Characterization of Ip6k1^−/− platelets. (A) Flow cytometry analysis of fixed platelets from WT and Ip6k1^−/− mice to determine forward scatter as a reflection of platelet size. Data are mean ± standard error (n = 5). (B) Representative (left) fixed and (right) whole mount transmission electron micrographs of platelets pooled from 3 mice of each genotype. In fixed samples, arrows indicate dense granules and arrowheads indicate α-granules. Dense granules are clearly visible in whole mount micrographs. Bars represent 0.5 µm. (C) Surface P-selectin expression in unstimulated and thrombin stimulated WT and Ip6k1^−/− platelets analyzed by flow cytometry. Data are mean ± standard error (n = 3). (D) Binding of Alexa Fluor 488–conjugated fibrinogen to integrin α_IIbβ₃ on the surface of unstimulated and ADP-stimulated WT and Ip6k1^−/− platelets, monitored by flow cytometry. Data are mean ± standard error (n = 5). (E) Thrombin stimulated aggregation of washed platelets from WT and Ip6k1^−/− mice measured spectrophotometrically as a decrease in percent light transmission over a period of 10 minutes. Samples were pooled from 3 mice of each genotype for the analysis. Data are mean ± standard error from 3 independent experiments. (F) Flow cytometry analysis of washed and fixed WT and Ip6k1^−/− platelets labeled with a serotonin antibody, followed by an Alexa Fluor 488–conjugated secondary antibody (stained) or the secondary antibody alone (unstained). Data are mean ± standard error (n = 6). Data were analyzed by a 2-tailed Student t test (n.s., not significant, P > .05). MFI, median fluorescence intensity.