Characterization of thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptors of rat platelets and their interaction with TXA2/PGH2 receptor antagonists.

Characterization of thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptors of rat platelets was performed on both intact platelets and crude membrane fractions. The binding of [3H]U46619, a stable TXA2 mimetic, to intact platelets was found to be saturable and displaceable. Scatchard analysis of equilibrium binding at 24 degrees revealed a single class of binding sites with a Kd of 37 nM and a Bmax of 160 fmol/10(8) platelets. The binding affinity of [3H]U46619 to the platelet membrane fractions was remarkably and specifically enhanced by addition of Mg2+ without alteration of the maximum density level. Kinetic analysis for [3H]U46619 binding to the membrane fractions in the presence of 20 mM MgCl2 gave a K1 of 6.9 x 10(6) M-1 min-1 and a K-1 of 0.25 min-1, yielding a Kd (K-1/K1) of 36 nM; the value corresponded well to Kd values from Scatchard analysis in both intact (37 nM) and crude membrane fractions (39 nM). A series of TXA2/PGH2 receptor antagonists completely suppressed U46619 binding to rat platelets as well as collagen-induced platelet aggregation. The rank order of binding affinities to rat platelets (intact platelets or crude membranes) among the respective antagonists correlated well with (a) that of human platelet membrane fraction and (b) the potencies for suppression of collagen-induced platelet aggregation in rat. These results may support our proposed mechanism of TXA2/PGH2 action in collagen-stimulated platelets [K. Hanasaki et al., Thromb. Res. 46, 425 (1987)] and also suggest that they may provide a simple technique for evaluating synthetic TXA2/PGH2 receptor antagonists.