Temperature dependence of platelet aggregation and agglutination by ristocetin-mediated binding of vWF. (A and B) Aggregation of live platelets was monitored simultaneously with platelet [Ca²⁺]_i. The platelet suspension (50,000 cells/μL) was incubated at 20°C (A) or 37°C (B) in the presence of 1 mmol/L CaCl₂. Aggregation was deduced from the transmitted light intensity (during the 380 nm fluorescence excitation phase). Aggregation patterns at 25°C and 30°C (not shown) were indistinguishable from those at 20°C and 37°C. The results illustrated were acquired in parallel with the [Ca²⁺]_i measurements shown in Fig 2; these aggregation data are typical of four such studies. (C and D) Agglutination of paraformaldehyde-fixed platelets (50,000 cells/μL) was evaluated at 20°C (C) and 37°C (D) in an analogous manner. Agglutination patterns at 30°C (not shown) were comparable to those at 37°C. These data are typical of three such studies. Panels A through D present the responses on incubation either with 1 mg/mL ristocetin and 5 μg/mL multimeric human vWF (solid traces) or with 1 mg/mL ristocetin alone (dotted traces).