Abstract
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 (Kd = 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.
- Copyright © 1981 by The American Society for Pharmacology and Experimental Therapeutics
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