title: Tissue- and species-specific differences in ligand binding to thromboxane A2 receptors.
abstract: The ligand binding site of vascular smooth muscle (VSM) and platelet thromboxane A2 (TxA2) receptors was characterized in humans and rabbits using the TxA2 mimetic [125I]BOP. Vessel contraction and platelet aggregation studies demonstrated that unlabeled I-BOP and the prostaglandin H2 (PGH2) mimetic U-46619 were potent agonists in rabbit aortas, human saphenous veins, and washed human and rabbit platelets. [125I]BOP bound saturably to a single site on cultured vascular smooth muscle (VSM) cells from rabbit aortas and human saphenous veins with dissociation constants (Kd) of 392 +/- 8 (n = 5) and 390 +/- 120 pM (n = 6) and binding capacities (Bmax) of 5,322 +/- 200 and 2,017 +/- 322 sites/cell, respectively. [125I]BOP also bound saturably to one site on rabbit platelets (Kd = 415 +/- 15 pM, Bmax = 594 +/- 43 sites/platelet, n = 4) but, in agreement with previous studies, to two sites on human platelets (high-affinity Kd = 118 +/- 24 pM, Bmax = 121 +/- 33 sites/platelet; low-affinity Kd = 1.1 +/- 0.47 nM, Bmax 232 +/- 23 sites/platelet, n = 4). [125I]BOP was displaced from its binding site on rabbit and human VSM and platelets by stable TxA2/PGH2 analogues possessing either agonist or antagonist activity but not by other prostaglandins. The rank orders of the binding inhibition constants (IC50) for the TxA2/PGH2 analogues were compared among the four tissues and were highly correlated (r = 0.963) in VSM and platelets from rabbits but not humans (r = 0.699), suggesting that human VSM TxA2 receptors may be distinct from platelet TxA2 receptors. The IC50 rank order was also highly correlated (r = 0.935) between human and rabbit platelets.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: University of Texas Health Science Center, San Antonio.
authors.forename: G W
authors.initials: GW
authors.lastname: Dorn
mesh_terms: D000818:Animals; D001667:Binding, Competitive; D001792:Blood Platelets / Q000378:metabolism; D001643:Bridged Bicyclo Compounds / Q000378:metabolism; D019086:Bridged Bicyclo Compounds, Heterocyclic; D002478:Cells, Cultured; D005231:Fatty Acids, Unsaturated / Q000378:metabolism; D006801:Humans; D008024:Ligands; D009131:Muscle, Smooth, Vascular / Q000166:cytology / Q000378:metabolism / Q000648:ultrastructure; D011982:Receptors, Prostaglandin / Q000378:metabolism* / Q000502:physiology; D017482:Receptors, Thromboxane; D013045:Species Specificity; D013931:Thromboxanes / Q000378:metabolism
chemical_list: D001643:Bridged Bicyclo Compounds; D019086:Bridged Bicyclo Compounds, Heterocyclic; D005231:Fatty Acids, Unsaturated; D008024:Ligands; D011982:Receptors, Prostaglandin; D017482:Receptors, Thromboxane; D013931:Thromboxanes; C061750:7-(3-(3-hydroxy-4-(4'-iodophenoxy)-1-butenyl)-7-oxabicyclo(2.2.1)heptan-2-yl)-5-heptenoic acid