[alpha]2-adrenoreceptors on human platelets: Selective labelling by [3H]clonidine and [3H]yohimbine and competetive inhibition by antidepressant drugs

Abstract

[3H]Clonidine and [3H]yohimbine have been used to characterize [alpha]2-adrenoreceptors on human platelets. At 25[deg]C binding was rapid (t1/2 of association, 1.8 and 2.7 min) and reversible (t1/2 of dissociation, 0.5 and 8.2 min). The binding sites for [3H]clonidine and [3H]yohimbine showed the specificity required for an [alpha]2-adrenoreceptor. The rank order of potency of inhibitors of [3H] clonidine binding was clonidine > yohimbine >> phenylephrine > prazosin and of [3H]yohimbine binding was yohimbine > clonidine >> phenylephrine > prazosin. Scatchard analysis of [3H]yohimbine binding indicated the existence of a single population of noninteracting sites (KD = 3.0 nM; Bmax = 188 fmol/mg protein). The high-affinity binding of [3H]clonidine had a lower affinity and a lower number of sites (KD = 5.0 nM; Bmax = 35 fmol/mg protein). [3H]Clonidine binding also showed evidence of a second site of much lower affinity and greater number (KD = 18.6 nM; Bmax = 77 fmol/mg protein) in 40% of the normal population. In vitro, antidepressant drugs competed with [3H]clonidine and [3H]yohimbine for the platelet [alpha]2-adrenoreceptor. The rank order of potency of inhibitors of [3H]clonidine binding was mianserin > amitriptyline >> iprindole > desipramine and of [3H]yohimbine binding was mianserin > amitriptyline >> desipramine > iprindole. The inhibition constants (Ki) of adrenergic drugs and of various antidepressant drugs in competing with [3H]clonidine were correlated with the inhibition constants of these drugs in competing with [3H]yohimbine (r=0.970; P2-adrenoreceptor on the human platelet. The inhibition of binding induced by all antidepressant drugs was competitive. In contrast, long-term administration of tricyclic antidepressant drugs to patients was recently found to be associated with a decrease in the number of binding sites for [3H]clonidine on platelet membranes. The present results indicate that both [3H]clonidine and [3H]yohimbine are useful tools for the quantification of [alpha]2-adrenoreceptors on blood platelets and suggests that the specific binding of radiolabelled [alpha]2-adrenoreceptor ligands to human platelet membranes might be used to monitor changes in [alpha]2-adrenoreceptors during tricyclic antidepressant drug treatment.