[3H]Dihydroergocryptine binding to alpha-adrenergic receptors of human platelets. A reassessment using the selective radioligands [3H]prazosin, [3H]yohimbine, and [3H]rauwolscine.

Which subtype(s) of the alpha-adrenergic receptor occurs on human platelets? Studies of platelet responsiveness to adrenergic compounds and indirect radioligand binding studies addressing this question have yielded contradictory conclusions. These bindings studies employed the ligand [3H]dihydroergocryptine ( [3H]DHE), an alpha-adrenergic antagonist that does not select between alpha 1- and alpha 2-adrenergic receptors and that also binds to other receptor types in some tissues. To determine the subtype of the platelet alpha-adrenergic receptor, we have examined the binding to intact human platelets of [3H]prazosin (alpha 1-selective), [3H]yohimbine (alpha 2-selective), and [3H]rauwolscine (alpha 2-selective), and we have compared the binding of these selective radioligands with that of [3H]DHE. [3H]Yohimbine and [3H]rauwolscine both bound with high affinity (Kd = 2.7 and 4.6 nM, respectively) to an equal number and a single class (Hill coefficient approximately 1.0) of sites ( approximately 300 per platelet), but [3H]yohimbine yielded lower nonspecific binding than did [3H]rauwolscine. In paired experiments, [3H]DHE bound to 1.5 times as many (phentolamine-displaceable) sites as did [3H]yohimbine or [3H]rauwolscine. Unlabeled yohimbine and epinephrine competed for fewer [3H]DHE binding sites than did phentolamine. Thus, in addition to binding to the alpha 2-adrenergic receptors identified by [3H]yohimbine and [3H]rauwolscine, [3H]DHE seems to bind to other sites on human platelets. The nature of these sites is not clear. We found that [3H]prazosin did not identify alpha 1-adrenergic receptors on platelets, and that phenoxybenzamine only inhibited [3H]yohimbine and [3H]DHE binding at higher concentrations than usually observed for alpha 1-adrenergic receptors. We conclude that (1) all alpha-adrenergic sites on human platelets are of the alpha 2 subtype, (2) [3H]DHE may bind to additional, as yet ill-defined, sites in addition to those sites identified by [3H]yohimbine and [3H]rauwolscine, and (3) [3H]yohimbine is the preferred antagonist radioligand for studying the alpha 2-adrenergic receptors on human platelets.