Importance of Tyr409 and Tyr414 in constructing the substrate pocket of human aminopeptidase B.

Aminopeptidase B (APB, EC 3.4.11.6) preferentially hydrolyzes basic amino acids of synthetic substrates and requires a physiological concentration of chloride anions for optimal activity. Several amino acid residues of APB responsible for its enzymatic activity have been elucidated. In this study, we further searched for residues critical to its enzymatic activity, especially toward peptide substrates. APB residues Tyr409 (Y409) and Tyr414 (Y414), both of which were critical to its hydrolytic activity toward synthetic substrates, were predicted by molecular modeling to be involved in cleaving peptide substrates via its interaction with amino acids in the P1' cleavage site. Using site-directed mutagenesis, several mutant APBs were prepared. In contrast to synthetic substrates, wild-type and Y409F/Y414F double mutant enzymes showed P1'-dependent cleavage of peptide substrates, indicating that both tyrosine residues were not indispensable for hydrolytic activity toward peptide substrates. Moreover, the Y409F/Y414F double mutant enzyme cleaved peptides with a Pro residue at the P1' site, which is uncommon among the M1 family of aminopeptidases. These results suggested that Tyr409 and Tyr414 of APB play important roles in enzymatic function and characteristic properties of APB via proper formation of the S1' site.

Renal functional effects of the highly selective AT2R agonist, ß-Pro7 Ang III, in normotensive rats.

Recently, we designed a group of peptides by sequential substitution of the naturally occurring α-amino acid throughout the Ang III peptide sequence with the corresponding ß-amino acid. ß-Amino acid substitution at the proline residue of Ang III (ß-Pro7-Ang III) resulted in a highly selective AT2R ligand, demonstrating remarkable selectivity for the AT2R in both binding and functional studies. To provide additional functional evidence for the suitability of ß-Pro7 Ang III as a novel AT2R agonist, we tested effects of acute systemic administration of ß-Pro7-Ang III on renal hemodynamic and excretory function in anesthetized normotensive male and female rats. We also compared the natriuretic effects of acute intrarenal administration of native Ang III and ß-Pro7-Ang III in the presence of systemic AT1R blockade in anesthetized female rats to allow for the differentiation of systemic versus direct intrarenal natriuretic actions of ß-Pro7-Ang III. In both male and female rats, acute systemic administration of ß-Pro7-Ang III elicited renal vasodilatation and natriuresis. Notably, greater renal vasodilatory effects were observed in female versus male rats at the highest dose of ß-Pro7-Ang III administered. Moreover, intra-renal administration of ß-Pro7-Ang III produced significant natriuretic effects in female rats and, like Ang III, evoked AT2R translocation to the apical plasma membrane in renal proximal tubular cells. Taken together, our findings support the use of ß-Pro7-Ang III as a novel AT2R agonist and experimental tool for exploring AT2R function and its potential as a therapeutic target. Furthermore, our findings provide further evidence of a sex-specific influence of AT2R stimulation on renal function.

Update on angiotensin AT2 receptors.

PURPOSE OF REVIEW: This review updates major new findings and concepts introduced during the past year on the role of angiotensin II (Ang II) subtype 2 receptors (AT2Rs) in the control of blood pressure and renal function. RECENT FINDINGS: AT2R activation prevents sodium (Na) retention and lowers blood pressure in the Ang II infusion model of experimental hypertension and prevents salt-sensitive hypertension in the obese Zucker rat model of obesity and the metabolic syndrome. Ang II metabolite, des-aspartyl-Ang II (Ang III) is the predominant AT2R agonist in the kidney and possibly also in the vasculature; a novel synthetic Ang III peptide, ß-Pro-Ang III, is vasodepressor and lowers blood pressure in conscious spontaneously hypertensive rats in the presence of low-level Ang II type 1 receptor (AT1R) blockade. Because nitric oxide is a product of AT2R activation, a potential feed-forward loop, wherein nitric oxide increases AT2R transcription, may reinforce the beneficial actions of AT2R in the long term. AT2R activation also reduces proteinuria and oxidative stress in glomerulosclerotic kidneys of high-salt obese Zucker rats. SUMMARY: Studies during the past year have helped to clarify the physiological and pathophysiological roles of AT2Rs and have enhanced the promise of AT2R agonists in cardiovascular and renal disease.

ß-Pro7Ang III is a novel highly selective angiotensin II type 2 receptor (AT2R) agonist, which acts as a vasodepressor agent via the AT2R in conscious spontaneously hypertensive rats.

We have previously shown that individual ß-amino acid substitution in angiotensin (Ang) II reduced Ang II type 1 receptor (AT1R) but not Ang II type 2 receptor (AT2R)-binding and that the heptapeptide Ang III exhibited greater AT2R:AT1R selectivity than Ang II. Therefore, we hypothesized that ß-amino-acid-substituted Ang III peptide analogues would yield highly selective AT2R ligands, which we have tested in binding and functional vascular assays. In competition binding experiments using either AT1R- or AT2R-transfected human embryonic kidney (HEK)-293 cells, novel ß-substituted Ang III analogues lacked appreciable AT1R affinity, whereas most compounds could fully displace (125)I-Sar(1)Ile(8) Ang II from AT2R. The rank order of affinity at AT2R was CGP42112 > Ang III > ß-Pro(7) Ang III=Ang II > ß-Tyr(4) Ang III ≥ PD123319 >> ß-Phe(8) Ang III >> ß Arg(2) Ang III=ß-Val(3) Ang III >> ß-Ile(5) Ang III. The novel analogue ß-Pro(7) Ang III was the most selective AT2R ligand tested, which was >20,000-fold more selective for AT2R than AT1R. IC50 values at AT2R from binding studies correlated with maximum vasorelaxation in mouse aortic rings. Given that ß-Pro(7) Ang III was an AT2R agonist, we compared ß-Pro(7) Ang III and native Ang III for their ability to reduce blood pressure in separate groups of conscious spontaneously hypertensive rats. Whereas Ang III alone increased mean arterial pressure (MAP), ß-Pro(7) Ang III had no effect. During low-level AT1R blockade, both Ang III and ß-Pro(7) Ang III, but not Ang II, lowered MAP (by ∼30 mmHg) at equimolar infusions (150 pmol/kg/min for 4 h) and these depressor effects were abolished by the co-administration of the AT2R antagonist PD123319. Thus, ß-Pro(7) Ang III has remarkable AT2R selectivity determined in binding and functional studies and will be a valuable research tool for insight into AT2R function and for future drug development.

The effect of the secondary structure on dissociation of peptide radical cations: fragmentation of angiotensin III and its analogues.

Fragmentation of protonated RVYIHPF and RVYIHPF-OMe and the corresponding radical cations was studied using time- and collision energy-resolved surface-induced dissociation (SID) in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially equipped to perform SID experiments. Peptide radical cations were produced by gas-phase fragmentation of Co (III)(salen)-peptide complexes. Both the energetics and the mechanisms of dissociation of even-electron and odd-electron angiotensin III ions are quite different. Protonated molecules are much more stable toward fragmentation than the corresponding radical cations. RRKM modeling of the experimental data suggests that this stability is largely attributed to differences in threshold energies for dissociation, while activation entropies are very similar. Detailed analysis of the experimental data obtained for radical cations demonstrated the presence of two distinct structures separated by a high free-energy barrier. The two families of structures were ascribed to the canonical and zwitterionic forms of the radical cations produced in our experiments.

Roles of brain angiotensins II and III in thirst and sodium appetite.

The current study examined the effects of intracerebroventricular (icv) infused aminopeptidase-resistant analogs of angiotensin II (AngII) and angiotensin III (AngIII) on thirst and sodium appetite. The analogs, [D-Asp1D-Arg2]AngII and [D-Arg1]AngIII, were further protected from degradation by pretreatment with the aminopeptidase A inhibitor, EC33, or the aminopeptidase N inhibitor, PC18. Prior to icv infusions, rats were sodium depleted with furosemide, followed by the angiotensin-converting enzyme inhibitor captopril, to block endogenous angiotensin formation. Both angiotensin analogs, at either of the two doses, were capable of eliciting fluid intakes of water and 0.3 M NaCl. Water and saline intakes were increased to a similar extent by 125 and 1250 pmol of [D-Asp1D-Arg2]AngII. [D-Arg1]AngIII produced a dose-dependent increase in water intake, whereas saline intake was equivalently increased by the 125 and 1250 pmol infusions. Pretreatment with EC33 or PC18 decreased water and saline intakes in response to [D-Asp1D-Arg2]AngII, while pretreatment with PC18 altered the time course of the [D-Arg1]AngIII-induced water and saline intakes. The ability of both inhibitors to decrease, but not completely block, AngII analog-induced intakes, coupled with the altered time course of the responses induced by the AngIII analog in the presence of PC18, supports the hypothesis that both AngII and AngIII are active ligands in brain angiotensin-mediated thirst and sodium appetite. However, these results do not resolve the primary question of whether conversion of AngII to AngIII is a prerequisite to dipsogenic and salt appetite responses in the brain.

Integrated protein microchip assay with dual fluorescent- and MALDI read-out.

A pore chip protein array (PCPA) concept based on a dual readout configuration, fluorescence imaging, and MALDI-TOF MS has been developed. Highly packed, (>4000 spots/cm2), antibody arrays were dispensed on the porous chip by using a piezo-electric microdispenser. Sandwich assay was made after blocking by addition of a secondary antibody either IgG-FITC-labeled or anti-Ang II. The antigen in the first system was a large protein (IgG), and in the other system, a FITC marked peptide Angiotensin II (Ang II) was used. Ang II antibodies showed specificity for Ang II, while the Ang I antibodies showed binding properties for Ang I, II, and Renin. Fluorescence and MALDI TOF MS read-out was made for IgG and Ang II. A major advantage of the dual read-out PCPA approach is that both affinity binding and mass identity are derived. Detection limits for Ang II on the chip is as low as 500 zmol (Ang II).

Tonic suppression of spontaneous baroreceptor reflex by endogenous angiotensins via AT(2) subtype receptors at nucleus reticularis ventrolateralis in the rat.

We evaluated the role of endogenous angiotensins at the rostral nucleus reticularis ventrolateralis (NRVL) in the modulation of spontaneous baroreceptor reflex (BRR) response and the subtype of angiotensin receptors involved using rats anesthetized and maintained with pentobarbital sodium. Bilateral microinjection of angiotensin II (ANG II) or its active metabolite angiotensin III (ANG III) (5, 10, or 20 pmol) into the NRVL significantly suppressed the spontaneous BRR response, as represented by the magnitude of transfer function between systemic arterial pressure and heart rate signals. The inhibitory effect of ANG III (20 pmol) was discernibly reversed by coadministration with its peptide antagonist, [Ile(7)]ANG III (1.6 nmol), or the nonpeptide AT(2) receptor antagonist, PD-123319 (1.6 nmol), but not by the nonpeptide AT(1) receptor antagonist, losartan (1.6 nmol). On the other hand, the peptide antagonist, [Sar(1), Ile(8)]ANG II (1.6 nmol) or both non-peptide antagonists appreciably reversed the suppressive action of ANG II (20 pmol). Whereas losartan produced minimal effect, blocking the endogenous activity of the angiotensins by microinjection into the bilateral NRVL of PD-123319, [Sar(1), Ile(8)]ANG II or [Ile(7)]ANG III elicited significant enhancement of the spontaneous BRR response. We conclude that under physiologic conditions both endogenous ANG II and ANG III may exert a tonic inhibitory modulation on the spontaneous BRR response by acting selectively on the AT(2) subtype receptors at the NRVL.

Angiotensin III modulates noradrenaline uptake and release in the rat hypothalamus.

1. Effects of angiotensin III (A III) on 3H-noradrenaline (3H-NA) total, neuronal and non-neuronal uptake, 3H intracellular distribution and release were studied in vitro in the rat hypothalamus. 2. A III (1 microM) decreased total, neuronal and non-neuronal 3H-NA uptake when hypothalamic slices were incubated with 3H-NA for 30 min. A III effects on neuronal and non-neuronal 3H-NA uptake were determined in the presence of 100 microM hydrocortisone or 10 microM cocaine hydrochloride, respectively. The effect of A III on total 3H-NA uptake was blocked by 10 microM Ile7 angiotensin III (Ile7 A III), an antagonist at A III receptors. In contrast, 100 nM A III had no effect on 3H-NA uptake. 3. The study of the 3H-NA uptake time course showed that 1 microM AIII decreased NA uptake at 1, 3, 7, 15 and 30 min incubation. 4. In hypothalamic slices preloaded with 3H-NA for 30 min, 1 microM AIII increased the 3H content in the granular pool and decreased it in the cytosolic pool. 5. Spontaneous 3H release was also modified by 1 microM A III when hypothalami were preloaded with 3H-NA for 30 min. A III increased the spontaneous output of 3H. This effect was receptor-mediated since the effect of A III on 3H release was antagonized by Ile7 A III. 6. The present results suggest that the effects of A III on NA neurotransmission, may be involved in the regulation of central angiotensin effects such as blood pressure control, hydrosaline balance and dipsogenesis, through modulation of central sympathetic activity.

Angiotensin-I- and -III-mediated cardiovascular responses in the freshwater North American eel, Anguilla rostrata: effect of Phe8 deletion.

Cardiovascular responses to synthetic eel [Asn1, Val5, Gly9]-ANG-I, ANG-III (2-8), and ANG-I (1-7) were measured in conscious and resting freshwater North American eels. Indwelling Doppler flow probes were placed on the ventral and dorsal aortas, a pressure catheter in the lienomesenteric artery, and a peptide delivery catheter in the caudal vein. Twenty-five and 150 ng.kg-1 ANG-III increased baseline cardiac output (CO) (15.23 +/- 0.31 ml.min-1.kg-1; n = 5) by 23 and 47%, respectively, by increasing stroke volume (SV) but not heart rate (HR). ANG-I (150 ng.kg-1) also elevated CO (62%) by increasing both SV (44%) and HR (14%). Estimated branchial shunting (BS) was increased by 150 ng.kg-1 ANG-I and -III suggesting that more blood perfused the arteriovenous pathway in the gills. Dorsal aortic blood pressure (PDA) (3.08 +/- 0.12 kPa) was elevated by 150 ng.kg-1 ANG-I (67%) and -III (52%). Pressor responses temporally preceded the blood flow increases and there was a significant increase in systemic vascular resistance (RSYS) at the peak pressor responses. At the peak flow responses, increased CO was solely responsible for the increase in PDA; RSYS had returned to baseline values. Pressor responses to ANG-III decayed more rapidly (18.6 min) compared with those of ANG-I and -II (36 min). ANG-I (1-7) had no measurable effect on cardiovascular function indicating that the carboxyl-terminal 8-phenylalanine is an absolute requirement for the hormonal activity of angiotensin in fishes.

Novel synthesis of cyclic amide-linked analogues of angiotensins II and III.

Cyclic amide-linked angiotension II (ANGII) analogues have been synthesized by novel strategies, in an attempt to test the ring clustering and the charge relay bioactive conformation recently suggested. These analogues were synthesized by connecting side chain amino and carboxyl groups at positions 1 and 8, 2 and 8, 3 and 8, and 3 and 5, N-terminal amino and C-terminal carboxyl groups at positions 1 and 8, 2 and 8, and 4 and 8, and side chain amino to C-terminal carboxyl group at positions 1 and 8. All these analogues were biologically inactive, except for cyclic [Sar1, Asp3, Lys5]ANGII (analogue 10) which had high contractile activity in the rat uterus assay (30% of ANGII) and [Lys1, Tyr(Me)4, Glu8]ANGII (analogue 7) which had weak antagonist activity (PA2 approximately 6). Precyclic linear peptides synthesized using 2-chlorotrityl chloride resin and N alpha-Fmoc-amino acids with suitable side chain protection were obtained in high yield and purity and were readily cyclized with benzotriazol-1-yloxytris(dimethylamino)-phosphonium hexafluorophosphate as coupling reagent. Molecular modeling suggests that the ring structure of the potent analogue can be accommodated in the charge relay conformation proposed for ANGII.

Differential neuronal responses to angiotensin III from the subfornical organ of normotensive and spontaneously hypertensive rats.

We previously reported that chronic central administration of angiotensin III (AIII) fails to produce sustained drinking behavior in spontaneously hypertensive rats (SHR), possibly because of the development of early desensitization of the angiotensin receptors. The present study extended these findings to the cellular level, using brain-slice preparation from Wistar-Kyoto rats (WKY) and SHR, in conjunction with single-neuron recording in the subfornical organ (SFO), a target site for angiotensin II-induced drinking. We found that a majority of the SFO neurons studied (13/18 in WKY, 20/28 in SHR) responded in a dose-related manner to AIII, given in the range of 10(-6)-10(-5) M. This excitation was receptor-specific, since it was reversed by Ile7-AIII (10(-4)-10(-3) M), the selective AIII antagonist. Bestatin (10(-5)-10(-4) M), an aminopeptidase B inhibitor, did not discernibly affect basal spike frequency when delivered alone. Nevertheless, given in combination with the heptapeptide, bestatin reduced the intensity and duration of SFO neuronal response in WKY to the higher dose (10(-5) M), and in SHR to both doses (10(-6) or 10(-5) M), of AIII. These data suggest that the SFO may also be a central site of action for AIII. Moreover, prolonging the action of AIII by protecting it from being metabolized with bestatin may produce desensitization of the angiotensin receptors on SFO neurons. This was particularly so in the SHR, which are thought to be defective in the degradation of the heptapeptide in the brain.

ANG II receptor expression and function during phenotypic modulation of rat aortic smooth muscle cells.

Angiotensin II (ANG II) receptors were investigated in primary cultured rat aortic smooth muscle cells (SMC) that expressed either a proliferative phenotype (during the growth phase) or a contractile phenotype (at postconfluence). For each phenotype, alpha-smooth muscle actin expression, 125I-labeled ANG II specific binding, D-myo-inositol 1,4,5-triphosphate [Ins(1,4,5)P3] production, and ANG II-mediated increases in intracellular calcium (Cai2+) were studied. In both phenotypes, 1) ANG II-specific high-affinity binding (KD 0.5 +/- 0.1 nM and Bmax 196 +/- 106 pmol/mg protein in proliferative state, KD 1.5 +/- 0.3 nM and Bmax 560 +/- 299 pmol/mg protein in postconfluent state) was entirely inhibited by the selective AT1-antagonist losartan as well as by [Sar1,Ala8]ANG II and ANG III; 2) the AT2-antagonist CGP 42112A was ineffective, except at very high concentrations (> or = 10 microM); 3) the specific binding of ANG II was inhibited by guanosine 5'-[gamma-thio]triphosphate; and 4) ANG II induced a losartan-sensitive increase in Ins(1,4,5)P3. In postconfluent cultures, ANG II elicited a rapid biphasic elevation in Cai2+, which was abolished by losartan, whereas in growing cultures, this response was either absent or greatly attenuated. It is concluded that AT1-receptors coupled to phospholipase C via a G protein are expressed in the proliferative as well as in the contractile SMC phenotype and that their coupling to Cai2+ release is impaired in the proliferative phenotype. No evidence for AT2-receptor expression during phenotypic modulation of SMC was found.

Participation of nucleus reticularis gigantocellularis in the antinociceptive effect of angiotensin III in the rat.

We evaluated the participation of nucleus reticularis gigantocellularis (NRGC), a medullary nucleus that plays an important role in the regulation of nociceptive processes, in the antinociceptive effect of angiotensin III (AIII), a biologically active peptide of the renin-angiotensin system. Adult, male Sprague-Dawley rats anesthetized with pentobarbital sodium (40 mg/kg, i.p., with 10 mg/kg/h i.v. infusion supplement) were used. Bilateral, site-specific microinjection of AIII (80 or 160 pmol) into the NRGC produced a dose-related increase in the latency of tail-flick response to noxious thermal stimuli (50 degrees C hot water). Such an antinociceptive action of AIII was blocked by concomitant administration of the AIII receptor antagonist, Ile7-angiotensin III (Ile7-AIII, 10 nmol). At the neuronal level, microiontophoretic application of AIII suppressed, Ile7-AIII reversibly, the responsiveness of nociception-related neurons in the NRGC to tail-clamping. These results demonstrated that central AIII may elicit antinociception via a process that may at least take place at the NRGC.

Identification and characterization of a novel angiotensin binding site in cultured vascular smooth muscle cells that is specific for the hexapeptide (3-8) fragment of angiotensin II, angiotensin IV.

This study demonstrates the existence of a previously unrecognized class of angiotensin binding sites on vascular smooth muscle that exhibit high affinity and specificity for the hexapeptide (3-8) fragment of angiotensin II (AngIV). Binding of [125I]AngIV is saturable, reversible and describes a pharmacologic profile that is distinct and separate from the classic AT1 or AT2 angiotensin receptors. Saturation binding studies utilizing cultured vascular smooth muscle cells obtained from bovine aorta (BVSM) revealed that [125I]AngIV bound to a single high affinity site with an associated Hill coefficient of 0.99 +/- 0.003, exhibiting a KD = 1.85 +/- 0.45 nM and a corresponding Bmax = 960 +/- 100 fmol mg-1 protein. Competition binding curves in BVSM demonstrated the following rank order effectiveness: AngIV > AngII(3-7) >> AngIII > Sar1,Ile8 AngII > AngII > AngII(1-7) > AngII(4-8), DuP 753, PD123177. The presence of the non-hydrolyzable GTP analog GTP gamma S, had no effect on [125I]AngIV binding affinity in BVSM. The presence of this novel angiotensin binding site on smooth muscle in high concentration suggests the possibility that this system may play an important, yet unrecognized role in vascular control.

Angiotensin II--derived peptides devoid of phenylalanine in position 8 have full psychotropic activity of the parent hormone.

In this work we compared in rats the influence of heptapeptide 1-7-angiotensin II, hexapeptide 2-7-angiotensin II, pentapeptide 3-7-angiotensin II and angiotensin II on motility, stereotypy, learning of conditioned avoidance responses and recall of passive avoidance behaviour allowing to avoid aversive stimulation. The 4 peptides administered 15 min before the experiment, tended to increase the number of crossings, rearings and bar approaches in open field, significantly accelerated acquisition of conditioned avoidance responses and improved recall of the passive avoidance. All the peptides applied immediately before the experiment intensified stereotypy evoked by apomorphine in the dose 1 mg/kg and amphetamine in the dose 6.5 mg/kg given intraperitoneally. These results show full psychotropic activity of the examined fragments of angiotensin II, comparable with the activity of the parent octapeptide. Our previous hypothesis that the Val-Tyr-Ile-His-Pro fragment of angiotensin II is responsible for the psychotropic activity evoked by angiotensins in rats is thus confirmed.

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.

Two distinct angiotensin II receptor binding sites in rat adrenal revealed by new selective nonpeptide ligands.

The nonpeptide angiotensin II antagonists Dup-89 and WL-19 displaced specific 125I-angiotensin II binding in rat whole adrenal in a clearly biphasic manner, indicating the presence of high (nanomolar) and low (micromolar) affinity sites, each representing approximately 50% of the total maximal number of binding sites. Displacement studies using sufficient concentrations of either antagonist to prevent binding to its respective high affinity site revealed that the high affinity binding sites for Dup-89 and WL-19 were distinct and corresponded to the low affinity site of the other. Both binding sites were also present in the adrenal capsule (cortex) and adrenal decapsulated (medulla) tissue. The two 125I-angiotensin II binding sites were also differentiated by their sensitivity to dithiothreitol and the relative affinities of angiotensin II, angiotensin III, and their respective Sar1,Ile8- and Ile7-substituted antagonist analogs. Only Dup-89 (KB = 13 nM) was effective in antagonizing angiotensin II-stimulated aldosterone release from dispersed adrenal capsular cells, indicating that this functional response is mediated by an action upon the 125I-angiotensin II binding site at which Dup-89 has high affinity. Collectively, the data provide additional strong support for the presence of two distinct angiotensin II receptor subtypes in the rat adrenal.

Structure-function analyses of brain angiotensin control of pressor action in rats.

The present investigation examined the relative pressor potencies of intracerebroventricularly infused angiotensin (ANG) II, successively shortened COOH-terminal fragments through ANG II(5-8), and the analogues [Sar1]ANG II through [Sar1]ANG II(5-8). The results indicate that ANG II, ANG III, [Sar1]ANG II, and [Sar1]ANG III were identical with respect to pressor responses in the alert free-moving rat. In addition, ANG II(3-8) and [Sar1]ANG II(3-8) exhibited 68-70% of the activity of the above compounds, whereas the activity of the shorter COOH-terminal fragments dropped to approximately 13-35%. Pressor responses caused by each of the active forms of angiotensin could be substantially reduced by pretreatment with the specific angiotensin receptor antagonist [Sar1,Thr8]ANG II (Sarthran), suggesting either that these ligands are acting at multiple receptors for ANG II and its fragments, which are all blocked by Sarthran, or that the ligands are acting at a common receptor site. These results, coupled with other recent findings, suggest that the brain angiotensin receptor may be designed to preferentially interact with ANG II and/or ANG III or other angiotensin analogues that structurally resemble ANG III such as [Sar1]ANG II. It is concluded that ANG III's importance as a centrally active ligand has been underestimated and that ANG III may be an active form of angiotensin in the brain.

Metabolism of angiotensin peptides by neuronal and glial cultures from rat brain.

The degradation pattern and rate of [Ile5]-Angiotensin (Ang) I, II, and III were studied in neuron-enriched and glia-enriched cells in primary cultures from rat brain. Metabolites were separated by HPLC, and their identities were evaluated by comparison of their retention times with those of synthetic Ang peptide fragments and by analysis of their amino acid composition. Major metabolites were identified as des-Asp1-[Ile5]-Ang I, des-Asp1-[Ile5]-Ang II, [Ile5]-Ang II (3-8) hexapeptide, [Ile5]-Ang II (4-8) pentapeptide, and [Ile5]-Ang II (5-8) tetrapeptide. Glia-enriched cells degraded [Ile5]-Ang I and [Ile5]-Ang III significantly faster than neuron-enriched cells, whereas no difference between the two types of cells was found in the degradation rate of [Ile5]-Ang II. Although the half-lives of [Ile5]-Ang I and [Ile5]-Ang III in neuron-enriched cells from normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR) were not significantly different, neuron-enriched cultures from WKY rats metabolized [Ile5]-Ang II about 2.6 times faster than neuron-enriched cells derived from SHR.