Effect of α2β1 density on platelet adhesion and [Ca²⁺]_i elevations. (A) The density of α2β1 on the platelet membrane was determined in 2 normal donors by flow cytometry using the monoclonal antibody R2-7E4. The difference in mean fluorescence intensity was significant (P < .001). (B) Washed blood cell suspensions, derived from donors whose platelets were known to have high or low α2β1 density, were prepared as described in the legend to Figure 1 and perfused over immobilized acid-soluble collagen type I at the shear rate of 250 seconds. The time course of the accumulation of firmly adherent platelets, measured every 10 seconds for 200 seconds, is shown. Data are the mean ± 95% CI of 3 different experiments. The 2 curves are significantly different (P < .01). (C-D) Typical α-like (short-lasting) and γ-like (long-lasting) [Ca²⁺]_i peaks were identified with both types of platelets. (E-F) Note that, in either case, γ-like peaks were essentially abolished after platelet incubation with the anti-GPVI antibody, Fab 9O12.2 (50 μg/mL), whereas α-like peaks could still be observed.