Use of [3H]trimetoquinol as a radioligand in human platelets: interaction with putative endoperoxide/thromboxane A2 receptor sites.

We have recently shown that trimetoquinol [1-(3,4,5-trimethoxybenzyl)-6,7-dihydroxy-1,2,3,4- tetrahydroisoquinoline; TMQ] is a potent and stereoselective [R(+)-isomer greater than S(-)-isomer] antagonist of aggregation induced by thromboxane A2 and stable epoxymethano PGH2 analogues (U46619; U44069) in human platelets. The present study was undertaken to characterize the pharmacological specificity of binding sites for racemic- [3H]TMQ in washed human platelets. Specific binding of [3H]TMQ, determined by addition of 100 microM R(+)-TMQ, accounted for 12-26% of the total binding at 1.0 microM. Saturation data suggested two binding sites with apparent dissociation constants of 2.8 nM and 1.4 microM for the high and low affinity binding sites, respectively. Maximal binding densities (pmol/mg protein) were 2.3 and 42.9 for the respective high and low affinity sites. The optical isomers of TMQ, cis and trans-isomers of 13-azaprostanoic acid (13-APA), and U46619 inhibited [3H]TMQ specific binding to the low affinity site. R(+)-TMQ was 32-fold more potent than S(-)-TMQ at inhibiting [3H]TMQ binding, and the stereoselective potency difference and concentrations needed to inhibit binding were similar to those required for inhibition of U46619-induced platelet aggregation and serotonin secretion. Similarly, trans-13-APA was 90-fold more potent than cis-13-APA as a competitor of [3H]TMQ binding. U46619 also inhibited TMQ binding at concentrations (IC50 = 0.4 microM) similar to those required for aggregation (EC50 = 0.16 microM) and secretion (EC50 = 0.23 microM). At 1 mM, arachidonic acid inhibited [3H] TMQ binding by only 25%. Our studies indicate that [3H]TMQ interacts with specific binding sites with characteristics indicative of putative endoperoxide/thromboxane A2 receptors in human platelets.