Collagen-induced platelet aggregation and granule secretion requires β1 integrins. (A) Washed platelets were stimulated with 5 μg/mL fibrillar collagen (black curves) or 50 ng/mL CVX (gray curves) and aggregation was recorded for 600 seconds. (B) Aggregation of β1-null (KO) and β1^TTAA (TTAA) platelets in the presence of 100 μg/mL fibrinogen (gray curves) or 0.75 mM Mn²⁺ (black curves). (C) ATP release from platelet dense granules after stimulation of platelets with collagen. ATP was measured by luminescence after addition of luciferase luciferin reagent using the Lumi-aggregometer (Chronolog, Havertown, PA). Platelets were stimulated with either 5 μg/mL fibrillar collagen or 50 ng/mL CVX for 10 minutes. The relative luminescence after 8 minutes (ratio of collagen to CVX stimulated cells) is shown (bars represent mean values ± SEM of n = 3 independent experiments). (D) Serotonin release after stimulation with collagen. Platelets were stimulated with either 5 μg/mL fibrillar collagen or 50 ng/mL CVX for 8 minutes. The relative serotonin level (ratio of collagen to CVX stimulation) is shown (bars represent mean values ± SEM of n ≥ 3 independent experiments). (E) Quantification of the cellular fibrinogen content in resting platelets or 8 minutes after stimulation with either fibrillar collagen (5 μg/mL) or CVX (50 ng/mL; bars represent mean values ± SEM; significance levels are indicated; *P < .05; wild-type (WT), n = 3; knockout (KO), n = 3; TTAA, n = 3; hypomorphic (Hpm), n = 3; representative blots are shown in supplemental Figure 4B). (F) Platelet content of the α-granule cargo proteins vWF and thrombospondin-1 (TSP-1) in wild-type (WT) and β1-null (KO) platelets upon collagen and stimulation with CVX. (G) Ratio of PF-4 release from platelets stimulated with either 5 μg/mL fibrillar collagen or 50 ng/mL CVX for 8 minutes (bars represent mean values ± SEM of n ≥ 3 independent experiments).