Specific binding of the new stable epoprostenol analogue beraprost sodium to prostacyclin receptors on human and rat platelets.

Binding of beraprost sodium (sodium dl-4-[(1R,2R,3aS,8bS)-1,2,3a,8b-tetrahydro-2-hydroxy-1-[(3S, 4RS)- 3-hydroxy-4-methyl-oct-6-yne-(E)-1-enyl] -5- cyclopenta[b]benzofuranyl]butyrate, TRK-100), a new potent antithrombotic agent, to washed platelets of humans and rats was studied. [11-3H]-TRK-100 binding was rapid, reversible, saturable, and highly specific. Scatchard analysis of concentration-dependent binding to human platelets revealed a single class of specific binding sites with an equilibrium dissociation constant (Kd) of 133 nmol/l and a maximal concentration of binding sites (Bmax) of 46 fmol/10(8) platelets (275 sites/cell). Similar binding was observed on rat platelets. The Kd and Bmax were 66 nmol/l and 124 fmol/10(8) platelets (750 sites/cell), respectively. Competitive studies indicated that TRK-100 was 1.5 times less active than prostacyclin (epoprostenol, PGI2), but was 3 times more potent than PGE1 in displacing [3H]-TRK-100 from the binding sites on rat platelets. PGE2, PGD2, PGF2 alpha, and pinane thromboxane A2, a stable thromboxane A2 analogoue, had no affinity for the binding sites. The relative affinity of the four enantiomers of TRK-100--APS-314d, 315d, 3141 and 3151--for the binding sites was 100: 14: less than 1: less than 1, respectively. These results suggest that TRK-100 is a useful tool for studying biological roles of PGI2 as well as for use as an antithrombotic agent since TRK-100 mimics its actions via specific interaction with PGI2 receptors.