Tissue- 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 [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 (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. [¹²⁵I]BOP also bound saturably to one site on rabbit platelets (K(d) = 415 ± 15 pM, B(max) = 594 ± 43 sites/platelet, 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 1/4 0.47 nM, B(max) 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. 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.