Binding of [³H]-SQ29548 to platelets from P1 and a healthy control and to CHO-K1 cells expressing either wild-type or variant D304N TxA₂R. (A) Washed fixed platelets from P1 from a related control (P3) who did not have the D304N TxA₂R substitution and from a healthy unrelated donor were incubated for 20 minutes with [³H]-SQ29548 (0.1μM) in the presence of unlabeled ligand (10μM) to determine specific receptor-dependent binding. Binding was then determined by measuring bound labeled ligand after a 20-minute incubation. Data are expressed as disintegrations per minute per 4 × 10⁶ platelets and represent the means ± SEM of 3 independent experiments. (B) Specific receptor-dependent [³H]-SQ29548 binding to either wild-type (WT TxA₂R) or variant D304N TxA₂R (D304N TxA₂R) expressed in CHO-K1 cells was determined as described in “Methods.” Data are expressed as disintegrations per minute per milligram of protein and represent the means ± SEM of 3 independent experiments.