This study demonstrates that thromboxane A2 activates human platelets via two distinct receptor-effector systems, providing mechanistic insights into platelet aggregation and shape change.
Thromboxane A2 (TXA2) induces platelet shape change, secretion, and aggregation. Using a novel TXA2/prostaglandin endoperoxide receptor antagonist, 1r-[1 alpha(Z),2 beta,3 beta,5 alpha]-(+)-7-5-[[(1,1'- biphenyl)-4-ylmethoxy]-3-hydroxy-2-(1-piperidinyl) cyclopentyl]-4-heptenoic acid hydrochloride (GR32191), we demonstrate that these responses are mediated by at least two receptor-effector systems. GR32191 non-competitively inhibited platelet aggregation to the TXA2 mimetics, (15S)-hydroxy-11,9-(epoxymethano) prostadienoic acid (U46619) and 1S-(1 alpha,2 beta(5Z),3 alpha (1E,-3S), 4 alpha)-7-3-(3-hydroxy-4-(p-iodophenoxy)-1-butenyl)7- oxabicyclo[2.2.1hept-2yl]-5-heptenoic acid by binding irreversibly to a TXA2/prostaglandin endoperoxide receptor. Dissociation of 3HGR32191 from human platelets demonstrated two specific binding sites, one which was rapidly dissociating and a site to which binding was essentially irreversible. Stimulation by U46619 of platelets incubated with GR32191 and subsequently washed to expose the reversible binding site failed to aggregate or to secrete 3H5-hydroxy-tryptamine; formation of inositol phosphates and activation of protein kinase C were markedly suppressed. In contrast, platelet shape change and calcium stimulation remained at 90% of control. Furthermore, stimulation of the reversible binding site with U46619 induced aggregation in the presence of ADP, demonstrating its functional importance in amplifying the response to other agonists. These data suggest that TXA2 mediates platelet activation through at least two receptor-effector systems; one linked to phospholipase C activation, resulting in platelet aggregation and secretion and a second site mediating an increase in cytosolic calcium and platelet shape change.
Takahara et al. (1990) studied this question.