[³H]-idazoxan binds with high affinity to two sites on hamster adipocytes: An α₂-adrenoceptor and a non-adrenoceptor site

[³H]-Idazoxan labels a single population of high affinity sites (K(d) 2.26 ± 0.02 nM; B(max) 372 ± 25 fmol mg^-1 protein) in hamster adipocyte membranes. In the presence of 1 µM yohimbine to preclude binding to a₂-adrenoceptors, the density of [³H]-idazoxan binding sites was reduced (287 ± 18 fmol mg^-1 protein) without an apparent decrease in the affinity (K(d) 2.19 ± 0.24 nM) of the radioligand. Displacement studies indicate that a-adrenoceptor ligands with an imidazoline side chain completely inhibit [³H]-idazoxan binding to hamster adipocyte membranes; in contrast, the a₂-adrenoceptor antagonists yohimbine, rauwolscine, BDF 6143 and phentolamine inhibited only 20-30% of the specific binding with affinity values consistent with an interaction at a₂-adrenoceptors. The low potency of noradrenaline and adrenaline in displacing [³H]-idazoxan binding to the second site on hamster adipocyte membranes indicates that it is unlikelky that this site is a type of adrenoceptor. These results suggests that [³H]-idazoxan binds with high affinity to two sites in hamster adipocytes: an a₂-adrenoceptor and a non-adrenoceptor imidazoline site.