Role of calcium in regulating thrombus contraction. Lepirudin-anticoagulated human whole blood, spiked with DiIC₁₂-labeled platelets, was perfused through collagen-coated microslides at 1800 s⁻¹. The decrease in distance between firmly adherent platelets was quantified as described in “Two-dimensional quantification of thrombus consolidation” and used as an indirect marker of thrombus contraction. (A) Whole blood was perfused in the presence of EGTA/Mg²⁺ (2 mM/1 mM). (B) For studies with 2-APB, whole blood was initially perfused for 30 seconds without inhibitor to allow for the initial formation of noncontracted thrombi (refer to “Two-dimensional quantification of thrombus consolidation”), followed by perfusion of whole blood in the presence of 2-APB (200 μM). (A,B) Results represent the mean plus or minus SEM (n = 5; *P < .05; **P < .005; ***P < .001). (C) To examine the importance of calcium flux for fibrin clot retraction, PRP was preincubated with 2-APB (200 μM), c7E3 (50 μg/mL), or EGTA/Mg²⁺ (2 mM/1 mM), followed by addition of thrombin (1 U/mL). Clot retraction was assessed as described under “Platelet-mediated fibrin clot retraction.” Results represent the mean plus or minus SEM (n = 3; ns = P > .05; **P < .005).