Distinct platelet thromboxane A2/prostaglandin H2 receptor subtypes. A radioligand binding study of human platelets.
Thromboxane A2 (TXA2) and prostaglandin H2 (PGH2) may aggregate platelets via a common membrane receptor(s). To further characterize this receptor, binding of the radiolabeled TXA2/PGH2 mimetic [125I]BOP to washed human platelets (WP) was investigated. [125I]BOP was competitively displaced from its platelet binding site by stable TXA2/PGH2 analogues. Competition curves were shallow with Hill coefficients of -0.73 +/- 0.05 (P less than 0.001 different from unity) (90 +/- 1% specific binding). Scatchard plots were curvilinear and most consistent with two binding sites; a high-affinity site with Kd of 234 +/- 103 pM, Bmax of 0.7 +/- 0.3 pM/mg protein (180 +/- 87 sites/WP), and a lower affinity site with Kd of 2.31 +/- 0.86 nM, Bmax of 2.2 +/- 0.3 pM/mg protein (666 +/- 65 sites/WP). [125I]BOP association and dissociation kinetics gave a Kd of 157 pM without evidence of negative cooperativity. The EC50 for I-BOP-induced initial Ca2+ increase was 209 +/- 24 pM, shape change was 263 +/- 65 pM, and aggregation was 4.4 +/- 0.5 nM. Parallel binding studies using the TXA2/PGH2 receptor antagonist [125I]PTA-OH showed a single binding site. The rank order for TXA2/PGH2 analogues to displace [125I]PTA-OH was identical to that for [125I]BOP. These studies indicate that [125I]BOP binds to two distinct sites on human platelets that may represent platelet TXA2/PGH2 receptor subtypes. The close correlation of IC50 values for I-BOP-induced platelet shape change and aggregation with the two Kds for [125I]BOP binding suggests that these platelet responses may be independently mediated by the two putative receptors.
Dorn
G W
GW
Division of Cardiology, University of Texas Health Science Center, San Antonio 78284.
D001665:Binding Sites; D001667:Binding, Competitive; D001792:Blood Platelets / Q000378:metabolism*; D001643:Bridged Bicyclo Compounds / Q000378:metabolism / Q000494:pharmacology; D019086:Bridged Bicyclo Compounds, Heterocyclic; D002118:Calcium / Q000097:blood; D005231:Fatty Acids, Unsaturated / Q000378:metabolism / Q000494:pharmacology; D006801:Humans; D007700:Kinetics; D010974:Platelet Aggregation / Q000187:drug effects; D011463:Prostaglandins H / Q000037:antagonists & inhibitors; D011982:Receptors, Prostaglandin / Q000378:metabolism*; D017482:Receptors, Thromboxane; D044244:Receptors, Thromboxane A2, Prostaglandin H2; D013928:Thromboxane A2 / Q000031:analogs & derivatives / Q000037:antagonists & inhibitors / Q000378:metabolism
D001643:Bridged Bicyclo Compounds; D019086:Bridged Bicyclo Compounds, Heterocyclic; D005231:Fatty Acids, Unsaturated; D011463:Prostaglandins H; D011982:Receptors, Prostaglandin; D017482:Receptors, Thromboxane; D044244:Receptors, Thromboxane A2, Prostaglandin H2; C061750:7-(3-(3-hydroxy-4-(4'-iodophenoxy)-1-butenyl)-7-oxabicyclo(2.2.1)heptan-2-yl)-5-heptenoic acid; D013928:Thromboxane A2; C046957:I-PTA-OH; D002118:Calcium