Calcium responses of single platelets evoked by adhesion to fibrinogen or collagen matrices. Fura-2–labeled platelets were allowed to adhere to fibrinogen (A through C) or collagen (D through F ) surfaces in the presence of 2 mmol/L CaCl₂ and 0.2 U/mL apyrase; 2 nmol/L α-thrombin was given during the experiments where indicated. Timepoints of attachment are marked by arrows. Representative traces of individual platelets are given from at least 10 independent experiments.