Identification of human platelet a2-adrenoceptors with a new antagonist [3H]-RX821002, a 2-methoxy derivative of idazoxan

Abstract

1 The binding of a new α₂-adrenoceptor antagonist, [³H]-RX821002 (2-(2-methoxy-l,4-benzodioxan-2-yl)-2-imidazoline), was investigated in human platelet membranes and compared with [³H]-yohimbine binding parameters. 2 Analysis of kinetic data revealed association and dissociation time courses consistent with a simple bimolecular reaction. Saturation isotherms showed that [³H]-RX821002 labelled a higher total number of α₂-binding sites (224 ± 31 vs 168 ± 24 fmol mg⁻¹ protein) than [³H]-yohimbine and with higher affinity (K_d: 0.92 ± 0.06 vs 1.51 ± 0.08nm). Moreover [³H]-RX821002 exhibited a lower percentage of nonspecific binding. 3 The difference in total binding is due to a better labelling of the α₂-adrenoceptors in the low affinity state by [³H]-RX821002 since the labelled receptors number in high affinity state was identical with the two radioligands. 4 [³H]-RX821002 binding displayed a specificity similar to that obtained with [³H]-yohimbine. The potency of various compounds acting on adrenoceptors was: yohimbine > oxymetazoline > UK 14304 > (−)−adrenaline > prazosin ≥ (+)-adrenaline > isoprenaline. This order of potency is classical for an α_2A-adrenoceptor. 5 RX821002 is a more potent α₂-adrenoceptor antagonist than yohimbine on adrenaline-induced platelet aggregation. 6 These results indicate that [³H]-RX821002 is a suitable ligand for the identification of human platelet α₂-adrenoceptors.