Imidazolinic radioligands for the identification of hamster adipocyte alpha 2-adrenoceptors.
Imidazolinic radioligands ([3H]UK 14304, [3H]idazoxan and [3H]RX 821002) were used for the identification of alpha 2-adrenoceptors on hamster fat cell membranes since there are limitations to the use of [3H]yohimbine and [3H]clonidine, which suggest alpha 2-adrenoceptor heterogeneity. Biological assays (lipolysis measurements) were performed on isolated fat cells and binding studies were carried out on fat cell membranes. The imidazolinic derivative, UK 14304, was a full agonist as compared to clonidine. Idazoxan and RX 821002 (2-(2-methoxy-1,4-benzodioxan-2yl)-2-imidazoline), a recently developed alpha 2-antagonist, were more potent alpha 2-antagonists than yohimbine in this fat cell model. [3H]UK 14304 was the most suitable agent for the quantification of the 'high-affinity state' alpha 2-adrenoceptors in binding studies since it did not exhibit the sensitivity to the composition of the buffer shown by [3H]clonidine. Although it is a potent alpha 2-antagonist, [3H]idazoxan had major limitations for use in the identification of alpha 2-adrenoceptors in this cell model since it also bound to 'non-adrenaline displaceable' binding sites which were revealed when imidazolinic derivatives (phentolamine) were used instead of adrenaline to determine the non-specific binding. We demonstrated that [3H]RX 821002 was a more suitable radioligand than [3H]yohimbine for labelling hamster fat cell alpha 2-adrenoceptors (KD = 1.0 +/- 0.1 nM, Bmax = 776 +/- 60 fmol/mg protein). Moreover, since it exhibited low affinity for 'imidazoline-preferring sites', it represents a valuable ligand even in tissues possessing such binding sites. We suggest that [3H]RX 821002 can be used to identify alpha 2-adrenoceptors in various tissues when these sites cannot be labelled with [3H]yohimbine.
Saulnier-Blache
J S
JS
I.N.S.E.R.M., Université Paul Sabatier, Toulouse, France.
Carpéné
C
C
Langin
D
D
Lafontan
M
M
D000273:Adipose Tissue / Q000187:drug effects / Q000378:metabolism*; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D000068438:Brimonidine Tartrate; D003000:Clonidine / Q000494:pharmacology; D006224:Cricetinae; D004146:Dioxanes / Q000494:pharmacology; D019329:Idazoxan / Q000031:analogs & derivatives; D007093:Imidazoles / Q000378:metabolism*; D066298:In Vitro Techniques; D007700:Kinetics; D050356:Lipid Metabolism; D008066:Lipolysis; D008297:Male; D008647:Mesocricetus; D011810:Quinoxalines / Q000494:pharmacology; D011869:Radioligand Assay; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism*; D015016:Yohimbine / Q000494:pharmacology
D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D004146:Dioxanes; D007093:Imidazoles; D011810:Quinoxalines; D011942:Receptors, Adrenergic, alpha; D015016:Yohimbine; D000068438:Brimonidine Tartrate; C058525:2-methoxyidazoxan; D003000:Clonidine; D019329:Idazoxan