Effect of fibrinogen on the maintenance of Ca⁺⁺ entry.

(A) Fura-2–loaded human platelets suspended in HBS containing 1 mM Ca⁺⁺ were stimulated with ADP (5 μM) (Control). Fibrinogen (1 mg/mL) was added 10 seconds later (FG), as shown by the arrow, or 45 seconds before the addition of ADP (FG [control]). (B) Human platelets suspended in HBS containing 1 mM Ca⁺⁺ were stimulated with TG (200 nM) (Control). The addition of fibrinogen (FG; 1 mg/mL) was made 1½ minutes after the addition of TG, as shown by the arrow. (C) Human platelets were stimulated in a Ca⁺⁺-free medium (100 μM EGTA) with ΤG (200 nM). Three minutes later, CaCl₂ (final concentration, 300 μM) was added to the medium to initiate Ca⁺⁺ entry (Control). The addition of fibrinogen (FG; 1 mg/mL) or lanthanum chloride (LaCl₃; 100 μg/mL) was made 2 minutes after the addition of Ca⁺⁺, as shown by the arrow. Traces shown are representative of 6 independent experiments.