title: Labelling of human platelet alpha 2-adrenoceptors with the full agonist [3H](-)adrenaline.
abstract: [3H](-)Adrenaline has been used to characterize alpha 2-adrenoceptors on human platelets. Although (-)adrenaline is a good substrate for the platelet enzyme MAO-B, enzymatic inhibition was not a prerequisite to quantify the specific binding of the radioligand to platelet membranes. At 25 degrees C the binding was rapid (t1/2 of association: 10.3 min), reversible (t1/2 of dissociation: 4.0 min) and linearly dependent on the amount of protein present in the assay. The binding sites for [3H](-)adrenaline showed the specificity required for an alpha 2-adrenoceptor. The rank order of potency of inhibitors of [3H](-)adrenaline binding was oxymetazoline greater than idazoxan congruent to phentolamine congruent to clonidine congruent to (-)adrenaline greater than (-)noradrenaline greater than yohimbine much much greater than phenylephrine much greater than prazosin greater than (+)propranolol. Moreover, the nucleotide guanosine triphosphate (GTP) inhibited in a concentration-dependent manner (10(-9)-10(-4) M) the specific binding of [3H](-)adrenaline, suggesting that the radioligand preferentially labelled the high affinity state of the alpha 2-adrenoceptor. Linear (Scatchard) and non-linear analyses of [3H](-)adrenaline binding indicated the existence of a single population of non-interacting sites (KD = 2.5-2.7 nM; Bmax = 49-53 fmol/mg protein). The binding characteristics for [3H](-)adrenaline were not affected by age and sex of the donors or by freezing of platelet-rich plasma. In the same subjects alpha 2-adrenoceptor density (Bmax) for the full agonist [3H](-)adrenaline was 2.9-fold lower than that quantitated by the selective antagonist [3H]yohimbine. The inhibition constants (Ki) of adrenergic drugs and of various antidepressant drugs in competing with [3H](-)adrenaline were correlated with the inhibition constants of these drugs in competing with [3H]clonidine (r = 0.96; P less than 0.001) which suggests that both radioligands labelled the same alpha 2-adrenoceptor on the human platelet. The binding of the full agonist [3H](-)adrenaline to human platelet membranes might be a useful tool for the study of dysfunctions related to the high affinity state of the alpha 2-adrenoceptor.
mesh_terms: D000328:Adult; D001667:Binding, Competitive / Q000187:drug effects; D001792:Blood Platelets / Q000378:metabolism*; D001798:Blood Proteins / Q000378:metabolism; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D004837:Epinephrine / Q000097:blood* / Q000494:pharmacology; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D008996:Monoamine Oxidase Inhibitors / Q000494:pharmacology; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000378:metabolism*
chemical_list: D001798:Blood Proteins; D008996:Monoamine Oxidase Inhibitors; D011942:Receptors, Adrenergic, alpha; D004837:Epinephrine