title: Comparison of ligand binding affinities at human I1-imidazoline binding sites and the high affinity state of alpha-2 adrenoceptor subtypes.
abstract: To identify selective compounds for nonadrenergic I1-imidazoline receptors (I1), the affinities of 22 ligands for [125I]p-iodoclonidine binding have been compared at human platelet I1-imidazoline binding sites (analyzed under norepinephrine mask of alpha-2 AR) and at human alpha-2A, alpha-2B and alpha-2C adrenoceptors stably expressed on transfected Chinese hamster ovary cells. Competition curves at the platelet I1-imidazoline binding site were biphasic for most compounds. Only tizanidine and BDF,6143 displayed monophasic I1 competition curves. Agmatine, an endogenous neurotransmitter candidate for the I1-imidazoline receptor, was identified as the most selective agent for a subcomponent of platelet I1 sites. The affinity of agmatine at the high affinity component of platelet I1 sites was 1400-fold selective over alpha-2A adrenoceptors, 5000-fold selective over alpha-2B adrenoceptors and 800-fold selective over alpha-2C adrenoceptors. Moxonidine and tizanidine also displayed selectivities for a high affinity component of the platelet I1 binding sites over alpha-2 adrenoceptors. Naphazoline was the most selective compound for the high affinity state of the alpha-2A adrenoceptor, displaying 7-, 23- and 9-fold higher affinity than alpha-2B, alpha-2C and platelet I1-midazoline binding sites, respectively. No single selective compound was identified for the alpha-2B adrenoceptor. Norepinephrine displayed, respectively, 18- and 31-fold selectivity for the high affinity state of the alpha-2C adrenoceptor as compared to alpha-2A- or alpha-2B adrenoceptors, and was > 100,000- fold selective over platelet I1-imidazoline sites. Thus, human alpha-2 adrenoceptors and the platelet I1-imidazoline binding site can be clearly discriminated based on their affinities for certain compounds.
authors.affiliation: Department of Psychiatry and Human Behavior, University of Mississippi Medical Center, Jackson, USA.
authors.forename: J E
authors.initials: JE
authors.lastname: Piletz
authors.forename: H
authors.initials: H
authors.lastname: Zhu
authors.forename: D N
authors.initials: DN
authors.lastname: Chikkala
mesh_terms: D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D001792:Blood Platelets / Q000378:metabolism*; D016466:CHO Cells; D003000:Clonidine / Q000031:analogs & derivatives / Q000378:metabolism; D006224:Cricetinae; D019204:GTP-Binding Proteins / Q000378:metabolism; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D006801:Humans; D054755:Imidazoline Receptors; D009278:Naphazoline / Q000494:pharmacology; D011869:Radioligand Assay; D018341:Receptors, Adrenergic, alpha-2 / Q000378:metabolism*; D011955:Receptors, Drug / Q000378:metabolism*
chemical_list: D054755:Imidazoline Receptors; D018341:Receptors, Adrenergic, alpha-2; D011955:Receptors, Drug; C053113:4-iodoclonidine; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D019204:GTP-Binding Proteins; D009278:Naphazoline; D003000:Clonidine