Rheology of fibrin clots. II. Linear viscoelastic behavior in shear creep

The elastic modulus of fibrin clots and fibrinogen gels: the effect of fibronectin and dithiothreitol

The elastic modulus (G') of factor XIIIa induced fibrinogen gels was found to be substantially lower than the G' of fibrin gels that were formed by clotting fibrinogen with thrombin. The addition of fibronectin and/or the reducing reagent dithiothreitol (DTT) to the factor XIIIa coagulation mixture led to the formation of a weaker gel structure, while the rigidity of thrombin induced clots was not appreciably affected by the inclusion of the DTT but increased somewhat in the presence of fibronectin. The reasons for the differing clot rigidities are discussed in terms of biochemical mechanisms.

Small-angle X-ray scattering studies of fibrin film: comparisons of fine and coarse films prepared with thrombin and ancrod

Measurements of small-angle x-ray scattering have been made on films prepared from fine and coarse (i.e., formed at high and low, respectively, pH and ionic strength) clots of bovine fibrin by osmotic shrinkage or compression in one dimension. Intensity profiles were obtained with pinhole geometry on films stretched up to a stretch ratio of 1.43. In unstretched coarse films, repeat spacings were seen at about 245, 120, and 77-80 A. These peaks can probably be identified with the first, second, and third orders of the well-known fibrin repeat of 225 A. In unstretched fine films, only the 77-80 A spacing was seen. In this case, the first two orders may be weak because the half-staggered arrangement of monomer units giving rise to the 225 A reflection is not reinforced by lateral aggregation of protofibrils; the third order may be strong since the molecular subdomains appear to divide the repeat roughly into thirds. After stretching, the 77-80 A spacing persisted in the meridional direction but almost disappeared in the equatorial. Experiments on unstretched films prepared with ancrod substituted for thrombin gave similar results.

Strain enhancement of elastic modulus in fine fibrin clots

Fine fibrin clots, prepared at pH 8.5, ionic strength 0.45, with minimal lateral aggregation of protofibrils, and ligated (cross-linked) by factor XIIIa, were subjected to constant static shear strain (gamma) with superposed small oscillating strains. The incremental shear modulus (dynamic storage modulus) measured in the oscillating deformations was strain-independent at small static strains (up to about 0.1) and approximately equal to the static modulus. At higher static strains, it increased rapidly, up by a factor of 5 to 8 at gamma = 0.35. Comparison with earlier data on unligated clots showed that the enhancement of stiffness was independent of ligation except at very high strains. The enhancement is attributed to additional forced contacts between network fibers as the strands are bent and oriented. When the static strain was maintained for up to one day, in a clot ligated by factor XIIIa the enhanced incremental modulus remained constant or decreased slightly, and after removal of stress the clot returned almost to its original shape. This contrasts with the behavior of unligated clots, where most of the enhancement was progressively lost as the incremental modulus fell toward its small-strain value, and there was a substantial permanent deformation after the removal of stress. The latter behavior has been attributed to gradual severance of network strands at high strains, followed by their rejoining in relaxed configurations, but leaving some structural damage that is only very slowly recovered in the resting state. Ligation of protofibrils evidently eliminates the possibility of strand rupture.

Thermally induced conformational changes in fibrin film

Fibrin film is prepared by compaction in one dimension of a fibrin clot (pH 6.3, ionic strength 0.15, fibrin concentration about 0.5%) by expulsion of fluid to reach a fibrin concentration of about 15%. Strips of film, equilibrated in the same buffer with very slowly increasing temperature, shrink in length in a narrow temperature range, as reported in 1962 by Loeb and Scheraga. The transition temperature was found to be 54 +/- 2 degrees C independently of whether the film was unligated or ligated (cross-linked) by Factor XIIIa and whether the film had previously undergone stretching with about 50% stress relaxation at a relative length of 1.23 to 1.44 and subsequent stress-free retraction. The percentage of linear shrinkage in buffer was about 32%. The transition corresponds to that observed calorimetrically by Mihalyi and Donovan in both fibrinogen and fibrin and by Medved' and Privalov in fibrinogen, localized in the D fragment. It is attributed to unfolding of structures in the D domain.

Contribution of fibrin stabilization to clot strength. Supplementation of factor XIII-deficient plasma with the purified zymogen

The contribution of fibrin stabilization to clot strength, measured as the static elastic modulus, was evaluated in human plasma by two independent procedures. In the first approach, amine inhibitors of fibrin stabilization were examined for their effects on the rigidity of normal plasma clots. It is a unique property of these inhibitors that they do not interfere with the reversible aggregation of fibrin molecules, i.e., do not delay clotting time, but selectively prevent only the formation of gamma-glutamyl-epsilon-lysine protein-to-protein linkages. Though the compounds tested were of different chemical structures and potencies, a fivefold reduction in clot strength was obtained in each instance. This value of 20% of normal seems to correspond to the rigidity of the Factor XIII-deficient plasma clot because, as demonstrated by the second approach, when a plasma specimen that genetically lacked the fibrin stabilizing factor was supplemented by the addition of measured amounts of the purified zymogen, a fivefold increase in clot strength could be achieved. The described procedure of evaluating Factor XIII in terms of correcting the elastic modulus of a deficient plasma clot is considered an important assay for the functional competence of purified preparations of the zymogen for the purpose of therapeutic application.

Modification of shear modulus and creep compliance of fibrin clots by fibronectin

Shear moduli and creep compliances have been measured for four types of clots of human fibrin (about 7 mg/ml) clotted with and without human plasma fibronectin (usually 1.2 mg/ml). Fine clots (with little lateral aggregation of the fibrin protofibrils) were found at pH 8.5, ionic strength 0.45; coarse clots (with substantial lateral aggregation) were formed at pH 7.5, ionic strength 0.15; in both cases with and without ligation by fibrinoligase. In fine clots, the addition of fibronectin without ligation scarcely affected the shear modulus; with ligation, the modulus was decreased by a factor of 0.48. In coarse clots, the shear modulus was increased by addition of fibronectin. The increase was by a factor of 2.0 without ligation and by a factor of 2.4 with ligation. Creep and creep recovery in clots formed with and without fibronectin were similar except for the scale factor represented by the change in modulus.

Effect of temperature on dynamic viscoelasticity during the clotting reaction of fibrin

The dynamic rigidity and loss moduli for fibrinogen-thrombin solution were determined during clotting in the temperature range between 15 and 45 degrees C. The rigidity of fibrin gel decreased with increasing clotting temperature, owing to the dissociation of cross-links. The rate constant of the dissociation of cross-links increased with increasing temperature. The rate constant of the cross-linking reaction increased and then decreased through a maximum with increasing temperature. It is explained by assuming that denaturation of fibrin occurs at high temperature. The irreversible denaturation becomes appreciable at high ionic strength. The activation energy and the enthalpy change for the cross-linking reaction of fibrin is about 35 and 15 kcal/mol, respectively. The enthalpy change for the reversible denaturation is about 46 kcal/mol.

Identification of fibrin oligomers in sonicated fibrin clots

Clots of bovine fibrin, with both coarse and fine structure, and ligated to different extents by fibrinoligase, have been broken up by ultrasonic agitation and the sonicates have been examined by ultracentrifugal sedimentation. Sonication is followed by gross aggregation of the fragments unless guanidine hydrochloride is introduced (order of 1 M). In that case, sonicates of gamma-ligated fine clots contain two species whose sedimentation coefficients correspond to fibrin monomer and an oligomer with twice the monomer cross-section area and at least 20 monomer units, presumably with the structure of lateral dimerization with staggered overlapping. If the gamma ligation is incomplete, shorter oligomers are identified. The monomer and oligomer with degree of polymerization greater than 20 appear also in sonicates of coarse clots, but in smaller amounts, the principal product consisting of larger aggregates. The implications of these results with respect to metastability of the fine clot and the pattern of polymerization are discussed.

The identification of staphylococci in clinical and food microbiology laboratories

A comprehensive review of the methods which have been utilized for the identification of staphylococci is presented. Biochemical characteristics which have assisted in the primary isolation of staphylococci, such as pigmentation, hemolytic activity, the egg yolk phenomenon, and deoxyribonuclease and coagulase production, are also analyzed. The potential applicability of advanced techniques to identify staphylococci, such as the detection of enterotoxin production, base ratio analysis, cell wall analysis, phage typing, and serology, is discussed. The following procedures are recommended for routine use: Idnetification of Staphylococcus sp. (clinical laboratories): microscopic observation, catalase activity, coagulase production, lysostaphin sensitivity, and (optional) facultative growth in thioglycolate medium. Identification of Staphylococcus aureus (food laboratories): microscopic observation, catalase activity, coagulase production, thermonuclease production, and (optional) lysostaphin sensitivity.

Rheology of fibrin clots. III. Shear creep and creep recovery of fine ligated and coarse unligated closts

Creep and creep recovery of human fibrin clots in small shearing deformations have been investigated over a time scale from 24 to 10(4) s. Coarse, unligated clots and fine clots ligated by fibrinoligase in the presence of calcium ions were studied to suppliement previous data on coarse ligated and fine unligated clots. Stress was found to be proportional to strain up to at least a maximum shear strain (in torsion geometry) of 6.2%. The initial modulus (25 s after imposition of stress) is proportional to approximately the 1.5 power of concentration for fine ligated and coarse unligated clots. For fine unligated closts there is comparatively little creep subsequent to the initial deformation; ligation (in this case involving mostly the gamma chains) reduces the creep to nearly zero. For coarse unligated clots, there is substantially more creep under constant stress, and creep recovery is not complete. Ligation (in this case involving both camma and alpha chains) alrgely supresses the creep and causes the recovery to be complete. If the structure if fully formed before creep begins, tests of creep recovery by the Boltzmann superposition principle show adherence to linear visoelastic behavior for all four clot types. Otherwise, the Boltzmann test fails and the recovery is much less than calculated. For fine ligated clots, the observed recovery agrees well with that calculated on the basis of a dual structure model in which an additional independent structure is built up in the deformed state, so that the state of ease after removal of stress is a balance between two structures deformed in opposite senses. It is postulated that the coherence and elastic modulus of the fine ligated clot are largely due to steric blocking of long protofibrils with a high flexural stiffness. In the coarse clot, it is proposed that the structure involves extensive branching of thick bundles of protofibrils, which become permanently secured by the ligation of the alpha chains of the fibrin.