title: Delineation of three pharmacological subtypes of alpha 2-adrenoceptor in the rat kidney.
abstract: 1. Simultaneous computer modelling of plain and ARC 239- and guanoxabenz-masked [3H]-RX821002 saturation curves, plain ARC 239 and guanoxabenz competition curves as well as ARC 239-masked guanoxabenz competition curves revealed that the drugs bound to three alpha 2-adrenoceptor subtypes in the rat kidney with grossly differing selectivities. These alpha 2-adrenoceptor subtypes were termed alpha 2 A, alpha 2B1 and alpha 2B2. The order of affinities for [3H]-RX821002 for the adrenoceptor sites was alpha 2A greater than alpha 2B1 greater than alpha 2B2, the KdS being 0.62 +/- 0.05, 2.52 +/- 0.11 and 6.74 +/- 1.21 nM, respectively. The order of affinities for ARC 239 was alpha 2B1 greater than alpha 2B2 much greater than alpha 2A with KdS 4.78 +/- 1.04, 28.8 +/- 4.1 and 1460 +/- 270 nM, respectively. For guanoxabenz the order of affinities was alpha 2A greater than alpha 2B1 much greater than alpha 2B2 with KdS 99.7 +/- 15.1, 508 +/- 135 and 25,400 +/- 2400 nM, respectively. 2. Binding constants for 14 compounds for the three rat kidney alpha 2-adrenoceptor subtypes were determined by the simultaneous computer modelling of plain and ARC 239- and guanoxabenz-masked drug competition curves, plain ARC 239 and guanoxabenz competition curves as well as ARC 239-masked guanoxabenz competition curves. Of the 14 compounds tested, oxymetazoline and guanfacine were found to bind with low affinities to both of the alpha 2B1- and alpha 2B2-adrenoceptor but with high affinity to the alpha 2A-adrenoceptor. 3. (-)-Adrenaline and (-)-noradrenaline showed dissimilar order of affinities for the three alpha2-adrenoceptors. For (-)-adrenaline the order of affinities was alpha2Bl >- alpha2A> alpha2B2 and for (-)-noradrenaline alpha2B2 > alpha2Bl > alpha2A. All three alpha2-adrenoceptors showed the expected stereoselective binding for adrenaline enantiomers, the (+)-form being 7-10 fold less potent than the (-)form. 4. [3H]-yohimbine was also used as radioligand. The data with this ligand were fully compatible with the [3H]-RX821002 data. However, [3H]-yohimbine appeared to label only alpha2Bl- and alpha2B2-adrenoceptors presumably because it had too low an affinity for alpha2A-adrenoceptors. 5. We conclude that three pharmacological subtypes of alpha2-adrenoceptors are labelled by [3H]-RX821002 in the rat kidney. Guanoxabenz and ARC 239 may be used in competition studies to delineate between these three alpha2-adrenoceptor subtypes. Moreoever, we here present a method allowing the determination of binding constants for an arbitrary drug to the three alpha2-adrenoceptor subtypes.
authors.affiliation: Department of Pharmacology, Umeå University, Sweden.
authors.forename: S
authors.initials: S
authors.lastname: Uhlén
authors.forename: J E
authors.initials: JE
authors.lastname: Wikberg
mesh_terms: D000818:Animals; D001667:Binding, Competitive / Q000187:drug effects; D003198:Computer Simulation; D004146:Dioxanes / Q000494:pharmacology; D006143:Guanabenz / Q000031:analogs & derivatives / Q000494:pharmacology; D019329:Idazoxan / Q000031:analogs & derivatives; D066298:In Vitro Techniques; D007668:Kidney / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D011869:Radioligand Assay; D051381:Rats; D011919:Rats, Inbred Strains; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects*; D015016:Yohimbine / Q000378:metabolism
chemical_list: D004146:Dioxanes; D011942:Receptors, Adrenergic, alpha; D015016:Yohimbine; C058525:2-methoxyidazoxan; D006143:Guanabenz; C028782:guanoxabenz; D019329:Idazoxan