α-Actinin-1 deficiency reduces platelet function, including platelet spreading, clot retraction, aggregation, and ATP secretion. (A) Platelets from Actn1^f/f and PF4-Actn1^−/− mice were allowed to adhere to, and spread on, fibrinogen-coated coverslips for 90 minutes without or with ADP (20 μmol/L) or thrombin (0.1 U/mL) and then stained with tetramethyl rhodamine isothiocyanate-labeled phalloidin. The data shown are representative pictures from 1 of 3 experiments with similar results. The scale bar is 10 μm. (B) The left panel shows the percentage of the surface area covered by spreading platelets. The right panel displays the surface coverage area of each platelet (n = 3 independent experiments). (C) The clots were photographed at different time points. The percentage of the clot size was generated by calculating the ratio of the surface area of the retracted clots to that of the initial clots (n = 3 independent experiments). The data are presented as the mean and SD of 3 independent experiments. (D) Platelet-rich plasma or washed platelets from Actn1^f/f and PF4-Actn1^−/− mice were stimulated with ADP (10, 20, and 40 μmol/L), thrombin (0.02, 0.05, and 0.1 U/mL), and collagen (0.5, 2, and 4 μg/mL). The results are expressed as the percent change in light transmission relative to the blank (platelet poor plasma/buffer without platelets), set at 100% (n = 4 independent experiments). (E) ATP secretion from dense granules in platelets stimulated with agonists, including ADP (0, 10, 20, and 40 μmol/L), thrombin (0, 0.02, 0.05, and 0.1 U/mL), and collagen (0, 0.5, 2, and 4 μg/mL). The data are shown as the mean ± SD (n = 12 mice per group). ∗P < .05; ∗∗P < .01; ∗∗∗P < .005. ns, not significant.