IDENTIFICATION OF HUMAN PLATELET ALPHA-2-ADRENOCEPTORS WITH A NEW ANTAGONIST [H-3] RX821002, A 2-METHOXY DERIVATIVE OF IDAZOXAN

The binding of a new α2-adrenoceptor antagonist, [3H]-RX821002 (2-(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline), was investigated in human platelet membranes and compared with [3H]-yohimbine binding parameters. 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 ± 0.08 nM). Moreover [3H]-RX821002 exhibited a lower percentage of non-specific binding. 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. [3H]-RX821002 binding displayed a specificity similar to that obtained with [3H]-yohimbine. The potency of various compounds acting on adrenoceptors was: yohimbine > oxymetazoline > UK14304 > (-)-adrenaline > prazosin ≥ (+)-adrenaline > isoprenaline. This order of potency is classical for an α(2A)-adrenoceptor. RX821002 is a more potent α2-adrenoceptor antagonist than yohimbine on adrenaline-induced platelet aggregation. These results indicate that [3H]-RX821002 is a suitable ligand for the identification of human platelet α2-adrenoceptors.