ceacam1^−/− platelets display a lower threshold in alpha and dense granule release in response to stimulation with GPVI-selective agonists. (A) Wild-type (■), ceacam1^+/− (), and ceacam1^−/− (□) platelets were stimulated with varying concentrations of GPVI-selective agonists including type I fibrillar collagen in the absence of magnesium and CRP, or with thrombin. The amount of 5-hydroxytryptamine released was measured as described in “Methods.” The assays were performed in triplicate, and the results expressed as the mean plus or minus SEM. These results are representative of 3 independent experiments. Note that the release of dense granular contents in response to stimulation at subthreshold levels of GPVI-selective agonists was greater in ceacam1^−/− platelets than in wild-type and ceacam1^+/− platelets. (B) Surface expression of P-selectin (alpha granule release) was determined for washed platelets stimulated by thrombin, PAR-4 agonist peptide, and CRP at different concentrations. The platelets were then stained with either a buffer control or FITC-P-selectin monoclonal antibody for both wild-type and ceacam1^−/− platelets. FITC-labeled samples were analyzed on a FACSCanto analyzer. Results are representative of 3 independent experiments.