Identification of human platelet alpha 2-adrenoceptors with a new antagonist [3H]-RX821002, a 2-methoxy derivative of idazoxan.
1. The binding of a new alpha 2-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. 2. Analysis of kinetic data revealed association and dissociation time courses consistent with a simple biomolecular reaction. Saturation isotherms showed that [3H]-RX821002 labelled a higher total number of alpha 2-binding sites (224 +/- 31 vs 168 +/- 24 fmol mg-1 protein) than [3H]-yohimbine and with higher affinity (Kd: 0.92 +/- 0.06 vs 1.51 +/- 0.08 nM). Moreover [3H]-RX821002 exhibited a lower percentage of nonspecific binding 3. The difference in total binding is due to a better labelling of the alpha 2-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 greater than oxymetazoline greater than UK14304 greater than (-)-adrenaline greater than prazosin greater than or equal to (+)-adrenaline greater than isoprenaline. This order of potency is classical for an alpha 2A-adrenoceptor. 5. RX821002 is a more potent alpha 2-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 alpha 2-adrenoceptors.
Galitzky
J
J
I.N.S.E.R.M. U.317, Université Paul Sabatier, Rue, Toulouse, France.
Senard
J M
JM
Lafontan
M
M
Stillings
M
M
Montastruc
J L
JL
Berlan
M
M
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology*; D000328:Adult; D001792:Blood Platelets / Q000737:chemistry / Q000378:metabolism*; D000068438:Brimonidine Tartrate; D004146:Dioxanes / Q000494:pharmacology*; D004837:Epinephrine / Q000494:pharmacology; D006801:Humans; D019329:Idazoxan / Q000031:analogs & derivatives; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D008566:Membranes / Q000187:drug effects / Q000378:metabolism; D010109:Oxymetazoline / Q000494:pharmacology; D011224:Prazosin / Q000494:pharmacology; D011810:Quinoxalines / Q000494:pharmacology; D011869:Radioligand Assay; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects*; D015016:Yohimbine / Q000494:pharmacology
D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D004146:Dioxanes; D011810:Quinoxalines; D011942:Receptors, Adrenergic, alpha; D015016:Yohimbine; D000068438:Brimonidine Tartrate; D010109:Oxymetazoline; C058525:2-methoxyidazoxan; D011224:Prazosin; D019329:Idazoxan; D004837:Epinephrine