Ligand binding to thromboxane receptors on human platelets: correlation with biological activity

The preparation of enantiomerically pure [3H]-15 (S) 9, 11-epoxymethano PGH2 (a thromboxane A2-like agonist) has enabled the binding of ligands to the thromboxane receptor of the human platelet to be studied. The binding of the radio-ligand to washed human platelets has 3 components. One component is not displaceable by 'cold' 9, 11-epoxymethano PGH2 and its concentration-binding plot is roughly linear. The other 2 components are displaceable and saturable, and the larger of the two, which is sensitive to the stereochemistry of the C15 secondary alcohol, appears to represent the thromboxane receptor. About 1700 15(S)9, 11-epoxymethano PGH2 molecules are specifically bound to a single platelet and 50% of this binding is achieved with a concentration of 75 nM. Displacement of [3H]-15(S)9, 11-epoxymethano PGH2 is effected by (a) TXA2 and PGH2 and a number of bicyclic stable analogues (e.g. 9,11-azo PGH2), all of which produce irreversible aggregation of human platelets; (b) analogues of PGF2 alpha with potent thromboxane-like activity (e.g. ICI 79939); (c) compounds with partial agonist activity on the platelet thromboxane system (e.g. CTA2); (d) Thromboxane/endoperoxide analogues which specifically antagonize thromboxane-like actions on the human platelet (e.g. PTA2 and EP 045). Displacement is not achieved with the natural prostaglandins PGE2, PGD2 and PGF2 alpha. Neither the thromboxane-synthetase inhibitor dazoxiben nor R(+)-trimethoquinol have high displacing activity. The correlation of radio-ligand displacement with the biological activity of the competing ligands is discussed in relation to the nature of the thromboxane receptor on the human platelet.