency to drink was similar for angiotensin III and angiotensin II. However, there was no dipsogenic response to angiotensin III 25 minutes after intracerebroventricular injection. These data do not support The Angiotensin III Hypothesis and suggest that conversion of exogenously applied angiotensin II to angiotensin III is not necessary to cause brain-mediated pressor or dipsogenic responses.

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.

Identification of a novel non-AT1, non-AT2 angiotensin binding site in the rat brain

Efforts to protect radiolabeled angiotensins from metabolism during receptor binding assays date back more than 30 years. However, this continues to be a problem. This study focused on the effects of a protease inhibitor, p-chloromercuribenzoate (PCMB), on the binding of (125)I-Ang II to rat brain membranes. Addition of PCMB to the incubation medium revealed a high affinity binding site for (125)I-Ang II in brain membranes (K(d)=1-4 nM) with a greater amount of binding than revealed in previous studies of brain Ang II receptors. Further characterization of this binding, revealed it to be insensitive to inhibition by losartan (an AT(1) receptor antagonist) and PD123319 (an AT(2) receptor antagonist). This non-AT1, non-AT2 binding site was not present in liver or adrenal membranes. It was activated by a limited range of concentrations of PCMB, with maximal activation at 0.3-1 mM. This binding site was equally abundant in cerebral cortex (a brain region with few Ang II receptors) and the hypothalamus (a brain region with abundant Ang II receptors). The binding site was also present in mouse brain, but not mouse liver. The binding site shows high affinity for Ang I, Ang II and Ang III (K(i) approximately 40-100 nM), but lesser affinity for smaller angiotensin fragments and other neuropeptides. This binding site shares some characteristics with the liver cytosolic Ang II binding proteins, later identified as endopeptidases EC 3.4.24.15 and/or EC 3.4.24.16. However, some unique characteristics of this non-AT1, non-AT2 binding site suggest that it may be a novel angiotensin binding substance.

Differential loss in function of angiotensin II receptor subtypes during tissue storage

In vitro receptor autoradiography was performed on rat brain and kidney sections stored frozen at -20 degrees C for extended time periods (17, 40, 64, 121, 183, 251, and 333 days). The results indicate that prolonged tissue storage has a differential effect upon 125I sar1ile8 angiotensin II binding to AT1 and AT2 receptor sites. Binding at AT1 receptor rich tissues studied (renal medulla, renal cortex, anterior pituitary, ventral hippocampus, spinal trigeminal nucleus, and nucleus of the solitary tract) shows a first order exponential decay pattern. The logarithmic linear regression slope (log(e) specific binding versus time), is significantly different from zero (p<0.05) in all AT1 rich tissues except for nucleus of the solitary tract (p=0.086). There is no detected loss of 125I sar1ile8 angiotensin II binding at the AT2 prominent regions in the superior colliculus, medial geniculate nucleus, and the inferior olivary nucleus. The half lives of AT1 receptors are highly variable, ranging from 36 days in the anterior pituitary to 442 days in the nucleus of the solitary tract, and this might be related to variable stability of AT1A and AT1B receptors. These observations should be taken into account when assessing and comparing AT1 and AT2 receptor subtype densities.

The neuronal role of angiotensin II in thirst, sodium appetite, cognition and memory

Within the past two decades, a great deal has been learnt about the renin-angiotensin system in the brain. The renin-angiotensin system is one of the best-studied enzyme-neuropeptide systems in the brain. The diversity of localization of this peptide throughout the brain has implied a variety of potential functions. Besides its classical role in the regulation of blood pressure and body-fluid homeostasis, it has more subtle functions involving complex mechanisms such as learning and memory. The profound effects on behaviour produced by angiotensin are of broad interest to neuroscientists. The mechanisms of action differ depending on whether angiotensin is locally synthesized and whether regulation is governed by neural or metabolic inputs impinging on the neurones. Its central action is mediated through peptidergic receptors present on neurones. The description of the receptor subtypes AT1 and AT2 for angiotensin II and the development of non-peptidic specific angiotensin receptor subtype antagonists have opened a new area in this field of research. The AT1 site, which preferentially binds to angiotensin II and angiotensin III, appears to mediate the classical angiotensin functions concerned with maintenance of blood pressure and body-fluid control. In addition, most of the behavioural effects described so far are linked with AT1, although so-called psychotropic effects are presumed to be mediated by receptor systems other than the known specific angiotensin receptors. In fact, evidence for the existence of such receptors with high-affinity binding has been reported. The central action of angiotensin II mediated by AT2 is as yet unclear. Most reports concerning this receptor subtype suggest a role in differentiation and development, since the number of binding sites is higher in fetal and young rats than in adults. Furthermore, the neuronal effect of angiotensin II in the inferior olivary nucleus which is blocked specifically by AT2 antagonists suggests an involvement in motor control. Over the next few years we should find answers to many of the questions currently unanswered about angiotensin function and, given the rapid progress in research on this neuropeptide, it may serve as a model for the action of peptides on neuronal function in general.

Angiotensin receptors in the brain

Angiotensin receptors have recently become a focus of scientific interest due to the recent development of specific receptor ligands which allow to distinguish between various angiotensin II receptor subtypes, notably the angiotensin II type 1 receptor (AT1) and angiotensin II type 2 receptor (AT2). Although both receptors belong to the seven transmembrane domain receptor family they feature less than 35% homology and differ in their signal transduction mechanisms and in the effects mediated. In the brain, both angiotensin receptor types and probably some further subtypes are present and have been localized in distinct regions. In the adult brain, the AT1 receptor dominates by far and is responsible for most of the known central actions of angiotensin peptides, for example blood pressure increase, release of vasopressin from the pituitary gland, natriuresis, drinking and induction of immediate early genes in distinct brain areas. Some of the AT1 receptor-mediated effects have been shown to be enhanced by blockade of AT2 receptors in the brain suggesting that the central AT2 receptor can exert an inhibitory control on AT1 receptor-mediated actions in the brain.

Quantitative autoradiography of angiotensin II AT2 receptors with [125I]CGP 42112

Most radiolabeled ligands for angiotensin II (Ang II) receptors do not discriminate between the AT1 and AT2 receptor subtypes, which must be distinguished by displacement with selective AT1 or AT2 ligands. We compared [125I]CGP 42112 with the non-selective agonist [125I]Sar1 Angiotensin II. We studied the inferior olive, medial geniculate nucleus and the adrenal medulla, areas rich in AT2 receptors, using both ligands with quantitative autoradiography and membrane binding techniques. [125I]CGP 42112 bound with high affinity (Kd = 0.07-0.3 nM, depending on the area studied). [125I]CGP 42112 binding was selective for AT2 receptors, as determined by lack of competition with the AT1 ligand losartan, and competition by the AT2 ligands PD 123177 and unlabeled CGP 42112 and the non-selective peptides Ang II and angiotensin III (Ang III). Using [125I]CGP 42112 binding, we found the same order of potency: CGP 42112 > Ang II = Ang III > PD 123177 using both quantitative autoradiography or membrane binding methods. Our results demonstrate that [125I]CGP 42112 is the most selective, highest affinity ligand available for AT2 receptors. Because of these characteristics, and low non-specific binding, quantitative autoradiography with [125I]CGP 42112 is the method of choice to selectively characterize AT2 receptors, especially in tissues like the brain, with a highly heterogeneous distribution of receptor subtypes.

Pharmacological characterization of angiotensin II AT(2) receptor subtype heterogeneity in the rat adrenal cortex and medulla

Adrenal angiotensin II (AII) receptors have been pharmacologically and structurally divided into two main subtypes, AT(1) and AT(2). Radioligand receptor binding assays with(125)I-sarcosine(1), isoleucine(8) angiotensin II ((125)I-SI AII) in the presence of losartan, an AT(1) selective ligand, and PD123177 an AT(2) selective ligand, indicated that the AT(1) subtype was predominant in membrane homogenates of the rat adrenal cortex (AT(1) Bmax=649 ± 62 fmol/mg protein; AT(2) Bmax=237 ± 29 fmol/mg protein). In membrane homogenates of the adrenal medulla, the AT(2) subtype was predominant (AT(1) Bmax=55 ± 5 fmol/mg protein; AT(2) Bmax=109 ± 29 fmol/mg protein). Overall 58% of the(125)I-SI AII binding in the rat adrenal was to the AT(1) subtypes, and 42% was to the AT(2) subtypes. The outer cortex contained 59% of the AH receptor binding sites in the adrenal, while the medulla accounted for the remaining 41%. The affinity of the AT(1) binding sites in membrane homogenates of the cortex and medulla (K( D )=672 ± 123 pM and 573 ± 85 pM, respectively) was not significantly different. The affinity for(125)I-SH AII of AT(2) binding sites in membrane homogenates was higher than that of AT, binding sites. The affinity for(125)I-SI All of AT(2) binding sites in membrane homogenates of the outer cortex (K( D )=265 ± 35 pM) was significantly less than that in the medulla (K( D )=133 ± 11 pM).In vitro receptor autoradiography also demonstrated that the AT(2) subtype in frozen sections of the cortex had a lower affinity (K( D )=1512 ± 191 pM) than that in the medulla (K( D )=867 ± 72 pM). The heterogeneous affinity of adrenal AT(2) binding sites may indicate existence of multiple AT(2) receptor subtypes in the rat adrenal.

Angiotensin II receptor subtypes in the human central nervous system

The distribution of the AT1 and AT2 subtypes of angiotensin II receptor was mapped in the adult human central nervous system using quantitative in vitro autoradiography. Binding in all forebrain, midbrain, pontine, medullary and spinal cord sites where angiotensin II receptors have previously been described is of the AT1 subtype, as is binding in the small and large arteries in the adjacent meninges and in choroid plexus. By contrast, both AT1 and AT2 receptors occur in the molecular layer of the cerebellum. Angiotensin II AT1 receptors in the brain show a moderate degree of conservation across mammalian species studied so far, whereas expression of AT2 receptors is more variable, and is more restricted in the human CNS than in many other mammals. These differences between the subtype distributions in humans and other animals indicate the need for care when extrapolating the results of animal studies involving the brain angiotensin system.

The brain renin-angiotensin system: molecular mechanisms of cell to cell interactions

The components of the Renin-Angiotensin System (RAS) have been found to be expressed in the brain. Angiotensinogen, the high molecular weight precursor of the system, is widely distributed and expressed in areas not related to control of blood pressure and body fluid homeostasis as well. It has been shown that it is regulated by steroid hormones independently from the liver and that it is also regulated in a different manner in several brain areas. Angiotensin II, the effector peptide of the system, may be generated in the brain via the classical pathway, using renin and angiotensin converting enzyme or directly from angiotensinogen by cathepsin G or tonin. N-terminal peptides of angiotensin II have been found in several brain areas with ANG (1-7) involved in vasopressin release however without influence on blood pressure and with ANG III acting as potent as ANG II. Transgenic animals may be used to study the pathophysiology of an activated brain RAS.

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.

Dithiothreitol, sodium chloride, and ethylenediaminetetraacetic acid increase the binding affinity of [125I]angiotensin IV to AT4 receptors in bovine adrenal cortex

The present studies demonstrate that the sulfhydryl reducing agent, dithiothreitol (DTT), increases the specific binding of [125I]angiotensin IV ([125I]AIV) to AT4 receptors in bovine adrenal cortical membranes. Both the degree of stimulation and the pharmacological selectivity of [125I]AIV binding in the presence of DTT were quantitatively different depending on the contents of the assay buffer. Similar effects were also observed using a different sulfhydryl reducing agent, 2-mercaptoethanol (2-MCE). These sulfhydryl reducing agents (100 mM) produced a 200% increase in specific [125I]AIV binding in an assay buffer that has been used to characterize the novel AT4 receptor subtype. A much larger stimulation (700%) of specific [125I]AIV binding was found when the assay was conducted in a buffer that has been used to characterize ligand binding to the AT1 receptor. Ligand association studies indicated that 0.3 nM [125I]AIV displayed similar equilibrium kinetics and stability in both the AT4 and AT1 buffers. Ligand saturation studies indicated that [125I]AIV bound with high affinity (Kd = 6 nM) in the AT4 buffer system, but bound with lower affinity (Kd = 32 nM) in the AT1 buffer system. Removal of NaCl and EDTA from the AT4 buffer also resulted in low-affinity [125I]AIV binding (Kd = 33 nM). The subsequent inclusion of NaCl, EDTA, or DTT resulted in higher-affinity [125I]AIV binding (KdS = 3-14 nM). No significant effects on the apparent density (Bmax) of AT4 receptors were observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduction of voluntary alcohol consumption in the rat by transplantation of hypothalamic grafts

Stimulation of the peripheral renin-angiotensin system has been shown previously to decrease the voluntary intake of ethanol in the rat. The existence of a separate brain renin-angiotensin system, independent from that of the periphery, has been widely demonstrated. The brain renin-angiotensin system plays an important role in the regulation of water and electrolyte balance and neuroendocrine function. However, the role played by this system in the regulation of voluntary alcohol consumption has not yet been studied. The goal of the present work was to assess the feasibility of decreasing the voluntary alcohol intake in a strain of rats (Rapp SS/Jr rats) that have a genetic deficiency responsible for a low activity of the renin-angiotensin system and elevated alcohol intake. Adult Rapp SS/Jr rats received intraventricular transplants of fetal hypothalamic grafts (from normal donors), known to contain angiotensin-immunoreactive cell bodies. Our studies revealed that angiotensin-immunoreactivity in the cell bodies and fibres in the paraventricular, supraoptic and suprachiasmatic nuclei of the hypothalamus in Rapp SS/Jr rats was markedly reduced. Animals that had surviving grafts containing angiotensin-immunoreactive cell bodies in the dorsal third ventricle--but not in the ventral third ventricle, in the lateral ventricles, or sham operated animals--had a 40% decrease of their voluntary alcohol intake, when compared to their intake before surgery, or to the control group. However, water consumption was not reduced in both the sham and transplanted animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and characterization of angiotensin II AT2 receptors from neonatal rat kidney

Angiotensin II (Ang II) AT2 receptors were purified 40,000-fold to a nearly homogeneous state after solubilization from neonatal rat kidney membranes with 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propane-sulfonic acid. Comparable IC50 values for the soluble extract (0.32 nM) and membranes (0.31 nM) were obtained by competition curves with 125I-labeled CGP42112, a selective AT2 ligand. Binding to AT2 receptors in the soluble extract was not sensitive to dithiothreitol. AT2 receptors were further purified by gel filtration and a CGP42112 Sepharose affinity column. Ang II AT2 receptors were selectively eluted with 5 microM CGP42112 at 4 degrees C, and a single band with an apparent molecular mass of 71 kDa was obtained after SDS/PAGE. Two-dimensional electrophoresis confirmed the purity of the protein and an isoelectric point of 5.3-5.5 was obtained. A highly selective elution of the AT2 receptors from the affinity column was performed with 5 nM 125I-labeled CGP42112 at room temperature after the column was treated with 1 microM losartan in the presence of high salt. After cross-linking, a major labeled protein with similar molecular mass and isoelectric point was obtained. Dissociation of the radiolabeled protein was insensitive to losartan but was enhanced by CGP42112, PD123177, Ang II, and [Sar1]Ang II. In summary, Ang II AT2 receptors were purified by CGP42112 affinity chromatography and selective elution and retain the pharmacological specificity of particulate receptors.

Angiotensin II and non-angiotensin II displaceable binding sites for [3H]losartan in the rat liver

By virtue of the more than 1000-fold selectivity of losartan (DuP 753) for the AT1 angiotensin II (AII) receptor subtype compared with the AT2 subtype, [3H]losartan may be a useful radioligand for studies of the AT1 receptor subtype. Comparison of Bmax values in the liver obtained from saturation isotherms using [3H]losartan (Bmax = 194 pmol/g tissue) and [125I]sarcosine1,isoleucine8 angiotensin II (Bmax = 20 pmol/g tissue) indicated that the AII receptor concentration was approximately 10% that of the [3H]losartan binding sites. In addition, AII at concentrations as high as 10 microM displaced less than one-third of specific [3H]losartan binding in the liver and less than 80% in the whole adrenal. The presence of non-AII displaceable [3H]losartan binding in the liver did not appear to result from metabolism of the radioligand since HPLC analysis of free and bound 3H revealed that greater than 90% of the 3H eluted at the same time as the parent [3H]losartan. This suggests that [3H]losartan binds with high affinity to a site(s) other than angiotensin II receptors in the rat liver.

Sheep hypothalamus contains a non-angiotensin ligand for type 1 and type 2 angiotensin II receptors

1. The aim of this study was to determine whether the brain contains an alternative ligand for angiotensin II (AII) receptors. 2. A radioreceptor assay based upon bovine cerebellar membranes (Type 2 AII receptors) was used to monitor the partial purification of an AII-like material from sheep hypothalami. 3. This material displaces 125I-[Sar1, Ala8]-AII from both type 1 (rat adrenal capsular membranes) and Type 2 AII receptors in a manner parallel to that of AII. It has a size of approximately 30,000 Da, is strongly cationic, is stable to boiling but is destroyed by trypsin. It is not recognized by AII antisera. 4. These data provide direct evidence for a non-angiotensin endogenous ligand for brain AII receptors. This novel ligand may play a role in the regulation of blood pressure and other actions mediated by brain AII receptors.

The renin-angiotensin system in the brain: an update 1993

The renin-angiotensin system is considered to be one of the most important hormonal systems in the regulation of blood pressure and body fluid homeostasis. Ever since this system has been demonstrated to be present also in the brain, vast efforts have been made in investigating its central impact and function. The last few years, and especially the development of non-peptidic angiotensin II receptor subtype specific antagonists and the subsequent pharmacological characterization of these subtypes, brought this field of research a large step forward. This progress also might have opened up new avenues of developing highly specific anti-hypertensive drugs and thereby new ways of treating hypertension. This paper intends to provide a summary of the knowledge about the brain renin-angiotensin system accumulated during recent years; an update 1993.

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)

Effects of peptidase inhibitors on binding at angiotensin receptor subtypes in the rat brain

Sulfhydryl reducing agents affect angiotensin II (AII) receptor binding differentially at AT1 and AT2 sites. Consequently, sulfhydryl reducing agents are now used infrequently in AII receptor binding assays. In this regard, the present autoradiographic study evaluates the effects of additional peptidase inhibitors on AII receptor binding and radioligand integrity. EDTA at 5 mM enhanced binding similarly, by about 70%, at both AT1 and AT2 binding sites, whereas bacitracin (10(-4) M) did not affect binding at either site. In contrast, addition of phenanthroline and bovine serum albumin (BSA) increased binding at AT1 sites 2.3-fold, whereas binding at AT2 sites was affected minimally. Degradation of 125I-[Sar1,Ile8]-AII (125I-SIAII) was determined by HPLC analysis of samples before and after incubation with tissue in each buffer. Omission of bacitracin from buffers reduced the recovery of intact radioligand to 83-87%, while recovery exceeded 94% in the presence or absence of all other buffer constituents. These results suggest that degradation of 125I-SIAII is minimal in large volume in vitro receptor autoradiography studies of rat brain AII receptors. Further, the beneficial effects on radioligand binding caused by buffer constituents such as EDTA, phenanthroline, and BSA were not due to their ability to protect the radioligand from enzymatic degradation. Because these constituents (and possibly others) had differential effects on binding with respect to receptor subtypes, caution should be used when interpreting or comparing binding data obtained from various laboratories utilizing different buffer components.

Roles of sulfhydryl and disulfide groups in the binding of CP-55,940 to rat brain cannabinoid receptor

The roles of sulfhydryl and disulfide groups in the specific binding of synthetic cannabinoid CP-55,940 to the cannabinoid receptor in membrane preparations from the rat cerebral cortex have been examined. Various sulfhydryl blocking reagents including p-chloromercuribenzoic acid (p-CMB), N-ethylmaleimide (NEM), o-iodosobenzoic acid (o-ISB), and methyl methanethiosulfonate (MMTS) inhibited the specific binding of [3H]CP-55,940 to the cannabinoid receptor in a dose-dependent manner. About 80-95% inhibition was obtained at a 0.1 mM concentration of these reagents. Scatchard analysis of saturation experiments indicates that most of these sulfhydryl modifying reagents reduce both the binding affinity (Kd) and capacity (Bmax). On the other hand, DL-dithiothreitol (DTT), a disulfide reducing agent, also irreversibly inhibited the specific binding of [3H]CP-55,940 to the receptor and about 50% inhibition was obtained at a 5 mM concentration. Furthermore, 5 mM DTT was abelt to dissociate 50% of the bound ligand from the ligand-receptor complex. The marked inhibition of [3H]CP-55,940 binding by sulfhydryl reagents suggests that at least one free sulfhydryl group is essential to the binding of the ligand to the receptor. In addition, the inhibition of the binding by DTT implies that besides free sulfhydryl group(s), the integrity of a disulfide bridge is also important for [3H]CP-55,940 binding to the cannabinoid receptor.

Immunohistochemical demonstration of angiotensin II receptors in rat brain by use of an anti-idiotypic antibody

In the present study we investigated the ability of an anti-idiotypic antibody which recognizes angiotensin II (AII) receptors to demonstrate the presence of such receptors under immunohistochemical conditions. The experiments revealed punctate immunoreactive granules on neurons of the nucleus supraopticus and of the nucleus paraventricularis of the hypothalamus. This localization of AII receptors is consistent with the findings obtained using other experimental approaches to the brain renin-angiotensin system. The conclusion of this study is that the applied anti-idiotypic antibody seems to be a reliable tool for mapping AII receptor distribution. The established experimental approaches to AII receptors are thus now supplemented with the possibility of immunohistochemical investigation. Moreover, the possible microscopic analysis of AII receptors on distinct cells will allow studies at an ultrastructural level.

[125I]CGP 42112 binding reveals differences between rat brain and adrenal AT2 receptor binding sites

The AT2 angiotensin II receptor selective ligand CGP42112 was radioiodinated and used to study AT2 receptor binding sites in the rat brain (combined olfactory bulb, septum, thalamus and midbrain) and whole adrenal. The [125I]CGP 42112 binding was of high affinity, saturable and specific in both tissues. Competition studies with nonselective and angiotensin II receptor subtype selective ligands, and evaluation of the effects of the sulfhydryl reducing agent beta-mercaptoethanol, confirmed that [125I]CGP 42112 bound selectively to the AT2 angiotensin II receptor subtype. [125I]CGP 42112 bound with higher affinity in the brain than in the adrenal. beta-Mercaptoethanol enhanced [125I]CGP 42112 binding in the brain, but did not alter its binding in the adrenal. A similar difference in binding affinity for [125I]sarcosine, isoleucine angiotensin II and enhancement of binding affinity by beta-mercaptoethanol was observed in the rat brain and adrenal. These observations suggest that the AT2 angiotensin II receptor subtypes in the brain and adrenal may differ.

Functional roles of brain AT1 and AT2 receptors in the central angiotensin II pressor response in conscious young spontaneously hypertensive rats

Areas of adult rat brain that mediate the cardiovascular effects of central angiotensin II (ANG II) predominantly express AT1 ANG II receptors. In contrast, AT2 receptor expression in young rats is transiently increased, reaching a maximum during the first few weeks of life. This study was designed to determine the roles of brain AT1 and AT2 receptors in mediating the central pressor effects of ANG II in young (4-week-old) conscious spontaneously hypertensive rats (SHR). Mean arterial pressure responses to intracerebroventricular (i.c.v.) ANG II (100 ng in 5 microliters) were determined 10 minutes after i.c.v. injection of either the AT1 receptor antagonist Losartan (1.0, 2.5, 5.0, and 10.0 micrograms), the AT2 receptor ligand PD 123319 (3.5 x [10(-6), 10(-4), 10(-2), 10(0)] micrograms), or both. In control rats, i.c.v. Losartan prevented the pressor response to i.c.v. ANG II in a dose-dependent manner (P < 0.05), while i.c.v. PD 123319 alone was without effect. In other animals, pressor responses caused by i.c.v. ANG II-induced vasopressin secretion (VP-component) and sympathetic nervous system activation (SNS-component) were studied individually, with similar result; Losartan prevented the SNS-component, but reduced the VP-component by only 45%, indicating that both pressor components involve AT1 receptor activation. However, doses of Losartan were more effective when combined with 3.5 micrograms of PD 123319 than when given alone (P < 0.05); nearly eliminating the VP-component. These results suggest that i.c.v. ANG-II-induced pressor effects may involve activation of multiple receptor subtypes.

Cellular localization of angiotensin type 1 receptor and angiotensinogen mRNAs in the subfornical organ of the rat brain

The cellular localization of angiotensin type 1 receptor (AT 1) and angiotensinogen mRNA expression in the subfornical organ (SFO) of the rat brain has been studied by means of non-radioactive in situ hybridization combined with immunocytochemistry for glial fibrillary acidic protein (GFAP) and Neutral red staining. The AT 1 receptor mRNA expression is shown to be within putative nerve cells without any association with the glial fibrillary acidic protein (GFAP)-immunoreactive (IR) cells. In contrast the angiotensinogen cRNA expression is associated predominantly with GFAP-IR cells. The results demonstrate that a neuronal AT 1 receptor mediates the actions of circulating angiotensin II on the SFO and that the angiotensinogen mRNA is predominantly expressed in the SFO astroglial cells.

Levels of angiotensin and molecular biology of the tissue renin angiotensin systems

The cloning of renin, angiotensinogen and angiotensin converting enzyme genes have established a widespread presence of these components of the renin-angiotensin system in multiple tissues. New sites of gene expression and peptide products in different tissues has provided strong evidence for the production of angiotensin independently of the endocrine blood borne system. In addition, the cloning of the angiotensin receptor (AT1) gene has confirmed the widespread distribution of angiotensin and suggested new functions for the peptide. This review of various tissues shows the variation in gene expression between tissues and angiotensin levels, and the fragmentary state of our knowledge in this area. As yet we cannot state that the gene expression of the substrates, enzymes and peptide products are involved in a single cell synthesis. This is not so much evidence against a paracrine function for tissue angiotensin, as lack of detailed, accurate intracellular information. The low abundance of renin in brain, spleen, lung and thymus compared to kidney, adrenal, heart, testes, and submandibular gland may suggest that there are both tissue renin-angiotensin systems (RAS) and nonrenin-angiotensin systems (NRAS). The NRAS could function through cleavage of angiotensinogen by serine proteinases such as tonin and cathepsin G to form Ang II directly. Although much angiotensinogen is extracellular and could therefore be a site of synthesis outside of the cell, intracellular angiotensinogen in a NRAS process could produce Ang II intracellularly without requiring extracellular conversion of Ang I to Ang II by ACE. In summary, renin mRNA is found in high concentrations in kidney, adrenal and testes and decreasing lower concentrations in ovary, liver, brain, spleen, lung and thymus. Angiotensinogen mRNA is found in the following tissues in descending order of abundance: liver, fat cells, brain (glial cells), kidney, ovary, adrenal gland, heart, lung, large intestine and stomach. It is debatable whether angiotensinogen and renin mRNA are expressed in blood vessels. The evidence that is lacking for a paracrine function of angiotensin is a complete description of the intracellular molecular synthesis and release of Ang II from single cells of promising tissues. Such tissues, SMG, ovary, testes, adrenal, pituitary and brain (neurons and glia) are potent sources of RAS components for future studies. Although the evidence for a paracrine function of angiotensin II is incomplete, it is an important concept for progressing toward the understanding of tissue peptide physiology and the significance of their gene regulation.

Identification and characterisation of angiotensin II receptor subtypes in human brain

Autoradiographic and homogenate binding studies using the radioligand, [125I]angiotensin II, identified a heterogeneous distribution of specific binding sites (defined by angiotensin II, 1.0 microM) throughout the human forebrain. Highest AT receptor densities were detected in the paraventricular nucleus, median eminence, substantia nigra, putamen and caudate nucleus (2.4, 1.2, 1.0, 0.30 and 0.24 fmol/mg tissue equivalent, respectively). The AT1 receptor antagonist, losartan (1.0 microM) competed for the majority of the specific binding. [125I]Angiotensin II-specific binding (although not consistently above non-specific binding levels) was also detected in various other brain regions (e.g. amygdala, entorhinal cortex, frontal cortex, hippocampus, inferior colliculus, nucleus accumbens, parietal cortex, periaquaductal grey, superior colliculus, striate cortex, temporal cortex, thalamus). In the presence of losartan (1.0 microM), angiotensin II, saralasin, losartan and PD123177 competed for [125I]angiotensin II binding to membranes prepared from the cerebellum or substantia nigra with a rank order of affinity; angiotensin II = saralasin > PD123177 > losartan. In the presence of PD123177 (1.0 microM), the rank order of affinity of losartan and PD123177 was reversed. These studies indicate the presence of both AT1 and AT2 receptor subtypes within various regions of the human forebrain.

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.

Changes in skin angiotensin II receptors in rats during wound healing

Angiotensin (AII) is associated with increased vascular smooth muscle growth and we have found increased levels of tissue AII during healing of wounded skin. Here we have determined changes in skin AII receptors during wound healing in adult male Sprague-Dawley rats. An abdominal surgical incision was made under anesthesia and rats were sacrificed at different times after wounding. Specific binding of 125I-AII was significantly decreased at 12, 18 and 24 hours in the wounded tissue compared to control tissue from the same rat. By 3 days the binding had recovered to baseline levels. Receptors were mostly AT1, with a high and a low affinity site in the skin both in control and healing tissue. The Bmax of the high affinity site was significantly decreased in healing tissue but there was no significant change in Kd. Our results demonstrate that adult rat skin contains predominantly AT1 receptors and also that these receptors are downregulated for 12-24 hours after wounding.

Effects of angiotensin II and its selective antagonists on inferior olivary neurones

On the basis of biochemical and autoradiographic studies it has been shown that the inferior olivary nucleus (ION) contains predominantly angiotensin II (Ang II) receptors of the subtype 2 (AT2). In the present investigation we used microiontophoretic techniques to test the effect of Ang II on the spontaneous firing rate of rat neurones in the ION in vivo. Ang II excited the majority of histologically identified ION neurones. Furthermore, the antagonism of this angiotensin-induced excitation by selective angiotensin receptor blockers of subtype 1 and 2 (AT1 and AT2) was examined. The excitation could be blocked by low doses of the AT2-antagonists PD 123177 and CGP 42112A, whereas the AT1-antagonist DuP 753 was ineffective even at high doses. On a few occasions, however, ejection of the AT1-antagonist resulted in a potentiation of angiotensin-induced excitation. The results suggest that Ang II has an excitatory effect on a considerable number of ION neurones and that this effect is mediated by AT2-receptors.

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 distribution of angiotensin II AT1 receptor subtype mRNA in the rat brain

The present study demonstrates the existence and regional distribution of angiotensin II AT1 receptor subtype mRNA expression in the rat brain by the use of in situ hybridization and RNase protection assay. Substantial expression levels in the brain have only been detected in certain distinct areas, such as the subfornical organ, the parvocellular part of the paraventricular hypothalamic nucleus, and the median preoptic nucleus. The results give further evidence for the involvement of the angiotensin II AT1 receptor subtype in the classical functions of central angiotensin II, like blood pressure control, body fluid homeostasis and in corticotropin-releasing factor (CRF) secretion.

The role of angiotensin, AT1 and AT2 receptors in the pressor, drinking and vasopressin responses to central angiotensin

Angiotensin II (Ang II) given centrally produces an increase in blood pressure and motivation to drink. The physiological mechanisms that mediate the pressor response include release of vasopressin (AVP) and activation of the sympathetic nervous system. Using 2 new Ang II receptor antagonists, we were able to investigate the role of AT1 or AT2 receptors in mediating these effects. Adult male Sprague-Dawley rats were cannulated in the lateral ventricle and 5 days later catheterized in the carotid artery for blood pressure measurements. All experiments were carried out in conscious rats. Three treatments were given intraventricularly (i.v.t.), in 2 microliters artificial cerebrospinal fluid (ACSF) at 30 min intervals: (1) 50 ng Ang II, (2) 0.7 micrograms AT1 antagonist Losartan or 7.0 micrograms AT2 antagonist PD123177, followed by 50 ng Ang II, and (3) 50 ng Ang II, to test for recovery. Blood pressure and drinking measurements were recorded. Also, blood samples for assay of AVP were drawn at 1 or 3 min post-injection in 2 separate groups of rats. We found that both Losartan and PD123177 significantly reduced release of AVP to Ang II 1 min post-injection. Losartan significantly blocked the pressor response (P less than 0.001), while PD123177 had no significant effect. Drinking was also antagonized by Losartan (P less than 0.05) and reduced (n.s.) by PD123177. The results suggest that the pressor response to Ang II (i.v.t.) is predominantly AT1 mediated, while the drinking and AVP responses may be mediated by both receptor subtypes.

Characterization and distribution of angiotensin II binding sites in fetal and neonatal astrocytes from different rat brain regions

Although angiotensin II (Ang II) binding sites have been extensively investigated in brain, revealing the presence of both AT1 and AT2 subtypes in various areas, the question as to which cells express AT1 and AT2 sites is still open. We report here that primary cultures of astrocytes obtained from various brain regions of fetal (F17) and one-day-old rats express Ang II binding sites belonging only to the AT1 subtype. The binding sites have the same binding profile in all regions tested; however, much less binding was observed in membranes of astrocytes derived from cortical than from subcortical regions and almost none were found in neonatal cortex. In addition, the dispersion method used at the onset of culture affects the number of binding sites present at the end of the culture period.

The AT2 subtype of the angiotensin II receptors has differential sensitivity to dithiothreitol in specific brain nuclei of young rats

We studied the effect of the sulfhydryl reducing agent dithiothreitol on the binding of the angiotensin II agonist [125I][Sar1]-angiotensin II to AT2 receptors in selected brain areas of young (2-week-old) rats. In the inferior olive and the hypoglossal nucleus, angiotensin II binding to AT2 receptors was insensitive to 5 mM dithiothreitol. Conversely, in the ventral and mediodorsal thalamic, medial geniculate, and oculomotor nuclei, the superior colliculus and the cerebellar cortex, incubation with 5 mM dithiothreitol significantly decreased angiotensin II binding to AT2 receptors to about 40% of control. These data suggest that brain AT2 receptors are heterogeneous with respect to their sensitivity to sulfhydryl reducing agents.

Autoradiographic visualization and characteristics of [125I]bradykinin binding sites in guinea pig brain

The present study was undertaken to localize and characterize bradykinin (BK) binding sites in 10 microns serial sections of guinea pig brain by in vitro quantitative receptor autoradiography. Specific binding of [125I-Tyr8]bradykinin ([125I]BK) was localized in the medulla oblongata to the regions of the nucleus of the solitary tract (nTS), the area postrema (AP), the dorsal motor nucleus of the vagus (X) and the caudal subnucleus of the spinal trigeminal nucleus. No significant specific [125I]BK binding was seen in other brain regions. The specific binding (85-90% of total binding) was of high affinity and saturable with a KD of 73.5 +/- 9.9 pM and a Bmax of 27.8 +/- 1.9 amol per mm2 of tissue. In competition studies, the rank order of potencies was: BK greater than Met-Lys-BK greater than Lys-BK much greater than Des-Arg9-BK. The B2 receptor antagonist D-Arg0-Hyp3-Thi5,8-D-Phe7-BK inhibited [125I]BK binding with a Ki value of 3.5 +/- 1.5 nM while Des-Arg9-[Leu8]-BK, a B1 receptor antagonist did not significantly inhibit [125I]BK binding in concentrations up to 10 microM. Our finding of specific high affinity [125I]BK binding sites in the nTS, AP and the X is important because these brain areas are known to be involved in central cardiovascular regulation. Moreover, our results suggest that the specific [125I]BK binding sites in the guinea pig medulla are of the bradykinin B2 receptor type.

The AT2 angiotensin receptor subtype predominates in the 18 day gestation fetal rat brain

The angiotensin II receptor subtype-specific antagonists Dup 753 (AT1) and PD 123177 (AT2) were used to characterize the angiotensin II receptor subtypes present in 18 day gestation fetal Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rat brain using in vitro receptor autoradiography. The AT2 subtype was predominant in the brain of both rat strains, even in areas that display predominantly the AT1 subtype in the adult rat brain.

Angiotensin II and the locus coeruleus

The locus coeruleus (LC) is a putative site of action for angiotensin II in the brain. Immunocytochemical studies have identified angiotensin II-like immunoreactive material in nerve terminals innervating the LC, and the LC contains one of the highest densities of angiotensin II receptor binding sites in the rat brain. Recent studies using selective neurotoxins suggest that the binding sites for angiotensin II in the LC are present on noradrenergic perikarya. Angiotensin II receptors are now known to exist as two subtypes that are distinguishable both pharmacologically and biochemically. Radioligand binding studies using agonists and antagonists selective for these angiotensin II receptor subtypes indicate that the rat LC contains a mixture of the two known angiotensin II receptor subtypes, but that the PD123177-sensitive AII beta receptor subtype is predominant. Comparisons of spontaneously hypertensive rats with normotensive rats indicates that angiotensin II and its receptors in the LC are elevated in the hypertensive rat strain. Studies of the biochemical and physiological actions of angiotensin II in the LC have not yet established an agreed-upon function for angiotensin II in this nucleus.