D3 binding isotherms to platelets.

Platelets (2.5 × 10⁸/mL) were incubated with¹²⁵I-D3 (■, n = 4) or mouse IgG (n = 4), final concentrations of 0.025, 0.05, 0.1, 0.2, and 5 μmol/L, for 5 minutes at 37°C. ADP (10 μmol/L) was added either simultaneously with the antibody (▴, n = 4) or was preincubated along with the antibody for 2 minutes (○, n = 5). In one set of tubes, D3 or IgG was preincubated with the platelets for 2 minutes, and the ADP was added for 3 minutes (▪, n = 4). Gamma emissions were counted. Specific binding was quantitated by subtracting the nonspecific binding (IgG) from the D3 binding. Binding isotherms were generated by plotting the number of antibody binding sites versus moles of antibody added. A representative graph is shown.