Identification and characterization of the imidazoline I2b-binding sites in the hamster brown adipose tissue as a study model for imidazoline receptors

The imidazoline-type compound, MPV-1743, has been found to activate nonshivering thermogenesis (NST) in brown adipose tissue (BAT) of the genetically obese Zucker rats. The regulation of NST in BAT is linked to the catecholamine metabolism, and the imidazoline I2-binding sites have been found on the monoamine oxidase, a catecholamine metabolising enzyme. In this study, the I2-binding sites of hamster BAT have been characterised using a receptor binding assay with 3H-idazoxan as a radioligand, and the interaction of MPV-1743 with these I2-binding sites has been studied using the enantiomers of MPV 1743, that is, MPV 2088 and MPV 2089. Cirazoline was used to determine the specific binding of 3H-idazoxan to the imidazoline I2-binding sites. Rauwolscine was added in the 3H-idazoxan binding assay in order to inhibit any binding to potential alpha2-adrenergic sites. In the presence of rauwolscine mask 3H-Idazoxan labelled a population of non-adrenergic binding sites expressing the properties of the imidazoline I2b-receptor subtype similar to that found in the rat liver (cirazoline >> guanabenz = amiloride >> clonidine). The binding of 3H-idazoxan to the I2b-binding sites could be displaced by the imidazole compounds with the following affinities: detomidine (KiHigh 9.2 nM; KiLow 3200 nM), MPV-2088 (KiHigh 19 nM; IKiLow 760 nM) and MPV-2089 (KiHigh 190 nM; KiLow 1300 nM), atipamezole (3500 nM) and dexmedetomidine (Ki 8400 nM). These results have shown that the hamster BAT contains the imidazoline I2b-binding sites with heterogeneous binding properties for some test compounds. In addition, the enantiomers of MPV 1743, that is, MPV 2088 and MPV 2089, had high affinity to these BAT imidazoline I2b-binding sites. Therefore, it is suggested that the regulation of NST in the hamster BAT may be an attractive model to study the role of imidazoline I2b-binding sites.

Molecular mechanism for agonist-promoted alpha(2A)-adrenoceptor activation by norepinephrine and epinephrine

We present a mechanism for agonist-promoted alpha(2A)-adrenergic receptor (alpha(2A)-AR) activation based on structural, pharmacological, and theoretical evidence of the interactions between phenethylamine ligands and alpha(2A)-AR. In this study, we have: 1) isolated enantiomerically pure phenethylamines that differ both in their chirality about the beta-carbon, and in the presence/absence of one or more hydroxyl groups: the beta-OH and the catecholic meta- and para-OH groups; 2) used [(3)H]UK-14,304 [5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine; agonist] and [(3)H]RX821002 [2-(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline; antagonist] competition binding assays to determine binding affinities of these ligands to the high- and low-affinity forms of alpha(2A)-AR; 3) tested the ability of the ligands to promote receptor activation by measuring agonist-induced stimulation of [(35)S]GTPgammaS binding in isolated cell membranes; and 4) used automated docking methods and our alpha(2A)-AR model to predict the binding modes of the ligands inside the alpha(2A)-AR binding site. The ligand molecules are sequentially missing different functional groups, and we have correlated the structural features of the ligands and ligand-receptor interactions with experimental ligand binding and receptor activation data. Based on the analysis, we show that structural rearrangements in transmembrane helix (TM) 5 could take place upon binding and subsequent activation of alpha(2A)-AR by phenethylamine agonists. We suggest that the following residues are important in phenethylamine interactions with alpha(2A)-AR: Asp113 (D(3.32)), Val114 (V(3.33)), and Thr118 (T(3.37)) in TM3; Ser200 (S(5.42)), Cys201 (C(5.43)), and Ser204 (S(5.46)) in TM5; Phe391 (F(6.52)) and Tyr394 (Y(6.55)) in TM6; and Phe411 (F(7.38)) and Phe412 (F(7.39)) in TM7.

Three-dimensional models of alpha(2A)-adrenergic receptor complexes provide a structural explanation for ligand binding

We have compared bacteriorhodopsin-based (alpha(2A)-AR(BR)) and rhodopsin-based (alpha(2A)-AR(R)) models of the human alpha(2A)-adrenengic receptor (alpha(2A)-AR) using both docking simulations and experimental receptor alkylation studies with chloroethylclonidine and 2-aminoethyl methanethiosulfonate hydrobromide. The results indicate that the alpha(2A)-AR(R) model provides a better explanation for ligand binding than does our alpha(2A)-AR(BR) model. Thus, we have made an extensive analysis of ligand binding to alpha(2A)-AR(R) and engineered mutant receptors using clonidine, para-aminoclonidine, oxymetazoline, 5-bromo-N-(4, 5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK14,304), and norepinephrine as ligands. The representative docked ligand conformation was chosen using extensive docking simulations coupled with the identification of favorable interaction sites for chemical groups in the receptor. These ligand-protein complex studies provide a rational explanation at the atomic level for the experimentally observed binding affinities of each of these ligands to the alpha(2A)-adrenergic receptor.

Alpha2-adrenoceptor agonists stimulate high-affinity GTPase activity in a receptor subtype-selective manner

Transfected Chinese hamster ovary cells expressing human alpha2A-, alpha2B- and alpha2C-adrenoceptor subtypes were used to monitor alpha2-adrenoceptor-stimulated GTP hydrolysis. Incubation with 100 microM (-)-adrenaline resulted in stimulation of pertussis toxin-sensitive GTPase by 380% after activation of the alpha2A-subtype, by 320% after activation of the alpha2B-subtype and by 110% after activation of the alpha2C-subtype. The agonists dexmedetomidine, UK14,304 (5-bromo-6-[2-imidazoline-2-ylamino]quinoxaline) and oxymetazoline showed subtype-dependent efficacy. Dexmedetomidine was a full agonist at the alpha2B-subtype and a partial agonist at the alpha2A- and the alpha2C-subtypes. UK14,304 was a full agonist at the alpha2A-subtype and a partial agonist at the other two. Oxymetazoline showed strong partial agonism at the alpha2B-subtype (63% of adrenaline), but did not significantly activate the alpha2A- and the alpha2C-subtypes. These results agreed with cAMP accumulation experiments carried out with cell lines endogenously expressing the alpha2A-subtype (human erythroleukemia, HEL) or the alpha2B-subtype (neuroblastoma-glioma, NG108-15). The GTPase assay may thus provide a valuable tool for the identification of subtype-selective alpha2-adrenoceptor agonists.

Protean agonism at alpha2A-adrenoceptors

The coupling of the endogenously expressed alpha2A-adrenoceptors in human erythroleukemia cells (HEL 92.1.7) to Ca2+ mobilization and inhibition of forskolin-stimulated cAMP production was investigated. The two enantiomers of medetomidine [(+/-)-[4-(1-[2, 3-dimethylphenyl]ethyl)-1H-imidazole]HCl] produced opposite responses. Dexmedetomidine behaved as an agonist in both assays (i.e. , it caused Ca2+ mobilization and depressed forskolin-stimulated cAMP production). Levomedetomidine, which is a weak agonist in some test systems, reduced intracellular Ca2+ levels and further increased forskolin-stimulated cAMP production and therefore can be classified as an inverse agonist. A neutral ligand, MPV-2088, antagonized responses to both ligands. Several other, chemically diverse alpha2-adrenergic ligands also were tested. Ligands that could promote increases in Ca2+ levels and inhibition of cAMP production could be classified as full or partial agonists. Their effects could be blocked by the alpha2-adrenoceptor antagonist rauwolscine and by pertussis toxin treatment. Some typical antagonists such as rauwolscine, idazoxan, and atipamezole had inverse agonist activity like levomedetomidine. The results suggest that the alpha2A-adrenoceptors in HEL 92.1.7 cells exist in a precoupled state with pertussis toxin-sensitive G proteins, resulting in a constitutive mobilization of intracellular Ca2+ and inhibition of cAMP production in the absence of agonist. This constitutive activity can be antagonized by inverse agonists such as levomedetomidine and rauwolscine. Levomedetomidine can be termed a "protean agonist" because it is capable of activating uncoupled alpha2-adrenoceptors in other systems and inhibiting the constitutive activity of precoupled alpha2-adrenoceptors in HEL 92.1. 7 cells. With this class of compounds, the inherent receptor "tone" could be adjusted, which should provide a new therapeutic principle in receptor dysfunction.

A series of 6-(omega-methanesulfonylthioalkoxy)-2-N-methyl- 1,2,3, 4-tetrahydroisoquinolines: cysteine-reactive molecular yardsticks for probing alpha2-adrenergic receptors

A series of 6-(omega-methanesulfonylthioalkoxy)-2-N-methyl-1,2,3, 4-tetrahydroisoquinolines (7a-d) was prepared and characterized as SH-reactive molecular yardsticks useful in probing alpha2-adrenergic receptors. Rapid displacement of the methanesulfonyl group by a cysteine residue in dilute aqueous solution with concomitant formation of a disulfide conjugate was verified by MALDI-TOF mass spectrometric analysis of the reaction of 7a with a cysteine-containing decapeptide. 7a-d all showed a marked affinity for the three different variants of human alpha2-adrenergic receptors: H alpha(2A)wt, H alpha(2B)wt, and mutant H alpha(2A)Ser201Cys197. However, only the mutated receptor (H alpha(2A)Ser201Cys197) was irreversibly inactivated, and the extent of inactivation in this case was linearly dependent on the length of the side chain of 7a-d. These results show that the molecular yardstick approach tested here can provide useful information for modeling receptor proteins.

Chloroethylclonidine binds irreversibly to exposed cysteines in the fifth membrane-spanning domain of the human alpha2A-adrenergic receptor

The alpha2-adrenergic receptors (alpha2-ARs) mediate signals to intracellular second messengers via guanine nucleotide binding proteins. Three human genes encoding alpha2-AR subtypes (alpha2A, alpha2B, alpha2C) have been cloned. Several chemical compounds display subtype differences in their binding and/or functional activity. Site-directed mutagenesis and molecular modeling are new tools with which to investigate the subtype selectivity of ligands. In this study, we introduce a new approach to mapping of the binding site crevice of the human alpha2A-AR. Based on a three-dimensional receptor model, we systematically mutated residues 197-201 and 204 in the fifth transmembrane domain of the human alpha2A-AR to cysteine. Chloroethylclonidine, an alkylating derivative of the alpha2-adrenergic agonist clonidine, binds irreversibly to alpha2A-ARs by forming a covalent bond with the sulfhydryl side chain of a cysteine residue exposed in the binding cavity, leading to inactivation of the receptor. Irreversible binding of chloroethylclonidine was used as a criterion for identifying introduced cysteine residues as being exposed in the binding cavity. The results supported a receptor model in which the fifth transmembrane domain is alpha-helical, with residues Val197, Ser200, Cys201, and Ser204 exposed in the binding pocket. Residues Ile198, Ser199, Ile202, and Gly203 face the lipid bilayer of the plasma membrane. This approach emerges as a powerful tool for structural characterization of the alpha2-ARs.

Evaluation of the effects of a specific alpha 2-adrenoceptor antagonist, atipamezole, on alpha 1- and alpha 2-adrenoceptor subtype binding, brain neurochemistry and behaviour in comparison with yohimbine

In the present study we evaluated the alpha 1- and alpha 2-adrenoceptor subtype binding, central alpha 2-adrenoceptor antagonist potency, as well as effects on brain neurochemistry and behavioural pharmacology of two alpha 2-adrenoceptor antagonists, atipamezole and yohimbine. Atipamezole had higher selectivity for alpha 2- vs. alpha 1-adrenoceptors than yohimbine regardless of the subtypes studied. Both compounds had comparable affinity for the alpha 2A-, alpha 2C- and alpha 2B-adrenoceptors, but yohimbine had significantly lower affinity for the alpha 2D-subtype. This may account for the fact that significantly higher doses of yohimbine than atipamezole were needed for reversal of alpha 2-agonist (medetomidine)-induced effects in rats (mydriasis) and mice (sedation and hypothermia). The effect on central monoaminergic activity was estimated by measuring the concentrations of transmitters and their main metabolites in whole brain homogenate. At equally effective alpha 2-antagonising doses in the rat mydriasis model, both drugs stimulated central noradrenaline turnover (as reflected by increase in metabolite levels) to the same extent. Atipamezole increased dopaminergic activity only slightly, whereas yohimbine elevated central dopamine but decreased central 5-hydroxytryptamine turnover rates. In behavioural tests, atipamezole (0.1-10 mg/kg) did not affect motor activity but stimulated food rewarded operant (FR-10) responding (0.03-3 mg/kg) whereas yohimbine both stimulated (1 mg/kg) and decreased (> or = 3 mg/kg) behaviour in a narrow dose range in these tests. In the staircase test, both antagonists increased neophobia, but in the two compartment test only yohimbine (> or = 3 mg/kg) decreased exploratory behaviour. The dissimilar effects of the antagonists on neurochemistry and behaviour are thought to be caused by non alpha 2-adrenoceptor properties of yohimbine. In conclusion, the alpha 2-antagonist atipamezole blocked all alpha 2-adrenoceptor subtypes at low doses, stimulated central noradrenergic activity and had only slight effects on behaviour under familiar conditions, but increased neophobia. The low affinity for the alpha 2D-adrenoceptor combined with its unspecific effects complicates the use of yohimbine as pharmacological tool to study alpha 2-adrenoceptor physiology and pharmacology.

Different apparent modes of inhibition of alpha2A-adrenoceptor by alpha2-adrenoceptor antagonists

The inhibition of alpha2A-adrenoceptor-mediated Ca2+ elevation by alpha2-adrenoceptor antagonists was measured in HEL human erythroleukemia cells. The antagonists could be divided in two classes: those that displayed surmountable inhibition (right-shift of the agonist dose-response curve), and those that displayed different degrees of insurmountable inhibition (depression of the maximum signal and a possible right-shift of the agonist dose-response curve). The degree of surmountability of the inhibition correlated well with the measured antagonist dissociation rates, suggesting that the hypothesis of the antagonist dissociation rate governing the mode of inhibition of fast responses, holds true. HEL cells thus provide a useful model system for the investigation of physiological consequences of different dissociation rates. Also, the dissociation rates of antagonists not available in radiolabelled form can be predicted from the functional data. The data stresses the importance of measurement of kinetic parameters of the drug-receptor interaction in addition to the equilibrium binding constants.

Alpha2-adrenoceptor regulation of adenylyl cyclase in CHO cells: dependence on receptor density, receptor subtype and current activity of adenylyl cyclase

Chinese hamster ovary (CHO) cells stably transfected to express different densities of the human alpha2A-, alpha2B- and alpha2C-adrenoceptor subtypes, were used to characterize the regulation of adenylyl cyclase activity by alpha2-adrenoceptor agonists. In isolated cell membranes, activation of alpha2A- and alpha2C-adrenoceptors did not affect basal enzyme activity, but activation of alpha2B-adrenoceptors stimulated adenylyl cyclase activity. The extent of stimulation was dependent on the receptor density and was insensitive to pertussis toxin treatment. In the presence of 10 microM forskolin all three receptor subtypes mediated inhibition of adenylyl cyclase activity in a pertussis toxin-sensitive manner. In experiments performed with intact cells the same pattern could be seen: the basal production of cAMP was not affected when alpha2C-adrenoceptors were activated, but activated alpha2B-adrenoceptors mediated stimulation of cAMP production. In the presence of forskolin, both receptor subtypes mediated inhibition of cAMP production. Our results suggest that alpha2B-adrenoceptors are coupled to both Gi and Gs proteins. The signal transduction pathway to which the receptor is coupled is not dependent on receptor density, but its effect on adenylyl cyclase regulation is dependent on the current activity of adenylyl cyclase. The results also suggest that the alpha2A- and alpha2C-subtypes are preferentially coupled to Gi and transduce only inhibition of adenylyl cyclase activity in transfected CHO cells. At low densities of alpha2C-adrenoceptors, clonidine was a partial agonist, but in clones expressing high levels of alpha2C-adrenoceptors, clonidine acted as a full agonist by inhibiting cAMP accumulation with the same efficacy as (-)-noradrenaline. This demonstrates that receptor reserve can mask partial agonist activity.

Anti-obesity effect of MPV-1743 A III, a novel imidazoline derivative, in genetic obesity

MPV-1743 A III ((+/-)-4-(5-fluoro-2,3-dihydro-1H-inden-2-yl)-1H-imidazole) is a novel imidazoline derivative. In this study, it was shown to bind with high affinity to alpha2-adrenoceptor subtypes alpha2A (IC50) = 0.66 +/- 0.06 nM), alpha2B (IC50) = 3.8 +/- 0.53 nM), alpha2C (IC50) = 3.1 +/- 0.61 nM) in the recombinant S115 cells and to alpha2D (IC50 = 0.94 +/- 0.10 nM) in the rat submandibular gland. MPV-1743 A III also showed remarkably high affinity to alpha1-adrenoceptors (IC50 = 150 +/- 12 nM) in the rat cerebral cortex and to imidazoline I2b-binding sites (IC50) = 150 +/- 5.0 nM) in the rat liver. The functional alpha2-adrenoceptor antagonistic effect of MPV-1743 A III was demonstrated by studying the ability of orally administered MPV-1743 A III to reverse and prevent the alpha2-adrenoceptor agonist detomidine-induced mydriasis in rat. The anti-obesity effect of MPV-1743 A III was investigated in genetically obese (fa/fa) Zucker rats in two different phases of obesity. Chronic treatment with MPV-1743 A III (0.3 3 mg/kg per day p.o. for 3 weeks) dose dependently decreased weight gain in early-phase obesity. In fully established obesity, GDP binding to mitochondria and expression of uncoupling protein mRNA were increased in brown adipose tissue by MPV-1743 A III indicating an activation of non-shivering thermogenesis. The present study shows that MPV- 1743 A III has a modest anti-obesity effect in the genetic rodent model of obesity. The relative importance of alpha2- and alpha1-adrenoceptors and imidazoline I2b-binding sites in mediating the effects of MPV-1743 A III needs further evaluation.

alpha 2A/D-Adrenoceptor subtype predominates also in the neonatal rat spinal cord

alpha 2-Adrenoceptors are remarkably regulated by developmental factors. In this study alpha 2-adrenoceptor subtypes have been characterised in neonatal and adult rat spinal cords. In saturation experiments, a 5% proportion of [3H]rauwolscine binding has a high affinity component, representing the alpha 2C-subtype in both tissues. Competition studies with [3H]RX821002 indicate that in both tissues the alpha 2A/D subtype is expressed similarly.

[3H]dexmedetomidine, an alpha 2-adrenoceptor agonist, detects a novel imidazole binding site in adult rat spinal cord

Binding properties of [3H]dexmedetomidine [(+)-(S)-4-[1-(2,3-dimethylphenyl)ethyl]-1H-imidazole] as an agonist-type radioligand for alpha 2-adrenoceptors were characterised for the first time in tissues relevant to its analgesic (spinal cord from neonatal or adult rats) and behavioural (rat cerebral cortex) actions. In membranes of rat cerebral cortex (KdHigh 0.2 +/- 0.03 nM, KdLow 8.8 +/- 1.4 nM with Bmax High 130 +/- 11 fmol/mg protein, RHigh 16%) and neonatal spinal cord (KdHigh 0.3 +/- 0.04 nM, KdLow 14 +/- 3.7 nM with Bmax High 290 +/- 40 fmol/mg protein, RHigh 25%) Gpp(NH)p modifies the biphasic binding to monophasic and binding is competed with specifically by alpha 2-adrenoceptor compounds. Binding to rat cerebral cortex is not modified by pretreatment with the noradrenergic neurotoxin, DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine). In contrast, [3H]dexmedetomidine binding to adult rat spinal cord membranes is more complex and both saturation analysis and competition experiments indicate the presence of a non-adrenergic component of binding (about 40% of total binding) which is sensitive to imidazole-type compounds. This non-adrenergic component of [3H]dexmedetomidine binding can be defined as a novel type of imidazole binding site such that, of the imidazoline I1 or I2 receptor ligands, only cimetidine has relatively high affinity. In conclusion, [3H]dexmedetomidine shows very complex binding characteristics that limit its use as an agonist-type radioligand for alpha 2-adrenoceptors but it may be a useful tool for imidazoline receptor characterisation.

Alpha 2A-adrenergic regulation of cyclic AMP accumulation and lipolysis in human omental and subcutaneous adipocytes

OBJECTIVE

To characterize differences in alpha 2-adrenergic regulation between subcutaneous and omental adipocytes which could offer a possibility of pharmacological intervention in the metabolic syndrome.

DESIGN

Both subcutaneous and omental adipocytes were isolated from 32 patients. Adipocytes were incubated in the presence of adrenoceptor agonists, and cyclic AMP and glycerol levels were measured. alpha 2-Adrenoceptors of isolated plasma membranes were characterized.

RESULTS

Adrenaline increased cyclic AMP levels about two-fold in omental adipocytes but had almost no effect in subcutaneous fat cells. The inhibition of cyclic AMP accumulation and glycerol release by UK-14304 and dexmedetomidine was less pronounced in omental adipocytes. The maximal effect of isoprenaline on cyclic AMP levels and glycerol release was similar at the two sites. The subcutaneous and omental alpha-adrenoceptors had similar affinities to 3H-RX821002 and showed characteristics of the alpha 2A subtype. The receptor densities were 220 +/- 21 and 460 +/- 84 fmol/mg of protein (means +/- s.e.m.) in omental and subcutaneous membranes, respectively.

CONCLUSION

Inhibition of cyclic AMP accumulation and lipolysis by alpha 2A-adrenoceptors is less pronounced in omental than subcutaneous adipocytes which could be due to differences in receptor number. These differences in alpha 2A-adrenergic regulation could be of value in the treatment of the metabolic syndrome.

Two human alpha 2-adrenoceptor subtypes alpha 2A-C10 and alpha 2B-C2 expressed in Sf9 cells couple to transduction pathway resulting in opposite effects on cAMP production

The baculovirus expression vector system utilizing the strong polyhedrin gene promoter of the Autographa californica nuclear polyhedrosis virus (AcNPV) was used for high level expression of the two alpha 2-adrenoceptor subtypes alpha 2A-C10 and alpha 2B-C2 in Spodoptera frugiperda (Sf-9) insect cells. For rapid screening of recombinant viruses the luciferase gene was expressed under the early ETL-promoter (early transcript large) in the same plasmid. Both receptor subtypes showed the same rank order of binding affinity for four agonists tested: dexmedetomidine > l-medetomidine = clonidine > noradrenaline. For the alpha 2A-C10 subtype, these agonists inhibited forskolin stimulated cAMP production through pertussis toxin sensitive G-proteins. In contrast, for the alpha 2B-C2 subtype the agonists stimulated both basal and forskolin stimulated cAMP production.

Recombinant human alpha 2-adrenoceptor subtypes: comparison of [3H]rauwolscine, [3H]atipamezole and [3H]RX821002 as radioligands

Kinetic, saturation and competition binding assays were employed to optimize and validate radioligand binding methods for characterization of recombinant human alpha 2-adrenoceptor subtypes and for screening of new subtype-selective ligands. Stable transfected lines of Shionogi 115 mouse mammary tumour cells (S115) and three structurally different antagonist radioligands, [3H]rauwolscine, [3H]atipamezole and [3H]RX821002, were used. Specificity of alpha 2-adrenergic binding was defined with 100 microM (-)-adrenaline. Steady-state was reached with all three radioligands within 15-30 min at 25 degrees C, and the binding was rapidly reversible. The receptor affinities (alpha 2-C10) were highest in glycylglycine, almost equally high in K(+)-phosphate, and lowest in Tris buffer for all three [3H]-ligands. This was mainly caused by different association rates. [3H]RX821002 was bound with high affinity and similar kinetic properties to all three alpha 2-adrenoceptor subtypes in K(+)-phosphate buffer, and had the highest proportion of specific binding (96-98%). [3H]RX821002 and K(+)-phosphate buffer were subsequently used in competition assays. The rank order of affinity of compounds selective for alpha 2-adrenoceptor subtypes was alpha 2-C10 > alpha 2-C4 > alpha 2-C2 for oxymetazoline, alpha 2-C4 > alpha 2-C2 > alpha 2-C10 for prazosin and alpha 2-C2 > alpha 2-C4 > alpha 2-C10 for chlorpromazine. The drug affinities (Ki values) determined in this system were in close agreement with earlier results with [3H]rauwolscine in Tris buffer (r = 0.94). Agonist competition for [3H]RX821002 binding was biphasic in K(+)-phosphate buffer supplemented with 10 mM MgCl2, indicating functional coupling of receptors to G-proteins. Accordingly high-affinity binding of the agonists (-)-noradrenaline and UK14,304 was eliminated by 10 microM Gpp(NH)p in the assays. Our results confirm that [3H]RX821002 is a suitable radioligand for the characterization of all three human alpha 2-adrenoceptor subtypes and for the determination of the subtype-selectivity of new alpha 2-adrenoceptor agonists and antagonists.

Different sensitivity of alpha 2A-C10 and alpha 2C-C4 receptor subtypes in coupling to inhibition of cAMP accumulation

Two human alpha 2-adrenergic receptor subtypes, alpha 2A-C10 and alpha 2C-C4, were compared with respect to their ability to inhibit stimulated cAMP-production. The inhibition was transduced with about one order of magnitude higher sensitivity in the alpha 2C-C4 subtype than in the alpha 2A-C10 subtype. The phorbol ester, TPA, known to desensitize alpha 2-adrenergic receptor function, possible through phosphorylation of Gi, almost completely abolished the inhibition of cAMP-production in the alpha 2C-C4 subtype, while only a partial effect was seen in the alpha 2A-C10 subtype. These results suggest that the receptor subtypes differ with respect to their coupling efficiency to adenylyl cyclase.

Coupling of human alpha 2-adrenoceptor subtypes to regulation of cAMP production in transfected S115 cells

Stable S115 mouse mammary tumour cell lines, expressing separately alpha 2A-C10, alpha 2B-C2 and alpha 2C-C4 adrenoceptors were used to compare the receptor binding properties of alpha 2-adrenoceptor agonists with their potency in inhibiting cAMP production. All tested agonists detected high and low affinity binding sites in all three receptor subtypes. In the presence of the GTP analogue Gpp(NH)p (10 microM), all displacement curves were shifted to the right and were best modelled by one-site fits, suggesting that the receptor subtypes are coupled to G-proteins. The extent of the Gpp(NH)p-induced shift was greatest in the alpha 2A-C10 subtype, smaller in alpha 2C-C4, and minimal in alpha 2B-C2. All three receptor subtypes were also coupled to inhibition of forskolin-stimulated cAMP production through pertussis toxin-sensitive G-proteins. For the full agonists noradrenaline, UK 14,304, and dexmedetomidine, the maximal inhibitory effect on cAMP production was smaller in the alpha 2B-C2 subtype (35%) than in the alpha 2A-C10 and alpha 2C-C4 subtypes (50-70%). After treatment of cells expressing alpha 2B-C2 receptors with pertussis toxin, cAMP production was increased by up to 58% by alpha 2-adrenoceptor agonists. Similar stimulation of adenylyl cyclase activity could not be demonstrated at the other two receptor subtypes. In conclusion, these results demonstrate that (1) alpha 2-adrenoceptor agonists may be characterized by an agonist-type binding pattern in homogenates of transfected S115 cells, (2) all three alpha 2-adrenoceptor subtypes are coupled to inhibition of adenylyl cyclase in S115 cells through pertussis toxin-sensitive G-proteins, (3) the receptor-effector coupling in S115 cells is different among the subtypes so that the alpha 2A-C10 subtype is coupled with high efficacy but with low sensitivity, the alpha 2B-C2 subtype with low efficacy but high sensitivity, and the alpha 2C-C4 subtype with both high efficacy and high sensitivity, and (4) at least alpha 2B-C2 receptors may also be coupled to stimulation of adenylyl cyclase activity, presumably through Gs.

Functional analysis of the human alpha 2C-C4 adrenergic receptor in insect cells expressed by a luciferase-based baculovirus vector

A click beetle luciferase-based baculovirus expression vector is described for functional analysis and high level expression of a human alpha 2-adrenergic receptor (alpha 2AR) in Sf9 insect cells. The resultant recombinant baculovirus construct, AcLucGR-alpha 2(C4), was isolated by utilizing the light emitting properties of luciferase and used for abundant expression of the alpha 2C-C4 receptor protein in this lepidopteran insect cell line. A maximal expression of alpha 2-receptors at a level of 1.370 pmol/mg protein was obtained at 48 h after infection as determined by ligand-binding experiments using the alpha 2-receptor antagonist, [3H]rauwolscine. The receptor agonists, noradrenaline and clonidine, displaced the [3H]rauwolscine binding with Ki values 12.3 +/- 1.54 microM and 1.23 +/- 0.11 microM, respectively. The recombinant receptors were functionally intact since the agonists inhibited forskolin-stimulated cAMP production. Here, however, the maximal inhibition was obtained at 36 h after the infection. The results presented here, suggest that the baculovirus expression vector system (BEVS) provides a simple method for abundant expression of functional alpha 2-receptor subtypes. In addition, co-expression of luciferase proved to be useful for screening and isolation of the recombinant baculovirus.

Characterization of [3H]atipamezole as a radioligand for α2-adrenoceptors

Atipamezole (MPV-1248, 4-(2-ethyl-2,3-dihydro-1H-inden-2-yl)-1H-imidazole), a potent alpha 2-adrenoceptor antagonist, was tritiated to high specific activity. We then compared [3H]atipamezole and [3H]rauwolscine as radioligands for alpha 2-adrenoceptors in rat cerebral cortex, neonatal rat lung, and human platelets. (-)-Noradrenaline and phentolamine were used to define specific alpha 2-adrenergic binding. Unlabelled atipamezole was used in a similar manner to define saturable, high-affinity non-adrenergic binding. [3H]Atipamezole binding to human platelets (Kd 1.3 nM) and rat brain membranes (Kd 0.5 nM) equilibrated rapidly and was displaced in the expected manner by alpha 2-adrenergic ligands. In contrast, [3H]atipamezole binding in neonatal rat lung membranes was only effectively inhibited by unlabelled atipamezole, and by high concentrations of idazoxan. The total density of binding sites for [3H]atipamezole was clearly in excess of the density of alpha 2-adrenoceptors in this tissue, as defined by [3H]rauwolscine binding. We conclude that [3H]atipamezole binds with high affinity to alpha 2-adrenoceptors in human platelets and rat cerebral cortex, and that the compound can be used to investigate alpha 2-adrenoceptor properties and drug actions in these tissues. In neonatal rat lung, [3H]atipamezole identified an additional population of binding sites, distinct from both classical alpha 2-adrenoceptors and idazoxan-defined imidazoline receptors. The pharmacological identity of these binding sites remains to be elucidated. This non-adrenergic component in the binding characteristics of [3H]atipamezole complicates its use as a general alpha 2-adrenoceptor radioligand.

Central alpha 2-adrenoceptors are highly stereoselective for dexmedetomidine, the dextro enantiomer of medetomidine

To determine whether different alpha 2-adrenoceptor-mediated functions have different stereochemical requirements in the central nervous system, we studied the pharmacological activities of the purified optical enantiomers of medetomidine (dl-4-[1(2,3-dimethylphenyl)ethyl]-1H-imidazole), a specific and selective agonist of alpha 2-adrenoceptors. We found that dexmedetomidine (the dextro enantiomer) had the pharmacological activity of medetomidine. Dexmedetomidine had hypotensive and bradycardic actions in anaesthetized rats as well as sedative (decreased spontaneous locomotor activity and prolonged sleep induced by hexobarbital in rats), analgesic (attenuated a writhing response induced by acetic acid in mice) and midriatic actions in rats. The potency of dexmedetomidine was slightly greater than that of medetomidine. 1-Medetomidine was generally without pharmacological activity, but it showed some sedative and analgesic properties at high doses. Although the findings obtained with l-medetomidine might indicate some deviation from strict homogeneity, these experiments demonstrate that the different alpha 2-adrenoceptor-mediated functions have similar stereochemical requirements in the central nervous system.

Alpha 2-adrenoceptor-mediated inhibition of gastric acid secretion by medetomidine is efficiently antagonized by atipamezole in rats

The effect of a novel highly selective alpha 2-adrenoceptor agonist, medetomidine (4-[1-(2,3-dimethylphenyl)ethyl]-1H-imidazole), was studied on gastric secretion in conscious and anaesthetized rats. Medetomidine (3-30 micrograms/kg s.c.) inhibited basal gastric acid and fluid output in conscious rats in a dose-dependent manner, while in anaesthetized rats no effect was observed when administered i.v. at the doses of 1-1000 micrograms/kg. Furthermore, medetomidine did not modify gastric acid output stimulated by infusion of histamine i.v. in anaesthetized rats, suggesting also that a medetomidine-induced change in gastric secretion does not involve any action on histamine H2-receptors. Furthermore, evidence was given indicating that alpha 2-adrenoceptors mediate the antisecretory action of medetomidine, since a low dose (0.1 mg/kg s.c., -30 min) of a selective alpha 2-adrenoceptor antagonist, atipamezole [MPV 1248, 4-(2-ethyl-2,3-dihydro-1H-inden-2-yl)-1H-imidazole] efficiently antagonized the effects of medetomidine (30 micrograms/kg s.c.) in conscious rats. In summary, medetomidine inhibits gastric secretion in the rat via alpha 2-adrenoceptor activation, the antisecretory action being blocked by atipamezole. Probably due to its high specificity for alpha 2-adrenoceptors, medetomidine did not show a stimulatory effect on gastric acid secretion in anaesthetized rats.

Highly selective and specific antagonism of central and peripheral alpha 2-adrenoceptors by atipamezole

The potency, selectivity and specificity of atipamezole [MPV-1248, 4-(2-ethyl-2,3-dihydro-1H-inden-2-yl)-1H-imidazole], as an alpha 2-adrenoceptor antagonist was studied. In receptor binding studies [( 3H]-clonidine and [3H]-prazosin displacement) an alpha 2/alpha 1 selectivity ratio of 8526 was obtained for atipamezole, while idazoxan and yohimbine showed ratios of 27 and 40, respectively. Atipamezole had also about a 100 times higher affinity on alpha 2-adrenoceptors than the reference compounds. In the electrically stimulated prostatic portion of rat vas deferens, atipamezole showed potent competitive antagonistic activity against clonidine (pA2 8.6) and medetomidine (pA2 8.7) at presynaptic alpha 2-adrenoceptors. In the epididymal portion of the rat vas deferens, atipamezole had only weak competitive antagonistic activity against phenylephrine (pA2 5.0). In binding studies and studies with isolated organs, atipamezole had no effect on beta 1-, beta 2-, H1-, H2-, 5-HT1-, 5-HT2-, muscarine, DA2-, tryptamine, GABA, opiate and benzodiazepine receptors. In the pithed rat, atipamezole, like idazoxan, had partial alpha 1-adrenoceptor-mediated vasoconstrictor effects in addition to potent alpha 2-adrenoceptor blocking activity. In mice, medetomidine-induced sedation was effectively antagonized by atipamezole. These results show that atipamezole is a potent, selective and specific antagonist of both centrally and peripherally located alpha 2-adrenoceptors.

Characterization of the selectivity, specificity and potency of medetomidine as an alpha 2-adrenoceptor agonist

Medetomidine (4-[1-(2,3-dimethylphenyl)ethyl]-1H-imidazole) was tested for alpha 2-adrenoceptor agonist activity and compared to several reference agents. In binding studies carried out with rat brain membrane preparations, medetomidine showed high affinity for alpha 2-adrenoceptors, as measured by the displacement of [3H]clonidine (Ki 1.08 nM compared to 1.62, 3.20, 6.22 and 194 nM for detomidine, clonidine, UK 14,304 and xylazine, respectively). The affinity of medetomidine for alpha 1-adrenoceptors, as measured by [3H]prazosin displacement, was much weaker, yielding a relative alpha 2/alpha 1 selectivity ratio of 1620 which is 5-10 times higher than that of the reference compounds. Medetomidine caused a concentration-dependent inhibition of the twitch response in electrically stimulated mouse vas deferens with a pD2 value of 9.0 compared to that of 8.6, 8.5, 8.2 and 7.1 for detomidine, clonidine, UK 14,304 and xylazine, respectively. The effect of medetomidine was antagonized by idazoxan. In anaesthetized rats, medetomidine caused a dose-dependent mydriasis which could be reversed by alpha 2-adrenoceptor blockade. In receptor binding experiments and isolated organs medetomidine had no affinity or effects on beta 1-, beta 2-, H1, H2, 5-HT1, 5-HT2, muscarine, dopamine, tryptamine, GABA, opiate and benzodiazepine receptors. Based on these results, medetomidine can be classified as a potent, selective and specific alpha 2-adrenoceptor agonist.

Cardiovascular and sedative alpha-adrenoceptor effects of detomidine-like arylalkyl imidazoles and associated derivatives

The effects of detomidine (4-[(2,3-dimethylphenyl)methyl]-1H-imidazole, Domosedan) and its congeners, MPV compounds, are evaluated in the cardiovascular system and related to their sedative action using clonidine and xylazine for reference purposes. The structure of the MPV compounds had been modified with methyl substituents in the phenyl ring, with differing lengths in the alkyl bridge, and with variations in the imidazole terminus. The lipophilicity of the test compounds had been estimated in terms of apparent partition coefficients in the octanol/buffer (pH 7.4, 24-26 degrees C) using the HPLC technique. The lipophilicity of the MPV compounds was found to be higher than for either clonidine or xylazine, suggesting a more rapid penetration into the central nervous system. Some of these 21 MPV compounds were hypotensive and bradycardic in anaesthetized rats. Interestingly a number of these MPV compounds were only bradycardic but not clearly hypotensive after i.v. administration to anaesthetized rats, this being most obvious in the case of the 2,5-dimethylphenyl derivative MPV 867. A further characteristic distinguishing between the central and peripheral alpha-adrenoceptors regulating cardiovascular tone was a distinct inability of some derivatives active at peripheral cardiac presynaptic (pithed rats) and central (young chicks) alpha 2-adrenoceptors to reduce blood pressure in anaesthetized rats. The sedative action of the compounds after i.m. injection into 2- to 5-day-old chicks was in general related to their central hypotensive and bradycardic effect in anaesthetized rats, and with peripheral vasopressor and sympatho-inhibitory activity in pithed rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for medetomidine as a selective and potent agonist at alpha 2-adrenoreceptors

The activity on alpha-adrenoreceptors of medetomidine ((+/-)-4-(alpha,2,3-trimethylbenzyl)imidazole), an alpha-methyl derivative of detomidine, has been characterized in vivo and in vitro using detomidine, MPV 207, MPV 295, azepexole, clonidine and xylazine for reference purposes. Medetomidine (1-100 micrograms/kg i.v.) was a hypotensive and bradycardic compound in anaesthetized rats. Furthermore, it induced vasopressor (PD50 1.7 microgram/kg) and sympatho-inhibitory (ID50 1.6 microgram/kg) actions in pithed rats, the effects being antagonized by idazoxan (0.3 mg/kg i.v.) but not by prazosin (0.1 mg/kg i.v.). Medetomidine (30-300 micrograms/kg i.m.) had an alpha 2-adrenoreceptor mediated sedative effect on chicks. Medetomidine was, overall, more potent than detomidine, MPV 207, clonidine, xylazine, MPV 295 or azepexole in central (sedation in the chick) and peripheral (cardiac presynaptic in the pithed rat) actions on alpha 2-adrenoreceptors. Clonidine had, however, about an equal potency to medetomidine in the vascular smooth muscle of the pithed rat. Like detomidine and MPV 295, medetomidine had no agonistic activity in the rat aortic ring, but high concentrations antagonized methoxamine-induced contractions, giving a pA2 value of 5.68 for alpha 1-adrenoreceptor antagonism. The overall lipophilicity (log P') of medetomidine in the octanol/buffer (pH 7.4, 24-26 degrees C, HPLC technique) was 2.80. In summary, the experimental data suggest that medetomidine is a lipophilic compound with selective alpha 2-adrenoreceptor-stimulating properties and high potency. It may, therefore, prove to be a suitable pharmacologic tool for interventions in alpha 2-adrenoreceptor mediated effects in the autonomic nervous system.

Alpha-adrenoceptor activity of arylalkylimidazoles is improved by alpha-methylation and impaired by alpha-hydroxylation

To investigate the effects of hydroxyl and methyl substitution of the alkyl bridge bond on the alpha-adrenoceptor activity of arylalkylimidazole derivatives, the cardiovascular effects of the molecules were studied in anaesthetized and pithed rats. The compounds studied were 4(5)-substituted imidazole derivatives with a methano, ethano or etheno bridge between the imidazole and the 2-, 2,3- or 2,6-methyl substituted phenyl rings. The hypotensive and bradycardic activities of the molecules in the anaesthetized rat were always reduced by alpha-hydroxylation and usually augmented by alpha-methylation of the bridge between the imidazole and phenyl rings. Hydroxylation was associated with a consistent, marked decrease in vasopressor and sympatho-inhibitory activity in the cardiovascular system of the pithed rat, but a methyl moiety as a "bulky substituent" in the alpha-position of the alkyl bridge did not decrease but even caused an increase in alpha-adrenoceptor activity in this test system. The detrimental effect of alpha-hydroxylation of the compounds at alpha 1- and alpha 2-adrenoceptors supports the notion that the interaction of the imidazoles at alpha-adrenoceptor is different from that of the classical, noradrenaline-like phenethylamines. The results also suggest that the alkyl bridge between the phenyl and imidazole rings of the imidazoles may contribute directly to the binding process.

Re-evaluation of drug-interaction with alpha-adrenoceptors in vivo and in vitro using imidazole derivatives

The critical spatial dimension requirements for drug interaction with alpha-adrenoceptors were examined using imidazole derivatives MPV 295 and its semi-rigid analogue MPV 305 T (= trans) or MPV 305 C (= cis). The ethenyl bridge bond between the phenyl and imidazole moieties of MPV 305 prevents it achieving the critical spatial dimensions of the phenethylamines (e.g. norepinephrine). MPV 295 (0.03-10 mg/kg i.v.) and the trans-extended MPV 305 T (0.01-1 mg/kg i.v.) were hypotensive and bradycardic in anesthetised rats. In pithed rats, MPV 295 and MPV 305 T induced vasoconstriction, the doses giving a 50 mmHg rise in mean arterial pressure being 34.5 and 11.5 ug/kg, respectively. The pressor activity of MPV 295 was antagonized by idazoxan (1 mg/kg i.v.) but not by prazosin (0.1 mg/kg i.v.), whereas that of MPV 305 T was antagonized by prazosin and to a greater extent by idazoxan. Both compounds inhibited the increase in heart rate produced by electrical stimulation of the cardioaccelerator sympathetic nerve fibres in the pithed rats. The doses which induced a 50% inhibition of sympathetic transmission were 49.0 and 38.0 ug/kg for MPV 295 and MPV 305 T, respectively. This peripheral sympatho-inhibitory action was antagonized by idazoxan. Both compounds inhibited the twitch response of electrically stimulated mouse vas deferens, the pD2 values being 7.59 and 7.89 for MPV 295 and MPV 305 T, respectively. In the rat anococcygeus muscle only MPV 305 T was active (pD2 4.84). The cis-folded MPV 305 C was practically inactive in pithed rats and in rat anococcygeus muscle. According to the results, the strengthening of the ethano bridge of MPV 295 to that of MPV 305 T, thus preventing it fitting into the proposed dimensions of alpha-agonists, does not lead to a decrease in alpha-adrenoceptor mediated activities. Therefore, the spatial dimension requirements among imidazoles are different from those among the phenethylamines, supporting the concept that imidazoles interact differently with alpha-adrenoceptors when compared to the phenethylamines.

Cardiovascular action of detomidine, a sedative and analgesic imidazole derivative with alpha-agonistic properties

The cardiovascular effects of detomidine, a new veterinary sedative and analgesic imidazole derivative were studied in rats and cats using as reference compound xylazine, a widely employed veterinary antinociceptive and sedative drug with alpha-agonistic potency. Detomidine (1-30 micrograms/kg i.v.) and xylazine (10-1000 micrograms/kg i.v.) had both dose-dependent hypotensive and bradycardiac effects in anaesthetized rats. After i.v. administration of 3-100 micrograms/kg detomidine and 0.1-3 mg/kg xylazine to conscious rats, detomidine was more active in reducing the heart rate than in lowering the blood pressure. In anaesthetized cats, detomidine (1-30 micrograms/kg i.v.) was hypotensive and bradycardiac in a dose-dependent manner. A low dose of detomidine into the vertebral artery was more effective than i.v. application in reducing blood pressure. Idazoxan (0.3 mg/kg i.v. and 0.03 mg/kg into the vertebral artery) antagonized the hypotensive and bradycardiac effects of detomidine injected into the femoral vein or vertebral artery, respectively. In pithed rats, detomidine and xylazine stimulated presynaptic and postsynaptic alpha 2-adrenoceptors, and to a lesser extent postsynaptic alpha 1-adrenoceptors. The results indicate that detomidine is an agonist of central and peripheral alpha 2-adrenoceptors which exerts its hypotensive and bradycardiac effects via activation of the central alpha 2-adrenoceptors.

Peripheral cardiovascular alpha- and beta-adrenergic effects of some hypotensive and bradycardic arylalkyl imidazole derivatives in the rat

In pithed rats, a series of four alkyl bridge analogues of 4(5)-substituted arylalkyl imidazole induced alpha-adrenoceptor-mediated vasoconstriction and inhibition of electrically stimulated tachycardia. These effects were induced in the order of potency clonidine = MPV 207 greater than MPV 295 greater than MPV 304 greater than MPV 390, correlating with the length of the alkyl bridge between the phenyl and imidazole moieties. The peripheral postsynaptic actions of MPV 207 and MPV 304 were attenuated by prazosin (0.1 mg kg-1 i.v.) and yohimbine (1 mg kg-1 i.v.). The pressor responses induced by MPV 295 were antagonized only by yohimbine (0.3 and 1 mg kg-1 i.v.). The peripheral sympathoinhibitory action of these compounds was antagonized by yohimbine (1 mg kg-1 i.v.). In spontaneously beating rat atria, the MPV compounds showed neither agonistic nor antagonistic activity at cardiac postsynaptic alpha- and beta-adrenoceptors. The results indicate that the hypotensive and bradycardic MPV compounds are agonists at peripheral cardiovascular alpha-adrenoceptors. The extension of the alkyl bridge between the phenyl and imidazole moieties reduces their activity at alpha-adrenoceptors. Finally, MPV 295 seems to be a selective agonist of peripheral alpha 2-adrenoceptors in the cardiovascular system of the pithed rat.