Preparation and one-step purification of mono-125I-angiotensin II for radioligand binding assays

Localization and quantification of angiotensin II (A II) binding sites in the kidney of Xenopus laevis--lack of A II receptors in the adrenal tissue

The distribution and properties of angiotensin binding sites in the kidney of the clawed toad Xenopus laevis were studied using quantitative in vitro autoradiography. Specific binding sites for [125I]-[Val5]-angiotensin II (A II) were located in the glomeruli of the kidney but not in the adrenal tissue. [125I]-[Val5]-A II binding was equilibrated after 45 min. Scatchard and Hill analyses of saturation experiments showed that [125I]-[Val5]-A II binds to a single class of binding sites with a dissociation constant (Kd) of 1.884 +/- 0.535 nM and a maximum binding capacity (Bmax) of 0.484 +/- 0.144 fmol/mm2 (n = 8). Various angiotensin analogues displaced [125I]-[Val5]-A II in the rank order [Sar1, Ile5]-A II greater than human A II greater than [125I]-[Val5]-A II = [Val5]-A II = human A III much greater than human A I. Unrelated peptides did not alter the binding of [125I]-[Val5]-A II. Acclimation to 1.5% seawater increased [125I]-[Val5]-A II binding in glomeruli after 12 hr but returned to control levels after 7 days. Steroidogenic and catecholaminergic actions of [Val5]-A II on the adrenal tissue were examined in vitro and in vivo. Compared with known interrenal stimulators [human ACTH(1-39) and AVT] minimal effects were obtained only in vitro with high doses of [Val5]-A II while catecholamine release was unaffected. In vivo a single injection of 3 nmol [Val5]-A II per 100 g body wt did not change serum levels of corticosterone, aldosterone, epinephrine, norepinephrine, or dopamine.

Angiotensin II binding sites in frog kidney and adrenal

Angiotensin II (AII) binding sites were localized and quantified in kidney and adrenal of the frog Rana temporaria by quantitative in vitro autoradiography. AII binding was present in kidney glomeruli and in interrenal tissue of the outer zone of the adrenal gland. Saturation experiments showed that [125I]-[Val5]AII binds to a single class of binding sites with a dissociation constant (Kd) of 548 +/- 125 pM in glomeruli and 593 +/- 185 pM in interrenal tissue (n = 8). The corresponding maximal binding capacities (Bmax) were 2.48 +/- 0.71 and 3.05 +/- 1.02 fmol/mm2, respectively. AII binding was displaced by unlabeled angiotensin analogues in the rank order: [Sar1]AII greater than human AII greater than [125I]-[Val5]AII = [Val5]AII = human AIII much greater than human AI. The AII binding sites in glomeruli and interrenal tissue suggest an influence of AII on glomerular filtration rate and adrenal steroid secretion to take part in osmomineral regulation of the frog.

Localization of binding sites for atrial natriuretic factor and angiotensin II in the central nervous system of the clawed toad Xenopus laevis

The distribution of binding sites for atrial natriuretic factor (ANF) and angiotensin II (A II) was investigated in the central nervous system (CNS) of the clawed toad Xenopus laevis by means of in vitro autoradiography using [125I]-rat ANF(99-126) or [125I] [Val5] A II and [125I]human A II as labeled ligands. The highest densities of specific ANF-binding were detected in the nucleus habenularis, thalamic regions, hypophyseal pars nervosa and nucleus interpeduncularis. Moderate ANF-binding was found in the bulbus olfactorius, pallium, septum, striatum, lateral forebrain bundle, nucleus infundibularis, hypophyseal pars distalis and tectum. The highest levels of specific A II binding sites were observed in the nucleus praeopticus, nucleus habenularis, hypophyseal pars nervosa and pars distalis, whereas the amygdala contained moderate A II binding. The existence of specific binding sites for ANF and A II in the CNS of Xenopus laevis suggests that both peptides act as neurotransmitters or neuromodulators in the amphibian CNS. The co-localization of dense binding sites for both peptides in the nucleus habenularis, hypophyseal pars nervosa and pars distalis supports the view that ANF and A II have opposite regulatory functions in these regions.

Localization and quantification of nonapeptide binding sites in the kidney of Xenopus laevis: evidence for the existence of two different nonapeptide receptors

The distribution and properties of nonapeptide binding sites in the kidney of the anuran Xenopus laevis were investigated using quantitative in vitro autoradiography. The binding studies were performed with [3H]arginine vasopressin (AVP) as ligand because [125I]arginine vasotocin (AVT) lacks biological activity. Specific binding sites for [3H]AVP are located in the glomeruli of the kidney. [3H]AVP binding results in a steady state of association and dissociation between ligand and binding sites. Scatchard and Hill analyses of saturation experiments showed that [3H]AVP binds to a single class of binding sites with a dissociation constant (Kd) of 430 +/- 109 pM and a maximum binding capacity (Bmax) of 5.306 +/- 1.379 fmol/mm2 (n = 8). Displacement studies demonstrated the same affinity of these [3H]AVP binding sites to [3H]AVP, unlabeled AVP, and AVT, whereas mesotocin possesses only weak affinity. Further nonapeptides like oxytocin and isotocin or the mammalian-specific V1 receptor antagonist [1-beta-mercapto-beta,beta-cyclopentamethylene propionic acid)-2-(O-methyl)-tyrosine)-AVP or the V2 receptor agonist (1-deamino-8-D-arginine)-vasopressin or unrelated peptides did not alter the binding of [3H]AVP. The localization of nonapeptide binding sites in the glomeruli with the same affinity to AVP as to AVT agrees with the finding that AVT causes antidiuresis in Xenopus laevis. An earlier study demonstrated Xenopus laevis interrenal tissue to possess a higher sensitivity for AVT than AVP which points to a nonapeptide receptor with a higher affinity for AVT than AVP.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II receptors in the fowl aorta

In the domestic fowl, angiotensin II (ANG II) causes an in vivo depressor response and in vitro relaxation of aortic rings which appear to be a direct action of ANG II on the blood vessels. Thus, we determined whether binding sites specific to ANG II exist in the membrane fraction of the fowl aorta. The particulate fraction of aortas from adult female fowl, Gallus gallus, exhibits high specific binding to ANG II ligand. 125I-[Ile5]ANG II (0.5 nM) binding to the above fraction (30 micrograms protein) in 50 mM Tris (pH 7.2), 10 mM MgCl2, and 0.2% bovine serum albumin at 12 degrees (1) is rapid, saturable, and reversible; (2) increases as a function of ligand or membrane concentration, time, and temperature; and (3) optimally fits to a two-site (high and low affinity) model. The equilibrium dissociation constant (0.15 +/- 0.03 nM) and binding site concentration (28.7 +/- 8.1 fmol/mg protein) of the high affinity site as well as association (0.055 nM-1.min-1) and dissociation (0.0122 min-1) rate constants are similar to those of mammalian vascular ANG II receptors. Both 125I-[Ile5]ANG II and 125I-[Val5]ANG II are competitively displaced by unlabeled ANG II. These results suggest that specific ANG II receptors exist in the fowl aorta.

Subpressor infusions of angiotensin II alter glomerular binding, prostaglandin E2, and cyclic AMP production

Angiotensin II (ANG II) has been postulated to have pathogenetic role in diminished glomerular function in a number of animal models of acute renal failure. The present studies were designed to test the hypothesis that modest elevations in circulating ANG II potentiate the ability of ANG II to reduce glomerular capillary surface area through an effect on ANG II binding to glomerular mesangial cells and/or influences on other modulators of function. Rat glomeruli isolated by a sieving technique were employed in vitro in an ANG II radioreceptor assay. Subpressor infusion of ANG II for 36 hours in rats increases the affinity and number of ANG II binding sites of isolated glomeruli. The ability of ANG II to influence function was tested by assessing its effect on glomerular surface area in vitro by image-analysis microscopy, a method of measuring mesangial cell contractility. The sensitivity and magnitude of ANG II-induced decrements in glomerular surface area were increased. ANG II infusion diminished glomerular prostaglandin E2 (PGE2) production, increased basal cyclic adenosine 3',5'-monophosphate (cAMP) production, and enhanced ANG II-induced decrements in cAMP production. In control glomeruli, only pharmacological concentrations of ANG II inhibited cAMP, but after ANG II infusion, physiological concentrations of ANG II were capable of inhibiting cAMP by as much as 57% (below basal values). In conclusion, continuous infusion of subpressor concentrations of ANG II in rats enhances the contractile response of the glomerular mesangial cell through effects on the cell's surface receptor for ANG II and on prostaglandin and cAMP production. These actions may be important mediators of the effects of ANG II on glomerular function associated with a number of experimental models of kidney disease.

Estrogen effects on angiotensin receptors are modulated by pituitary in female rats

The present studies were designed to test the hypothesis that changes in angiotensin II (ANG II) receptors might modulate the altered target tissue responsiveness accompanying estradiol administration. Estradiol was infused continuously in oophorectomized female rats by employing minipumps to achieve plasma estradiol levels and simulating pregnancy levels in the rat. Aldosterone was also infused in control and experimental animals to avoid estrogen-induced changes in renin and ANG II. ANG II binding constants were determined in radioreceptor assays. Estradiol increased binding site concentration in adrenal glomerulosa by 76% and decreased binding sites of uterine myometrium and glomeruli by 45 and 24%, respectively. There was an accompanying increase in the affinity of ANG II binding to adrenal glomerulosa and uterine myometrium. Because estrogen is a potent stimulus of prolactin release from the pituitary of rodents, studies were also designed to test the hypothesis that prolactin may mediate some or all of the estrogen-induced effects observed. Hypophysectomy abolished estradiol stimulation of prolactin release and most ANG II receptor changes. The only effect that persisted was a 41% decrease in the density of uterine receptors. Prolactin administration to pituitary intact rats was associated with a 50% increase in receptor density of adrenal glomerulosa simulating estradiol administration. However, the changes in glomeruli and uterine myometrium were opposite in that both tissues also increased receptor density, suggesting that prolactin was not the sole mediator of the estrogen-induced receptor changes. In conclusion, regulation of ANG II receptors in a number of diverse target tissues by estradiol is complex with contributions from estrogens and pituitary factors, which include but do not exclusively involve prolactin.

Quantitative autoradiography of angiotensin II receptors in the SHR brain

Several lines of evidence indicate brain angiotensin II is associated with the elevation of blood pressure seen in the spontaneously hypertensive rat (SHR). These include an increased pressor response to intracerebroventricularly administered angiotensin II and a reduction of blood pressure in response to centrally administered angiotensin II receptor antagonists. Using quantitative receptor autoradiography, we have detected greater angiotensin II receptor binding in a number of discrete brain nuclei of the 6-week-old SHR when compared to age-matched Wistar-Kyoto controls. Tissue sections from various brain regions were labeled with [125I]-angiotensin II according to a previously described method. Autoradiograms were generated by apposing the labeled tissue sections to LKB Ultrofilm along with brain paste standards which contained known amounts of [125I]. Quantitation of the binding, utilizing computer-assisted microdensitometry, indicated greater [125I]-angiotensin II binding in several brain areas implicated in cardiovascular control including the subfornical organ, nucleus of the solitary tract, dorsal motor nucleus of the vagus, locus coeruleus, supraoptic nucleus and the organum vasculosum of the lamina terminalis. Scatchard analysis of the binding in the nucleus of the solitary tract indicated an increased receptor number (Bmax) was responsible for the change while binding in two forebrain structures, the subfornical organ and supraoptic nucleus, showed alterations in receptor number and affinity (Kd). Several other brain regions, unrelated to cardiovascular control, exhibited no change in [125I]-angiotensin II binding. Since the increased receptor binding was present primarily in brain regions related to cardiovascular control, we conclude that an increased angiotensin II receptor affinity and density is indicated as a factor in the etiology of the high blood pressure seen in the SHR.

Distribution of [125I]angiotensin II binding sites in the rat brain: a quantitative autoradiographic study

Angiotensin II receptors have been localized by quantitative autoradiography in the rat central nervous system after labeling with [125I]angiotensin II. A highly discrete distribution of these receptors was found throughout the rat brain. The highest density was seen in regions of the medulla, hypothalamus and circumventricular organs where angiotensin II could potentially produce cardiovascular, dipsogenic and neuroendocrine responses. The distribution of angiotensin II receptors correlates relatively well with the previously reported distribution of angiotensin immunoreactive nerve terminals as well as areas determined by various physiological techniques to be sensitive to angiotensin II. Finally, the anatomical localization of angiotensin II receptor populations has revealed several areas of the brain where the effects of this peptide have not been investigated. Many of these nuclei are involved in the transmission and processing of somatic and visceral sensory information. These results suggest a broader role for the central renin-angiotensin system in modulating several types of sensory input.

Effect of intraventricular infusion of an angiotensin II antagonist on 125I-angiotensin II binding in rats

The effects of chronic (six day) intracerebroventricular (i.c.v.) infusion of an angiotensin II antagonist, sarcosine1, isoleucine8 angiotensin II ([Sar1, Ile8]Ang II), (500 ng/microliter per hour) was studied. Specific 125I-Ang II binding site density and binding affinity in the hypothalamus-thalamus-septum-midbrain (H-T-S-M) region of the brain and the adrenal medulla did not differ significantly between [Sar1, Ile8]Ang II treated and control (0.9% saline) rats. However, 125I-Ang II binding to the adrenal cortex was significantly reduced by i.c.v. infusion of [Sar1, Ile8]Ang II. The drinking response to microinjection of Ang II was blunted for up to seven days of [Sar1, Ile8]Ang II infusion. Thus, although [Sar1, Ile8]Ang II effectively blocked the central Ang II receptors, chronic infusion of this Ang II antagonist did not appear to cause alterations in brain H-T-S-M Ang II receptors, suggesting that brain Ang II receptors in normal rats do not undergo homologous regulation.

Angiotensin II receptor binding sites in brain microvessels

We assessed the specific binding of 125I-labeled angiotensin II (125I-Ang II) to particulate fractions of the cerebral cortex and cerebellum and to microvessels obtained by bulk isolation from these two brain regions in the dog. 125I-Ang II binds to cerebral and cerebellar microvessels in a specific, saturable, and reversible manner and with high affinity (dissociation constant about 1 nM). Maximal binding of 125I-Ang II to brain microvessels was about 2-fold higher than the maximal binding to particulate fractions of the cerebellum and more than 15-fold higher than that of the cerebral cortex. No significant differences were noted between cerebral and cerebellar microvessels in their specific binding of Ang II. Furthermore, our finding that analogues of Ang II displace specific 125I-Ang II binding to brain microvessels in a rank order that correlates with their pharmacological activities confers biological relevance on the ligand-binding studies. These results strongly suggest that specific Ang II receptor binding sites are present in brain microvessels. Such Ang II receptors may have an important role in regulating the microcirculation of the brain.

Angiotensin II receptor localization in the canine CNS

Specific binding of [125I]angiotensin II [(125I]Ang II) to sections of dog brain was determined by in vitro receptor autoradiography. Highly discrete, dark images representing specific binding of [125I]Ang II were observed in areas corresponding to the nucleus of the solitary tract, dorsal motor nucleus of the vagus, area postrema, ventrolateral medulla, pineal, subfornical organ, nucleus medianus, septum, organum vasculosum of the lamina terminalis and the anterior pituitary. The specific binding was frequently present either as a narrow band or tiny spot within a small portion of the nuclei to which the binding corresponded. The location of these Ang II recognition sites in regions associated with regulation of autonomic and neuroendocrine function provides further evidence for a role of this peptide within the central nervous system.

In vitro autoradiographic localization of [125I]-angiotensin II binding sites in the rat and dog kidney

Light microscopic autoradiographic techniques have been utilized to demonstrate specific regions of the rat and dog kidney where angiotensin II receptors exist. Slide mounted tissue sections were labeled with [125I]-angiotensin II using conditions which provided for highly specific binding. These angiotensin II binding sites were localized to several distinct renal structures. In the renal cortex, angiotensin II binding sites were found concentrated in all parts of the glomeruli including the vascular components, the macula densa and the juxtaglomerular apparatus. Angiotensin II binding in the medulla was more diffusely associated with the vasa recta, and to a lesser extent, the thick ascending segment of the loop of Henle. Binding sites specific for angiotensin II were also found in the smooth muscle laminae of the ureter. Scatchard analysis of the binding kinetics allowed the demonstration of two subpopulations of binding sites which differ slightly in their affinities for [125I]-angiotensin II. These subpopulations appear to be associated with distinct components of the renal structure.