Inhibitory effect of anti GPIb (6D1) and anti-GPIIbIIIa (7E3) antibodies. (A) Representative plot of recorded platelet aggregation subsequent to addition vWF–A₁–expressing COS-7 cells. The effect of anti-GPIb (αGPIb) on vWF–A₁-domain expressing COS-7 cell–induced aggregation was determined at relatively high ratios (1:200 COS-7 cells/platelets). HWP were preincubated with the MoAb 6D1 (23.0 μg/mL), known to block GPIb-mediated aggregation by to vWF (see Materials and Methods). (B) Aggregation and simultaneously recorded intracellular Ca⁺⁺ uptake (Δ [Ca²⁺]_i) of human platelets due to porcine A₁-domain (P_473-713) transfected COS-7 cells after preincubation of human washed platelets with 7E3 (1.0 μg/mL). Washed platelets were prepared as described in Materials and Methods. The upper (+)-tagged lines show human platelet aggregation as percentage of light transmission. The lower (×)-tagged lines give the simultaneously recorded change in luminescence due to platelet Δ[Ca²⁺]_i . The representative experiment for nonblocked platelets (left panel) is shown for COS-7 cell to platelet ratios of 1:280. The ratios had to be increased (1:140, right panel) to show platelet Δ[Ca²⁺]_i in the presence of GPIIbIIIa antibodies.