European Journal of Pharmacology, 159 (1989) 199-203 199 Elsevier EJP 20276 Short communication [3H]Idazoxan binding at non-a2-adrenoceptors in rabbit adipocyte membranes Dominique Langin and Max Lafontan * INSERM U.317, Institut de Physiologie, Unioersit~ Paul Sabatier, Rue Francois Magendie, 31400 Toulouse, France Received 23 September 1988, accepted8 November 1988 The imidazoline ligand, [3H]idazoxan, labels a large population of high-affinity binding sites in rabbit fat cell membranes (Bmax = 1370_ 91 fmol/mg protein; KD= 1.6 + 0.6 nM) when imidazoline derivatives are used for definition of non-specific binding. [3H]Idazoxan sites are not a2-adrenoceptors as assessed by competition studies which showed that epinephrine, norepinephrine and yohimbine do not inhibit [3H]idazoxan binding. Naphazoline, tramazoline and the Na+/H + exchange inhibitor, amiloride, completely inhibited [3H]idazoxan binding. The K i values were 9, 27 and 48 nM, respectively. [3H]Idazoxan; Imidazoline; Amiloride; ct2-Adrenoceptors; Adipocytes; (Rabbit) 1. Introduction The definition of the role of fat cell a2-adreno- ceptors, mainly in humans, has attracted the at- tention of various laboratories because these re- ceptors could be responsible for the hypore- sponsiveness to physiological amines of the adipocytes of some fat deposits. The search for reliable animal models to characterize the role and the regulation of fat cell a2-adrenoceptors has been one of the aims of our laboratory in the last years. Previous data from our group have high- lighted the originality of rabbit fat cells, which are characterized by a weak lipolytic responsiveness to physiological amines. This response could be explained by an increased a2-responsiveness (Lafontan, 1981). Binding studies and aE-adreno- ceptor characterization have still not been at- tempted for rabbit fat cells although the rabbit * To whomall correspondenceshould be addressed: INSERM U.317, Institut de Physiologie, Universit~Paul Sabatier, Rue Francois Magendie,31400 Toulouse, France, could represent a good model for a a2-adrenocep- tor investigations in fat cells. In recent years selective 3H-antagonists ([3H] yohimbine, [3H]rauwolscine, [3H]idazoxan) or 3H- agonists ([3H]clonidine, [3H]paraminoclonidine, [3H]UK-14,304) have been used as probes for aE-adrenoceptor identification in a large variety of tissues and species. Biological assays and binding analyses have revealed species differences in a 2- adrenoceptor characteristics. However, not only is there no explanation of this apparent heteroge- neity but the physiological relevance of these dis- crepancies is quite unclear (Bylund, 1985). In this study, an attempt to characterize rabbit fat cell a2-adrenoceptors revealed some striking differences in the binding properties of some well recognized a2-3H-ligands. We studied binding with crude plasma membranes from rabbit fat cells with [3H]yohimbine, the imidazoline antag- onist [3H]idazoxan, and the imidazolidine agonist [3H]UK-14,304. A [3H]idazoxan binding compo- nent of large capacity and high affinity to non-a 2- adrenoceptors in rabbit fat cells was identified. Preliminary characterization of these binding sites 0014-2999/89/$03.50 © 1989 Elsevier SciencePublishers B.V. (BiomedicalDivision)