Alpha 2-adrenoreceptors on human platelets: selective labelling by [3H]clonidine and [3H]yohimbine and competitive inhibition by antidepressant drugs.

[3H]Clonidine and [3H]yohimbine have been used to characterize alpha 2-adrenoreceptors on human platelets. At 25 degrees C binding was rapid (t 1/2 of association, 1.8 and 2.7 min) and reversible (t 1/2 of dissociation, 0.5 and 8.2 min). The binding sites for [3H]clonidine and ]3H]yohimbine showed the specificity required for an alph 2-adrenoreceptor. The rank order of potency of inhibitors of [3h[clonidine binding was clonidine greater than yohimbine greater than phenylephrine greater than prazosin and of [3H]yohimbine binding was yohimbine greater than clonidine greater than phenylephrine greater than 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 greater than amitriptyline greater than iprindole greater than desipramine and of [3H]yohimbine binding was mianserin greater than amitriptyline greater than desipramine greater than 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; P less than 0.001) which suggests that both radioligands labelled the same alpha 2-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.