Binding of an 125I-labeled thromboxane A2/prostaglandin H2 receptor antagonist to washed canine platelets

A binding site for the thromboxane A2/prostaglandin H2 (TXA2/PGH2) antagonist 125I-PTA-OH (9,11-dimethylmethano-11,12-methano-16-(4-methoxyphenyl)-13,14-dih ydro-13-aza-1 5 alpha beta-w-tetranor-TXA2) to washed canine platelets is described. 127I-PTA-OH competitively antagonized aggregation induced by the TXA2/PGH2 mimetic U46619. A Schild analysis of the pharmacologic study revealed pA2 of 7.97 and a slope of -0.95. The pA2 value yielded a Kd of 11 nM. Specific binding in Tris-NaCl buffer (pH 7.4) is not affected by extracellular Ca2+ or Mg2+ in concentrations up to 750 microM. The pH optimum for binding resides between 7.0 and 7.4. The association rate constant, k1, was 4.5 X 10(6) M-1 min-1, and the dissociation rate constant, k-1, was 1.45 X 10(-1) min-1, yielding a kinetically determined Kd (k-1/k1) of 32 nM. Scatchard analysis of I-PTA-OH binding to washed canine platelets revealed two classes of binding sites, a high affinity site (Kd = 24 nM, Bmax = 71 fmol/10(7) platelets) (4400 binding sites/platelet) and a low affinity site (Kd = 2.1 microM). Several TXA2/PGH2 receptor antagonists competed with specific 125I-PTA-OH binding, and the rank order of potency for displacing the ligand correlated (r = 0.97) with the rank order of potency for their ability to inhibit U46619-induced aggregation in canine platelet-rich plasma. Prostaglandins F2 alpha and E2 also displaced the ligand, but only at much higher concentrations. Binding of I-PTA-OH or the TXA2/PGH2 mimetic U46619 was unaffected by the aggregating agents epinephrine (10 microM) or ADP (5 microM). The similarity in the Kd values obtained kinetically, by equilibrium binding studies for the high affinity site and by Schild analysis, suggests that this high affinity site mediates TXA2/PGH2 induced platelet aggregation. In addition, the close correlation between the abilities of the antagonists to displace the ligand and to inhibit U46619-induced aggregation suggests that this site may represent a TXA2/PGH2 receptor.