Figure 2.

Ligand binding to platelet TXA₂ receptors. (a) [³H]U46619 binding to intact washed platelets. Platelets, pretreated with aspirin (1 mM) to prevent endogenous TXA₂ production, were incubated with thrombin, thrombin plus hirudin, ADP, A23187, plasmin, or TRAP at the concentrations indicated. Binding was performed as described (19), and results are expressed as percentage increase in [³H]U46619-specific binding relative to unstimulated platelets. All experiments were done in triplicate at least five times, and the values represent means ± SEM of all results. The average specific binding in the absence of agonist pretreatment was 18.2 ± 2.4 fmol/10⁹ platelets. Statistical significance was evaluated using a Kruskal–Wallis analysis of variance followed by Dunn’s multiple comparison test (∗ P < 0.05; ∗∗ P < 0.01). (b) [³H]SQ29,548 binding to solubilized platelet membranes. Platelet membranes were solubilized in CHAPS (15) and pretreated with thrombin, thrombin plus hirudin, ADP, A23187, plasmin, or TRAP at the concentrations indicated. Binding was performed as described (15), and results are expressed as the percentage increase in [³H]SQ29,548-specific binding relative to unstimulated, solubilized platelet membranes. All experiments were done in triplicate at least four times, and the values represent means ± SEM of all results. The average specific binding in the absence of agonist pretreatment was 138 ± 15 fmol/mg protein. Statistical significance was evaluated by using a one-way analysis of variance followed by Bonferroni’s multiple comparison test (∗ P < 0.05; ∗∗ P < 0.01).