TISSUE-SPECIFIC AND SPECIES-SPECIFIC DIFFERENCES IN LIGAND-BINDING TO THROMBOXANE-A2 RECEPTORS

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 [I-125]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. [I-125]BOP bound saturably to a single site on cultured vascular smooth muscle (VSM) cells from rabbit aortas and human saphenous veins with dissociation constants (K(d)) of 392 +/- 8 (n = 5) and 390 +/- 120 pM (n = 6) and binding capacities (B(max)) of 5,322 +/- 200 and 2,017 +/- 322 sites/cell, respectively. [I-125]BOP also bound saturably to one site on rabbit platelets (K(d) = 415 +/- 15 pM, B(max) = 594 +/- 43 sites/platelets, n = 4) but, in agreement with previous studies, to two sites on human platelets (high-affinity K(d) = 118 +/- 24 pM, B(max) = 121 +/- 33 sites/platelet; low-affinity K(d) = 1.1 +/- 0.47 nM, B(max) 232 +/- 23 sites/platelet, n = 4). [I-125]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. However, the human and rabbit VSM IC50 rank orders differed significantly (r = 0.466), suggesting that VSM TxA2 receptors are dissimilar in these species. Collectively, these receptor binding studies strongly suggest the existence of TxA2 receptors that are both tissue and species specific.