Effect of R5421 on Ca²⁺-dependent scrambling of NBD-PS in human platelets stimulated with Ca²⁺-ionomycin. Platelets were preincubated with 50 μmol/L R5421 for 60 minutes. At arrow 1, dithionite (5 mmol/L) was added; at arrow 2, ionomycin (1 μmol/L); at arrow 3, CaCl₂ (1 mmol/L); and at arrow 4, Triton X100 (1%). Curve a: control platelets in the absence of R5421, curve b in the presence of R5421. Curve c represents Scott platelets stimulated with Ca²⁺-ionophore. Insert: effect of preincubation time with R5421 (20 μmol/L) on rate of scrambling.