Fibrin formation: effect of calcium ions.

Using laser fluctuation spectroscopy, a technique that measures particle size change in solution, the kinetics of fibrin clot formation from fibrinogen can be studied. With this technique the effect of calcium on the three distinguishable phases of clot formation, (1) proteolysis of fibrinogen, (2) fibrinogen-fibrin monomer complex formation, and (3) fibrin monomer polymerization, were investigated. Only a small change in the length of the induction period that results from the fibrinogen-fibrin monomer interactions was observed. However, there was a marked increase in the rate of fibrin monomer polymerization in the presence of calcium ions. These data show that calcium decreases the time required for fibrin formation from fibrinogen by markedly accelerating the phase of fibrin monomer polymerization.

Fibrin formation: the role of the fibrinogen-fibrin monomer complex.

The process of fibrin formation using highly purified fibrinogen and thrombin was studied using laser fluctuation spectroscopy, a method that rapidly determines particle size in a solution. Two periods in fibrin clot formation were noted: an induction period during which no fibrin polymerization occurred and a period of rapid increase in particle size. Direct measurement of fibrin monomer polymerization and fibrinopeptide release showed no evidence of an induction period. These observations were best explained by a kinetic model for fibrin clot formation incorporating a reversible fibrinogen-fibrin monomer complex. In this model, the complex serves as a buffer system during the earliest phase of fibrin formation. This prevents the accumulation of free polymerizable fibrin monomer until an appreciable amount of fibrinogen has reacted with thrombin, at which point the fibrin monomer level rises rapidly and polymerization proceeds. Clinically, the complex may be a homeostatic mechanism preventing pathological clotting during periods of elevated fibrinogen.