RX 821002 hydrochloride

Antagonists that differentiate between alpha 2A-and alpha 2D-adrenoceptors.[Pubmed:8692278]

Naunyn Schmiedebergs Arch Pharmacol. 1996 Feb;353(3):245-9.

Four antagonists were examined for their ability to differentiate alpha 2A-from the orthologous alpha 2D-adrenoceptors. The antagonists were (2S,12bS)1',3'-dimethylspiro(1,3,4,5',6,6',7,12b-octah ydro-2H- benzo[b]furo[2,3-a]quinolizine)-2,4'-pyrimidin-2'-one (MK912), 2-[2-(methoxy-1,4-benzodioxanyl)imidazoline (RX 821002), efaroxan and benoxathian. The alpha 2-autoreceptors in rabbit brain cortex were chosen as alpha 2A-and the alpha 2-autoreceptors in guinea-pig brain cortex as alpha 2D-adrenoceptors. Slices of the brain cortex were preincubated with 3H-noradrenaline and then superfused and stimulated electrically by brief pulse trains (4 pulses, 100 Hz) that led to little, if any, alpha 2-autoinhibition. 5-Bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) was used as an alpha 2-adrenoceptor agonist. UK 14, 304 decreased the stimulation-evoked overflow of tritium. The antagonists shifted the concentration-inhibition curve of UK 14, 304 to the right in an apparently competitive manner. Dissociation constants of the antagonists were calculated from the shifts. MK 912, RX 821002 and efaroxan had markedly higher affinity for (guinea-pig) alpha 2D-adrenoceptors (pKd values 10.0, 9.7 and 9.1, respectively) than for (rabbit) alpha 2A-adrenoceptors (pKd 8.9, 8.2 and 7.6, respectively). Benoxathian had higher affinity for alpha 2A-(pKd 7.4) than for alpha 2D-adrenoceptors (pKd 6.9). Ratios calculated from the Kd values of the four compounds differentiated between alpha 2A and alpha 2D up to 100 fold. It is concluded that MK 912, RX 821002, efaroxan and benoxathian are antagonists with high power to differentiate alpha 2A-from alpha 2D-adrenoceptors.

Does [3H]2-methoxy-idazoxan (RX 821002) detect more alpha-2-adrenoceptor agonist high-affinity sites than [3H]rauwolscine? A comparison of nine tissues and cell lines.[Pubmed:7791100]

J Pharmacol Exp Ther. 1995 Jun;273(3):1287-94.

The authors compared [3H]2-methoxy-idazoxan (RX 821002) and [3H]rauwolscine binding in rat cerebral cortex, spleen and kidney; guinea pig kidney; porcine kidney; human kidney and platelets and HEL and NG 108-15 cells. [3H]RX 821002 had less nonspecific binding and higher affinity than [3H]rauwolscine in most models. Although both ligands detected similar alpha-2 adrenoceptor numbers in rat, porcine and human kidney and in NG 108-15 cells in saturation experiments, [3H]RX 821002 detected more alpha-2 adrenoceptors than [3H]rauwolscine in rat cerebral cortex and spleen, guinea pig kidney, human platelets and HEL cells. These differences were seen in Tris and in glycylglycine buffer regardless of whether EDTA, MgCl2, MgCl2 plus GTP or GTP plus NaCl was added to the former and were not explained by additional labeling of serotonin or dopamine receptors or nonadrenergic sites; in contrast, [3H]rauwolscine also labeled nonadrenergic sites in porcine kidney. In prazosin competition experiments, both ligands differentially recognized alpha-2-adrenoceptor subtypes but this could not account for the observed differences in detected receptor numbers. In epinephrine competition experiments, both ligands labeled similar numbers of agonist low affinity sites in all models; [3H]RX 821002, however, labeled more agonist high-affinity sites than [3H]rauwolscine did in models in which it detected a greater total number of receptors. It was concluded that [3H]RX 821002 is a more suitable ligand for the detection of alpha-2 adrenoceptor than [3H]rauwolscine because of less nonspecific binding, higher affinity and greater specificity for alpha-2 adrenoceptors; moreover, [3H]rauwolscine appears not to detect all agonist high-affinity sites of alpha-2 adrenoceptors.

Characterization of [3H]RX821002 binding to alpha-2 adrenergic receptor subtypes.[Pubmed:7908054]

J Pharmacol Exp Ther. 1994 Mar;268(3):1362-7.

Alpha-2 adrenergic receptors have been divided into four pharmacological subtypes based on their differences in affinity for several drugs. Previous studies showed that [3H]RX821002 has a high affinity for the alpha-2A subtype. The current study characterized the binding properties of [3H]RX821002 [2-(2-methoxy-1,4- benzodioxan-2yl)-2-imidazoline] to the alpha-2A receptor in CHO-C10 cells, alpha-2B in neonatal rat lung, alpha-2C in OK cells and alpha-2D in bovine pineal gland. Membrane binding studies of [3H]RX821002 were done in 25 mM glycylglycine buffer at room temperature. The nonspecific binding rates at the KD concentration were 4.9%, 20%, 14% and 8.3% of the total for CHO-C10, neonatal rat lung, OK cells and bovine pineal, respectively, which were determined by adding 100 microM norepinephrine. Saturation curves indicate that [3H]RX821002 has a high affinity for all alpha-2 adrenergic subtypes. The KD values were 0.29, 1.05, 0.37 and 0.19 nM for CHO-C10, neonatal rat lung, OK cells and bovine pineal, respectively. [3H]Rauwolscine has affinities of 0.34, 0.55 and 0.24 nM for the alpha-2A, -2B and -2C subtypes. By contrast, [3H]rauwolscine has a much lower affinity for alpha-2D subtype with a KD value of 5.2 nM. The binding site density for [3H]RX821002 was significantly lower in the neonatal rat lung compared with [3H]rauwolscine. The correlation coefficients of pKi values of adrenergic compounds against [3H]RX821002 versus [3H]rauwolscine were close to unity for each tissue. These data clearly show that the two ligands label the same alpha-2 adrenergic receptor population.(ABSTRACT TRUNCATED AT 250 WORDS)