[H-3]RX821002 (2-methoxyidazoxan) binds to alpha(2)-adrenoceptor subtypes and a non-adrenoceptor imidazoline binding site in rat kidney

The binding of [H-3]RX821002 (2-methoxyidazoxan) was evaluated in rat kidney membranes. [H-3]RX821002 (0.13-16 nM) recognized a single, saturable binding site with high affinity. Different binding site densities were calculated depending on non-specific binding as defined by (-)-adrenaline or RX821002 (10 mu M). Competition assays using (-)-adrenaline and the subtype-selective drugs ARC 239 (2-[2-[4-(o-methoxyphenyl)-piperazin-1-yl]-ethyl]-4,4-dimethyl-1,3(2H,4H)-isoquinolindione), BRL 44408 (2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidazole), oxymetazoline or prazosin for [H-3]RX821002 binding sites revealed the presence of alpha(2B)-adrenoceptors (33-51%), alpha(2D)-adrenoceptors (15-28%) and an adrenaline-insensitive population (34-40%), sensitive to imidazolines. After the addition of(-)-adrenaline (3 mu M) to mask alpha(2)-adrenoceptors, [H-3]RX821002 specifically identified a saturable binding site with high affinity (K-d = 4.9 +/- 1.5 nM). The pharmacological profile of this non-adrenoceptor, [H-3]RX821002 binding site (potencies: efaroxan > clonidine > guanabenz > BRL 44408 > ARC 239 > BU 224 (2-(4,5-dihydroimidaz-2-yl)quinoline) > moxonidine > (-)-noradrenaline > cimetidine) is different to that of imidazoline I-1 or imidazoline I-2 binding sites. Alternative incubation in the presence of ARC 239 (50 nM) to mask alpha(2B)-adrenoceptors or BRL 44408 (100 nM) to mask alpha(2D)-adrenoceptors confirmed the existence of both alpha(2)-adrenoceptor subtypes and a non-adrenoceptor imidazoline binding site.