Uptake and inactivation of thrombin by the fresh, glutardialdehyde or heparin treated human umbilical cord vein endothelium

Heparinization of Biological Vascular Graft Reduces Fibrin Deposition

An alternative graft is needed for coronary bypass operations in patients lacking suitable autologous vessels. We therefore studied Denaflex™, a biologic graft, in a dog ex-vivo shunt model to determine whether heparin treatment makes this graft less thrombogenic. Comparison was also made to Bioflow™, a nonheparinized biologic graft. Fibrinogen deposition during high flow (593 ± 202 ml/min) decreased from 672 ± 467 ngl mm2 in nonheparinized Denaflex grafts to 448 ± 298 ng/mm2 (p<0.05) in heparinized Denaflex grafts. At low flow (117 ± 13 ml/min), heparinization of Denaflex grafts similarly decreased fibrinogen deposition from 1102 ± 601 ng/mm2 to 703 ± 405 ng/mm2 (p<0.05). At both flow rates fibrinogen deposition in Bioflow grafts was less than in nonheparinized Denaflex, but was similar to heparinized Denaflex grafts. Platelet deposition was not influenced by heparinization of Denaflex grafts and was similar among Denaflex and Bioflow preparations. Whether Denaflex performs acceptably in vivo as a xenograft requires extensive study.

Adsorption of coagulation proteins from whole blood on to polymer materials: relation to platelet activation

A combination of methods, immunoassays of plasma proteins and platelet release of beta-thromboglobulin and chromogenic substrates for enzymatically active coagulation factors, was used to measure the reactions of coagulation proteins upon contact between whole blood and artificial surfaces as a function of time and surface material. Four types of well-known polymer surfaces, polyvinylchloride, polytetrafluoroethylene, polyurethane and silicone rubber, were investigated to elucidate if a simple and fast in vitro experimental set-up can be of guidance in the selection of materials for use in vivo. Platelets were activated at the polymer surfaces whereas the coagulation enzymes showed little activity on the polymer surfaces tested. There was a correlation between the adsorption of adhesins (fibrinogen, fibronectin and factor VIII-related antigen) at the surfaces and the release of beta-thromboglobulin from platelets, suggesting that adsorption of adhesins is a major determinant of blood compatibility of polymer materials. Significant differences between the surfaces were seen--polyurethane being the surface with the least protein adsorbed and least platelet activation initiated. This study shows that it is possible to make a first in vitro choice of possible blood compatible artificial surfaces before expensive and cumbersome in vivo experiments.

Adsorption of coagulation proteins and adhesion and activation of platelets at the blood-solid interface. An experimental study of human whole blood

A procedure is presented allowing detailed studies of the adsorption of coagulation factors from whole blood on to surface. Anticoagulant (citrate or hirudin) was added to fresh venous blood. The blood was incubated in hydrophilic or hydrophobic glass tubes without contact with air. The adsorption of fibrinogen, fibronectin and factor IX was measured with an enzyme immunoassay using specific antibodies directed against these proteins. Adsorption of enzymically active kallikrein was measured using a chromogenic peptide substrate. Adhesion and activation of platelets was measured by direct examination in a scanning electron microscope and by measurement of release of beta-thromboglobulin. The results show that the adsorption of plasma proteins at the blood-solid interface is dependent on the anticoagulant used, surface energy of the test surface and incubation time. In experiments using hirudin a specific inactivator of thrombin, as anticoagulant, we found dynamic changes of the adsorbed protein film which could not be studied using citrated blood.

Cell biology of endothelial cells

Endothelial cells are a source of physiologically important molecules synthesized therein and secreted to the blood and/or to the subendothelial extracellular matrix. These molecules participate in formation of platelet and fibrin thrombi (e.g., von Willebrand factor and tissue factor) and contribute to antithrombotic properties of the endothelium (e.g., prostacyclin, thrombomodulin, and heparan sulfate). Endothelial cells synthesize and secrete plasminogen activator and inhibitors. They are the source of molecules regulating the growth of other cells; they synthesize angiotensin-converting enzyme, and bind lipoproteins and hormones. Finally, they are the target for, and participant in, immune reactions. Thus, endothelial cells constitute not only the first barrier between the blood and the extravascular space but also serve as a source of molecules influencing the structural and functional integrity of the circulation.

Why small caliber vascular grafts fail: a review of clinical and experimental experience and the significance of the interaction of blood at the interface

The results using polytetrafluoroethylene (PTFE) and tanned human umbilical cord vein (HUV) grafts are discussed. These two grafts perform well for reconstructions above the knee and the results are similar to the autogenous saphenous vein (ASV) grafts in several series; however, in more distal reconstructions the PTFE and HUV are far inferior to ASV grafts. The factors responsible for graft failure including the events which take place at the blood interface are reviewed. New material surfaces under investigation such as degradable grafts, heparin-bonded surfaces, and endothelial-seeded fabrics are also addressed.

Thrombin uptake and inhibition on heparinized polytetrafluoroethylene (PTFE) grafts and native sheep vessels

Non-heparinized and covalently-heparinized PTFE grafts, sheep aorta, carotid artery and jugular vein were evaluated according to their ability to adsorb and inactivate thrombin. All surfaces, except non-heparinized PTFE caused considerable losses of enzymatically active thrombin from the solution. Covalently-heparinized PTFE and jugular veins adsorbed thrombin, which was inactivated on subsequent contact with antithrombin III. Despite the same loss from the solution as was encountered with veins arteries adsorbed significantly smaller amounts of thrombin. Protamine treatment of covalently-heparinized PTFE almost totally abolished the ability to adsorb thrombin. It is concluded that 1) arteries possess a stronger thrombin inhibitory capacity than veins, possible explanations to this being a different glucosaminoglycan or the presence of antithrombin III in the arterial wall and 2) that heparinizing PTFE results in a pronounced ability to inactivate thrombin.

In vitro evaluation of a biologic graft surface. Effect of treatment with conventional and low molecular weight (LMW) heparin

Human umbilical vein grafts were treated with either conventional or LMW heparin, followed by exposure to alcohol. The grafts were investigated for their ability to adsorb and inactivate thrombin, and comparison was made with non-heparinized and saline-alcohol treated grafts and grafts supplied with a covalently bonded layer of conventional heparin. In addition, the effect of protamine exposure to heparin-alcohol and LMW heparin-alcohol treated grafts as well as native human umbilical veins (HUV) was studied. Native HUV and heparin treated graft surfaces adsorbed and inactivated thrombin, whereas non-heparinized and saline-alcohol treated grafts inactivated surface-bound thrombin to only a small degree. Surface-bound LMW heparin exhibited a significantly lower ability to inactivate thrombin as compared with conventional heparin, but LMW heparin-alcohol surfaces were better than non-heparinized ones. Protamine treatment of "heparinized" surfaces impaired the thrombin inhibiting ability of the heparin-alcohol surface, whereas this property was totally abolished for the LMW heparin-alcohol surface. The findings indicate that LMW heparin, despite its weaker thrombin inhibiting capacity, may be an alternative to conventional heparin, for "heparinizing" the human umbilical vein graft. Protamine exposure may be potentially harmful for a heparin treated surface, although protamine concentrations used in the present in vitro study may not be reached in vivo. The native HUV was not at all affected by protamine exposure regarding its ability to inactivate thrombin.

Thromboxane production in umbilical vein grafts

Umbilical vein grafts were placed in the carotid arteries of sheep and removed after 10 or 90 days for determination of in vitro production of 6-keto-PGF1 alpha and thromboxane B2 (TxB2) in the anastomoses and in the middle of the grafts. At both intervals the grafts produced much more TxB2 than carotid arteries, but no significant difference in production of 6-keto-PGF1 alpha was found between grafts and carotid arteries. The strongly decreased 6-keto-PGF1 alpha/TxB2 ratio indicates a low resistance to platelet adhesion and aggregation, which may be a contributory cause of the thrombogenicity of vascular grafts and may possibly be involved in the pathogenesis of neointimal fibrous hyperplasia.

A new non-thrombogenic surface prepared by selective covalent binding of heparin via a modified reducing terminal residue

A new method for the covalent binding of heparin to artificial surfaces has been developed. The heparinized surface releases insignificant amounts of heparin and can be regarded as stable. The blood contact properties as studied in vitro revealed that the surface was highly thromboresistant in terms of reduced platelet adhesion, surface catalyzed adsorption and inhibition of thrombin and capacity to prevent clotting of nonanticoagulated blood.

Uptake and inhibition of thrombin by the vascular wall

Thrombin activity and inhibition were assayed on the aortic surface of dogs and pigs. After sacrifice, the aortae were excised and thrombin activity was measured with an amidolytic assay. A small thrombin activity was found on the endothelium. Heparinization of the animals lowered endothelial thrombin activity. After exposure of the aortic endothelium to a thrombin/albumin solution in vitro, thrombin activity disappeared from the solution and was recovered on the surface. De-endothelialized vessels took up more thrombin than those with intact endothelium. Endothelium confined thrombin was rapidly inactivated when exposed to plasma. A slow inactivation was seen upon exposure to a modified Ringer's solution. Thrombin confined to de-endothelialized aortae, i.e. to medial structures, was always inactivated at a slow rate irrespective as to whether it was exposed to plasma or Ringer's solution. It is concluded that endothelium and subendothelium bind thrombin and subsequently inactivate it. The inactivation proceeds faster on endothelium when exposed to plasma. The possible role of glycosaminoglycans is discussed.