NONADRENERGIC SITES FOR IMIDAZOLINES ARE NOT DIRECTLY INVOLVED IN THE ALPHA-2-ADRENERGIC ANTILIPOLYTIC EFFECT OF UK-14304 IN RAT ADIPOCYTES

The binding of the α2-agonist [3pH]UK 14304 on Wistar rat adipocyte membranes was separated in two distinct components: one was displaceable by adrenaline or other α2-adrenergic agents and possessed the characteristics of α2-adrenoceptors while the other, non-adrenergic in nature, was only recognized by some imidazoline derivatives. [3pH]Idazoxan binding shared the same characteristics. The non-adrenergic sites labeled by both radioligands are similar to those described for [3pH]idazoxan on other tissues such as brain cortex, smooth muscle and kidney. Even though they were about 10-fold more numerous than the true α2-adrenoceptors, the non-adrenergic binding sites were not directly involved in the antilipolytic action of UK 14304 since α2-antagonists devoid of interaction with these sites (yohimbine, phentolamine) totally blocked the UK 14304 effect. However, the existence of such a type of site impairs direct quantification of α2-adrenoceptors in rat adipocytes. The use of [3pH]RX 821002 (2-(2-methoxy-1, 4-benzodioxan-2yl)imidazoline) allowed an accurate quantification of rat adipocyte α2-adrenoceptors (Bmax = 35 ± 2 fmol mg protein, Kd = 2.6 ± 0.6 nM) since it did not interact with non-adrenergic binding sites and exhibited the highest α2-blocking properties among the various α2-antagonists tested. [3pH]RX 821002 binding analysis revealed that α2-adrenoceptors are, on rat adipocytes; (i) less numerous than in other species well known for their α2-adrenergic inhibitory regulation of lipolysis (human, hamster, rabbit); (ii) slightly different in nature from the receptors of these species since they had weaker affinity for clonidine and yohimbine; and however (iii) not of the typical α2-B subtype since the affinity of prazosin was lower than that of oxymetazoline in displacing [3pH]RX 821002 or [3pH]yohimbine binding. © 1990.