Induced secretion of α granule contents from HPS model platelets is impaired at low doses of thrombin. Washed platelets (8 × 10⁷) isolated from WT, pallid, pearl, or light ear mice were stimulated with the indicated concentrations of thrombin in 100 μL of phosphate-buffered saline/0.1% bovine serum albumin for 10 minutes in the absence or presence of 10 μM adenosine 5′-diphosphate (ADP) as indicated, and then supernatants were collected and analyzed by enzyme-linked immunosorbent assay for PF-4 (A) or PBP (C). Untreated platelets (8 × 10⁷) were lysed in 100 μL of lysis buffer, and lysates were analyzed directly for content of PF-4 (B) or PBP (D). In panels A,C, the percentage of PF-4 and PBP in releasates relative to untreated cell lysates for each sample was plotted relative to the highest percentage of release observed in a single assay for WT platelets (range: 50% to 84% for WT). Data represent mean ± standard deviation from at least 3 independent experiments. *P < .05; **P < .01; ***P < .005.