Discrimination between alpha 2-adrenoceptors and [3H]idazoxan-labelled non-adrenergic sites in rabbit white fat cells

Previous results indicated that the rabbit could represent a suitable model for investigations on the functional role of alpha 2-adrenoceptors in fat cells, but the characterization of these receptors was not resolved yet. In the present report, imidazoline compounds were used to attempt a better definition of rabbit adipocyte alpha 2-receptivity by means of lipolysis and binding studies. Lipolysis data showed that UK14304 is a full alpha 2-adrenoceptor agonist promoting a strong antilipolysis in rabbit fat cells. Moreover, the methoxy derivative of idazoxan, RX821002, is a more potent antagonist of UK14304-induced antilipolysis than idazoxan or yohimbine. Whereas [3H]yohimbine failed to bind at rabbit adipocyte alpha 2-adrenoceptors, [3H]UK14304 and [3H]RX821002 are valuable tools to study this receptor. Analysis of binding data demonstrated that [3H]UK14304 labels the high-affinity-state receptor while [3H]RX821002 binds to the whole alpha 2-adrenergic population. Inhibition studies of [3H]RX821002 and [3H]UK14304 binding by various compounds confirmed the alpha 2-adrenergic nature of the sites labelled by both radioligands. The other alpha 2-adrenoceptor radioligand, [3H]idazoxan, labelled binding sites which are insensitive to catecholamines. Competition studies of [3H]idazoxan binding with imidazoline derivatives revealed structure-activity relationships different from those of alpha 2-adrenoceptors. The most striking observation is that substitutions in the 2-position of idazoxan markedly reduce the affinity for the non-adrenergic sites, whereas the alpha 2-potency is increased or unchanged.