A new approach to angiotensin antagonists: Methylation of the tyrosine hydroxyl in angiotensin II

[Sar¹, Tyr(Me)⁴] angiotensin II, synthesized by the solid phase method and purified by ion-exchange chromatography and reversed-phase HPLC, was found to inhibit the contractile response to angiotensin II in the rat isolated uterus and inhibit the pressor response to angiotensin II in the vagotomized ganglion-blocked rat. In the rat isolated uterus Schild plots gave a pA₂ of 8.1, and a slope of 0.9 indicative of competitive inhibition. In the rat pressor assay, infusion of the analogue at a rate of 500ng/kg/min caused a parallel displacement of the dose-response curve to ANG II to the right. In contrast, the classical angiotensin inhibitor [Sar¹, Ile⁸] ANG II appeared to demonstrate non-competitive inhibition in both the rat isolated uterus and the pressor assays. The phenolic hydroxyl or phenoxide anion of Tyr⁴ in angiotensin II appears to be critical for the activation of angiotensin receptors in smooth muscle. Alkylation of the tyrosine residue in angiotensin analogues provides a new route for the synthesis of potent competitive antagonists of angiotensin.     

[DES-ASP1] angiotensin II in the sheep: Blood levels and its effect on plasma renin concentration

The present study describes an improved method for measuring angiotensin III in arterial blood. This was accomplished by SE-sephadex column to separate angiotensin II from angiotensin III prior to radioimmunoassay. The arterial concentration of angiotensin III measured before and after 24 to 48 hours sodium depletion by acute cannulation of parotid gland was 12.4 ± 1.7 fmol/ml (SEM, n=7) and 49.8 ± 10.3 fmol/ml (SEM, n=7) respectively. The arterial concentration of Val⁴-angiotensin III obtained from continuous infusion of Val⁴-angiotensin III at rates of 24 and 48 nmol/h in sodium deficient sheep were 245 ± 32.5 fmol/ml (n=6) and 330 ± 11.4 fmol/ ml (n=7) respectively. The clearance rate of exogenous Val⁴-angiotensin III in sodium deficient sheep after correction for endogenous level was calculated to be 140 ± 13.6 L/h (SEM, n=13). This was in the same order as Ile⁵-angiotensin II and Ile⁴-angiotensin III reported earlier in sodium replete sheep. Prolonged intravenous infusion of Val⁴-angiotensin III at a rate of 48 nmol/h in sodium- deficient sheep suppressed plasma renin concentration to the same extent as equimolar infusions of angiotensin II. This suggests that angiotensin III may inhibit renin secretion by a similar mechanism to angiotensin II.     

Effects of ANG II and III and angiotensin receptor blockers on nasal salt gland secretion and arterial blood pressure in conscious Pekin ducks (Anas platyrhynchos)

The vertebrate renin-angiotensin system controls cardiovascular, renal and osmoregulatory functions. Angiotensin II (ANG II) is the most potent hormone of the RAS but in some vertebrate animals angiotensin III (Val⁴-ANG III) may be a hormone. We studied the effects of some angiotensins and mammalian ANG II receptor antagonists on nasal salt gland function and arterial blood pressure in conscious white Pekin ducks. Nasal salt gland fluid secretion (NFS) was induced by a 10 ml · kg⁻¹ bw i.v. injection of a NaCl solution (1000 mosmol · kg⁻¹ H₂O) and maintained by a continuous i.v. infusion of the same solution at a rate of 0.97 ml · min⁻¹. There was a positive linear correlation between nasal fluid [Na⁺] and osmolality, between [Na⁺] and [K⁺], and also between the rate of NFS and [Na⁺] and [K⁺]. [Asp¹,Val⁵]-ANG II (1 nmol · kg⁻¹ i.v.) inhibited NFS but did not change ionic concentrations. Val⁴-ANG III (1 or 5 nmol · kg⁻¹) and ANG I (1-7) (20 nmol · kg⁻¹) had no effect on NFS. [Sar¹, Ile⁸]-ANG II (SARILE) acted as an ANG II receptor agonist and resulted in a prolonged and complete inhibition of NFS. The AT₁ receptor antagonist, losartan (DuP 753) and the AT₂ receptor antagonist, PD 123319 both failed to block the inhibitory effect of [Asp¹, Val⁵]-ANG II on the nasal salt glands. [Asp¹,Val⁵]-ANG II (2 nmol · kg⁻¹ i.v.) increased mean arterial blood pressure (MABP), whereas the same dose of [Asn¹,Val⁵]-ANG II (teleost) had only 30% of the pressor potency of the avian ANG II. Neither 1 nor 5 nmol · kg⁻¹ of Val⁴-ANG III i.v. nor 20 nmol · kg⁻¹ of ANG I (1-7) had any measurable effect on MABP. SARILE blocked completely the pressor response to [Asp¹,Val⁵]-ANG II but the AT₁ antagonists losartan and CGP 48933 and the AT₂ antagonist PD 123319 all failed to block the pressor response to [Asp¹,Val⁵]-ANG II. These results have substantiated an important role of the nasal salt gland in potassium regulation and highlighted a pharmacological dimorphism of saralasin, namely agonist and antagonist to angiotensin II-mediated inhibition of nasal salt gland function and pressor response, respectively. Using specific nonpeptidergic angiotensin II receptor antagonists, we have confirmed the distinct pharmacology of the avian angiotensin II receptors in a nongallinaceous species and the absence of significant angiotensin I (1-7) and angiotensin II effects on the cardiovascular system and nasal salt gland. Accepted: 6 November 1997     

The steroidogenic response to angiotensin II in the Australian lungfish,Neoceratodus forsteri

Corticosterone, aldosterone and cortisol were found to be present in lungfish plasma. Plasma levels of these hormones were measured in lungfish following separate single intramuscular injections of three forms of angiotensin II; [Asp¹, Ile⁵], [Asp¹, Val⁵] and [Asn¹, Val⁵]. Aldosterone levels were significantly elevated in response to [Asp¹, Ile⁵] AII and [Asn¹, Val⁵] AII injection. [Asp¹, Val⁵] AII increased plasma corticosterone levels. The difference between these data and the negative results previously reported by Blair-West et al. (1977) are discussed.Abbreviations AII angiotensin II - bw body weight - DOC deoxycorticosterone - RAS renin-angiotensin system - RIA radioimmuno assay     

Biological activity of the novel cyclic angiotensin II analogue [Sar1,Lys3,Glu5]ANG II

Summary This study was undertaken to investigate the biological activity of the cyclic amide-linked analogue of angiotensin II (ANG II), [Sar¹,Lys³,Glu⁵]ANG II, in both ex vivo and in vivo experiments. This constrained analogue was designed on the basis of a recently suggested conformational model for ANG II-induced receptor activation, which is characterized by a Tyr-Ile-His backbone bend and the clustering of the three aromatic rings (Tyr, His, Phe). After [Sar¹,Lys³,Glu⁵]ANG II was found to have contractile activity (15% of ANG II in the rat uterus assay), it was administered in anesthetized rabbits where it produced an immediate and dose-dependent increase in blood pressure, which peaked within minutes, was sustained as long as the drug was given, and was gradually returned to baseline after discontinuation of the drip. The blood pressure response to the cyclic analogue was of less magnitude compared to that elicited by an isovolemic and equimolar solution of ANG II. These data confirm the importance of a properly oriented ring cluster, allowing the charge-relay conformation proposed for ANG II.     

NMR studies on angiotensin II: Histidine and phenylalanine ring stacking and biological activity

The conformations of angiotensin II and the antagonist [Sar¹, Ile⁸]angiotensin II in dimethylsulfoxide have been examined by high resolution proton magnetic resonance spectroscopy at 400MHz. The chemical shifts for the aromatic protons of the phenylalanine residue in angiotensin II are consistent with shielding and restricted rotation for this side-chain. The chemical shifts for the histidine C₂ and C₄ protons in angiotensin II also indicate shielding, whereas these same protons in the antagonist [Sar¹, Ile⁸]angiotensin II do not demonstrate this shielding influence. These findings suggest a stacking interaction for the histidine and phenylalanine side-chains in angiotensin II which is important for activating angiotensin receptors.     

Conformational analysis of cyclic angiotensin II analogues

Summary Conformationally restricted cyclic analogues of angiotensin II (ANG II), Asp¹-Arg²-Val³-Tyr⁴-Val⁵-His⁶-Pro⁷-Phe⁸, with a link between positions 3 and 5 have considerable biological activity. It is proposed that the spatial arrangement of the pharmacophore groups of Tyr⁴, His⁶ and Phe⁸ side chains and the C-terminal carboxyl group in ANG II and active analogues is similar. Conformational analysis of ANG II and two cyclic analogues c[Sar¹, Lys³,Glu⁵]ANG II and c[Sar¹,Hcy³,Mpt⁵]ANG II was performed, and a geometrical comparison of the low-energy conformations of these compounds allowed one to propose a model of receptor-bound conformation in terms of the spatial arrangement of the pharmacophore groups. This model is characterised by the close spatial location of the His⁶-Phe⁸ side chains and the Tyr⁴ C-terminal carboxyl group and is stabilised by the electrostatic interaction of Arg² and the C-terminal carboxyl group.Abbreviations ANG II angiotensin II - Hcy homocysteine - Mpt trans-4-mercaptoproline     

Comparison of the biological effects of two angiotensin II analogues in hypertensive patients with sodium depletion

The effects on blood pressure (BP), plasma aldosterone concentration (PAC) and plasma renin activity (PRA) of two angiotensin II analogues (AII A), i.e., 1-sarcosine, 8-isoleucine angiotensin II (Sar¹, I1e⁸-AII) and 1-sarcosine, 8-alanine angiotensin II (Sar¹, Ala⁸-AII), were investigated in patients with hypertension with various etiologies on sodium depletion. The changes of BP, PAC and PRA on infusion of Sar¹, Ile⁸-AII and Sar¹, Ala⁸-AII were very similar. With both compounds, there were significant inverse correlations between the pre-infusion PRA and the changes in BP and PAC, and a significant positive correlation between the pre-infusion PRA and change in PRA. The slopes of the regression lines for the correlations of changes in BP, PAC and PRA, plotted as functions of the pre-infusion PRA for Sar¹, Ile⁸-AII and Sar¹, Ala⁸-AII were not statistically different. In clinical investigations, the two compounds seemed equally useful for detecting renin-dependency in hypertension.     

Receptor interactions of the position 4 side chains of angiotensin II analogues: Importance of aromatic ring quadrupole

A triad of interacting group (TyrOH? His$ \underline\ominus$O₂C) in angiotensin II (ANG II) has been postulated to create the tyrosinate anion pharmacophore (tyanophore) responsible for receptor activation/triggering (Biochim. Biophys. Acta 1991, 1065, 21). In the present study we investigated the effects on bioactivity of substituting the Tyr⁴ residue in [Sar¹]ANG II with other anionic or electronegative amino acids, and with a number of aromatic amino acids lacking a hydroxyl group. [Sar¹ Nva(δ-OH)⁴]ANG II, [Sar¹ Nva(δ-OCH₃)⁴]ANG II, [Sar¹ Met⁴]ANG II, [Sar¹ Gln⁴]ANG II, [Sar¹ Glu⁴]ANG II and [Sar¹ DL -Alg⁴]ANG II had agonist activities in the rat isolated uterus assay of 4, 3, 19, 10, > 0.1 and > 0.1%, respectively, of that of ANG II. [Sar¹ Nal⁴]ANG II, [Sar¹ Pal⁴]ANG II, [Sar¹ DL -Phg(4′-F)⁴]ANG II, [Sar¹ Phe(4′-F)⁴]ANG II, [Sar¹ Phe(F₅)⁴]ANG II and [Sar¹ His⁴]ANG II had agonist activities of 4.5, 7, < 0.1, 0.2, 1 and 0.6%, respectively. All peptides investigated were devoid of measurable antagonist activity except [Sar¹] Phe(4′-F)⁴ ANG II (pA₂ = 7.7). These findings illustrate that anionic or electronegative aliphatic side chains replacing tyrosinate at position 4 can partially activate the angiotension receptor. For ANG II analogues containing an aromatic amino acid other than Tyr at position 4, ligand binding and agonist activity are not dependent on the electronegativity or dipole moment of the aromatic ring, or on the ability of the 4′ ring substituent to accept a proton. Modelling based on ab initio calculations of aromatic ring multipoles illustrate that the apparent binding affinity (PA₂) of ANG II analogues is associated with a perpendicular electrostatic interaction of the position 4 aromatic ring with a receptor-based group. In addition, intramolecular interactions providing for the conformation of the ligand as it approaches its receptor appear to have a role in determining agonist vs antagonist activity.     

Kallikrein generates angiotensin II but not bradykinin in the plasma of the urodele,Amphiuma tridactylum

Incubation of heat-denatured plasma from the urodele, Amphiuma tridactylum (three-toed amphiuma) or from the anurans Rana ridibunda (European green frog) and Rana catesbeiana (American bullfrog) with either glass beads, porcine pancreatic kallikrein or trypsin did not generate bradykinin-like immunoreactivity. However, peptides were generated in kallikrein-treated amphiuma plasma that contracted vascular rings from the bullfrog systemic arch and had a spasmogenic action on the bullfrog urinary bladder. These peptides, which were not generated in trypsin-treated plasma, were purified to homogeneity by reverse-phase HPLC and their primary structures established as: Asp-Arg-Val-Tyr-Val-His-Pro-Phe ([Asp¹,Val⁵]angiotensin II) and Asn-Arg-Val-Tyr-Val-His-Pro-Phe ([Asn¹,Val⁵]angiotensin II). Incubation of synthetic [Asn¹,Val⁵]angiotensin II with amphiuma plasma resulted in deamidation to [Asp¹,Val⁵]angiotensin II. The data suggest, therefore, that amphiuma plasma contains an l-asparagine amidohydrolase (asparaginase), as previously described for the eel. Although bradykinin-related peptides have been isolated from frog skin, this study provides evidence that the kallikrein-kinin system may be absent from the blood of amphibia.     

Conformation and activity in angiotensin II peptides

Angiotensin II and its competitive inhibitor [Sar¹, Ile⁸]-angiotensin II, as well as several analogs of these two compounds specifically chosen for their well-defined pharmacological properties, were studied by circular dichroism and nuclear magnetic resonance methods at various pH values in aqueous solution and in d₆-dimethylsulfoxide. The results were compared with their biological activities. This allowed us to establish relationships between conformation and pressor activity, explaining most of the properties of angiotensin II, its inhibitor, and the analogs successively substituted in positions 3 and 5.     

Effects of angiotensin II on the spontaneous activity of rostral ventrolateral medullary cardiovascular neurons and blood pressure in spontaneously hypertensive rats

The interactive role of rostral ventrolateral medulla (RVL) cardiovascular neurons and brain angiotensin II (Ang II) in regulating the arterial blood pressure was examined by recording simultaneously the spontaneous activity of these spinal projecting neurons and the arterial blood pressure in the pentobarbital-anesthetized spontaneously hypertensive rat (SHR) and its normotensive control, the Wistar Kyoto rat (WKY). It was found that Ang II elicited dose-dependent excitatory responses in a subpopulation of RVL cardiovascular neurons, followed by a subsequent increase in blood pressure. These effects of Ang II were significantly greater in SHR than in WKY. The effects were attenuated or abolished by co-administration of Ang II antagonist, [Sar¹, Ile⁸]-Ang II. Pre-administration of [Sar¹, Ile⁸]-Ang II to RVL using bilateral microinjection attenuated the blood pressure effects of intracerebroventricularly administered Ang II by as much as 70%. These results indicated that spinal projecting RVL cardiovascular neurons are important in mediating the pressor action of Ang II. The enhanced sensitivity and responsiveness of RVL cardiovascular neurons to Ang II may be pertinent to the genesis of hypertension in adult SHR.     

Angiotensin stimulates oxytocin release

A sensitive and specific radioimmunoassay for oxytocin (OT) was developed to study the effect of angiotensin II (ANG II) upon neurohypophyseal OT release in conscious rats. ANG II injected into the lateral cerebral ventricle (i.c.v.) produced within 60 seconds a steep increase in plasma OT concentration from a control value of 10.46 ± 1.35 fmol/ml to 88.95 ± 5.06 fmol/ml and 119.56 ± 11.46 fmol/ml following 10 and 100 ng i.c.v. ANG II, respectively. Inhibition of OT release by simultaneous application of the ANG II antagonist {Sar¹, Ile⁸}-ANG II suggests that ANG II acted via specific angiotensin receptors in the brain.     

The Arrestin-selective Angiotensin AT1 Receptor Agonist [Sar1,Ile4,Ile8]-AngII Negatively Regulates Bradykinin B2 Receptor Signaling via AT1-B2 Receptor Heterodimers

The renin-angiotensin and kallikrein-kinin systems are key regulators of vascular tone and inflammation. Angiotensin II, the principal effector of the renin-angiotensin system, promotes vasoconstriction by activating angiotensin AT₁ receptors. The opposing effects of the kallikrein-kinin system are mediated by bradykinin acting on B₁ and B₂ bradykinin receptors. The renin-angiotensin and kallikrein-kinin systems engage in cross-talk at multiple levels, including the formation of AT₁-B₂ receptor heterodimers. In primary vascular smooth muscle cells, we find that the arrestin pathway-selective AT₁ agonist, [Sar¹,Ile⁴,Ile⁸]-AngII, but not the neutral AT₁ antagonist, losartan, inhibits endogenous B₂ receptor signaling. In a transfected HEK293 cell model that recapitulates this effect, we find that the actions of [Sar¹,Ile⁴, Ile⁸]-AngII require the AT₁ receptor and result from arrestin-dependent co-internalization of AT₁-B₂ heterodimers. BRET₅₀ measurements indicate that AT₁ and B₂ receptors efficiently heterodimerize. In cells expressing both receptors, pretreatment with [Sar¹,Ile⁴,Ile⁸]-AngII blunts B₂ receptor activation of G_q/11-dependent intracellular calcium influx and G_i/o-dependent inhibition of adenylyl cyclase. In contrast, [Sar¹,Ile⁴,Ile⁸]-AngII has no effect on B₂ receptor ligand affinity or bradykinin-induced arrestin3 recruitment. Both radioligand binding assays and quantitative microscopy-based analysis demonstrate that [Sar¹,Ile⁴,Ile⁸]-AngII promotes internalization of AT₁-B₂ heterodimers. Thus, [Sar¹,Ile⁴,Ile⁸]-AngII exerts lateral allosteric modulation of B₂ receptor signaling by binding to the orthosteric ligand binding site of the AT₁ receptor and promoting co-sequestration of AT₁-B₂ heterodimers. Given the opposing roles of the renin-angiotensin and kallikrein-kinin systems in vivo, the distinct properties of arrestin pathway-selective and neutral AT₁ receptor ligands may translate into different pharmacologic actions.     

Role of angiotensin II in oral contraceptive hypertension in anesthetized rats

Rats fed a diet containing an oral contraceptive estrogen-progestogen mixture (Enovid) for 26 weeks developed a significant (P<0.01) elevation in mean arterial pressure (145 ± 6 mm Hg) when compared to a control group of rats fed the same diet without oral contraceptive steroids (mean arterial pressure averaged 117 ± 6 mm Hg). Infusion of [Sar¹, Ala⁸] angiotensin II, a competitive antagonist of angiotensin II, reduced the mean arterial pressure (P<0.01) in the oral contraceptive-treated rats to 117 ± 9 mm Hg, while infusion of this angiotensin II analogue into the control rats did not lower the arterial pressure. These studies provided evidence that angiotensin II may be involved in the hypertension produced by Enovid treatment in rats.     

Identification and characterization of angiotensin II receptors in cardiac sarcolemma

We have used [¹²⁵I] angiotensin II to investigate the presence of specific angiotensin II receptors in beef heart sarcolemmal membranes. The observed binding is saturable, reversible and specific. The apparent equilibrium dissociation constant is 2.23 ± 0.15 ($\text{x}$ ± SEM) and the maximal number of binding sites per mg membrane protein is 32.8 ± 5.4 fmol ($\text{x}$ ± SEM). The specific binding is 80–100% of the total [¹²⁵I] angiotensin II bound and is directly proportional to membrane protein concentration over the range of 33–173 μg protein per ml. Angiotensin II and its antagonists competed for binding in a potency order of (agent, K_i): angiotensin II, 0.9nM > Sar¹ Ala³, 7 nM > Sar¹-Ile³, 51 nM > Sar¹-Leu³, 427nM > angiotensin I, 1709 nM. The ability to characterize and quantify these receptors should now provide a method for investigating the mechanisms underlying the effects of angiotensin II on myocardial tissues.     

Angiotensin II antagonists versus drinking induced by bombesin or eledoisin in pigeons

The angiotensin II analogues Sar¹,Ile⁸-,Sar¹,Leu⁸- and, to a lesser extent, Sar¹,Ala⁸- and Sar¹-Gly⁸-angiotensin II proved to inhibit angiotensin-induced drinking in the pigeon, but did not significantly affect drinking induced by eledoisin or bombesin. The results suggest that the dipsogenic response elicited by bombesin or eledoisin, although almost identical to that evoked by angiotensin II, is mediated by the activation of central receptors different from those of angiotensin-induced drinking. These findings are consistent with the hypothesis that several peptidergic mechanisms are involved in the regulation of water intake.     

Biochemical pharmacology of total retro-inverso analogues of bradykinin and angiotensin II: Molecular recognition by G-protein-coupled receptors and angiotensin converting enzyme

Summary Total retro-inverso (TRI) analogues of bradykinin (BK), the B_2a -selective kinin antagonistd-Arg⁰[Hyp³,d-Phe⁷,Leu⁸]BK, angiotensin II (AT II) and the AT II antagonist Saralasin ([Sar¹, Val⁵, Ala⁸]AT II) were prepared by conventional solid-phase synthesis. Molecular recognition of TRI peptidomimetics by G-protein-coupled receptors was studied by competitive radioligand displacement experiments. TRI analogues ofd-Arg⁰[Hyp³,d-Phe⁷,Leu⁸]BK specifically bound to the kidney medulla B_2a bradykinin receptor with affinities (K _d ) ranging from 64 μM to 4 μM. Conversely, TRI analogues of BK, AT II and Saralasin did not bind to either the B_2a bradykinin receptor or the rat AT_1a AT II receptor, respectively. These studies indicate that the TRI strategy is more compatible with the synthesis of antagonists than ‘agonists’. Three TRI peptidomimetics ofd-Arg⁰[Hyp³,d-Phe⁷,Leu⁸]BK were weak inhibitors of angiotensin converting enzyme. All other TRI peptidomimetics had no effect upon ACE activity. These data endorse the utility of the TRI strategy for the synthesis of protease-resistant antagonists of peptide hormones and neuropeptides.     

Biochemical pharmacology of total retro-inverso analogues of bradykinin and angiotensin II: Molecular recognition by G-protein-coupled receptors and angiotensin converting enzyme

Total retro-inverso (TRI) analogues of bradykinin (BK), the B₂a-selective kinin antagonist d-Arg⁰[Hyp₃,d-Phe⁷,Leu⁸]BK, angiotensin II (AT II) and the AT II antagonist Saralasin ([Sar¹,Val⁵,Ala⁸]AT II) were prepared by conventional solid-phase synthesis. Molecular recognition of TRI peptidomimetics by G-protein-coupled receptors was studied by competitive radioligand displacement experiments. TRI analogues of d-Arg⁰[Hyp³,d-Phe⁷,Leu⁸]BK specifically bound to the kidney medulla B₂a bradykinin receptor with affinities (K_d) ranging from 64 M to 4 M. Conversely, TRI analogues of BK, AT II and Saralasin did not bind to either the B_2a bradykinin receptor or the rat AT_1a AT II receptor, respectively. These studies indicate that the TRI strategy is more compatible with the synthesis of antagonists than agonists. Three TRI peptidomimetics of d-Arg⁰[Hyp³,d-Phe⁷,Leu⁸]BK were weak inhibitors of angiotensin converting enzyme. All other TRI peptidomimetics had no effect upon ACE activity. These data endorse the utility of the TRI strategy for the synthesis of protease-resistant antagonists of peptide hormones and neuropeptides.     

Effects of angiotensin II and its blockers Sar1-lle8-angiotensin II and DuP 753 on drinking in ducks in relation to properties of subfornical organ neurons

Properties of systemically applied angiotensin II in stimulating water intake of normally hydrated ducks were studied and the results compared with properties of angiotensin II-responsive neurons of the subfornical organ which are considered as targets for circulating angiotensin, II acting as a dipsogen. Following intravenous infusion of hypertonic saline (2000 mosmol·kg^-1 at 0.3 ml·min^-1 for 1 h), intravenous infusion of 0.3 ml·min^-1 isotonic saline with angiotensin II (200 ng·min^-1), starting 1 h later, stimulated drinking in each case at an angiotensin II plasma level of about 1400 pg·ml^-1. Without hypertonic priming, the same angiotensin II infusion did not stimulate drinking in each experiment; however, if effective, repeated infusions of ANGII induced stable dipsogenic responses. Angiotensin II infusions did not alter plasma levels of antidiuretic hormone. Sar¹-Ile⁸-angiotensin II, a non-selective angiotensin II antagonist, acted weakly as a partial agonist when injused at a dose 200-fold higher than angiotensin II and effectively blocked the dipsogenic action of angiotensin II; this corresponds to the inhibition of angiotensin II-induced excitation by Sar¹-Ile⁸-angiotensin II observed in duck subfornical organ neurons. DuP 753 (losartan), an angiotensin II antagonist specifically blocking AT₁ receptors in mammals, had equivocal effects on angiotensin II-induced drinking in ducks at rates 50- and 200-fold higher than angiotensin II, which corresponds to the weak inhibitory action of this compound on angiotensin II-induced neuronal excitation in the duck SFO. Blood pressure was only marginally elevated by the applied angiotensin II dose and Sar¹-Ile⁸-angiotensin II had no effect.Abbreviations ANGII angiotensin II - AVT arginine vasotocin - DuP 753 losartan - EDTA ethylene diamine tetra-acetic acid - HR heart rate - ICV intracerebroventricular - IV intravenous - MAP mean arterial pressure - SARILE Sar¹-Ile⁸-angiotensin II - SFO subfornical organ