Coupling of the alpha 2-adrenergic receptor to the inhibitory G-protein Gi and adenylate cyclase in HT29 cells

Previous studies have established that the human colon carcinoma cell line HT29 expresses an alpha 2-adrenergic receptor of the alpha 2A subtype, which is negatively coupled to adenylate cyclase. The purpose of the present study was to examine the mechanisms of alpha 2-adrenergic signal transduction in these cells. [32P]ADP-ribosylation with pertussis toxin and immunoblots using antibodies specific for the Gi alpha-subunits indicated that two distinct Gi-proteins (Gi2 and Gi3) were present in HT29-cell membranes. Treatment of intact cells with pertussis toxin resulted in a time-dependent decrease in the amount of [32P]ADP-ribosylatable Gi2 and Gi3, which coincided with a diminution in the number of alpha 2-adrenergic receptors in high-affinity state for agonists and with a progressive loss of ability of UK14304 to inhibit forskolin-stimulated accumulation of cyclic AMP. When membranes were [32P]ADP-ribosylated with cholera toxin in the absence of exogenous added guanine nucleotides, radioactivity was incorporated into a 45 kDa polypeptide representing Gs, as well as into 40-41 kDa polypeptides corresponding to Gi3 and Gi2. The amount of radioactivity incorporated into the two GiS under basal conditions was decreased by addition of the alpha 2-antagonist RX821002. It was not significantly affected by addition of clonidine (partial alpha 2-agonist), but was doubled by the addition of UK14304 (full alpha 2-agonist). This effect was blocked by RX821002. Study of adenylate cyclase activity indicated that preincubation of HT29 membranes with the antibody AS/7 (anti-alpha i1/alpha i2), but not with the antibody EC/2 (anti-alpha i3), attenuated the inhibitory effect of UK14304 on forskolin-stimulated adenylate cyclase. These data demonstrate that the alpha 2A-adrenergic receptor is coupled to both Gi2 and Gi3, and identify Gi2 as the major mediator of inhibition of adenylate cyclase in HT29 cells.

Alpha 2A adrenoceptors and non-adrenergic idazoxan binding sites in calf brain and retina are distinct from those in human brain

alpha 2 Adrenoceptors in membrane preparations of human and calf frontal cortex and of calf retina can be labelled by the antagonists [3H]idazoxan, [3H]rauwolscine and [3H]RX 821002. Present and previous data indicate that [3H]idazoxan possesses the highest affinity for the alpha 2 adrenoceptors in the calf tissues, whereas [3H]rauwolscine displays the highest affinity for those in the human frontal cortex. Competition binding experiments with adrenergic and serotonergic drugs further support the notion that the alpha 2 adrenoceptors in calf frontal cortex and retina are similar, but distinct from the receptors in human frontal cortex. The alpha 2 adrenoceptors in the three investigated tissues display low affinity for the antagonist prazosin, which suggests that they all belong to the alpha 2A subclass. The competition binding curves of the alpha 2A adrenoceptor subtype-selective agonist oxymetazoline are shallow, but undergo a rightward shift and steepening in the presence of GTP. The shallow curves can therefore be attributed to the coupling of the alpha 2 adrenoceptors to G proteins. The different binding characteristics of the alpha 2A adrenoceptors from the investigated human and bovine tissues are likely to reflect species-related differences in protein structure. [3H]Idazoxan binds also to non-adrenergic sites in membrane preparations from the three tissues. However, the affinity of [3H]idazoxan for these sites in calf cortex and retina is appreciably lower than for those in human cortex. The species-related differences of the non-adrenergic idazoxan binding sites may be due to differences in protein structure or even to differences in gene-product.

In vitro metabolism of 1,3-dioxane, 1,3-oxathiolane, and 1,3-dithiane derivatives of theophylline: a structure-metabolism correlation study

Correlation between structure and metabolism was studied within a series of cyclic acetal and thioacetal theophylline derivatives. All the compounds showed marked regioselectivity in in vitro metabolism, the metabolites arising only from 7-cycloalkyl side chain transformation. The 1,3-dioxane derivative, besides N-dealkylation to theophylline, underwent enzymatic ring cleavage, through the oxidation of the acetal carbon and subsequent rearrangement. Thus the acetal group was converted enzymatically to an ester. A similar transformation, catalyzed by cytochrome P450-dependent monooxygenases, was previously found for the 1,3-dioxolane ring of doxophylline. The cyclic thioacetal derivatives (i.e. 1,3-oxathiolane and 1,3-dithiane) were not cleaved during oxidative metabolism. The metabolites arise only from the oxidation of the sulfur atom, the major nucleophilic center in the molecule. No N-dealkylation to theophylline was observed. Enzymatic sulfoxidation proceeded diastereoselectively in both the 1,3-oxathiolane and 1,3-dithiane rings, the trans isomers being the major ones with a ratio trans: cis 75:25 and 60:40 respectively. The sulfoxides were stable to hydrolysis and were not further metabolized. Neither disulfoxides nor sulfones were detected in the incubations.

Characterization and distribution of alpha 2-adrenergic receptors in the human intestinal mucosa

The subtype and the expression of the alpha 2-adrenergic receptor were investigated in the normal mucosa from human intestine by means of radioligand binding, RNase mapping, and measurement of adenylate cyclase activity. The study of the binding of the alpha 2-adrenergic antagonist, [3H]RX821002, to epithelial cell membranes indicated the existence of a single class of noninteracting sites displaying a high affinity for the radioligand (Kd = 1.1 +/- 0.5 nM). The rank order of potency of antagonists to inhibit [3H]RX821002 binding (RX821002 > yohimbine = rauwolscine > phentolamine approximately idazoxan >> chlorpromazine > prazosin) suggested that the receptor is of the alpha 2A subtype. A conclusion which is confirmed by the fact that only alpha 2C10 transcripts were found in the human intestine mucosa. Competition curves with (-)-norepinephrine demonstrated that 60% of the receptor population exhibited high affinity for agonists. This high-affinity state was abolished by the addition of GTP plus Na+ or by prior treatment of the membranes with pertussis toxin indicating it corresponded to G protein-coupled receptors. [32P]ADP-ribosylation and immunoblotting experiments identified two pertussis toxin-sensitive G proteins corresponding to Gi2 and Gi3. The study of the distribution of the receptor indicated that (a) the proximal colon is the intestine segment exhibiting the highest receptor density and (b) the receptor is predominantly expressed in crypts and is preferentially located in the basolateral membrane of the polarized cell. The distribution of the receptor along the crypt-surface axis of the colon mucosa can be correlated with a higher level of alpha 2C10-specific mRNA and a higher efficiency of UK14304 to inhibit adenylate cyclase in crypt cells.

Alpha 2-adrenoceptor antagonist potencies of two hydroxylated metabolites of yohimbine

1. The alpha 2-adrenoceptor antagonist capacities of two hydroxylated metabolites of yohimbine in man (10-OH-yohimbine and 11-OH-yohimbine) were investigated on the alpha 2-adrenoceptors of human platelets and adipocytes and compared to those of yohimbine. 2. Yohimbine and 11-OH-yohimbine exhibited similar alpha 2-adrenoceptor affinity in biological studies i.e. inhibition of adrenaline-induced platelet aggregation and inhibition of UK14304-induced antilipolysis in adipocytes. 3. Yohimbine and the two metabolites displaced [3H]-RX 821002 binding with equivalent affinities in platelet and adipocyte membranes with the following order of potency: yohimbine > 11-OH-yohimbine > 10-OH-yohimbine. However, when binding studies were carried out in binding buffer supplemented with 5% albumin, the apparent affinity of yohimbine was reduced about 10 fold and was similar to that of 11-OH-yohimbine. 4. Yohimbine and its metabolites were bound to different extents to plasma proteins, the bound fraction being 82%, 43% and 32% respectively for yohimbine, 11-OH-yohimbine and 10-OH-yohimbine. 5. These results show that the main hydroxylated metabolite of yohimbine in man (11-OH-yohimbine) possesses alpha 2-adrenoceptor antagonist properties. The discrepancies found in binding studies (i.e. 10 fold lower affinity of 11-OH-yohimbine than yohimbine for alpha 2-adrenoceptors but similar capacities in blocking biological alpha 2-adrenoceptor effects in cells) are attributable to the higher degree of binding of yohimbine to plasma protein.

Production and characterization of antibodies specific for the imidazoline receptor protein

Polyclonal antibodies were raised in rabbits against a 70-kDa ligand-binding protein of the imidazoline receptor purified from solubilized bovine adrenal chromaffin cell membranes by ligand affinity chromatography. The antibodies labeled a single protein (approximately 70 kDa) in Western blots of bovine adrenal chromaffin cell membranes, inhibited 40% of specific [3H]idazoxan binding to imidazoline receptors in chromaffin cell membranes, and specifically immunoprecipitated 75% of all imidazoline-binding activity of solubilized chromaffin cell membrane proteins. The antibodies specifically immunostained heterogeneous subsets of cultured bovine chromaffin cells. They stained subpopulations of chromaffin cells of rat adrenal medulla but not the cells of adrenal cortex. We conclude that the antibodies recognize with high specificity and selectivity a approximately 70-kDa binding protein associated with or representing the imidazoline receptor that is expressed in mammalian species. Highly specific antibodies against the imidazoline receptor protein will permit mapping of the distribution of imidazoline receptors in brain and periphery and also may be useful as probes in cloning genes encoding the imidazoline receptors.

Chronic treatment with the monoamine oxidase inhibitors clorgyline and pargyline down-regulates non-adrenoceptor [3H]-idazoxan binding sites in the rat brain

1. The binding of [3H]-idazoxan in the presence of 10(-6) M (-)-adrenaline was used to quantitate non-adrenoceptor idazoxan binding sites (NAIBS) in the rat brain after treatment with various psychotropic drugs. 2. Chronic treatment (14 days) with the monoamine oxidase (MAO) inhibitors clorgyline (0.3-10 mg kg-1, i.p.) and pargyline (10 mg kg-1, i.p.), but not with Ro 41-1049 (1 mg kg-1, i.p.), markedly decreased (30-50%) the density of NAIBS in the cerebral cortex without any apparent change in the affinity of the radioligand. 3. Acute (1 day) and/or chronic treatments (14 days) with other psychotropic drugs such as desipramine (3 mg kg-1, i.p.), cocaine (10 mg kg-1, i.p.), reserpine (0.12 mg kg-1, s.c.), haloperidol (1 mg kg-1, i.p.) and diazepam (10 mg kg-1, i.p.) did not alter the density of NAIBS in the cerebral cortex. 4. In vitro, the propargylamines clorgyline, pargyline and deprenyl displaced the binding of [3H]-idazoxan to NAIBS from two distinct sites, but only clorgyline displayed an apparent very high affinity for a relevant population of NAIBS (KiH = 40 pM; KiL = 10.6 microM). The structurally diverse MAO inhibitors Ro 16-6491 (selective for MAO-B) and Ro 41-1049 (selective for MAO-A), as well as the other psychotropic drugs (desipramine, cocaine, reserpine and haloperidol) displaced the binding of [3H]-idazoxan to NAIBS monophasically and with very low potencies. As expected, the MAO inhibitors clorgyline and Ro 41-1049 displaced the binding of [3H]-Ro 41-1049 to MAO-A monophasically and with high potencies (Ki values: 0.18 nM and 22 nM, respectively). In contrast, idazoxan displayed very low affinity (Ki =40 microM) against the binding of pH]-Ro 41-1049 to MAO-A. These results disprove a direct interaction between [3H]-idazoxan and the enzyme MAO.5. Preincubation of cortical membranes with clorgyline (10-9M or 10-6 M for 30 min) or pargyline(10-6 M or 10-5M for 30 min), reduced by 30-50% and by 17-30%, respectively, the total density of NAIBS without any apparent change in the affinity of the radioligand. Preincubation with 10-6M clorgyline did not alter the affinity of cirazoline for the two populations of NAIBS, but reduced by 60%the binding of [3H]-idazoxan to the high affinity site without affecting the binding of the radioligand to the low affinity site. These results indicate that the two MAO inhibitors irreversibly block the binding of[3H]-idazoxan to NAIBS.6. In vivo, however, various acute treatments with clorgyline (1-20 mg kg-1, i.p.) for different time intervals (6-48 h) did not alter the density of NAIBS. In vivo, only very high doses of clorgyline (40 and 80 mg kg-1, i.p.) induced modest decreases (21-28%) in the density of NAIBS in the cerebral cortex.7. Together the results indicate that the irreversible binding of clorgyline and pargyline to NAIBSfound in vitro does not fully explain the marked decreases in the density of NAIBS found in vivo after the chronic treatments. It is suggested that the down-regulation of NAIBS induced in vivo by clorgyline and pargyline, through a direct or indirect mechanism, may have functional implications.

Discrimination and pharmacological characterization of I2-imidazoline sites with [3H]idazoxan and alpha-2 adrenoceptors with [3H]RX821002 (2-methoxy idazoxan) in the human and rat brains

The alpha-2 adrenoceptor antagonist idazoxan has been shown to also recognize with high affinity nonadrenoceptor sites (I2-imidazoline sites). In contrast, the 2-methoxy derivative of idazoxan, 2-methoxy idazoxan (RX821002), binds almost exclusively to alpha-2 adrenoceptors. The purpose of this study was to assess and extend the pharmacological characterization of I2-imidazoline sites and alpha-2 adrenoceptors in the human and rat brains. Competition studies with several imidazoli(di)ne/guanidine drugs and other nonrelated structures were performed in cortical membranes against [3H]idazoxan (4 nM in the presence of 10(-6) M I-epinephrine to prevent binding to alpha-2 adrenoceptors) or [3H]RX821002 (1 nM). Drugs such as cirazoline, guanoxan, naphazoline, tolazoline, clonidine, bromoxidine (UK 14,304) and phenylbiguanide displaced [3H]idazoxan from two distinct binding sites, which suggested the existence of two affinity states for I2-imidazoline sites that were not modulated by MgCl2 or the nucleotide analog guanylyl-5'-imido-diphosphate. Binding affinities at the low-affinity site (KiL) were consistently more than 2 orders of magnitude lower than binding affinities at the high-affinity site (KiH), and there was a good correlation between KiH and KiL values for a given drug in the human (r = 0.89) and rat (r = 0.92) brains. For 18 to 22 drugs, the Ki values in the human brain correlated well with the corresponding Ki values in the rat brain both for I2-imidazoline sites (r = 0.94) and alpha-2 adrenoceptors (r = 0.97). However, the Ki values for I2-imidazoline sites did not correlate with the Ki values for alpha-2 adrenoceptors in human and rat brains. The order of drug potency for the I2-imidazoline sites was: guanoxan (1.3 nM) approximately cirazoline > idazoxan approximately naphazoline > clonidine > phentolamine > RX821002 > (8aR, 12aS, 13aS)-3-methoxy-12-methanesulfonyl-5,6,8a,9,10,11,12,12a,13,13a- decahydro-8H-isoquino[2,1-g]-naphthyridine (RS 15385-197) (> 10 microM). In contrast, the potencies at the alpha-2 adrenoceptor were: RS 1538-197 (0.3 nM) > RX821002 > clonidine > phentolamine > idazoxan approximately naphazoline > guanoxan approximately cirazoline (307 nM). The results demonstrate that I2-imidazoline sites (labeled by [3H]idazoxan) and alpha-2 adrenoceptors are different pharmacological entities with similar characteristics in the human and rat brains. In both species, I2-imidazoline sites are markedly heterogeneous in nature.

Evidence for the existence of two forms of alpha 2A-adrenoceptors in the rat

The alpha 2A-adrenoceptors in rat spleen, kidney, spinal cord and cerebral cortex were studied using [3H]-RX821002 radioligand binding. In the spleen, spinal cord and cerebral cortex, the ligand bound to saturable sites with a Kd of about 1 nmol/l and capacities of 134, 240 and 290 fmol/mg protein, respectively. Computer modelling competition curves for 39 drugs, including those for alpha 2A-, alpha 2B- or alpha 2C-adrenoceptor selective drugs, indicated that the sites labelled by [3H]-RX821002 in the spleen consisted of a single population of alpha 2A-adrenoceptors. However, the competition curves for guanoxabenz were definitely biphasic and resolved into two site fits, indicating that guanoxabenz was binding to both high affinity (Kd = 35 nmol/l) and low affinity (Kd = 8900 nmol/l) alpha 2A-adrenoceptor sites in the proportions 57% and 43%, respectively. The KdS for a number of alpha 2-adrenoceptor subtype selective drugs, measured in competition with [3H]-RX821002 in cerebral cortex and spinal cord, were highly correlated with those obtained in the spleen indicating their alpha 2A-adrenoceptor nature. However, by contrast to the results with the spleen, the guanoxabenz competition curves for the spinal cord and cerebral cortex were monophasic and resolved only into one site fits, the Kd of guanoxabenz being about 4000 nmol/l for both tissues. Drug KdS for kidney alpha 2A-adrenoceptors were also determined using [3H]-RX821002. For nearly all drugs tested, the KdS were highly correlated with those found for the alpha 2A-adrenoceptors in the other rat tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

Heterogeneity of the specific imidazoline binding of [3H]idazoxan in the human cerebral cortex

The aim of the present study was to verify whether [3H]idazoxan can be considered as a highly selective ligand for imidazoline preferring receptors (IPR). In human frontal cortex membrane preparations [3H]idazoxan at a low concentration (2 nM) only labelled imidazoline sensitive, catecholamine insensitive sites. Binding was of high affinity, saturable and stereospecific. The rank order of potency of different compounds able to inhibit this binding was cirazoline > (+/-)-idazoxan > guanoxan > (-)-idazoxan > tolazoline > UK-14304 > clonidine. Amiloride, imidazol-4-acetic acid and histamine had no significant affinity for IPR labelled by [3H]idazoxan. [3H]idazoxan bound to 2 different sites (KD1 = 1 nM and KD2 = 16.4 nM). Clonidine behaved as a non competitive, non allosteric inhibitor of [3H]idazoxan binding. Both [3H]idazoxan binding sites were equally affected by clonidine. In membrane preparations obtained from the Nucleus Reticularis Lateralis region (NRL) of the brainstem, [3H]idazoxan binding was similar to that in cortical membranes, particularly with regard to specificity and kinetics. However, in the NRL region binding sites were 4-5 times more numerous than in the frontal cortex. Non linear analyses of saturation data obtained with NRL membrane preparations were compatible with both a one site and a two sites model. No significant effects of 1 mM MgCl2 alone or with 100 microM Gpp(NH)p were observed on either [3H]idazoxan binding or the competition with clonidine or rilmenidine. As in the cortical membrane, clonidine was a non competitive inhibitor of [3H]idazoxan binding to membranes from the NRL region. In conclusion, we show that when a low concentration is used, [3H]idazoxan binding to human brain involves sites almost completely insensitive to catecholamines and specific for imidazolines or related compounds. This binding involves two distinct sites. We also report that [3H]idazoxan imidazoline binding sites are not coupled with a G protein. Because of the non competitive interaction between clonidine and [3H]idazoxan for the binding sites of the latter, we are unable to conclude that the binding sites of the two drugs are identical. However, the non competitive, non allosteric interaction suggests a complex model of multiple binding sites.

Decrease in locus coeruleus [3H]idazoxan binding site density in genetically epilepsy-prone (GEPR) rats

Deficits in norepinephrine synthesis, transmitter level, turnover and reuptake have been reported in the brain of genetically epilepsy-prone (GEPR) rats. We investigated the hypothesis that these alterations may trigger a compensatory downregulation of locus coeruleus alpha 2-adrenergic receptors and an upregulation of postsynaptic alpha 2-adrenergic receptor density in forebrain regions of GEPR rat brain. alpha 2-adrenergic receptor density was measured in the locus coeruleus and 7 forebrain regions of control and GEPR rats by in vitro [3H]idazoxan autoradiography. Specific [3H]idazoxan binding site density was decreased significantly in the locus coeruleus of both GEPR-3 and GEPR-9 rats compared to controls. No significant differences in specific [3H]idazoxan binding were observed in the 7 forebrain regions of GEPR-9 rats compared to control. Reduced locus coeruleus alpha 2-adrenergic receptor density in GEPR rats may produce a net increase in locus coeruleus noradrenergic cell firing, an effect which could, in part, offset the impact of reduced noradrenergic influence in GEPR rat forebrain. Additionally, decreased norepinephrine levels in GEPR rat brain may be a long-term consequence of reduced alpha 2-adrenergic receptor-mediated inhibition of locus coeruleus firing activity.

Effect of chronic administration of antidepressants on alpha 2-adrenoceptors in the locus coeruleus and its projection fields in rat brain determined by quantitative autoradiography

The density of alpha 2-adrenoceptors, using 3H-idazoxan as the radioligand, was determined by quantitative autoradiography in the locus coeruleus and in 13 noradrenergic projection fields following chronic administration of drugs acting on noradrenergic and/or serotonergic neurons. Protriptyline, an inhibitor of the uptake or norepinephrine, and mianserin, an alpha 2-adrenoceptor antagonist, reduced the binding of 3H-idazoxan only in the locus coeruleus. Phenelzine, an inhibitor of both type A and type B monoamine oxidase (MAO), reduced the binding of 3H-idazoxan in the locus coeruleus and in several areas with noradrenergic innervation from tegmental cell bodies. Clorgyline, a selective inhibitor of type A MAO, had no effect. Of the two selective inhibitors of serotonin uptake, citalopram caused a modest increase in binding only in one terminal field area, whereas sertraline had no effect. Although these antidepressants did not produce consistent effects on alpha 2-adrenoceptors, protriptyline, mianserin, and phenelzine were similar in that they all decreased the binding of 3H-idazoxan in the locus coeruleus without widely affecting its binding in the coerulean terminal fields. Deprenyl, a selective inhibitor of type B MAO, the only drug in this study without proven antidepressant efficacy, differed from all other drugs in that it decreased the binding of 3H-idazoxan both in the locus coeruleus as well as in most terminal fields with primarily coerulean noradrenergic innervation.

Determination of alpha 1-adrenoceptor subtype selectivity by [3H]-prazosin displacement studies in guinea-pig cerebral cortex and rat spleen membranes

1. [3H]-prazosin homogeneously labels alpha 1-adrenoceptors in guinea-pig cerebral cortex and rat spleen membranes with dissociation constants of 1.28 and 1.49 x 10(-10) M respectively. 2. Phentolamine and WB 4101 displacement studies show that guinea-pig cerebral cortex contains 30% alpha 1A- and 70% alpha 1B-adrenoceptor subtypes, whereas rat spleen contains a virtually homogeneous alpha 1B-adrenoceptor subtype population. The alpha 1-adrenoceptor population of rat thoracic aorta is predominantly of the alpha 1A-adrenoceptor subtype, and in guinea-pig thoracic aorta it is mainly of the alpha 1B-adrenoceptor subtype. 3. Half of the compounds displacing [3H]-prazosin bound to guinea-pig cerebral cortex membranes display alpha 1A-adrenoceptor selectivity. Among these compounds, WB 4101 and methoxamine are most selective, displaying selectivity ratios of approximately 38 and approximately 26 respectively. 4. The affinity constants of the non-selective compounds for the alpha 1-adrenoceptor in guinea-pig cerebral cortex membranes correlate well with the affinity constants obtained for alpha 1B-adrenoceptors in rat spleen membranes. The affinities of selective compounds for the alpha 1B-adrenoceptor subtype in guinea-pig cerebral cortex correlate very well with their affinity for alpha 1B-adrenoceptor in the rat spleen homogenate. Both regression lines coincide with the line of identity. The affinity constants of selective compounds for the alpha 1A-adrenoceptors in guinea-pig cerebral cortex only apparently correlate with the affinity for either the alpha 1B-adrenoceptors in guinea-pig cerebral cortex or in the rat spleen. Regression analyses indicate a straight line relationship (r2>0.9) between pKEA and Pk1B but the regression lines deviate from the line of identity.

Atypical alpha-1 adrenergic receptors on the rat parotid gland acinar cell

Subtypes of alpha-1 adrenergic receptors on rat parotid gland acinar cell membranes were characterized using subtype selective alpha adrenergic receptor antagonists. The alpha-1 adrenergic receptor antagonist beta-iodo-[125I]-4-hydroxyphenyl-ethyl-aminomethyl-tetralone (125IBE) had an equilibrium dissociation constant for specific binding sites on these membranes of 0.241 +/- 0.03 nM and a total number of specific radioligand binding sites of 41 +/- 4 fmol bound/mg of protein. Displacement of 125IBE binding by subtype-selective alpha-1 adrenergic receptor antagonists 2-(2,6-dimethoxyphenoxyethyl)-aminomethyl-1,4-benzodioxane HCl (WB4101) and 5-methylurapidil fit best to biphasic competition curves. The high- and low-affinity inhibition equilibrium dissociation constant for WB4101 were 0.45 +/- 0.1 and 27 +/- 6 nM, respectively. Similarly, the high- and low-affinity inhibition equilibrium dissociation constants for 5-methylurapidil were 0.16 +/- 0.03 and 71 +/- 20 nM, respectively. These affinities for 125IBE binding sites suggest the presence of alpha-1A and alpha-1B adrenergic receptor subtypes on acinar cell membranes. The irreversible alpha-1 adrenergic receptor antagonist chloroethylclonidine was used to inactivate alpha-1B adrenergic receptors on acinar cell membranes. After treatment with chloroethylclonidine, saturation binding analysis demonstrated no change in the total number of 125IBE binding sites. In addition, competition curves for WB4101 and 5-methylurapidil again showed two sites of 125IBE displacement, with no change in antagonist affinities in membranes treated with chloroethylclonidine.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ischemia on regional ligand binding to adrenoceptors in the rat brain

Changes in ligand binding to adrenoceptors ([3H]prazosin to alpha 1-receptors, [3H]idazoxan to alpha 2-receptors and [125I]cyanopindolol to beta-receptors) following transient cerebral ischemia were investigated using autoradiographic methods. The binding was quantified in brain sections from control rats, rats subjected to 15 min of 2-vessel occlusion ischemia, and rats with recirculation times of 1 h, 1 week or 4 weeks after ischemia. No significant change in alpha 1-receptor binding was observed during and immediately following ischemia, but a decrease was noted in the vulnerable hippocampal CA1 region following 1 week's survival. In the parietal cortex, the ligand binding to alpha 1-receptors increased at 4 weeks. A reduced [3H]idazoxan binding was observed 1 h after ischemia in the temporal cortex and amygdala. No change in ligand binding to beta-receptors was seen in the early phase postischemia, but a marked increase had occurred in the hippocampal CA1 region at 1 and 4 weeks after ischemia (+163% and +142%, respectively), presumably due to accumulation of macrophages expressing beta-receptors. The early postischemic changes in receptor binding may represent downregulation of the adrenoceptors by processes activated during ischemia, while neuronal degeneration, compensatory mechanisms in surviving neurons and proliferation of non-neuronal cells may account for the subsequent changes.

Quantification of in vivo binding of [3H]RX 821002 in rat brain: evaluation as a radioligand for central alpha 2-adrenoceptors

On the basis of its established in vitro characteristics, [3H]RX 821002 was evaluated in rats as an in vivo radioligand for central alpha 2-adrenoceptors. Estimates for in vivo binding potential, obtained by compartmental analyses of time-radioactivity data, ranged between 1.9 for hypothalamus and 0.2 for cerebellum, with a regional distribution in brain which was similar to that observed in vitro. Selectivity and specificity of the signal were checked by predosing with either the alpha 2-antagonists, idazoxan or yohimbine, the alpha 2-agonist, clonidine, or the alpha 1-antagonist, prazosin. Pretreatment of the rats with the selective neurotoxin, DSP-4, had no significant effect on [3H]RX 821002 binding, suggesting that the majority of labelled sites were situated post-junctionally. The studies indicate that [3H]RX 821002 can be used experimentally as an in vivo marker for central alpha 2-adrenoceptors. The size and rate of expression of the specific signal encourage the development and assessment of [11C]RX 821002 for clinical PET studies.

Characterization and autoradiographical localization of non-adrenoceptor idazoxan binding sites in the rat brain

1. In rat whole brain homogenates, saturation analysis revealed that both [3H]-idazoxan and [3H]-RX821002, a selective alpha 2-adrenoceptor ligand, bound with high affinity to an apparent single population of sites. However, the Bmax for [3H]-idazoxan was significantly (P less than 0.01) greater than that for [3H]-RX821002. 2. In competition studies, (-)-adrenaline displaced 3 nM [3H]-idazoxan binding with an affinity consistent with [3H]-idazoxan labelling alpha 2-adrenoceptors. However, this displacement was incomplete since 23.68 +/- 1.11% of specific [3H]-idazoxan binding remained in the presence of an excess concentration (100 microM) of (-)-adrenaline. In contrast, unlabelled idazoxan promoted a complete displacement of [3H]-idazoxan binding with a Hill slope close to unity and an affinity comparable with its KD determined in saturation studies. 3. Displacement of [3H]-idazoxan binding by the alpha 2-adrenoceptor antagonists yohimbine, RX821002 (2-(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline) and RX811059 (2-(2-ethoxy-1,4-benzodioxan-2-yl)-2-imidazoline) was more complex, with Hill slopes considerably less than unity, and best described by a two-site model of interaction comprising a high and low affinity component. The proportion of sites with high affinity for each antagonist was similar (60-80%). 4. The rank order of antagonist potency for the high affinity component in each displacement curve (RX821002 greater than RX811059 greater than yohimbine) is similar to that determined against the binding of [3H]-RX821002 to rat brain, suggesting that these components reflect the inhibition of [3H]-idazoxan binding to alpha 2-adrenoceptors.The remaining component in each displacement curve exhibiting low affinity towards these antagonists is attributable to the displacement of [3H]-idazoxin from a non-adrenoceptor idazoxan binding site (NAIBS) since a comparable amount of [3H]-idazoxan binding was not displaced by an excess concentration of (-)-adrenaline.5. The displacement of [3H]-idazoxan binding by RX801023 (6-fluoro-(2-(1,4-benzodioxan-2-yl)-2-imidazoline) was also best described by a model assuming a two site interaction with 20.07 +/- 3.11% of the sites labelled displaying high affinity for RX801023. The Ki of RX801023 for the remainder of the sites labelled was similar to its Ki versus [3H]-RX821002, indicating that this drug displays improved affinity and NAIBS/z2-adrenoceptor selectivity compared with idazoxan.6. In autoradiographical studies, the distribution of 5 nM [3H]-idazoxan binding to sections of rat whole brain was consistent with that reported from previous studies and resembled the distribution ofM2-adrenoceptors. However, when sections of brain were coincubated with concentrations of alpha2-adrenoceptor agonists or antagonists predicted to saturate alpha2-adrenoceptors, there remained distinct areas of binding corresponding to discrete brain nuclei. This remaining binding was however displaced by unlabelled idazoxan (3 microM) or RX801023 (3 microM) indicative of the labelling of NAIBS.7. Quantitative autoradiography of NAIBS revealed several brain nuclei which contained higher densities of these sites than alpha2-adrenoceptors, notably the area postrema, interpeduncular nucleus,arcuate nucleus, ependyma and pineal gland.

Isolation and characterization of imidazoline receptor protein from bovine adrenal chromaffin cells

We sought to isolate and partially purify proteins corresponding to the binding element of the imidazoline receptor (IR) from adrenal chromaffin cell membranes. These cells express IRs of the I-2 subclass and not alpha 2-adrenergic receptors. Proteins were solubilized in 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate-containing buffer and were assayed by binding of [3H]idazoxan, an imidazoline radioligand. Two ligand affinity resins, p-aminoclonidine-Trisacryl GF-2000 (PAC-ReactiGel) and idazoxan-PharmaLink agarose (IDA-agarose), were synthesized. These allowed purification by single-step affinity chromatography of a major receptor binding protein component of 70 kDa, as assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis and [3H]idazoxan binding assay. The purified imidazoline-binding proteins from IDA-agarose and PAC-ReactiGel had similar affinities for the radioligand [3H]idazoxan (Kd = 3.7 and 4.9 nM, respectively) and a displacement profile, showing sensitivity to imidazoline agents (cirazoline > clonidine) and insensitivity to catecholamines and adrenergic agents (epinephrine approximately rauwolscine), that was similar to that of the intact membrane receptor. The imidazoline-binding protein did not bind to concanavalin A, suggesting that it may not be glycosylated or that the sugar moieties present are not recognized by this lectin. The results indicate that IR and alpha 2 receptor proteins may be biochemically distinct and that IDA-agarose and PAC-ReactiGel columns are useful for purification of sufficient quantities of imidazoline-binding proteins to allow for structural and functional studies of the IR.

Quantitative in vitro and ex vivo autoradiography of the α2-adrenoceptor antagonist [3H]atopamezole

Atipamezole is an alpha-adrenoceptor antagonist with high affinity and selectivity for the alpha 2-receptor. In vitro autoradiography of [3H]atipamezole on rat and human brain sections revealed a pattern virtually identical to the one observed using 3H-labeled antagonists such as idazoxan or methoxyidazoxan or agonists such as p-aminoclonidine and 5-bromo-6-(2-imidazolin-2-yl-amino-quinoxaline (UK-14304). In vivo studies of [3H]atipamezole demonstrated good penetration into the brain (0.3-1.8% injected dose/g at 5 min, depending upon brain region). In addition, [3H]atipamezole displayed rapid in vivo clearance of nonspecific binding such that at 1 h following an i.v. injection of the drug (100 microCi/animal, rat tail vein administration) the pattern of radioactivity in the brain correlated very well with the receptor distribution as seen by in vitro autoradiography. Atipamezole binding in vivo was displaceable by idazoxan. These results demonstrate the potential of suitably labeled atipamezole for regional studies of brain alpha 2-adrenoceptors in vitro and in vivo, in experimental animals as well as in human positron emission tomography (PET) studies.

Purification and characterization of mitochondrial imidazoline-guanidinium receptive site from rabbit kidney

The imidazoline-guanidinium receptive site (IGRS) is a membrane-bound protein that may mediate some of the pharmacological effects of imidazoline and guanidinium compounds. The structure and functionality of this protein are unknown but, in addition to its location at the plasma membrane, it is found in high density in the outer membrane of mitochondria (Tesson, F., Prip-Buus, C., Lemoine, A., Pegorier, J.-P., and Parini, A. (1991) J. Biol. Chem. 266, 155-160). Using a two-step procedure, we report the purification of mitochondrial IGRS from rabbit kidney to the apparent homogeneity. After solubilization of mitochondrial membranes with digitonin, an apparently homogeneous IGRS preparation was obtained by two sequential purification steps, chromatofocusing and hydroxylapatite-agarose chromatography. One- and two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the purified preparation after silver staining or radioiodination indicated that IGRS binding subunit was purified at the apparent homogeneity since a single band (M(r) approximately 60,000) was observed. IGRS behaves as an acidic protein (pI 5.5) whose binding activity is regulated by H+ concentration near a physiological pH of 7.4. The ability to achieve rapid purification of IGRS should facilitate efforts to define molecular properties and functionality of this protein.

Tissue-specific localization of mitochondrial imidazoline-guanidinium receptive sites

In the present report, we studied the distribution of the imidazoline-guanidinium receptive site in mitochondrial fractions from different rabbit and human tissues. Binding studies of the imidazoline-guanidinium receptive site ligand [3H]idazoxan, allowed to distinguish two groups of tissues: the first one, including kidney, brain and liver, displays a high density of imidazoline-guanidinium receptive site; the second one, consisting of striated and smooth muscle, enterocytes, lung, spleen and heart, is characterized by 4- to 16-fold lower binding site density. The demonstration that mitochondrial imidazoline-guanidinium receptive sites are not equally expressed in all tissues can be considered as a further progress towards the characterization of their functional activity.

Novel indolodioxanes with antihypertensive effects: potent ligands for the 5-HT1A receptor

The synthesis and biological evaluation of a new family of tricyclic indolodioxanes is described. These compounds all contain the 2,3-dihydro-7H-1,4-dioxino[2,3-e]indole nucleus and bear substituents at the 2 and/or 8 positions. Thirteen members of this class were prepared and shown to be potent ligands for the 5-HT1A receptor, with several compounds displaying subnanomolar inhibition constants. These compounds also bind to the dopamine D-2 receptor, but generally with higher inhibition constants than those for 5-HT1A. Certain members of this novel structural class show in vivo activity in the mouse hypothermia assay. One of these compounds, U-86192A, has been shown to have antihypertensive effects in the cat, completely eliminating sympathetic nerve discharge at 1 mg/kg iv and lowering mean arterial pressure to 50% pretreatment levels. These effects can be reversed by the administration of spiperone, indicating that U-86192A is acting via a central serotonergic mechanism.

Increased density of alpha-adrenoceptors in vas deferens of spontaneously hypertensive rats (SHR), indicated by functional and receptor binding studies

Pharmacological parameters were determined from contractile responses mediated by alpha-adrenoceptors in vas deferens from spontaneously hypertensive rats (SHR) and corresponding normotensive controls, Wistar Kyoto rats (WKY), and compared with data obtained from radioligand binding assays. Contractile responses induced in longitudinal and circular muscle layers by the alpha-adrenoceptor agonist noradrenaline (NA) and by barium chloride were recorded as described previously. In both muscle layers the maximal effects induced by NA, but not by BaCl2, were significantly greater in SHR. As a consequence, the relative responsiveness ratio (rho) for the alpha-adrenoceptor was also larger for SHR than for WKY. NA-induced contractions of both muscle layers were competitively antagonized by indoramine. The pA2 values for indoramine and pD2 values for NA were the same in SHR and WKY, indicating that alpha-adrenoceptor affinity was not changed in SHR. Additionally, binding studies with the alpha-adrenoceptor ligand [3H]WB4101 revealed that Bmax values were greater in the vas deferens of SHR, whereas Kd values were not significantly different from those of WKY controls. In summary, although differences could not be detected for affinity-related parameters, a greater density of alpha-adrenoceptors was shown for SHR in receptor binding studies and this was corroborated by functional studies.

Imidazoline binding sites in human placenta: evidence for heterogeneity and a search for physiological function

1. An alpha 2-adrenoceptor antagonist, idazoxan, that binds to both alpha 2-adrenoceptors and to imidazoline sites (IR), has been used to characterize human placental IR. Human placenta is shown to be the richest source of IR (1800 +/- 100 fmol mg-1 protein; Kd 38.9 +/- 3.4 nM). 2. Primary cells derived from human placenta and grown in monolayers, also displayed a high density of receptors (3209 +/- 136 fmol mg-1 in cytotrophoblasts and 3642 +/- 144 fmol mg-1 protein in syncytiotrophoblast enriched cell culture). 3. [3H]-idazoxan did not show binding characteristics of alpha 2-adrenoceptors in human placental membranes or human trophoblastic cells, thus making it a ligand of choice to study the imidazoline site. The tissue appeared to be lacking alpha 2-adrenoceptors in that other alpha 2-adrenoceptor ligands, [3H]-rauwolscine and [3H]-clonidine, do not bind to alpha 2-adrenoceptors in human placenta. 4. IRs are localized on the cell surface, as determined by the release of bound [3H]-idazoxan from cells, when washed with high ionic/acidic medium. 5. Imidazoline receptors of human placenta display high affinity for amiloride (72 +/- 27 nM). The high affinity was used as a criterion to classify IR to IRa subtype (placenta, rabbit kidney, rabbit liver and rabbit adipose cells) as opposed to the IRb subtype which display low affinity for amiloride (greater than 2 microM, in all the other tissues).6. Several novel ligands comprising a guanido functional group attached to an aromatic residue (e.g. benziliden-amino-guanidine (BAG), guanido pyrole) display pronounced selectivity for IR over the M2-adrenoceptors as the affinity of BAG is about 40 fold higher (Kd= 18.9 +/- 13.8 nM in human placenta), than the affinity for M2-adrenoceptors (Kd = 768 +/- 299 nM in human platelets). Imidazoline sites bind selectively BAG and other guanido ligands thus indicating a distinct structural requirement at its site of binding.7. K+ channel blockers and monovalent ions (e.g. Cs' and NH4+) interfere with idazoxan binding to IR, indicating a possible involvement of IR in K+ transport.