Non-adrenergic sites for imidazolines are not directly involved in the alpha 2-adrenergic antilipolytic effect of UK 14304 in rat adipocytes.
The binding of the alpha 2-agonist [3H]UK 14304 on Wistar rat adipocyte membranes was separated in two distinct components: one was displaceable by adrenaline or other alpha 2-adrenergic agents and possessed the characteristics of alpha 2-adrenoceptors while the other, non-adrenergic in nature, was only recognized by some imidazoline derivatives [3H]idazoxan binding shared the same characteristics. The non-adrenergic sites labeled by both radioligands are similar to those described for [3H]idazoxan on other tissues such as brain cortex, smooth muscle and kidney. Even though they were about 10-fold more numerous than the true alpha 2-adrenoceptors, the non-adrenergic binding sites were not directly involved in the antilipolytic action of UK 14304 since alpha 2-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 alpha 2-adrenoceptors in rat adipocytes. The use of [3H]RX 821002 (2-(2-methoxy-1,4-benzodioxan-2yl)imidazoline) allowed an accurate quantification of rat adipocyte alpha 2-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 alpha 2-blocking properties among the various alpha 2-antagonists tested. [3H]RX 821002 binding analysis revealed that alpha 2-adrenoceptors are, on rat adipocytes; (i) less numerous than in other species well known for their alpha 2-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 alpha 2-B subtype since the affinity of prazosin was lower than that of oxymetazoline in displacing [3H]RX 821002 or [3H]yohimbine binding.
Carpéné
C
C
I.N.S.E.R.M. Unité 317, Institut de Physiologie, Université Paul Sabatier, Toulouse, France.
Galitzky
J
J
Larrouy
D
D
Langin
D
D
Lafontan
M
M
D000273:Adipose Tissue / Q000378:metabolism*; D000316:Adrenergic alpha-Agonists / Q000378:metabolism / Q000494:pharmacology*; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D000068438:Brimonidine Tartrate; D003994:Bucladesine / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D005576:Colforsin / Q000494:pharmacology; D004146:Dioxanes / Q000378:metabolism / Q000494:pharmacology; D019329:Idazoxan / Q000031:analogs & derivatives; D007700:Kinetics; D008066:Lipolysis / Q000187:drug effects*; D010646:Phentolamine / Q000494:pharmacology; D011810:Quinoxalines / Q000378:metabolism / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism*; D015016:Yohimbine / Q000494:pharmacology
D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D004146:Dioxanes; D011810:Quinoxalines; D011942:Receptors, Adrenergic, alpha; D005576:Colforsin; D015016:Yohimbine; D000068438:Brimonidine Tartrate; D003994:Bucladesine; C058525:2-methoxyidazoxan; D019329:Idazoxan; D010646:Phentolamine