Molecular Pharmacology, Vol 19, 179-183, Copyright © 1981 by the American Society for Pharmacology and Experimental Therapeutics

Characterization of Alpha-Adrenergic Receptors in Human Platelets Using [³H]Clonidine

¹ Hematology-Oncology Section, Department of Medicine, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104, and Division of Pharmacology, Department of Medicine, University of California, La Jolla, California 92093

Pharmacological studies of human platelets indicate that epinephrine initiates platelet aggregation and secretion by interacting with alpha-adrenergic receptors on the platelet surface. Moreover, direct binding experiments using nonselective antagonists, such as [³H]dihydroergocryptine, indicate a single class of platelet alpha-adrenergic receptors. Thus, we sought to examine the interaction of the alpha₂-selective compound, clonidine, with human platelets in both pharmacological and radioligand binding studies. Although clonidine is considered an agonist in some tissues, we found that this compound was a mixed agonist-antagonist for alpha-adrenergic responses in platelets. Furthermore, [³H]clonidine bound to a single class of noncooperative binding sites on platelet membranes. Binding was rapid, reversible, of high affinity (K_d = 24.5 ± 2.1 nM) and low capacity (63.7 ± 4.2 fmoles/mg of protein). Alpha-adrenergic agonists and antagonists competed for [³H]clonidine binding sites with a rank order of potency typical for interaction at an alpha₂-adrenergic receptor. The binding of [³H]clonidine was enhanced in the presence of Mg²⁺ and decreased in the presence of GTP. The total number of [³H]clonidine binding sites was only 18% of the number of [³H]dihydroergocryptine binding sites in platelet membranes. These studies demonstrate that [³H]clonidine, a mixed agonist-antagonist, can be used to identify high-affinity alpha₂-adrenergic receptors on human platelets. Because [³H]clonidine sites are regulated by Mg²⁺ and GTP, these alpha₂-adrenergic receptors are likely to be the sites mediating alpha-adrenergic agonist responses in platelets.