Parameters of β1-tubulin^—/—platelet secretion and aggregation. (A) Normal [¹⁴C]5-HT secretion by stimulated β1-tubulin^—/— platelets. Wild-type (□) and mutant (▪) platelets preloaded with [¹⁴C]5-HT were stimulated with the indicated concentrations of thrombin for 2 minutes, and [¹⁴C] release was determined as a fraction of total platelet content. Results are aggregated from 3 or 5 independent experiments and expressed as the mean ± SD for each dose of thrombin. (B-C) Results of aggregometry assays on fresh platelets from mice of the indicated β1-tubulin genotype stimulated in vitro with 10 μM ADP (B; platelet-rich plasma) or with 1 U/mL thrombin (C; washed platelets). To highlight the blunted shape change in the absence of β1-tubulin, washed platelets (inset in C) were separately stimulated with a low dose of thrombin (0.05 U/mL) that is insufficient to aggregate washed platelets.