The fibrinogen-fibrin complex: stoichiometry and negative cooperativity

Effect of cold storage on shear-induced platelet aggregation and clot strength

BACKGROUND

Platelets (PLTs) participate in hemostasis and save lives following trauma. PLTs for transfusion are maintained at room temperature (RT, 22°C), limiting viability to 5 days because of metabolic compromise and high risk of bacterial contamination. RT storage may result in weaker clots, delaying hemorrhage control, whereas cold storage (4°C) could permit longer PLT shelf life and result in a more hemostatic product. In this study, we characterized the effect of storage temperature on shear-induced PLT aggregation, clot formation, and strength.

METHODS

PLTs obtained from phlebotomized blood or by apheresis were stored at RT or 4°C for 5 days, and PLT aggregation and clot strength were assessed at 37°C. We studied PLT aggregation at steady and complex patterns of shear rates (500-2,500 per second) by flow cytometry, and the kinetics of clot formation and strength were measured using turbidity and dynamic mechanical analysis, respectively.

RESULTS

PLT aggregation was higher in 4°C-stored samples on Day 5 compared with fresh or RT-stored samples at all shear rates tested (fresh vs. 4°C and RT vs. 4°C, p < 0.05). PLTs stored at 4°C for 5 days formed significantly stronger clots compared with fresh or RT-stored samples as quantified by turbidity and elastic moduli measurements (fresh vs. 4°C and RT vs. 4°C, p < 0.05).

CONCLUSION

Our results show that cold-stored PLTs are more responsive to aggregation stimuli and form stronger clots, presumably because of thicker fibrin strands. These data suggest that the superior functionality of cold-stored PLTs may support faster hemostasis for acutely bleeding in trauma patients compared with RT-stored PLTs.

Simulating fibrin clotting time

The clotting time (CT) of fibrinogen mixed with thrombin decreased, then increased with increasing fibrinogen levels. By contrast, log CT decreased monotonically with respect to the log level of activating enzyme (thrombin or reptilase). Here, the CT was determined over a large range of fibrinogen concentration (to 100 mg ml(-1)) at a fixed level of enzyme. A new parameter, [Fib]min, the minimal fibrinogen concentration required for thrombin or reptilase-instigated phase change (coagulation), was determined as [Fib]min = 0.2 +/- 0.05 microM fibrinogen. A dynamic simulation program (Stella) was employed to organize simulations based on simple and complex coagulation mechanisms, which generated CT values. The successful simulation aimed at forming [Fib]min and "recognized" the binding of unreacted fibrinogen with intermediate fibrin protofibrils. The "virtual data" mimicked the biphasic experimental CT values over a wide range of concentrations. Fibrinogen appeared to act in three modalities: as a thrombin substrate; as a precursor of fibrin; and as a competitor for fibrin protofibrils. The optimized simulation may provide a basis for predicting CT in more complex systems, such as pathological plasmas or whole blood or at high concentrations encountered with fibrin sealant.