Dynamics of clot growth induced by thrombin diffusing into nonstirred citrate human plasma

The dynamics of clot formation was studied in a two-compartment chamber designed to allow free diffusion of thrombin according to its concentration gradient into nonstirred citrate plasma or fibrinogen solution. Fibrin clots in fibrinogen solutions increased progressively until the substrate was depleted. In plasma, the clot weight dynamics significantly depended on the concentration of thrombin in the thrombin compartment. When the thrombin concentrations were extremely low (25-40 nM), the clot weight increased throughout the experiment (sometimes 20-24 h). At higher thrombin concentrations, the clot weight increased for 1-2 h and then stopped growing for the following 3-4 h. The clot weight observed at the plateau varied only slightly in the range of thrombin concentrations of 50-770 nM. In this range, high thrombin concentrations (250-770 nM) caused a second increase in the clot weight 4-8 h after the start of diffusion, which was followed by the second plateau in the curve of clot weight against time. The time to the plateau and the plateau duration decreased with increasing thrombin concentrations. The abundant plasma inhibitors of thrombin cannot account for these results. It was hypothesized that an as yet unknown mechanism is responsible for the inhibition of clot growth.