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Kinetic studies on stereospecific recognition by the thromboxane A2/prostaglandin H2 receptor of the antagonist, S‐145

1 The mechanism for the stereospecific recognition of the antagonist S‐145 by the thromboxane A2 (TXA2)/prostaglandin H2 (PGH2) receptor was examined by ligand‐binding techniques in rat vascular smooth muscle cells (VSMCs) and in human platelet membranes. 2 Scatchard analysis revealed the existence of a single class of binding sites with the same maximum number for both [3H]‐(+)‐S‐145 and [3H]‐(−)‐S‐145 in both cell types. The dissociation constants (Kd) for the binding of the (+)‐isomer in rat VSMCs and human platelet membranes were, respectively, 0.40 ± 0.03 and 0.20 ± 0.02 nm, each value being lower than that for the (−)‐isomer (3.57 ± 0.74 and 2.87 ± 0.08 nm, respectively). 3 The rank orders of potency (Ki) for a series of TXA2/PGH2 ligands at inhibiting [3H]‐(+)‐S‐145 binding were highly correlated with those determined for [3H]‐(−)‐S‐145 binding in both cell preparations. 4 Kinetic analysis of the binding of both radioligands revealed a much lower dissociation rate constant (k−1) and a slightly greater association rate constant (k1) for the (+)‐isomer compared to those for the (−)‐isomer. 5 These results suggest that it is at the stage of dissociation from the TXA2/PGH2 receptor that the stereochemistry of the optical isomers of S‐145 confers their difference in affinity for these receptors in rat VSMCs and human platelet membranes.